The Elevation and Surface Morphology of Microatolls on the Cocos (Keeling) Islands, Indian Ocean Scott Geoffrey Smithers University of Wollongong

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1997 The elevation and surface morphology of microatolls on the Cocos (Keeling) Islands, Indian Ocean Scott Geoffrey Smithers University of Wollongong

Recommended Citation Smithers, Scott Geoffrey, The lee vation and surface morphology of microatolls on the Cocos (Keeling) Islands, Indian Ocean, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 1997. http://ro.uow.edu.au/theses/1988

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THE ELEVATION AND SURFACE MORPHOLOGY OF MICROATOLLS ON THE COCOS (KEELING) ISLANDS, INDIAN OCEAN

A thesis submitted in fulfilment of the requirements for the award of the degree of

Doctor of Philosophy

from t

THE UNIVERSITY OF WOLLONGONG

by

Scott Geoffrey Smithers B. Env. Sci. Hons. (Wollongong)

School of Geosciences

1997 ABSTRACT

This thesis examines the elevation and surface morphologies developed by massive

Porites microatolls growing in reef flat, interisland passage and lagoonal habitats around the

Cocos (Keeling) Islands, Indian Ocean. It has long been known that corals adopt the microatoll

form near to the sea surface, and microatolls are widely recognised as important and relatively

precise indicators of sea-level position, particularly through the mid to late Holocene. Recently

it has also been suggested that the upper surface microtopographies of modern microatolls

may yield valuable information about recent sea-level change. However, rarely has the

elevation of microatolls been accurately surveyed to a known datum, or the microtopographies

of their upper surfaces accurately measured. This thesis has addressed this shortcoming by

accurately surveying the upper limit to coral growth around the rims of 282 microatolls from 19

separate sites around the Cocos (Keeling) Islands, Indian Ocean to a known datum (MSL at the

Home Island tide-gauge), and by systematically sampling and documenting the morphological

development of more than 120 microatolls. Relationships between microatoll surface form and

sea-level and environmental change were also explored.

The height of living coral (the 'aHLC') around individual microatolls was found to vary

'intrinsically' by approximately 2cm, irrespective of where the microatoll grew, representing the

precision level with which the aHLC and the constraining water level (ra) are typically coupled.

The aHLC did not vary systematically around microatoll rims; the majority of microatolls on

Cocos have relatively horizontal upper surfaces. Separate elevational groups were statistically

discriminated at most survey sites, and were attributed to the combined occurrence of

microatolls that were confined at sea-level and/or one or more ponded water levels, or because

some microatolls were not water-level limited. Reef flat microatolls were typically constrained at

greatest depth, with interisland passage microatolls gradually becoming higher as the

passages shallowed toward the lagoon. The aHLC of lagoonal microatolls was variable, and appeared to be controlled by the depth of the underlying substrate. The upper limit to coral growth around microatolls freely connected to the open sea at low tide was restricted at a level approximately midway between MHLW and MLLW, though their actual elevations varied around the atoll, reflecting variations in reef morphology and tidal hydrodynamics.

Most microatolls had statistically similar microtopographies across separate growth axes. Lagoonal and reef flat microatolls were the most consistently symmetric, and typically developed prominent concentric 'bumps' over their upper surfaces. In contrast, interisland passage microatolls were least often symmetric, and typically developed flat upper surfaces with little topographic relief, ft was concluded that surface symmetry is promoted in free- draining reef flat and lagoonal habitats where the potential amplitude of water-level change is large relative to both the noisiness of the water level surface and the range of intrinsic aHLC variability, because a strong and persistent water-level change signal is produced that is reflected in the direction, if not the magnitude of rim growth. Where the microtopographies of individual microatolls are symmetric, adjacent microatolls were also most likely to be similar. The similarity of microatoll microtopographies in like habitats around the atoll was generally low. It was speculated that this divergence reflects the variety of physiographic conditions possible within each habitat type, and the complexity of biological responses to this diversity.

The surface microtopographies of two long-lived open water reef flat microatolls from different sites, dominated by broad undulations of 5-10cm amplitude developed over 18-20 year cycles, were determined to be similar statistically. The independent development of this congruent morphology by these separated microatolls implies the influence of a common environmental signal. Statistical correlations between microatoll surface morphology and the short and fragmented tide-gauge record available for Cocos are not strong, but this was expected given the extent and quality of the tide-gauge data and the fact that microatoll response to water-level change is biologically mediated and unlikely to be sensitive to short- term or subtle fluctuations. Nevertheless, a visual comparison between microatoll surface microtopography and the tide-gauge record shows similar trends. Microatoll microtopography and a range of other environmental variables for which instrumental data were available were very weakly correlated. An intriguing temporal coincidence between the 18.6 year cycle of lunar tides (related to slight variations in the tilt of the moon's orbit relative to the sun's equator) and the broad undulations that dominate the microtopographies of large open water reef flat microatolls on Cocos was identified, suggesting the possibility that the broad undulations develop in response to tidal adjustments to the 18.6 year lunar cycle. A tentative link between

ENSO events and low water levels on Cocos may also be speculated from the timing of stronger events and marked falls in the upper limit to coral growth represented by depressions over the microatoll plane, though this hypothesis requires further investigation. It was concluded that the surface morphologies of open water reef flat microatolls on Cocos were principally influenced by sea-level, though their microtopographies are more likely to preserve filtered, longer-term sea-level trends than interannual variation. The surface microtopographies of large microatolls on Cocos are clearly dominated by the cyclic undulations described above, suggesting that the water level that constrains upward coral growth on Cocos has fluctuated through a series of regular cycles over at least the last century.

The surface form of microatolls on Cocos does not support the 10-20cm rise in global sea level over the last one hundred years, indicated in aggregated tide-gauge records, but instead indicates sea level at Cocos has undergone little net change over this period. It was concluded that microatoll upper surfaces preserve subdued, low temporal resolution records of sea-level change, and represent a valuable source of retrospective sea-level data in areas where tide- gauge records are short or lacking. IV

ACKNOWLEDGEMENTS

Thanks are due to many people for their support and encouragement during the completion of this thesis. I particularly wish to thank my supervisor, Associate Professor Colin

Woodroffe, for his guidance, detailed and timely feedback, patience, perseverance and friendship. I am similarly indebted to Professor Roger McLean for his invaluable advice, assistance in the field, and for organising access to X-ray facilities at ADFA. Dr Paul Kench also deserves special mention for his companionship and assistance on Cocos, and for his sympathetic ear and constructive comments in our many discussions. Many thanks must go to

Eugene Wallensky for his enthusiasm, strength and culinary abilities in the field, and to Steve

Cox for his strength and enthusiasm only. Thank you Richard Miller for your advice and patience in all matters cartographic, and to the other staff and students at the Department of Geography,

University of Wollongong who helped out along the way.

I am grateful to the National Greenhouse Advisory Committee for funding for this project, and to the Adminstration of the Cocos (Keeling) Islands, Indian Ocean for logistical support in the field. I particularly would like to thank John Stevenson and Jeff Tranter, the ANPWS

Conservators on Cocos. I thank Major John Mobbs for assistance in surveying the temporary benchmarks around the atoll, Sergeant Ray Bird for his X-ray expertise, and the various

Australian Airforce personnel who facilitated the rapid transport of my samples to the mainland on board their aircraft. I am thankful to Dr Anita Andrew and staff for access to the mass spectrometer and assistance at the Centre of Isotope Studies, CSIRO Mineral Research

Laboratories, Ryde.

Finally, I would like to thank my family and friends for their confidence and support which reinforced my resolve at critical stages, and helped me keep it all in perspective. I owe my deepest gratitude to my wife, Phillippa, without whose understanding, support, and sacrifice none of this would have been possible. She has never complained, and I know I am a lucky guy. V

TABLE OF CONTENTS.

A BSTR A CT i ACKNOWLEDGEMENTS iv TABLE OF CONTENTS v LIST OF FIGURES x i LIST OF TABLES xv

CHAPTER ONE: INTRODUCTION.

1. INTRODUCTION 1

2. MICROATOLLS 6

2.1. DEFINITION AND DESCRIPTION 6 2.2. MICROATOLL FORMATION 7 2.2.1. Response to Subaerial Exposure 10 2.3. MICROATOLL MORPHOLOGY: RESPONSE TO WATER-LEVEL CHANGE 12 2.4. MICROATOLLS AS SEA-LEVEL MONITORS 14 3. MASSIVE PORITES MICROATOLLS 17

3.1. DESCRIPTION AND GROWTH 17 4. STUDY AREA 1 9

4.1. LOCATION AND GENERAL DESCRIPTION 19 4.2. GEOLOGIC DEVELOPMENT AND TECTONIC HISTORY 21 4.3. CLIMATE 22 4.4. TIDAL AND WATER-LEVEL CHARACTERISTICS 23

5. THESIS OUTLINE 2 5

CHAPTER TWO: CORAL GROWTH. CORAL SKELETONS. AND CORALLINE RECORDS OF ENVIRONMENTAL CHANGE.

1. INTRODUCTION 2 7

2. CORAL SKELETONS 2 8

2.1. GROWTH AND ARCHITECTURE 28 VI

3. SKELETAL RECORDS 2 9

3.1. STRUCTURAL RECORDS - DENSITY BANDING 30 3.1.1. The Physical Nature of Density Bands 31 3.1.2. The Environmental Significance of Density Band Formation 38 3.2. INCLUSIVE RECORDS 49 3.2.1. Isotopic Signatures in Coral Skeletons 49 3.2.2. Trace Elements 56 3.2.3. Fluorescent Banding 60 3.3. The Influence of Skeletal Structure on Inclusive Records 62 4. SUMMARY 6 3

CHAPTER THREE: ESTABLISHING THE GROWTH CHRONOLOGIES OF Pontes MICROATOLLS FROM THE COCOS (KEELING) ISLANDS.

1. INTRODUCTION 6 5

2. SAMPLE COLLECTION 6 5

2.1. SITE SELECTION 65 2.2. FIELD SAMPLING 67 3. ESTABLISHING CORAL GROWTH CHRONOLOGIES 6 9

3.1. SKELETAL DENSITY BANDS 69 3.1.1. Slice Preparation 69 3.1.2. X-Radiography 70 3.1.3. Densitometry 71 3.2. FLUORESCENT BANDS 74 3.2.1. Illumination and Detection 74 3.2.2. Image Analysis 74 3.3. ESTABLISHING BAND PERIODICITY AND SEASONALITY 76 3.3.1. Alizarin Staining 76 3.3.2. The 1983 Stress Band 77 3.3.3. Stable Oxygen and Carbon Isotope Analysis 78 3.4.4. Annual Skeletal Density and Fluorescent Bands: A Synthesis of the Evidence 81 4. SUMMARY 8 7 vii

CHAPTER FOUR: GEOGRAPHICAL VARIATION IN THE HEIGHT OF LIVING CORAL.

1. INTRODUCTION 8 9

2. METHODOLOGY 9 0

2.1. SURVEY BENCHMARKS 90 2.2. SURVEYING THE HEIGHT OF LIVING CORAL 91 2.3. STATISTICAL ANALYSIS 92 3. VARIATION IN THE HEIGHT OF LIVING CORAL AROUND THE RIMS OF INDIVIDUAL MICROATOLLS 9 2

3.1. ANALYSIS AND RESULTS 93 3.2. DISCUSSION 95 4. WITHIN-SITE VARIATION IN THE MEAN HEIGHT OF LIVING CORAL BETWEEN MICROATOLLS 9 8

4.1. ANALYSIS AND RESULTS 98 4.2. DISCUSSION 108 4.2.1. Multiple Constraining Low Water Levels (Multiple ros) 1 09 4.2.2. Microatolls Beneath the Constraining Low Water Level (aHLC

5. VARIATION IN THE HEIGHT OF LIVING CORAL AROUND THE ATOLL. ...12 5

6. VARIATION IN THE HEIGHT OF LIVING CORAL BETWEEN 1991 AND

1992 131

7. SUMMARY 134

CHAPTER FIVE: TEMPORAL VARIATION IN THE HEIGHT OF LIVING CORAL.

1. INTRODUCTION 137

2. METHODOLOGY 137

2.1. aHLC TIME-SERIES 138 viii

3. COMPARISON OF THE SURFACE MORPHOLOGY OF DIFFERENT GROWTH AXES ON INDIVIDUAL MICROATOLLS 146

3.1. ANALYSIS AND RESULTS 147 3.2. DISCUSSION 151

4. COMPARISON OF THE SURFACE MORPHOLOGY OF DIFFERENT MICROATOLLS FROM THE SAME HABITAT AT EACH SITE 161

4.1. ANALYSIS AND RESULTS 162 4.2. DISCUSSION 175

5. COMPARISON BETWEEN THE SURFACE MORPHOLOGY OF MICROATOLLS GROWING IN THE SAME HABITAT-TYPE AT DIFFERENT SITES AROUND THE ATOLL 178

5.1. ANALYSIS AND RESULTS 179 5.2. DISCUSSION 182 6. SUMMARY 186

CHAPTER SIX: MICROATOLL SURFACE MORPHOLOGY AND ENVIRONMENTAL CHANGE.

1. INTRODUCTION 189

2. METHODOLOGY 190 2.1. MICROATOLL DATA 190 2.2. ENVIRONMENTAL DATA 195 3. RESULTS 197

3.1 MICROATOLL MORPHOLOGY AND SEA-LEVEL 197 3.2 MICROATOLL MORPHOLOGY AND ENVIRONMENTAL VARIABLES 198

4 DISCUSSION 199 5. A CENTURY OF MICROATOLL SURFACE MORPHOLOGY AND SEA-LEVEL CHANGE ON THE COCOS (KEELING) ISLANDS 201

6. MICROATOLLS AND WATER-LEVEL: A SYNTHESIS 2 04

7.SUMMARY .210 IX

CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS.

1. INTRODUCTION 212

2. CONCLUSIONS 213

2.1. ESTABLISHING THE GROWTH CHRONOLOGIES OF PORITES MICROATOLLS FROM THE COCOS (KEELING) ISLANDS 213 2.2. MICROATOLL ELEVATION 213 2.3. MICROATOLL MORPHOLOGY 217 2.4. MICROATOLL MORPHOLOGY AND ENVIRONMENTAL CHANGE 219

3. RECOMMENDATIONS FOR FUTURE RESEARCH 223

REFERENCES 225

APPENDICES

APPENDIX A: SITE DESCRIPTIONS 258 APPENDIX B: CLIMATE SUMMARY 285

APPENDIX C: SURVEY DATA 287

APPENDIX D: aHLC TIME-SERIES FOR SAMPLED MICROATOLLS 298

APPENDIX E: MATRIX OF RAW AND NORMALISED R-VALUES FOR MICROATOLLS IN EACH HABITAT AT EACH SITE 352 APPENDIX F: MATRIX OF RAW AND NORMALISED R-VALUES FOR CORRELATIONS BETWEEN ALL MICROATOLLS 360 LIST OF FIGURES.

Captions Page

Porites microatoll growing on reef flat, Cocos (Keeling) 7 Islands.

Microatolls growing in the interislands passages on the 9 eastern rim of the Cocos (Keeling) Islands.

Schematic response of microatolls to different water levels. 13

Schematic representation of moated (GJJ and open water (xnu) 16 tidal curves and microatoll upper surface morphology.

The Cocos (Keeling) Islands. 20

Mean monthly sea level at the Cocos (Keeling) Islands, 1963- 24 1991.

Schematic corallite morphology. 29

Schematic growth of coral skeleton within the tissue layer. 35

Density profiles generated by numerical models of coral 37 density band formation in which thickening and extension are driven by a sine curve forcing function.

Microatoll sample site locations. 66

Field sampling a reef flat microatoll with the motorised saw. 68

Densitometer trace on X-radiograph negative of thin vertical 73 slice of microatoll CKI162 from Pulu Pandan.

Linear profile of pixel brightness values along transect on 75 black and white photograph of fluorescing vertical slice of microatoll CKI162.

Photograph of microatoll CKI91/10F2(N) from which isotope 79 samples were extracted using fluorescent banding to establish chronology.

4n 4Q __ Diagram showing fluctuations in 8 C and 8 O from 1991 to 79 1981.

Relationship between reproduction, environmental variables 85 and skeletal and fluorecent banding patterns in microatolls from the Cocos (Keeling) Islands, Indian Ocean.

Determination of the height of living coral (aHLC) using the 91 laser level.

Analysis of variance statistical procedure flowchart. 100

Mean aHLC heights for surveyed corals at each site ± one 104 standard deviation. LIST OF FIGURES (continued).

Captions Page

Schematic representation of multiple comparison (MCP) 106-107 analyses for site samples where significant differences in microatoll mean aHLC values were detected.

Elevation of microatoll groups discerned from survey 112 samples 1991.

Elevation of microatoll groups discerned from survey 112 samples 1992.

Longitudinal bathymetric profile and mean microatoll heights 117 through sites XIV, XV, and XVII on the eastern atoll rim.

Interisland passage between Pulu Pandan and Pulu Wak 118 Banka at low spring tide approximately half way between reef crest and the lagoonward end of the island.

Fluorescent image of a cross-section through microatoll 119 CKI92/10PP9 from the interisland passage approximately half way between reef crest and the lagoonward end of the islands (the area shown in Fig. 22).

Longitudinal bathymetric profiles and mean microatoll rim 121 heights at reef flat sites II, IV, and XI backed by steep sandy beaches.

Mean aHLC for each habitat at different sites -1991. 129

Mean aHLC for each habitat at different sites -1992. 129

Change in aHLC around the rims of microatolls at five sites 132 around the atoll.

Porites microatoll growing on the reef flat at site XV, 1992. 133

Microatoll aHLC time-series 1970-1992, sites I to IV. 139

Microatoll aHLC time-series 1970-1992, site V. 140

Microatoll aHLC time-series 1970-1992, site VI. 141

Microatoll aHLC time-series 1970-1992, sites VII to XII. 142

Microatoll aHLC time-series 1970-1992, sites XIII to XIV. 143

Microatoll aHLC time-series 1970-1992, sites XIV to XV. 144

Microatoll aHLC time-series 1970-1992, sites XV to XIX. 145

Microatoll aHLC time-series 1970-1992, site XIX. 146

3 0009 03201222 6 xii

LIST OF FIGURES (continued).

Figure Number Captions Page

29 Confidence levels of correlations between aHLC time-series 150 derived from different growth axes of individual microatolls, grouped according to habitat.

30 aHLC time-series graphs of the two microatolls with the most 150 highly, most equivocally, and most negatively correlated surface morphologies across separate growth radii from each habitat.

31 Vertical cross-section through a tilted interisland passage 153 microatoll (CKI91/10B9) from site V.

32 Schematic model of the morphologic development of a : a) 156 reef flat; b) lagoonal; and c) interisland passage microatoll as a function of the temporal pattern of GJ change,TO noisiness, and inherent variability in the aHLC-ro juxtaposition.

33i Raw and derived index time-series for each habitat, sites I to 164 VI.

33ii Raw and derived index time-series for each habitat, sites VI 165 to XIV.

33iii Raw and derived index time-series for each habitat, sites XIV 166 to XVII.

33iv Raw and derived index time-series for each habitat, site XIX. 167

34i Normalised and derived index time-series for each habitat, 168 sites I to VI.

34ii Normalised and derived index time-series for each habitat, 169 sites VI to XIV.

34iii Normalised and derived index time-series for each habitat, 170 sites XIV to XVII.

34iv Normalised and derived index time-series for each habitat, 171 site XIX.

35 Index time-series from each habitat sample, with habitat 181 indices also plotted.

36 Vertical cross-sections through microatolls F2 and PP30 192 showing synchronous fluctuations in surface morphology.

37 aHLC time-series developed from the surface 194 microtopographies of microatolls F2 and PP30.

38 Microatoll surface morphology (F2 and PP30), coral index 198 master, and sea-level fluctuations relative to long-term MSL. LIST OF FIGURES (continued).

Figure Number Captions

39 Plot showing relationships between temporal fluctuations in the surface morphology of open water reef flat microatolls on the Cocos (Keeling) Islands (represented by the master index) and 18.6 year lunar tidal cycles and ENSO events.

40 X-radiograph of cross-section through microatoll CKI91/10T1 from site III, West Island, showing internal growth structure. XIV

LIST OF TABLES.

Title Page

Sea-level signatures of selected reef features. 3

Results summary of Kendall's coefficient of concordance analysis. 94

Height of living coral (aHLC) data summary. 101

Summary of statistical analyses. 102

Summary of statistical analyses (continued). 103

Microatoll groups determined for each site sample based on mean 105 heights of the living rims by MCP analysis.

Summary of height of living coral for water-level limited groups 110 (1991).

Summary of height of living coral for water-level limited groups 111 (1992).

Summary of statistical analyses: Mean aHLC levels for habitats at 127 each site.

Correlation coefficients between aHLC time-series derived from 149 different growth axes of individual microatolls.

Percentages of correlations in each habitat sample that are 172 statistically significant at the 95% confidence level.

ANOVA analyses to examine the proportion of variation between 173 yearly normalised aHLC values relative to within year aHLC variation.

ANOVA analyses to examine the proportion of variation between 174 yearly normalised aHLC values relative to within year aHLC variation (continued).

Correlations between index time-series from each habitat-type. 180

ANOVA analyses to examine the proportion of variation between 182 yearly normalised aHLC values relative to within year aHLC variation of index time-series from each habitat-type.

Correlation coefficients between microatoll time-series (two data 194 points per year) 1991-1917.

Correlations (Spearman's rank correlation coefficient) between 197 aHLCs reconstructed from microatolls F2 and PP30, the master index chronology, and annual sea-level minima 1986-1991.

Multiple regression of selected environmental variables against 199 master index aHLC data (1990-1952 (1966-1973 data missing)). 1

CHAPTER 1

INTRODUCTION.

1. INTRODUCTION.

Coral reefs are most prolific in shallow, clear and warm tropical seas. The reef-building, or hermatypic, corals (order Scleractinia) that compose these reefs consist of colonies of polyps that are characterised by two distinctive features: i) hard calcareous exoskeletons; and ii) the presence of unicellular photosynthetic algae known as zooxanthellae within their living tissues

(Yonge, 1940; Goreau, 1961). Zooxanthellae exist in an endosymbiotic relationship with the coral polyps, using the host polyp's waste-products for their own metabolism and receiving protection from grazing organisms. In return photosynthates produced by zooxanthellae are a significant source of polyp nutrition (Goreau and Goreau, 1960; Trench, 1986) and it is well known that photosynthetic zooxanthellae enhance coral calcification (Pearse and Muscatine,

1971; Vandermeulen et al., 1972; Muscatine, 1990). Coral reefs thus grow best in the photic zone, where algal photosynthesis and the mutualistic benefits to corals are optimal. Confined within this relatively narrow 'depth window' coral reefs are excellent gross indicators of sea level

(Davies and Montaggioni, 1985).

In recent decades research has been initiated to resolve the sea-level signatures of many biologic and geologic coral reef features with greater precision. At the broad end of the spectrum, late Quaternary glacioeustatic sea-level fluctuations have been estimated from the relative elevations of radiometrically-dated fossil reefs, with regional adjustments for isostatic, tectonic and erosional (both marine and subaerial) factors (e.g. Bloom et al., 1974; Chappell,

1974, 1983a, 1983b; Neumann and Moore, 1975; Chappell and Veeh, 1978; Taylor et al.,

1985; Aharon and Chappell, 1986; Bard et al., 1990; Chappell and Polach, 1991; Gvirtzman et al., 1992; Smart and Richards, 1992; Ota etal., 1993; Hearty and Kindler, 1995; Chappell er al.,

1996a, 1996b). Holocene reef-growth curves have been determined for many reefs by radiometrically dating in situ corals from varying depths (e.g. Great Barrier Reef: Davies and

Marshall, 1979; Hopley, 1982; Marshall and Davies, 1982; Davies and Hopley, 1983; Pacific: 2

Glynn and Macintyre, 1977; Schofield, 1977; Montaggioni, 1988; Yonekura et al., 1988; Miyata et al., 1990; Woodroffe et al., 1990a; Cabioch et al., 1995; Kan et al., 1995; Indian Ocean:

Woodroffe et al., 1990b; Woodroffe, 1993; Atlantic/Caribbean: Neumann, 1971; Macintyre and

Glynn, 1976; Adey, 1978) and used to develop minimum sea-level curves and envelopes (e.g.

Easton and Olson, 1976; Macintyre et al., 1977; Lighty et al., 1982; Davies and Hopley, 1983;

Montaggioni and Pirazzoli, 1984; Davies etal, 1985; Marshall and Jacobson, 1985; Pirazzoli et al, 1988; Pirazzoli and Montaggioni, 1988a, 1988b, Woodroffe etal, 1990b; Kayanne et al,

1993; Larcombe etal, 1995). Precise Holocene sea-level histories can rarely be established because the exact depth below the sea surface at which the dated material grew can seldom be determined. Sea-level indicators on coral reefs are either diagnostic (also referred to as finite or indicative) or directional] the former having a definite relationship with the sea-surface whilst the later can only be used to establish whether sea level was above or below the indicator at the time of its formation (Chappell et al, 1983; Davies and Montaggioni, 1985). Diagnostic indicators normally occur within a narrow (<2m) depth range and within 5m of the sea-surface (Davies and

Montaggioni, 1985) and can be used to construct more accurate sea-level histories than directional indicators.

Davies and Montaggioni (1985) comprehensively reviewed the coral reef literature and concluded that microatolls - discoid intertidal corals with living vertical sides and predominantly flat, dead tops that lie near to the level of the lowest tide (Abe, 1937; Scoffin and Stoddart,

1978; McLean et al, 1978; Woodroffe and McLean, 1990) - are among the most precise, geologically persistent, and useful diagnostic sea-level indicators found on coral reefs (see

Table 1). Former sea-level stands and rates of tectonic uplift have been reconstructed from the relative elevations of contemporary and fossil microatolls (Easton and Ku, 1980; Hopley, 1982;

Chappell etal, 1983; Pirazzoli and Montaggioni, 1986; Woodroffe etal, 1990b; Yonekura etal,

1988; Miyata er al, 1990; Nunn, 1995), and dated sequences of fossil microatolls across reef flats have provided evidence of a gradual fall of late Holocene sea level in the Pacific and Indian

Oceans (Chappell etal, 1982; Chappell, 1983a; Pirazzoli etal, 1987; Woodroffe et al, 1990c).

At a finer scale, concentric undulations over the upper surfaces of individual microatolls have 3

Table 1: Sea-level signatures of selected reef features (after Davies and Montaggioni, 1985).

Sea Level Criteria Condition Signal Precision Examples Relevance Transg^ Still- Diag1 Direct- ression stand nostic ional Gross Features. Discrete reefs / / MLWS + 25cm Most atolls, GBR. Interpretation of physical zonation Diffuse reefs Many m Morinda Shoals, GBR. difficult. Upper growth Physical zonation MSL ± 2m Most Indo-Pacific Reefs. limits only useful in Upper growth limits MLWS + 25cm Belize and Florida. open water. Biological Distribution. 1. Corals A. palmata 0.5m Caribbean. Morphology useful. A. danii 0.1m Indo-Pacific. Limited A. robusta Vertical species zonation 0>30m Caribbean & Pacific Morphologic variation useful. Horizontal species zonation MSL Caribbean, Indo-Pacific Application difficult. 2. Corallines P. pachydermum 0.2m Caribbean. The algal ridges of L congestum Pacific and Atlantic reefs may not be P. oncodes MLWS ± 0.5m Indo-Pacific. preserved. P. gardenerii 3. Molluscs V V Only oysters, large Certain types only MSL Indo-Pacific and clams and chitons Caribbean. 4. Foraminifera V V useful. Benthonic MLWS Indo-Pacific. Limited. Encrusting Indo-Pacific and Reef fronts destroyed Caribbean. during low sea levels. Growth Facies. 1. Framework Acropora / / -10m Mauritius. Porites -15m Branching and head corals / V <10-15m Mostly Caribbean. Distinct potential but Encrusting corals / / 0-2m difficult to retrieve Branching corals / V 2-10m GBR. and interpret cores. Head corals / V 10-20m Plate corals / V7" >20m Microatolls V V Open Water: Indo-Pacific. High geologic MLWS potential. Algal flats V V MoatedMSL :± 0.5MLWm N Atlantic and Indo-Pacific. Very useful if preserved. 2. Detrital Transgressive: i) subtidal ~10m Atolls, GBR. Poorly understood. ii) intertidal / / MLWS-MLWN Caribbean, Red Sea. High geologic relevance. Stillstand: Cays MSL Common. Not often preserved. Beach rock / / To MHWS Common. Limited distribution - low potential. Prograding reef flats / / LWN-MHWN Wide distribution Rubble ramparts / / MLWS-MLWN Pacific and Belize. - high potential. Conglomerate platform / / MSL+ 1m Ichno-facies Subtidal Caribbean and GBR. Limited use in / / Pleistocene. Micro-borings / / Intertidal Caribbean. High potential.

Abbreviations

GBR - Great Barrier Reef

LWN - Low Water Neaps MLWN - Mean Low Water Neaps

MHWN - Mean High Water Neaps MLWS - Mean Low Water Springs

MHWN - Mean High Water Springs MSL - Mean Sea Level 4

been used to detect smaller changes in relative water-level that have occurred over shorter periods, including those attributed to the cyclonic destruction of moats (Isdale, 1974; Hopley and Isdale, 1977), seismic uplift (Easton and Ku, 1980; Buskirk etal, 1981; Taylor et al, 1987), and interannual variations in sea-level driven by oceanographic and climatic factors (Woodroffe and McLean, 1990).

Much research in recent decades has focused on the extraction and interpretation of proxy records of environmental change preserved chemically and structurally within coral skeletons. Annual, monthly and daily skeletal density bands have been identified and attributed to seasonal, lunar monthly and diurnal cycles in environmental parameters (e.g. Knutson et al,

1972; Dodge and Vaisnys, 1975; Weber et al, 1975a; Buddemeier and Kinzie, 1975; Hudson etal, 1976; Wellington and Glynn, 1983; Barnes and Lough, 1989; Risk and Pearce, 1992;

Dodge et al, 1993), and variations in skeletal isotope and trace element chemistry along coral growth axes have been correlated with temporal changes in environmental conditions during skeletogenesis (e.g. Schneider and Smith, 1982; Chivas et al, 1983; Shen et al, 1987; Cole and Fairbanks, 1990; Aharon, 1991; Beck etal, 1992; Chakraborty and Ramesh, 1993; Cole et al., 1993; Rasmussen etal, 1993a, 1993b; McCulloch etal., 1994;Tudhope etal., 1995; 1996;

Wellington and Dunbar, 1995; Allison etal, 1996; Swart etal, 1996a; 1996b). These structural and chemical signatures of environmental change can be used to establish coral growth chronologies (e.g. Buddemeier and Kinzie, 1976; Hudson et al, 1976; Fairbanks and Dodge,

1979; Patzold, 1984; Carriquiry etal, 1988; Winter etal, 1991; Klein et al, 1993; Gagan er al,

1994) that could be applied to date topographic fluctuations across a microatoll's upper surface, from which changes in the lowest tide level over the coral's lifespan can be inferred. To date, however, few detailed and systematic investigations of microatoll morphology and its relationship to water level and local environmental conditions have been undertaken.

There is widespread concern that the level of the sea is rising and that the rate of rise will accelerate as greenhouse gases accumulating in the atmosphere cause global temperatures to increase. Current rates of average global sea-level rise of between 1-3mm/yr have been 5

calculated from existing aggregated tide-gauge records (Gornitz etal, 1982; Barnett, 1984;

Gornitz and Lebedeff, 1987; Peltier and Tushingham, 1989; Douglas, 1991, Emery and

Aubrey, 1991; Gornitz, 1995); however these estimates are based on typically short records that are spatially biased and often contaminated by vertical land movements (Pirazzoli, 1986, 1989,

1993; Wyrtki, 1990; Grogerand Plag, 1993; Gornitz, 1995). Nevertheless, regional variation in the pattern and rate of relative sea-level change is evident even from these data, bringing into

question the validity of extrapolating sea-level estimates where tide-gauge data are lacking.

Long-term tide-gauge records are particularly scarce for the mid-ocean areas of the South Pacific

and Indian Oceans, where there are many low-lying atolls that are especially vulnerable to sea-

level rise. Microatolls are common on the reef flats and in the lagoons of many of these atolls. If

sea-level changes can be accurately interpreted from microatoll upper surfaces they may

represent an important geographical supplement to tide gauges, a common source of

retrospective sea-level data, and an inexpensive and widespread means to monitor future sea-

level change. It would appear that microatolls may be able to satisfy the urgent need for sea-

level data in mid-ocean areas.

This thesis is a detailed study of the elevation and upper surface morphology of modern

microatolls at several sites around the Cocos (Keeling) Islands, a mid-ocean atoll in the eastern

Indian Ocean. The research has three aims: (i) to investigate variability in microatoll elevation; ii)

to investigate variability in microatoll upper surface morphology; and (iii) to provide a modern

calibration for palaeosea-level records reconstructed using fossil, particularly mid to late

Holocene, microatolls. In undertaking this research three primary objectives were proposed:

1. To establish by accurate survey the relative elevations of the upper surfaces and the limit to living coral growth around the rims of microatolls on the Cocos (Keeling) Islands.

2. To investigate variability in microatoll elevation, upper surface morphology and growth chronologies at a range of spatial scales.

3. To investigate the environmental significance of microatoll upper surface morphology. 6

2. MICROATOLLS.

2.1. Definition and Description.

Microatolls are discoid coral colonies with predominantly flat, dead tops and living sides

that occur in intertidal reef flat and lagoonal environments (Stoddart and Scoffin, 1979;

Woodroffe and McLean, 1990; Rodda, 1991). Microatolls most commonly develop from

massive corals, particularly Porites spp., however branching and foliaceous corals may also

adopt this form. Some 43 coral species form microatolls on the northern Great Barrier Reef

(Rosen, 1978).

Microatolls were described by many of the pioneering reef workers (Darwin, 1842; Dana

1872; Semper, 1880; Guppy, 1886; Agassiz, 1895). Guppy (1886) first compared coral

colonies with this morphology to 'miniature atolls'; the presence of analogous subcircular coral

rims led Agassiz (1895 - 'diminutive atolls') and Krempf (1927 - 'dwarf atolls') to repeat the

comparison. The specific term microatoll was first used by Wood-Jones (1910), who not only

compared the appearance of microatolls he observed on the Cocos (Keeling) Islands to atolls

but also argued that they developed through a similar sedimentation mechanism to that he

proposed in his now largely discounted theory of atoll formation. Abe (1937, p.305) referred to

microatolls as 'table forming corals', focusing on their flat dead tops. Patch reefs with raised

annular rims and sunken, often sand-filled centres have also been referred to as microatolls

(Kornicker and Boyd, 1962; Hoskin, 1963; Larkum and Steven, 1994) but are better described

as 'mini-atolls' (Scheer, 1971).

Microatolls are generally less than 0.5m thick but may grow to several metres in diameter

and be quite long-lived. A fossil microatoll exceeding 9m in diameter and dated at

2195±80years BP at its centre and 1535±130 years BP on its margin has been reported from

the Mariana Islands (Siegrist and Randall, 1989). Though microatolls exhibit a characteristic

discoid form, variations in both surface convolution and radial symmetry occur. The internal

structures of true microatolls should indicate a constructional rather than erosional heritage for

their flat upper surfaces, termed the 'microatoll plane' by Scoffin and Stoddart (1978). Microatoll 7

pseudomorphs - corals with flat planated tops caused by factors such as erosion, predation, sedimentation or disease - can usually be easily identified by their internal structure and the variable upper surface elevations of adjacent colonies (Chappell et al., 1983; Kobluk and Noor,

1990).

Figure 1. Porites microatoll growing on reef flat, Cocos (Keeling) Islands.

2.2. Microatoll Formation.

Microatolls develop when the upward growth of a coral colony is constrained, after which living coral is restricted to the sides of the colony and lateral growth predominates. Excessive sedimentation, nutrient depletion, and prolonged subaerial exposure have been proposed by various authors as likely constraints to upward coral growth. 8

Sediment accumulation on the upper surfaces of coral colonies has been nominated as the principle cause of microatoll formation by several authors (Dana, 1872; Wood-Jones, 1910;

Krempf, 1927; Guilcher etal, 1965 cited in Guilcher, 1988), Guilcher (1988, p.30) arguing that

'in the coral world as a whole' excessive sedimentation is the primary cause of microatoll formation. Excessive sedimentation inhibits coral growth by reducing available light to photosynthetic endosymbionts (Dodge et al, 1974; Dodge and Vaisnys, 1977; Foster, 1977;

Chappell, 1980; Rogers, 1990) and by increasing the energetic costs of sediment rejection

(Bak, 1978; Lasker, 1980; Brown et al, 1986). However, despite Guilcher's assertion, the effects of sedimentation on coral growth have been shown to be patchy and cannot easily account for the usual symmetry of microatolls, nor the consistent elevation at which the tops of neighbouring microatolls usually lie. Several studies describe Porites (a common microatoll forming genus) colonies killed by excessive sedimentation interspersed with others that show no adverse effects (Hudson et al, 1982; Brown et al, 1986), whilst Scoffin et al (1992) found rates of linear extension were greatest where turbidity was high, the converse of predictions based on photosynthetic and energetic criteria (Aller and Dodge, 1974; Dodge et al, 1974).

Furthermore, massive Porites corals debilitated by heavy sedimentation were observed by

Brown etal. (1990) to fully recover in less than two years, suggesting that where corals survive, excessive sedimentation may only temporarily constrain coral growth. Finally, it would seem reasonable to discount excessive sedimentation as the principal cause of microatoll formation because the tops of many are bare of sediment and it cannot easily explain the development of microatolls formed by open branching corals (Scoffin and Stoddart, 1978).

Cary(1931) proposed an alternative hypothesis for microatoll formation whereby the progressive depletion of nutrients from waters flowing from a coral's margins toward its centre would cause a commensurate fall in polyp vigour that would promote lateral colony growth more than upward colony growth. The symmetry of most microatolls is clearly difficult to explain using

Cary's hypothesis, particularly those growing in habitats where unidirectional currents prevail.

Cary's hypothesis has received little support, although the possible role of nutrient shortages in killing polyps, especially on coral tops not continuously flushed by water, has been recognised 9

(Scoffin and Stoddart, 1978; Woodroffe and McLean, 1990). The tops of many microatolls are encrusted by coralline algae, however the algae encrusts only dead surfaces and does not kill the living coral polyps or promote microatoll development (Scoffin and Stoddart, 1978).

The debilitating effects of prolonged subaerial exposure on coral polyps are well documented, and it has long been argued that microatolls develop when upward coral growth is limited very near to the low tide air-water interface (Darwin, 1842; Semper, 1899; Kuenen, 1933;

Manton, 1935; Abe, 1937; Stoddart and Scoffin, 1979; Hopley, 1982; Woodroffe and McLean,

1990). The narrow elevational range for the uppermost extent of coral growth within many microatoll fields, the consistency with which coral tops lie close to the local low water level, and the congruent morphological response of adjacent microatolls when this water level changes

(Fig. 2) have been interpreted as strong support for this view (Hopley and Isdale, 1977; Scoffin and Stoddart, 1978; Taylor et al., 1987; Woodroffe and McLean, 1990). On the strength of such evidence it is generally accepted that prolonged subaerial exposure around the low water level is the principal causative factor in microatoll formation, and not excessive sedimentation as purported by Guilcher (1988).

Figure 2. Microatolls growing in the interisland passages on the eastern rim of the Cocos (Keeling) Islands. Note the similarity of surface form. 10

22.1. Response to Subaerial Exposure.

Although coral growth is clearly inhibited by prolonged subaerial exposure the physiological constraints and critical levels of exposure remain unclear. However, desiccation, overheating, chilling and overexposure to ultra-violet (UV) radiation have been shown to restrict coral growth (Yamaguchi, 1975; Brown and Howard, 1985; Gleason and Wellington, 1993;

Fadlallah etal, 1995) and have been cited as probable limiting factors.

Most corals can endure several hours subaerial exposure before being damaged

(Stoddart, 1969; Scatterday, 1977; Hopley, 1982); certain genera (e.g. Goniastrea) are more resilient than others (Abe, 1937; Moorehouse, 1936; Motoda, 1940; Fishelson, 1973). Corals adopt several strategies to cope with emersion. Many species secrete mucus over exposed surfaces to reduce evaporation and slow desiccation (Scatterday, 1977; Hopley, 1982; Krupp,

1984; Stafford-Smith and Ormond, 1992). Drollet etal (1993) have also shown that the mucus produced by Fungia fungites effectively absorbs UV-radiation, presumably offering some protection to polyps when they are subaerially exposed. Tissue retraction has also been proposed as a mechanism by which certain species minimise the deleterious effects of subaerial exposure; a number of species, including several Porites spp, have been observed to retract polyps into the calix when emersed (Brown and Le Tissier, 1993; Brown et al, 1994a). Genera in which polyps are more superficially attached to the exoskeleton (e.g. Pocillopora) appear particularly susceptible to desiccation and die quite quickly when subaerially exposed

(Moorehouse, 1936). Hopley and Isdale (1977) estimated that polyps on moated Porites microatolls on the Great Barrier Reef may be prevented from drying out up to 5cm above the still low water position by capillary action, mucus secretions and wavelets.

Coral mortalities caused by prolonged emersion during extreme low tides associated with exceptionally calm sea and weather conditions are reported from several areas, including the Gulf of Aqaba, Red Sea (Fishelson, 1973) and Bonaire, Netherlands Antilles (Scatterday,

1977). Mass mortalities were similarly linked to the exceptionally low tides associated with the

1972 El-Nino when reef flat organisms on Guam were exposed for prolonged periods 11

(Yamaguchi, 1975). Several workers have noted that the height to which living coral grows

(refered to hereafter as the 'HLC') is most stringently constrained by subaerial exposure when tidal emergence coincides with midday high temperatures, when desiccation and photochemical damage are likely to be most severe (Loya, 1976; Buskirk et al, 1981; Taylor et al, 1987). For example, Taylor et al. (1987) contend that the HLC for microatolls growing on the reef flats of

Vanuatu, where spring low tides occur only at night during summer and only during the day in winter, is higher than it would be if low spring tides occurred during the daytime in summer.

Fadlallah et al. (1995) have linked the mortality of reef flat corals in the western Arabian Gulf to

'clustered' exposure events during extreme low tides induced by seasonal winter winds, though they suggest the seasonally low air and water temperatures and windy conditions that accompany these exposures are also critical. From November through to April the western

Arabian Gulf is swept by the strong and cold shamal winds that blow from the northwest which are capable of generating a negative surge at the northern end of the Gulf of up to 3m amplitude. Reef flat topography and hydrodynamic factors such as the exposure to and persistence of waves and currents may affect the elevation of the low water level over reefs and the HLC at more local scales (Abe, 1937; Scatterday, 1977; Woodroffe and McLean, 1990).

Exposure to high intensity UV-radiation damages most zooxanthellae and reduces coral vigour (Jokiel and York, 1984), though some resistant strains have been identified (Jokiel, 1980;

Gleason and Wellington, 1993). However, clear seawater is a poor filter of UV light and shallow water corals subject to high ambient UV intensities have been shown to be less sensitive to lethal UV damage than deep water colonies (Siebeck, 1981, cited in Falkowski et al, 1990).

Though reflectance at the sea surface has been shown to reduce irradiance by some shorter UV wavelengths by more than 30% (Brown et al, 1994a), it would appear that the shallow reef settings in which microatolls occur offer little protection from most UV-radiation, and that ambient levels would not be significantly increased whilst the coral is exposed. Recent work by Brown et al. (1994b) concluded that UV-radiation had little effect on the growth of coral polyps on the tops of intertidal Goniastrea colonies on the reef flats of Phuket, Thailand. 12

2-3. Microatoll Morphology: Response to Water-Level Change.

If prolonged subaerial exposure above a critical low water level (TO) curtails the continued upward growth of microatoll rims, then the upper surface morphology of microatolls may reflect changes in the elevation of this water level through time (Isdale, 1974; Hopley and Isdale, 1977;

Scoffin and Stoddart, 1978; Stoddart and Scoffin, 1979; Woodroffe and McLean, 1990). A summary of commonly observed microatoll morphologies and their water-level interpretations is presented in Figure 3.

Massive corals that remain submerged at low tide are generally hemispherical in shape

(Fig. 3a). Eventually the uppermost surface of these corals will grow up to the low tide air-water interface and further upward growth will be constrained. However, the outward growth of polyps on the vertical sides of a colony is not restricted, and the microatoll will continue to expand laterally. Where TO remains constant the coral will adopt the 'classical' microatoll form (Isdale,

1974) (Fig. 3b). Classical microatolls are common in permanently moated locations where the low water level is fixed by the height of the sill. If the sill height and TO subsequently rise to a higher, stable level, the living polyps on a microatoll's rim will resume upward growth until once again constrained near to the new air-water interface. Whilst growing up to the new, higher TO a microatoll's living coral rim may begin to overgrow its dead upper surface, forming an elevated rim of living coral that encircles a lower, dead central surface formed at the earlier low water level

(Figs. 2c, 2d, 2e). Once the higher living rim becomes constricted near the new TO it may bifurcate, one side growing back towards the centre of the coral whilst the other continues to build outwards. Microatolls with raised outer rims and lower central areas are known as 'upgrown'

(Isdale, 1974).

When the constraining water level (TO) falls a microatoll's living vertical sides may be partially or fully exposed. Three scenarios are possible: i) the coral is wholly emersed and dies, i.e. becomes a fossil microatoll (it should be noted that factors other than emersion may also kill microatolls); ii) the TO fall is gradual and of a small magnitude and the microatoll continues to grow outwards with its upper surface at a progressively lower elevation, eventually developing a 13

a) Water W1 Level *1

Time vftss* b) Water W1 — Wi Level

Time

c) Water *2 w, Level Wi

Time d) Water ^ w2 Level

Time

e) 1-- —_W 3 Water 1 Level I— W,

Time

f) Water Level Wi w W, £^ Time g) Water Level W1 W\TT~ S Time -a

h) w2 *2 Water Wi/w y' / 4 W1/W4 Level *3 - wa

Time

Figure 3. Schematic response of microatolls to different water levels, (after Woodroffe and McLean, 1990). a) sea level not limiting, coral hemispherical; b) constant water level; c) water level constant then rising; d) water level constant then rising then constant; e) three phases of water level constant and then rising then constant; f) water level falling gradually; g) water level falling episodically; h) fluctuating water level. 14

domelike colony (Fig. 3f); or iii) TO falls episodically and a series of terracettes form across a microatoll's top (Fig. 3g). Such terracettes have been used to detect and date coseismic uplift of reef flats (Taylor etal, 1987). When TO is abruptly lowered 'top hat' (Isdale, 1974) microatolls with raised centres encircled by lower outer rims develop. Top hat microatolls have been linked to water-level falls associated with the cyclonic destruction of moats on the Great Barrier Reef

(Hopley and Isdale, 1977) and with tectonic uplift in the Kokor Islands, Palau (Easton and Ku,

1980).

The most complex microatoll morphology develops where TO rises and falls through time, either abruptly or gradually, to form 'multiple-ringed' microatolls (Isdale, 1974) that are characterised by concentric ridges and troughs over their upper surfaces (Fig. 3h).

Figure 3 is a simple schematic model of microatoll response to water-level (TO) changes.

However, it is important to note that not all water-level changes to which microatolls respond are sea-level related. As will be discussed below, the upper surface morphology of many microatolls develops in response to changes in local water levels that are not related to sea-level fluctuations.

2.4. Microatolls as Sea-Level Monitors.

Microatolls may occur in either open water or moated habitats, the former being freely connected to the open ocean and the later comprising habitats where the fall of the ebb tide is held above the open water low tide level (Hopley and Isdale, 1977; McLean et al, 1978; Scoffin and Stoddart, 1978; Stoddart and Scoffin, 1979; Hopley, 1982). Previous work, largely from the

Great Barrier Reef, indicates that the tops of open water microatolls are usually elevated around mean low water springs (MLWS) and those of moated microatolls near to mean low water neaps

(MLWN) (i.e. TO for open water microatolls is MLWS and TO for moated microatolls is MLWN)

(McLean etal, 1978; Scoffin and Stoddart, 1978; Hopley, 1982). However, these heights have rarely been directly established by survey to a precise datum. 15

Open water microatolls with unimpeded connections to the open sea appear best suited to function as sea-level monitors. Woodroffe and McLean (1990) showed that the temporal pattern of rim height variation (preserved as undulations across the microatoll plane) developed by open water microatolls growing on free-draining reef flats in the central Pacific closely tracked interannual changes in mean sea level (MSL) recorded at a nearby tide gauge, though they recognised several limitations to interpreting sea-level from microatoll upper surface morphology. They stressed that the upper limit to coral growth occurs at a level where prolonged subaerial exposure above the constraining low water level (TO) prevents further upward growth, and not MSL, and emphasised that the record of sea-level change preserved by open water microatolls is subdued, with small magnitude and short-term fluctuations not likely to be morphologically represented. It is also important to note that the height to which a microatoll's living coral rim actually grows (the actual height of living coral - aHLC) may lag beneath the level at which it may potentially survive (the potential height of living coral - pHLC) if the rate of sea-level rise exceeds the maximum coral growth rate (i.e. in Fig. 3c the aHLC is the highest point of coral growth approximately midway between w^ and W2, where W2 is the raised TO which must equal or be slightly below the pHLC depending on polyp tolerance to the subaerial exposure). In these cases a microatoll's surface morphology may incompletely record both the magnitude and direction of the sea-level change .

In contrast, moated microatolls preserve a record of the height at which water is ponded above the open water low tide level, usually behind structures such as rubble ramparts and algal rims. Rubble ramparts and associated moats are generally most common and of greater size in high-energy environments (Hopley, 1982). Coralline algae are more tolerant of subaerial emersion than most corals and may extend above the upper limit to open water coral growth to form algal rims. Algal (Porolithon, Lithothamnion) rims that extend up to 2.6m above the open water low tide level occur on the central Great Barrier Reef (spring tidal range approximately

4.3m), and have been shown to moat water for up to five hours a day and sustain living microatolls 2.3m above the open water low tide level (Hopley, 1982). Microatolls moated by these rims are less than 8cm below MSL, however the high and impervious algal rims of the 16

central Great Barrier Reef province are exceptional, and large fields of microatolls are normally not higher than MLWN (Scoffin and Stoddart, 1978; McLean et al, 1978; Hopley, 1982). Many moats function only at extreme low tides and are impounded by more subtle features such as depressions on reef flats or crevices on shore platforms (Hopley, 1982). An important consequence of moating is that the tide falls to a similar level on every ebb tide, and the surface morphology of moated microatolls is normally quite planar (Fig. 4). However, it is not uncommon to observe terracing of these surfaces caused by changes in the height and/or permeability of the moat sill (Isdale, 1974; Hopley and Isdale, 1977; Scoffin and Stoddart, 1978). The control of sill height on the morphology of moated microatolls, and their isolation from and insensitivity to fluctuations in open water levels, has led many researchers to discount them as useful sea-level indicators.

Figure 4. Schematic representation of moated (TOJ and open water (TOU) tidal curves and microatoll upper surface morphology, (after Hopley, 1982). Note that the upper surface of the moated microatoll is flat, reflecting the constant low water level (m) determined by the moat rim, whilst the open water microatoll may develop a more complex surface morphology that reflects interannual variations in open water low tide in levels. In this model aHLC = w = pHLC, but it is possible for aHLC < m <, pHLC, aHLC = w < pHLC; aHLC £ m < pHLC; aHLC = pHLC> m. 17

The advantages of using microatolls that are firmly fixed to a technically stable platform for sea-level reconstructions are obvious, to ensure that the signal preserved in microatoll skeletons reflects changes in water level rather than microatoll position. Microatolls from areas infrequently affected by high energy storms, where they are less likely to be tilted or moated behind high rubble ramparts and algal rims are thus most useful as sea-level indicators, as are those growing in inaccessible or sparsely populated areas where they are less prone to disturbance by humans. Mid-ocean microatolls are also preferred, being remote from continental influences that may obscure and/or compound the sea-level signal, such as river discharge and hydroisostatic shelf loading and deformation. Furthermore, mid-ocean microatolls experience microtidal regimes, where the magnitude of interannual sea-level variation is likely to be smaller compared to coral growth rates, and thus more easily detected against the daily tidal fluctuations than in areas of higher tidal range (Woodroffe and McLean, 1990). Finally, mid- ocean sea-level records are particularly scarce and it is these areas that the need for past sea- level data and future monitoring is greatest.

In summary, though detailed studies are lacking it would appear from the literature that microatolls function most effectively as sea level monitors when they are freely connected to the open ocean, are firmly fixed to a stable substrate, and the influences of factors that may obscure or compound the sea-level signal are minimised. Though open water microatolls preserve a subdued and conservative record of sea-level change, those which remain undisturbed and unmoated on hard substrate reef flats of tectonically stable, microtidal mid-ocean islands outside of the tropical storm belt would appear to be the most suitable for monitoring sea-level change.

3. MASSIVE PORITES MICROATOLLS.

3.1. Description and Growth.

Corals of the genus Porites are widespread throughout the Indo-Pacific and form microatolls in shallow water reef environments such as occur on reef flats and in lagoons. The 18

microatolls examined in this study comprised three species of massive Porites; P. lobata Dana,

1846, P. lutea Edwards and Haime, 1860 and P. solida Forskal, 1775. These corals are by far the most common species encountered in the intertidal reef flat and lagoonal areas of the Cocos

(Keeling) Islands. All three species of Porites microatolls examined in this study are very similar in gross morphological character, reaching several metres in diameter and developing a hummocky outer growth surface (Fig. 1, see Veron, 1982; 1986). It is usually necessary to examine corallite architecture to separate these species. Even at this level P. lobata is very difficult to distinguish from P. solida, and P. lutea has also been confused with P. solida, although they are now known to be quite distinct (Veron, 1982). Veron (1982) is less certain of the distinction between P. lutea and P. somaliensis Gravier, 1911, and suggests that they may be synonymous. The growth and elevation of reef flat P. somaliensis microatolls in Iwayama Bay,

Palau were examined by Abe (1937).

Porites are resilient and quick to recover from a variety of environmental stresses (Brown and Howard, 1985; Brown et al, 1990), although it has been demonstrated that they are more sensitive to subaerial exposure and less likely to extend above the low tide level than genera such as Goniastrea (Abe, 1937). Published annual growth rates (linear extension) for massive

Porites corals vary between 0.5cm-2.5cm/yr (Isdale, 1977; Buddemeier and Kinzie, 1975,

Patzold, 1984; Aharon, 1991), though Isdale (1981) concluded that P. lobata and P. lutea are

'slow growers' and at the lower end of this range. Significant interannual variation in growth rate has been identified in massive Porites corals and conflicting evidence exists as to whether growth rates are deterministic (Buddemeier and Kinzie, 1975; Isdale, 1981; Hudson, 1985;

Klein and Loya, 1991). Porites microatolls examined in this study had a mean annual growth rate of 1.35cm ± 0.4.4cm (n = 66 from six coral slices; significant difference was detected between individual coral means) and no systematic relationship between growth rate and age was detected in any of these corals (r2 values between 0.004 and 0.309). 19

4. STUDY AREA.

4.1. Location and General Description.

The Cocos (Keeling) Islands are an isolated Australian Territory situated in the eastern

Indian Ocean consisting of the main atoll known as the South (Keeling) Islands (12°04'-13'S;

96°48'E) (Fig. 5), and North Keeling Island (11°50'S; 96°49'E), a small atollon 27km to the north.

They lie on the Cocos Rise, a section of the Venning-Meinesz Seamount Range that rises approximately 5000m from the sea floor (Jongsma, 1976). The North and South Keeling Islands are connected by a submarine ridge that reaches a maximum depth of approximately 1000m

(Urquart, 1960; Colin, 1977).

This research was principally focused on the South (Keeling) Islands. The South

(Keeling) Islands consist of 26 reef islands of varying size which surround a shallow lagoon of approximately 150km2 (Fig. 5). The islands lie on a reef rim that is horse-shoe shaped in plan and exposed or shallowly flooded at low tide. At the south of the atoll the reef flat is more than

1.5km wide, but it rarely exceeds 80m in width elsewhere. The lagoon can be divided into two bathymetric provinces, the deeper (>8m) northern basin and the shallower southern flats (<3m), large areas of which are intertidal. At the north of the atoll wide and deep passages either side of

Horsburgh Island connect the lagoon to the open ocean. Other exchange between the lagoon and ocean is restricted to 11 shallow interisland passages situated on the eastern and southern atoll rim. Good biologic and physiographic descriptions of the Cocos (Keeling) Islands exist elsewhere (Gibson-Hill, 1950; Smithers, 1990; Williams, 1994; Woodroffe and McLean, 1994;

Woodroffe, 1994), and detailed site descriptions are provided in Appendix A.

The Cocos (Keeling) Islands occupy an historically significant position in the evolution of reef science. They are the only atoll on which Darwin landed (he visited briefly in April 1836), and are where he found 'tolerably conclusive evidence' of atoll subsidence that confirmed in his mind his theory of reef development (Darwin, 1842; see Hopley, 1982; Guilcher, 1988 for good 96°49'E 96°50t

NORTH KEELING

96°50'

Figure 5. The Cocos (Keeling) Islands. 21

accounts of Darwin's theory). Other eminent naturalists to have visited the islands include

Forbes (in 1879) and Guppy (in 1888), who was searching for evidence to disprove Darwin's hypothesis. The observations of Wood-Jones (1907, 1909, 1910), the resident medical officer from 1905 until 1906, are of particular relevance to this study. Wood-Jones (1910) proposed an alternative to Darwin's theory of atoll development based on his observations on Cocos in which atolls formed because reef growth and sediment production were vigorous on the reef rim whereas coral growth was slower toward the centre of the reef platform where sediments were deposited. Furthermore, Wood-Jones argued that this theory could also explain the formation of the discoid massive corals with dead flat tops that he commonly observed in the intertidal areas of the Cocos (Keeling) Islands, for which he coined the term 'microatolls'.

The Cocos (Keeling) Islands are a suitable location to study the relationship between microatoll morphology and sea level for several reasons. First, microatolls are common. Second, the atoll experiences a microtidal regime and relatively large areas of the atoll are intertidal. Third, the Cocos (Keeling) Islands are a mid-ocean atoll and distant from continental influence. Fourth, the islands are rarely affected by severe cyclones. Finally, though difficulties are inevitably encountered when working in remote locations, on the Cocos (Keeling) Islands some logistical support was available.

4.2. Geologic Development and Tectonic History.

The Cocos (Keeling) Islands have not been deep drilled; however, several lines of evidence suggest that the atoll comprises areefal carbonate capping on a volcanic seamount.

Magnetic and gravity surveys reveal anomalies consistent with an underlying volcanic basement

(Chamberlain, 1960; Finlayson, 1970) and igneous rocks have been dredged from the western end of the Cocos Rise (Bezrukov, 1973). Although the thickness of the overlying carbonate veneer is unknown, the depth to the last interglacial limestone has been established for a range of sites around the atoll (Woodroffe et al, 1990b; 1994; Searle, 1994) and reconciled with estimates of the last interglacial sea level to calculate a subsidence rate for the atoll of <0.12mm/yr (Woodroffe et al, 1990b). Based on seismic investigations of the lagoon, Searle

(1994) has suggested that the subsidence rate may be as low as 0.02mm/yr.

Although the atoll's structure appears to be controlled by the slow and long-term subsidence of its foundations, recent patterns of reef growth and sea-level change have shaped its surface morphology (Woodroffe et al, 1994). The Holocene history of Cocos has been examined in detail (Woodroffe et al, 1990a; 1990c; 1994; Smithers etal, 1993; McLean and Woodroffe, 1994), and it has been established that atoll development during the Holocene occurred in three main phases that can be summarised as: (1) an initial phase of rapid reef growth and vertical accretion as the last interglacial platform was drowned by a rapidly rising post-glacial sea-level; (2) lateral expansion of the reef top and formation of the reef flat as reefs caught-up to a stable sea-level approximately 4000 years ago; and (3) a subsequent fall in sea-level of 50-

90cm some time after 3000 years ago with subsequent erosion of the higher reef flat and development of the contemporary one. The reef islands formed after the sea-level decline, largely over remnants of the former reef flat preserved as the conglomerate platform.

4.3. Climate.

An excellent review of the climatic data available for the Cocos (Keeling) Islands is provided by Falkland (1994), and climatic averages are provided in Appendix B. The Cocos

(Keeling) Islands lie within the southeast trade wind belt and the climate is dominated by their influence for approximately 85% of the year. These winds are fairly steady (mean daily wind speed around 7m/second), but strengthen in August and September (mean daily wind speed around 8m/second). The cyclone season on Cocos extends from November to May, however strong cyclones infrequently affect this atoll. Only ten cyclones with a minimum central pressure of less than 1000 hectopascals and winds of more than 100km/hr have passed within 100km of these islands in the last 30 years, and only 6 have passed within 50km. Cyclone Doreen, with a central pressure of 970 hectopascals and wind gusts of 176km/h passed directly over the atoll in

1968 and was by far the most devastating cyclone during this period. Doreen extensively damaged the atoll's infrastructure and coconut palms, however no accounts of its geomorphic influence exist. Three recent cyclones (Frederic: 30/1/1988, max. wind 111km/hr; Graham:

5/12/91, max. wind 98km/hr; Harriet: 27/1/1992; max. wind 163km/hr) had little noticeable affect on the atoll's geomorphology (Woodroffe (Frederic) and Kench (Graham; Harriet) pers. comm.) although Forbes (1879) reported that large portions of conglomerate platform were excavated and moved during the severe cyclone of 1876. Tropical cyclones affecting the Cocos (Keeling)

Islands have usually approached from the northwest.

Rainfall averages around 2000mm/yr, with most falling between February and July.

Cloud cover varies little throughout the year (monthly daily mean 5-5.3 oktas) and the annual range of temperatures on the atoll seldom exceeds 8°C.

4.4. Tidal and Water-Level Characteristics.

The closest amphidromic point to the Cocos (Keeling) Islands occurs off the southwest coast of Australia (Platzman, 1984), and anti-clockwise rotation of the tidal wave around this point suggests that tides on Cocos set from the east-northeast. A mainly mixed semidiurnal microtidal regime affects the Cocos (Keeling) Islands, characterised by large inequalities in range and time between consecutive high and low tides. The maximum spring tidal range is approximately 1.4m at the Home Island tide gauge, however tidal amplitudes are markedly attenuated at the southern end of the lagoon and lag those at the north by 15-55 minutes dependent on the tidal range and lunar phase (spring or neap tides) (Kench, 1994).

Kench (1994) determined that strong tidal currents penetrate the lagoon from the northeastern passage to within 1km of the shallow passages of the eastern and southern atoll rim, and dominate circulation within the lagoon. He proposed a general circulation model in which water entering through the northeast passage moves down the eastern side of the lagoon before being deflected westward by north flowing currents from the south and exiting through the northwestern passage. Currents through the shallow interisland passages are predominantly unidirectional into the lagoon and are dominated by translator/ wave motion. However, during low spring tides currents have been observed to temporarily flow seaward

through passages on the eastern atoll rim (Kench, 1994).

Tidal observations have been made on the Cocos (Keeling) Islands since 1963 by the

CSIRO Division of Oceanography, however tidal records for the atoll are discontinuous, with

large gaps from 1967-1968 and from 1971 until 1986 (Fig. 6). From these data it would appear

that the tidal range in 1963 and 1964 was greater than two metres and has subsequently

decreased, though it is more likely that this larger range is a function of instrumental error. In late

1992 a NOAA tide gauge was installed on the Home Island jetty as part of a global network of

base-line sea-level monitoring stations and is linked by satellite to the National Tidal Facility at

Flinders University, South Australia.

200 150 100 TYfo Maximum y fVf/vA 50 Mean co 0 irv^ ^v-&H jfitAt Minimum E -50 •J T^Wrft o -100 -150

-200 II i i i i i i—n- i i i i i i i i i i i I i i i i i i i i i i i i 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 Year

Figure 6. Mean monthly sea level at the Cocos (Keeling) Islands, 1963- 1991. Source: Home Island tide gauge.

No long-term sea-level rise is indicated by the record of monthly mean sea level (Fig. 6),

although a slight rising trend is apparent for the periods of record collected whilst the gauge was functioning between 1968-1971 and 1986-1991. However, both intraannual and interannual fluctuations in mean sea level are evident. Intraannual fluctuations appear to range between 20 and 40cm in magnitude, with the highest monthly mean sea levels generally occurring in 25

October or November and the lowest monthly mean sea levels from March to May. Perigaud and

Delecluse (1992) identified a strong annual sea-level cycle in the southern tropical Indian Ocean using Geosat altimetry which they attributed to Rossby waves generated by the seasonal trade wind cycle. They calculated that the annual variation in sea level reaches a peak of approximately

12cm in November at 12°S, 90°E, just to the west of Cocos.

A heavy surf driven by the persistent and strong southeast trade winds breaks on the windward reef crests of the Cocos (Keeling) Islands for most of the year. Whilst the trade winds prevail wave conditions on the western atoll rim are often relatively calm, although large waves do affect this shore. During cyclonic conditions large waves break around all sides of the atoll. No data on wave height exist for the Cocos (Keeling) Islands, however during the research for this thesis waves more than 3m high were observed around most of the atoll rim, and ocean swells exceeding 3m were experienced at the northern end of the lagoon. It has been demonstrated elsewhere that open ocean swells are attenuated as they pass over reefs, with up to 97% of wave energy being dissipated at the reef crest (Roberts etal, 1975; 1992). As indicated above,

Kench (1994) suggests that the predominantly unidirectional currents that flow through the interisland passages on the eastern and southern atoll rim are dominated by translatory wave motion, and are possibly enhanced by an hydraulic gradient from the reef crest into the lagoon that develops as wave-induced currents are slowed as they pass over the reef flat and water is

'piled up' at the reef crest (Tait, 1972).

5. THESIS OUTLINE.

This thesis takes the following form. A review of the literature on coral growth, records of environmental change preserved within coral skeletons, and the techniques used to extract these records is presented in chapter 2. The methods used to construct growth chronologies for Porites microatolls on Cocos are described in chapter 3, where the seasonality and periodicity of skeletal density bands and fluorescent bands are also established. Geographical variability in microatoll rim elevation is examined at a range of spatial scales in chapter 4, and variability in microatoll upper surface form is investigated in chapter 5. In chapter 6 the upper surface morphologies developed by two exceptionally large microatolls are compared with instrumental records of sea-level and meteorological conditions to establish whether or not microatoll form can be correlated with environmental conditions. The major conclusions of this thesis are presented in the final chapter, chapter 7, together with suggestions for further study. 27

CHAPTER 2

CORAL GROWTH. CORAL SKELETONS, AND CORALLINE RECORDS OF ENVIRONMENTAL CHANGE

1. INTRODUCTION.

Coral growth and skeletogenesis are influenced by environmental factors, and records of past environmental conditions are structurally and chemically preserved within coral skeletons (e.g. Barnard etal, 1974; Buddemeier etal, 1974; Dodge and Vaisnys, 1975; Weber etal, 1975b; Hudson etal, 1976; Smith etal, 1979; Chivas etal, 1983; Wellington and Glynn,

1983; Dodge and Gilbert, 1984; Isdale, 1984; Shen and Boyle, 1987; Lea etal, 1989; Cole and

Fairbanks, 1990; Druffel et al, 1990; Lough and Barnes, 1990b; Shen and Sanford, 1990;

Aharon, 1991; Beck etal, 1992; Cole etal, 1993; Dunbar et al, 1994; McCulloch et al, 1995;

Tudhope et al, 1995; Wellington and Dunbar, 1995; Swart et al, 1996a; Swart et al, 1996b;

Tudhope etal, 1996). Where such records can be detected and correlated with causative environmental factors of known periodicity they can be used to establish coral growth chronologies (e.g. Fairbanks and Dodge, 1979; Dunbar and Wellington, 1981; Carriquiry et al,

1988; Chakraborty and Ramesh, 1993; Klein etal, 1993; Gagan etal, 1994; Quinn etal, 1996;

Tudhope etal, 1996). A principal objective of this thesis is to reconstruct past water levels from the skeletons of massive Porites microatolls. To achieve this objective topographic fluctuations across the upper surfaces of microatolls must be dated. In this chapter the records of environmental change preserved within coral skeletons that can be used to develop coral growth chronologies, and to reconstruct water-level histories from microatoll skeletons, are reviewed. 28

2. CORAL SKELETONS.

2.1. Growth and Architecture.

Coral skeletons are composed of calcium carbonate precipitated as needle-like aragonite crystals by coral polyps (Goreau, 1961; Barnes, 1970; Barnes and Chalker, 1990).

Despite considerable research, the mechanics of skeleton formation remain unclear. There is some agreement that the aragonite crystals are precipitated over an organic matrix which controls the order of skeletal growth (Goreau and Hayes, 1977; Johnston, 1977); however, the origin and operation of this matrix are poorly understood (Highsmith, 1979; Barnes and Chalker,

1990).

Coral skeletons are colonial aggregations of corallites - aragonitic exoskeletons secreted by individual coral polyps. Corallites consist of four main skeletal elements (Fig. 7); the basal plate located immediately beneath the living coral polyp; the septa which radiate as ridges from the thecae (corallite walls) towards the corallite centre; and the columella which is an upwards extension of the basal plate situated in the centre of the corallite. Basal plates left behind as the coral grows form dissepiments. Thecae are composed of aligned and connected rod-like trabeculae. Skeletal extension takes place at the outer ends of trabeculae which terminate in a splay of fine spikes. These spikes occasionally continue to extend and fuse with those on adjacent rods to form horizontal members termed synapticulae. Synapticulae consolidate the skeletal rods into the major elements listed above. In Porites skeletons adjacent corallites share common, relatively porous thecae that allow neighbouring, but nevertheless individual, polyps to be connected (Barnes and Lough, 1990).

Coral growth may be measured as the increase per unit time in the size (linear extension: cm/yr), mass per unit area (calcification rate: g/cm2/yr), or mass per unit volume

(skeletal bulk density: g/cm^/yr) of a colony. This study is primarily concerned with the vertical extension of a coral's skeleton relative to changes in the constricting water level, and therefore linear extension per unit time is the definition of coral growth adopted here. 29

Columella

Septa

•Thecae 21 Basal Plate

Figure 7. Schematic corallite morphology.

Coral growth rates were originally measured simply by remeasuring selected corals at

known temporal intervals (e.g. Abe, 1940; Shinn, 1966), but are now established using a range

of more sophisticated techniques. The most common methods are of two main types; the first

being the measurement of annual density variations revealed by X-radiography, and the second

being by reference to a range of natural and artificial skeletal markers incorporated into a coral's

skeleton at a known time. Good reviews of these methods are provided by Buddemeier and

Kinzie (1976) and Buddemeier (1978). In recent years more advanced technologies have been

applied to determine coral growth rates, including reflectance optical microscopy (Risk and

Pearce, 1992), scanning electron microscopy (Naqvi, 1994), and computerised tomography and

image analysis (Kenter, 1989; Logan and Anderson, 1991; Bosscher, 1993; Vago er al, 1994).

The methods used to determine coral growth rates in this study are described in detail in the

following chapter.

SKELETAL RECORDS.

Coral skeletons preserve evidence of environmental conditions in two forms, structural and inclusive. Structural records normally involve variations in the balance between 30

the linear extension rate and the calcification rate, and are usually manifested as incremental bands of varying skeletal density, or as hiatuses within the coral skeleton. Inclusive records may be derived from the chemical composition of the skeleton itself, or from material held within the skeletal matrix.

3.1. Structural Records - Density Banding.

Alternate dense and less dense skeletal bands concordant with the growth surface are characteristic of many scleractinian corals. Knutson etal. (1972 p.271) described density bands revealed on X-radiographs as 'distinct and fairly regular alternating dark and light bands, reflecting cyclic variations in the bulk density of deposited skeletal material'. Density band pairs

(couplets) consisting of one dense and one less dense skeletal band are typically 5-20mm wide

(Knutson et al, 1972; Hopley and Isdale, 1977; Patzold, 1984; Barnes and Lough, 1989;

Lough and Barnes, 1990a; Quinn et al, 1996), though the relative widths of, and density gradients between the dense and less dense bands can be quite variable (Highsmith, 1979;

Schneider and Smith, 1982; see Taylor et al, 1993; Heiss, 1996). Density band couplets may comprise either a narrow high density (HD) and wide low density (LD) portion or vice versa (see

Taylor etal, 1993). They may be clear and unambiguous (e.g. Hudson etal, 1976; Swart et al,

1996), or indistinct and difficult to detect (Buddemeier et al, 1974; Isdale, 1981; Dodge and

Vaisnys, 1975; Lough and Barnes, 1992; Heikoop, et al, 1996). Density bands are generally most difficult to detect in corals with lenticulate outer growth surfaces such as Porites (see

Barnes etal, 1989; Barnes and Lough, 1989, 1990; Lough and Barnes 1990a, 1990c, 1992;

Le Tissier et al, 1994 for discussions of the difficulties of detecting density bands). It is generally agreed that dense skeleton is deposited when tissue growth, skeletal extension and calcification rates are slow, and that less dense skeleton is secreted when tissue growth, skeletal extension, and calcification rates are faster (Barnes, 1970; Buddemeier etal, 1974; Highsmith,

1979; Wellington and Glynn, 1983; Dodge and Brass, 1984). Until recently however, a mechanistic account, and the biological and environmental causes of skeletal density band formation remained unclear, despite more than two decades of investigation. 31

A diverse range of contradictory results characterises the coral density band literature and has no doubt slowed progress towards an understanding of their nature and significance.

Conflicting results exist regarding almost all aspects of coral density bands, including the physical structure of the bands themselves, the periodicity and synchroneity with which they form, the relationship between adjacent dense and less dense bands, and the environmental and/or endogenic conditions responsible for their formation.

3.1.1. The Physical Nature of Density Bands.

Several studies have demonstrated that density bands in coral skeletons are constructional rather than erosional features (Buddemeier et al, 1974; Macintyre and Smith,

1974). Three main physical explanations have been proposed for density band construction at the microstructural scale (the scale of individual skeletal elements - Barnes and Deveraux, 1988): i) temporal variation in aragonite needle packing tightness; ii) temporal variation in skeletal element thickness; and iii) variable spacing of dissepiments.

Few direct microstructural investigations of aragonite needle packing densities within dense and less dense skeletal bands exist (Barnes, 1970; 1972); they have usually been inferred from measurements of skeletal density (Buddemeier et al, 1974; Emiliani et al, 1978).

Estimates derived in this way are not very accurate because it is difficult to differentiate changes in packing tightness from changes in the porosity of the skeletal structure. However, where packing densities have been directly observed a positive correlation between skeletal density and the tightness of aragonite needle packing has been demonstrated (Barnes, 1970).

Skeletal thickening has also been proposed as the structural basis for density bands in corals. Trabecular thickening was shown to coincide with HD bands in Pavona gigantea colonies by Macintyre and Smith (1974), and has been speculated to be the cause of HD bands in other massive corals (Buddemeier and Kinzie, 1975; Barnes and Lough, 1989; Dodge et al,

1993). Scanning electron microscopy (SEM) has shown that skeletal elements are thicker through the HD bands of massive Porites (Barnes and Deveraux, 1988; Risk and Sammarco, 32

1991); Patzold (1984) specifically reported that dissepiments were thicker in the HD bands. Le

Tissier etal. (1994) observed a general thickening of all skeletal elements through the HD bands of Porites using high power photomicroscopy.

Dissepiment spacing is determined by the distance over which the base of the tissue layer is periodically, and abruptly uplifted as a coral grows (Barnes and Lough, 1989), and intra­ annual variation in interdissepimental distance has been inferred as a possible factor in density band formation (Barnes and Lough, 1993). However, though marginally variable interdissepimental spacings have been reported in massive Porites (Barnes and Lough, 1993;

Le Tissier et al, 1994) dissepiments in Montastrea annularis and Diploria colonies are regularly spaced (Dodge and Vaisnys, 1980; Dodge et al, 1993), and no change could be detected between dense and less dense bands in Pavona gigantea (Macintyre and Smith, 1974).

At the mesostructural scale (the scale at which the arrangement of skeletal elements comprising a corallite (the skeletal meso-architecture) can be examined - Barnes and Deveraux,

1988) several authors have proposed that the annual banding pattern is caused by periodic

'groupings' of finer subannual bands that have been described mostly from massive Porites

(Barnes and Lough, 1989). A lunar periodicity has been widely proposed for the formation of the fine bands despite annual counts varying between 10 and 14 (Buddemeier and Kinzie,

1975; Houck, 1978 cited in Buddemeier and Kinzie, 1975). Variance about the lunar frequency of 13 yr1 is generally attributed to the difficulty in accurately defining an annual growth increment, and/or in detecting fine bands superimposed over the main banding pattern. X- radiographs reveal that fine bands can form at the skeletal perimeter (Barnes and Lough, 1989), and it has been speculated that they are generated by the extension and thickening of synapticulae at the growth surface in response to environmental and/or endogenic parameters under lunar control (Buddemeier and Kinzie, 1975). Barnes and Lough (1989) recognised that the periodicity and spacing of fine bands and dissepiments in Porites were similar, and hypothesised that a functional relationship between the two may exist. However, they realised that the relationship was unlikely to be simple for several reasons. First, dissepiments form at the 33

base of the tissue layer and fine bands have been observed at the skeletal perimeter. Second, dissepiments are generally very thin (usually <0.03mm) and do not contain enough bulk to form the fine bands, even when grouped. Finally, unlike dissepiments, the spacing between fine bands is often noticeably irregular.

More recently Barnes and Lough (1993) have suggested that two series of fine bands may be deposited over the main density band pattern; one related to the occasional thickening of skeletal elements close to the growth surface in response to episodic variations in environmental and/or endogenic variables, and another related to the regular uplift of the tissue layer base and dissepiment formation combined with within-tissue skeletal thickening. They contend that the occasional bands formed at the skeleton surface are seldom distinguishable on

X-radiographs because they are obscured by the 'dissepimental' fine bands that are later superimposed over them as the tissue layer moves outward. Based on experimental evidence and observations of Porites corals from the Great Barrier Reef, Barnes and Lough (1993) have developed a mechanistic model of density band formation that incorporates linear skeletal extension, skeletal thickening within the tissue layer, dissepiment spacing, and the thickness of the tissue layer. This model can explain the formation of both the fine and main density band patterns in Porites. Taylor et al. (1993) have extended this work, formulating mathematical models that show how different density band characteristics (e.g. HD/LD band width ratios, shape of the density profile) can develop in response to seasonal cycles in a hypothetical forcing function. Brief reviews of these models are presented below.

Continuous extension at the skeletal perimeter of living corals is widely accepted

(Gladfelter, 1984; Barnes and Lough, 1992), and calcification and extension at the colony surface have long been assumed to be the principal mechanisms by which coral skeletons grow

(e.g. Highsmith, 1979; Barnes and Lough, 1989; Klein and Loya, 1991). Supported by Alizarin staining evidence and SEM observations of element thickness through the tissue layer, Barnes and Lough (1993) concluded that skeletal thickening within the tissue layer is another significant dimension of skeleton growth. Such a theory challenges the long-held notion that skeletal 34

growth is confined to the skeletal perimeter, and implies that calcification continues when the skeleton is invested with living tissue. Barnes and Lough (1993) argue that although skeletal thickening within the tissue layer is comparatively slow, the duration of tissue investment over a given point on a skeleton can mean that its contribution to skeletal bulk may rival calcification at the growing front. Based on average tissue depths, Barnes and Lough (1993) have calculated that skeletal thickening commonly continues five months after initial calcification in Porites on the

Great Barrier Reef.

Once a month the lower margin of the tissue layer is abruptly uplifted and a dissepiment is formed (Barnes and Lough, 1989), immediately reducing the thickness of the tissue layer by 20-25%. Because the lower margin is abruptly translated over the skeleton whilst the outer edge gradually extends the duration of tissue investment and amount of intra-tissue skeletal thickening varies within dissepiment spacings; skeleton immediately behind a dissepiment is enveloped by tissue and subject to thickening for a shorter period than that immediately in front. Assuming a constant rate of within-tissue thickening above and below dissepiments should vary by more than 20%. Barnes and Lough (1993) propose that this process generates the fine banding pattern (Fig. 8).

Varying periods of tissue investment and skeletal thickening within the tissue layer can also explain the development of the main density band pattern (Barnes and Lough, 1993;

Taylor etal, 1993). The duration of tissue investment is governed by the difference in the rate of linear extension at the colony perimeter and the rate of uplift of the lower tissue margin.

Because the rate of linear extension varies seasonally whilst the uplift rate of the lower tissue margin remains relatively constant, the thickness of the tissue layer varies through the year as, therefore, does the amount of skeleton potentially thickened within the tissue layer.

Calcification rates and the rate of skeletal thickening within the tissue layer also vary seasonally

(Barnes and Lough, 1993; Barnes et al, 1995). Such variability results in different parts of the skeleton being invested for different lengths of time and thickened at different rates, forming bands of dense and less dense skeleton. Throughout this variability the regular uplift of the 35

lower margin of the tissue layer and emplacement of dissepiments develops the fine banding pattern (Fig. 8), irrespective of the density of the skeleton over which they are formed.

a) Time = t days b) Time = t + 15 days c) Time = t + 30 days

Skeleton Tissue Dissepiment

Figure 8. Schematic growth of coral skeleton within the tissue layer. (after Barnes and Lough, 1993). Vertical skeletal elements are shown without linking synapticulae and the ratio of skeletal thickening to extension has been greatly exaggerated, a) Time = t days: newest dissepiments have just closed, tissue layer extends to depth of most recent dissepiments, b) Time = t +15 days: vertical elements are being both extended and thickened within the tissue layer above the most recent dissepiments, c) Time = t + 30 days: new dissepiments are almost closed, thickening soon to be restricted to the tissue layer above these dissepiments.

The contention that skeletal thickening and density increases continue behind the skeleton surface within the tissue layer is an important facet of Barnes and Lough's (1993) model of skeletal growth. Density bands may continue to 'mature' behind the growth surface several months after initial calcification took place at the growth surface (Barnes and Lough,

1993), and thus density bands formed within the tissue layer may appear to predate their actual time of formation in retrospective analyses that assume skeletogenesis occurs only at the skeletal perimeter (e.g. Schneider and Smith, 1982; Wellington and Glynn, 1983). Barnes and

Lough (1993) surmise that much of the conflict surrounding the timing and seasonality of 36

density band formation (discussed in the following section) may stem from this assumption, and may be resolved by their model.

Taylor et al's (1993, 1995) mathematical models examine how cyclic variations in a forcing function may influence relationships between extension rates, uplift of the tissue layer base, tissue layer thickness, and calcification and skeletal thickening within the tissue layer.

They also demonstrate how different density band patterns can form and how discrepancies between the apparent and actual time of density band formation can arise (Fig. 9). By mathematically modelling the variable affects of differences in the rate of linear extension and skeletal thickening, the latter due to changes in the calcification rate and the duration of tissue investment (a function of the ratio between the thickness of the tissue layer and the extension rate), Taylor and colleagues have produced models which eloquently explain how the myriad density band patterns described in the literature may develop in response to a simple cyclic forcing function which approximates the seasonal oscillations of many of the environmental parameters commonly linked to density band formation.

It is important to recognise that though these models advance our understanding of density band formation in Porites corals, they are, by necessity, simplifications of a complex process that is yet to be fully understood. For example, though the hypothesis that within- tissue thickening contributes to skeletal bulk generally been accepted, several recent studies indicate that most skeletogenesis occurs at the outer growth surface (Gagan ef al, 1994;

McCulloch et al, 1994; Wellington et al, 1996), thus questioning the significance of within- tissue thickening to skeletal bulk. It should also be noted that because skeletal architecture varies between corals of different genera (Dodge et al, 1993) it is likely that the models may need to be modified or replaced to accommodate genera other than Porites. For example, though it has been hypothesised that fine banding contributes to the development of annual bands in Porites and other genera (Barnes and Lough, 1989, 1993), Dodge et al. (1993) have argued that fine subannual bands observed in Montastrea annularis corals are episodic 'stress bands' and are not analogous to the regular fine banding pattern most often described in Porites 37

1f ' I I I II II ™ I I I i II II i i " ! ll II i— i i i II II "- i i i i II II I.I i . i I.I I.I Jf ' ' [ ' ' f ' " r i i g ' ' ii II II ^%J4^ •*XJ<'^/. "*x^^ ^vj^^x/ ' ' if II II 1 .....i..- i . iI.I i . Ii I r ii i I ' ' I i

i i i i I 5 i i i i i i i >- i i i i i i I l I.I I.I i.i I.I ^1I !^l I I

o . i i I I ^ I I i i I I l l Fi I I i i ! I I i I- I I i i I I i I I.I I.I I.I I.I i i

I i "o 1 1 i I I i I it! • ii i i I I I I p! I 1 i i I I l l H- 1 1 i i I I l I I.I I.I I.I i , i

I I I I l i. JK i i I ] I I I i I I I I i l I I i I K I I i i I I i I i . i I I I.I 0 12 24 0 12 24 0 12 24 0 I.12I 24 R = 1.0 R = 0.5 R = 0.25 R = 0.0 Distance along growth axis (mm)

Figure 9. Density profiles generated by numerical models of coral density band formation in which thickening and extension are driven by a sine curve forcing function (after Taylor ef al, 1993). Dashed vertical lines represent the mid-winter trough (left) and mid-summer peak (right) of the sine curve forcing function. TTL=Thickness of the tissue layer, E0=annual extension; R=ratio of wintensummer extension (R=1: no difference, R=0: no extension in mid-winter). The ratio of tissue thickness to annual extension increases down the page. Values of R decrease from left to right. In this model it is assumed that 50% of the skeleton is deposited at initial calcification and 50% as within tissue thickening. Note that the model demonstrates how by varying the depth of the skeleton invested by tissue relative to the extension rate (TTL/EJ, and the ratio of winter to summer extension, it is possible using the same sine curve forcing function to generate density banding patterns with i) apparent 6 month differences in the timing of HD and LD band deposition; ii) variable transition patterns between adjacent HD and LD bands; and iii) variable density differences between adjacent HD and LD bands. 38

(Buddemeier, 1974; Buddemeier and Kinzie, 1975; Barnes and Lough, 1989). It is clearly possible that some of the confusion regarding the formation and structure of density bands may stem from the quest for a universally applicable model, and that specific models tailored for genera with particular skeletal meso-architectures may be more appropriate. As indicated in chapter one, Porites microatolls are examined in this thesis, and the following discussion will thus focus on studies of the physical nature of density bands in corals of this genera.

3.1.2. The Environmental Significance of Density Band Formation.

Ma (1933) was one of the first to recognise the environmental significance of cyclic variations in the density of coral skeletons, correlating the width of what he considered were annual cycles of thick and thin skeletal elements visible in coral sections with seawater temperature. Other early work was also based on the visual identification of periodic thickening and thinning of skeletal elements (Ma, 1937; Abe, 1940; Wells, 1963; 1966). The utility of X- radiography for detailed investigations of the structure of coral skeletons was later demonstrated by Knutson et al (1972), renewing interest in the environmental significance of coral density bands (e.g. Buddemeier et al, 1974; Macintyre and Smith, 1974; Dodge and Vaisnys, 1975,

1980; Weber et al, 1975a, 1975b; Hudson, 1981; Wellington and Glynn, 1983; Barnes and

Lough, 1989; Lough and Barnes, 1990a, 1990b; Gagan etal, 1994).

Density band couplets have been shown to form annually using several types of independent evidence, including the incorporation of Alizarin stains in live corals (Macintyre and

Smith, 1974; Dustan, 1975; Isdale, 1981; Scoffin et al, 1992), isotopic analysis of skeletal material (Weber etal, 1975a; Emiliani etal, 1978; Patzold, 1984; Klein etal, 1993; Tudhope et al, 1996), radiometric traces of skeletal deposition (Dodge et al, 1974; Moore and

Krishnaswami, 1974), autoradiography (Knutson et al, 1972; Noshkin et al, 1975; Hudson,

1985), and by reference to 'natural' markers of known age such as scars caused by volcanic eruptions and anoxic water conditions (Woodroffe and McLean, 1990; Heikoop et al, 1995).

Density band couplets have subsequently been widely used to delineate annual increments of skeletal growth, and to reconstruct the growth histories of massive corals (Hopley and Isdale, 39

1977; Weber and White, 1977; Dodge and Lang, 1983; Isdale, 1984; Woodroffe and McLean,

1990). However, the universal acceptance of an annual periodicity for density band couplets has been questioned in several studies that described corals that formed more than one density band couplet per year (Druffel, 1985; Brown etal, 1986; Scoffin etal, 1992).

Although many studies in the last twenty years have linked various environmental and/or endogenic parameters with intra-annual shifts in skeletal density, the long-standing lack of a mechanistic account of density band formation (see section 3.1.1, this chapter) has fuelled confusion regarding when during the year, and in response to what factors, density bands form.

The need to establish clear quantitative relationships between environmental parameters and density band patterns before the full potential of corals as environmental monitors can be realised has been repeatedly stressed (Weber etal, 1975a; Wellington and Glynn, 1983; Taylor etal, 1995). However, with few exceptions (Buddemeier et al, 1974; Buddemeier and Kinzie,

1976; Barnes and Lough; 1989; Lough and Barnes, 1990b; Dodge et al, 1993; Taylor et al,

1993; Le Tissier et al, 1994), now density bands form has been largely overlooked. Most studies that have linked density band formation to coral growth factors have been based on assumptions regarding both the nature and timing of shifts in skeletal density, and it is not surprising that many contradictory conclusions have been reached.

Associations between density bands and environmental and/or endogenic factors are usually premised on the apparent synchrony of shifts in skeletal density and fluctuations in the assumed causative parameter. As Weber et al. (1975b, p.139) state 'It would seem at first that relating skeletal density to season at a particular locality, and subsequently identifying the environmental parameter(s) whose variation(s) correspond most closely with the production of density bands, would be a relatively simple and straightforward task. This is, however, not the case.' Three difficulties are encountered; detecting the bands, determining when the bands formed, and establishing the principal forcing function. 40

The problems of detecting coral density bands with X-radiographic techniques have been well covered by several authors (Barnes and Deveraux, 1988; Barnes et al, 1989; Lough and Barnes, 1990a; Le Tissier et al, 1994). It is generally concluded that most problems of density band detection arise because X-rays pass perpendicularly through a coral slice but density bands may be slanted through its thickness, and seldom is the slant consistent along an entire sample. Where bands are slanted the average density vertically through the slice is represented on the X-radiograph, obscuring the magnitude and exact position of shifts in skeletal density - the so called 'Venetian blind effect" (Barnes and Deveraux, 1988). Clearly it is difficult to establish when a density band formed if its exact position cannot be defined. The seasonality of skeletal density variation has generally been determined by reference to the outermost band at the time of sampling and/or by reference to skeletal markers of known age

(Buddemeier et al, 1974; Weber et al, 1975b; Wellington and Glynn, 1983; Charuchinda and

Chansang, 1985; Brown et al, 1986; Klein et al, 1993). However, curved growth surfaces commonly make it difficult to accurately determine skeletal density at the colony perimeter from

X-radiographs because the outer edge of a skeletal slice is seldom square to the X-ray beam.

The detection of skeletal markers can also be problematic, various authors reporting that markers such as Alizarin stains could not be reliably detected in sectioned samples

(Buddemeier and Kinzie, 1975; Barnes and Lough, 1993; Wellington et al, 1996). Errors can also arise where density bands are slanted through a sample and the position of the marker visible on the upper slice surface is offset from its true position relative to a skeletal density band on the resultant X-radiograph (Barnes et al, 1989; Lough and Barnes, 1990a). Furthermore,

Barnes and Lough (1993) have shown that Alizarin stains are concentrated at the skeletal front where calcification is quickest, and may not mark where density bands are being formed by skeletal thickening within the tissue layer. The suggestion that calcification continues within the tissue layer also has significant implications for studies that have verified the timing of density shifts in coral skeletons using chemical proxies of seasonal change (e.g. stable isotopes and trace elements). These proxies may not provide the instantaneous records of coral growth conditions at the skeletal front as has long been assumed, but because the skeleton 41

progressively thickens within the tissue layer may instead yield a smoothed record akin to a running mean, where contributions to the mean vary as calcification rates seasonally fluctuate

(Barnes et al, 1995; Taylor et al, 1995). However, as previously mentioned, recent research suggests that skeletal growth processes typically distort inclusive records to a lesser extent than predicted by Barnes and Lough (1993).

In addition to the fact that skeletal density may have been incorrectly linked to assumed causative factors in many studies because density band formation was poorly understood and inappropriate dating techniques were used, direct mechanistic links between these factors and coral density banding are yet to be empirically established. Barnes and Lough

(1989, p.121) lament 'there has been an unfortunate tendency in the literature to accept annual cycles in environmental variables as evidence for a causal effect for the annual cycle of skeletal density' and go on to describe the correlations between environmental variables and coral density reported in the literature as 'often ambiguous' and in many cases 'tenuous'.

A review of the literature reveals that the deposition of HD bands has been linked to excessively high or low water temperatures (Dodge and Vaisnys, 1975, 1980; Weber et al,

1975a, 1975b; Hudson etal, 1976; Highsmith, 1979; Dodge and Lang, 1983; Patzold, 1984); low light levels (Knutson et al, 1972; Dodge and Thomson, 1974; Buddemeier and Kinzie,

1975; Wellington and Glynn, 1983; Charuchinda and Chansang, 1985; Brown et al, 1986); sedimentation and turbidity (Dodge etal, 1974; Dodge and Vaisnys, 1977, 1980; Brown et al,

1986, 1990), hydrodynamic and wind stress (Scoffin et al, 1992); nutrient availability

(Wellington and Glynn, 1983; Klein et al, 1993), reproduction (Loya, 1985; Wellington and

Glynn, 1983), and stress due to freshwater influx or bleaching events (Hudson et al, 1976;

Hudson, 1981; Leder etal, 1991). Early studies divide almost evenly between seasonal fluctuations in water temperature and light intensity as the primary cause of coral density banding, and present rather polarised views. Recent studies are generally more accommodating of the possible roles of both of these factors, and in many instances suggest the possible influence of others. 42

Light intensity and water temperature have been widely nominated as key factors in the development of density bands (see Highsmith, 1979; Wellington and Glynn, 1983), probably because they vary seasonally and their effects on coral growth and calcification have been well studied (e.g. Shinn, 1966; Baker and Weber, 1975; Clausen and Roth, 1975; Dustan,

1975; Coles etal, 1976; Chalker, 1977; Houck et al, 1977; Coles and Jokiel, 1977, 1978;

Chalker, 1981; Huston, 1985; Barnes and Chalker, 1990; Falkowski et al, 1990; Jokiel and

Coles, 1990). Coral growth has been positively correlated with both light intensity and water temperature up to optimal tolerance limits, beyond which it stabilises or declines. Optimum temperatures generally lie within the range of 24°C and 28°C (Clausen and Roth, 1975; Houck et al, 1977), though species and geographic variation has been demonstrated (Coles et al, 1976;

Coles, 1988). Light enhances calcification via the photosynthetic activities of endosymbiotic algae (see Barnes and Chalker, 1990 for a recent review). Extreme light intensities may induce coral bleaching and photoinhibition (Huston, 1985; Gleason and Wellington, 1993), though several studies report photoadaption to increased illumination (Chalker and Dunlap, 1983;

Falkowski etal, 1990). Jokiel and Coles (1990) suggest that increased light intensities will not directly stress corals, but it may heighten their sensitivity to other factors.

The relative importance of light and temperature in density band formation has been well debated (Buddemeier et al, 1974; Weber et al, 1975b; Wellington and Glynn, 1983;

Lough and Barnes, 1990a), with some speculation that the process is controlled by their interaction (Highsmith, 1979). However, because they often fluctuate in phase and/or their effects are rather imprecisely derived from secondary sources, their relative influence on coral growth is difficult to establish. Negative correlations between seasonal changes in skeletal density and light intensity under relatively stable water temperature conditions have been cited as evidence that light intensity is the primary environmental cue involved in density band formation (Wellington and Glynn, 1983; Charuchinda and Chansang, 1985; Brown et al, 1986), as have increases in the HD/LD band width ratio with depth, where light intensity is progressively diminished but temperature remains relatively constant (Baker and Weber, 1975; Huston,

1985). Several studies have inferred a negative relationship between skeletal density and light 43

intensity from skeletal §13C isotope profiles; skeletal carbonate becomes 13C enriched as 12C is more rapidly removed by zooxanthellae during high light, rapid growth phases when LD bands are deposited (Patzold, 1984; Cole etal, 1993; Klein etal, 1993). Regional coincidence in the timing of shifts in skeletal density and water temperature has been interpreted as evidence that water temperature is the primary control of skeletal density (Weber et al, 1975b; Highsmith,

1979), though more recent studies usually cite synchronised cycles in skeletal density and oxygen isotope or strontium/calcium ratios as support for this claim (e.g. Weber et al, 1975a;

Schneider and Smith, 1982; Patzold, 1984; Wang and Huang, 1987, 1989; Druffel et al, 1990;

Klein et al, 1993). A positive correlation between water temperature and coral density is generally reported (see Lough and Barnes, 1990a), however there are several notable exceptions (Schneider and Smith, 1982; Chivas et al, 1983; Carriquiry et al., 1988; Aharon,

1991; Klein and Loya, 1991; Klein etal, 1993). Lough and Barnes (1990a) suggest that these exceptions may reflect errors in dating density shifts due to coral geometry, analytical technique, or observer error. They note that most of the disparate studies involved Porites, a genus with characteristically bumpy surfaces that can make estimations of skeletal density at the skeletal perimeter, and thus correlations with environmental/endogenic factors, difficult. Clearly, it is also possible that these discrepancies are real, or reflect dating errors stemming from the assumption that calcification only occurs at the colony surface (see 3.1.1, this chapter). The development of density bands within the tissue layer may also explain the apparently synchronous deposition of

HD and LD bands in neighbouring corals, an observation that has troubled several investigators because it questions a fundamental tenet of much of the density band literature - that density bands form in direct response to environmental cues (e.g. Isdale, 1977; Stearn et al, 1977;

Charuchinda and Chansang, 1985; Brown etal, 1986; Goenaga pers. comm. cited in Winter et al, 1991; Lough and Barnes, 1990a; Barnes and Lough, 1992). Taylor et al (1993) have demonstrated how Barnes and Lough's (1993) model of density band formation can explain this conundrum in terms of colony differences in tissue layer thickness and annua! extension rates.

Several authors have recognised that it is difficult to link density band formation solely to seasonal fluctuations in light intensity or water temperature in equatorial corals which 44

experience little seasonality, particularly because intra-monthly variability in these parameters often exceeds that between monthly means (Weber etal, 1975b; Highsmith, 1979). Searching for an explanation, Highsmith (1979) hypothesised that water temperature and light intensity may be proximate rather than ultimate causes for cyclic changes in skeletal density, serving as environmental cues for seasonal shifts in coral physiology which then control skeletal density.

Sedimentation and turbidity have also been linked to coral density band formation, either by periodically reducing light intensities and/or by imposing a metabolic drain through the energetic costs of sediment dispersal (Dodge and Vaisnys, 1975, 1977; Charuchinda and

Chansang, 1985; Brown etal, 1986). Regular density band formation is, however, predicated on the cyclic influx of sediments. Several studies of shallow water corals have speculated that

HD bands may be regularly generated by turbidity associated with the resuspension of sediments by seasonal winds (Charuchinda and Chansang, 1985; Brown et al, 1986). In practice the effects of sedimentation and turbidity on coral growth are often difficult to isolate from those of light intensity (Dodge and Lang, 1983).

Seasonal fluctuations in nutrient availability, hydraulic energy, and endogenic processes such as those associated with reproduction, have also been linked to density band formation. The development of HD bands has been connected to nutrient influx during upwellings (Dodge and Vaisnys, 1975; Wellington and Glynn, 1983), and lunar cycles of phytoplankton abundance (an organic food source for many corals) have been implicated in the development of the fine subannual band series in Porites (Buddemeier and Kinzie, 1975).

Highsmith (1979) has argued that nutritional constraints are not important in density band formation on the basis that it cannot account for increasing HD/LD band width ratios with depth at sites flushed by well-mixed oligotrophic waters. Loya (1985) similarly argued that nutrient availability could be discounted as a control of skeletal density in corals growing in the Gulf of

Eilat, Red Sea because nutrient levels are constantly low throughout the year (Loya, 1985).

However, Klein etal (1993) has described a reversal of the seasonal timing of band deposition with depth in Porites from the same area, and has speculated that a depth-related autotrophy- 45

heterotrophy gradient may be responsible. Klein and co-workers postulated that shallow corals are more dependent on autotrophic nutrition whilst poorly illuminated corals at depth derive their energy largely by feeding on demersal zooplankton, and that the times of year when these

'energy' maxima occur may be six months out of phase. If their hypothesis is correct, and is found to be a general phenomenon, much of the existing literature will have to be re-evaluated, and more detailed attention paid to energy sources in future investigations.

Corals subject to higher hydraulic energy levels generally deposit more robust skeletal elements (Foster, 1977; Scoffin et al, 1992). Dodge and Vaisnys (1980) found that spatial variability in coral growth rates over the Bermuda platform is best explained by local differences in wind and wave energy. They suggest coral growth is inhibited under high energy conditions because food capture is more difficult and energy expensive, and entrained sediment reduces illumination and may physically buffet and bury colonies. The possibility that seasonal changes in wave and current energy may cause seasonal changes in a coral's skeletal density, either directly or indirectly, is presently being investigated by Scoffin and co-workers

(see Scoffin etal, 1992).

Endogenic causes for skeletal density fluctuations have also been investigated, with several authors proposing a link between HD band deposition and the energetic costs of reproduction (Buddemeier and Kinzie, 1975; Wellington and Glynn, 1983, Hudson, 1985;

Loya, 1985). Hudson (1985) noted reports that Montastrea annularis appeared to produce HD bands only after sexual maturity was reached (Szmant-Froehlich unpublished data cited in

Hudson, 1985), and suggested that linear extension is reduced during reproductive phases because energy is diverted to gamete propagation and release. Hudson (1985) also postulated that the lack of density bands in Porites from Enewetok lagoon for several years after the 1958 nuclear tests was due to their sterilisation by the blast. Wellington and Glynn (1983) provide further, though anecdotal, evidence of a distinctive shift from LD to HD in Hawaiian Porites skeletons coincident with their reported time of spawning. In contrast, Klein and Loya (1991) 46

found that shifts in skeletal density were not in phase with reproduction in Red Sea Porites, and concluded that reproduction had little influence on density band formation.

High density 'stress' bands have been reported in several studies and are thought to develop when calcification and linear extension rates are suppressed (Hudson et al, 1976;

Wellington and Glynn, 1983; Brown etal, 1986; Leder etal, 1991; Swart etal, 1996). A variety of unfavourable environmental conditions have been associated with stress bands, including abnormally cool water (Hudson et al, 1976; 1982), unusually warm water (Leder et al, 1991), and excessive light intensities due to subaerial exposure (Buskirk et al, 1981). Increased sedimentation, higher turbidity and poor illumination have also been demonstrated to reduce coral growth rates (Dodge and Vaisnys, 1975, 1977; Loya, 1976; Bak, 1978; Edmunds and

Spencer-Davies, 1989), and have been hypothesised as possible causes of subannual stress bands (Brown etal, 1990). However, not all stressful events are recorded as HD stress-bands, suggesting that caution is required when interpreting histories of environmental stress from coral skeletons. For example, Brown et al. (1990) found no stress-bands, changes in linear extension, or calcification rates in corals adjacent to those killed by excessive sedimentation caused by dredging, and Dodge and Brass (1984) reported that corals subject to high turbidity had relatively high linear extension rates and did not accrete a HD stress band.

Lough and Barnes (1990a, p49) examined the results of 21 papers which specifically included observations of the intra-annual timing of shifts in skeletal density and concluded

There is a general consensus, across species and regions, that HD bands form in summer, late summer or during the period of warmest water. There is also general agreement that LD bands form in winter or during the period of lower water temperature.' They cite five exceptions to this general trend, and suggest that methodological problems may be responsible. However, it is now evident that dating errors may be common throughout the literature, and in addition to the exceptions noted by Lough and Barnes (1990a) there are several other 'anomalous' papers

(Carriquiry etal, 1988; Aharon, 1991; Winter etal, 1991; Klein and Loya, 1991; Klein et al,

1993). In an earlier review Highsmith (1979) identified latitudinal differences in the timing of HD 47

band formation and its presumed environmental correlates. Isdale (1981) also identified a latitudinal shift in the apparent timing of density band formation in Porites corals on the Great

Barrier Reef, but he could not unequivocally correlate this change with any systematic environmental variation. At a smaller scale, Lough and Barnes (1992) could find no evidence of a common density band signal in Porites growing on reefs less than 100km apart on the central

Great Barrier Reef, in colonies growing on the same reef, nor between transects from the same colony. As noted earlier, several studies describe the apparently synchronous deposition of HD and LD bands in corals growing on single or nearby reefs (see Highsmith, 1979; Charuchinda and Chansang, 1985; Brown, et al, 1986; Lough and Barnes, 1990a), though most indicate that corals from a given locality exhibit a common density band pattern (Buddemeier, et al,

1974; Weber et al, 1975b). Where a similar banding pattern is recognised over a large area control of skeletal density by large-scale factors, such as seasonal fluctuations in light and temperature is usually inferred, whilst localised departures from the pattern are attributed to local influences. A number of attempts have been made to integrate a range of environmental/endogenic factors known to affect coral growth to explain observed variability in density band patterns (Highsmith, 1979; Schneider and Smith, 1982; Wellington and Glynn,

1983; Klein and Loya, 1991), however the causal relations on which these models are founded are usually inferred, and they cannot account for all of the variability reported in the literature. It would seem that taken as a whole the evidence increasingly suggests that the cause(s) and timing of density band formation may vary with location, and at some sites, through time.

An important principle of tree-ring research (dendrochronology), to which coral band research (sclerochronology) is often compared, is that not all trees will preserve a clearly defined environmental signal, and that the strength of that signal will be strongest where one of the many factors known to influence tree growth dominates the record. Another important principle is that sites are selected where growth will be most sensitive to a single factor and that the factor most clearly expressed in the dendrochronological record will vary with location (see Fritts, 1976;

Hughes etal, 1982; Schweingruber, 1988 for discussion of dendrochronological principles and procedures). Few authors appear to have recognised the significance of this analogy for 48

sclerochronological research (Lough and Barnes, 1990a, 1990b). It is possible that variability in density band patterns on the Great Barrier Reef reflects the generally benevolent coral growth conditions which prevail, analogous to a mesic forest in tree-ring studies, where unlike nearer to the latitudinal limits of coral growth (eg. Hawaii, Abrolhos Islands) steep environmental gradients do not impose strong signals on the density band pattern.

Ambiguity regarding the timing and cause(s) of density band formation has several implications for studies which employ these bands to establish coral growth chronologies.

Lough and Barnes (1990a, p54) conclude that 'the density banding pattern in corals is unlikely to be a faithful record of time', concurring with Buddemeier's earlier statement that the annual banding pattern is made up of 'soft' years. In effect these authors suggest that points of equivalent density in successive density bands need not be deposited at the same time of year and thus annual chronologies derived from them must be considered somewhat imprecise, a conclusion also reached by others (Winter et al, 1991; Swart et al., 1996a). For example, if density bands develop in response to seasonal changes in light intensity Lough and Barnes

(1990a) have calculated that the interval over which light intensity could vary enough to develop

HD bands would be about seven months where strong monsoonal conditions prevail, but around seventeen months during drought years. Furthermore, they note that differences in the timing and proportion of the year over which successive HD bands are deposited are likely to vary with latitude. Where two or more factors synergistically act to control density band formation then estimation of the 'softness' of the annual chronology becomes even more complex, particularly in view of likely differences in the influence of these factors between and even within sites (i.e. at different depths, aspects, etc), and because of the differing responses of individual colonies and different species. Where dissepiments can be identified within a skeletal slice they may allow the chronology of colony growth to be more precisely determined because they form at regular lunar monthly intervals (Barnes and Lough, 1992; 1993).

In summary, clear relationships are yet to be established between any particular environmental factor(s) and the accretion of high and low density bands in coral skeletons. Light 49

and water temperature are most often nominated as key controlling factors, with sedimentation, water turbidity, nutrient supply, wind stress, and reproductive cycles also possibly being involved at more localised scales. Evidence in the literature suggests that no single environmental factor can universally account for seasonal changes in coral skeleton density.

Despite uncertainties as to how and when during the year density band couplets form, they have been overwhelmingly accepted as 'annual' features, though the annual chronology must be considered somewhat elastic.

3.2. Inclusive Records.

Corals preserve within their skeletons a range of inclusive and structural clues about environmental conditions during their lifespans. In the last twenty years much research has focused on reconstructing palaeoenvironmental conditions from coral cores using annual density bands as chronometers. Typically this research has involved the systematic sampling of a coral's skeleton along a growth axis and establishing how various proxy indicators of environmental conditions have varied through time. Stable oxygen and carbon isotopes, trace and minor elements, and organic compounds such as humic and fulvic acids are the most common proxy indicators used in these studies.

3.2.1. Isotopic Signatures in Coral Skeletons.

Stable oxygen and carbon isotope ratios in coral skeletons have been shown to be useful indicators of environmental conditions and coral vigour during skeletogenesis (see

Weber and Woodhead, 1970; Land etal, 1977; Erez, 1978; Aharon and Chappell, 1983; Cole and Fairbanks, 1990; Chakraborty and Ramesh, 1993; Quinn etal, 1993; Dunbar etal, 1994;

Gagan etal, 1994). Stable oxygen isotopes have been used to reconstruct historical changes in seawater temperature (Weber and Woodhead, 1972; Fairbanks and Dodge, 1979; Dunbar and Wellington, 1981; Wang and Huang, 1987, 1989; Carriquiry etal, 1988; Leder et al, 1991;

Winter etal, 1991; McCulloch etal, 1994; Tudhope etal, 1995; Wellington and Dunbar, 1995;

Tudhope et al, 1996; Wellington, et al, 1996), and salinity and rainfall (Swart and Coleman, 50

1980; Cole and Fairbanks, 1990; Cole et al, 1993; Swart et al, 1996a, 1996b). Stable carbon isotopes have been used to hindcast light intensity, coral growth rates, and the isotopic signature of the surrounding seawater (e.g. Emiliani et al, 1978; Fairbanks and Dodge, 1979;

Weil etal, 1981; McConnaughey; 1989). Fairbanks and Dodge (1979) demonstrated that the stable oxygen and carbon isotope composition of coral skeletons fluctuated in annual cycles, which they related to seasonal variations in sea-surface temperature and cloudiness (light intensity) respectively. The existence of seasonal stable isotope cycles has been confirmed in many subsequent studies (Chivas et al, 1983; McConnaughey, 1989; Aharon, 1991), and has been used to infer the timing and frequency of skeletal density band accretion (Patzold, 1984;

Leder etal, 1991; Klein etal, 1993; Gagan etal, 1994; Tudhope etal, 1995).

The history and theory of the stable isotope analysis of coral skeletons have been comprehensively reviewed by several authors (Goreau, 1977; Rye and Sommer, 1980; Swart,

1983; Aharon, 1991). It is generally assumed that precipitated skeleton and ambient seawater are close to isotopic equilibrium (Keith and Weber, 1965; Erez, 1978), and that shifts away from the equilibrium are caused by 'vital effects' (Erez, 1978) which develop in response to kinetic and metabolic factors (Swart, 1983; McConnaughey, 1989). Kinetic effects occur when lighter isotopes (12C and 160) are preferentially utilised by CO2 hydration and hydroxylation during skeletogenesis (McConnaughey, 1989; Aharon, 1991). Kinetic effects are characterised by the simultaneous depletion of both 813C and 8130, and are greatest during phases of rapid skeletal growth (McConnaughey, 1989; Aharon, 1991). In contrast metabolic effects are caused by photosynthetic and respiratory modification of the internal carbon pool, and can be recognised where depletions in 513C and 5180 are not positively correlated (Weil et al, 1981;

McConnaughey, 1989).

Plots of 513C against 8130 usually show much scatter and have been interpreted as evidence that different processes control the fractionation of carbon and oxygen isotopes in coral skeletons (see McConnaughey, 1989). The temperature and isotopic composition of seawater are generally believed to be the primary controls of skeletal 8130 (e.g. Weber and 51

Woodhead, 1972; Goreau, 1977; Aharon, 1991; Dunbar etal, 1994), whilst coralline 513C is most commonly linked to seasonal fluctuations in light intensity, mediated by endosymbiotic photosynthesis (e.g. Emiliani ef al, 1978; Fairbanks and Dodge, 1979; Weil et al, 1981;

McConnaughey; 1989; Cole and Fairbanks, 1990; Aharon and Chappell, 1983; Quinn et al,

1993; Allison etal, 1996). Swart etal (1996c) have recently suggested that seasonal changes in the 813C of local seawater also strongly influences both the S180 and 813C coral record. The hypothesis that photosynthesis has little affect on skeletal 8180 is supported by similar skeletal

8180 values in photosynthetic and non-photosynthetic corals (McConnaughey, 1989). Aharon

(1991) demonstrated that coralline S^O reflects environmental rather than biological controls by comparing synchronous 8180 records from neighbouring corals and clams (organisms with different calcification pathways).

Epstein et al. (1953) formulated an expression relating skeletal 8180 values to seawater temperature based on the isotopic equilibrium between seawater and calcite. This equation was later modified for coralline aragonite by Dunbar and Wefer (1984), and predicts a negative correlation between 8180 and seawater temperature. After examining 4291 corals from 44 genera Weber and Woodhead (1972) concluded that temperature versus coralline

8180 curves were essentially parallel to the calcite curve, but the curve for each genus was displaced from it by a unique, species particular constant, thought to reflect taxonomic variability in vital effects.

Many studies have established good agreement between measured temperatures and those derived from skeletal 8^80, and 8180-temperature equations have been widely used to reconstruct palaeoseawater temperatures from coral skeletons (Fairbanks and Dodge, 1979;

Dunbar and Wellington, 1981; Weil etal, 1981; Chivas etal, 1983; Patzold, 1984; Carriquiry et al, 1988; Wang and Huang, 1989; Leder etal, 1991; Winter etal, 1991; Klein et al, 1993;

Chakraborty and Ramesh, 1993; Gagan et al, 1994; McCulloch et al, 1994; Wellington and

Dunbar, 1995 Quinn etal, 1996). McConnaughey (1989) has calculated accuracies of ±0.6°C can be achieved using these equations. However, other researchers are less convinced of the 52

accuracy of coralline 8^^0 palaeothermometry and argue that seasonal 5180 ranges, and thus reconstructed temperatures, can be affected by metabolic processes (Goreau, 1977; Emiliani er al, 1978), changes in seawater salinity (Swart and Coleman, 1980; Swart et al, 1983), and sampling difficulties (Aharon, 1991; Leder etal, 1991; Patzold, 1993). Significant differences between recorded temperatures and those calculated from skeletal 8180 data from Great Barrier

Reef Porites using Weber and Woodhead's (1972) and McConnaughey's (1989) S180- temperature equations have prompted Aharon (1991) to suggest that they may need revision.

Coralline 8180 is also influenced by the isotopic composition of the surrounding seawater during skeletogenesis, requiring adjustments to the simple 8180-temperature relationship.

Changes in seawater 8180 are principally related to salinity variations; seawater becoming 8180 enriched when salinity is increased by evaporation and S^80 depleted when diluted by rainfall

(Rye and Sommer, 1980; Swart and Coleman, 1980; Dunbar and Wellington, 1981; Aharon and

Chappell, 1983; Cole and Fairbanks, 1990; Quinn et al, 1993; Gagan et al, 1994; Swart et al,

1996a, 1996b). Several authors have proposed that the temperature sensitivity of coralline

8180 may be dampened by salinity variations; enhanced 8180 depletion caused by seawater warming possibly being cancelled out by 8180 enrichment caused by lower rainfall, increased evaporation, heightened salinity, and possibly bleaching events (Swart and Coleman, 1980;

Swart etal, 1983; Leder et al, 1991; Halley er al, 1994). Where rainfall is markedly seasonal, salinity effects can significantly influence 8180 values; Dunbar and Wellington (1981) estimated that around 30% of seasonal 8180 variation in Panamanian corals was due to rainfall induced salinity variation. Skeletal 8^ 80 has been shown to be a good proxy for rainfall in many reef settings (e.g. Great Barrier Reef: Aharon, 1991; Gagan et al, 1994; Caribbean: Winter et al,

1991; Cole et al, 1993). Indeed, the strength of the rainfall coralline 8180 signal may overwhelm the temperature record in certain areas. Cole and Fairbanks (1990) concluded that skeletal 8180 is principally a proxy for precipitation in the central equatorial Pacific and that the temperature signal is secondary. Swart et al. (1996b) similarly used the S180 composition of a long-lived Montastrea colony to reconstruct a 240-year rainfall history for southern Florida. Most studies to date have focused on the dampening effects salinity changes have on the skeletal

S180 record, however Aharon (1991) argues that when seawater salinity falls during monsoonal 53

downpours and is increased six months later by dry season evaporation, the amplitude of seasonal 8180 variations may exceed temperature-driven values.

Causative links between coral 813C values and specific environmental variables are more difficult to define. Skeletal 813C is both synthesised by the coral itself and derived from the water column, the isotopic composition of which may be modified by photosynthesis and respiration in the vicinity of living polyps (Weil et al, 1981; McConnaughey, 1989; Aharon,

1991). The relative representation of autotrophic and heterotrophic carbon at calcification sites, differences in carbon residence times in various biological and biochemical reservoirs, and the modulation of photosynthesis by the corals themselves, all complicate environmental interpretations of 813C. Nevertheless, despite these uncertainties a strong positive correlation between coralline 813C and insolation is generally described (Fairbanks and Dodge, 1979;

Patzold, 1984; McConnaughey, 1989; Shen et al, 1992; Wellington and Dunbar, 1995), both seasonally and along depth gradients. Cummings and McCarty (1982) established that a positive correlation also exists between skeletal 813C and the density of algal symbionts, and it is now widely accepted that coralline 813C is largely a function of light intensity regulated by endosymbiotic photosynthesis (Fairbanks and Dodge, 1979; Dunbar and Wellington, 1981;

Weil et al, 1981; Swart, 1983; Patzold, 1984; Wang and Huang, 1989). The generally lower skeletal S13C values established for non-photosynthetic corals support this hypothesis (Weber and Woodhead, 1972; Land et al, 1977). Contrary reports of 813C depletion coincident with intense insolation have been attributed to bleaching and photoinhibition (McConnaughey,

1989; Leder et al, 1991), when algal symbionts are expelled or debilitated (Falkowski and

Dubinsky, 1981; Dustan, 1982). McConnaughey (1989) suggests that workers advocating that skeletal 813C and photosynthesis are negatively correlated (e.g. Erez, 1978) are flawed because they fail to separate the influence of metabolic and kinetic effects. McConnaughey

(1989) argues that kinetic effects dominate the fractionation of isotopic carbon during skeletogenesis is rapid, when photosynthetic rates are usually high, causing an apparent link between photosynthesis and 813C depletion. 54

As indicated above, skeletal 813C depletion is most pronounced in rapidly deposited skeleton. Land etal. (1975) concluded that 813C depletion is caused by the active movement of organic compounds to sites of rapid calcification. McConnaughey (1989) alternatively suggested that kinetic effects are responsible. Aharon (1991) determined that S13C was increasingly depleted as growth rates accelerated in Porites from the Great Barrier Reef, and noted that within a single colony 8^3C depletion was greatest in the more vigorously growing parts. Aharon (1991) concluded that seasonal cycles in skeletal 813C can serve as a useful proxy of seasonal variation in coral growth rate, a conclusion also reached by others (Goreau,

1977; Dunbar and Wellington, 1981). Various studies have used seasonal 813C variations to reconstruct coral growth chronologies, and have recognised that 813C is a particularly useful indicator of coral growth hiatuses because of its relationship to coral vigour (e.g. Fairbanks and

Dodge, 1979; Patzold, 1984; Coles and Fairbanks, 1990; Cole etal, 1993). Gagan etal. (1994) also examined Porites from the Great Barrier Reef, but argued that concurrent smooth S180 and episodically fluctuating 813C cycles suggest that 813C is not controlled by growth rate. Gagan and colleagues found that 813C enrichment occurs as gametes form and depletion follows spawning, and proposed that coralline 8^3C is controlled by changes in coral metabolism associated with gamete production. Furthermore, they recognised that because corals on the

Great Barrier Reef spawn at predictable times related to lunar and tidal cycles (Babcock et al,

1986), the timing of S^C peaks, and coral growth chronologies, can be accurately established.

Although an annual periodicity for skeletal stable isotope cycles can usually be detected, it is not always strongly evident. It has been argued that the seasonality of the controlling parameters may be weak (Cole and Fairbanks, 1990), and/or that the influence of various parameters may act in opposition and obscure the annual pattern (Swart and Coleman,

1980; Swart etal, 1983; Leder etal, 1991). Furthermore, various workers have suggested that common sampling procedures may contaminate the isotope signal or be inappropriate for revealing maximum seasonality. First, little consensus exists regarding the correct pre-treatment of biological aragonites prior to isotopic analysis, and it is likely that in some cases aberrant results reflect the non-standardisation of analytical techniques (Weil er al, 1981; Aharon, 1991). 55

Second, frictional heat generated when sampling by dry drilling has been shown to cause

'substantial 8180 and 813C depletions' (Aharon, 1991, p86), and is obviously unsuitable for accurate environmental interpretations (Gill et al, 1995). Third, sampling frequencies and procedures must be re-evaluated in light of recent evidence regarding the formation, structure and geochemistry of coral skeletons. Recent work by Quinn et al. (1996) suggests that quarterly sampling is sufficient to detect interannual and decadal trends, and that bimonthly sampling can account for most of the variance explained by higher density sampling regimes. If coral skeletons do progressively thicken within the tissue layer over several months as proposed by

(Barnes and Lough, 1993) seasonal variations in skeletal geochemistry will be smoothed. This would seem to negate the usefulness of high resolution sampling, though a number of recent studies reveal no significant reduction in signal amplitude as a result of within tissue skeletal thickening (Gagan etal, 1994; McCulloch etal, 1994; Wellington et al, 1996). Patzold (1993) identified systematic differences in stable isotope fractionation between different corallite elements and similarly speculated that sampling across different elements would smooth the seasonal isotope signal. To overcome these problems it would appear that the truest isotope values can be derived from dissepiments which are deposited relatively quickly every lunar month, and are reasonably easy to distinguish, date, and sample in isolation. It is important to recognise that structural and growth constraints may also complicate the environmental interpretation of coralline isotopes. Environmental disturbances that reduce growth rates may be difficult to detect, especially in slower growing deep water corals, simply because slow rates of skeletogenesis limit the amount of skeleton available for sampling, and sampling frequencies that would ensure sensitivity to the causative event are difficult to achieve (Leder et al, 1991).

Furthermore, the coralline isotopic signatures need not reflect large-scale factor(s), but may reflect the influence of small-scale physical and biologic effects. For example, McConnaughey

(1989) attributed distinct changes in skeletal 813C and S180 in a Pavona colony to changes in its depth and orientation after it toppled over (McConnaughey, 1989). 56

3.2.2. Trace Elements.

A variety of trace elements are incorporated into coral skeletons at concentrations that reflect those of the surrounding seawater (Veeh and Turekian, 1968; Thompson and

Livingston, 1970; Livingston and Thompson, 1971); and trace element concentrations in coral skeletons have been widely recognised as useful proxy records of environmental change

(Smith etal, 1979; Lea et al, 1989; Shen and Sanford, 1990). The process of trace element incorporation and the distribution of trace elements within coral skeletons has been addressed in various studies (e.g. Amiel et al, 1973; Goreau, 1977; Swart, 1981; Allison and Tudhope,

1993) and will not be covered here. Several studies have identified cycles in certain trace element concentration(s) that can be related to seasonal changes in water chemistry associated with upwelling, salinity/run-off and temperature (Smith et al, 1979; Lea et al, 1989; Shen and

Sanford, 1990; Shen etal, 1991; Delaney etal, 1993; Sholkovitz and Shen, 1995). A brief review of the trace elements most commonly examined in coral skeletons is presented below.

The ratio of strontium (Sr) to calcium (Ca) in a coral's skeleton has been shown to be temperature sensitive in both laboratory and field studies (Weber, 1973; Houck et al, 1977;

Schneider and Smith, 1982; McCulloch etal, 1994), though skeletal Sr concentrations are also affected by the Sr/Ca ratio of the incubatory seawater, the concentration of other elements in the water column, and biological variables such as growth rate and taxonomic differences in fractionation (Livingston and Thompson, 1971; Smith et al, 1979; Swart, 1981; Schneider and

Smith, 1982; Cross and Cross, 1983; Muir, 1984; Rasmussen, 1988; De Villiers et al, 1994,

1995). Several studies have had difficulty identifying subannual fluctuations in the Sr/Ca ratio that can be related to seasonal variations in seawater temperature, and have concluded that skeletal Sr/Ca is a poor palaeothermometer (Goreau, 1977; Chivas et al, 1983; De Villiers et al,

1995). Schneider and Smith (1982) managed to identify seasonal Sr/Ca cycles in Porites corals from off the West Australian coast, but considered that the Sr/Ca thermometer developed from these corals was imprecise (±1.5°C) and of limited value in view of the annual seawater temperature range in that locality. The recent development of more sophisticated analytical techniques has significantly improved the precision of temperature reconstructions from 57

coralline Sr/Ca ratios (better than ±0.5°C: Beck etal., 1992, 1994). However, De Villiers et al.

(1995) calculated that temperature uncertainties of between 2° and 4°C can be generated by realistic variations in seawater Sr/Ca content and coral growth parameters and argue that Sr/Ca values is neither a precise nor simple proxy for seawater temperature. The extent to which seawater chemistry affects coral Sr/Ca ratios is unresolved, though it would appear that the Sr/Ca thermometer in nearshore corals is more vulnerable to discursions attributable to changes in seawater chemistry. Several authors have concluded that anthropogenically elevated seawater nutrient concentrations may blur the seasonal seawater temperature signal preserved by skeletal Sr/Ca (Muir, 1984; Rasmussen, 1988), possibly as a result of their influence on skeletogenesis (Barnes et al, 1995). Swart (1981) hypothesised that seasonal variation in seawater salinity and/or a coral's biological demands may have the same outcome. Although

DeVilliers and colleagues (1994, 1995) have recently argued that the influence of biological controls and seawater chemistry on the Sr/Ca palaeothermometer are underestimated, it is generally accepted that Sr/Ca ratios in oceanic waters are well buffered and reasonably constant

(see Schneider and Smith, 1982; Beck et al, 1992). If it is accepted that Sr/Ca ratio faithfully records seawater temperature independent of salinity effects then the Sr thermometer has distinct advantages over 8^80 for palaeotemperature investigations. Seawater 8^80 can obscure the S180 temperature record, however as Beck etal. (1992; 1994) point out, the more precise Sr/Ca ratios now achievable may allow the temperature and seawater 8180 components to be discriminated.

The concentration of cadmium (Cd) in coral skeletons has been shown to vary in close association with phosphate and nitrate levels, and has been used as a proxy indicator of sea-surface nutrient levels (Shen et al, 1987; Shen and Sanford, 1990). Cd/Ca ratios vary seasonally with the upwelling of cold nutrient rich waters during non-ENSO years in the eastern

Pacific (Linn etal, 1990; Delaney et al, 1993). Close coupling of upper ocean nutrient levels and water temperature in upwelling zones has led to speculation that Cd concentration may also act as a temperature proxy in suitably located corals (Shen and Sanford, 1990). Investigation of corals from Tarawa in the central Pacific suggests that rainfall may affect Cd/Ca ratios, however 58

how it does so is not yet known (Shen and Sanford, 1990). Barium (Ba) has been linked to the upwelling of nutrient rich waters in the eastern Pacific (Lea et al, 1989) and in the Arabian Sea

(Tudhope etal, 1996), although Ba/Ca ratios appear to be less sensitive to these events than

Cd/Ca ratios (Lea etal, 1989). The barium content of coral skeletons has also been used as a palaeoflow indicator. For example, the barium concentration of discharge from the Amazon

River is approximately three times the ambient level of the Atlantic Ocean, and leaves a detectable signature in nearshore corals (Boyle, 1976 cited in Shen and Sanford, 1990). Clearly

Ba/Ca ratios have considerable potential when used in concert with other markers of terrestrial run-off such as fulvic and humic acids (Isdale, 1984; Boto and Isdale, 1985). In contrast to cadmium and barium, manganese (Mn) concentrations are highest near to the ocean surface and are depleted during upwellings (Linn etal, 1990; Shen et al, 1991; Delaney er al, 1993).

Manganese fluctuates in opposing phase to Cd and Ba, and has also been used as a nutrient proxy (Shen and Sanford, 1990; Shen etal, 1991). Recently several studies have recognised seasonal variations in skeletal U/Ca and have concluded that these cycles principally develop in response to seawater temperature fluctuations (Min etal, 1995; Shen and Dunbar, 1995). This research indicates that U/Ca has several advantages over Sr/Ca thermometry, including the fact that the fractional shift per degree Celsius for U/Ca is six times larger than that for Sr/Ca, meaning that lower analytical precision is required to achieve a given precision in temperature. As a result, lower precision, higher throughput inductively coupled plasma - mass spectrometry (ICP-MS) can be effectively used in place of more complex thermal ionisation mass spectrometry (TIMS) techniques. Furthermore, U/Ca appears minimally affected by Vital' effects, though there is evidence to suggest that at least one other factor besides seawater temperature affects U/Ca values.

The historical influx of industrial and military pollutants have also been inferred from trace element concentrations in coral skeletons; several studies correlating increases in lead

(Pb) (e.g. Dodge and Gilbert, 1984; Shen and Boyle, 1987) and Cd (Shen er al, 1987) with increased industrial activity since the turn of the century. Radionuclides released during nuclear testing in the central Pacific have been incorporated in coral skeletons and provide a useful 59

temporal benchmark against which coral growth chronologies constructed using other means can be confirmed and calibrated (Knutson et al, 1972; Hudson, 1985; Cole and Fairbanks,

1990). The natural decay of several isotopes (e.g. 228Ra-210Pb; 234U-230Th) has proven an effective means of establishing the age of coral growth bands (Moore and Krishnaswami, 1972).

Individual coral bands may be independently dated with accuracies of less than a few years (see

Taylor etal, 1987) using the 234U 230Th mass-spectrometric technique described by Edwards etal (1987).

In addition to elements incorporated within the aragonite lattice, 'detrital' inclusions within the skeletal structure have also been described (Barnard et al, 1974; Goreau, 1977,

Howard and Brown, 1984; Brown et al, 1991; Davies, 1993; Budd et al, 1993; Heikoop et al,

1996). Where the influx event is documented such inclusions may function as useful markers within a coral's skeleton, however the incorporation of detrital material is neither consistent nor predictable (Budd et al, 1993; Davies, 1993), and further work is required before detrital markers will have a practical use in establishing coral growth chronologies.

Most studies of trace element concentrations in coral skeletons have used bulk methods of analysis. However, studies using ion microprobes to sample individual skeletal elements (Allison and Tudhope, 1993; Allison, 1996) have shown that trace element concentrations were highly variable at a local scale (<100um). For example, Sr concentrations were found to vary by 280ppm over <100um (estimated to be about two days growth), which would be interpreted as a difference of 4°C using Smith etal's (1979) equation. They could not explain the observed variation, but concluded that it is extremely unlikely that such local heterogeneity would be caused by a change in seawater temperature or chemistry. This work has significant implications for previous studies that have reconstructed environmental conditions from the trace element geochemistry of bulk samples taken from coral skeletons.

Although certain trace elements within coral skeletons exhibit clear cycles that can be related to seasonal variations in environmental conditions when bulk skeletal samples are analysed, Allison and co-workers have shown that the concentrations of a particular trace element in two different 60

but simultaneously deposited corallite elements may differ by more than the annual range of variation detected by bulk skeletal analyses. Further work is clearly required on the incorporation and fractionation of trace elements in coral skeletons before the environmental significance of coralline trace element geochemistry can be fully understood.

3.2.3. Fluorescent Banding.

The discovery that nearshore massive corals preserved within their skeletons a series of fluorescent bands which become visible under long-wave ultraviolet light was first made by

Isdale (1984), who demonstrated that the periodicity, width and intensity of fluorescent bands in corals growing on Great Barrier Reef near to the Burdekin River were well correlated with its discharge. Fluorescent bands are typically described as bright yellow-green bands against a duller yellow or blue-green background (Isdale, 1984; Scoffin etal, 1989; Smith, etal, 1989;

Klein er al, 1990), and have been shown to be largely caused by the presence of humic and fulvic acids leached from organic soils within the crystal structure of coral skeletons (Boto and

Isdale, 1985; Susie etal, 1991; Fang and Chou, 1992). The potential application of fluorescent banding from long-lived and fossil corals for reconstructing past fluvial and pluvial regimes was quickly recognised (Isdale, 1984; Smith et al, 1989), and several studies have inferred interactions between flood plumes and oceanic water masses based on spatial patterns of coral fluorescence (Scoffin et al, 1989; Fang and Chou, 1992). Recent detailed investigations by

Neil etal. (1995) confirm that coral skeletal fluorescence is a useful proxy indicator of streamflow from humid tropical catchments, though these authors conclude that the temporal resolution of the fluorescence record of streamflow is coarser than previously recognised and that skeletal fluorescence is best correlated with annual total streamflow rather than discrete flood events.

Evidence presented in several studies suggests that the resolution of fluorescent bands within coral skeletons reflects the proximity of the coral to the source(s) of river discharge, the discreteness (both spatial and temporal) and size of flood events, and the mixing of nearshore waters. A general pattern is evident whereby fluorescent bands become more difficult to detect with distance offshore (Isdale, 1984, Lough and Barnes, 1990a), although 61

Scoffin et al. (1989) note that fluorescent banding in close inshore (<2-5km) corals can be diffuse and indistinct, and is often difficult to correlate with discrete rainfall events. Fang and

Chou (1992) found that the overall concentration of fulvic acids in corals was dependent on the strength of oceanic flushing near to the source of fluvial discharge, although they make no comment about the resolution of fluorescent bands. At the other extreme Susie et al. (1991) report faint fluorescent banding in a coral from Christmas Island in the central Pacific, several thousand kilometres from the nearest riverine source. They suggest that fluorescence is a general feature of all scleractinian corals and demonstrate through chromotographic analysis that humic and fulvic acids are not solely responsible for coral fluorescence. Fang and Chou (1992) speculate that other organics within the water column, or even compounds produced as by­ products of coral metabolism may also contribute to coral fluorescence, and have established that the influence of these secondary source(s) is greater in offshore corals. Tudhope et al.

(1996) similarly hypothesise that the seasonal breakdown of marine organic matter is responsible for the fluorescent banding observed in corals from the Gulf of Oman, where the aridity of the adjacent Arabian landmass and subsequent lack of terrestrial vegetation and permanent streams limits the possibility of a terrestrial source for the fluorescence. Fluorescent banding has been described in a variety of massive coral species (e.g Porites lutea : Isdale,

1984; Montastrea annularis: Teal, 1986; Scoffin et al, 1989; Fang and Chou, 1992; Favia maxima /Fang and Chou, 1992; Solenastrea boumoni: Smith et al, 1989). Work by Fang and

Chou (1992) suggests that species effects may occur.

Research investigating the relationship between fluorescent and skeletal density banding patterns has demonstrated that no universal model relating skeletal density and fluorescent bands is appropriate. For example, bright yellow-green fluorescent bands have been shown to coincide with less dense skeletal bands in South-east Asian and Middle Eastern

Porites (Scoffin et al, 1989; Klein er al, 1990) whilst the converse has been reported for the same species on the Great Barrier Reef (Isdale, 1984). Scoffin et al. (1989) did, however, observe that very thin and bright fluorescent bands coincide with very thin dense bands which they interpreted as 'stress' bands deposited during high run-off episodes. Teal (1986) could 62

detect no systematic relationship between fluorescent and skeletal density banding in

Montastrea annularis colonies from the West Indies. Scoffin etal (1989) reported that the width of bright/dull fluorescent band couplets and skeletal density band couplets in the Indonesian corals are very similar, and concluded that the coral growth increments demarcated by successive fluorescent bands were annual.

3.3. The Influence of Skeletal Structure on Inclusive Records.

The dearth of knowledge on how coral skeletons grow has recently been recognised as a major hurdle to the reliable interpretation of proxy records of environmental conditions preserved within coral skeletons (Barnes and Lough, 1993; De Villiers et al, 1994; Le Tissier et al, 1994; Barnes etal, 1995; Quinn etal, 1996; Wellington et al., 1996). As discussed earlier in this chapter, models of coral skeletal growth developed by Barnes and Lough (1993) and

Taylor et al. (1993) which incorporate both linear extension and skeletal thickening within the tissue layer (see section 3.1.1. of this chapter) represent important first steps in surmounting this obstacle. Further work by these authors demonstrates how the distribution and concentration of inclusions used to reconstruct past environmental conditions annual cycles may vary in response to subannual fluctuations in skeletal thickening, linear extension, and phase shifts between cycles in coral growth and the environmental concentration of trace inclusions (Barnes etal, 1995; Taylor etal, 1995).

Taylor et al. (1995) and Barnes et al. (1995) classified inclusive records in coral skeletons into three types according to the duration of the causative event; long term (more than a few years), annual (annual cycles), and pulse events (occasional events lasting a few days to a few months). Their numerical models suggest that inclusive proxies most reliably and accurately reflect the causative environmental factor(s) when the tissue layer is thin, the extension rate remains constant throughout the year, and the duration of the environmental cycle or event exceeds skeletal growth cycles. Under these circumstances skeletogenesis will be dominated by initial deposition, inclusive records will be less 'contaminated' by within tissue thickening, and linear distance along the growth axis will most closely approximate the temporal 63

depositional sequence. The models predict that the concentration of inclusions deposited in response to long-term environmental signals faithfully track those of the local environment provided the later change slowly and continuously. In contrast, shorter duration pulse events are most vulnerable to discrepancies between the magnitude and duration of the delivery event and interpretations derived from the concentrations of trace inclusions (Taylor et al, 1995).

Pulse events may appear longer than the causative environmental event when they coincide with periods of rapid linear extension; the amplitude of the skeletal signal (represented by inclusion concentration) will be biased by the density of skeleton deposited at the time (HD or

LD) and the extent of subsequent within tissue thickening, the later being a function of tissue thickness (thickening duration) and seasonal variations in calcification rate. According to the models developed by Taylor, Barnes and Lough the amplitude of the skeletal signal is increasingly diminished as the tissue layer becomes thicker (their models predict that only 45% of the actual amplitude will be represented at high tissue thickness) and the degree to which the environmental signal and the forcing function for skeletal growth are offset. This research illustrates the varied and dynamic ways that environmental signals can be 'encoded' by the growth mechanisms of individual coral colonies. In doing so it also provides a systematic framework within which seemingly contradictory palaeoenvironmental histories reconstructed from coral skeletons may begin to be deciphered (Barnes etal, 1995).

4. SUMMARY.

As corals grow their skeletons preserve information about themselves and their environment. This information can be stored structurally or chemically within the skeleton.

Chronologies for these proxy records have largely been established by comparison with cyclic fluctuations in skeletal density that have been demonstrated to be roughly annual using a range of techniques. However, exactly how and in response to what environmental parameters these bands form is still debated. Uncertainty about the mechanisms by which corals record information has hindered the ability of researchers to routinely reconstruct past environments from environmental proxies preserved in coral skeletons. A recent review of the literature by

Taylor et al. (1995) indicated that though the fundamentals of skeletal growth mechanisms 64

remain unclear, papers attempting to interpret inclusive records are being published at twice the rate of papers investigating the nature of density bands themselves, a strategy that they considered 'seriously flawed'. Though Taylor etal. (1995) are clearly correct, an investigation of the mechanics of coral skeletal growth is beyond the scope of this study. In the following chapter the growth chronologies of Porites microatolls growing on the Cocos (Keeling) Islands will be established using a range of techniques. Supporting evidence from several techniques allows reasonable confidence to be held in the veracity of the established chronologies. 65

CHAPTER 3

ESTABLISHING THE GROWTH CHRONOLOGIES OF Porites MICROATOLLS FROM THE COCOS (KEELING) ISLANDS.

1. INTRODUCTION.

To reconstruct water-level histories from Porites microatolls growing on the Cocos

(Keeling) Islands it is necessary to establish their growth chronologies. The various records preserved within coral skeletons that can be used to reconstruct growth histories have been examined in the previous chapter. Coral growth chronologies were established in this study using skeletal density and/or fluorescent bands visible in vertical skeletal slices extracted from across a colony's diameter. In this chapter the methods used to observe and analyse these banding patterns will be described, and the periodicity and seasonality of the density and fluorescent banding patterns will be established

2. SAMPLE COLLECTION.

2.1. Site Selection.

Study sites were chosen to ensure adequate geographic coverage of the atoll (Fig.

10). Microatolls grow in reef flat, interisland passage, or lagoonal habitats on Cocos; some sites may include microatolls from several habitats (i.e. sites XIV, XV, and XVII; see Appendix A). Reef flat habitats are variable in character but generally comprise a seaward algal pavement behind which a sandy coralgal surface stretches shoreward. The algal pavement may extend to the shoreline at high energy reef flat sites. Where the reef flat is wide, or the inner reef flat is relatively protected, a sandy zone that is often colonised by seagrass abuts the shore.

Interisland passage habitats are also variable but typically consist of an undulating substrate of coral gravels and sands, with microatolls scattered throughout. The lagoonal habitats in which microatolls grow on Cocos are low energy depositional environments with predominantly sandy 66 substrates (Smithers, 1994; Williams, 1994). Detailed site descriptions are presented in

Appendix A.

1— o Horsburgh 96 55'E Island

XIX -^"i/w^ Direction "Island

b

-12°10'S

V ^Xll \^3 .x i ji>£^: 0 Kilometres 5 J L Figure 10. Microatoll sample site locations (see Appendix A for detailed descriptions). 67

2.2. Field Sampling.

Ideally whole microatolls would be collected and taken back to the laboratory for detailed analysis. However, logistical, environmental, and budgetary constraints render this strategy impractical. Except close to West and Home Islands, microatoll fields are difficult to reach. Most are only accessible by boat at high tide when waves and currents make sampling difficult and dangerous. Sites XII and XIII on the southeastern atoll rim could not be directly reached by boat with safety, requiring that equipment and samples be carried several kilometres to and from these sites. Microatolls accessible by boat during lower tidal stages were generally deep bodied (>50cm thick) and too massive to sample using the techniques available. These constraints combined with the need to work the tides, time available in the field, and the cost of freighting samples back to Wollongong limited the area from which samples could be retrieved and the number of samples that could be collected. Nevertheless, nineteen sites were examined and more than 120 microatoll samples were systematically collected during fieldwork for this thesis.

In most cases, a microatoll's upper surface morphology was accurately surveyed to a temporary benchmark related to the Home Island MSL datum before sampling began (see section 2.2 in the following chapter). Samples consisted of 5-15cm wide vertical slabs cut across a microatoll from its seaward to leeward living edge, passing through the growth origin where that could be determined. Most slabs were cut using a 52cm diameter diamond saw blade attached to a rotivator head mounted on a standard 2-stroke brushcutter. The long shaft of the brushcutter allowed the motor to be kept out of the water and enabled sampling to progress at lower stages of the tide. At higher tides waves and currents caused the blade to flex and jam in the coral, damaging the saw blades and the gears in the rotivator head. The motorised saw most effectively cut samples from microatolls less than 15cm thick (from base to top), but it frequently broke down. A handsaw with hardened cutting tips was consequently used to cut many samples, and to complete the cuts through thicker specimens begun with the motorised saw.

Handsaws were also used at the most remote sites because of their portability. Sampling with the handsaw was slow and exhausting compared to using the motorised saw, reducing the rate 68

at which samples could be collected. Unfortunately, during the 1992 field trip the motorised saw irreparably seized after cutting only three samples, requiring that sampling proceed exclusively by hand sawing.

At least one sample was collected from all sample sites except site XVIII, where the microatolls were too massive to retrieve a suitable sample using available techniques. The number of samples collected was generally largest at the most accessible sites and where suitable microatolls were abundant. Microatolls were selected for sampling based on their preservation (not eroded or degraded), firmness of attachment to the substrate (not moved or tilted), and the anticipated success of extracting a suitable sample. Samples were washed and soaked in a dilute bleach solution for several days before being dried. The clean, dry samples were then wrapped in plastic for transportation.

Figure 11. Field sampling a reef flat microatoll with the motorised saw. 69

3. ESTABLISHING CORAL GROWTH CHRONOLOGIES.

As indicated in the introduction to this chapter, the growth chronologies of the

Porites microatolls examined in this study were established using skeletal density and fluorescent banding patterns visible in vertical slices cut across their diameters and passing through their centres. The methods used to detect these bands and verify the chronologies derived from them, are described in the following section.

3.1. Skeletal Density Bands.

Skeletal density bands were revealed by X-radiographic analysis of uniformly thin (5-

10mm) coral slices cut parallel to the coral's growth axis. Density band definition on X- radiographs varied within and between samples; largely reflecting differences in internal skeletal structure and geometry. The methods used most successfully in this study to detect skeletal density bands are described below.

3.1.1. Slice Preparation.

Thin slices were cut from coral slabs collected in the field using several techniques.

The field samples from which slices were cut were inevitably irregular due to the typically undulating upper and lower surfaces of most microatolls, and samples were often too long and/or deep bodied to be successfully sliced in one piece.

Many slices were cut using a custom made aluminium saw-table in which a circular diamond saw blade protrudes above a flat bench top. Slices were cut by pushing samples across the bench top against guides that could be adjusted to determine slice thickness. This system worked well for slices less than 40cm long and between 8-15cm from base to top; however, slices of greater dimensions frequently broke whilst being cut or the slabs had to be sectioned prior to slicing to fit on the saw-table. Where possible sectioning of field samples into smaller pieces was avoided to minimise the possibility of losing pieces or distorting the sample's original form during reassembly. Circular, diamond-tipped geological saw blades were found to 70 be the most suitable for cutting coral slices, the 52cm diameter blade being the largest available without the radial grooves that shattered coral samples during slicing. A problem with using circular blades to cut thin slices from the coral slabs was that the rotation of the blade set up opposing stresses in the slice as the slab passed beyond the centre of the blade. These stresses often caused the slice to fracture, particularly in very thin slices. Thin slices were alternatively cut by hand using tip-hardened handsaws, however it was difficult to achieve straight and parallel cuts along long slices using this method. Furthermore, the surface of slices cut by hand were often scored by the saw teeth, producing grooves that hindered band detection.

The best results were achieved by first carefully cutting one flat surface from the coral slab using the field saw (diamond saw blade attached to brushcutter), either whilst it was on the ground or the saw-table, and then doing another roughly parallel cut to form a more sturdy slice approximately 2-3cm wide. Slices were then soaked in water until saturated. The carefully cut flat side of the slice was then laid face down on aflat surface and firmly anchored using pegs.

Guides as to thickness of the desired slice were then set around the slice and the 'rough' upper surface ground down to the required thickness using an abrasive sanding disc attached to a hand-held electric drill. The slice was regularly rewetted to reduce dust and frictional heat. Final sanding to the correct thickness was carried out using a finer sanding disk. By far the largest complete thin slices were achieved using this technique.

All slices were thoroughly rinsed and scrubbed in freshwater to remove fine cuttings, and then ultrasonically cleaned until no further fine cuttings were flushed from the sample.

Slices were then air dried and smaller fragments glued together using superglue, before being bagged with hygroscopic desiccant for storage.

3.1.2. X-Radiography.

X-radiographic images of coral slices collected during this thesis were taken using either the facilities at Wollongong Hospital or at the Hospital at the Australian Defence Force 71

Academy, in Canberra. Both machines used Kodak T-mat G film in Kodak X-omatic cassettes with LANGX regular screens. Trials on each machine indicated that the optimum settings for revealing skeletal structure and density banding patterns were 50KvP/100ma for 0.03 seconds, with a focal distance of 1.0m. X-radiographs of sixteen samples donated by Woodroffe and

McLean were taken using the hospital facilities on Cocos, also using Kodak T-mat G film in either regular or detailed screens. Settings for these X-radiographs were 45-60KvP/6MAS for 0.2 seconds. After cleaning and trimming to uniform thickness additional X-radiographs of the donated samples were taken using the same facilities and settings as the corals collected in this study.

The detection of skeletal density bands in coral samples by X-radiography is dependent on several factors (see section 3.1 in chapter 2, Barnes et al, 1989; Lough and

Barnes, 1990a for discussion). The best resolution is achieved when density variations through the thickness of skeletal slices are aligned parallel to the incident X-ray beams. Where density bands are slanted through the thickness of a skeletal slice an average density is represented on the X-radiograph (the Venetian blind effect' - Barnes and Deveraux, 1988), and distinct banding patterns can be obscured. A consequence of the hummocky outer growth surfaces characteristic of massive Porites corals is that orientation of growth axes and density bands seldom remain constant through long skeletal sections, and thus it is often difficult to detect long sequences of discrete skeletal bands in these corals from a single X-radiograph. Some success has been achieved by X-raying samples progressively tilted at different angles and constructing a collage of images that best show the density band pattern over the entire sample

(Barnes and Lough, 1989; Lough and Barnes, 1990b). Because Porites growth surfaces are typically convoluted and rarely parallel to the slice surface or X-ray beams, the skeletal density bands near to the growth surface are often poorly defined on X-radiographs.

3.1.3. Densitometry.

Densitometric analysis was used to objectively determine density fluctuations in microatoll skeletons detected from X-radiograph negatives. Using a LKB Ultrascan XL 72 densitometer, X-radiographs (negatives) were scanned along transects from the coral's outer growth surface toward its growth origin. Densitometer settings were selected to give the best definition between the dark and light areas on the X-radiograph plates, determined by trial and error to be aline beam with ax-width of 1.6-2.4mm, ay-step of 80mm, and smoothing set at 2

(twenty-one point running mean). Densitometer traces were generated in which light areas on the scanned X-radiograph (dense skeleton) were represented by troughs (low absorbence) and dark areas (less dense skeleton) by peaks (high absorbence) (Fig. 12). No quantitative density measurements were made as determination of calcification rates was not an objective of this study.

It is important to recognise that densitometers detect differences in the darkness or exposure along an X-radiograph negative, and that they represent a second order approximation of density variations in the original skeletal sample. As such the quality of information yielded by densitometry is dependent on the quality of the X-radiograph, and densitometry cannot discriminate density variations not detected by X-radiography.

Densitometry is, however, useful for objectively determining where density peaks and troughs occur along a skeletal slice, and thus for delineating adjacent density bands on X-radiographs.

Density band slanting through X-radiographed slices means that peaks and troughs represented on densitometer traces represent maximum and minimum average densities through the thickness of the coral slice, the positions and amplitudes of which reflect not only changes in skeletal density along the slice but also the extent to which bands are slanted through it. As previously noted, the slant of density bands in Porites rarely remains constant along longer coral sections, and thus the position of peaks and troughs on the densitometer trace may reflect this variable slanting. The error generated by averaging skeletal density through the thickness of the slice was compounded by difficulties in cutting slices of uniform thickness. It became apparent that for most microatolls examined in this study densitometric analysis did not greatly improve the accuracy with which skeletal density bands could be 73 recognised and located by eye. As a result densitometric analysis was only used as a back-up to visual discrimination where banding patterns were ambiguous.

Densitometer Scan Results f. 5"

«

,j™_ ,-,^. ~/.*•-,JX#. •••r"" "i "*——r- T r :/ Distance (cm)

Figure 12. Densitometer trace on X-radiograph negative of thin vertical slice of microatoll CKI162 from Pulu Pandan. 74

3.2. Fluorescent Bands.

When illuminated with long-wave UV light many vertical slices and slabs extracted from Cocos microatolls exhibited a fluorescent banding pattern consisting of bright yellow-green and dull yellow couplets similar to those described in nearshore corals (Isdale, 1984; Scoffin et al, 1989; Klein etal, 1990; see chapter 2, section 3.2.3). Fluorescent bands can be seen on the surface of illuminated samples, and problems related to the skeletal slice thickness and variable band slanting that confound density band discrimination are avoided. Fluorescent bands can also be detected using relatively simple and portable technology, and can be studied at most field bases. Fluorescent bands are most effectively revealed when a clean, flat dry surface is illuminated. It is critical that samples to be fluoresced are thoroughly bleached and dried after collection, so that mould does not grow on samples, as mould fluoresces and can obscure the fluorescent banding pattern.

3.2.1. Illumination and Detection.

Skeletal slices were placed on black card and illuminated with two banks of two 40W black-light-blue fluorescent tubes mounted 30cm apart and 80cm above the skeletal sample.

Illuminated samples were photographed with an SLR camera fitted with two UV (Hoya Y44) and a standard yellow filter, using Kodak Technical Pan film rated at 50 ISO. An exposure of 8 seconds at /5.6 was determined through trial and error to produce the best results. The film was developed in H.C 110 developer at a dilution of 1:3 for 3 minutes at 23°C and agitated at 30 second intervals. Black and white prints were processed and used as working 'hard copies' for the analysis of the fluorescent bands.

3.2.2. Image Analysis.

A selection of the black and white photographs of fluorescent banding in these corals were scanned using a Sharp JX-320 scanner and Adobe Photoshop software loaded on a

Macintosh Centris computer. Images of the photographs were saved as TIFF files and imported into DIMPLE image analysis software, where linear profiles were performed over the coral's 75 maximum growth axis, beginning at the outer growth edge. The linear profile function assessed the relative brightness (a 256 grey-scale image was used) of pixels along the growth axis transect and produced a trace similar to that produced by the densitometer in which high brightness values coincided with the location of the bright fluorescent bands and lower brightness values were recorded for areas along the sample transect which fluoresce less strongly (Fig. 13).

Figure 13. Linear profile of pixel brightness values along transect on black and white photograph of fluorescing vertical slice of microatoll CKI162. 76

3.3. Establishing Band Periodicity and Seasonality.

Density banding was observed in most of the microatolls examined during this study, though rarely were they clearly distinguishable over the entire length of a sample. In contrast, fluorescent banding was ubiquitous, usually clear and distinct, and generally more easily observed over a sample's entire length. In order to establish the periodicity and seasonality of skeletal and fluorescent bands in microatolls on the Cocos (Keeling) Islands the frequency and time of year at which they are deposited was established by reference to a series of

independent temporal markers of known age preserved within their skeletons.

3.3.1. Alizarin Staining.

In November 1991 nine microatolls from three sites around the atoll (sites IV, VII, XIV -

see Fig. 10) were stained using the vital stain Alizarin Red S following the method described by

Barnes (1972). Alizarin Red S is incorporated during skeletogenesis to form a distinct red-

purple band within the coral skeleton that acts as an independent time marker of known age. To

stain corals it is necessary to enclose colonies in plastic bags injected with the dye for 24 hours,

and it was thus necessary to use relatively small specimens and to dislodge them from the

substrate in order to enclose them in the bags. Staining was undertaken to determine the timing

and periodicity of both skeletal density and fluorescent band formation, and these corals were

not used for water level investigations. Stained corals were marked by pegs driven either into

the corals themselves or into the adjacent reef flat.

In November 1992 five of the stained colonies were relocated and slices cut from

them as described in section 2.2 of this chapter. Unfortunately, four of the stained samples

could not be located. Cyclone Harriet affected the atoll in January 1992, and it is probable that

some of the marking pegs, or even the corals themselves (which were not attached to the

substrate), were moved during this storm. Furthermore, despite the identical staining

procedures used in each case, a distinct Alizarin marker was recognisable in only four of the five

stained colonies that were found. Reference to the Alizarin skeletal marker introduced in 77

November 1991 suggests that the deposition of less dense skeletal and brighter fluorescence bands in Cocos corals begins around November-December. However, if we accept Barnes and

Lough's (1993) model of density band formation, and assume that the Alizarin stain marks the outer growth surface and not necessarily the main site of density band formation, then based on average measurements of tissue layer thickness (0.65cm, n=104) and annual extension

(1.35cm, n=250) (determined from skeletal density banding) the bands aligned with the Alizarin marker may not have been completely formed until February-March. Although a discrepancy may exist between the apparent and actual time at which both the fluorescent and skeletal density bands were formed, the incorporation of the fluorescing organics within the skeletal crystal lattice (Boto and Isdale, 1985) suggests that the fluorescent and density bands which occur over a given section of skeleton form at the same time (i.e. the fluorescent banding does not develop after the skeleton has been constructed).

Although the tissue layer often obscures banding patterns near a coral's growing edge and density bands in that area may not be completely formed (Barnes and Lough, 1993), the distance over which the stained coral's skeletons grew following the introduction of the

Alizarin marker one year earlier corresponds well with the width of both skeletal density and fluorescent couplets. This close correspondence of width suggests that both banding patterns are annual.

3.3.2. The 1983 Stress Band.

In March 1983 a mass mortality of fish and corals was reported from the Cocos lagoon

(Blake and Blake, 1983). At this time a sustained relaxation of the normally persistent southeast trade winds occurred, and flushing of the shallow areas at the south and east of the lagoon failed. Water in these areas became discoloured, foul smelling, and killed or severely debilitated all organisms with which it came in contact (Blake and Blake, 1983). It has been speculated that this event developed as water stagnated in the lagoon and became anoxic due to the respiratory demands of sessile or territorial organisms, and that it was possibly made worse by the aerobic demands of coral spawn that entered the water column at around this time (Bunce, 1988; 78

Simpson et al, 1993). The foul water was observed to flow seaward through the interisland passages and to discolour the surrounding sea (Bunce, 1988), and many corals from outside of the lagoon were also affected.

Evidence of this event is preserved in the skeletons of many Cocos corals as a distinct scar (Fig. 14). It appears that this scar formed when polyps then at the outer growth surface were severely stressed. In many cases the resultant scar is plainly visible in coral slices, or else can be detected by the presence of a distinct high density 'stress' band in X-radiographs or a thin, bright fluorescent band. This feature forms a useful independent time marker within the skeletons of Cocos microatolls, and the number of density and/or fluorescent bands between this stress band and the skeletal perimeter at the time of sampling can be used to establish the annual nature of both skeletal density and fluorescent band couplets. Microatolls collected by Woodroffe and McLean in 1986, 1988 and 1989 were also examined during the course of this study, and the number of density and fluorescent bands deposited since 1983 was generally consistent with the number of years, further supporting the conclusion that both banding patterns have an annual periodicity.

3.3.3. Stable Oxygen and Carbon Isotope Analysis.

Detailed stable oxygen and carbon analysis was undertaken on the skeleton deposited over the last eleven fluorescent band cycles along the northern radii of a large microatoll (CKI91/10F2) from the southern atoll rim. These analyses were undertaken to investigate whether seasonal cycles in water temperature, light intensity or coral vigour could be related to the development of the density and fluorescent banding patterns (Patzold, 1984;

Carriquiry et al, 1988; McConnaughey, 1989; Aharon, 1991; Cole etal, 1993; Klein etal,

1993; Tudhope etal, 1995). Up to 20 samples were extracted from each fluorescent band couplet using a fine coping saw (mean number of samples per band: 14). A new blade was used to extract each sample and extreme care was taken to sample parallel to the band surface to minimise cross-contamination. Standard techniques (see Aharon, 1991 for good description) were used to determine the stable carbon and oxygen isotope concentrations of these samples 79 using the laboratory facilities of the Division of Exploration Geoscience, CSIRO, Sydney. The results of these analyses are plotted in Figure 14.

a) Microatoll CKI91/1

b) S13C and 5180 isotope analyses of CKI91/10F2(N), 1981-1991.

-7 i ii h IIII mil ii iii'i illi u mifii iii'ft | inn if lYiiiiir II i n II n II if II ii|im ii i ii|ili n nm IIII JIII irI III I iiii|iiiiniii|ir t- o 1C D C—O r- co m >* CO CM T- CD CD CO CO CO CO CO CO CO 00 CO CD CD CD CD CD CD Year CD CD CD CD CD

Figure 14. a) Photograph of microatoll CKI91/10F2(N) from which isotope samples were extracted using fluorescent banding to establish chronology. Note the distinctive scar formed during the 1983 mortality event. b) Diagam showing fluctuations in 513C and 5180 from 1991 to 1981. Dashed vertical lines mark the late fluorescent band edge, stipple marks denote high density skeletal bands. The number of samples per band varies with band width. 80

It appears from Figure 14 that the stable oxygen isotope composition of microatoll

CKI91/10F2 varies irregularly and exhibits no seasonal trends. Other authors have reported similar erratic 5180 fluctuations in shallow reef flat corals, and have attributed this pattern to short- term water temperature and salinity fluctuations associated with ambient weather conditions

(Swart and Coleman, 1980; Swart etal, 1983). Corals growing on shallow reef flats where water may stagnate and become heated at low tide may experience daily temperature ranges which exceed the annual range of well-flushed deeper corals. They are also vulnerable to salinity changes associated with heavy rainfall or evaporation. Water temperatures around the Cocos

(Keeling) Islands typically fluctuate by 3-4°C daily, with a maximum variation of 7°C recorded from the Southern Passage (Paul Kench, pers. comm.). Thus it is not surprising that the 5180 record derived from CKI91/10F2 does not show seasonal fluctuations. In contrast 513C appears cyclic.

513C cycles in coral skeletons are not well understood (see section 3.2.1), however seasonal variations in endosymbiotic photosynthetic vigour, coral growth rates and seawater 813C have been nominated as likely controls (Land etal, 1975; McConnaughey, 1989; Aharon, 1991). In mid-ocean areas where oceanic productivity is low and shows little seasonal variation, the influence of seasonal changes in seawater 513C can be discounted (Cole and Fairbanks, 1990) and 513C is generally accepted as an indicator of seasonal fluctuations in coral vigour (Fairbanks and Dodge, 1979; McConnaughey, 1989; Wang and Huang, 1989). Gagan et al (1994) have recently suggested that 513C cycles in massive corals from Pandora Reef on the Great Barrier

Reef may be controlled by gamete production and spawning, which occurs annually following the October/November full moon (Harrison etal, 1984; Babcock etal, 1986). Both fluorescent and skeletal density band couplets have equivalent periodicities to the seasonal 813C cycle, further suggesting that these band series are annual. 81

3.4.4. Annual Skeletal Density and Fluorescent Bands: A Synthesis of the

Evidence.

Several lines of evidence indicate that both skeletal density and fluorescent band couplets in microatolls from the Cocos (Keeling) Islands are annual. This evidence can be summarised as:

1. The annual linear extension of stained corals, defined by the distance between the

Alizarin stain marker and the skeletal perimeter 12 months after the stain was introduced, approximates the width of earlier skeletal density and fluorescent band couplets observed in the same corals, and corresponds well with published annual growth rates for this species (Patzold, 1984; Lough and Barnes, 1990a).

2. Where band series are clearly defined the number of couplets between the 1983 stress band and the living edge at the time of sampling equals the number of years passed. It is necessary to note, however, particularly in the case of skeletal density banding, that bands are often obscured and difficult to detect due to the convoluted skeletal structure.

3. The width and periodicities of both skeletal density and fluorescent band couplets closely correspond with cyclic fluctuations in skeletal stable carbon isotope composition, presumed to reflect seasonal variation in coral vitality (Fairbanks and

Dodge, 1979; Wang and Huang, 1989; Tudhope et al, 1995), or reproductive cycles (Gagan etal, 1994).

It is accepted, therefore, that both skeletal density and fluorescent band couplets in

Porites microatolls on the Cocos (Keeling) Islands have an annual periodicity and can be used to reconstruct annual growth chronologies. As stated above, annual skeletal density band couplets are often difficult to detect over long sections of coral skeleton, largely due to the confounding effects of convoluted skeletal structure on X-radiograph resolution, and 82

fluorescent banding was found to be a more effective means by which to date coral growth in most of the microatolls observed in this study. Fluorescent banding reported in the literature has been related to seasonal and episodic flood events in nearshore corals (Isdale, 1984;

Scoffin etal, 1989; Tudhope etal, 1995) and has been shown to correlate with both dense and less dense skeletal bands in studies from different areas. The recognition of an annual fluorescent banding pattern in mid-ocean corals, and the relationship between density and fluorescent banding in corals from the Cocos (Keeling) Islands, therefore requires further comment.

The Cocos (Keeling) Islands are located more than 1000 kilometres from the nearest river, clearly suggesting that the distinctly annual fluorescent bands observed in Cocos microatolls are not directly related to humic and fulvic acids fluvially exported from terrestrial catchments. The seasonal influx of these compounds from local terrestrial sources would also seem unlikely, because soils on the reef islands are poorly developed and corals windward of the vegetated islands also exhibit clear fluorescent banding. Furthermore, surface run-off from the reef islands is minimal, rain falling on the islands percolating into the sea via the mediating influence of a freshwater lens. Humic acids are resistant to leaching from the soil surface and high concentrations are normally associated with high riverine sediment loads (Susie et al,

1991). Reef island water lenses would therefore seem likely to buffer surrounding seawater from rapid influxes of these compounds. Moreover, the mean monthly rainfall at Cocos is highest in April (Bureau of Meteorology records) whilst Alizarin staining evidence suggests that fluorescent bands are deposited between December-February (Fig. 15). The possibility that organic compounds derived from coral spawn are responsible for the fluorescent bands present in these mid-ocean coral also seems unlikely in view of reports that suggest coral spawning on

Cocos occurs in March (J. Tranter, pers. comm.).

An oceanic source for fluorescing organics is consequently implied. Oceanic seawater contains organic acids at low concentrations (Klein etal, 1990; Susie etal, 1991), and it is possible that seasonal upwellings of relatively cool, nutrient rich water may generate 83

seasonal cycles in organic productivity and humic acid concentration at the ocean surface.

However, NOAA satellite sea-surface temperature data indicate that minimum water temperatures around the Cocos (Keeling) Islands occur from August to October, before the fluorescent bands form in December- February. Tudhope et al. (1996) similarly speculated that marine organics may be the source of fluorescent banding in Porites from reefs off the arid southern coast of Oman, where both run-off and terrestrial vegetation are depauperate.

Comparisons of X-radiographs and fluorescent pictures of the same sample, or of densitometric and fluorescent profiles along the same transects indicate that the bright yellow portion of the fluorescent band couplet generally coincides with the less dense part of the skeletal density couplet (see Figs. 12 and 13), a relationship repeated in most samples which clearly display both band series. It is widely accepted that LD bands form when coral growth is optimal, suggesting that bright fluorescent bands are also deposited under these conditions.

The systematic depletion of skeletal S^c coincident with LD and bright fluorescent band formation determined for microatoll CKI91/10F2 supports this postulate (Fig. 14b). Cyclic fluctuations in the 513c of microatoll CKI91/10F2 presumably document seasonal fluctuations in its growth rate (Land et al, 1975; Aharon, 1991; Wellington and Dunbar, 1995). As indicated above, 513c, skeletal density, and fluorescent band cycles have equivalent periodicities in sample CKI91/10F2, suggesting that both band series are annual. It is probable that microatolls are effectively light-saturated throughout the year due to their occurrence in shallow water settings, and though no direct evidence is available it is unlikely that seawater 8^^C concentrations at a low productivity mid-ocean site such as the Cocos (Keeling) Islands would show marked seasonality. It is therefore unlikely that seasonal variations in the 5^C signature of surrounding seawater or seasonal differences in insolation affect the skeletal 51^c record depicted in Figure 14.

The coincidence of bright fluorescent and less dense skeletal bands, the latter deposited when coral growth rates were highest, suggests the possibility that in the absence of a clear external source for the fluorescent bands observed in the Cocos corals their appearance 84 may to some extent be related to the growth of the coral itself. Susie et al. (1991) have presented data that suggest humic acids are typically concentrated approximately two orders of magnitude above that of the ambient seawater during skeletogenesis, and it is possible that during rapid growth (LD) phases fluorescing organics are more highly concentrated than in adjoining HD bands without any significant fluctuation in seawater concentrations. Because of low background fluorescence in these mid-ocean corals it is possible that only small increases in organic concentration would be required for banding to develop. Alternatively, fluorescent bands may occur due to the presence within the skeletal matrix of fluorescent non-humic metabolites (Susie etal, 1991) which would also presumably be most abundant during periods of rapid skeletal growth. Studies of fluorescent banding in nearshore corals typically describe the duller fluorescent band as 'bluish' and have identified differences in the compounds resident in the bright and dull bands (Scoffin et al, 1989). However, in Cocos corals both the bright and dull portions of fluorescent couplets are yellow-green, though the intensity with which they fluoresce noticeably differs. Identification of the compounds responsible for fluorescent banding in the Cocos corals was beyond the scope of this study, but will no doubt shed considerable light on their origins, and will hopefully be pursued in the near future. Taylor et al. (1995) have suggested that differences in the intensity of fluorescent banding in some corals are largely a function of variations in skeletal architecture, opacity and density and not the concentration or composition of skeletal inclusions, i.e. that the pattern of fluorescent banding is actually generated by the underlying skeletal density. Although the factors responsible for fluorescent banding patterns are not yet known, correlations with other temporal markers present within coral skeletons convincingly suggest that fluorescent couplets in Cocos corals are annual features, and that they can be reliably used to establish coral growth chronologies.

An annual periodicity for skeletal density band couplets in Cocos corals has been firmly established above; however, which environmental and/or endogenic factors are most closely related to seasonal shifts in skeletal density remains equivocal. The relationships between skeletal density and a range of factors commonly linked to it are depicted in Figure 1 5 and are briefly discussed below. Figure 15 shows two seasonal cycles of skeletal density, db(1) 85

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db(2) —i 1 1 1 1 1 1 1 1 1 1 1— Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month

Figure 15. Relationship between reproduction, environmental variables and skeletal and fluorescent banding patterns in microatolls from the Cocos (Keeling) Islands, Indian Ocean. 86 representing skeletal density dated by direct alignment with the Alizarin stain (i.e. it is assumed that density bands form at the skeletal perimeter), and db(2) where it is assumed that the Alizarin stain marks the skeletal growing front at the time of sampling but density band formation was not complete at the stain marker until 3 months later (calculated using equation (1) in Barnes and

Lough (1993) using average tissue thickness and linear extension rates for Cocos corals). In effect db(1) represents the 'apparent' time of density band formation and db(2) Barnes and

Lough's (1993) 'actual' time of band formation. Db(1) is included here so that comparisons with the literature which predominantly correlate density fluctuations and possible controlling influences according to the apparent time of band formation can be made. Figure 15 shows an apparent shift from HD to LD from September-November. During these months oceanic water temperatures begin to rise above 25°C and cloud levels and rainfall are near their nadir. The

'actual' shift from HD to LD occurs around December-February, when sea-surface temperatures rise above 27°C and cloud cover is relatively low. Though light intensity and/or seawater temperature have been identified as likely controls of density band formation in other studies based on the synchroneity of seasonal fluctuations, it would seem unlikely that either is the ultimate cause of density band formation on Cocos. Regardless of seasonal seawater temperatures, water temperatures over the shallow intertidal areas of the Cocos (Keeling)

Islands typically undergo daily oscillations that exceed the annual oceanic range (and generate an erratic 8180 profile), suggesting that a temperature control for density banding in microatolls is improbable. Furthermore, seasonal fluctuations in light intensity at Cocos are marginal and are an unlikely cause of annual density bands, particularly in view of the shallow, light-saturated habitats in which microatolls exist.

The formation of skeletal density bands in Cocos corals is also unlikely to be related to coral reproductive cycles as they occur out of phase. The apparent formation of HD bands during cooler winter water temperatures is at odds with the consensus reported in the literature

(see Lough and Barnes, 1990a), though several other studies that do not fit with this consensus have also focused on Porites from the Indian Ocean (Schneider and Smith, 1982; Scoffin et al,

1989), or from high latitude reef areas (Bermuda: Dodge and Thomson, 1974; Hawaii: 87

Buddemeier and Kinzie, 1975; Schneider and Smith, 1982; Abrolhos: Schneider and Smith,

1982; Gulf of Eilat: Klein and Loya, 1991). It is also important to note that microatolls are intertidal corals whilst most density band studies have focused on subtidal colonies. Klein er al. (1993) found that Porites corals may simultaneously deposit bands of opposing density at differing depths, reporting that shallow Porites deposited LD and HD bands during summer and winter respectively, in phase with Cocos microatolls. There appears to be no obvious environmental or endogenic correlate with shifts in skeletal density in Cocos microatolls, and although the demonstrated annual periodicity of these bands suggests a seasonal control, the factor(s) which dictate the skeletal density pattern in these corals remain(s) elusive.

It is also necessary to emphasise that although both fluorescent and density band couplets are without doubt annual features, the annual nature of growth increments defined by each couplet are likely to comprise 'soft years' (Buddemeier, pers. comm., cited in Barnes and

Lough, 1990a). In effect, points of equivalent density or fluorescent intensity in successive bands may not be deposited at the same time of year, and it is possible that the duration over which annual increments are deposited may be several months shorter or longer than a calender year, dependent on interannual differences in the influence of exogenic factor(s) to which coral skeletogenesis responds.

4. SUMMARY.

Growth chronologies for the corals examined in this study were established using skeletal density bands revealed by X-radiography and fluorescent bands visible when coral slices are illuminated with long-wave ultra-violet light. The annual periodicity for both band series was verified by comparison with stable carbon isotope cycles and by reference to an Alizarin stain marker and a skeletal scar introduced/deposited within coral skeletons at a known date.

This is the first report of an annual fluorescent banding pattern in mid-ocean corals. A general pattern is evident whereby bright fluorescent bands correspond with the less dense portion of the skeletal density band couplet which is deposited in the summer months, however the 88 environmental and/or endogenic factor(s) which promote the development of both band series remains uncertain. Nevertheless, available evidence suggests that both band series have an annual periodicity and may be reliably used to reconstruct coral growth histories. In chapter five these band series will be used to assign to the (approximate) nearest year topographic variations across the upper surfaces of microatolls and to reconstruct historical fluctuations in the height to which living coral polyps on the rims of corals grew (the aHLC). First, however, geographic variations around the atoll in the aHLC at the time of sampling (1991 and 1992) will be examined in the following chapter. CHAPTER 4

GEOGRAPHICAL VARIATION IN THE HEIGHT OF LIVING CORAL.

1. INTRODUCTION.

Fossil microatolls are well known as important and relatively precise indicators of palaeo-sea-level change (e.g. Easton and Ku, 1980; Hopley, 1982; Chappell et al., 1983;

Davies and Montaggioni, 1985; Pirazzoli etal, 1987; Woodroffe etal., 1990a; Nunn, 1995) and several authors have recognised the potential of modern microatolls to function as biophysical monitors of recent water level fluctuations (Isdale, 1974; Hopley and Isdale, 1977; Scoffin and

Stoddart, 1978; Stoddart and Scoffin, 1979). However, there have been few detailed investigations of the relationship between water level and microatoll elevation. In perhaps the earliest and most systematic of these studies Abe (1937) identified species and spatial differences in the relationship between water level and the tops of microatolls growing in

Iwayama Bay, Palau. On the Great Barrier Reef, two distinct classes of microatoll have been identified based on the elevation of their upper surfaces; open water microatolls with tops close to the mean low water spring tide (MLWS) level and moated microatolls with tops near to the mean low water neap tide (MLWN) level (McLean et al., 1978; Scoffin and Stoddart, 1978;

Hopley, 1982). These elevations are usually estimated by reference to a predicted tidal curve, a practice which McLean etal. (1978) note inevitably reduces survey accuracy, and which makes comparisons between sites difficult.

If microatoll morphology is to be correctly interpreted and the full potential of microatolls as palaeo- and recent sea-level indicators is to be realised, then the relationship between the height of living coral (aHLC) around microatoll rims and sea level must be accurately determined. An understanding of variability in the aHLC of microatolls is thus critical, and is examined in this chapter at four levels; i) variation in the aHLC around the living rims of individual microatolls; ii) variation in the mean aHLC of microatolls growing within individual survey sites; iii) variation in microatoll mean aHLC heights between survey sites; and iv) temporal variation in the aHLC at sites surveyed in both 1991 and 1992. Two-hundred and eighty-two Porites microatolls from nineteen survey sites around the Cocos (Keeling) Islands, Indian Ocean (Fig. 10) were examined in the field and are analysed in this chapter.

2. METHODOLOGY.

The investigation of microatoll elevation around the atoll involved four main tasks: i) the establishment of a network of benchmarks; ii) site selection; iii) surveying the height of living coral (aHLC) around the periphery of microatolls at each site; and iv) the statistical analysis of survey data. Site selection has been covered earlier in section 2.1 of chapter 3, and tasks i, ii, and iv are outlined below.

2.1. Survey Benchmarks.

Permanent survey marks (PSMs) exist on West Island, Home Island, Direction Island and Horsburgh Island. West Island PSMs relate to aerodrome datum (0.146m above MSL) and on Home Island, where the tide gauge is located, the datum is known as Mean Sea Level and is

0.70m above chart datum. Most of the bathymetric data for the atoll have been related to chart datum, and tidal predictions were quoted against chart datum until 1993. In order to achieve a greater coverage of benchmarks around the atoll for this and continuing scientific work, the atoll rim between Home and West Islands was accurately surveyed with a total station by a professional surveyor (Major John Mobbs of the Australian Army Survey Corps) and 'temporary' benchmarks consisting of steel bolts or brass cartridges set in cement were installed (ADFA1-

34) on the intervening islands. Most of the temporary benchmarks have proven durable, with 26 out of 34 remaining intact after one year. On Direction and Horsburgh Islands temporary benchmarks were also established distant from permanent survey marks that were only recently established by the Department of Lands Administration (DOLA) using a global positioning 91

system. All benchmarks (both temporary and permanent) and levels quoted in this thesis have

been reduced to MSL (Home Island datum) and can be related to the Home Island tide gauge.

2.2. Surveying the Height of Living Coral.

A Wild LNA-2 laser level was used to survey microatoll rim heights. The laser level emits a horizontal laser beam detectable to within ±0.8mm by a sensor attached to a survey staff.

The laser level has a range of up to 450m but is most accurate within 250m (i.e. over a 500m range from the benchmark). The laser level can be operated by one person and data are recorded at the staff, allowing the operator to accurately select the features for which heights are recorded. Because the laser level cannot measure distance or direction the positions of surveyed microatolls were determined by reference to recognisable features on aerial photographs and orthophoto maps. The survey procedure is schematically depicted in Figure

16.

Figure 16. Determination of the height of living coral (aHLC) using the laser level. Equation to calculate the height of living coral (aHLC) relative to the datum referred to as 'MSL' on the Home Island jetty: aHLC = (z + b) - (a + y). where z = the elevation of the benchmark relative to the local datum b = the elevation of the laser beam above the benchmark a = the distance between the laser beam and the top of the coral y = the height of MSL relative to the local datum. For most microatolls the aHLC was measured by placing the staff base on the upper surface of the living coral rim at eight compass points (N, NE, E, SE, S, SW, W, NW) around its circumference, and recording the vertical distance between the staff base and the laser level beam, the elevation of which was calculated by reference to temporary benchmarks and PSMs.

Where living coral did not fully encircle a coral no reading was recorded for the relevant compass point(s). Where a microatoll was greater than 1.5m in diameter up to twelve heights were measured around its circumference. Two hundred and eighty-two Porites microatolls were surveyed in this way. Survey data indicate that almost all of the modern microatolls growing on the Cocos (Keeling) Islands, irrespective of habitat, have tops constrained within a relatively narrow elevational range (-50cm MSL to -20cm MSL), which corresponds to a range between

MLLW and slightly deeper than MHLW (1993 estimate of MSL was 69.8cm above the lowest astronomical tide (LAT) - Australian National Tide Tables, 1993). The aHLC relative to MSL around the circumferences of the microatolls surveyed in this study are presented in Appendix

C.

2.3. Statistical Analysis.

Non-parametric statistical techniques were used to assess the significance of variability in the aHLC around the rims of individual microatolls, and between the rims of separate microatolls. Non-parametric statistical techniques were used because the number of microatolls surveyed at some sites and the number of surveyed heights on some microatolls were small, and replication and variation about the mean rim height for each microatoll was not uniform. The statistical tests used are outlined in the relevant sections below.

3. VARIATION IN THE HEIGHT OF LIVING CORAL AROUND THE RIMS OF

INDIVIDUAL MICROATOLLS.

The upper surfaces (hereafter referred to simply as the surfaces) of Porites microatolls on Cocos appear to be reasonably horizontal when observed in the field. However, survey data (see Appendix C) indicate that the aHLC around the rims of individual Porites

microatolls varies, but generally by less than 4cm (the mean aHLC variation for all corals is 3.3cm

± 1.7cm (outliers included)). Fields of microatolls with tops that are conspicuously and uniformly

inclined and orientated have been described from other reefs, and are generally attributed to

systematic variation in the aHLC around their circumference caused by differences in polyp

exposure to direct solar radiation (Buskirk et al, 1981; Taylor et al, 1987; David Hopley, pers.

comm.), or current and sediment influx (Abe, 1937; Scoffin and Stoddart, 1978). Whether or

not any systematic variation in the height of living coral (aHLC) occurs around the rims of

individual microatolls on Cocos is investigated in the following section.

3.1. Analysis and Results.

To statistically test whether or not the living rims of individual Porites microatolls at

each survey site on Cocos were consistently inclined toward a particular direction a null

hypothesis was constructed that stated:

H0: There is no agreement in the rank order of rim (aHLC) heights between microatolls

HA: There is agreement in the rank order of rim (aHLC) heights between microatolls (1) a = 0.05

Microatolls without living coral at more than two survey points around their rims were

excluded from the analysis, and where living coral was missing from microatoll rims at two or less

survey points the substrate depth was substituted for the missing values. The heights for each

microatoll were ranked from the highest to lowest and then Kendall's coefficient of concordance

(Zar, 1984 p352-357) was calculated to determine the intensity of agreement in the rank order of heights between microatolls. Where the null hypothesis (1) was rejected the data were re­ examined to identify the direction to which the rims are inclined. The results of these analyses are summarised in Table 2. Table 2. Results summary of Kendall's coefficient of concordance analysis. M is the number of variables (microatolls) being correlated; Number of ties refers to the number of tied data (i.e. equal aHLCs); Wc is the calculated Kendall's coefficient of concordance; x2c is the calculated Chi-square. In all cases n=8, a = 0.05, Xo.o5.7= 14.067.

Number of 2 Direction Site M W Y Conclusion ties c X c of Inclination

I 7 1 0.1158 5.6742 Accept H0 - Accept H II 23 28 0.0628 10.1108 0 • ill Only 3 microatolls, none with entire coral rims

IV 7 3 0.4078 19.9829 Reject H0 SW

V 15 14 0.1363 14.3102 Reject H0 N

VI 22 14 0.0427 6.5725 Accept H0 -

VII (1991) 8 3 0.1090 6.1040 Accept H0 -

VII (1992) 11 7 0.0989 7.6153 Accept H0 - Accept H VIII 2 - 0.6070 8.5000 0 • Accept H IX 6 3 0.2246 9.4332 0 •

X 13 8 0.1241 11.2931 Accept H0 -

XI (1991) 11 10 0.0511 3.9347 Accept H0 - Accept H XI (1992) 19 19 0.0188 2.5083 0 •

XII 5 2 0.1967 6.8845 Accept H0 -

XIII (1991) 3 - 0.3280 6.8890 Accept H0 - Accept H XIII (1992) 15 4 0.3280 8.1270 0 •

XIV (1991) 15 2 0.1652 17.3460 Reject H0 NE

XIV (1992) 8 3 0.1588 8.8939 Accept H0 -

XV (1991) 14 10 0.0749 7.3402 Accept H0 -

XV (1992) 17 6 0.2257 26.8583 Reject H0 NE

XVI 73 - 0.3280 6.8880 Accept H0 -

XVII 9 3 0.0774 17.434 Reject H0 NE

XVIII 2 3 0.7394 10.3516 Accept H0 -

XIX 14 12 0.0567 5.5566 Accept H0 -

Significant agreement in the rank order of aHLC heights around microatoll rims was detected in only five (IV, V, XIV (1991), XV (1992) and XVII) of twenty-four site samples (Table 2), supporting the impression gained in the field that most microatoll tops are relatively flat. With the exception of microatolls at site V, where microatolls tops were found to consistently dip in a particular direction, they were not orientated relative to the direction of current flow into the lagoon, suggesting that current-directed sedimentation was not a prevalent controlling factor.

Solar radiation was also discounted as a likely universal cause because the direction of inclination varied between sites.

3.2. Discussion.

The results of the preceding analysis indicate that though the aHLC around the rims of individual microatolls varies on average by 3.3cm ± 1.7cm, in most survey samples rim height did not vary in a spatially systematic fashion. Uniformity in the aHLC around microatoll rims was not expected in view of the numerous subtle environmental and oceanographic factors that influence the height to which living polyps on microatoll rims can extend, and the variability inherent in such a biological response. As discussed in chapter 1, variability in aHLC levels around microatoll rims may partly reflect the fact that water levels in reef environments are seldom still. They may change rapidly and over short distances due to wave and current action, and thus an exact water-level position that constrains upward coral growth can be difficult to define.

Indeed, it is unlikely that such a precise water level actually exists in most reef settings.

Furthermore, waves, mucous coverings and capillary action may introduce additional variability by enabling coral polyps to variably extend slightly above the critical low tide level without becoming desiccated (Hopley and Isdale, 1977; Hopley, 1982; see chapter 1). The living rims of many (but not all) Porites microatolls on Cocos were observed to be exposed by several centimetres during maximum low spring tides during this study; Hopley and Isdale (1977) have reported that moated Porites microatolls on the Great Barrier Reef are also commonly emerged by up to 5cm during low tide. Variability in the aHLC around a microatoll's rim may also occur where coral growth around its margin is locally affected by a variety of factors, including sedimentation, bioerosion or physical damage. It is noteworthy that if outliers obviously not related to water level are excluded from the aHLC data then height variation around microatoll rims on Cocos is typically and consistently about 2cm, comparable in size to the lenticular protrusions that characteristically develop on massive Porites corals (Isdale, 1977; Barnes and 96

Lough, 1992). This suggests the possibility that skeletal geometry imposes an intrinsic limit on the precision with which a microatoll's living rim may respond to a low water level, and on the potential uniformity of rim height around a microatoll's circumference.

Of the sites where microatoll rims were found to be consistently inclined toward a particular direction, only at site V, where the leeward margins of many of microatolls were depressed or buried by sediment, can this phenomenon be associated with an easily recognisable environmental factor. Scoffin and Stoddart (1978) similarly observed that the leeward rims of microatolls on the Great Barrier Reef were generally lower than windward rims.

They speculated that this pattern developed because coral polyps can extend higher on windward margins due to currents 'backing-up' against them, and because sediment scoured from the windward margin is deposited on the leeward rim and suppresses upward coral growth.

However, it must be emphasised that sediment deposits were not observed over the leeward rims of all microatolls at site V, and it is possible that microatolls have developed inclined planes in response to other, unknown factors. The evidence at site V does not support Abe's (1937) hypothesis that the upward growth of a microatoll's windward rim is inhibited by sedimentation.

Abe observed that many Goniastrea aspera microatolls in Iwayama Bay, Palau ('Station B') had tops that dipped toward the rising tide, which he concluded resulted from the continued inhibition of upward coral growth by sediment influx over their 'upstream' rims. Abe argued that tide level could not be a significant factor in the development of the inclined surface planes by these G. aspera microatolls as most were fully exposed at low tide.

The inclination and orientation of the rims of microatolls at sites IV, XIV (1991), XV

(1992), XVII are more difficult to link to obvious environmental factors. At sites XIV (1991), XV

(1992) and XVII on the eastern atoll rim microatoll planes generally dip to the northeast, possibly reflecting the influence of current-directed sedimentation over their 'upstream' margins as described by Abe's (1937) scheme outlined above. Net sediment movement on Cocos is from the reef flat toward the lagoon (Smithers et. al, 1993), and thus sediment influx over the northeastern margins of microatolls on the eastern atoll rim is broadly consistent with this hypothesis. Alternatively, these microatolls may be inclined toward the reef crest due to tilting following erosion of the substrate beneath their windward edges as described for some microatolls on the Great Barrier Reef (Scoffin and Stoddart, 1978). Differences in rim height around these microatolls are, however, generally small (around 2cm when obvious outliers are omitted), and evidence to suggest that they are systematically inclined is not convincing, particularly because surveys in consecutive years yielded conflicting results at two of these sites

(no significant inclination XIV -1992; XV -1991). The dip toward the southwest detected for microatolls at site IV is also of questionable significance if corals omitted from the statistical analysis because of incomplete living rims are considered; the northern rims of the excluded microatolls are most often barren. Clearly to selectively accept Abe's (1937) or Scoffin and

Stoddart's (1978) contrasting schemes of current-directed sedimentation as responsible for the development of inclined microatoll planes at different sites appears somewhat contradictory.

However, it is certainly conceivable that each of these schemes, and possibly even others, are more appropriate at separate sites where environmental conditions may be different (e.g. sediment production, current strength, site size and habitat diversity). Indeed, considering the physiographic complexity of most sites, it is possible that within-site variability in the inclination of microatoll tops revealed in this study may be attributed to subtle within-site differences in environmental conditions.

It should be noted that Goniastrea sp. microatolls (mostly G. aspera and G. retiformis) feature prominently in reports of microatolls with conspicuously and consistently slanted surfaces. As mentioned earlier, microatolls of this genera are often fully emergent at low tide

(Abe, 1937; Motoda, 1940), a characteristic which suggests that their sloping tops are not a function of water level. Several studies have linked inclined Goniastrea microatoll planes to the angle of incidence of solar radiation experienced when they are exposed at low tide (Buskirk ef al, 1981; Taylor etal, 1981; Brown etal., 1994b); the inclination of Goniastrea microatoll planes on Vanuatu has been correlated to the noon sun angle during the winter solstice, the only time of year when they are emergent during the day (Buskirk et al, 1981; Taylor et al, 1981). Buskirk et al. (1981) have argued that the inclination developed by these colonies is especially 98

pronounced due to the acute winter solstice solar angles incident on this high latitude reef.

Abe's (1937) observation that the tops of Porites somaliensis (syn. P. lutea, see chapter one) microatolls in Iwayama Bay 'do not show distinct inclination as in the case of Goniastrea aspera'

(p.313) is particularly relevant to the present study, suggesting that their surfaces are, like those on Cocos, reasonably flat. It would seem reasonable to assume that the more conspicuously and consistently slanted tops of Goniastrea microatolls reflect their more frequent emersion and exposure to solar radiation. The identification of species effects clearly has important implications for studies aiming to reconstruct environmental conditions from microatoll morphology. Though many coral genera form microatolls when further upward growth is constrained (see chapter 1), different species may exhibit different sensitivities to emergence and/or other environmental parameters that must be recognised and considered when interpreting past environmental conditions from their surface form. Indeed, recognition of the particular sensitivities of different genera may be used to target corals for the investigation of particular environmental parameters, a fundamental tenet of dendrochronology based palaeoenvironmental reconstructions (Lough and Barnes, 1990a; 1990b).

In summary, the above analyses indicate that the majority of microatolls surveyed on

Cocos had relatively horizontal surfaces that were not systematically inclined relative to the sun, though occasionally those growing in zones of active sediment transport and accumulation exhibited a weak trend, dipping away from the predominant current flow.

4. WITHIN-SITE VARIATION IN THE MEAN HEIGHT OF LIVING CORAL

BETWEEN MICROATOLLS.

4.1. Analysis and Results.

Non-parametric analysis of variance techniques were used to investigate whether or not the mean aHLC varies significantly between microatolls at each site. A flow chart of the procedure is shown in Figure 17. Where a section of a microatoll's living rim was markedly lower than the rest of the rim the heights for this section were omitted from the statistical analysis. As discussed in the previous section, microatolls where the living coral margin is locally depressed are not uncommon and can be caused by partial burial, bioerosion or physical damage. After removing these outliers Kruskal-Wallis tests (Zar, 1984 p.176-179) (or Mann-Whitney test for sites VIII and XVIII where only 2 microatolls were sampled (Zar, 1984 p135-144)) were applied to test each site sample for the null hypothesis that:

H0: The mean height of living coral (aHLC) is the same for all microatolls at a site

HA: The mean height of living coral (aHLC) is not the same for microatolls at a site (2) a = 0.05

Where the null hypothesis (2) was accepted the power of the test was calculated to assess the likelihood that all true differences among population means were detected. Because means are sensitive to outliers a median test testing whether or not median aHLC values varied between microatolls at each site was performed as a check (see Table 4 for results). Where the null hypothesis that the mean aHLC was the same for all corals at a site was rejected a 'Tukey- type' (Zar, 1984 p.199-201) non-parametric multiple comparisons procedure (MCP) was applied to identify which microatolls significantly differed from others. At sites where microatolls were surveyed during both 1991 and 1992 the results for each year are treated as separate survey samples. Mann-Whitney tests were performed using Statview 512 statistical software and all other statistical analyses were performed using JMP version 2 statistical software. Summaries of the aHLC data and of the statistical tests are presented in Tables 3 and 4. The mean aHLC values for each microatoll at each site (± one S.D) are presented in Figure 18.

As the mean rim heights of the microatolls at many sites gradually ranged through a continuum rather a series of distinctive elevational steps, the MCP analyses could not discriminate mutually exclusive 'groups' of microatolls with statistically indistinguishable mean rim heights in many samples (individual microatolls were often statistically indistinguishable from others that were themselves significantly different, i.e. a type II error committed in each case). In 1

these cases 'groups' of microatolls with rims at similar elevations were defined by reconciling the divisions determined by MCP with the mean rim heights plotted for each site (Fig. 18). The statistical associations between microatolls at each site where significant differences were detected are depicted in Figure 19 by MCP 'bars', and the derived elevational groups are summarised in Table 5.

Plot data and remove outliers

Bartlett's Test for equality of variance ^ Variance j Variance not C equal Mann-Whitney or Kruskal-Wallis Tests to test the null hypothesis that:

Ho: No significant difference exists between the mean height of living coral for the mien

Tukey-type' MCP to determine which corals are Check Power significantly different from others of the test

AreI singl e corals Are groups of microatolls delineated by significantly different from MCP correlated with specific habitats? others? c ** ) LEJ

Examine confidence Do these microatolls differ limits for mean significantly from others in similar habitats from different sites?

Figure 17. Analysis of variance statistical procedure flowchart. 101

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Fiaure 18. Mean aHLC heights for surveyed corals at each site ± one standard deviation. Sequence of microatolls along the X-axis indicates order of samples away from the reef crest (distance is, however not relative), except for sites where no directional environmental gradients appear to occur. Habitat Key: RF: reef flat; IIP: interisland passage; LAG: lagoon. 105 Table 5. Microatoll groups determined for each site sample based on mean heights of the living rims by MCP analysis.

Water -Level Limited Corals Unconstrained

Number Group 2 Site Number of of Group 1 (Intermediate) Group 3 Microatolls Groups (High) 2a 2b 2c 2d (Low)

i 7 4 1(D 3 (4, 3, 5) 1(2) 2(6,7) II 23 3 2 (17,18) 15 (19. 20, 2. 6, 4 (3.10, 5, 8, 21.11, 15, 13, 23) 12, 7,1, 9.14, 22.4) m 3 2 2(1,3) 1(2) IV 12 2 8 (10*. 2, 7, 4(11.6,5,4) 12. 9,1, B, 3) V 15 3 3(3,1.9,2,5, 5(8,12,11,14, 1 (13) 6, 7, 10, 4) 15) VI 22 3 5 (20, 22, 19. 11 (14. 17,13, 6, 6 (11, 2. 3, 8.10) 9, 21. 16. 12, 1, 4,5,7) IB, 15)

VII 11 3 3 (6, 8, 9) 4(5,11,7,1) 4 (2, 4, 3, (1991) 10) VII 13 2 11 (21,23,26, 2 (31, 30) (1992) 28, 27, 33, 25, 24, 29, 32. 22)

VIII 2 1 2(1,2) K 6 2 1(6) 5 (3,1, 2, 5, 4) X 13 3 2 (10,13) 6(1,2.12,3,7.11) 5 (9, 6. 4, 8,5) XI 11 1 1 (1, 2, 3, 4, 5, 6 (1991) 7. 8, 9, 10, 11) XI 19 3 3 (31, 30. 38) 8 (35, 37. 34, 32, 8 (28, 21, (1992) 36. 23, 24, 33) 27, 20. 25, 26. 22. 29) XII 8 3 4 (2, 7. 8. 6) 2(1,3) 2(5,4)

XIII 9 3 4 (2, 1, 4, 5) 1(3) 4 (7, 8. 9, 6) (1991) 1(31) XIII 17 3 3(34, 36, 35) 13 (21, 23. 20, (1992) 22. 24, 25, 27, 32, 26, 29. 28, 30,33) 3 (13. 11, XIV 16 4 5 (10. 5, 9,15, 3 (7,14, 6) 5(1,4.3, 12) (1991) 8) 2.16)

XIV 8 4 2 (24, 23) 2(21,22) 2(20.19) 2 (17.18) (1992) 2 (1, 20) XV 19 5 2 (9,10) 7(11,12,7,8, 2(6,5) 6 (16. 2, (1991) 14, 13, 15) 17, 3, 4, 18) 6(13, 2 (1. 16) XV 18 6 1(7) 3 (8,10, 9) 2 (17,6) 4(4,11, 15, 14, (1992) 12,2) 3. 30, 5)

XVI 5 3 1(5) 2(3.4) 2(1.2) 1(10) XVII 9 3 1(6) 7 (5, 4, 2.1, 3, 8, 9)

XVIII 2 1 2(1,2) 4(11, 12. XIX 14 2 10 (9,1, 2. 3, 10, 8, 5, 6, 4, 14. 13) 7) *it is suspected that this microatoll has been disturbed Site I M 9911 "Group" 2b | 2M I i

Habitat 111111111111111 i-n

Site 11(1992) •Group" 2b

WlRK>WB>CKHKKHKKK»«KKKKKKKHKKKKKHK»*HKmB«« aHRftSBRHRRtRRHftHHRHHJWHHHHHftftftRHHttRJ^^ £^Ui^,??M^

Site III M 9911 Site IV M 9911 "Group" 2a I

"Group" 3 nom tans««w»waHW*tf«««»W'««»«

4—5 R 11 ' 3 8 1 9 12 7 2 10 2 • 3 1 • Habitat '"lllllllll Habitat Site Vt1991) "Group" 2a —^ *.'.\A\.\.\t • I.I.I.I.«.!.!.!.!JLLL1J|' ''' 13 'Ts 14 iT'lzlB 4 10 7 6 5 2 9 1 3 Habitat U H.i.l.l_.l^--I •i-8-l..^.«..J..4..*..4..lL.tl '.^A.i.k.^Sga

Site VI M991) "Group" a ^'V^:•^•:^'^"^'I*i*^ :i ! !l!lil!: i:fr:i:i:i:i:i:o:':'.i.'..'.'.!jI'I'I'I'I'I'I'I'Ig j:':i:L - - *" lRftnBftftHRflflSn«KRftfCK>cK«RRJO«fl M.M.*.***^ •-•-•-•-•-•-•••-•.•.jn:i:!:!:!:!:i:!:i:»:i:i:griii "7-T" 4 3 2 77ll5 18 1 12 16 21 9 6 13 17 14 10 8 19 22 20 ja HaMat •»»•••••• •••-••••• -•- • • '••••' •'•'•'•'

Site VII f19911 "Group" 2b

"TH—g—I—5 ' 1 7 11 5 '9 B 6

Site IX M 992) Site VII (1992) "Group" 3 "Group"

c30TP2 2 32 29 24 25 33 27 28 26 23 21 pli-Jipj j8 Jd JJ -&i -••! J.M Habitat IJ'mJJS/t/ft/ttSttll Habitat

Site XM992) "Group" 3

MSflflHftBRBttHBRHWlftfiBKJMraWW l!.!.!.'.'.|.:-|JI*l;.;!;!:!;W;!!!:!:!:':':':J:: :j:::ii ! l 1 = Ij ooHHflflflftftftHttftftttftH»WVi*i*i*i*rtI 3ft«flttf t¥*Vf * *^* ' ' * ' "

Site XI (1992) "Group" 2b ..<:i:!:i:i:i:i:^ wKKioaaanfiaaAaniutiuuHttftWHBH} ? 37 HaWtat ,» » » " » »' " **« " " ^ ° » faMA

Site XII M 992) Site XIII M 991) "Group" 2b i 2» "Group" 2b

C o q fi 5 4 1 Habitat • ••••••••••••••I • ••• • • miaini Habitat

Fiaure 19. Schematic representation of multiple comparison (MCP) analyses Figure i*. ^ ^ sampres where significant differences in microatoll mean aHLC values were detected. See over page for key and description. Site XIII f 1992» "Group" 3 2a .•.•.ij II i i l!i'i:ili i '.'.'.'.'.'.'.'.'.'.'.'.'•'•LL'i.i i.i'i.ili'i.i.'.i.i.'.'.'.'.'.'.'.'.'.'.j; l • i i I'I'IVI 0!iJ.i.i.i.i.i.i;i.i.i.i.i.i.i.i.i.Ll.i.i.if ' ' LI 11 iii;i;i;i;i;i:i'i:i:i:i.i.i:j:i:i.i.i.i.i:i Ti • * ** !•. •.'.?:?:»!':':':':'.'i:':':':1:'!!''''''''''''''''''' 35 36 34 Habitat .13 3(1 ?fl 99 26 32 27 25 24 22 20 23 21

Site XIV M991) "Group" 3

Site XIV (1992) "Group" 2c 2b 2a EZZ ::::::< HabitaMt£gkh3x&&&& Site XV M991) "Group" 3 2c 26 p.'.i.i.'.'.'.'AU

.l.l.l.l.M.'.'.'.'.'.M fi'.i.i.'.'.^ •i 11111 i i i.i.i.i.'.a III- ii 18 4 17 2 16 15 13 14 8 7 12 11 10 H*** j,iw-.iS*...i..y..«.^iTMfi^

Site XV M 992) "Group" 3 2ri 2c 2b

MRHRJBtRKRHa««> 11 «HHR»tt« CfFEP r T 6 17 9 10 8 H^ • \\.• ••..,r.•y'iB'• JT.,.V.lV.i.^.i.U.U.A..L.^.'3"' 14 15 13 ' 2 12 11 4 v vvs s-vm1111mi

Site XVI M992) Site XVII 11991) "Group" 3 2* . ....•Jj;i.j:j''' • • " 2a"Group" 2b I t'.'.'.lijlji'if****'' '.'.•.'.•••J I iIi I I ii*i*i;ri*H I 5 !!;8!!i;:!:!:Jjljlj;{ "^ Habitat V *}//'//'J3 4 'J ? X Habitat dT^^dl*i*C*Z*iVX*UyZ*&fyZ**+, K

Site XIX f1 992) "Group" 3

fissflaftflflnsaaBSftfissft^^ i. •.••I|i!ij':i:':'i'i'.;:':!ij:!ij:j:i:ii'''' ia I 1 hi::::•!»:>:•:<• !:!J ri 1412 11 7 4 6 5 8H W1( 1 3 2 1 9 H,biut •mM/iiiiiiiiii"imiMiiiimiii illinium

KEY "Groups"(refer to Table 5) 7. High corals 2. Intermediate corals 3. Low corals MCP Bars. Habitat • Significantly low corals ED Reef Flat i Interisland Passage ] Intermediate corals Lagoonal Sand Apron I Significantly high corals

Figure 19. Schematic representation of multiple comparison (MCP) analyses 9 for site samples where significant differences in microatoll mean aHI T values were detected. Coral colonies are represented by the numbers above

S S.sS?Slhan the other groups, these separate groups »ere consrfered to belong to the one low "group'. 4.2. Discussion.

It is clear from Table 3 that at most survey sites the range of microatoll mean aHLC values can be large (even after omitting low outliers), varying between 3.0cm at site VIII and

33.7cm at site XV(1992). It is therefore not surprising that only three survey samples (VIII, XI

(1991), XVIII) contained microatolls all with rims at statistically indistinguishable heights (Table 4), or that these survey samples were characterised by small sample sizes and the power of statistical tests was low. The discovery that significant within-site variability in mean aHLC values is commonplace around the Cocos (Keeling) Islands may appear to challenge the premise proposed earlier in this thesis (chapter 1) that water level is the principal determinant of microatoll rim height. However, as argued previously exact conformity of rim heights is unlikely in view of subtle spatial differences in environmental and oceanographic conditions and the inevitable variability of biological (rim) response. As indicated in the foregoing section the mean variation in aHLC around the rims of Porites microatolls on Cocos was 3.3 ± 1.7cm (outliers included), but is reduced to approximately 2cm after outliers unlikely to reflect water level were excluded. It is interesting to note that variance in the aHLC (outliers excluded) around the rims of individual microatolls was statistically indistinguishable between colonies at all sites (a=0.05; see Table 4), even those extending from seaward reef flats into the lagoon. This constant level of variance independent of environmental conditions may indicate that the precision of coral response to a constraining water level is endogenically determined. As already discussed, the lenticular protrusions that characteristically develop on massive Porites corals are of a similar magnitude to the level of aHLC variance detected at most sites, suggesting the possibility that skeletal geometry is an important constraint on the uniformity of aHLC response (Isdale, 1977; Barnes and Lough, 1992).

The relatively high proportion of survey samples statistically determined to comprise microatolls with rims at different elevations (twenty-one from twenty-four) no doubt partly reflects the inability of the applied ANOVAs to accommodate the inherent level of variance expected in the natural systems involved, with several statistically determined groups (and microatolls in these separate groups) separated by depths of less than 3.3cm, or the mean aHLC variance around individual microatoll rims. ANOVAs test whether microatoll rims are at the same elevation by comparing the relative variance in the aHLC around the rims of individual microatolls with that between microatolls in the survey sample. The depth range in which microatoll rims are statistically defined as indistinguishable thus varies between samples according to the relative ranges of intra and inter microatoll aHLC variability, and takes no account of what is a realistic aHLC depth envelope (intra microatoll aHLC variability). Based on the results of the previous section a vertical range of approximately 2cm would appear to be a realistic aHLC envelope if outliers not related to water level are recognised and excluded. Despite these shortcomings, the ANOVA analyses clearly identified discrete groups of microatolls at different elevations in several site samples (e.g. site XIII; site XIV (1991); site XV (1991 and 1992); see Figs. 18 & 19), with the microatolls included in separate elevational groups also tending to be spatially clustered within the survey area. An investigation of the complex biological conditions which generate within-site variability in microatoll rim heights is beyond the scope of this study, however the clustering of microatolls with similarly elevated rims within the survey areas suggests that much of the variability in mean rim heights within the survey samples may be more simply explained by i) the occurrence of several different constraining water levels in the survey areas; and/or ii) the presence of microatolls with rims that had not grown up to the constraining water level (w) at the time of survey i.e. microatolls where aHLC

4.2.1. Multiple Constraining Low Water Levels (multiple ros).

The mean aHLC levels for the microatoll groups discerned from each site sample

(Tables 4 & 5; Figures 18 & 19) are presented in Tables 6 and 7, and the group mean heights are plotted in Figure 20. As indicated above, the statistical analyses divided several of the survey samples into more groups than could be easily justified in view of the inherent variability in the biological, environmental and oceanographic factors involved, complicating the realistic interpretation of the results. Some of the difficulties encountered include i) the isolation of

'groups' separated by a depth of less than the variance that occurs around the rims of individual microatolls (Tables 6 & 7; Fig. 20), though it is possible that different groups of microatolls may 110

Table 6. Summary of height of living coral for water-level limited groups (1991). Habitats : RF - Reef Flat; IIP - Interisland Passage; LAG - Lagoon. See Figure 19 and Table 5 for group divisions.

Site Group Number of Highest Lowest Grand Habitat(s) Microatolls Mean Mean Mean (measurements) (cm MSL) (cm MSL) (cm MSL) 1 1(8) -27.2 RF 2a 3(23) -31.3 -31.4 -31.3 RF 2b JJZL -33.4 RF III 1 2(11) -27.6 -28.2 -27.9 RF IV 1 8(43) -27.0 -30.4 -29.5 RF 2a 4(22) -31.7 -34.4 -32.6 RF 1 10(77) -19.7 -21.5 -20.7 IIP 2a 4(31) -22.9 -24.0 -23.4 IIP VI 1 4(39) -23.5 -25.7 -24.8 IIP 2a 11(74) -26.4 -29.7 -27.8 IIP 2b 6(42) -29.9 -32.7 -31.7 IIP VII 1 3(20) -21.5 -21.8 -21.6 LAG 2a 4(15) -22.8 -23.7 -23.3 LAG 2b 4(24) -23.8 -25.1 -24.2 LAG VIM 1 2(19) -25.6 -27.5 -26.4 IIP XI 1 11 (82) -32.5 -34.2 -33.3 RF XII 1 4(17) -29.7 -30.8 -30.1 RF 2a 2(11) -32.4 -33.1 -32.7 RF 2b 2(14) -33.8 -34.5 -34.2 RF XIII 1 4(20) -18.5 -22.7 -20.5 RF 2a 1(5) - - -29.7 RF 2b 4(20) -33.1 -34.2 -33.5 RF XIV 1 5(30) -25.7 -26.8 -26.4 IIP 2a 3(23) -28.1 -29.5 -28.9 IIP 2b 5(39) -31.4 -35.5 -33.5 RF XV 1 2(15) -19.1 -19.5 -19.3 IIP 2a 7(46) -21.1 -23.8 -22.9 IIP 2b 2(12) -27.2 -27.7 -27.6 RF 2c 6(34) -32.4 -35.8 -33.9 IIP/LAG XVII 1 1(7) -21.4 LAG 2a 11511 -28.3 -35.5 -31.9 IIP XVIII 1 2(16) -31.5 -32.0 -31.8 RF 111

Table 7. Summary of height of living coral for water-level limited groups (1992). Habitats : RF - Reef Flat; IIP - Interisland Passage; LAG - Lagoon. See Figure 19 and Table 5 for group divisions.

Site Group Number of Highest Lowest Grand Habitat(s) Microatolls Mean Mean Mean (measurements) (cm MSL) (cm MSL) (cm MSL) II 1 2(16) -24.7 -25.6 -25.5 RF 2a 16(126) -27.0 -30.4 -29.3 RF 2b 4(31) -30.9 -31.2 -30.4 RF VII 1 11(73) -27.3 -29.8 -29.4 LAG IX 1 1(7) - - -9.8 IIP 2a 5(34) -11.6 -12.8 -12.2 IIP X 1 2(11) -11.1 -11.6 -11.3 IIP 2a 6(42) -12.7 -14.1 -13.4 IIP XI 1 3(23) -27.5 -28.6 -28.1 RF 2a 8(63) -35.1 -36.5 -35.7 RF 2b 8(64) -36.6 -38.0 -37.3 RF XIII 1 3(19) -29.2 -34.4 -32.4 RF 2a 13(96) -43.4 -49.0 -45.9 RF XIV 1 2(14) -34.3 -35.0 -34.7 IIP 2a 2(16) -36.7 -37.2 -36.9 IIP 2b 2(15) -42.1 -42.8 -42.4 RF 2c 2(14) -43.6 -44.8 -44.2 RF XV 1 1(7) - - -11.9 IIP 2a 3(24) -17.2 -18.0 -17.5 IIP 2b 2(14) -23.7 -24.5 -24.1 IIP 2c 4(29) -26.4 -29.5 -27.9 IIP 2d 6(41) -33.4 -36.3 -34.5 RF/LAG XVI 1 1(4) - - -14.5 LAG 2a 2(12) -19.3 -19.6 -19.5 LAG 2b 2(10) -23.7 -24.8 -24.5 LAG XIX 1 10(76) -38.5 -39.8 -39.1 RF 112

a) 1991 Survey Sites

-10-

-20- o O • co o o o o + E -30- + o + + + o o + o o + + o -40-

-50 T 1 1 1 I 1 1 1 1 1 1 1 1— III IV V VI VII VIII XI XII XIII XIV XV XVII XVIII Site

b)1992 Survey Sites

-10- O o o o o o -20 • CO + o • o E -30- + u o + -40- o +

-50 1 1- ~7~ —I- T T I VII IX x X XIII XIV XV XVI XIX Site

KEY + Reef Flat "Groups" O Interisland Passage "Groups"• Lagoonal "Groups"

Figure 20. Elevation of microatoll groups discerned from survey samples relative to MSL: a) 1991; b) 1992. 113

be separated by small depth intervals; ii) the inclusion of microatolls in separate groups that are statistically indistinguishable from other microatolls in other groups, and to which they may be closer than to microatolls within their own group; and iii) significantly different microatolls in one sample may be closer than microatolls deemed to be statistically indistinguishable in another

(e.g. microatolls with mean aHLC values 1.6cm apart at site VII are deemed to be significantly different whilst microatolls 1.9cm apart at site VIII are not). These difficulties stem from the precision of the data relative to the variability of the measured parameter (aHLC). Though the discrimination of too many groups from a survey sample can make interpretation of the results difficult, these difficulties can usually be resolved by reference to the data and are not insurmountable. Furthermore, that the composition of groups defined by the MCP analyses often reflects the location of microatolls within the survey area suggests that in many cases the group divisions may be valid, and that subtle differences in rim height may be controlled by an environmental parameter.

If it is assumed that water level is the primary constraint to the upward growth of microatoll rims, the detection of separate groups of microatolls with statistically indistinguishable rim heights within survey samples could be interpreted as indicating the presence of multiple constraining low water levels over the survey area, or it may be that some microatolls lag beneath the constraining water level (see section 4.2.2. following). As indicated in chapter 1, microatolls may grow in open water or moated habitats, the former usually being free-draining and well connected to the open ocean at low tide, whereas ebb tide drainage is impeded and water is held above the open water low tide level in the latter (Isdale, 1977; Hopley and Isdale, 1977;

Hopley, 1982) (see discussion in chapter 1). Where multiple constraining water levels are speculated to occur over a sample area they may comprise the open water low tide level and one or more moated levels, or multiple moated water levels only. These possibilities are examined further below. 114

Microatolls Constrained at the Open Water Low Tide Level.

The mean rim aHLC for the lowest groups surveyed during 1991 regularly lay 30cm to

35cm below the MSL datum at the Home Island tide gauge (i.e. 35-40cm above chart datum)

(Fig.20a), and fell to around -35cm to -45cm MSL in 1992 (Fig. 20b). This elevation corresponds to a level approximately midway between predicted MHLW and MLLW at the Home Island gauge

(MHLW is 12.2cm and MLLW is 59.8cm below the datum known as MSL at the Home Island tide gauge (Paul Davill, NTF, pers. comm.)). It is necessary to re-emphasise that the data used to calculate predicted tidal levels for Cocos are limited, and estimates of standard tidal levels for

Cocos have been revised several times in recent years. Furthermore, the usefulness of such predicted levels is questionable for sites away from the tide gauge. Recent investigations of the hydrodynamics of the atoll by Kench (1994) indicate that the tidal range varies around the atoll; the spring tide low water level at the shallower southern part of the lagoon is estimated to be more than 10cm higher than at the deeper northern end. The microatolls included in the low groups surveyed at sites XIII and XVIII meet the criteria for open water microatolls outlined by

Scoffin and Stoddart (1978); they are steep sided and grow in association with other corals to form a reef framework at the outer margin of the reef flat. The other groups within this range are predominantly composed of microatolls from reef flat or lagoonal areas which remain connected with the ocean during most low tides. It would thus seem reasonable to speculate that the upward growth of open water microatolls on Cocos is constrained around 30cm to 35cm below the Home Island MSL datum and that the upward growth of microatolls in the lower groups at other free-draining sites is also constrained by the open water low tide level (e.g. groups 2a and

2b, site I; group 2a, site IV; group 1, site XI; group 2b, site XIV), but that this is not at the same absolute height as occurs at the Home Island tide gauge. Small variations in mean rim height between these open water groups at different sites can be attributed to local differences in tidal range and wave conditions around the atoll identified by Kench (1994).

Microatolls Growing Above the Open Low Water Level.

Survey data indicate that microatolls with rims elevated above their open water counterparts also occur on Cocos, suggesting the possibility that microatolls survive in water 115

held above the open water low tide level. Microatolls growing in pools held above the open water low tide level behind conspicuous 'moating' (see discussion chapter 1) structures such as cemented rubble ramparts and prominent algal rims have been described on other reefs by various workers (e.g Hopley and Isdale, 1977; Scoffin and Stoddart, 1978; Stoddart and Scoffin,

1979; Hopley, 1982), most often from reefs in mesotidal and macrotidal settings that are also exposed to frequent high energy storms. Moated pools are common in these settings and are usually quite large and clearly visible at low tide. Microatolls contained within moated pools typically exhibit uniform upper surface morphologies that document fluctuations in the height and/or permeability of the moat sill, and hence the moated water level (Isdale, 1974; Hopley and

Isdale, 1977). Though large areas of the atoll rim on Cocos are emergent during low spring tides, conspicuous moats are rare. Instead, much of the exposed surface is covered by small, shallow ponds that form where water accumulates in dips and hollows defined by substrate topography as the tide falls, and ponding is a more appropriate term than moating. Many ponds become active only during extreme low tides. The elevation of ponded water on Cocos largely reflects the elevation of the bed above the open water low tide level, whereas the elevation of moats is usually determined by the height of substantial topographic structures deposited or constructed over the reef surface. Microatolls growing in ponds above the open water level can be observed on Cocos during spring low tides, though it must be emphasised that the ponding is very subtle. Nevertheless, it would appear that bathymetric variation and water ponded at different levels is a significant contributor to within-site variability in microatoll rim heights at many sites. Hopley (1982) has described similar subtle ponding of microatolls in depressions and crevasses over reef platforms on the Great Barrier Reef, however they have received little attention elsewhere.

Three 'schemes' of microatoll ponding are common on the Cocos (Keeling) Islands; i) ponding through the interisland passages and over the lagoonal sand aprons; ii) ponding over sand deposits at the rear of reef flats backed by steep sandy beaches; and iii) ponding over hard erosional or constructional reef flat surfaces. 116

/) Ponding through the Interisland Passages and over the Lagoonal SandApmnR.

The three sample areas (XIV, XV and XVII) which traverse from the reef flat through interisland passages toward the lagoon on the eastern atoll rim have similar longitudinal bathymetric profiles and spatial patterns of microatoll rim height variation (Fig. 21). The bathymetry at these sites typically rises slowly from an algal reef flat pavement of between -80cm and -30cm MSL to around -20cm to -30cm MSL approximately halfway through the passages, before shallowing further over the lagoonward parts of the interisland passages. The interisland passages are predominantly floored by unconsolidated coral sands and gravels derived from the outer reef tract (Smithers, 1990), and their bathymetry reflects the transport, sorting and deposition of this material as hydrodynamic energy dissipates toward the lagoon. Kench (1994) determined that current flow through the interisland passages is principally unidirectional from the ocean toward the lagoon, though a temporary seaward reversal of flow occurs during spring low tides in passages on the eastern atoll rim. Kench hypothesised that this seaward flow was generated by the differences between tidal height at the reef crest and the elevation of the sand aprons lagoonward of the passages. During spring low tides water drains slowly off the seaward side of the sand aprons and from the interisland passages of the eastern atoll rim toward the reef crest, where friction and topographic irregularities impede the seaward flow and commonly maintain a shallow 'meniscus' of water above the bed. Channels of varying depth traverse the passages from the reef flat to the lagoon, but they are commonly reduced to a string of unconnected pools during low spring tides (Fig. 22).

The good agreement between bathymetry and microatoll mean rim height at sites

XIV, XV and XVII implies that small-scale ponding over the substrate is a significant cause of microatoll rim height variation through the interisland passages of the eastern atoll rim (Fig. 21).

Correspondence between microatoll rim elevation and bed depth across the interisland passages supports this hypothesis (e.g. at site XIV microatolls CKI91/10D7, D8 and D14 grow on a shoal on the northern side of the passage and have higher rims than CKI91/10D1, D2, D3 located in a deeper channel near the southern shore), as does the fact that the range of microatoll rim heights accords well with the magnitude of topographic relief above the open 117

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ii 118

Figure 22: Interisland Passage between Pulu Pandan and Pulu Wak Banka at low spring tide approximately half way between the reef crest and the lagoonward end of the island. Note the sandy rubble substrate and the shallowly flooded microatolls.

water low tide level. Though each pond raises the local low water and aHLC level only slightly, the rims of ponded microatolls on shallower parts of the atoll rim are commonly 10-20cm above their open water counterparts. The gradual and progressive elevation of microatoll aHLC levels with shallowing through the interisland passages can account for the difficulty encountered in discerning distinct elevational groups from the MCP analyses, and for the proliferation of type I errors. Similar but more subtle associations between bed depth and microatoll rim height are also apparent elsewhere around the atoll. For example, at site XVI microatoll rims become progressively lower as the lagoonal sand apron deepens westward, ranging from -14.5cm MSL at the western end of the interisland passage to -24.7cm MSL approximately 400m further lagoonward.

The dynamic nature of bed morphology through the interisland passages is well documented by the undersides of many microatolls which record histories of sediment accumulation and erosion around their margins (Fig. 23). However, moderately sized (50-60cm diameter) microatolls are common through the interisland passages, suggesting that though 119

Figure 23: Fluorescent image of a cross-section through microatoll CKI92/10PP9 from the interisland passage between Pulu Pandan and Pulu Wak Banka, approximately half way between the reef crest and the lagoonward end of the islands (the area shown in Fig. 22). Note the changing depth of the substrate beneath the microatoll evidenced by the 'bumpy' undersurface.

interisland passage bed microtopography may be ephemeral, their overall morphology is reasonably stable. Furthermore, uniformity of size and presumably age is characteristic of microatolls within most fields on Cocos, suggesting the possibility that microatoll fields are episodically colonised and destroyed. Although many interisland passage microatolls appear to survive in ponds constructed from unconsolidated sands and gravels, these sediments form relatively permeable and ineffective sills, suggesting that the underlying consolidated substrate imposes greater control on the nature of ponding through the interisland passages, particularly at their seaward ends. As such, discrete ponds defined by unconsolidated sediments may act to 'fine tune' a larger and more permanent hydraulic system which operates as the interisland passages drain at low tides, and individual ponds may be destroyed, reconfigured, or replaced without fatally affecting included microatolls. However, the atoll periodically experiences high energy storm waves capable of eroding and resculpting passage bathymetry, and tilting, moving, or overturning microatolls. Microatolls on a shoal at the northeastern end of site XIV are exposed to high energy waves, and provide evidence of such an event. These microatolls exhibit a 'top-hat' morphology that was interpreted in the field as indicating that the sill(s) of pond(s) over the shoal were abruptly lowered. Internal examination of these microatolls revealed, however, that their 'top hat' morphology has developed after they were overturned, probably by either storm-waves or people.

//) Ponding over Nearshore. Reef Flat Sands.

Groups of nearshore microatolls at sites I, II, IV and XI (1992) that are elevated above others at the same site may also be explained by ponding. At sites I, II, IV and XI the reef flat is backed by a steep sandy beach from the foot of which sand bars extend seaward. The surface of these sand bars lie above the adjoining reef flat, and they are locally stabilised by seagrass. At sites I, IV and XI (1992) the mean aHLC levels for microatolls growing over the nearshore sand bars are distinctly higher than those growing on the adjacent reef flat, the spatial demarcation of the two elevational groups supporting the notion that the higher aHLC are related to the sandy substrate (Fig. 24). At each of these sites the higher microatolls located over the nearshore sand bars all have mean aHLC values around -28cm MSL, approximately 4-5cm above the estimated open water low tide level. It is possible that the higher living rims of these microatolls are sustained by water held over the sandy surface as the tide falls, or alternatively the sandy substrate may be wetted by seepage as terrestrial water tables exit the beach face at low tide and capillary action may keep exposed polyps sufficiently wetted.

iii) Ponding nn Hard Rp.p.f Flat Surfaces.

Microatolls also survive above the open water low tide level in ponds formed in depressions and undulations over hard reef flat surfaces as the tide ebbs. This ponding is usually shallow and recessed into the reef surface, and the elevations of ponded water levels and microatoll rims are principally controlled by the height of the surface. Survey data and field observations suggest that at site XIII, a narrow reef flat site on the eastern atoll rim (Fig. A13), the occurrence of a group of microatolls elevated more than 13cm above another group less than

150m away can be attributed to the higher group being ponded in pools retained over a hard reef flat surface. The deeper group at site XIII all have mean rim heights of around -35cm MSL

(1991), they are all steep sided (>20cm above the substrate), and they are well connected with 12

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O) u. the open ocean at low spring tides, strongly suggesting that their upward growth is constrained by the open water low tide level. In contrast, the microatolls in the high group all have mean rim heights of around -20cm MSL, are typically thin bodied (<15cm), and grow in ponds retained at low spring tides over the erosional surface of an emergent fossil reef flat, represented near the shore by conglomerate platform. Water and microatolls are ponded in a similar fashion within shallow low tide pools over an eroded surface at site XIV (Fig. A14), where a narrow channel dissects the conglomerate platform between Pulu Siput and Pulu Jambatan, the small island immediately to its south (Fig. 5). The microatolls surveyed in this channel (CKI91/10D9, D10) are firmly attached to the substrate and have aHLC levels which lie around 10cm above the open water low spring tide level. Resurvey of 1991 sites in 1992 indicated that microatolls growing on reef flats that are deep and well connected to the open ocean experienced the greatest lowering in their aHLC levels between 1991 and 1992 (Fig. 18), suggesting that the shallow reef flat which encircles most of the atoll effectively holds water above open water levels during extreme low tides. At these times the level of the reef flat, or water ponded over it, effectively comprises a floor below which the rims of resident microatolls cannot fall.

Although ponding can explain the growth of microatolls above the open water low tide level at many survey sites, microatolls with rims elevated above the open water low tide level can also occur for other reasons. Near to the outer edge of the reef flat on the eastern atoll rim, where survey data indicate that microatolls with relatively high aHLCs are not uncommon (e.g.

CKI91/10P6, P7) ponding over the algal rim/pavement near to the reef crest, direct wetting by waves, and elevation of the constraining water level by wave set-up ( Paul Kench, pers. comm.) are all possible explanations. Away from the reef crest waves are less likely to influence microatoll rim heights because wave penetration into the interisland passages is limited at low tides. However, wind generated wavelets may effectively wet the rims of ponded microatolls above the still water level and produce some variation in microatoll rim heights within survey areas. The height to which microatoll rims grow may also be affected by pond size; smaller ponds provide a limited fetch for wavelet generation and are more vulnerable to overheating when exposed at low tide. 4.2.2. Microatolls Beneath the Constraining Low Water Level (aHLC

In three survey samples (III, VII (1992), XIX) within-site variability in microatoll mean aHLC values can be entirely explained by the presence of microatolls with rims that had not reached the potential upper limit to coral growth (the pHLC) and were continuing to grow up vertically (i.e. they lagged beneath the constraining low water level). In three other survey samples the presence of microatolls with rims not yet constrained can explain part of the within- site variability in mean aHLC values (e.g. sites I and XVII). Where microatolls with rims below the limiting water level are interspersed with those that are constrained they can usually be quickly identified. Typically microatolls with rims at the constraining water level have mean aHLCs that are confined within a narrow depth range (Hopley and Isdale (1977) suggest the rims of moated

Porites microatolls lie within 5cm of the constraining water level). In contrast, the rims of microatolls not yet limited by subaerial exposure are conspicuously lower and often exhibit the noticeably raised rims characteristic of'upgrown'microatolls (Isdale, 1977; see Fig. 2c) (e.g. site

I: X6, X7; site V: B13; site XIX: S13, S14).

Microatolls with rims below the level at which most others at a site are constrained may occur because of spatial variation in the intensity of environmental factors that limit coral growth; colony differences in sensitivity to the environmental factors that restrict upward coral growth; or because individual colonies have been lowered. As discussed earlier in this thesis, spatial variation in the intensity of certain environmental factors (e.g. light intensity, sedimentation, wave exposure) may affect the height of living coral around microatoll rims, and where these factors are locally intense the upward growth of colonies may be constrained. The Porites which form microatolls on Cocos are difficult to distinguish in the field (chapter 1, section 3), however no conspicuous depth-species patterns were recognised. Nevertheless, the possibility that microatolls with rims lower than most others at a site may stem from genotypic variation in the tolerance of individual colonies to the factors that restrict upward coral growth is raised by reports of disparate reactions to environmental stresses in neighbouring Porites corals of the same species. Apparently healthy corals growing next to others that have been severely debilitated by excessive sedimentation are described in the literature (Charuchinda and Chansang, 1985; Brown etal, 1990), and it is certainly possible that differential sensitivities to sedimentation, or other environmental factors, may lead to the occurrence of low, 'sensitive' microatolls scattered amongst those constrained by water level.

Many microatoll habitats on Cocos are zones of active sediment transport and deposition (Smithers, 1994), though it must be emphasised that microatoll tops are not universally affected by excessive sedimentation as propounded by Wood-Jones (1910 - see discussion in chapter 1). Observations made during this study suggest that where excessive sedimentation can be implicated in the occurrence of sporadic microatolls with anomalously low rims on Cocos it is via the encroachment of small sediment lobes over individual corals rather than a differential sensitivity to a widespread sedimentation stress. Excessive sedimentation is unlikely to be responsible for isolated low microatolls growing on deeper reef flats, where microatoll tops are usually elevated well above the bed and are flushed by waves and currents.

Turbulence and turbidity are more likely limiting factors in these areas; various authors propose that turbulence and turbidity inhibit coral growth by lowering light intensities, hindering polyp feeding, and by physically buffeting colonies (Dodge and Vaisnys, 1980; Scoffin et al, 1992).

However, though individual colonies may be more or less resilient to these conditions, it is difficult to attribute low microatolls interspersed amongst seemingly unaffected colonies to turbulence and turbidity alone. Marine organisms on Cocos are periodically affected by mass mortality events (Guppy, 1889; Wood-Jones, 1910; Blake and Blake, 1983), and it is possible that different reactions to and rates of recovery from these phenomena are responsible for the presence of isolated, low microatolls at many sites.

As indicated earlier, relatively deep microatolls scattered amongst others constrained at a common higher level may have been lowered. Microatolls can be lowered when their substrate is eroded, or if their central pedestals collapse. Substrate erosion can be inferred as the likely cause of the low rims on microatolls CKI92/10S12 and S14 from their surface morphology; erosion of the sandy substrate upon which they grow (see Fig. A19), causing them to settle below the constraining water level, to which they have not yet regrown. This hypothesis is supported by the apparent good health of their upgrown rims at the time of

sampling, and their relatively horizontal surfaces that are indicative of previous confinement

beneath a limiting water level.

In summary, statistically significant variation in microatoll rim heights was identified in

most survey samples, and though the number of groups and the precision with which they were

discriminated appears excessive given the range of inherent variability expected in coral growth,

much of the variation could be explained by the growth of microatolls in shallow ponds at varied

elevations as a result of subtle ponding across low gradient but typically rugged reef flat,

interisland passage and lagoon floors. Although microatolls with higher rims grow in ponds

elevated above the open water low tide level at a number of reef flat sites (site XIII; site XV), a

general pattern is evident in which the microatolls with the lowest rims grow on the free-draining

reef flats and the rims of interisland passage microatolls become gradually higher as the

passages become shallower toward the lagoon. The elevation of lagoonal microatolls is more

variable, and appears to be dependent on the elevation of the bed where they grow.

Specimens growing in the channels which connect the lagoon and reef flat during low tide and

further into the lagoon where they may be free-draining generally have lower rims than those

located on shallower areas of the sand aprons.

5. VARIATION IN THE HEIGHT OF LIVING CORAL AROUND THE ATOLL.

In the previous section significant variation in microatoll rim height was identified at most survey sites, and a general elevational trend between habitats at these sites was recognised. However, survey data also indicate that the mean height of microatoll rims in each habitat type differs between sites (Fig. 20). Recent work by Kench (1994) has indicated that hydrodynamic factors elevate the low tide level at the south of the lagoon above that at Home

Island jetty, and it is inevitable that these factors also cause local variability in the elevation of the water level (w) that constrains upward coral growth elsewhere around the atoll. In this section between-site variability in the mean height of microatoll rims growing in each habitat type is examined.

In view of the fact that reef flat, interisland passage, and lagoonal habitats are not encountered at all of the study sites, and as the number of microatolls surveyed from each habitat at the different sites was not equal, the data were not suitable for a two-way ANOVA to test for interactions between sites and habitats. Furthermore, the variances in aHLC values for a given habitat were not homogenous between sites (see Table 8), dictating that a non-parametric test be adopted. Therefore, Welch's one-way non-parametric ANOVAs for testing between means with unequal variances (JMP User's Guide, p288) were performed to test the equality of mean aHLC values between like habitats at different sites (those microatolls deemed not to have reached the constraining water level were once again omitted). This was considered a valid procedure because in most cases the MCPs performed in section 4 of this chapter divided the survey samples into groups of microatolls which grew within different habitats. Where significant differences were detected MCP analyses were carried out to determine which sites differed.

The results of these analyses (Table 8) indicate that the mean aHLC for each habitat type is not consistent between sites, though microatolls growing in free-draining reef flat habitats appear more likely to have rims at similar elevations to microatolls in the same habitat at other sites. Almost no agreement was detected between the mean heights of microatolls growing in separate interisland passage or lagoonal habitats around the atoll. With few exceptions, there is a general trend in which the mean elevation of reef flat microatoll habitats are deepest, and interisland passage and lagoonal microatoll habitats are more shallow. The habitat means for the different sites are plotted in Figure 25.

Subtle topographic irregularities control ponding and were identified as an important determinant of microatoll rim height in the foregoing section, and it is likely that the results of the present analysis reflect the relative extent to which the elevation and microtopography of reef flat, interisland passage and lagoonal habitats controls microatoll rim height. The elevation and 127

microtopography of microatoll habitats is controlled by factors such as the topography of the

underlying substrate, sediment supply, and wave and current energy, all of which can differ with

location. Therefore, the elevation of low tide ponds over the shallower parts of the atoll rim, and

the microatolls they contain, may also vary around the atoll. In contrast, reef flat habitats are

usually less variable in character and elevation, and reef flat microatolls are more likely to grow

within a relatively narrow depth envelope.

Table 8. Summary of statistical analyses: Mean aHLC levels for habitats at each site. (*: Xllla: refers to the high, moated microatoll population identified at site XIII). Ba+ denotes Bartlett's test for equality of variance.

H01: The variance in aHLC for H02: The mean aHLC for each each habitat type was the habitat type was the same same at all sites at all sites

Habitat Number of Test Calc. Prob Conc­ Test Calc. Prob Conc­ MCP Sites F >F lusion F >F lusion Groups (meas­ (DF) (DF: urements) Num/Den)

1991

RF 9(305) Ba+ 5.25 0.00 Reject Welch 206.41 0.00 Reject XV=III; (8) (8/76.7) IV=XVIII=I= XII=XIII; XVIII=I=XII= XIINXI; XIV. IIP 5(485) Ba+ 48.28 0.00 Reject Welch 406.51 0.00 Reject V; VI; XIV, (4) (4/195.7) XV; XVII.

LAG 4(91) Ba+ 4.02 0.01 Reject Welch 205.07 0.00 Reject VINXVII; (3) (3/16.4) VIII; XV.

1992

RF 6 (583) Ba+ 63.43 0.00 Reject Welch 829.50 0.00 Reject II=XV= (5) (5/124.8) Xllla*; XI; XIX; XIII. IIP 4(249) Ba+ 89.16 0.00 Reject Welch 1529.74 0.00 Reject IX=X; (3) (3/98) XIV; XV,

LAG 2(106) Ba+ 25.63 0.00 Reject Welch 127.89 0.00 Reject XVI; VII. (1) (1/28.7) The influence of habitat elevation and microtopography on microatoll rim height is evident in the interisland passage habitats of the Southern Passage which have the highest mean rim heights on the atoll (site IX: 11.8cm; X: 13.8cm; V: 20.7cm). Large areas of the

Southern Passage are exposed at low spring tides, isolating microatolls in shallow ponds over the passage bed. Wave set-up is most active at low tides (Roberts et al, 1975; Frith, 1983) and has been demonstrated to raise low tide water levels by around 50cm over windward reefs on

Enewetok (Munk and Sargent, 1954), though wave set-up is unlikely to affect microatoll rim heights in areas that are emergent at low tide. Sites IX, X and V are all located over the shallowest part of the passage, approximately 600m from the reef crest, where sediments delivered from the windward reef are deposited as hydrodynamic energy dissipates toward the lagoon (Smithers, 1994). The habitat mean elevations of interisland passage microatolls on the eastern atoll rim are generally lower than those from the Southern Passage (around -25cm to -

30cm MSL) (Fig. 25), though considerable within-site variation exists. On the eastern atoll rim interisland passages do not extend as far from the reef crest as in the Southern Passage, and lagoonal sand apron crests are normally amongst the first features to become exposed as the tide falls.

The elevation of lagoonal microatolls varies markedly around the atoll (Fig. 25).

Lagoonal microatolls growing on sand aprons that are exposed at spring low tides also have relatively high rims that can be related to sand apron surface elevation where correspondence between sand apron topography and microatoll rim elevation is demonstrated (e.g. sites XVI &

XVII). Where highly elevated lagoonal microatolls appear to grow in free-draining habitats it is possible that lagoonal low tide levels (Kench, 1994) may be hydrodynamically elevated. For example, the surface morphology of microatolls surveyed at site VII suggests that they are not ponded, and levelling indicates that they lie approximately 10cm above their open water counterparts; 10cm corresponds with Kench's (1994) estimate of tidal attenuation for this part of the lagoon. 1

The mean elevation for reef flat microatoll habitats is less variable than for interisland passage and lagoonal habitats, suggesting the possibility that the upward growth of microatolls in reef flat habitats around the atoll is controlled by a common constraining low water level.

n a) J 99.1.

-10-

-20- A

_J ^ -30- E •* - - _ ^ y <• ~ ' ' U -40- v X - - T/

-50- a— Reef Flats (1991) - -fr-- IIP(1991) 1--- LAG(1991)

-60- NW I W | S — 1 SE | E 1 NE | N 1 1 1 1 1 i i i i 1 1 i i i i i i i i i i i III — == > >- r; ^?J = x XPN; -NN-N^N^ r; = x. — — P* ?* > — oic»x*c«?c»oi?^xJ>x > > xx = = = 2: 2: x x SiteX X ^ X X

0- b) 1992

-10-

-20------I"

__ _*v- - — ~~ " _i ^ -30- • ---_ +"' E ° -40-

J. -50- »-- Reef Flats (1992) ---*--- ||P(1992) \-~~ LAG(1992)

-60- MIA/ I W I R — 1 SE I E 1 IMF I N 1 I"""I ' i" 1—i—i—r i i —i—i—i—1—i—1—i—i i i JL JL ' 1 J ' ~~ = = > > r; zr?7 = — crO>><0>0C7>C7»X 1 — — P f >• ir-br- i='i^'3'5^5'>> X > > xx x X X Site

Figure 25. Mean aHLC for each habitat at different sites; a) 1991; b) 1992. Dotted lines joining plotted points from like habitats are to allow easier comparison between different sites only. Although water may pond over some reef flat sites during extreme low tides, at others

(e.g. I, II, XIII, XVII) the reef crest is poorly developed and discontinuous, and the reef flat was observed to remain flooded during low spring low tides. The rims of deep bodied reef flat microatolls at sites II, XI, XIII and XV (and also on North (Keeling) Island, 27km to the north) were observed to fall approximately 9cm below the microatoll plane between 1991 and 1992, supporting the proposition that microatolls in these habitats are free-draining and respond to an open water low tide level (see below). The similar heights at most other reef flat sites suggest that they too respond to an open water constraining water level. Though statistically significant differences were detected between a number of reef flat samples from various sites (Table 8) the magnitudes of significant differences (maximum 4cm) are usually well within the range that could be explained by geographic variations in an open water low tide level caused by local reef morphology and hydrodynamic conditions (Abe, 1937; Munk and Sargent, 1954; Marsh et al,

1981; Kench, 1994). The habitat mean elevation of reef flat microatolls does not vary in a geographically systematic fashion, although in both the 1991 and 1992 surveys the narrow windward reef flats at the east-southeast of the atoll (sites XIII and XIV) were noticeably lower.

The reef flat on this part of the atoll is comparatively deep (approx. 0.5-1 m at low spring tide) and it is unclear why microatolls on this part of the atoll rim are low. The upward growth of corals may be restricted by turbulence and turbidity, however most microatolls at site XIII exhibit a similar concentric pattern of surface morphology that would be unlikely to develop under these constraints. Under normal conditions oceanographic factors such as waves and wave set-up would be expected to elevate low tide levels on the windward reefs. However, if prevailing winds were to ease or fail it is possible that the relative drop in water level could be enhanced on this windward atoll rim.

As indicated previously, a general trend is apparent where reef flat microatolls are lower, and interisland passage and lagoonal microatoll rims are higher. However, there are exceptions to this general trend where lagoonal and interisland passage microatolls have deeper rims at elevations close to those of reef flat colonies. The elevational accord between these deeper lagoonal and interisland passage groups and free-draining reef flat groups may suggest that the same low water level limits upward coral growth in all three habitats. However, it is also conceivable that the coincidence of rim height between habitats has developed under the influence of other, possibly different limiting factors. For example, at site XV (1991) lagoonal microatolls grow in a deep, free-draining channel incised into the sand apron surface which connects the interisland passage and the lagoon, and have rims at similar elevations to reef flat microatolls. Though it may be inferred that the constraining water level is similar, the low tide water level on the reef flat is likely to be affected by waves and currents whilst the lagoonal water level may be elevated by shallow water effects on tidal circulation (Kench, 1994).

6. VARIATION IN THE HEIGHT OF LIVING CORAL BETWEEN 1991 AND 1992.

Temporal variation in the height of living coral (aHLC) around the rims of individual microatolls is covered in detail in the following chapter and will only be briefly dealt with here.

Microatolls at five sites (VII, XI, XIII, XIV and XV) were surveyed in both 1991 and 1992, and fluctuations in the height to which live coral polyps reach within these samples over this period are shown in Figure 26 and are discussed below.

Where microatolls were surveyed in both 1991 and 1992 their rims were in most cases lower in 1992, independent of habitat type (Fig. 26). The fall in rim height was particularly evident on microatolls growing in free-draining reef flat habitats, where the aHLC in 1992 was clearly lowered relative to the microatoll plane (Fig. 27). Falls in the aHLC around the rims of free- draining reef flat microatolls between 1991 and 1992 can be explained by interannual fluctuations in the oceanic lowest low water level over this period, however the drop in rim height on interisland passage microatolls ponded above the oceanic low tide level is more difficult to account for. As only one microatoll could be definitely identified two years running

(CKI91/10D11; 92/10D20) it is possible that the sampling of different individuals combined with within-site variability in microatoll rim height could underlie some of the differences detected between 1991 and 1992, particularly in ponded interisland passage and lagoonal sand apron -10 Site VII: Lagoonal Habitat. -20-

Il^ylll hh M Mm — :.* I * • "• f * i.. CO -30 - V E -40"- 1991 -50"- 1992 -60" -10>" Site XI: Wide Windward Reef Flat

-20'

-30'" - co E -40'" - u 1991 -50'" - Reef Crest 1992 -60'" -10'" SiteXIII: Narrow Windward Reef Flat 1991

1992

co E o High L 50 -r * —• " ' " moated Outer reef corals crest -60'" -10" Site XIV: Reef Flat-lnterisland Passage.

-20"-

c^"30 H

-40'- 1991 -50"- Reef Flat Interisland Passage 1992

-60 -10' ". Site XV: Reef Flat-lnterisland Passage-Lagoojr_ 1991 -20'" 1992

co -30' - o -40r A t "*••

-50'" -| Interisland Passage Reef Flal Lagoon

-60 Fiqure 26 Change in aHLC around the rims of microatolls at five sites around the atoll Error bars denote ± one standard deviation. Horizontal scale represents proportional distance from the reef crest (except for the lagoonal site VII). Note that in the majority of cases, different microatolls were survwed in 1991 aid 1992 ateach site. Alignment of corals above the horizontal axis does not indicate the same coral was surveyed in both years, but indicates they are located a similar distance from the reef crest. To identify corals represented in this figure the site diagrams in Appendix A should be consulted. 1

habitats, though the consistency with which microatolls rims were lowered at most sites remains puzzling. Surveys between the temporary benchmarks and other fixed reference-points in consecutive years did not deviate, indicating that the fall in microatoll rim heights is real and not an artefact of the survey procedure.

Upper limit of living coral tissue

Figure 27. Porites microatoll growing on the reef flat at site XV, 1992. Note the level to which the living coral rim has been lowered relative to the microatoll plane.

The length of time that ponds remain active over the reef rim reflects the level to which the tide falls beneath the substrate. It is possible that the fall recorded by the rims of ponded microatolls between 1991 and 1992 indicates that they were contained above the open water level for a prolonged period between 1991 and 1992, and that ponded water levels fell more than usual as a result of seepage through unconsolidated substrates and pond sills, or simply due to evaporation. Other factors may also inhibit coral growth at these times, such as high water temperatures and/or increased salinity and bleaching, though reaction to these stresses is unlikely to produce the distinctly horizontal rims (see Fig. 27) observed on microatolls right around the atoll. Where the 1992 aHLC levels were higher than those surveyed in 1991 the microatolls were typically growing over a relatively impermeable substrate. The three high

1992 interisland passage microatolls surveyed at site XV, for instance, are located over the eroded remnants of the conglomerate platform. High 1992 microatolls were also located close to the reef crest at sites XI and XV, and possibly reflect ponding over the algal rim or wave set-up close to the reef crest (Roberts et al, 1975; Munk and Sargent, 1954). As indicated earlier, a convincing explanation for the lower rims surveyed on microatolls at site XIII remains elusive.

Where microatoll rims were lowered in free-draining sites the magnitude of the fall generally reflects the connection of site with the open ocean and the depth of the substrate as indicated elsewhere in this chapter. The most marked and consistent fall in the height of living coral around individual microatoll rims was recorded at site XIII (approximately 9cm from the near horizontal microatoll plane to the top of the living coral rim) and on the reef flats at sites XIV and

XV. A more subdued fall in rim height was recorded by microatolls growing at site VII at the southern end of the lagoon (approximately 5cm), possibly reflecting the attenuated tidal levels in this part of the lagoon (Kench, 1994). The rims of reef flat microatolls at site XI were also lowered to a lesser extent than those on the eastern atoll rim, most probably due to geographic differences in the low-tide level around the atoll attributable to either differences in hydrodynamic conditions or the extent to which the drainage of water off the reef flat is impeded as the tide falls. Geographic differences in the pattern of rim height variation through time are investigated in more detail in the following chapter.

7. SUMMARY.

In summary, surveys of 282 microatolls to a datum related to the Home Island tide gauge has demonstrated that living microatolls on the Cocos (Keeling) Islands are restricted to a narrow elevational range in the lower intertidal, and supports the view that it is water level and not sedimentation or nutrient limitation that restricts further upward coral growth. However, precise levelling indicates that the height of living coral on microatoll rims (the actual height of living coral - aHLC) in different microatoll fields (both within and between survey sites) may vary by up to

40cm. Statistical analyses further indicate significant variation in the height of living coral around the rims of individual microatolls, between microatolls within fields at survey sites, and between sites around the atoll. In view of the numerous biological (e.g. genotypic variation in vigour and tolerance to exposure), environmental (e.g. sedimentation, elevation of bed, bed topography) and oceanographic (e.g. connections with the open ocean, exposure to waves and currents) factors that may influence the height to which corals may grow, and the complex and generally poorly understood interactions between these factors, some variation is to be expected.

Investigations reveal that the aHLC around the rims of the majority of Porites microatolls on Cocos is variable (mean range 3.3 ± 1.7cm); however no systematic spatial pattern of variability could be detected (i.e. microatoll rims on Cocos are not systematically tilted). Rim height is not consistently aligned with current flow or solar exposure, nor are the tops of microatolls inclined relative to the influence of these factors. The statistical analysis confirmed the impression gained in the field that the upper surfaces of the majority of Porites microatolls on

Cocos are relatively flat. Differences in microatoll rim height within survey samples can generally be attributed to the occurrence of at least two of the following: i) ponded microatolls growing in ponds held above the oceanic low tide level at heights controlled by habitat bed elevation and topography; ii) 'unconstrained' microatolls that have not yet grown up to the constraining water level; and iii) free-draining microatolls that remain directly connected to open water at all tidal stages.

The mean aHLC for free-draining reef flat microatolls was generally within a relatively narrow range of between -30cm and -35cm MSL in 1991 and between -35cm and -45cm MSL in

1992 right around the atoll, which corresponds to a level approximately midway between MHLW and MLLW calculated at the Home Island tide gauge. The mean aHLCs for most interisland passage and many lagoonal microatolls were found to be elevated above their reef flat counterparts and to closely correspond with the topographic elevation of the surface on which they grew. It was concluded that ponding enabled these elevated microatolls to survive above the oceanic low tide level. Attenuation of tidal amplitude within the lagoon can account for the presence of free-draining microatolls with rims elevated above the lowest oceanic low tide level at the south of the lagoon; where spring low tides are approximately 10cm higher than the open water level (Kench, 1994). Microatolls at the south of the lagoon are elevated a commensurate amount above their open water counterparts. Although the rims of free-draining reef flat and lagoonal microatolls were lowered between 1991 and 1992, lagoonal microatoll rims fell by a lesser amount possibly due to the mediation of tidal fluctuations within the lagoon. The rims of microatolls from the deep free-draining reef flats were lowered by the greatest amount over this period, reflecting their better connections with the open ocean and their sensitivity to interannual fluctuations in the open water low tide levels. 137

CHAPTER 5

TEMPORAL VARIATION IN THE HEIGHT OF LIVING CORAL.

1. INTRODUCTION.

In the previous chapter geographic variation in the aHLC around the rims of Porites microatolls growing on Cocos was examined at several spatial scales and was found to vary both within and between most sample sites, though some consistency in the pattern of rim height variation with habitat was evident. In this chapter the investigation of geographic variability in the surface morphology of Porites microatolls on Cocos is expanded to include a temporal dimension. Annual density and fluorescent bands were used to generate radial time-series of

past aHLC levels that are documented by the upper surface morphology of intact microatolls.

Radial aHLC time-series are visually and statistically compared to ascertain whether a coherent

morphological signal of past aHLC change exists over different growth axes on individual

microatolls, and over the tops of microatolls within and between different sample sites and

habitats on the Cocos (Keeling) Islands.

2. METHODOLOGY.

To construct an aHLC time-series from a microatoll it is necessary to determine the elevation and age of microtopographic variations across its upper surface. The living rim, centre, and conspicuous morphological features over the tops of microatolls were surveyed relative to

MSL using the laser-levelling procedure outlined in chapter 4 (section 2.2), and field samples were collected and laboratory samples prepared using the techniques described in chapter 3.

Growth chronologies were established by counting the number of annual skeletal density and/or fluorescent bands inward from the outer growth edge, banding being detected by X-raying or illuminating with UV-light thin vertical slices cut across a microatoll from its leeward to seaward margin (see chapter 3, section 3). 138

2.1. aHLC Time-Series.

The principal aim of the research reported in this chapter was to determine whether temporal changes in coral aHLC around the Cocos (Keeling) Islands varied systematically, and thus comparisons of aHLC time-series form the focus of this study. aHLC time-series were compiled by aligning the upper surfaces of microatolls depicted on X-ray plates and fluorescent photographs to MSL using the surveyed heights of their living rims and prominent topographical features on their upper surfaces, and then measuring the aHLC at the start and approximately midway through each annual skeletal density and/or fluorescent band. aHLC data were arranged chronologically from the most recent to the oldest, the most recent being the height of the living rim when the sample was collected. The length of derived data series is dependent on the size of the microatoll and the definition of annual growth bands. Most time-series constructed here span 10-20 years (20-40 data points), however four markedly longer records were extracted from two large microatolls (CKI91/10F2: 3.3m diameter; and CKI92/10PP30:

2.7m diameter) which extend back to the turn of the century. For most samples two aHLC data series were compiled, one for the oceanward and another for the lagoonward growth axis.

However, only a single radial time-series could be compiled for a number of microatolls because only half of the colony was successfully sampled, the sample was damaged during transport or slice preparation, or because banding over one axis was ambiguous. Approximately 15% of the samples returned to the laboratory were rejected from further analysis because clear banding could not be detected on any axis. Conversely, more than two time-series were compiled for several microatolls where larger samples (half colonies) were collected. A total of 168 radial aHLC time-series ranging from 4 to 98 years long were derived from 90 separate colonies.

These time-series essentially depict time-adjusted radial topographic profiles, the majority of which do not extend back beyond 1970. The time-series constructed for the period 1970-1992 are graphically presented in Figure 28, and full time-series are presented in Appendix D. 139

Site I -15 92a 93a 94a -20 92b 93b 94b _j -25- co *°~ 2 E-30- -35 4 -40

R21(E)

73T.

Site IV -15 811a E11(W) E12(S)

-20 E12(N) 811b -D EH(E) co-25 B-30 Irf -35-

-40 -15- 812 -20 4

-25

E-30-

-35-

-40 —I T1 T r T T T T 1 I i I T~" 1970 1975 1980 1985 1990 1995 1970 1975 198810 1985 1990 1995 1970 1975 1980 1985 1990 1995 Year Year Year 140

Site V -15

-20 Jfe&u

co1"2^ E -30 o B1(SE) B2(NW)

-35 B1(NW) B2(SE) B3(S) -40

-15'

-20. -25. ^ CO £ -30 V o B4(S) B5(N) B6(SE) -35 B4(N) B5(S) B6(NW) -40 -15

-20

w -25 I -30 4 B7(N) B8(SE) B9(S) -35- B7(S) B8(NW) B9(N) -40 -15

-20 : aP g-25 E ° -30 4 B10a(S) B10b(S) B11(NW) -35 B10a(N) B10b(N) B11(SE)

B14(S)

B13(S) B14(N) H—i—i—i—i—I —i—i—i—i—I I ' ' ' > l 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 Vonr vYeaQar Year Figure 28H: Microatoll aHLC time-series 1970-1992. 141

Site VI -15 A10(N) A11(N) A12(SE) -20- A10(S) A11(S) A12(NW) -25- co 2.-3 0 r E a -35- -40 -15- A13a(SE) A13b(SE) A15(SE) -20- A13a(NW) A13b(NW) A15(NW) co-2 5 4 E °-30 -35- -40 -15 A16(NW) A19(SE) -20 A16(SE) A19(NW)

A20a(SSE)

E-30-J A20a(NNW) o A20b(SE) -35 A20b(NW) -40

-15^ A22b(N) 51a. 52a. -20 51b. 52b. -25 r co E -30-: o -35 -i

-40 -15 53a. -20- 53b. w-25

§-30-

-35 4

-40 T T T T T 1970 1975 1980 198T 5 199T 0 1995 1970 1975 198T 0 198 T 5 199T 0 1991 5 197r0 1975 1980 1985 1990 1995 Year Year Year

Figure 28iii: Microatoll aHLC time-series 1970-1992. 1

Site VII -15

-20

co"-25

E-30 o 89a. -35 87 88 89b. -40 Site VIII

-9C\ —

AA^M^ /**%*&* P in — b_d0~ - 133a 134a -oo- - 133b 134b -AC\ - Site X 5.0

-10 Ssi -15 w -20 § -25

-30-4 M13(S) 132a -35 M13(N) 131 132b -40 -15-Site XI F1(N) F3(N) -20- F1(S) F3(S) w-25- §-30- -35- -40- Site XII -15- H1(E) H7(E) -20- H1(W) H7(W) | -25 § -30 -35 -40 I T T T T 1 1970 1975 1980 1985 1990 1995 1970 197T 5 198T—0 198r 5 199T 0 1995 1970 1975 1980 1985 Year 81Yea r Year Year Figure 28iv: Microatoll aHLC time-series 1970-1992. 143

Site XIII -15- I2(W) -20 -25 co -30 E o -35 -40 -45 Site XIV -15 02(E) D3(W) D2(W) D3(E)

D5(SE)

D5(NW)

D7(E)

D7(W)

-15 -20 "^ttqjl co -25

§ -30 D8(E) D14(E) -35 D8(W) D14(W) D15(E) -40

-15 D16(E) 61a. 62a. -20- D16(W) 61b. 62b. co-2 5

u -30

-35'

-40- —I 1 1 1— T T ~n 1—i—r- 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 Year Yea- Year Figure 28v: Microatoll aHLC time-series 1970-1992. 144

Site XIV -15 63a. -20 CO 63b. -25 E -30U -35 -40 Site XV -15 P1(E) _, -20 co P1(W) r*W/r fW f-25. *»$0%*>^ O -30 - P7(E) - P11(E) -35 - P7(W) - P11(W) -40'

-45' -15 P16(E) -ft PP4(W) . -20 CO P16(W) -o PP4(E) -25

-30-

-35-j

-40 -10

-15 -a PP13(E) -20 CO -a PP13(W) E -25 u -30 PP9(E) -35 41 PP9(W) -40-

-15 81 -t. 82 83 -20-f _i CO 2 -25 E °-304 -35 ^^Hft^iAM^fa. ^^^^J^-

-40 i 1 1 1 1 i 1 1 1 1 1 I 1 1 1 r 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 Year Year Year Figure 28vi: Microatoll aHLC time-series 1970-1992. 145

Site XV -15 84 85 86a. -20- co 86b. -25- E o -30

-35

-40 Site XVI -15' 161a 163a 162 -20' 161b 163b CO -25- E o -30-

-35-

-40- -15 CSa -20 CSb CO -25 E o -30

-35

-40 Site XVII -15 K1(E) 121 122a -20 K1(W) 122b CO -25 E u -30 ECttr

-35

-40' Site XIX -15' S5(N) 171a 172a -20- S5(S) 171b 172b CO -25- E o -30-

-35-

-40-

-45- T 1 1 1 1 ~l 1 1 1 1 1 I I ' I I— 1970 1975 1980 1985 1990 19951970 1975 1980 1985 1990 1995 1970 1975 198° 1985 1990 1995 Year year Year

Figure 28vii: Microatoll aHLC time-series 1970-1992. 146

SITE XIX (continued) -15 -20 _i CO 2 -25 E o -30 -35 -40 -45 1970 1975 1980 1985 1990 1995 Year Figure 28viii: Microatoll aHLC time-series 1970-1992.

3. COMPARISON OF THE SURFACE MORPHOLOGY OF DIFFERENT GROWTH AXES ON INDIVIDUAL MICROATOLLS.

It was demonstrated in the previous chapter that the aHLC around most Cocos

microatolls lies near to the mean lowest low tide level, except where other inhibitory or

destructive factors restrict coral growth. This suggests that the ability of coral polyps to withstand

subaerial exposure principally defines the height to which living coral rims extend. Other

researchers on other reefs have reached the same conclusion (e.g. Abe, 1940; Isdale, 1974;

Scoffin and Stoddart, 1978; Taylor et al., 1987; Rodda and Nunn, 1990; Woodroffe and

McLean, 1990). Following this it has been widely inferred that with the continued absence of

other limiting factors a microatoll will develop an essentially symmetric upper surface that

documents the temporal pattern of relative changes in the juxtaposition of the aHLC and a water

level (ro) above which prolonged subaerial exposure arrests upward coral growth (Stoddart and

Scoffin, 1979; Woodroffe and McLean, 1990; see chapter 1). Although several authors have

suggested that the aHLC around a microatoll's margin may vary systematically as a result of

spatial variability in vs related to wave and current direction (Abe, 1940; Scoffin and Stoddart,

1978) and/or as a result of differences in the orientation of polyps around a microatoll's rim to direct exposure to debilitating environmental phenomena (Buskirk et al., 1981; Taylor et al,

1987, see chapter 3.2 of chapter 4 for discussion), no systematic variation in the aHLC around 147

Porites microatolls on Cocos was identified in the previous chapter (section 3). It would therefore seem reasonable to assume that a common TO limits upward coral growth completely around the margins of these microatolls, and the same temporal pattern of water-level (TIT) change affects a microatoll's entire living circumference. However, it is necessary to re- emphasise that the response of coral polyps to the limiting conditions is not precise; the mean aHLC variability around Porites microatolls surveyed on Cocos is 3.3 ± 1.7cm (reduced to approximately 2cm when outliers obviously not related to water level are excluded). This section aims to assess the coherence of temporal patterns of aHLC-change (i.e. microatoll surface morphology) developed under uniform conditions of water-level change by statistically comparing 87 pairs of radial aHLC time-series derived from 72 microatolls growing in a range of sites and habitats on the Cocos (Keeling) Islands.

3.1. Analysis and Results.

Sophisticated statistical techniques exist to assess the coherence of pairs of time- series (e.g. cross power spectrum analysis - see Fritts (1976) for description). However, linear correlation analysis was considered an appropriately rigorous statistical technique for assessing the similarity of a microatoll's upper surface form over separate growth axes in view of expectations regarding the nature of the data. Some variability in the juxtaposition of the aHLC relative to a given m is to be expected given the complex interaction of environmental and biological factors that determine coral growth and because microatoll surfaces may be eroded after deposition. Furthermore, aHLC chronologies may also vary due to difficulties in precisely detecting annual bands and between colony differences in the manifestation and elasticity of their 'annual' periodicity (Buddemeier's soft' chronologies - see Lough and Barnes, 1990a). The linear correlation coefficient (r) provides a simple measure of the similarity of two time-series, and is used here to compare the height of a microatoll's upper surface deposited at a given time along two separate growth axes. The results of the correlation analyses are presented in Table

9. 148

Table 9 reveals that most of the microatolls examined had radial topographic profiles across different growth axes that were reasonably similar, with 65.5% of the growth axes pairs analysed being significantly and positively correlated at the 95% confidence level (69% at 90% level), 16.1% showing no significant positive correlation and 14.9% being negatively correlated.

The upper surface morphologies of lagoonal microatolls appear to be the most consistently symmetric (9 from 9: 100%), followed by reef flat (15 from 22: 68.2%) and then interisland passage corals (34 from 56: 60.7%). Of the corals that generated negative correlation coefficients 85% were interisland passage microatolls and 15% reef flat specimens. These relationships are evident in Figure 29 which plots, by habitat, r-value confidence levels for correlations between aHLC time-series from different growth axes of individual microatolls. The unequal samples are a good approximation of the relative abundance of microatolls within each habitat on Cocos, and were unavoidable given the logistical difficulties of retrieving large samples from which banding could be detected and chronologies established over multiple growth axes. Nonetheless, it is acknowledged that the habitat-microatoll symmetry relationships identified here could be more confidently accepted if habitat sample sizes were even.

The aHLC time-series pairs for the microatolls that are most highly correlated, most equivocally correlated and most negatively correlated from each habitat are plotted in Figure 30.

Figure 30 (see also App. D) shows that in all three habitats the most highly correlated radial topographic profiles document prominent, synchronous fluctuations in the pattern of aHLC change. Microatolls with lower r-values typically develop flatter, more subdued radial topographic profiles that fluctuate out of phase to varying degrees. It is important to note that the radial profiles of many microatolls determined to be poorly correlated (r-values near to zero) are reasonably similar over most of their length, but are punctuated by short intervals where they digress more markedly. These deviations may reflect the localised influence of one or more of the numerous factors that can lower a microatoll's rim (i.e. excessive sedimentation, disturbance, bleaching, etc, see chapter 1), or alternatively they develop through erosion of the microatoll plane. The surfaces of many microatolls on Cocos have been eroded by boring organisms, particularly Tridacna, however where the influence of boring organisms was recognised in the 149

Table 9: Correlation coefficients between aHLC time-series derived from different growth axes of individual microatolls. Bold correlation coefficients are positive and significant at the 95% level. Those marked with superscripta are positive and significant at the 90% level. "5HT Coral Axes Habitat CKmyyyya vs b TIT -srr TT7T CKI89/93a vs b RF 18 0.92 CKI91/10T1(SSWCKI89/94a vs b ) RF 25 0.91 vs TtF TT76" 1(SSW) rs (SW) ~38~ CKI91/10T1(S)vs(SW) RF 34 0.78 TV CKI91/10E11(E)vs(W) RF 314 0.80.419 CKI91/10E12(S)vs(N) RF 20 0.25 CKI91/10B1(SECKI89/811avsb) vs(NW) RIIFP 2119 -0.00.583 CKI91/10B2(SE) vs (NW) IIP 20 -0.44 CKI91/10B4(S) vs (N) IIP 22 0.94 CKI91/10B5 S)vs N IIP 18 0.87 CKI91/10B7(S)vs(NCKI91/10B6SE)vs(NW) ) IIP 13 0.74 CKI91/10B8(SE) vs NW) IIP 21 0.08 CKI91/10B9(S)vs(N) IIP 22 0.22 CKI91/10B10(S) vs(N) IIP 10 -0.18 CKI91/10B10S) vs(SE) IIP 21 0.92 CKI91/10B10(S) vs(NW) IIP 15 0.67 CKI91/10B10(N)vs(SE) IIP 15 0.48a CKI91/10B10(N)vs(NW) IIP 15 0.80 CKI91/10B10(SE) vs NW) IIP 15 0.72 CKI91/10B11(SE vs NWJ IIP 2135 0.20.904 VI CKI91/10B12SE vs(NW) IIP IIP 2109 -0.70.985 CKI91/10B14(SE) vs (W) IIP IIP 272 0.00.967 CKI91/10A10(S) vs(N) IIP IIP 2330 0.853 CKI91/10A11(S)vs(N) IIP 23 0.62 CKI91/10A12(SE)vs(NW) IIP 22 0.70 CKI91/10A13SE)vs(NW) IIP 25 0.37a CKI91/10A13 SE vs (SSE) IIP 22 0.83 CKI91/10A13(SE) vs(NW (NNW) ) IIP vs (N 25 0.43 CKI91/10A13(NW) vs(NW (SSEJ ) IIP vs(NV 21 -0.34 CKI91/10A13(NW) vs(NW (NNW) ) IIP vs (NW) 21 0.69 CKI91/10A13JSSE) vs (NNW) IIP vs SSE) 22 0.69 CKI91/10A15(SE) vs(NV" IIP vs (NNW) 32 0.37 CKI91/10A16SE IIP 24 0.65 CKI91/10A19(SE) IIP 26 -0.42 CKI91/10A20(SE IIP 24 0.97 CKI91/10A20 SE) IIP 26 0.15 CKI91/10A20(SE) IIP 15 inr 24 0.20 CKI89/133CKI91/10A20(NWa vs b ) vs'(SSE) LAIIPG 23 0.60 "vnr 18 0.70 CKI91/10A20(NWCKI89/134a vsb ) vs (NNW) LAIIPG 29 0.50 CKI92/10M13(S)vs(N) RF 214 0.02a CKI91/10A20(NNW) vs (SSE) IIP 0.39 CKI89/132a vs b RF 3127 0.38 CKI89/51a vs b IIP 0.56 XI CKI91/10F1(S) vsi TATRF T 23 "TTSS-0.58- CKI91/10F2(S)vsCKI89/52a vs b i RF 173 0.37 CKI89/53CKI91/10F3(a vsS bv s i RF 22 0.83 Tdl CKI91/10H1(E)CKI89/89a vs b' RF 29 -0.52 CKI91/10H7(E); RF 27 0.18 36 0.33 TIV CKI9l/10Dl(E)vs(W) ~TTP~ 28 -0.72 CKI91/10D2(E vs W IIP 46 0.86 CKI91/10D3E vs W IIP 27 0.50 CKI91/10D4 E) vs (W) IIP 27 0.91 CKI91/10D5(SE)vs(NW) IIP 25 0.97 CKI91/10D7E) vs(W) IIP 26 0.93 CKI91/10D8(E[vs (W) IIP 30 -0.02 CKI91/10D14(E) vs (W) IIP 21 -0.12 CKI91/10D16(E)vs(W) IIP 22 0.92 CKI9l/lOPl(E)vs(WCKI89/61a vs b ; LAG 25 15 0.95 ~w CKI91/10P7(E)vsCKI89/62a vs b ( LAG 25 CKI91/10P11(E)vs(W) LAG 237 0.98 CKI91/10P16(CKI89/63a vs Eb (W) TATT 28 0.91 CKI92/10PP4I IIP 30 -0.74 CKI92/10PP5(E IIP 74 0.73 CKI92/10PP9(E IIP 21 0.05 CKI92/10PP13(! IIP 36 0.84 CKI92/10PP30(E RF 151 0.90 TVT CKI89/86a vs b IIP 20 0.62 CKI89/161a vsb IIP -0.07 CKI89/163avsb "T" TVT RF 0.78 CKI89/162avsb 43 0.01 CKId27lOS5(N)vs(S) IIP CKI91/10K1(E) vs (\A/) - 37 0.91 CKI89/171avsb TAG CKI89/122a vs b 0.62 CKI89/172avsb LAG IT RF 0.81 17 0.87 ~W ~38~ 0.98 IIP 24 irrr "RFRF - 27 0.00.853 150

O-i o 10- o • o Positive Correlations x 20- x Negative Correlations :« * 30- > _j 40- 4> « 50- o O o e 2 60- O § 70- O 95% Confidence Level o o 80- o ^^ 90- 100 - Reef Flat Interisland Passage Lagoon Habitat Figure 29: Confidence levels of correlations between aHLC time-series derived from different growth axes of individual microatolls, grouped according to habitat. Data from Table 9.

a) Reef Flat

S5(N) 94(a) H7(E> F1(N) H1(E) S5(S) S«b) H7(W) F1(S) H1(W)

r-0.91 052 TtZiZT^&^ttA

A

b) Interisland Passage

A11(N) 51a 86a.

A11(S) 122b 51b • 86b.

r-0.02

r«0.01 W< B11(NW) P7(E)

B11(SE) P7(W)

1970 1975 199T"~"T0 196T6 1990 1965,970 1975 1980 1985 1990 1995 c) Lagoon YMT YMT -15. 62a 63a

62b 63b

r-0.95

t'OS

1970 1975 1980 1985 1990 0 1995 Excellent Correlations Poor Correlations Negative Correlations

Figure 30: aHLC time-series graphs of the two microatolls with the most highly, most equivocally, and most negatively* correlated surface morphologies across separate growth radii from each habitat. *There are no lagoonal microatolls with aHLC time-series that are poorly or negatively correlated. 151

field alternative microatolls were selected for sampling. Apart from these short-term aberrations the tops of many 'poorly correlated' microatolls are also quite planar, however data associated with the discordant intervals often dominate statistical calculations and promote the calculation of

low r-values. Whilst such r-values correctly indicate a poor correlation exists between two radial

profiles taken as a whole, they do not necessarily indicate that two profiles are poorly correlated

over their entire lengths.

3.2. Discussion.

Statistical analyses indicate that i) approximately two-thirds of the 72 microatolls for

which aHLC time-series pairs were compared have surfaces that are radially symmetric across at

least two growth axes (i.e. aHLC times-series generated across two growth axes are statistically

equivalent), ii) the most symmetric microatolls exhibit prominent synchronous aHLC fluctuations

across their surfaces, and iii) interisland passage microatolls are more likely to have developed

asymmetric patterns of surface morphology than reef flat or lagoonal microatolls. Accepting that

the temporal pattern of water-level change is unlikely to vary significantly over the area

encompassed by a single microatoll, the development of disparate radial topographic profiles

over a microatoll's surface must reflect the single or combined influence of: i) the partial or

variable debilitation of the microatoll rim and/or destruction or realignment of the microatoll plane

by a factor(s) other than water level, and/or ii) an ambiguous temporal pattern of relative water-

level change and subsequent weak control of the aHLC response (the discoid shape of most

microatolls suggests that directional differences in growth rate are unlikely to be a significant

factor).

Excessive sedimentation is the most conspicuous cause of partial rim debilitation and

surface asymmetry on Cocos, particularly in the interisland passages where partially buried

microatolls incompletely circled by living polyps are common, as observed by Wood-Jones

(1910, see discussion in chapter 1, site descriptions in App. A). Field observations confirm that sediment influx inhibits microatoll rim growth; the debilitating effects of excessive sedimentation have been discussed in chapter 1. Interisland passage microatolls on Cocos are particularly 152 vulnerable to sediment influx because the interisland passage beds are dominated by sands and gravels (Smithers, 1990), and most microatolls grow in shallow ponds and are thin bodied

(<10cm), and are therefore easily buried. Sediment influx over a microatoll's surface has been linked with current direction (Abe, 1940; Scoffin and Stoddart, 1978), however no systematic pattern was demonstrated on Cocos (chapter 4, section 3). Furthermore, the aHLC time-series constructed for many asymmetric microatolls indicate that the lowered rim section(s) were often only temporarily suppressed, suggesting that the area affected by sedimentation (or any other causative factor), and/or the severity of its influence, may change through time. Sediments do not appear to impinge upon and differentially affect living polyps around the rims of lagoonal microatolls, despite the lagoon being a major depositional environment (Smithers, 1990;

Smithers et al., 1993). The lagoon is alow hydrodynamic energy environment, and sediment accumulation is typically slow. Sediments are less likely to bank up against microatoll rims in these areas, and polyps may disperse sediments deposited at slower rates (Marshall and Orr,

1931; Rogers, 1990; Stafford-Smith and Ormond, 1992). Greater wave and current action generally prevents sediment accumulation over or against reef flat microatolls.

Physical damage, bioerosion, adverse water quality, and disturbance are among other factors that may singly or synergistically cause microatolls to develop irregular upper surfaces (see chapter 1). However, the vulnerability of microatolls to specific debilitating/destructive influences differs between habitats. For example, because many interisland passage microatolls are confined within shallow ponds at low tide they are particularly vulnerable to critical salinity reductions associated with heavy downpours, and to bleaching caused by high water temperatures during prolonged daytime exposures. Rims lowered by these factors are more susceptible to sediment influx. Interisland passage microatolls are also often loosely anchored to the unconsolidated substrate, and cross-sections through asymmetric colonies often indicates that those have been tilted (Fig. 31). In contrast, microatolls growing on high energy reef flats are usually firmly attached to a solid substrate and well flushed at low tide, but they are particularly susceptible to physical damage. 153 154

Although conspicuous irregularities caused by the factors outlined above can account for the lack of radial symmetry observed in many microatolls, the radial topographic profiles shown in Figure 28 indicate that in many instances topographic variation between separate growth axes is more subtle. Poor correlations between radial topographic profiles on many microatolls reflect the erratic occurrence of small-magnitude undulations out of phase across different growth axes; particularly where topographic relief over the microatoll plane is limited and dominated by these small-scale fluctuations. It is proposed here that microatoll surface symmetry is also a function of the persistence and clarity of the water-level change signal, and the extent to which it is morphologically expressed compared to these small, random aHLC fluctuations, and that this varies as environmental and hydrodynamic conditions change between habitats.

It is implicitly assumed in simple models that relate microatoll surface morphology to water-level changes that the water level above which prolonged exposure constrains upward coral growth (tn) can be readily defined and that the juxtaposition between TO and the aHLC is relatively precise (e.g. Taylor etal, 1987; see chapter 1). However, in reality an exact w position is difficult to identify as the water surface in reef settings is rarely horizontal or 'still' because of waves and currents, so that generally only an envelope within which ra oscillates can be recognised. Furthermore, though there is ample evidence to suggest that the aHLC and prolonged exposure during low tides are intimately linked (Abe, 1940; Scoffin and Stoddart,

1978; Rodda and Nunn, 1990; see chapter 4), some deviation in the aHLC around a microatoll is inevitable as a consequence of a variability in a range of factors including polyp tolerance to exposure, the effectiveness of protective mucous coatings, and possibly even the structural characteristics of a microatoll's skeleton (Isdale, 1977; Hopley and Isdale, 1977; Hopley, 1982).

Indeed, it was determined in chapter 4 that the aHLC around Porites microatolls on Cocos may inherently vary by around 2cm (outliers excluded) irrespective of habitat. This suggests that the aHLC achieved by microatolls relative to a particular xn may inherently fluctuate within this range, though individual corals may respond more precisely. As a consequence, it is possible that a microatoll growing where m is relatively stable may form a near-horizontal but nevertheless 155 asymmetric upper surface due to small, random undulations attributable to intrinsic aHLC variance that occurs out of phase across separate growth axes (i.e. changes in the aHLC can be attributed to inherent 'noise').

By comparison, in the absence of other limiting factors, it would seem reasonable to assume that a microatoll is more likely to develop a symmetric surface morphology where temporal changes in the critical low water level (TO) are large and/or persistent enough to produce distinctive changes in the aHLC that overwhelm the effects of smaller scale, random aHLC variance (i.e. the signal to noise ratio is high). This assumption is supported by the observation that though the noisiness of the TO signal in free-draining reef flat and lagoonal habitats is high compared to that in interisland passage ponds (vertical range measured against a survey staff for

15 minutes in the field: free-draining reef flats >5cm; free-draining lagoonal habitats ~3cm, interisland passage ponds <2cm), microatolls in free-draining reef flat and lagoonal habitats may also experience relatively large interannual changes in TO and typically develop more pronounced and consistently symmetric surface morphologies (Table 9, Fig. 28). For example, the most negative sea-level anomalies for 1993 and 1994 recorded by the Home Island tide- gauge differed by almost 10cm (Australian Baseline Sea Level Project Monthly Reports) whereas the TO in the interisland passage ponds tend to be relatively invariant as they are fixed by the height and durability of pond sills. A schematic model showing the response of microatoll growth and morphology to the different hydrodynamic conditions experienced in reef flat, lagoonal and interisland passage ponds is presented in Figure 32 and explained below.

According to the model presented in Figure 32, a microatoll's surface symmetry is a function of the relative magnitudes of the ambient noisiness of the TO (represented by the breadth of the stippled envelope) and the amplitude of ©-changes, mediated by coral growth rate and ro-aHLC sensitivity. The ambient noisiness of TO is largely a function of exposure to winds and waves and thus varies between habitats. It is hypothesised that the pronounced and predominantly symmetric surface morphology typical of most reef flat and lagoonal microatolls

(Figures 29 & 30; App. D) reflects the relatively large vertical range of TO fluctuations they 156

Figure 32: Schematic model of the morphologic development of a: a) reef flat; b) lagoonal; and c) interisland passage microatoll as a function of the temporal pattern of TO change; TO noisiness, and inherent random variability in the aHLC-tn juxtaposition. Note: the amplitude of TO fluctuations and the 'noisiness' of theTO positio n (chiefly caused by waves and currents and indicated by the breadth of theTO envelope) decreases from a to c. The left hand side of the figure depicts the surface form developed when the aHLC is constrained at the still low water level, indicated by the lower limit of the TO envelope, whilst on the right hand side of the figure the aHLC may lie anywhere within its range. A constant growth rate of 1cm/yr is assumed in all three habitats. 157 can experience and the strong relative w-change signal this imparts, even though the exact position of the ra is often poorly defined in these settings. As indicated in chapter 1, a microatoll's response to TO rise is mediated by its growth rate (approximately 1 cm/yr for massive

Porites), though only a few days of low water levels are required to lower microatoll rims

(Fishelson, 1973; Scatterday, 1977; Buskirk et al., 1981; Fadlallah et al., 1995). Therefore, where TO rises markedly and at a rate that exceeds coral growth, it is possible for a microatoll's rim to become submerged beneath the confounding influence of waves and currents that may obscure the tn-change signal. In such circumstances a clear and strong signal of relative TO rise is conveyed that the microatoll may respond to. During the ensuing 'catch-up' phase a submerged microatoll will usually become strongly and distinctly upgrown, the most pronounced upgrown morphology developing where the magnitude and persistence of the relative TO rise is large, and the duration of the catch-up phase is prolonged. Interisland passage microatolls that have sunk below a relatively stable TO (presumably following erosion of the substrate or disturbance by fishers) and have then grown back up to it over several years demonstrate this effect, developing prominently raised rims that are highly symmetric across separate growth axes (e.g.

CKI91/10A11: Fig. 30; see App. D). Cross-sections through many reef flat microatolls show that their rims have overgrown the microatoll plane during phases of continued upgrowth, suggesting that the TO lay considerably above the aHLC and did not constrain rim growth during these periods.

In contrast, ponded interisland passage microatolls experience relatively invariant TOS and are largely isolated from waves and currents at low tide. As a result, most have developed near-horizontal surfaces, though precise levelling indicates that small, erratically distributed undulations occur across the tops of many (Fig. 28; Table 9; App. D). Given the relatively fixed nature of ponded TOS and the comparable magnitudes of these undulations and the range of intrinsic aHLC variability it would seem fair to assume that the asymmetric radial microtopography developed by these overtly flat-topped corals is largely a function of the latter. The effects of intrinsic aHLC variability on microatoll microtopography are demonstrated in Figure 32; on the right side of Figure 32 upward coral growth can fluctuate randomly through the TO 'noise 158 envelope', whilst on the left side upward growth (the aHLC) is arrested at its lower limit. The lower limit was chosen for emphasis and it should be noted that it is possible for both sides to variably fluctuate within the TO envelope. The model demonstrates that greater coherence of surface form between separate growth axes is likely where the tu-change signal exerts a dominant influence on rim growth, and the direction, if not the magnitude, of rim growth is repeated over separate growth axes. These conditions are best met where the amplitude of TO changes is large relative to both the ambient noisiness of TO and the range of intrinsic aHLC variability. Where the w-change signal is weak, such as is the case under stable ponded TO conditions, random inherent aHLC variability about even a well defined TO can generate irregular, asymmetric microtopographies over separate growth axes. The fact that lagoonal microatolls were found to be most consistently symmetric offers further corroboration for the model.

Though the ro envelope is broader within the lagoon than in the interisland ponds, and the amplitude of the TO changes smaller than experienced in free-draining reef flat settings, the exceptional conformity of surface form evident between lagoonal microatolls suggests that the ro-change signal to m-noise ratio is greatest in the lagoon.

It is important to emphasise that unless upward coral growth has kept pace with the rate of water-level rise the distinctive surface form of 'highly correlated' microatolls growing in free-draining habitats may nonetheless preserve a rather subdued, incomplete and possibly incorrect record of the relative pattern of local water-level change, and may less accurately record the position of past TOS than microatolls with flatter, less well correlated radial topographic profiles growing where TO changes are smaller. A microatoll that lags beneath TO will not immediately record the magnitude of water-level changes, nor necessarily their direction, as it is possible for a microatoll's rim to be growing up to a falling TO. Conversely, the subdued and irregular surface microtopographies typical of most interisland passage microatolls can provide a good approximation of the long-term position(s) of relatively stable TOS, but are generally less useful for accurately reconstructing the temporal pattern of past water-level changes. It should, however, be noted that gradual changes in the position of a relatively 'stable' TO may be documented by 159

the trend of surface variation, and abrupt changes, particularly TO falls, may manifest as discrete terracettes (see section 2 of chapter 1).

Earlier studies of microatoll form have typically relied on visual comparisons and conclude that open water microatolls develop irregular surface morphologies due to wave action, and that moated microatolls are more likely to the develop symmetric surfaces due to the relative stability of the TO (Scoffin and Stoddart, 1978; Hopley, 1982; Scoffin, 1993). The preceding quantitative analyses clearly contradict this hypothesis. The elevational and temporal precision achieved in this study exceeds that of earlier quantitative descriptions of microatoll surface morphology (Isdale, 1974; Taylor etal., 1987; Woodroffe and McLean, 1990); therefore it is possible that the high proportion (40%) of ponded interisland passage microatolls with asymmetric surface microtopographies across different growth axes is a function of the survey precision and the use of formal statistics to define surface symmetry. The precision with which surface microtopographies were surveyed exceeds the range of intrinsic aHLC variability (see chapter 4), and thus where radial profiles are poorly correlated but fluctuate within this range the statistical determination that they are asymmetric may be unrealistic. However, it must be emphasised that the range of intrinsic aHLC variability for mid-ocean microatolls was unknown prior to this investigation; indeed, its quantification is an important contribution of this research.

Furthermore, because massive Porites growth rates are in the order of 0.5-2cm a year (Isdale,

1981; Patzold, 1984; Aharon, 1991) aHLC responses to changes in TO are subtle, requiring that surface microtopographies are precisely measured and discounting the application of smoothing filters. Though the use of a statistical definition of similarity that accommodates a base level of intrinsic aHLC variability is problematic, an alternative method that allows microtopographic coherence between separate growth axes to be objectively assessed is not readily apparent.

Differences in the results of this and earlier investigations may also reflect the varied environmental settings in which microatolls grow. Most of the earlier work was conducted on the

Great Barrier Reef, a relatively high energy, mesotidal setting, where relatively large, deep 160

(commonly 40-50cm deep -Scoffin and Stoddart, 1978; Hopley, 1982), and highly elevated moats (regularly around 1m above tidal datum - Stoddart et al., 1978) are common. The greater depth and volume of these moats buffers moated microatolls against many of the phenomena that may differentially inhibit microatoll rim growth and cause surface asymmetry on Cocos (i.e. sediment influx, critical water temperatures and salinities, etc). Furthermore, moated microatolls on the Great Barrier Reef often grow up to a level near to MLWN (see chapter 1). Though some of the higher ponded microatolls on Cocos stand at an equivalent tidal elevation, most lie below it and are therefore exposed for shorter periods between tides. As a consequence of their prolonged exposure, it is possible that the aHLC on microatolls from the Great Barrier Reef is more tightly constrained by TO. Alternatively, microatolls on the Great Barrier Reef commonly develop slightly raised outer rims that hold pools over their upper surfaces in which chemical and biophysical erosion occurs (Isdale, 1974; Hopley 1982) which together with the algal encrustation that generally follows may act to 'level-out' microtopography and give the appearance of greater surface symmetry. Though the living margins of some Cocos microatolls are slightly raised above the microatoll plane this morphology is not characteristic (see Figs. 28,

33 & 34; App. D).

It is more difficult to explain why free-draining reef flat and lagoonal microatolls on

Cocos achieve greater symmetry than those described from other reefs, though once again it may reflect the quantitative methods applied in this study. Species effects are another possibility; the microatolls examined in this study were exclusively Porites though many species adopt the microatoll form. For example, 43 different species form microatolls on the Great Barrier

Reef (Rosen, 1978), where Goniastrea is the most common genera forming massive microatolls in windward open water settings (Scoffin and Stoddart, 1978). It may also be that the Porites species and/or genotypes that form microatolls on Cocos have less 'hummocky' skeletal architectures than those reported from other reefs. In view of the numerous biological, environmental, and oceanographic factors that influence the height to which coral will grow, the complexity of their interactions, and spatial variability in their influence, it is likely that this explanation is too simplistic. Before an adequate explanation can be offered further research is 161 required to identify and quantify which environmental and oceanographic parameters critically influence the upper limit coral growth, and to understand the ways in which they do so. The volume and variety of such research is clearly beyond the scope of this thesis.

4. COMPARISON OF THE SURFACE MORPHOLOGY OF DIFFERENT MICROATOLLS FROM THE SAME HABITAT AT EACH SITE.

In this section the surface morphology of separate microatolls growing within each habitat at each sample site are compared (see Figure 10 for sample site locations; App. A for microatoll positions and site descriptions). Site samples were divided into habitat-based subsamples because evidence presented in chapter 4 suggests that the elevation of TO and the temporal pattern of TO change (see previous section; App. D) differs between habitats, presumably in response to differences in environmental and oceanographic conditions away from the reef crest. Several authors have noted that moated microatolls often develop concordant surface morphologies (Isdale, 1974; Hopley and Isdale, 1977; Scoffin and Stoddart,

1978), and have attributed the conformity of surface form to synchronous and uniform coral growth response to periodic changes in the moated TO. However, it must be re-emphasised that conspicuous moating in the sense described from the Great Barrier Reef does not occur in the

Cocos (Keeling) Islands. In an earlier, less extensive study of microatoll morphology on Cocos,

Woodroffe and McLean (1990) demonstrated good agreement between the surfaces of three adjacent interisland passage microatolls, and between two adjacent microatolls from the southern reef flat; however they also noted geographical variation in surface form around the atoll. The aim of the analyses undertaken in this section is to establish whether microatolls growing in a specific habitat at a site preserve a coherent pattern of aHLC change over their surfaces. 162

4.1. Analysis and Results.

To compare the surface morphologies of microatolls that may lie at different depths the 'raw1 aHLC time-series (Fig. 33) were normalised by dividing each raw aHLC datum by the axes mean (i.e. normalisation against the mean) (Fig. 34). Following this procedure each microatoll growth axis has a standardised mean of 1.00 and departures from the mean are expressed as percentages. Normalisation is a standard dendrochronological technique used when comparing time-series (Fritts, 1976; Schweingruber, 1981; Hughes etal., 1982) that has also been applied to coral growth studies (Dodge and Thomson, 1974; Dodge and Vaisnys,

1977; Dodge and Lang, 1983; Hudson etal, 1994; Heiss, 1996). By comparing pairs of normalised time-series the temporal trend of aHLC change, and thus the overall shape of each microatoll's surface is emphasised rather than the relative elevation of the microatoll plane and the magnitude of topographic fluctuations over it. Another advantage of normalisation is that it scales the variance between the time-series being compared so that statistical procedures are not dominated by highly variable samples (Fritts, 1976; Dodge and Lang, 1983).

Simple correlation analysis was used to assess the similarity of pairs of time-series.

Each normalised aHLC time-series pair was correlated individually rather than computing a correlation matrix between all time-series from a given habitat sample because the temporal interval spanned by individual microatolls may vary to the extent that the interval common to the entire sample is only a few years. The r-values calculated for the correlations between the 'raw'

(unnormalised) and normalised time-series' in each habitat sample are presented in Appendix E.

The r-values calculated for the correlations between each of the 167 raw time-series, and those for the 167 normalised time-series are presented in Appendix F. Slight differences in the raw and normalised time-series r-values arise because normalised time-series are normalised against the mean calculated for the entire radial profile, and not just the portion in common with the time- series with which they are being compared. The time-series included in each habitat sample were combined and averaged to form an index time-series that represents an 'average' description of the local pattern of aHLC change/microatoll surface morphology (Figs. 33 & 34)

(Fritts, 1976; Dodge and Thomson, 1974; Dodge and Lang, 1983; Hudson ef al, 1994). It 163 should be noted that because the time-series used to compile an index series may vary in length, portions of the index may be weighted differently, with the index being most representative over the interval common to the most growth radii. The representativeness of the index time-series is examined in more detail in section 5 of this chapter. 164

-10 -10-1 A) Site I: Reef Flat B) Site II: Reef Flat -15- . 92a 94a -15 -20- -20 93a 94b -25- -25 CO 93b Index _ -30- -30 §-35- -35 R1(W) -40- -40 R21(E) -45- -45 Index -50- -50 r T

-10 -10 C) Site HI: Reef Flat r>) SJle IV: Reef Flat -15- -15-1 -20- -20 _,-25- -25- I-30-I -30- 35- T1(S). 73T -35- E11(W) 811a •40- -40- E11(E) 811b T1(SW).- Index -45- -45 E12(S) 812 E12(N) Index T1(SSW). -50-1 -50- 1— 1 1 1 -10-i Interisland Passage -10-,Pi)5it e V- Interisland Passage EII)Site_V: -15- -15- -20- -20- _j-25- -25- i-30- -30- - B5(N) . _ B1JSE) •__- - -B?Lsi -35-" E-35- B5(S) B7(S) — B1(NW) B4(S) -40- B6(SE) B8(SE) -40- B8(NW) — B2(NW) » B4(N) -45- B6(NW) -45- B7(N) Index — B2(SE) —— Index -so-; -50- T T T -10 -10-i Eiii) Site V: Interisland Passage ci»}S|tft V: Interisland Passage -15- -15 -20- -20-1 _,-25- -25 5-30- -30- —r__B9£SJ. -35- |-35- B10b(N) B9(N) B12(SE) B14(S) -40- B11(NW] -40- B10a(S) B12(NW) B14(N] B11(SE) -45- -45- B10a(N) B13(S) Index Index -50- B10b(S) -50- T 1 1 1

-10 -i -10-, vl: F')?t»" Vi; |ntPri"»and Passaoe FiQSJis Interisland Passage -15- -20- -25- -30- " A13a(SE) -35- A15(NW) A13a(NW) A12(SE) "40~ A13b(SE) A16(NW) A12(NW) -45- A13b(NW) A16(SE) Index Index A15(SE) 50- - T T —I " 1 r 1980 1985 1990 1995 1970 1995 1970 1975 Year Year Figure 33i: Raw and derived index time-series for each habitat (correlation co­ efficients between each time-series are presented in Appendix E). 165

-10-. -10- Fiii) Site VI: Interisland Passage Fiv) Site VI: Interisland Passage -15- -15- -20- -20- -25- -25- •30 c_o -30 - •35 § -35- 51a 53. A19(SE) A20b(SE] -40 51b 53) -40- A19(NW) A20b(NW) -45 -45- A20a(SE) A22b(N) Index -50-J -50 • A20a(NW) T 1

-10- -10-] G) Site VII: Lagoonal H) Site VIH: Lagoonal -15- -15- -20- -20- _,-25. -25 W on -30-| Jj -30- -35 133a 134b E -35. B7 89b. •40- -40 Index Index 133b 88 -45- -45 -50 134a -50 89a. T r

-10-1 -10-, .1) Sjte XI: Reef Flat -15- -15 -20- -20- I) Site X: Reef Flat -25- -25- W -30 - _ -30- -35- §35- M13(S) 132a F1(N) F3(N) -40- •40- M13(N) 132b F1(S) F3(S) •45- -45- F2(S) Index 131 — Index F2(N) -50- -50- T ~l J— r -10-1 -10-1 K) S^A XII: Reef Flat 1 ) SK» XIII: Reef Flat -15- -15- -20- -20- _,-25- -25- i-30- •30- §-35- -35- -40- -40- H1(E) -45- H1(W| — H7(W) -45- I2(W) H7(E) — Index -50- -50- T T T T T T

10 Mi) gjlff f¥lvI ln^rl*land Passage "• Mil) Site XIV: Infwrlsland Passage -15-1

-30- -35- D5(SE) D8(E) •40- D5(NW) -45- D7(E) 0B(W| D7(W) Index -50- T 1 1980 1985 1990 1995 1970 1980 1985 1995 1970 1975 Year Year Figure 33ii: Raw and derived index time-series for each habitat (correlation co­ efficients between each time-series are presented in Appendix E). 166

-10-i Mil) Site XIV: Interisland Passage Miii) Site XIV: Lagoonal -15- -20- -25- -30- -35- -40- 61 a 61b 63a 63b -45- 62a Index -50- 62b T T

-10-i -10-i Ni) Site XV: Reef Flat Nii) site XV: Interisland Passage -15- -15- -20- -20- _,-25- -25 i-30. -30- BDOi '«i^*»^««eUBe>ibaii5eJ^5»5« §-35- -35- P7(E) -40- — P16(W) -40- PP5(E) -» PP4(W) PP30(E) -45- -45- PP5(W) P11{W) -o PP4(E) Index -50- PP30(W) -50- PI 6(E) —^— Index r 1 r 1 -10-, -10-i Niii) Site XV: Interisland Passage Miv)Site XV: Lagoonal -15- -20- -25- CO 30- E-35-^ WSS o PP9(EJ 85 -40- PP9(W) 86a. -45- PP13(E) 86b. -50- PP13(W) Index 84 1 -10-, ni) Site XVI: Lanoonal run Sit* XVI: Interisland Passage

Pil) Site XVII: Laaoonal

121 122b

122a Index —T— -r— -I T- 1995 1970 1975 1970 1975 1980 1985 1990 Year Year Figure 33iii: Raw and derived index time- series for each habitat (correlation co- efficients between each time-series are presented in Appendix E). 167

10-, Q) Site XIX; Roef Flat -15- S5(N) 172a -20- S5(S) 172b 171a 173 171b Index Master 2-30- 1-35- -40- -45- -50 -iI I 1 1 1 1 1970 1975 1980 1985 1990 1995 Year

Figure 33iv: Raw and derived index time-series for each habitat (correlation co­ efficients between each time-series are presented in Appendix E). 168

0.7-, D.7-j B) Site II: Reef Flat A) Site I: Reef Flat 0.8-

0.9

1-

1.1 R1(W) 1.2- R21(E) Index 1.3-

v.t- D) Site IV: Reef Flat 08-

0.9- 1- -"^M 1.1- • E11(W) 1.1- T1(S). 73T 811a LI l (LJ 811b 12- T1(SW). - Index 12- T1(SSW). 112(b) 812 1.3- ia- • E12(N) Index

0.7 -i 0.7. E) Site V: Interisland Passage Fii) Site V: Interisland Passage 08- OB-|

0.9- 0.9 x TJ 1 - 1-4 1.1 - 1.1 B5(N) B7(N) B2(SE) — B1(SE) B5(S) B7(S) 12- B3(S) 12 H — B1(NW) B6(SE) B4(S) B8(SE) 1.3- — B2(NW) 13 B6(NW) B4(N) B8(NW) — Index Index 0.7' 0.7-j Fiii) Site V: Interisland Passage fiv) SH*, V: Interisland 08 H 0.8- 0.9-

1-.

1.1- B12(SE) B14(S) B10b(S) 12- B9(N) B10b(N) B12(NW)- B14(N) B13(S) • Index u-J B10a(S) B11(NW) 13- B10a(N) B11(SE) 17-1 0.7 Index Fi) Site VI: Interisland Passage Fji) Site VI: Interisland Passage oe- 0.8- 0.9- 0.9-| x c i.i' A13a(SE) - • A15(SE) 1.1-I A10(N) - A12(SE) A13a(NW) A15(NW) 12- 12- A10(S) - A12(NW) A13b(SE) - * A16(NW) A11(N) — Index 13-J A13b(NW)- • A16(SE) 1.3-J A11(S) ——- Index r T T 1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995 Year Year

Figure 34i: Normalised and derived Index time-series for each habitat (correlation coefficients between each time-series are presented in Appendix E). 169

0.7-, 0.7-, Fiii) Site VI: Interisland Passage^ Fiv) Site VI: Interisland Passage 0.8-

0.9- 0.9- x 1- eg •21 511 53i 1.1- A20WSQ 1-1 - A20bfNW) 51b 12- 1.2- waxNl 52i 52> 1.3- Mm 1.3-1

0.7 0.7-. G. Site VII: Laqoonal. H. Site VIII: Laooonal 0.8- 0.8-

0.9-1 0.9 x -Si—^A^^- * E 1.1 12 12 Ma 134* 13 _ • 89a. 13-1

0.7 0.7-, |) Site X: Reef Flat lt) Site XI: Reef Flat 0.8

0.7-i K) Site XII: Reef Flat 0.8-

0.9- x •o • -

c H7

0.7-i 0.7-, Ml) Site XIV; Interisland Passage mn Site XfV: interisland Passage 0.8- 0.8-

1995 1980 1985 Year Figure 34ii: Normalised and derived index time-series for each habitat (correlation coefficients between each time-series are presented in Appendix E). 170

0.7-, 0 7 Miii. Site XIV: Interisland Passage^ Miv. Site XIV:Laggon 0.8 0.9- 0.9- 1- 1.1- 1.1- D14(E) D1B(E1 1.2- 12- DHftV) D16(W|

1.3- D15|E| 1.3-

0.7-, 0.7-, Ni. Site XV: Reef Flat Nil. Site XV: Interisland Passage

P16(E| 0.7 07-i Niii) Site XV: Interisland Passage Miv. Site XV: Lagoonal 0.8

0-7-1 Ol) Site XVI: Laaoonal Oii. Site XVI: Interisland Passage 0.8-

1S2 cs»

Pll) Site XVII: Lagoonal Pj) Site XVII: Reef Flat 0.8 0.8 0.9-1 0.9 x 1- S i •o 1 1.1- c 123) 1.1- 12- 12- 1.3- 13- —T" n i r— —r— —i r— —I J— T 1975 1980 1965 1990 1995 1970 1975 1980 1985 1990 1995 1970 Year Year Figure 34111: Normalised and derived index time-series for each habitat (correlation coefficients between each time-series are presented in Appendix E). 171

0.7-, Q) Site XIX: Reef Flat 0.8- 0.9-

12- 171a ° 173 13 J 171b ——— index —i 1 1 r 1970 1975 1980 1985 1990 1995 Year Figure 34iv: Normalised and derived index time-series for each habitat (correlation coefficients between each time-series are presented in Appendix E).

The percentages of significantly and positively correlated r-values (P^O.05) calculated between the time-series from each habitat sample (App. E) are shown in Table 10.

The results show that microatolls growing within lagoonal fields are most likely to develop concordant surface morphologies (71.6% of r-values F£0.05), those growing within reef flat fields slightly less so (approximately 66% of r-values P^0.05), whilst microatolls within interisland passage fields are most likely to have different surface morphologies (45% of r-values P^O.05).

These results are in good accord with those of the previous section, and suggest that where individual microatolls develop radially symmetric patterns of surface morphology nearby microatolls are more likely to develop similar surface morphologies. 172

Table 10: Percentages of correlations in each habitat sample that are statistically significant at the 95% confidence level. Number of time- series compared at each habitat in brackets (includes index time-series).

Site Reef Flat Interisland Passage Lagoon

Raw % Normalised % Raw% Normalised % Raw% Normalised %

I 71(7) 71(7)

II 33(3) 33(3) III 100(5) 100(5) IV 39(8) 57(8) V 51(29) 54(29) VI 37(28) 34(28) VII 70(5) 70(5) VIII 80(5) 80(5) X 40(6) 47(6) XI 52(7) 52(7) XII 47(5) 40(5) XIV 59(20) 58(20) 100(7) 100(7) XV 100(5) 100(5) 28(17) 29(17) 40(6) 40(6) XVI 50(4) 50(4) 80(5) 80(5) XVII 100(3) 100(3) 50(4) 50(4) XIX 68(8} 68(8)

Total 65% 66.8% 45% 45% 71.6% 71.6%

ANOVA analyses were also used to quantify the similarity of aHLC time-series from each habitat subsample where more than one coral was sampled. These analyses compared the variation between yearly (normalised) aHLC values to variation in the aHLC values for a given year from separate growth axes. The results of these analyses (Table 11) indicate that the proportion of variation between years is high (and statistically significant: P< 0.05) relative to that derived for a given year from different microatoll growth axes in all habitat subsamples except the interisland passage habitat at site XV. The lack of coherence in the m signal - aHLC response between colonies in this location is clearly evident in Figs. 34iii (Nii & Niii), and reasons for the disparity have been hypothesised in section 3.2. 173

Table 11: ANOVA analyses to examine the proportion of variation between yearly normalised aHLC values relative to within year aHLC variation. Entire time-series were used in analyses except where indicated. (RF: Reef Flat; IIP: Interisland Passage; LAG: Lagoonal). Ho: There is no difference in variation between years as opposed to within years.

Where Ho was accepted at the 5% level tests are italicised.

Site Habitat Source DF SS MS F-Ratio Prob>F

l RF Between Years 24 0.2053 0.0086 9.3642 0.0000 Within Years 133 0.1215 0.0009 Total 157 0.3267

ll RF Between Years 26 0.1414 0.0054 2.4066 0.0038 Within Years 50 0.1130 0.0026 Total 76 0.2544

III RF Between Years 47 0.2667 0.0057 19.0657 0.0000 Within Years 150 0.0446 0.0003 Total 197 0.3114

IV RF Between Years 28 0.0639 0.0023 6.2034 0.0000 Within Years 136 0.0500 0.0004 Total 164 0.1139

V IIP Between Years 34 0.6709 0.0197 10.7862 0.0000 Within Years 611 1.1177 0.0018 Total 645 1.7886

VI IIP Between Years 34 0.2492 0.0073 7.9925 0.0000 Within Years 652 0.5979 0.0009 Total 686 0.8472

VII LAG Between Years 32 0.03107 0.0009 4.1119 0.0000 Within Years 101 0.0239 0.0002 Total 133 0.0549

VIM LAG Between Years 28 0.0856 0.0031 7.0954 0.0000 Within Years 105 0.0452 0.0004 Total 133 0.1308

X RF Between Years 47 0.3906 0.0083 2.7260 0.0000 Within Years 170 0.5183 0.0030 Total 217 0.9089

XI RF Between Years 194 1.2750 0.0066 6.2898 0.0000 Within Years 470 0.4911 0.0010 Total 664 1.7661 XI RF Between Years 42 0.2349 0.0056 6.188 0.0000 (1970-1992) Within Years 188 0.1699 0.0009 Total 230 0.4049 X|| RF Between Years 36 0.0998 0.0028 3.4526 0.0000 Within Years 128 0.1028 0.0008 Total 164 0.2025 XIV IIP LAG Between Years 53 0.8904 0.0168 14.7130 0.0000 Within Years 797 0.9100 0.0011 Total 850 1.800 continued on following page. 1

Table 11: ANOVA Analyses to Examine the Proportion of Variation Between Yearly Normalised aHLC Values Relative to Within Year aHLC Variation (continued). Site Habitat Source DF ss MS F-Ratio Prob>F

XIV UP Between Years 53 0.6782 0.0128 10.3705 0.0000 Within Years 659 0.8131 0.0012 Total 712 1.4914

XIV LAG Between Years 26 0.3180 0.0122 38.8921 0.0000 Within Years 138 0.0434 0.0003 Total 164 0.3614

XV RF, IIP, Between Years 189 3.2219 0.0173 6.9123 0.0000 LAG Within Years 1112 2.7850 0.0025 Total 1301 6.0570

XV RF, IIP, Between Years 44 0.76 0.0173 6.7483 0.0000 (1970-1992) LAG Within Years 799 2.0491 0.0026 Total 843 2.8106

XV RF Between Years 189 4.7300 0.0250 13.1096 0.0000 Within Years 490 0.9354 0.0019 Total 679 5.6655

XV RF Between Years 44 1.3040 0.02964 24.7627 0.0000 (1970-1992) Within Years 180 0.2154 0.0012 Total 224 1.5195

XV IIP Between Years 42 0.0625 0.0015 1.2809 0.1183 Within Years 449 0.5220 0.0012 Total 491 0.5846

XV LAG Between Years 44 1.3041 0.0296 24.7627 0.0000 Within Years 180 0.2154 0.0012 Total 224 1.5195

XVI IIP, LAG Between Years 49 0.0971 0.0019 1.4643 0.0338 Within Years 229 0.3099 0.0014 Total 278 0.4071

XVI IIP Between Years 39 0.2246 0.0058 10.2117 0.0000 Within Years 100 0.0564 0.0006 Total 139 0.2810

XVI LAG Between Years 49 0.1550 0.0032 6.3798 0.0000 Within Years 129 0.0639 0.0005 Total 178 0.2190

XVII RF.LAG Between Years 30 0.2812 0.0094 7.4964 0.0000 Within Years 51 0.0638 0.0013 Total 81 0.3450

XVII RF Between Years 39 0.4519 0.0116 11.1869 0.0000 Within Years 77 0.0796 0.0010 Total 116 0.5316

XVII LAG Between Years 39 0.2933 0.0075 3.4885 0.0001 Within Years 37 0.0798 0.0022 Total 76 0.3730

XIX RF Between Years 60 0.3216 0.0054 5.7172 0.0000 Within Years 247 0.2316 0.0009 Total 307 0.5532 175

4.2. Discussion.

Both the linear correlation and ANOVA analyses demonstrate that the surface morphologies of microatolls included in the majority of habitat subsamples share common features. However, as noted above, the tops of microatolls in lagoonal and reef flat subsamples are usually more alike than those in interisland passage subsamples; in good agreement with the results of section 3 (i.e. where individual microatolls are radially symmetric the tops of separate microatolls are also more likely to be alike). This outcome suggests the possibility that the same factors argued previously to encourage the development of radial symmetry (clear and persistent relative ro-change signal, minimisation of the factors that may obscure this signal - see section 3.2 for a full discussion) also promote similarity of form between adjacent specimens.

The above conclusions are strengthened by comparisons of the aHLC plots from the different habitat-types (Figs. 33 & 34); the tops of microatolls in reef flat and lagoonal habitat subsamples usually lie at similar depths (generally <5cm range) and exhibit prominent, synchronous microtopographic fluctuations, whilst the tops of individual microatolls within interisland passages may be separated by considerable (>10cm) depths and typically possess subdued but erratic surface microtopographies. The conformity of surface form and elevation evident within the majority of reef flat and lagoonal subsamples suggests that upward coral growth in most reef flat and lagoonal habitats is controlled by a single m, and that the ro-change signal and aHLC response is unambiguous. This proposal is consistent with the free-draining character of most reef flat and lagoonal habitats on Cocos, and the more morphologically effective water-level changes incident within them (see discussion in section 3.2 of this chapter).

By contrast, the variable elevations at which interisland passage microatolls occur largely reflects their almost universal occurrence within shallow ponds held at a variety of elevations over an uneven passage bathymetry as the tide recedes (see Fig. 21), where despite differences in absolute elevation ponded tns are relatively calm and level, causing microatolls to develop similar near-horizontal or shallowly-terraced surfaces. The almost parallel alignment of 176

individual aHLC time-series at different depths demonstrates this relationship (Fig. 31).

However, though the near-horizontal upper surfaces of interisland passage microatolls appear very similar, detailed levelling reveals that surface microtopographies can be very different due to the presence of small-magnitude, irregularly distributed undulations (best indicated by the normalised aHLC plots - Fig. 34). In the foregoing section ft was argued that these undulations are the conspicuous morphological manifestation of intrinsic aHLC variability developed under stable ponded w conditions, where by definition the ur-change signal is weak. The aHLC plots of interisland passage habitat subsamples shown in Figs. 33 & 34 support this interpretation, with individual time-series in most habitat subsamples showing similar long-term trends but small- magnitude, often asynchronous fluctuations along their length. The aHLC plots for the lagoonal and reef flat habitats where microatoll surfaces were found to be less alike (i.e. where <50% of correlations were significant - see Table 10) offer further corroboration for this hypothesis. For example, microatolls at site X (47% of correlations between normalised aHLC time-series significant) stand well above their open water counterparts (approximately 20cm) and have rather planar tops (Fig. 33ii), strongly suggesting that they are ponded at low tide, and that the limited congruence between separate colonies reflects the enhanced relative influence of intrinsic aHLC variability under stable ponded w conditions. It is necessary to once again re-emphasise that where microatoll surface morphologies appear similar but were statistically determined to be unalike (poorly correlated), the discord is probably due to the influence of intrinsic aHLC variability and the statistical conclusion is unrealistic. In other words, given the expected range of aHLC variability that will occur under stable ro conditions it is unreasonable to expect coherence of the precision required by the statistical analysis, and it is probable that many ponded microatolls determined to be statistically different are in fact similar within the limits of expected 'natural' variability.

Where disparities of surface form are more striking (e.g. CKI91/10A11 at site VI, Fig.

33i) it is likely that one or more of the numerous factors that can debilitate microatoll rim growth

(eg. sedimentation, bleaching, bioerosion, disturbance, etc -see chapter 1) is/are responsible; interisland passage microatolls are especially vulnerable to many of these factors. Indeed, as 177

separate microatolls within a habitat subsample need not be synchronously affected by these debilitating factors they may be a significant source of morphologic discord, though in the present study microatolls that had been obviously affected were not selected for sampling.

Local variation in pond preservation is another possible cause of disparate surface form within passage habitats. However, ponded TOS in most passages on Cocos appear to be remarkably stable (see chapter 4, section 4.2.1), and because most ponds are shallow the vertical range of

TO change potentially caused by modifications to pond structure is small. Furthermore, the responses of a microatoll to such changes may be obscured by background intrinsic aHLC variability. Therefore it is usually difficult to attribute discrepancies of surface form between separate passage microatolls to spatial variance in pond preservation. Moreover, catastrophic alterations of pond structure capable of distinctively affecting microatoll surface morphology are likely to have occurred during high energy storms whose impacts are unlikely to be spatially restricted. Nevertheless, there is a tendency in some passages for neighbouring microatolls to be more alike than widely separated specimens (e.g. sites XIV and XV). Where such colonies lie at a similar depth it is possible that they grew in the same pond, or in separate ponds at a similar elevation subject to equivalent physical conditions. Where nearby interisland passage microatolls have developed similar surface form but stand at different depths they may grow in separate ponds that have been similarly altered; a plausible explanation given the largely unconsolidated substrate, the similarity of wave and current conditions over small areas, and the likelihood that the most marked morphologic alterations occur during high magnitude storms when changes at all levels are likely.

Woodroffe and McLean (1990) reported a common pattern of terracettes on three adjacent microatolls growing within an interisland passage on Cocos's eastern rim, though from their descriptions it is difficult to firmly establish whether this pattern developed in response to episodic falls in i) the open water level (i.e. the microatolls were unponded), ii) a single ponded TO

(i.e. they grew in a single pond), or iii) equivalent and synchronous falls in the TO within several separate ponds (i.e. they grew in separate ponds of similar character under similar physical conditions). However, given the depth through which their rims were progressively lowered, 178

their location at the lagoonward end of the passage, and the resemblance of their surface form to nearby lagoonal microatolls, it is likely that the terracettes reflect episodic falls in the 'free- draining' lagoonal water level.

5. COMPARISON BETWEEN THE SURFACE MORPHOLOGY OF MICROATOLLS GROWING IN THE SAME HABITAT-TYPE AT DIFFERENT SITES AROUND THE ATOLL.

In this section geographic variation in the surface morphology of microatolls within each habitat-type is examined by comparing the normalised index (or average) time-series developed for the different habitat subsamples at different sites for the period 1970-1992 (Figs.

33 & 34). Normalised time-series were used because they emphasise the pattern of relative changes in the aHLC over a given growth axis or series of growth axes (Fig. 34), and complications due to variations in the magnitude of aHLC fluctuations are minimised. It is assumed in these analyses that the normalised habitat index time-series reliably describe the general pattern of surface morphology developed by microatolls within a particular habitat subsample. In the previous section it was demonstrated that the majority of lagoonal and reef flat microatolls have surface microtopographies composed of prominent, synchronous undulations, and derived index time-series usually expose a common pattern of aHLC-change within a habitat sample (Figs. 33 & 34). In contrast, though the near-horizontal tops of most interisland passage microatolls look alike in the field, a subdued but irregular pattern of microtopographic variation is typically superimposed over their upper surfaces (Figs. 33 & 34), with the result that individual aHLC time-series are often poorly correlated (Table 10). However, though the fine resolution microtopographies of individual interisland passage microatolls may be markedly different (Fig.

33), relief over their upper surfaces is typically small and their overall shape is often rather similar

(Fig. 33). The averaging of individual time-series to form the habitat index series emphasises the common pattern of long-term change in the aHLC position whilst suppressing the influence of shorter-term variability. The assumption that index time-series adequately describe the general history of aHLC-change for most habitat subsamples can therefore be confidently accepted. A 179

habitat-type index master chronology was constructed (see section 4.1 for calculation procedure) which shows the average pattern of aHLC change for each habitat type. The aim of these analyses is to establish whether microatolls growing in a specific habitat type at different geographic locations around the atoll develop similar patterns of surface morphology, and to identify the nature of this pattern where one exists.

5.1. Analysis and Results.

The similarity of the surface morphologies of microatolls growing in like habitats at different sites was assessed by comparing the normalised habitat index time-series developed for each particular habitat subsample (Fig. 35) using linear correlation analysis (Table 12). A relatively low proportion of the habitat index time-series derived for subsamples from each habitat-type were found to be significantly correlated (lagoonal habitats: 52% r-values P^0.05; reef flat: 39% r-values F£0.05; interisland passages: 60% r-values PsO.05), suggesting that the general pattern of surface morphology developed by microatolls in a given habitat-type varies around the atoll. As for the previous section, ANOVA analyses were also used to quantify the similarity of habitat subsample time-series, comparing the amount of variation between yearly aHLC values in an index time-series to the variation within each year between the time-series included in each subsample. These analyses show that the proportion of variation between years is high relative to that within years (lagoonal habitats P = 0.010; reef flat P = 0.081; interisland passages P = 0.007), indicating that the habitat index time-series express common features, with the same rank order as that determined by the linear correlation analyses (i.e. a higher proportion of interisland passage habitat time-series are alike than lagoonal habitat time- series, reef flat time-series are alike least often). The apparent conflict represented by these results (i.e. linear correlation analyses indicate poor to moderate correspondence between the individual habitat index time-series for a given habitat-type whilst ANOVA analyses indicate they share common features) may be partly explained by the nature of the variation between the time- series plotted in Figure 35, which shows that though pairs of time-series from each habitat-type may vary, deviations are generally small-scale and/or short-term. As already discussed (section

3.1), such variations may dominate linear correlation analyses and generate low r-values 180

Table 12: Correlations between index time-series from each habitat-type (bold r-values indicate a positive, significant correlation exists at the 95% level).

Laaoonal Habitats

VII VIII XIV XV XVI XVII Index

VII -

VIM -0.04 -

XIV 0.17 0.86 - XV -0.06 0.58 0.84 . - XVI 0.43 0.21 0.09 -0.35 - XVII -0.33 0.09 0.51 0.66 -0.59

Index -0.07 0.61 0.90 i 0.83 0.40 0.81 _

Reef Flat Habitats

I 1 III IV X XI XII XIII XV XVII XLX Index

I ll 0.21 III 0.61 0.77 - IV 0.14 -0.20 -0.15 X 0.38 -0.47 -0.32 -0.20 - XI 0.54 -0.06 0.00 0.51 -0.32 - XII 0.14 0.76 0.58 -0.66 0.25 -0.71 - XIII 0.71 0.75 0.74 0.19 -0.08 0.57 0.22 - XV 0.00 0.25 0.00 0.58 -0.59 0.39 -0.43 0.47 XVII -0.46 -0.24 -0.64 0.62 -0.49 0.40 -0.64 -0.22 0.67 XIX 0.79 0.26 -0.12 -0.24 0.36 -0.09 -0.13 0.59 -0.39 -0.05 - Index 0.62 0.61 0.09 0.43 -0.21 0.74 -0.27 0.92 0.75 0.42 -0.07

Interisland Passage Habitats

V VI XIV XV XVII Index

V -

VI 0.28 - XIV -0.79 0.11 -

XV -0.10 0.69 0.56 -

XVII 0.91 0.81 -0.66 -0.15

Index 0.53 0.86 0.65 0.79 0.96 _ 181

0.7-, Laooonal index time-series.

xv Habits Index

0.7 -, Reef Flat index time-series

0.7 -, Interisland Pa^e "de* timtyseries 02-

0.9-

XV

Habitat Index 13. —I r— "~1 1990 1970 1975 1980 1985 1995 Year

Figure 35: Index time-series from each habitat sample, with habitat indices also plotted. 182

Table 13: ANOVA analyses to examine the proportion of variation between yearly normalised aHLC values relative to within year aHLC variation of index time-series from each habitat type. Ho: There is no difference in aHLC variation between years as opposed to within years.

Habitat Source DF ss MS F-Ratio Prob>F

REEF FLAT Between Years 55 0.1848 0.0034 1.3042 0.0811 Within Years 393 1.0127 0.0026 Total 448 1.1975

INTERISLAND PASSAGE Between Years 55 0.0771 0.0014 1.7147 0.0069 Within Years 128 0.1046 0.0008 Total 183 0.1816

LAGOON Between Years 5 0.1397 0.0026 1.6047 0.0100 Within Years 162 0.2662 0.0016 Total 215 0.4059

between pairs of time-series that are reasonably similar over most of their lengths. By comparison, the ANOVA procedure examines the variation within and between all of the time- series for each habitat-type, and is less affected by individual differences.

5.2. Discussion.

The above findings indicate that microatolls growing in different interisland passage habitats around the atoll are most likely to develop similar overall patterns of surface morphology, microatolls in different lagoonal habitats are slightly less likely to develop comparable patterns of surface form, and that the general pattern of surface morphology developed by microatolls in different reef flat habitats is most likely to differ. These findings appear sensible in view of the diversity of environmental and hydrodynamic conditions that may affect the TO within each habitat at low tide, and the expected variability between like habitats around the atoll. Regrettably, adequate site and habitat specific environmental, hydrodynamic, and coral growth data are not available to more rigorously test these hypotheses. 183

The TO within interisland passage ponds is largely controlled by the height and permeability of the pond structure, and it has been shown already that microatolls in separate ponds within a particular passage, even those at different elevations, often develop similar surface microtopographies that reflect the comparable preservation histories of the ponds in which they grow. It was speculated earlier in this thesis that microatolls in separate ponds develop conforming surface microtopographies because they usually grow in shallow (5-1 Ocm deep) ponds where the potential for marked TO changes due to changes in the height and permeability of pond sills is low. This contrasts with microatoll research from high energy reef settings with larger tidal ranges where substantive features such as storm ramparts and prominent algal rims moat water over the reef flat at low tide. In these high energy settings settings modifications to pond structure produce microatolls with distinctive surface morphologies (Isdale, 1974; Hopley and Isdale, 1977; Scoffin and Stoddart, 1978; Hopley,

1982). There is no evidence to suggest that the nature of ponding varies between passages on

Cocos, and thus it is not unexpected that ponded interisland passage microatolls have upper surfaces with a similar overall shape irrespective of the particular passage in which they occur.

Most interisland passage habitat index time-series are near horizontal or show a gently and mildly fluctuating general pattern (Fig. 35). Little long-term change over the period

1970 to 1992 is evident, presumably reflecting the relative stability of ponded TOS. Where gradual changes in the aHLC are indicated they may reflect subtle changes in drainage off the passage bed as the tide recedes; falling aHLC levels (e.g site XIV) possibly occurring as water is more quickly shed by widened channels, rising aHLC levels (e.g site V) possibly reflecting a decrease in the permeability of the pond structure as the substrate becomes more consolidated through time. Though the general pattern of surface morphology developed by the majority of microatolls in most interisland passage habitats is near horizontal, there is some evidence to suggest that common pattern of subtle aHLC changes can be detected at several sites (e.g. sites VI, XV, XVII) in which the aHLC steadily fell from the early to mid 1970s until the mid 1980s, after which it gradually rose again until around 1991. 184

In contrast to the relative uniformity of ponded conditions in the various interisland passages around the atoll, the environmental and hydrodynamic conditions experienced in different reef flat habitats around the atoll may vary markedly. For example, some reef flats are directly exposed to the prevailing southeast trade winds (e.g. sites XII, XIII) whilst others are not

(e.g. sites I, II, III, XIX); some reef flats are backed by reef islands (e.g. sites IV, XII, XII) whilst others adjoin interisland passages (e.g. sites X, XIV, XV, XVII); some reef flats are quite narrow (e.g. sites I, II, XII, XII!) whilst others are relatively wide (e.g. sites IV, XI); some reef flats are quite shallow (e.g. sites IV, X) and others quite deep (e.g. sites XIII, XVIII, XIX); and substrates may vary in complexity and type (ranging from algal pavement to sand - see App. A). In addition to the greater diversity of environmental and hydrodynamic conditions experienced in free-draining reef flat habitats, local differences in TO conditions are more likely to manifest conspicuously to produce distinctive patterns of surface morphology (Figs. 32, 33, 34) due to the larger depth range through which water levels may fluctuate and the greater vertical thickness of microatolls in most reef flat settings. This interpretation is supported by the greater microtopographic relief developed over the surfaces of most reef flat microatolls (Figs. 33 & 34), and the questionable agreement between many of the habitat index time-series plotted in Figure 35. However, although the habitat index time-series developed for different reef flat habitats are variable u p until the mid-1970s, most indicate lower aHLC levels until the mid-1980s followed by a steady rise until 1991, with a fall evident at most sites from 1991 to 1992.

A visual comparison of the lagoonal habitat index time-series indicates that with the exception of site XVII the general pattern of surface morphology developed by microatolls at the different lagoonal habitat sites around the atoll is remarkably similar, showing a rising aHLC trend from 1970 to 1975 and a progressive fall in the aHLC over the last 18 years. It is noteworthy that for all three habitat-types the general pattern developed at site XVII differs conspicuously from those of like habitats, suggesting that fishers may disturb microatolls in this passage near to

Home Island. The conformity of microatoll surface form between lagoonal habitats in various parts of the atoll strongly suggests the influence of the same controlling factor(s), presumably a common TO signal. The lagoonal habitat samples are all located on the eastern and south eastern 185 parts of the atoll (Fig. 10) and are very similar in most respects (free-draining, sandy substrate, free-draining at low spring tides -see App. A for more detailed descriptions). However, one important difference between the lagoonal habitat samples is they lie at different elevations; at the south of the lagoon the tops of microatolls lie at approximately -20cm to -25cm MSL (sites VI and VIII) whilst those further north along the eastern atoll rim have tops closer to -30cm MSL (Fig.

33). The similarity of surface form developed by microatolls at these different lagoonal habitats despite their differing elevations is initially somewhat perplexing. However, as outlined earlier, the open water tidal range is attenuated over the shallows in the southern part of the lagoon

(Kench, 1994), and it is possible that the elevational differences reflect this phenomenon.

Kench estimated that the spring tidal range at the south of the lagoon is attenuated by approximately 10cm, in good agreement with the difference in height between microatoll planes in the northeast and south of the lagoon described here. Further tentative support for this hypothesis is provided by geographic variation in the convexity of lagoonal microatoll planes; the surfaces of lagoonal microatolls in the southern part of the lagoon are generally less prominently domed than those along the eastern atoll rim, as would be crudely predicted on the basis of

Kench's (1994) results.

An important finding of the preceding analyses is that the habitat-master series developed for lagoonal and reef flat habitats are very different (Fig. 35). As already discussed, both lagoonal and reef flat habitats appear to be free-draining, open water settings, and thus the development of such divergent patterns of surface morphology is puzzling. If microatoll form is primarily a response to TO in both habitat types, a reasonable assumption given the general consistency of surface form evident within each habitat type, these results appear to indicate that the pattern of TO-change within the lagoon differs from that experienced in free-draining reef flat sites. Spatial variation in hydrodynamic conditions within and without the lagoon may account for some discrepancies between the lagoonal and reef flat rat-signals (see Kench, 1994 for discussion of lagoonal hydrodynamics on Cocos), though this cannot be verified with the data available. This finding clearly has significant implications for workers attempting to use microatoll microtopographies to reconstruct historical sea-level changes, with very different 186

characteristic trends being displayed by microatolls growing in free-draining reef flat and lagoonal

habitats.

It is also noteworthy that a similar general pattern of surface morphology is developed

by both free-draining reef flat and ponded interisland passage microatolls, in which aHLC levels

fell in the early 1980's and subsequently began to rise again toward the 1990s. As discussed in

chapter 3, many microatolls on Cocos were killed or severely debilitated by a mass mortality

event in early 1983 (Blake and Blake, 1983), evidence of which is preserved as a distinctive scar

visible in skeletal cross-sections (see Fig. 14). It is possible that the fall in aHLC levels around

this time was related to the mortality event, and that the rising aHLC trend that commonly follows

this period documents subsequent rim recovery. Anoxic water conditions have been proposed

as the primary cause of coral death during this event (Blake and Blake, 1983; Simpson et al,

1993), though the equivalent level to which microatoll rims fell both around the circumferences

of individual colonies and within many free-draining habitat samples suggests water level control.

This interpretation is problematic for the interisland passage habitats where this pattern was

observed because TO is principally controlled by pond structure. One possible explanation is

that during the 1983 event open water levels beneath their normal level exposing interisland

passage microatolls for a prolonged period, during which ponded water levels were also lowered

due to more complete drainage and/or greater evaporative losses. The fact that this pattern was

not as commonly observed in lagoonal samples may reflect a sampling bias. Many microatolls

within the lagoon are dead, possibly as a result of the 1983 mass mortality event, and it is

possible that the preference for microatolls with complete living rims effectively excludes many

lagoonal microatolls that show evidence of it.

6. SUMMARY

The results presented in this chapter suggest that i) lagoonal microatolls are more likely to develop symmetric upper surfaces than reef flat microatolls, which in turn are more likely be symmetric than interisland passage microatolls; ii) that adjacent lagoonal microatolls are more 187 likely to have similar upper surface morphologies than adjacent reef flat microatolls, that in turn are more likely to be more similar than adjacent interisland passage microatolls; and iii) that differences in surface morphology developed over microatolls growing within the same habitat type occur around the atoll rim, though some common patterns are evident.

One hundred and sixty-eight aHLC time-series ranging from 4-98 years in length were developed for 90 separate microatolls by aligning their upper surfaces as depicted on X-ray plates and fluorescent photographs to MSL, and then measuring the aHLC at the start and approximately halfway through each annual band. Investigations revealed that 65.5% of the time-series developed from separate growth axes of individual microatolls were significantly and positively correlated at the 95% confidence level; 16.1% were positively but not significantly correlated; and 14.9% were negatively correlated. Lagoonal microatolls were the most consistently symmetric across separate growth axes (100%, n=9), reef flat microatolls were the next most likely to be radially symmetric (68.2%, n=22), and interisland passage microatolls the least so (60.7%, n=56). Eighty-five percent of the microatolls that had negatively correlated aHLC time-series across separate growth-axes were interisland passage microatolls.

It was speculated that two main factors caused microatolls to develop asymmetric upper surface morphologies. First, the partial or variable debilitation of the microatoll rim and/or destruction or realignment of the microatoll plane by a factor other than water level.

Sedimentation is the most conspicuous cause of partial rim debilitation on Cocos, though physical damage, bioerosion, adverse water quality and disturbance by humans may also occur.

Second, it was proposed that an ambiguous temporal pattern of relative water level change and subsequent weak control of the aHLC response may promote the development of asymmetric radial microtopographic profiles. Where the directional signal of relative tn-change is not strong, which occurs where TO is relatively stable or the magnitude of fluctuations is small, intrinsic random aHLC variation may dominate the surface morphology of affected microatolls (i.e. changes in the aHLC can be attributed to inherent 'noise'). Microatolls subject to larger 188

magnitude and more persistent relative water-level fluctuations were observed to develop the most symmetric upper surfaces, such as those in free-draining reef flat settings.

Where individual microatolls develop radially symmetric patterns of upper surface morphology nearby microatolls are more likely to develop similar surface morphologies.

Microatolls growing in lagoonal fields are most likely to develop concordant surface morphologies (71.6% of r-values P£0.05); reef flat microatolls are slightly less so (66% of r- values P^O.05); and those growing in interisland passage fields are most likely to develop differing surface morphologies. This outcome suggests that the same factors argued to encourage the development of radial symmetry (clear and persistent ro-change signal, minimisation of the factors that may obscure the signal) also promote similarity of form between adjacent specimens.

A relatively low proportion of habitat-index time-series derived from subsamples for each habitat-type were found to be significantly and positively correlated (lagoonal habitats: 52% of r-values P^O.05; reef flats: 39% of r-values P£0.05; interisland passages 60% of r-values

P£0.05), suggesting that the general pattern of surface morphology developed by microatolls within a given habitat-type varies around the atoll. The findings above were considered sensible in view of the diversity of environmental and hydrodynamic conditions likely to occur between different habitats; conditions in interisland passage ponds are unlikely to differ markedly between sites whereas the nature of lagoonal, and in particular reef flat habitats around the atoll is quite varied. An important finding of this chapter is that the general patterns of surface morphology developed by free-draining lagoonal and reef flat microatolls can be very different.

The development of such divergent morphologies under free-draining conditions is puzzling, though it is likely that spatial variation in hydrodynamic conditions within, inside and outside the lagoon is an important contributing factor. 189

CHAPTER 6

MICROATOLL SURFACE MORPHOLOGY AND ENVIRONMENTAL CHANGE.

1. INTRODUCTION.

A vast and growing literature exists on coral skeletons as proxy recorders of environmental change (see chapter 2); more than 200 scientific papers have been published in this field in the last 20 years (see Taylor er al, 1995 for a recent review). Strong evidence that coral density banding is annual has no doubt provided the impetus for much of this scientific interest, allowing accurate chronologies to be established for chemical and physical changes in a coral's skeleton, and by proxy, the environment in which it grows or grew. Coral skeletal records have been identified as important, high resolution stores of information about a range of dynamic environmental phenomena, and have been used to extend both the temporal and geographic coverage of data where instrumental records are short and/or rare (Dunbar et al,

1994; Gagan etal, 1994; Halley etal, 1994; see chapter 2).

Predictions of sea-level behaviour in response to the enhanced greenhouse effect have suffered from the spatial bias of long tide-gauge records in the northern hemisphere, and the dearth of information from mid-ocean stations. Several researchers have recognised that microatolls may potentially function as natural tide gauges capable of supplementing instrumental sea-level data sets in tropical oceans (Isdale, 1977; Hopley and Isdale, 1977; Taylor et al, 1987; Woodroffe and McLean, 1990), allowing a better understanding of sea-level behaviour in areas where monitoring instruments are lacking. Woodroffe and McLean (1990) established a crude visual correlation between the surface morphology of a microatoll growing on a free-draining reef flat on Abemama, an atoll in the central Pacific, and interannual sea level 190

fluctuations recorded on a nearby tide gauge, that could in turn be linked to El Nino-Southern

Oscillation (ENSO) events.

Microatolls are also valuable indicators of longer-term Holocene sea-level change

(see Table 2, chapter 1). Microatolls are widespread, have a high geological preservation potential, can be radiometrically dated, and compared to other geological sea-level indicators their relationship with sea-level is relatively precise (Davies and Montagionni, 1985). Many researchers consider fossil microatolls the best geological sea-level indicators and they are widely sought as evidence by researchers aiming to establish sea-level histories from the geomorphology of coral reefs (e.g. Hopley, 1982; Chappell etal, 1983; Woodroffe et al, 1994;

Nunn, 1995).

In this chapter the surface morphologies of two particularly long-lived modern microatolls on the Cocos (Keeling) Islands are compared with instrumental records of several environmental variables to investigate whether any significant relationships between temporal changes in the aHLC and these factors can be detected. Following this the use of microatolls as geologic indicators of Holocene sea-level change is evaluated and discussed in light of the nature of microatoll/sea-level relationships revealed by this study.

2. METHODOLOGY.

2.1. Microatoll data.

Geographical differences in microatoll elevation and surface morphology around the

Cocos (Keeling) Islands have been investigated in detail in this thesis, and variation in form has been demonstrated between microatolls growing in reef flat, interisland passage and lagoonal habitats (chapter 5). However, as already discussed, the majority of reef flat microatolls enjoy unobstructed interaction with the open ocean during most low tides, and survey data (1991) indicate that the mean elevation of microatolls growing in free-draining reef flat habitats varies 191

little around the atoll (maximum variation between groups 5.6cm; overall mean rim height of -

31.8cm MSL). Reasonable reproducibility in the surface morphology of reef flat microatolls was also demonstrated both within and between separate fields (see Figs 33, 34 & 35).

Congruence of rim height and surface morphologies within and between separate fields suggests a common environmental control for the upper growth limit for free-draining reef flat corals on Cocos, and they were therefore used in the following analysis.

aHLC time-series constructed from two large massive Porites microatolls from two separate reef flat sites on Cocos were used in this analysis. Microatoll F2 (3.3m in diameter, Fig.

36a) was collected from site XI on the southern atoll rim (App A, FigA11) in 1991 and microatoll

PP30 (2.7m diameter, Fig. 36b) was collected from site XV on the eastern atoll rim in 1992 (App

A, FigA15). Both microatolls were i) radially symmetrical and exhibited concentric upper surface morphologies; ii) firmly attached to the underlying reef flat; and iii) freely connected to the open ocean. Both microatolls were accurately surveyed relative to the datum MSL at the Home Island tide gauge before being sampled and aHLC time-series were constructed as described in section 2.1 of chapter 5. The aHLC time-series compiled from microatolls F2 and PP30 extend beyond the turn of the century and are shown in Figure 37.

Records from both radii of PP30 agree well (r=0.77; P<0.0001), as do those of F2

(r=0.37; PO.0001). Good, statistically significant correlations exist for the years common to all four growth radii (1991-1917), which become highly significant if only the last 40 years of record are considered (1950-1991) (Table 14). The leeward (northern) F2 radius appears planated for the interval 1925 to 1935 and shifts out of phase with the seaward (southern) radius between the late 1930s and 1950. Despite their relative displacement however, both curves are similarly shaped which suggests that difficulties in the detection of distinct annual skeletal density or fluorescent bands through this section of F2's skeleton may be responsible for the phase shift.

The general symmetry of both F2 and PP30 suggests that the tops of neither have been significantly eroded or bored (e.g. by Tridacna), although as discussed above some localised erosion of F2's upper surface may have occurred. There is no evidence of a growth hiatus in 192

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either F2 or PP30. Annual fluorescent and density bands confirm that following the severe

1983 stress event neither coral ceased growing for a prolonged period, and there is no reason to suspect that either coral has previously stopped growing or that the coral growth histories are missing any years. Open water Porites microatolls on Cocos have a mean growth rate of

13±3mm/yr (range: 7-22mm/yr), a rate compatible with the diameters and reconstructed ages of

F2 and PP30, and with published rates for massive Porites (Hopley and Isdale, 1977; Knutson etal, 1972; Patzold, 1984).

The conformity of surface form evident between microatolls F2 and PP30 is striking, particularly the coincidence of broad undulations developed across their upper surfaces. Cross- sections through microatolls F2 and PP30 indicate that the undulations develop when the aHLC falls markedly and then slowly recovers at a rate limited by the coral growth rate. Several synchronous falls and recoveries in the aHLC in both microatolls F2 and PP30 are depicted in

Figure 36 by arrows b, c, d, and e. By counting back annual bands from the living outer rim at the time of sampling (r) it was established that the aHLC in both microatolls fell markedly in 1982-83

(arrow e), 1961 (arrow d), 1940-41 (arrow c), and 1925-26 (arrow b). The growth origin is represented by arrow a. The formation of these broad undulations of 5-10cm amplitude over approximately 20 year cycles is intriguing and will be discussed later in this chapter.

Considerable variability in the elevation and surface morphology of microatolls growing around the Cocos (Keeling) Islands was identified earlier in this thesis (chapters 4 and

5), raising reservations about the usefulness of microatolls as monitors of interannual sea-level change. However, microatolls growing in free-draining reef flat sites on Cocos lie within a relatively narrow depth range (mean aHLC's for open water reef flat habitats range from approximately -30cm to -40cm MSL) and develop similar surface morphologies both within and between separate fields (Figs 33, 34, 35), suggesting that microatolls in these habitats may respond to a common relative sea-level signal. As noted earlier, varying environmental conditions within and between different microatoll fields may promote the development of differing microatoll surface morphologies, and it is important to recognise that microatoll 194

response to water-level change is biologically mediated, and short-lived and/or low magnitude fluctuations are unlikely to be represented (Woodroffe and McLean, 1990). Nevertheless the congruent surface form developed by microatolls F2 and PP30 strongly supports the hypothesis that where microatolls grow in free-draining habitats with unobstructed connections to the open ocean their surface morphologies can coherently document a filtered local history of relative sea-level change.

-20 PP30(W) F2N)

PP30(E) Master Index -25- F2(S)

co -30- E u

-35-

-40 1875 1900 1925 1950 1975 2000 Year

Figure 37. aHLC time-series developed from the surface microtopographies of microatolls F2 and PP30.

Table 14. Correlation coefficients between microatoll time-series (two data points per year) 1991-1917 (normal text) and 1991-1950 (italics). ro.05(1),145=0136; ro.05(1), 82=0181. Coral Record PP30(W) PP30(E) F2(S) F2(N) PP30(W) 0.7686 0.4712 0.2999 PP30(E) 0.7731 0.4327 0.5824 F2(S) 0.6581 0.7234 . 0.3704 195

In addition to the Yaw' time-series, a normalised master index chronology of aHLC change was constructed by producing an index series for each coral growth axis (see section

4.1 previous chapter for procedure) and then averaging the index values for all four growth axes.

Indices generated in this fashion best expose common trends and have been used in several studies of coral growth histories (Dodge and Lang, 1983; Heiss, 1996). The index time-series constructed for microatolls F2 and PP30 is shown in Figure 37, and is compared with the environmental data in the following analyses.

2.2. Environmental Data.

It has been argued through earlier sections of this thesis that water level is the principal environmental control of microatoll surface form, and that in free-draining reef flat settings microatoll morphology responds to changes in sea level. As already pointed out,

Woodroffe and McLean (1990) were satisfied that the surface morphology of a reef flat microatoll on Abemama and gauged sea-level fluctuations were correlated, though they noted that microatolls do not respond to mean sea level itself and may not be sensitive to low magnitude and/or short duration water-level changes. Regrettably, the success with which microatoll surface morphology can be compared with measured changes in sea level on Cocos is limited by the discontinuous tidal records available (see section 5, chapter 1). The longest continuous record of sea-level variation presently available extends from 1986 until 1991, though the installation of the NOAA gauge in 1992 should herald the onset of more reliable tidal monitoring on the atoll. Due to the short and discontinuous nature of the instrumental sea-level record available for Cocos smoothing is difficult and formal statistical analyses (Spearman's rank correlation analyses - Table 15) of the relationship between microatoll surface morphology and sea-level change are not very meaningful. As a result the following discussion is based largely on a visual comparison of the temporal pattern of sea-level change and microatoll surface morphology presented in Figure 38. 196

Several meteorological and climatic variables are known to influence sea level, and thus these variables may be indirectly linked to microatoll surface morphology. Data are available from the Bureau of Meteorology for Cocos from 1952 to 1966 and then 1974 until present on rainfall (annual total), temperature (annual maximum and minimum), wind strength (annual average, annual maximum), surface pressure (annual maximum and minimum), and cloudiness

(annual average, annual total number of days). Annual and June-August averages for the

Southern Oscillation Index (SOI) (corresponding with the lowest phase of the intra-annual sea- level cycle (Perigaud and Delecluse, 1992)) are available from other sources (Rasmussen er al,

1993b). These independent environmental variables were selected for comparison with the master aHLC time-series because data were available and they have been speculated or shown to affect sea level or influence coral growth in other studies (e.g. Yamaguchi, 1975; Swart and

Coleman, 1980; Brown, 1987; Brown and Howard, 1985; Falkowski etal, 1990; Woodroffe and

McLean, 1990, Rasmussen etal, 1993b). It is acknowledged that these variables are unlikely to be the only environmental parameters which affect microatoll growth, and that more comprehensive records of environmental factors would be needed for more detailed study. The data for the eleven available environmental terms were standardised, and then regressed against the standardised aHLC index chronology. Time-series data are commonly serially correlated (confirmed by Durban-Watson statistic of 0.6636) and thus the following regression coefficients should only be regarded only as indicators of relationships rather than definitive measurements. Sea-level data were excluded from the multiple regression due to the limited data set. The aim of this analysis was to explore whether any of the variables were functionally related to the pattern of upper surface morphology developed by microatolls in an open water reef flat setting on Cocos, represented by the coral master index chronology. Results are presented in Table 16. 197

3. RESULTS.

3.1. Microatoll Morphology and Sea-Level.

The statistical correlation between the short sea-level record available for Cocos and the surface morphology developed by free-draining reef flat microatolls is weak (sea level against master index: r2= 0.445; see Table 15 for others), however as foreshadowed above the brevity of the tide-gauge record limits the power of the analyses. Visual comparison suggests that reasonable agreement exists between the growth trend represented by the surface morphologies of microatolls F2 and PP30 and sea level behaviour (Fig. 38), though as indicated by the statistical analyses the relationship is far from perfect (Table 15). Several periods can be recognised where sea level shows a rising trend at the Home Island gauge but the aHLC over microatoll surfaces shows a declining trend, and as sea level fell between 1987 and 1989 microatoll rims continued to grow further upward. Nonetheless, despite such discrepancies, it would appear that the general temporal trends in microatoll surface form and sea-level change are sympathetic. Recent work by Kench (1994) has identified significant differences in tidal elevation and behaviour around the atoll, suggesting that correlations between microatoll morphology and water-level change may be improved if water level was gauged adjacent to the sampled microatolls.

Table 15: Correlations (Spearman's rank correlation co-efficient) between aHLC's reconstructed from microatolls F2 and PP30, the master index chronology, and annual sea-level minima 1986-1991.

F2(s) F2(N) PP30(E) PP30(W) Master Index

Sea-Level Data -0.263 0.459 0.462 0.726 0.667 (annual minimum) 0.069 0.210 0.216 0.526 0.445 R. squared 0.219 P- value 0.966 0.437 0.434 0.165 198

-20 40

i - 30 3 3 CD 20 g < CD 10 g> c CO - 0 CD co --10

-40 i i i i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I•-2 0 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 Year F2S PP30W Coral Master Index F2N PP30E Gauged Sea Level

Figure 38. Microatoll surface morphology (F2, PP30), coral index master, and sea-level fluctuations relative to long-term MSL. Note that coral growth curves are in cm and sea-level fluctuations are in mm. It is the trend that is important in this diagram, not the magnitude of the changes.

3.2 Microatoll Morphology and Environmental Variables.

Multiple regressions of the annual aHLC values of the master index time-series with the independent environmental variables indicate that the total portion of the variability in the upper limit to coral growth that could be explained in terms of those variables was very low (Table

16). Only total annual cloud cover could explain more than ten percent of the variability in the aHLC data (master index), with the remaining variables showing very weak or negligible associations with microatoll surface morphology. 199

Table 16: Multiple regression of selected environmental variables against master index aHLC data (1990-1952 (1966-1973 missing)).

Mult. R R. Square B®ta" Environmental Parameter w Values

Cloud Cover (Oktas Annual Total). 0.5440 0.2959 0.5593

Wind Strength (knots Annual Average). 0.2747 0.0755 0.2850

Southern Oscillation Index (June-August Average). -0.2077 0.0431 -0.1997

Atmospheric Pressure (HPA Minimum). 0.1656 0.0274 0.1728

Southern Oscillation Index Anomaly. 0.0665 0.0044 0.0641

Wind Strength (knots Maximum). 0.0637 0.0040 0.0617

Atmospheric Temperature (Minimum). -0.0560 0.0031 -0.0583

Atmospheric Pressure (HPA Maximum). 0.0509 0.0025 0.0524

Southern Oscillation Index (Annual Average). -0.0480 0.0023 -0.0494

Atmospheric Temperature (Maximum). 0.0200 0.0004 0.0188

Rainfall (Annual Total). -0.0047 0.0000 -0.0048

4. DISCUSSION.

The conformity of surface morphology formed independently by microatolls F2 and

PP30 from separate free-draining reef flat sites on Cocos suggests that their development is

beyond coincidence. However, none of the environmental variables regressed against the

aHLC master index series were significantly correlated with it. Of the twelve variables tested only two could explain more than 10% of the variability in the aHLC data; sea level (44.44%) and

annual total cloud cover (29.59%). Sea level appears to explain a reasonable amount of the variance, though it should be noted that the data set was much shorter than that of the other eleven variables (1991-1986 compared to 1990-1952), and greater confidence in the result would necessitate a longer and more complete sea-level record. It should also be recognised that it is highly unlikely that microatoll morphology would exactly track sea level given the limited sensitivity of these corals to short-term and subtle events, and their biologically limited response rates. Nonetheless, on the basis of available data the pattern of surface morphology developed 200

by open water reef flat microatolls on Cocos appears to be most strongly related to sea level, visual comparisons confirming some correspondence between microatoll surface form and sea- level trends. This is consistent with the hypothesis that corals maintain a microatoll form when their upward growth is constrained by exposure near to the air-water interface (see chapter 1).

The results of the multiple regression analysis suggest that temporal variations in most of the selected environmental variables (listed in Table 16) have little in common with microatoll surface form, a result that may be interpreted as further support for the hypothesis that sea level is the major control of microatoll surface form. Only total annual cloud cover and annual average wind speed show relationships with microatoll surface morphology worthy of mention.

Total annual cloud cover was weakly and positively correlated with microatoll form (r2 =0.2959; beta-value 0.5593) suggesting that the effective limit to upward coral growth was higher during years when cloud cover was also high. In the absence of other mitigating factors desiccation would presumably be slower when conditions are overcast, and this relationship seems reasonable. Average wind speed was similarly found to be weakly and positively correlated with microatoll form (r2 =0.0755; beta-value 0.2850), and it would also seem plausible to assume that increased wave and wavelet activity would occur in windier years, wetting corals sufficiently above TO to raise the effective limit to upward coral growth.

Whilst the low r-values in Table 16 indicate a general lack of correspondence between temporal variations in the tested environmental terms and microatoll morphology, they do not necessarily indicate that an environmental variable never influences microatoll surface form. Rather, they indicate that each of the tested terms is unlikely to continuously influence microatoll surface morphology. It is possible that many environmental variables periodically influence microatoll growth, but due to the transitory nature of this influence they may be poorly correlated with microatoll form. For example, it is possible that high atmospheric pressure suppresses the m and causes microatoll rims to fall in one year, but it is unlikely that this influence would be persistently maintained, and therefore statistical correlations between this variable and surface form for the entire data set would be low. This point is particularly important 201

in view of the fact that the window of potential influence for most of the selected environmental variables is limited to the short period coincident with the critical low tides. A more powerful analysis could ideally be achieved if only data values that coincide with critical low tides were included, however such temporal resolution was impossible with the data sets available.

Synergistic effects are also likely to be important and may obscure the influence of single environmental factors. Empirical studies have identified synergistic effects between light, salinity and temperature that affect coral growth (Coles and Jokiel, 1978), and similar effects between these and other variables, including those for which data were not available, are possible on Cocos. In short, the results of the multiple regression analysis can only be viewed as a general measure of possible relationships between the selected environmental variables and microatoll surface form, and cannot be regarded as definitive indicators. As noted above, problems exist with the temporal resolution of the data relative to the times for which each variable is likely to be critical, and it is possible that the independent environmental variables for which data are available do not include the most influential factors. Sea level has been excluded from the multiple regression analysis (due to the short and discontinuous data set), and the low regression coefficients for the independent environmental variables listed in Table 16 may simply reflect the dominant influence of sea level on microatoll surface morphology. To satisfactorily unravel the possible influence of independent environmental conditions on microatoll surface morphology laboratory and manipulative field experiments are required to identify which environmental parameters are relevant, and longer, more complete and more comprehensive site-specific data sets must be compiled.

5. A CENTURY OF MICROATOLL SURFACE MORPHOLOGY AND SEA-LEVEL CHANGE ON THE COCOS (KEELING) ISLANDS.

Figure 37 illustrates that i) the aHLC on microatolls F2 and PP30 from Cocos has fluctuated in broad cycles of approximately 20 years duration and approximately 5-10cm amplitude, with more erratic and shorter term variability superimposed over these oscillations; 202

and ii) there has been little net change in the water level that limits further upward coral growth on

Cocos over the last century.

It is tempting to speculate a link between the broad undulations over the upper surface of microatolls on Cocos and temporal changes in m associated with either 18.6 year lunar cycles or long-term climatic/oceanographic phenomena, such as El Nino-Southern Oscillation

(ENSO) cycles (Figure 39). As can be seen in Figure 39 there appears to be a tantalising temporal correspondence between peaks in the 18.6 year lunar cycle and topographic highs over the tops of open water reef flat microatolls on Cocos. Microatoll morphology appears to lag slightly behind the lunar cycle in several instances (e.g. 1973 lunar peak), which would be expected given the limitations to microatoll response imposed by coral growth rate. Bryant

(1991) estimates that the influence of 18.6 year lunar cycle on tidal range is only 3.7% of the daily effect, which in microtidal settings like Cocos would be in the order of 5cm. This range is similar to the vertical range of topographic relief developed over the microatoll plane, suggesting the possibility that at a broad scale microatoll surface morphology tracks this lunar tide. Erratic shorter-term variability attributable to other factors such as wind and wave exposure, cloud cover, etc may be expressed as the smaller random variations of surface morphology superimposed over the longer-term pattern of broad undulations.

Woodroffe and McLean (1990) found that the rims of microatolls from the central

Pacific are lowered during ENSO events, and fell markedly during the exceptional 1982-83

ENSO. The rims of many Cocos microatolls were also conspicuously lowered during 1983 (Figs.

30, 33, 34). A tentative link between ENSOs and low water levels is implied by the coincidence of marked depressions across microatoll upper surfaces and strong and very strong El Nihos in

1982-83, 1940-41 and 1925-26 (Quinn and Neal, 1992) (Figs. 36, 39), and less severe falls coincident with more moderate events (e.g. 1987 -Quinn and Neal, 1992). However, several relatively strong El Ninos (e.g. 1972-73, 1957-58) are not represented in the surface morphology of either F2 or PP30, and a number of marked depressions do not correspond with recorded El Nihos(e.g. 1961). There is evidence that throughflow between the western Pacific 203

and Indian Oceans is reduced when southeast trade winds relax during ENSO events resulting in lower sea levels adjacent to the West Australian coast (Allan, 1988), however it is unknown whether these effects are felt as far west as Cocos. It is premature to link ENSOs with microatoll morphology and sea-level change on Cocos in view of the present lack of tide gauge data and the poor understanding of oceanic response to ENSO events in the eastern Indian Ocean.

Nonetheless, Tourre and White (1995) have recently speculated that an El Nino pattern occurs in the central Indian Ocean that is locked in phase with that of the Pacific Ocean, suggesting links between ENSOs and microatoll morphology on Cocos cannot be discounted.

X -20 •a

O) c 5- * K * K --25 CD 1— -t—' CO O co CO --30 z E LU o

--35 y = 0.037x -102.384 r2 = 0.357

El Nino Index Master Index JK Lunar Cycle Peak •-40 T 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 Year

Figure 39. Plot showing relationships between temporal fluctuations in the surface morphology of open water reef flat microatolls on the Cocos (Keeling) Islands (represented by the master index) and 18.6 year lunar tide cycles and ENSO events. * represent peaks of the lunar cycle; ENSO events indexed according to strengths indicated by Quinn et al, (1987) where 2=W or weak events; 4=W/M or near moderate strength; 6=M or moderate strength; 8=S or strong events; 10=VS or very strong events.

If it is accepted that the upper surfaces of microatolls F2 and PP30 (Figs. 36, 37 & 39)

have been constrained at or close to a water level (co) between MLLW and MHLW, then it must

be concluded that there has been little net change in sea level in this area over the last 100

years. A regression of the master index in Figure 39 indicates a net rise of 0.37mm/yr

(r2=0.357), though the record is clearly dominated by the cyclic fluctuations discussed above. 204

The large microatolls examined from Cocos do not support the 10-20cm rise in global sea level over the last century indicated in globally aggregated tide-gauge records (Barnett, 1984;

Gornitz, 1995). Indeed, little, if any, net change in sea level is evident over the last 100 years, and relatively stable sea-levels are indicated if the data for both corals in Figure 37 are adjusted for estimated rates of atoll subsidence (e.g. 0.04mm/yr (Searle, 1994); 0.12mm/yr (Woodroffe ef al, 1991). This conclusion is clearly important in view of predicted greenhouse induced sea- level changes and concerns regarding the inundation of low-lying mid-ocean atolls by rising sea levels.

6. MICROATOLLS AND WATER LEVEL: A SYNTHESIS.

The relationships between microatoll morphology and sea level hypothesised in

Figure 3, are based upon the assumptions that the upper limit to coral growth (the aHLC) is closely coupled to a critical water level (co), and that co fluctuations are accompanied by changes in the aHLC. However, the results of this thesis demonstrate that these assumptions need not hold. The long-held view that corals adopt the microatoll form when their continued upward growth is constrained by subaerial exposure during low tides is still supported; however upward coral growth is not limited by the tidal plane co, per se, but is actually curtailed by the subaerial exposure of polyps beyond a critical length of time during unfavourable conditions such as intense insolation. Water level is the principal component of the suite of factors that constrain co and influence the aHLC, but the extent to which the aHLC and co equate also reflects the cumulative net effects of a range of other environmental and biological factors. The critical limit may vary dynamically according to factors such as the timing of extreme low tides (both diurnally and seasonally) and the coincidence of desiccating winds or cloudy skies (statistical analyses suggest that cloudiness and average wind strength may be important environmental factors).

Furthermore, the co-change signal (the extent to which the aHLC can recognise a co-change trend) may vary through time and with location from a clear and persistent signal to one that is weak, irregular, and dominated by noise. The results presented in chapter 5 suggest that the co- change signal is most clear and strong in free-draining habitats where the amplitude of water- 205

level changes may be relatively large. Though local co-change exerts most influence on microatoll morphology, the variable influence of other factors means that microatoll morphology develops in response to a changing environmental signal dominated by but not solely a reflection of co (ie. the environmental conditions that determine the level of the aHLC may vary from year to year). A change in co in a specific year need not indicate a commensurate change in mean sea level in that same year, though over decades it is likely they would have highly correlated trends.

Despite the above limitations, the similarity of form evident in free-draining reef flat microatolls such as F2 and PP30 suggests that their development is a response to a common environmental signal, and statistical analysis implies that sea-level fluctuations are a major contributor to this signal. Therefore, it would appear that though microatolls may record interannual variations in sea-level less closely than predicted by early models, their surface morphologies can nevertheless reflect trends in the pattern of relative water-level change, particularly those that persist for several years (i.e. if a strong and persistent water-level change signal is imparted).

The conclusion that microatoll surface form is broadly but not precisely comparable with the temporal pattern of gauged sea-level change is consistent with earlier work that concluded that 'microatolls produce substantially more subdued sea level records' than tide- gauges (Woodroffe and McLean, 1990p. 533). The inability of microatolls to respond to short- term sea-level fluctuations is clearly evident in Figure 38, and obviously contributes to the relatively weak correlation (r2= 0.445; P= 0.219) established between gauged sea level and microatoll surface form on Cocos. As already noted (chapter 5, section 4), divergence between sea level and microatoll growth trends may also occur where the rate of to rise exceeds the coral growth rate, and it is possible for a 'lagging' microatoll rim to grow up toward a falling m.

Several authors have noted that a microatoll may require many years to document the total amount of submergence whereas they may respond to emergence within a few days, and 206

thus records of sea-level change reconstructed from microatoll surfaces are likely to be biased toward relative sea-level falls (Taylor er al, 1981; Woodroffe and McLean, 1990). For example, the aHLC around the rims of microatolls on the Cocos (Keeling) Islands was markedly lowered between 1991 and 1992, in some instances by more than 10cm (see Fig. 27). Corals affected by this event would need more than 7 years to recover if they were to grow at the mean growth rate established for Porites microatolls on Cocos (1.35cm/yr). Earlier, similar events are recognisable in cross-sections through larger reef flat microatolls (Figure 40), and suggest that for much of their lifespans living polyps on free-draining microatoll rims lie beneath co. Given their magnitude it would seem reasonable to presume that marked falls in TO are likely to cause the exceptional lowering of the aHLC as witnessed in 1991-1992, suggesting that the juxtaposition between co and aHLC can be most accurately determined for such years. In effect it is in those years that the aHLC is strikingly lowered that the relationship between co and the aHLC can be most precisely determined.

Figure 40- X-radiograph of cross-section through microatoll CKI91/10T1 from site III, West Island showing internal growth structure. Note marked fall in the elevation of the microatoll rim (aHLC) in 1982-83 and a smaller magnitude event in 1987, both followed by periods of 'upgrowth' recovery. 207

Geographic variations in the temporal pattern of water-level change around the atoll may also contribute to the weak correlation determined between the surface form of open water reef flat microatolls and sea level gauged at the Home Island jetty. Open water reef flat and lagoonal microatolls were shown to have developed distinctly different patterns of surface morphology in the previous chapter (section 5), implying that the effective water level that limits upward coral growth, and the pattern of changes in it, differs inside and outside the lagoon.

Furthermore, though reasonable consistency in the elevation of tn on open water reef flats around the atoll was demonstrated in chapter 4, as indicated above, w is not fixed in time or space at a particular tidal level, but may vary under the influence of other environmental factors, the identities and intensities of which may also change through time. In other words, microatoll morphology develops in response to temporal variations in the relative elevation of the effective upper limit to coral growth, which may differ from the temporal pattern of sea-level change recorded at the tide gauge. Consequently, though sea-level appears to be the principal factor governing microatoll surface form, only a broad first-order relationship between microatoll growth and gauged sea level trends is to be expected.

The upper limit to coral growth around the rims of modern microatolls on Cocos is elevated over a depth range of more than 40cm, which represents approximately one third of the spring tidal range. Microatolls at the lower end of this depth range typically occur in free- draining reef flat and lagoonal habitats in which co corresponds to a level midway between MLLW and MHLW. Though the aHLC of the majority of free-draining reef flat microatolls lies within a relatively narrow vertical range between -30cm and -40cm MSL, the aHLCs of interisland passage and lagoonal microatolls span a large depth range. Ponding over the substrate is the principal cause of elevated interisland passage microatolls, while elevated lagoonal microatolls may be ponded or in the south of the lagoon affected by attenuated tidal levels (Kench, 1994).

Earlier workers argued that microatolls growing in free-draining reef flat habitats were unlikely to develop the symmetric surface microtopographies that they observed on moated microatolls because the water-level signal is noisy in these wave and current swept settings. 208

However, microatolls growing on free-draining reef flats on Cocos commonly develop pronounced and symmetric surface microtopographies (chapter 5), and these have been attributed to the occurrence of a strong and persistent water-level change signal capable of imparting a clear directional cue to coral growth irrespective of wave and current 'noise'. It is important to recognise that unless upward coral growth has kept pace with the rate of water-level rise the pronounced symmetry and form of these microatolls will nevertheless preserve a subdued, possibly incomplete and incorrect record of water-level change. In comparison, ponded microatolls are isolated from the open water level at low tide, and co remains fixed by the height and permeability of the pond sills. Thus ponded interisland passage microatolls experience relatively stable water level conditions and rather weak co-change signals, and their surface morphology is dominated by small-scale random fluctuations about co that may reflect the variable influence of factors other than water level as described above. Although the co to which ponded microatolls respond is isolated at most low tides from sea level, it is important to note that their irregular upper surfaces may in many cases more closely approximate the position of their local co than is possible in open water corals.

Clearly, to reconstruct water-level histories from microatoll surface morphologies selection of the appropriate microatolls is critical, and an awareness of the limitations of the record is required. The finding that microatolls growing in different habitats and locations may develop different patterns of upper surface morphology convincingly demonstrates the importance of microatoll selection. Results of this thesis suggest that free-draining reef flat microatolls are the best indicators of longer term water-level trends, with the most accurate instantaneous reconstructions being possible from the aHLC in years when microatoll rims are markedly lowered. Microatoll upper surface morphology preserves a filtered record of longer- term relative water-level change. Though the resolution of this record is lower than that of tidal records from which sea-level trends are normally reconstructed, microatolls nevertheless preserve important records of longer term water level behaviour. Furthermore, as stated in chapter one, microatolls are more widespread than tide gauges through tropical mid-ocean areas, and they comprise a valuable source of retrospective data. 209

The determination that living microatolls on Cocos are not conspicuously moated but grow through a depth range of approximately one third of the spring tidal range has implications for investigations where microatolls have been used to reconstruct past sea levels. As outlined in chapter 1, fossil microatolls have typically been considered finite sea-level indicators, with the tops of the highest open water microatolls being related to MLWS and the tops of moated microatolls being related to MLWN (Scoffin and Stoddart, 1978; Hopley, 1982). Results presented in this thesis suggest that investigations that relate microatoll elevations to tidal datums on distant gauges may introduce considerable error into estimates of absolute elevation.

Though microatoll elevation might approximate the same tidal level (i.e. somewhere between

MLLW and MHLW) in two different locations, the absolute elevation of microatolls in these locations may differ due to local modification of the tidal hydrodynamics by atoll bathymetry. For example, microatolls growing in the southern parts of the lagoon on Cocos may be unmoated, but they may experience a spring low tide level more than 14cm higher than that experienced in the northern part of the lagoon (Kench, 1994). Furthermore, ponded microatolls on Cocos are not easily recognised in the field except at extreme low tides, and it is likely that fossil representatives would be at least equally as difficult to recognise and distinguish from open water corals. As outlined above, the surveyed heights of microatolls that are not conspicuously moated may vary by more than a third of the spring tidal range, which even in a microtidal setting like Cocos represents an elevational difference exceeding 40cm. The underlying substrate may provide some indication of the habitat in which the coral grew, though ponded microatolls may occur in all habitat types. If microatoll surface morphology is well preserved it may better indicate whether a microatoll grew in a free-draining habitat; if the surface is prominently and synchronously undulating then it may be possible to determine that a fossil microatoll grew on an open water reef flat from its internal structure.

In summary, the results of this thesis show that the upper limit to coral growth around the rims of modern microatolls on Cocos can are elevated through a depth range of more than one third of the spring tidal range. This variation can largely be attributed to i) local differences in tidal levels related to modification of tidal hydrodynamics by atoll bathymetry, and ii) subtle 210

ponding as a result of hydrodynamic responses to complex substrate topography and gradients.

Both of these phenomena are difficult to recognise in the modern context without accurate survey, and it is likely that the effects of subtle ponding or locally modified tidal levels would be at least as difficult to identify in the context of fossil microatoll evidence. Based on this evidence it would seem that the notion that fossil microatolls can be used to accurately reconstruct former water level positions should be cautiously accepted, though it must be emphasised that fossil microatolls represent the most precise indicators of water position available in reef settings (see

Table One). Caution is advocated when using the relative elevations of fossil and modern microatolls to establish the magnitude of water-level changes; several centimetres error may be introduced if changes in the nature of the habitat and/or tidal conditions have accompanied changes in sea level.

7. SUMMARY.

Statistical investigations indicate that the surface morphology of open water reef flat microatolls on Cocos is only weakly correlated with the very fragmented temporal pattern of sea- level change recorded at the Home Island tide gauge. However, visual inspection reveals that sea level and microatoll growth trends favourably correspond. Divergence between gauged sea level and microatoll surface form largely reflect physiological constraints on the sensitivity of microatoll growth to more dynamic sea-level fluctuations, though other factors such as geographic variation in exposure to waves and currents, and the elevation of TO relative to the aHLC are also likely to be important. Nevertheless, remarkable correspondence was demonstrated between the upper surface morphologies of two particularly long-lived open water reef flat microatolls growing in separate reef flat sites, implying a common sea-level control.

It was concluded that the upper surface morphologies of open water reef flat microatolls on

Cocos can provide a subdued record of past sea-level change; microatoll upper surfaces preserve evidence of decadal trends in sea-level behaviour but are unlikely to accurately record interannual sea-level fluctuations. 211

Multiple regression analysis indicates that the environmental variables listed in Table

16 had little in common with microatoll surface form, suggesting that none of the chosen variables regularly fluctuated in phase with microatoll upward growth. This result was interpreted as support for the hypothesis that sea level is the most persistent and principal control of microatoll upper surface morphology. It should be noted that meaningful comparisons between microatoll form and the independent environmental variables were hampered by the quality of the available data, and further work to develop a reliable and more comprehensive data base from which to choose environmental variables is essential. There appears to be a tentative association between broader undulations over the upper surfaces of microatolls on Cocos and the 18.6 year lunar cycle, though more work is required to establish a firm association. An important finding of this chapter is that based on a century of microatoll growth, sea level on the

Cocos (Keeling) Islands has undergone little net change, and there is certainly no evidence to support sea level rise over the last 100 years of the magnitude suggested from globally aggregated tide-gauge records.

These data imply that microatolls maintain a long term, filtered relationship with sea level and are capable of tracking sea-level trends at decadal time-scales. Based on this evidence it would appear that future monitoring of microatolls at 5-10 year time-scales would enable trends in future sea level to be monitored. The determination of this close relationship between microatoll morphology and sea level supports the use of fossil microatolls in the reconstruction of Holocene sea-level change, though it is apparent that for greatest accuracy the habitat in which the microatoll grew (reef flat, interisland passage, lagoon) should be differentiated, and the possibility of ponding should be addressed. 212

CHAPTER 7

CONCLUSIONS AND RECOMMENDATIONS.

1. INTRODUCTION.

It has long been recognised that corals adopt the microatoll form near to the sea surface, and that the relationship between morphology and sea-level makes them important sea-level indicators. Microatolls have most often been used as indicators of sea-level change through the

Holocene (e.g. Chappell etal, 1983; Pirazzoli et al, 1988; Woodroffe et al, 1990c), though recently attention has been given to their potential use as recorders of interannual sea level fluctuations (Woodroffe and McLean, 1990). This thesis set out to improve our understanding of microatoll development and its relationship with sea-level. To achieve this aim 282 Porites microatolls from 19 separate sites around the Cocos (Keeling) Islands (Fig. 10) were accurately surveyed to a known datum (MSL at the Home Island tide gauge), and more than 120 microatoll samples were collected systematically to investigate the development of their upper surface morphologies. The Cocos (Keeling) Islands, where microatolls are common, are an appropriate location to undertake such a study because they are located in mid-ocean and are subsequently not affected by continental effects such as lithospheric flexure and river run-off, they are tectonically stable, they experience a microtidal regime, they are rarely affected by cyclones, and logistical support such as boating and workshop facilities is available. A notable difference between the Cocos (Keeling) Islands and the reefs of the Pacific, where most other studies of microatolls have been carried out, is that the conspicuous rubble ramparts and associated moats common in these high energy settings do not occur on Cocos.

This chapter outlines the main findings of the preceding chapters, emphasising those that are most relevant to the use of massive Porites microatolls as indicators of past sea levels. 213

2. CONCLUSIONS.

2.1. Establishing the growth chronologies of Porites microatolls from the

Cocos (Keeling) Islands.

• Microatoll growth chronologies were reconstructed using fluorescent and skeletal density banding patterns visible in vertical slices cut across their diameters and passing through their centres (chapter 3). The annual fluorescent banding pattern is visible when coral slices are viewed under UV light, and was more easily distinguished than skeletal density bands which are revealed when skeletal slices are X-rayed. An annual periodicity for both banding patterns was verified by comparison with annual 813C and 8180 cycles, and by reference to both an Alizarin stain marker introduced into several microatolls during the course of this investigation, and to a distinctive scar formed within the coral skeleton during a stress event in 1983.

• The annual fluorescent banding pattern was ubiquitous within sampled microatolls. The presence of fluorescent bands in mid-ocean microatolls diverges from the widespread view that organic acids seasonally flushed from terrestrial catchments and incorporated into coral skeletons are responsible for their development. It was speculated that fluorescent bands in

Cocos corals develop from marine organics associated with seasonal upwelling because they form soon after the annual sea surface temperature minima; however, more work is required to test this hypothesis. Fluorescent bands were generally more distinct than skeletal density bands and they could be viewed using less elaborate technical facilities in the field.

Fluorescent bands were therefore used to establish the growth chronologies of the majority of microatolls examined in this study.

2.2. Microatoll elevation.

This study has reaffirmed that corals adopt the microatoll form when further upward coral growth is constrained by subaerial exposure during low tides. Few studies have established the elevation of the upper limit to coral growth by accurate survey to a known datum. A primary 214

objective of this thesis was to accurately survey the elevation of the actual height of living coral

(the aHLC) at the top of microatoll rims, and to examine variability in the aHLC at a variety of spatial scales; i) around the rims of individual colonies; ii) variation in the mean aHLC of microatolls growing within individual survey sites; iii) variation in microatoll mean aHLC elevation between survey sites; and iv) temporal variation in the aHLC at sites surveyed in both 1991 and 1992 (see chapter 4).

To achieve this objective microatolls at nineteen separate sites around the atoll rim were surveyed to temporary benchmarks related to the datum MSL at the Home Island jetty. A single operator laser-levelling technique was used (to an accuracy of ± 0.8mm) for these surveys.

i) The elevation of living coral (the aHLC) around the rims of individual microatolls.

• The aHLC around the rims of individual microatolls on Cocos varies by a mean range of 3.3cm

± 1.7cm, which if obvious outliers that are not related to water level are excluded is typically and consistently reduced to approximately 2cm (statistically indistinguishable over entire sample population (see Table 4)), representing an 'intrinsic' level of aHLC variability that constitutes the precision level with which microatolls typically respond to their constricting water levels (w) on Cocos.

• Variation in aHLC around the rims of individual Porites microatolls may be attributed to the localised influence of factors such as sedimentation, bioerosion or physical damage; it may reflect the fact that water levels in reef settings are themselves rarely 'still' and may vary rapidly and over short distances due to wave and current action; or it may develop as a consequence of the characteristically 'bumpy' skeletal geometry of the Porites microatolls themselves. The occurrence of a constant level of variance throughout the entire sample population independent of environmental conditions lends support to speculation that skeletal geometry is an important component of aHLC variation. 215

• The majority of microatolls on Cocos have reasonably horizontal surface planes, despite the variation in aHLC detected around their rims. Statistically significant agreement in the rank order of aHLC heights was detected in only five of the twenty-four survey samples, suggesting that most microatolls on Cocos do not develop surface planes that are systematically orientated relative to the incidence angle of solar radiation or current flows, as described in other studies.

ii) Within-site variation in the elevation of the aHLC of microatolls.

• The mean aHLC values for microatolls surveyed at each site were found to vary by between

3cm (site VIII) and 33.7cm (site XV -1992 survey). The mean elevations of all microatolls were statistically indistinguishable in only three site samples (sites VIII, IX (1991), and XVIII).

• The relevance of statistically discriminated 'elevational' microatoll groups in many site samples was questionable; several groups were separated by less than the 'intrinsic' aHLC, and the statistically significant depth range dividing discrete groups varied according to the relative ranges of aHLC variation around individual microatolls between separate colonies at a site.

• Groups of microatolls with statistically indistinguishable mean aHLCs tend to be clustered within sites, suggesting that within-site variance in microatoll rim height is a function of environmental heterogeneity. It was argued that most of the variability detected within site samples could be attributed to i) the occurrence of multiple constraining water levels (TO) within an site and/or ii) the presence of microatolls with rims that had not yet reached TO.

Multiple constraining water levels (co) may occur where some, or all of the microatolls at a site have been ponded above the open water low tide level, possibly at several different levels.

• Microatolls with upper surfaces conspicuously beneath TO (i.e. they are not water-level limited) contribute to the within-site variability in mean aHLC values detected at several sites. Such microatolls are easily recognised when interspersed with microatolls confined by TO because they lie at noticeably greater depth than neighbouring colonies and they are usually 216

'upgrown'. Microatolls not constrained by TO may occur due to spatial variation in the intensity of environmental factors that limit coral growth, because of colony differences in sensitivity to these environmental variables, or because the colony has been disturbed and lowered.

• The mean rim aHLC for the lowest free-draining reef flat and lagoonal microatoll groups surveyed in 1991 regularly lay between -30 and -35cm MSL (corresponding to an elevation approximately halfway between MHLW and MLLW), suggesting that the open water TO was close to this level. It was speculated that variation in the mean aHLC between these open water microatolls reflects spatial differences in the degree to which hydrodynamic conditions and tidal levels are modified around the atoll.

• Many microatolls on Cocos survive above the open water TO because they are subtly ponded, in a manner analogous to, but far less pronounced than moating as it is normally described from high-energy reefs. Three general schemes of ponding were identified on Cocos; i) ponding through the interisland passages and over the lagoonal sand aprons; ii) ponding over sand deposits at the rear of reef flats backed by steep sandy beaches; iii) and ponding over hard erosional or constructional reef surfaces.

• A general elevational trend was apparent within most sites whereby free-draining reef flat microatolls have the deepest rims and the elevation of interisland passage microatolls gradually increases as the passages shallow toward the lagoon. The elevation of lagoonal microatolls is more variable, and appears dependent on the elevation and microtopography of the substrate over which they grow.

iii) Habitat variation in the elevation of microatoll rims around the atoll.

• A two-way ANOVA indicates that the mean aHLC for each habitat type (reef flat, interisland passage, lagoon) was not consistent around the atoll. The data show that microatolls in separate free-draining reef flat habitats are most likely to be similarly elevated, and indicate considerable diversity exists in the mean aHLC of microatolls growing in different interisland 217

passage and lagoonal habitats. It was speculated that this discord reflects i) the variable elevations of the shallow interisland passage and lagoonal substrates and thus the ponds formed over them, and ii) the variable bathymetric modification of tidal levels around the atoll.

2.3 Microatoll morphology.

The aim of this section was to describe and quantify the patterns of upper surface morphology developed by microatolls, and to assess whether coherent patterns were developed at a range of spatial scales (chapter 5). In order to do this aHLC time series were reconstructed by establishing microatoll growth chronologies using skeletal density and fluorescent bands which were then used to date accurately surveyed microtopographic fluctuations across microatoll upper surfaces.

i) Correspondence of upper surface morphology across separate growth axes.

• The coherence of temporal patterns of aHLC-change developed across separate growth axes on individual microatolls was assessed by comparing 87 pairs of aHLC time-series developed from 72 different microatolls from a range of sites and habitats (e.g. reef flat, interisland passage, lagoonal). Correlation analysis indicated that most of the sampled microatolls had similar surface morphologies across separate growth axes; 65.5% of growth axes pairs were significantly and positively correlated at the 95% confidence level. Lagoonal microatolls were the most consistently symmetric (100%, n = 9) followed by reef flat microatolls (68.2%, n = 18) and then interisland passage microatolls (60%, n = 56). Of the microatolls that had negatively correlated microtopographic profiles across separate growth axes 85% were interisland passage microatolls and 15% were reef flat microatolls.

• aHLC time-series plots indicate that symmetric microatolls tend to have upper surfaces characterised by prominent synchronous 'bumps', and that negative correlations are typically generated by small-scale undulations over essentially flat surfaces that occur out of phase across different growth axes. The latter morphology was considered characteristic of sites 218

with relatively invariant (e.g. ponded) water levels, where intrinsic variability may generate random fluctuations in the absence of a strong water-level change signal, though other factors such as physical damage and sedimentation may also lead to surface asymmetry. A model was developed that shows how microatolls with coherent surface microtopographies over separate growth axes are most likely to form where the amplitude of ©-change is large relative to both the ambient noisiness of the TO surface and the range of intrinsic aHLC variability. Under these circumstances a strong and persistent signal of relative to-level change is imparted to microatolls and is reflected in the direction, if not the magnitude of rim growth over separate growth axes. Such conditions are most likely to occur in free-draining habitats where interannual fluctuations in water level may be relatively large, and are optimised in lagoonal habitats where the ambient noisiness of the TO surface is relatively low.

ii) Correspondence of upper surface morphology developed by separate microatolls at a site.

• Correlation analysis between normalised aHLC time-series was used to compare the pattern of surface morphology developed by different microatolls at each site. Normalisation was carried out so that the shape of each microtopographic profile was emphasised rather than the relative elevation of the microatoll plane, or the magnitude of fluctuations over it. The analyses indicate that lagoonal microatolls are most likely to develop concordant surface morphologies (71.6%), reef flat microatolls slightly less so (66%), whilst interisland passage microatolls with similar microtopographies are least common (45%). These results are consistent with those of the preceding section, and indicate that where individuals develop symmetric surface morphologies under a strong and persistent signal of water-level change then nearby microatolls are also likely to develop similar surface form.

iii) Correspondence of upper surface developed by microatolls growing in similar habitats at separate sites.

• The similarity of surface morphologies developed by microatolls growing in like habitats at different sites was assessed by comparing normalised habitat master index time-series developed for each habitat subsample. The similarity of surface form developed by 219

microatolls in like habitats at different sites was low (reef flat 39% r-values P< 0.05; lagoonal habitats 52% r-values P< 0.05; interisland passage 60% r-values P< 0.05), suggesting that the general pattern of surface morphology developed by microatolls in a given habitat-type varies around the atoll. The divergence of surface form detected between microatolls in like habitats around the atoll was attributed to the physiographic diversity experienced in each habitat type. For example, the conditions in interisland passage ponds at low tide are likely to be more similar around the atoll than those experienced in different reef flat habitats, where a variety of hydrodynamic, sedimentation, and other environmental regimes may be experienced.

• An important finding of this study is that the surface morphologies developed by free- draining lagoonal and reef flat microatolls may indicate very different growth trends; free- draining lagoonal microatolls suggest a gradual fall in TO over the last twenty years, whilst a fluctuating TO may be implied from the surface microtopographies of free-draining reef flat microatolls over the same period. This finding has important implications for studies aiming to interpret sea-level change from microatoll upper surfaces.

2.4. Microatoll morphology and environmental change.

The surface morphology of two particularly long-lived microatolls from separate free- draining reef flat sites on Cocos were compared with instrumental records of several environmental variables to investigate whether any significant relationships exist between microatoll morphology and environmental change (chapter 6).

i) Microatoll morphology: Coherence between century-old microatolls.

• Radial aHLC time-series that extend beyond the turn of the century were reconstructed from two large microatolls growing in separate free-draining reef flat sites (microatolls F2 and

PP30). The conformity of surface form evident between these two microatolls is beyond coincidence, with good, statistically significant correlations existing for the years common to 220

all growth radii (1991-1917), which become highly significant if only the last 40 years are compared. Particularly intriguing is the remarkable temporal coincidence of broad undulations across the upper surfaces of both corals of approximately 5-10cm amplitude over

20 year cycles. The formation of such consonant surface morphologies independently at different sites strongly suggests that both microatolls were responding to a common environmental signal.

ii) Microatoll morphology and interannual sea-level variations.

• The master index time-series developed from the upper surfaces of microatolls F2 and PP30 was only weakly correlated with the short and very fragmented gauged sea-level record available for Cocos (r2 = 0.455; P= 0.219), though the brevity of the sea-level record limits the power of the statistical analyses. It should also be recognised that exact correspondence was not expected because microatoll response to sea-level change is biologically mediated and unlikely to respond to short-term or subtle fluctuations. A visual comparison suggests reasonable agreement exists between the growth trend represented by the surface morphologies of microatolls F2 and PP30 and sea-level change, though several periods can be recognised where a declining sea-level trend may be interpreted from microatoll upper surfaces whilst the Home Island gauge indicated sea level was rising.

iii) Microatoll morphology and other environmental factors.

• A multiple regression of eleven independent environmental variables (sea-level was excluded from this analysis due to the brevity of the record) against the master index time- series developed from microatolls F2 and PP30 suggests that the selected environmental parameters can only account for a small proportion of the microtopographic change developed by these microatolls. Only total annual cloud cover could explain more than ten percent of the variability in the aHLC master index.

• A striking temporal coincidence exists between peaks in the 18.6 year lunar cycle and topographic highs developed over the upper surfaces of open water reef flat microatolls on 221

Cocos, suggesting the possibility that the broad undulations formed across the tops of these microatolls are related to variations in tidal levels associated with this lunar cycle.

• The analyses yield some support for a relationship between ENSO and low water levels, indicated by the widespread mortality or stress experienced by corals on Cocos in March

1983, when the surface morphologies of many corals indicate that water level fell significantly. Marked depressions across microatoll surfaces also coincide with strong and very strong El Nihos in 1940-41 and 1925-26, and less severe falls have occurred during more moderate events (e.g. 1987). There are, however, several relatively strong El Nihos that are not represented in the surface morphology of either microatoll (e.g. 1972-73, 1957-

58), and a number of marked depressions do not correspond with recorded El Nihos (e.g.

1961). Further work is required to test the hypothesis that ENSOs and low water levels on

Cocos are related.

• The upper surfaces of microatolls F2 and PP30 suggest that there has been little net change in sea level in the eastern Indian Ocean over the last century, and they certainly do not support the 10-20cm of global sea-level rise estimated from globally aggregated tide-gauge records. It is, however, necessary to recognise that the tidal curve over the atoll may differ from the open-ocean curve due to hydrodynamic factors, and the wate- level/time function that controls the aHLC on the atoll may not sensitively respond to sea-level changes of 10-

20cm. An average rate of sea-level rise of 0.37mm/yr (r2 = 0.357) can be estimated from a line of best fit through the master index aHLC time-series, though the record is dominated by the cyclic fluctuations discussed above. If this rate of rise is corrected for estimated rates of subsidence of the atoll, it must be concluded that sea level in the eastern Indian Ocean has undergone little net change over the last 100 years.

This thesis has yielded information with significant implications for both the potential use of microatolls as palaeosea-level indicators and as biophysical monitors of recent and future sea- level change. Research for this thesis has reaffirmed the hypothesis that corals adopt the 222

microatoll form when upward coral growth is constrained by prolonged subaerial exposure at low tide, but has established that the elevations at which microatolls growing on the Cocos (Keeling)

Islands are constrained may vary spatially by approximately one third of the open water spring­ tide range due to bathymetric modification of tidal hydrodynamics and/or subtle ponding as a result of hydrodynamic responses to complex substrate topography and gradients. As a result, though the upper surfaces of seemingly free-draining microatolls may be elevated at equivalent positions in the tidal spectrum, their absolute elevations may vary substantially. Though microatolls are undoubtedly the most robust geologic indicator of past sea levels found in reef settings, the occurrence of elevational variability of this magnitude in a microtidal environment dictates that caution is required when using fossil microatolls to precisely reconstruct sea-level histories, particularly on reefs exposed to higher tidal ranges, where the depth through which microatolls may develop is potentially larger. Where microatolls on Cocos could be firmly identified as 'free-draining', their upper surfaces were typically constrained at a level approximately midway between MLLW and MHLW. However, in view of the difficulties experienced in recognising 'free-draining' microatolls in the modern context, it is likely that to conclusively identify whether or not fossil microatolls were free-draining would be extremely difficult.

This research has also shown that it is unlikely that the upper surfaces of massive Porites microatolls will reliably document sea-level histories with an interannual temporal resolution, though carefully selected microatolls in appropriately sensitive habitats may preserve subdued records of longer-term sea-level trends. Spatial and temporal irregularities in the nature of the sea-level signal and microatoll sensitivity to it limit the likelihood that microatoll surface microtopographies will retain a consistent, high temporal resolution record of sea-level change.

However, the exceptional correspondence of broad undulations developed independently by two large microatolls growing in separate open water reef flat sites strongly implies that the upper surfaces of microatolls in these settings are capable of preserving a biologically-filtered record of longer-term (decadal) sea-level change. Though the sea-level histories preserved by microatoll microtopographies are not as detailed as tide-gauge records, they may nevertheless yield 223

valuable information regarding longer sea-level trends, and represent a valuable source of retrospective sea-level data in areas where tide-gauge records are short or lacking.

3. RECOMMENDATIONS FOR FUTURE RESEARCH.

• The fieldwork for this thesis represents an invaluable source of baseline data for the continued monitoring of microatoll development on Cocos. In view of the conclusion that open water reef flat microatolls track decadal sea-level trends, the morphological development of microatolls on Cocos should be re-examined at five to ten year intervals. It is also suggested that if Cocos is affected by any catastrophic events, such as cyclones and mass mortality episodes, that microatoll response and recovery is monitored.

The conclusions that could be drawn regarding the relationships between microatoll development, sea level, and environmental change were limited by the lack of high quality data on relevant environmental variables. Greater insight would be gained if instrumental monitoring of relevant environmental variables including water level, wave height, current strength and direction, sea surface temperature, air temperature, and solar radiation was initiated. Though our understanding of regional and open water sea level and environmental variation will be greatly improved by activities such as the installation of the NOAA tide-gauge on the Home Island jetty, remote sensing of sea-surface topography and temperature by initiatives such as the TOPEX/Poseiden program, and the co-ordination of improved data collection networks through the World Ocean Circulation Experiment (WOCE) and TOGA-

COARE (Coupled Ocean Atmosphere Response Experiment), the results of this thesis indicate that in situ at-a-microatoll monitoring is required to fully test the correlation between microatoll morphology and sea-level/environmental change. Furthermore, at-a-microatoll monitoring will facilitate an improved understanding of how microatolls develop under different environmental conditions and enable the records of past environmental change preserved within the skeletons of longer-lived microatolls to be more reliably interpreted. 224

• Uncertainty as to how microatolls develop under different environmental conditions could also be investigated through controlled, manipulative, aquarium based experiments. Such experiments should systematically investigate what frequencies, intensities and durations of exposure to stress are the critical controls of aHLC variability, and how interactions between different environmental variables heighten or relax the precision with which the aHLC and TO are coupled. A major focus of this investigation should be to identify how variations in the relative water-level change signal and the noisiness of the TO surface affect microatoll surface morphology.

• The detailed investigation of microatoll development could be geographically extended to include sites where longer and more detailed instrumental records of important environmental variables exist, and/or to include sites where microatolls are likely to be particularly sensitive to a specific environmental variable, a standard approach for analogous dendrochronology based environmental reconstructions. Tarawa would be an appropriate initial location to extend this work; a tide-gauge has operated there since the early 1970's and

ENSO related interannual variations in sea level of approximately 40cm amplitude are known to occur.

• Annual fluorescent bands in mid-ocean corals are previously unreported. The development and environmental significance of the annual fluorescent banding patterns found to exist in mid-ocean corals on Cocos could be further investigated. 225

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APPENDIX A

SITE DESCRIPTIONS. APPENDIX A

SITE DESCRIPTIONS.

1. INTRODUCTION.

Descriptions of the geology, geomorphology, climate, hydrodynamics and biota of the

Cocos (Keeling) Islands are contained in a recently published special issue of the Atoll Research

Bulletin (Woodroffe, 1994). The marine habitats of the entire lagoon are mapped in detail by

Williams (1994) and lagoonal sediment fades by Smithers (1994). The location of sites described in this appendix are shown in Figure 10. Sites were mapped from the 1987,1:10 000 scale colour aerial photographs of the atoll (Australian Survey Office). The mapping units depicted in the site diagrams relate to substrate cover and reef geomorphology interpreted from the aerial photographs and verified by extensive field ground truthing. A common legend is used for all site diagrams. Where the substrate/geomorphology at a site was considered to be intermediate between one or more of the classification units 'hybrid' patterns are depicted on the site diagrams.

2. MAP KEY. Algal Pavement: A relatively barren substrate, normally occurring in high energy reef flat areas comprising a cemented surface encrusted by algae. Algal pavement near to the reef crest is often strewn with boulders thrown up from the reef front.

Beachrock: Cemented beach sands that exhibit bedding and usually dip seaward. Linear beachrock outcrops on some reef flats may document former shore positions.

Conglomerate Platform: Cemented coral sticks and boulders deposited as reef flat facies during higher sea level stand approximately 3000-4000 years ago. Conglomerate platform underlies many of the reef islands and commonly outcrops on seaward shorelines.

Coralgal Substrate: Characterised by a firm algal encrusted substrate overlain by localised and generally thin patches of unconsolidated sediment. Scattered living and dead corals may overlie this substrate. 260

Intertidal Coral Bommie: Reef structure rather than individual coral colony, in intertidal areas.

Intertidal Sand: Sands deposited on or against reef island shores that are exposed during most low tides.

Mud: Fine, unconsolidated sediments that are often heavily bioturbated. Normally found in low energy environments in the lee of reef islands.

« i Reef Crest: The most seaward extent of the reef flat. An incipient and discontinuous algal rim marks the reef crest around some areas of the atoll. Spur and groove morphology may occur at the seaward limit of the reef crest. Debris thrown up from the reef front is common in this zone.

Sandy Coralgal Substrate: Similar to coralgal substrate, but with thicker and more widespread deposits of unconsolidated sediment.

Seagrass: The most extensive and densely covered seagrass areas occur over the bioturbated muds and sands lagoonward of the reef islands; seagrass areas on the reef flats tend to be more sparse. Thalassia hemprichii is the dominant species, often in association with the alga, Caulerpa sp. Shingle Bank: Ramparts formed predominantly of coral sticks in high energy areas.

Subtidal Coral Bommie: Submerged reef.

Subtidal Sand: Unconsolidated subtidal sands are the dominant substrate immediately lagoonward of the interisland passages. These deposits are generally rippled, and small mounds indicate the presence of a burrowing biota. Subtidal sands are sparsely covered by seagrass in some areas, and may be exposed during maximum low spring tides. Vegetated Island: Island surface dominated by coconut woodland, with occasional Calophyllum inophyllum and Toumefortia argentea (syn.: Argusia argentea) also occurring as overstorey species. A belt of Scaevola taccada scrub commonly grows on the windward coasts, whilst the smaller islets lagoonward of the larger reef islands are predominantly covered by Pemphis acidula.

Surveyed Microatoll: Sampled Microatoll, not surveyed.

At sites where microatolls surveyed in both 1991 and 1992 have the same final two numbers, those surveyed in 1992 are underlined. 261

3. SITE DESCRIPTIONS.

Site I.

Site I (Figure A1) is located on the reef flat at Trannies Beach on the northern tip of West

Island (Figure 10). The reef flat at site I is approximately 250m wide. A prominent but discontinuous algal rim marks the seaward perimeter of the reef flat behind which a barren algal pavement stretches shoreward for approximately 30m. At the northern end of site I the reef

Figure A1. Site I: Geomorphology, substrate cover and microatoll locations. 262 flat is approximately 0.8m below MSL, and immediately shoreward of the algal pavement a vigorous reef of predominantly branching Acropora extends to within 100m of the beach. A coralgal surface locally overlain by sand deposits of variable thickness occurs between the

Acropora reef and the beach. A shore parallel sand bar confines water in a shallow channel

approximately 10m wide against the shore during low spring tides. Site I is generally well

protected from high energy waves and currents, though during cyclones large waves and strong

currents can wash across the reef flat. Microatolls at this site are mostly massive Porites of

between 40 and 80cm in diameter. Intact living microatolls that are firmly attached to the

substrate are uncommon at this site. Seven microatolls were surveyed in November 1991 (see

Figure A1 for locations), three of which were sampled. Microatoll survey data are provided in

Appendix C. Samples from three microatolls collected in 1989 from this site by Woodroffe and

McLean were also used in this study.

Site II.

Site II (Figure A2) is located on the reef flat at the northern end of West Island, opposite

the road to Rumah Baru (Figure 10). The reef flat at site II is approximately 200m wide. A low

discontinuous algal rim occurs at the seaward limit of the reef flat at site II, behind which an algal

surface thinly covered by loose sediment stretches approximately 30m shoreward. A sandy

coralgal substrate dominates the reef flat for the following 50m, with several moderately large

massive corals present in this zone. None of these corals are presently living, or form

microatolls. Further shoreward undulating sand and gravel shoals predominate, with a hard algal

substrate occasionally outcropping. Sands locally stabilised by seagrass cover the reef flat for

the 40-50m adjacent to the West Island shore. A topographic profile across the reef flat at site I

is presented in Figure 24. Site II is located on the leeward side of the atoll and is normally

protected from persistent high energy waves and currents, though during cyclones large waves

and strong currents can occur. Most of the microatolls at site II are massive Porites between

30cm and 150cm in diameter. Twenty-two microatolls were surveyed at site II in November 1992

(see Figure A2 for locations), two of which were sampled. Microatoll survey data are provided in

Appendix C. Figure A2. Site Ii: Geomorphology, substrate cover and microatoll locations.

Site ill.

Site III (Figure A3) is located on the reef flat seaward of the transmitter towers north of the

West Island settlement (Figure 10). The reef flat at site III is approximately 200m wide and predominantly consists of a sediment free, hard algal substrate. Large waves and strong currents sweep north across the reef flat at site III, and it was too dangerous to survey to the reef crest at this site. The dangerous conditions restricted the search for microatolls to within 100m of the shore; the only complete and undisturbed microatolls encountered were located within

50m of the shore. The reef flat at site III is relatively deep, the algal substrate being submerged 264 by 0.5-1 .Om, even during low spring tides. Three Porites microatolls were surveyed at site III in

November 1991 (see Figure A3 for locations), one of which was sampled. Microatoll survey data

are provided in Appendix C. Another sample (CKI73T) collected at this site in 1989 by

Woodroffe and McLean was also used in this study.

Figure A3. Site III: Geomorphology, substrate cover and microatoll locations.

Site IV- Site IV (Figure A4) is located on the wide reef flat at the southern end of the atoll,

extending approximately 400m from the reef rim to the southern shore of West Island (Fig. 10) 265

Figure A4. Site IV: Geomorphology, substrate cover and microatoll locations.

The reef flat a site IV consists of a series of zones parallel to the reef crest which are principally related to the abatement of wave energy towards the shore. For approximately 100m shoreward of the reef crest the substrate chiefly consists of a hard algal pavement over which relatively large (1.5m to 2.5m diameter) and deep bodied Porites microatolls are scattered amongst coral blocks and boulders thrown up from the reef front. Unconsolidated sediments increasingly dominate the substrate shoreward of this zone. Between approximately 120m and

220m from the reef crest the reef flat typically consists of a relatively thin veneer of loose sediment over a firm, cemented surface. Moderately sized (40-80cm diameter) microatolls are relatively common over this zone, but are often partially covered by sediment and seldom have living coral completely around their rims. Approximately halfway across the reef flat sediment deposits over the cemented substrate become thinner, and a belt of cemented substrate 266 approximately 50m wide is exposed. Between this feature and West Island the reef flat consists of a sandy bed punctuated by shallow pools and subtle longshore channels. Microatolls are less common through this zone, and tend to be confined to pools and channels at low tide. Near to the shore the sandy substrate shallows and is sparsely covered by seagrass. A topographic profile of the reef flat at site IV is presented in Figure 24. Twelve Porites microatolls were surveyed at site IV in October 1991 (see Figure A4 for locations), three of which were sampled.

The survey data for these microatolls are provided in Appendix C. Samples from two microatolls collected at this site in 1989 by Woodroffe and McLean were also used in this study.

Site V.

Site V consists of a 400m2 area of interisland passage habitat located in the Southern

Passage, immediately to the southeast of Pulu Maria (Figure 10). The substrate at site V is primarily composed of undulating coral gravels and sands, swept by unidirectional currents which flow northward into the lagoon.

The bed at site V is only shallowly flooded at low tide, except within the occassional current aligned surge channels incised into the bed. Massive Porites and branching Montipora microatolls are common, although massive microatolls with live coral completely around their periphery are rare. Agglomerations of coalesced microatolls have formed where individual microatolls have grown and fused with neighbouring colonies. Living polyps are generally restricted to the outer edges and in fissures and depressions on the upper surfaces of these coalesced masses. Individual Porites microatolls at site V are typically 30-50cm in diameter.

Fifteen microatolls were surveyed at site V in October 1991 (see Figure A5 for locations), 14 of which were sampled. Microatoll survey data are presented in Appendix C. 267

Figure A5. Site V: Geomorphology, substrate cover and microatoll locations.

Site VI.

Site VI is located in the Southern Passage approximately 200m due north of Pulu Maria

(Figure 10). The substrate at site VI is undulating and principally comprised of variable coral sands and gravels (Smithers, 1990), with localised patches of rhodoliths and their debris also present. Unidirectional currents flow over this site into the lagoon. Massive Porites and branching Montipora microatolls are common, the Porites microatolls typically being around 30-

80cm in diameter. Twenty-two microatolls were surveyed at this site in October 1991 (see

Figure A6 for locations), ten of which were sampled. Microatoll survey data are provided in

Appendix C. Samples from three microatolls collected at this site in 1989 by Woodroffe and

McLean were also used in this study. 268

Figure A6. Site VI: Geomorphology, substrate cover and microatoll locations.

Site VII.

Site VII is a lagoonal site located approximately 200-400m north of the northeastern tip of West Island (Figure 10). Rippled coral sands dominate the substrate at site VII, although locally sediments may become more gravelly and/or be comprised of rhodolith debris (Smithers,

1990).

The predominant current direction over site VII is northward, however currents occasionally slacken off and may be reversed for short periods. Scattered massive Porites and branching Montipora microatolls occur throughout site VII. The Porites microatolls at site VII are typically 30-80cm in diameter, although several larger but degraded specimens also occur.

Eleven microatolls were surveyed at site VII in October 1991 and a further thirteen in 1992, (see

Figure A7 for locations). Microatoll survey data are provided in Appendix C. Three samples were 269 collected at site VII during the course of this thesis, and another three collected in 1989 by Woodroffe and McLean were also used.

Figure A7. Site VII: Geomorphology, substrate cover and microatoll locations.

Site VIII.

Site VIII is an lagoonal site located approximately 500m to the west of Pulu Blan in the

Southern Passage. The substrate at site VIII is relatively homogenous, predominantly consisting of rippled coral sands. Massive Porites microatolls are the dominant coral at this site, most of which are deep bodied and between 100-150cm in diameter. Unfortunately, the distance of these microatolls from land required that the laser level be set up standing in the lagoon, and only two microatolls could be surveyed (in October 1991) before the risk that the electronic survey equipment could be swamped was considered too great and surveying was 270

abandoned. Marker buoys were installed to mark the site for later revisitation, however they

could not be relocated and were presumably washed away. Despite several attempts the site

could not be relocated. The surveyed microatolls were 4m apart, CKI91/10C1 located to the

east of CKI91/10C2. Microatoll survey data are provided in Appendix C. Two samples collected

in 1989 by Woodroffe and McLean from microatolls growing in the vicinity of site VIII were also

used in this study.

Site IX.

Site IX is located in the Southern Passage (Figure 10), extending approximately 120m

from the interisland passage adjacent to Pulu Blan Madar towards the reef crest (Figure A9).

Coral sands and rubbles are the predominant substrate through this area. The site is swept by

unidirectional currents flowing toward the lagoon and is shallowly flooded at low tide, with some

areas being subaerially exposed. Massive Porites and branching Montipora microatolls are

common at this site, although microatolls with live coral completely around their periphery are

rare. Porites microatolls in this area are typically between 20cm and 60cm in diameter, and

usually have tops raised less than 15cm above the bed. Agglomerations of coalesced

microatolls are occasionally observed at site IX where individual microatolls have laterally

extended and fused with neighbouring colonies. Living coral polyps are generally restricted to

the periphery of these agglomerations. Six microatolls were surveyed at site IX in November

1992 (see Figure A9 for locations), one of which was sampled. Survey data for these microatolls

are provided in Appendix C.

Site X.

Site Xis located in the Southern Passage (Figure 10), extending approximately 120m from the interisland passage adjacent to Pulu Klapa Satu towards the reef crest (Figure A9).

Most of the site is shallowly flooded at low tide, with unidirectional currents flowing toward the lagoon during higher tidal stages. The substrate at site X is mainly composed of undulating coral sands and rubbles, with eroded conglomerate platform remnants sporadically outcropping. 271

Figure A9: Site IX and X: Geomorphology, substrate cover and microatoll locations.

Several deeper pools occur towards the reef crest, usually with steep sides and bottoms

0.5-1.0m below the surrounding reef flat. Massive Porites and branching Montipora microatolls are common, although microatolls with live coral completely around their periphery are rare.

Porites microatolls at site X are typically between 20cm and 60cm in diameter, and have tops raised less than 15cm above the bed, however microatolls growing in the deeper pools are deeper bodied and have more complete living coral rims. Agglomerations of coalesced microatolls occur in the shallower parts of site X, forming as microatolls grow outwards and fuse 272 with neighbouring colonies. Living coral polyps are generally restricted to the periphery of these

agglomerations. Thirteen microatolls were surveyed at site X in November 1992 (see Figure A9 for locations), two of which were sampled. Microatoll survey data are provided in Appendix C.

Samples from three microatolls collected from this site by Woodroffe and McLean in 1989 were also used in this study.

Site XI.

Site XI is located on the wide reef flat at the southern end of atoll (Figure 10), extending approximately 550m from the reef crest to the shore of South Island (Figure A10). A series of sub-zones can be distinguished over the breadth of this reef flat site. A hard algal pavement forms the substrate for 80-100m immediately behind the reef crest upon which high energy waves consistently break. Coral blocks and boulders are common in this area, and living corals are scarce. Continuing shoreward an approximately 50m wide zone of coralgal substrate occurs, with thin pockets of unconsolidated sands, gravels and rubbles lying over the cemented reef surface. Relatively deep bodied and often fractured microatolls occur in small numbers through this zone, several of these specimens approaching 2m in diameter. Dead and algal encrusted microatolls are more commonly observed. Further shoreward the substrate becomes increasingly sandy and living microatolls more common, especially through the region around

200m to 400m from the reef crest where several large living microatolls were surveyed.

Continuing toward South Island the reef flat substrate is dominated by sand and microatolls become smaller and less common. Near to South Island seagrass (Thalassia) stabilises the sand deposits and a deeper channel runs parallel to the beach.

A topographic profile of the reef flat at site XI is presented in Figure 24. Eleven Porites microatolls were surveyed at site XI in November 1991, and a further nineteen in October 1992

(see Figure A10 for locations). Microatoll survey data are provided in Appendix C. Five microatolls were sampled at this site, including CKI91/10F2 which had a diameter of 3.3m. Figure A11. Site XI: Geomorphology, substrate cover and microatoll locations.

Site XII.

Site XII is located on the reef flat seaward of South Island, at the south eastern corner of the atoll (Figure 10). Situated on the windward atoll rim this site is exposed to persistent and often high energy wave action. The reef flat at site XII is approximately 130m wide (Figure A12); stretching from a discontinuous low algal rim on its seaward edge over an undulating, predominantly hard coralgal surface towards a conglomerate and beachrock lined shore. Coral blocks and boulders are common across the breadth of the reef flat on this part of the atoll, but 274

are chiefly concentrated near the reef crest. Most of the microatolls at site XII are massive Porites and are located close to the shore.

Figure A12. Site XII: Geomorphology, substrate cover and microatoll locations.

A topographic profile of site XII is presented in Figure A20. Eight Porites microatolls

were surveyed at this relatively inaccessible site in November 1991 (see Figure A12 for

locations), three of which were sampled. Microatoll survey data are provided in Appendix C.

Site XIII.

Site XIII (Figure A13) is located on the reef flat seaward of South Island, approximately

2.8km south of its northern tip (Figure 10). The reef flat at site XIII is narrow, stretching around

50-80m from the reef crest to the conglomerate platform and beachrock outcrops on the shoreline. The reef flat here is affected by high energy waves, with coral blocks and boulders 275

35, 36

0 200 © metres Figure A13. Site XIII: Geomorphology, substrate cover and microatoll locations.

strewn across the reef flat and high onto the island shore. A hard calcareous algae forms the substrate over most of this reef flat, with little unconsolidated sediment deposited over the reef flat surface. Two distinct microatoll populations were surveyed at this site; art elevated population over a shallow reef flat surface at the northern end of the site, and a population of deeper microatolls approximately 50m to the south. Microatolls located on the shallower northern part of the reef flat are most abundant away from the reef crest whilst the deeper microatolls to the south are distributed over the breadth of the reef flat, with larger individuals concentrated near the reef crest. The microatolls encountered at site XIII were exclusively massive Porites. Microatolls included in the shallow population were generally thin bodied and 276

less than 1m in diameter; those in the deeper population were mostly around 1.5-2.0m in diameter with rims elevated up to 50cm above the reef flat. A topographic profile over the reef flat at site XIII is presented in Figure A20. Nine microatolls were surveyed at this site in November

1991 and another seventeen in November 1992 (see Figure A13 for locations). Microatoll survey data are provided in Appendix C. Three microatolls were sampled at site XIII.

Site XIV.

Site XIV is located on the eastern atoll rim (Figure 10), extending from the reef flat between Pulu Labu and Pulu Siput through the interisland passage separating these islands towards the lagoon (Figure A14). The narrow passage between Pulu Siput and Pulu Jambatan is also included in this site. Two main habitat types are traversed; the reef flat and interisland passage. The reef flat at site XIV consists of a cemented surface that is relatively bare of sediment. The seaward limit is marked by a broad algal rim immediately behind which boulders are scattered. Dead and living microatolls are sparsely distributed over this reef flat, those that do occur typically being around 30cm thick and 1-1.3m in diameter. Most appear to be firmly attached to the underlying substrate. The substrate through the interisland passage consists of unconsolidated gravels and sands, becoming more sandy toward the lagoon. A longitudinal bathymetric profile through site XIV is presented in Figure 21 which shows that substrate shallows though the interisland passages before slowly deepening toward the lagoon. Massive

Porites microatolls are scattered through the interisland passage, occurring in clusters rather than being distributed evenly. These microatolls are typically between 30cm and 100cm in diameter, less than 15cm thick, and are often loosely attached to the underlying substrate.

Sixteen microatolls were surveyed at site XIV in October-November 1991 and a further eight one year later (see Figure A14 for locations). Complete data records for these microatolls are provided in Appendix C. A total of 13 microatolls were sampled from this site during the research undertaken for this thesis, and three samples collected during 1989 by Woodroffe and

McLean were also used. 277

Figure A14. Site XIV: Geomorphology, substrate cover and microatoll locations.

Site XV.

Site XV is located on the eastern atollrim (Figur e 10), extending from the reef flat between Pulu Pandan and Pulu Siput through the interisland passage separating these islands towards the lagoon (Figure A15). This site includes reef flat, interisland passage and lagoonal habitat types. The characteristics of the reefflat and interisland passage habitats, and of the distribution and appearance of microatoll at this site are very similar to those described for site

XIV above. The lagoonal habitat at site XV is covered by a sandy substrate, with occasional patches of sparse seagrass. Most of the lagoonal microatolls at site XV grow in the shallow 278 channel which flows westward over the lagoonal sand apron. A longitudinal bathymetric profile through site XV is presented in Figure 21. Nineteen microatolls were surveyed at site XV in

October-November 1991 and a further eighteen in 1992 (see Figure A15 for locations).

Microatoll survey data are provided in Appendix C. Ten microatolls were sampled from site XV during the course of this thesis, and were supplemented by six collected from this area in 1989 by Woodroffe and McLean.

Figure A15. Site XV: Geomorphology, substrate cover and microatoll locations. 279

Site XVI.

Site XVI is located on the lagoonal sand apron west of the interisland passage between

Pulu Pandan and Pulu Wak Banka (Figure 10). The substrate at site XVI is consists of a prograding lobe of coral sand which is stabilised locally by patches of sparse seagrass. Much of this sand apron becomes exposed during low spring tides, although water remains in shallow channels and isolated pools over this surface. Living microatolls are not common through this site and occur in greatest number toward the interisland passage. Most of the microatolls which occur at site XVI are dead or have living coral only sporadically around their rims. The microatolls which occur here are massive Porites and vary in diameter from 30cm to 2m. The larger specimens often have incomplete living coral rims. Five microatolls were surveyed at site XVI in

November 1992, the positions of which are shown in Figure A16. Microatoll survey data are provided in Appendix C. Three microatolls collected from this site by Woodroffe and McLean in

1989 were also used in this study.

Figure A16. Site XVI: Geomorphology, substrate cover and microatoll locations. 280

Site XVII.

Site XVII is located on the eastern atoll rim (Figure 10), extending approximately 500m westward from the reef flat between Home Island and Pulu Ampang through the interisland passage separating these islands and into the lagoon (Figure A17). Three habitat types are included at this site; the oceanward reef flat, the interisland passage and the lagoonal sand apron. The reef flat at site XVII is generally less than 50m wide and strewn with coral boulders and blocks. Conglomerate platform remnants occur on both the reef flat and in the interisland passage. Coral sands and rubbles arranged into a series of deeper channels and linear bars normal to the reef crest form the predominant substrate through the interisland passage. A shallow sand apron extends from the western end of the interisland passage into the lagoon.

Figure A17. Site XVII: Geomorphology, substrate cover and microatoll locations.

A longitudinal bathymetric profile through site XVII is presented in Figure 21. Living and dead massive Porites microatolls are scattered throughout site XVII, being most common in the 281 interisland passage and typically around 50-100cm in diameter. No living microatolls were fou nd on the reef flat. Nine microatolls were surveyed at site XVII in November 1991 (see Figure A17 for locations). Microatoll survey data are provided in Appendix C. Two microatolls collected from this site by Woodroffe and McLean in 1989 were also used in this study.

Site XVIII.

Site XVIII is located on the reef flat at the northern end of Direction Island (Figure 10).

The reef flat at site XVIII is approximately 100-200m wide with a firm algal surface forming the substrate at the outer edge (Figure A18). Largely unconsolidated and undulating deposits of rubble and sand veneer the reef flat between the reef crest and the shore, with several deeper pools interspersed.

Figure A18. Site XVIII: Geomorphology, substrate cover and microatoll locations. 282

The steep rubble beach at the rear of this reef flat suggests that much of this rubble is being transported shoreward; the slope and textural composition of the beach physically documenting the exposure of this site to high energy waves. Massive Porites microatolls and bommies are concentrated at the outer edge of this reef flat, with live coral scarce over the inner reef flat. The outer reef flat at site XVIII is relatively deep; microatoll tops averaging around 50cm above the bed. The microatolls at site XVIII are usually around 1.5-2m in diameter. Unfortunately only two microatolls could be surveyed at site XVIII due to dangerous sea conditions, neither of which could be successfully sampled. The positions of the two surveyed microatolls are shown in Figure A18. Microatoll survey data are provided in Appendix C.

Site XIX.

Site XIX is located on the reef flat adjacent to Horsburgh Island's western coast (Figure

10). The reef flat at site XIX is approximately 300m wide and consists of a hard coralgal substrate overtopped with coral sand and gravel deposits which become thicker towards the shore (Figure

A19). A low, discontinuous algal rim occurs at the outer edge of this reef flat, however water is not moated behind the reef crest by this feature. The 150m of reef flat immediately behind the reef crest is dominated by a distinctly algal substrate upon which deep bodied massive corals grow. Microatolls are restricted to the sandy, comparatively shallow reef flat area which extends around 150m from the shore. Most of the microatolls observed at this site are massive Porites, however branching microatolls also occur. The Porites microatolls are typically from 50-100cm in diameter and occur in clusters rather than being evenly distributed over the reef flat. Completely dead microatolls of a similar size are not uncommon, especially towards the seaward extent of their range. Fourteen microatolls were surveyed at this site during November 1992 (see Figure

A19 for locations), two of which were sampled. Microatoll survey data are provided in Appendix

C. Samples from three microatolls collected in 1989 from this site by Woodroffe and McLean were also used in this study. 283

Figure A19. Site XVIII: Geomorphology, substrate cover and microatoll locations. 284

Topographic profiles of sites II, IV, XI, XIV, XV and XVII have been presented in Figures

21 and 24. Additional profiles for sites II, XII and XIII are presented below.

Topographic Profile: Site III.

4 w CO Rubble/Shingle E Beach c Rubble/Shingle o Reef Crest '15 Algal Pavement > LU T ^ r i i 80 —r" 60 100 120 140 160 180 200 Distance (m) Topographic Profile: Site XII.

2- W Ui Conglomerate Platform Reef Crest

g Boulders I UJ South Island 1 1 1 r" i^ —[— —i 1— —r 0 10 20 30 40 50 60 70 80 90 100 110 120 Distance (m) Topographic Profile: Site XIII.

2- w Conglomerate Platform CO Reef Crest 1- c Beach - South Island o > ill

"T -T" -| 1 1 1 10 20 30 40 50 60 70 80 90 100 110 120 Distance (m)

Figure A20. Longitudinal topographic surveys through sites III, XII, and XIII. See also Figures 21 and 24 for topographic profiles of selected other sites. 285

APPENDIX B

COCOS (KEELING) ISLANDS CLIMATIC SUMMARY. 286

APPENDIX B

COCOS (KEELING) ISLANDS CLIMATIC SUMMARY.

1. CLIMATIC AVERAGES

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Ann.

Mean daily max. 29.6 29.9 29.9 29.6 292 28.5 28.0 28.0 282 28.7 29.0 29.4 29.0 av. temp. (°C) + [32.1] [32.4] [32.1] [322] [31.4] [30.71 [299] [29.8] [30.0] [30.6] [31.1] [322] [highest record] 2 Mean daily min. 24.4 24.8 24.9 24.9 24.7 4.1 23.6 235 23.4 233 24.1 24.3 24J?av. temp. (°C) + [21.1] [20.1] [19.8] [19.6] [19.4] [20.1] [20.4] [18.3] [19.0] [20.6] [19.3] [21.1] [lowest record] Av. number of 11 13 15 12 5 1 0 0 0 13 7 days>30°C Mean 9a.m. rel. 75 75 77 78 78 77 78 76 73 72 73 73 75.4 av. humidity (%) 251.1 184.9 2022 201.9 143.1 Av. rainfall (mm) 205.9 162.6 2352 85.1 74.1 103.1 128.0 19772 (total) Av. no rain days 11 15 17 18 16 17 18 16 12 10 10 11 Av. no. days 1 1 2 2 10 0 0 0 0 0 0 thunder heard Av. no. days. 2 2 3 3 5 6 10 10 8 5 4 2 wind >35km/hr Mean 9a.m. sea- 10109 10103 1011.1 1011.4 1011.7 10127 10131 10137 10144 10142 10129 10119 level air pressure (mb) (source: Bureau of Meteorology records; rainfall data since 1901; other variables since 1952).

2. PERCENTAGE OCCURRENCE OF WIND DIRECTION (9 a.m.). Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Calm 7 8 6 4 2 1 2 2 - - 1 2 N 4 3 3 1 1 1 1 1 - 1 1 1 NE 4 3 5 4 4 4 5 3 3 3 3 4 E 19 17 25 44 34 36 37 48 47 41 31 20 SE 45 39 41 41 51 49 49 42 47 53 59 61 S 15 15 12 4 7 7 5 4 2 2 4 10 SW 4 5 4 1 1 - 1 - - - 1 - W 2 5 3 1 ------NW 1 4 2 1 1 . . - . - - -

(source: Bureau of Meteorology).

3. MAXIMUM WIND GUSTS AND DIRECTION (Knots). Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Speed 95 66 55 55 54 60 47 70 47 47 78 51 Dir. W SW NNW E W ENE ESE W SE ESE NNW S

(source: Bureau of Meteorology). 287

APPENDIX C

MICROATOLL SURVEY DATA. 288

APPENDIX C

MICROATOLL SURVEY DATA.

The survey data (heights) in this appendix are in millimetres relative to the datum referred to as Mean Sea Level at the Home Island tide gauge (0.7m above chart datum).

Column headings N, NE, E, SE, S, SW, W and NW indicate the compass bearing relative to the centre of the microatoll plane (C) along which the heights were surveyed (see chapter 4, section 2.2 for surveying procedure). The height of living coral (aHLC) at a microatoll's outer rim is listed under the first column for each compass bearing (e.g. the height listed in column N is the height of the living coral rim at the northern perimeter of the colony). Where heights are listed in subsequent columns with the same bearing prefix (e.g. N2, N3, N4, etc) they relate to prominent topographic fluctuations over the the microatoll plane on this bearing, numbered from the rim toward the microatoll's centre. These heights allowed the upper surfaces of slices derived from sampled corals and used to develop aHLC time-series to be aligned with greater precision relative to the MSL datum. A tick in the 'Coll.' (collected) column indicates that a sample has been collected from the microatoll. The benchmark to which the microatoll was surveyed is listed in the BM column. The year in which a microatoll was surveyed is indicated by the first two digits of its label, microatoll 91/10A10, for example, was surveyed and collected in

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1 1 1 I 1 1 1 1 i ill i 1 1 1 1 1 i S S §|S S § i S |9 —I— III—1—1—1—1—1 1 1 1 1 ...... 296

s- -4- >' - ,, 3 ^ S ? ? H ! 1 ; M ? ? ? l ? H H 38338S??S88833?33 1 s =

1 „ -t- I 8 1 9. - - • 1 . i „ 2 3S8SS g g s 5 8 8 3 § S 8 8 I s s 8§?3 « 8 « 8 8 3 3 6 3 I * 8383 38 i 8 8 2 * . - . _ i s s 8 8 15 8 a s s - 8 8 -4-8' - - -h -27 0 -341 •246

S ...... -27 3 E -28 1 -24 1 -22 8 -21 8 W 2 W 2 19 3 23 3 18 6 •42 4 •3 S •33 . -34 1 -2 8 -22 4 -JB 7 -27 8 -24 0 -24 2 •24 2 -32 6 15 4 •41 4 -39 8 •30 2 -26 4 -26 0 -12 3 -17 8 -17 8 -33 8 -34 S -28 5 -33 8 -24 4 •25 8 -20 4 W W i 1 i i i T 1 T 15 i ' i HsS; 5 8 i i i 225888 I ! ; 9 s 2 c sggsgssssESSsasBgsss sae

s US 8 8 8 8 3 8 8 s s

a o o « S c^i sl a cs , 83388 5 88S88 Sis I a §838i588*8g 8 8*888 !i 5 ? H E H s 5 s S § ? 8 3 s 88SS?

8 i S $ 3 9! S § 3 S8S88 88 "T~ S §1

# tn ^833 3 8 8 8 a 8' 3 § 3' 2 a 3 RSISSS SB 9! 3 •rf c? m co ey — c? c? 33888 8 8 UsN 88 a ?? J3| s« 3* »i;*Sg«?|H«i5?^!J»ssl!si1?S? a 8 S * 8 E

1 1 ' Is » S 5 s} a a ? 8 3 8 a a § 8 8 ? 8 8 ;» 8 S 8 S 3 SR88S»8E 88 883383888 ESSE 38838 8 5 5 8 2 8 3 2 2 8 8 i [ j? 2 2 9 8 1 8 s 2 8 8 §.88558 88 S § 3 S 5 s 8 8 8 55S8J8 3 g| 5 ? H J « s E 5 6 a S ? 5 9 3 ? 2 8 B i §l§§||§§§8§l§§§§§§g|s1§§l|l||i§§ig§§g| § § § § § i 1 1 1 1 i g,5SSiSifSSlIIfiiIs£lIlliiiililliiiIiiiil : a,liiii ; 1 I 1 i i III 1 1 I 1 1 III I 1 i I lis I 5 1 a § § i §l§ £ g §|g|g § a i i i 19 llaSISia 9 297 siiiliilli a§§ • § § § 888S5888588 3 — 3 3

i i i i i i i i i 3 8 i i i 3 88838? 33 888?S83888§?99 i i i £ S S * * s 8 E 8 i 5 5 -31 0 -29 6 -37 9 -25 9 W 3 w a W 3 8 8 S 88 5 3 | s 3Sgs883§# s 8s s 38l3?8333' ' 3 = ^sJ3 3 i i i 8 i 8 si i i i 8 i i i 8 e B I ' £ I ' ' 388S8533!? 83 833?833833?ff? s SB a as a s s X S3 ey c? z 8 3 a 3 3 z 8 8 8 8 3 83 S 3§3 s 8?88S833 a 88 a 388?8?S838?8?3 M MM 9 S3 3 8 3 3 3 3 3 S S c?

si ? as

8 8? 3 3 8 3 3 a 8838 a 38 a 338 a §33339533 a ? 8 a 838338§^83T???

a Si i si a a s a ! S « "7 M « ^ ^ §88 & 3 § 3 i? 38 83?88??838???? i £ i 8 3 ™ c? 2 8 3 as • S = S K SI 2 12 8 8 8 3 S3 a 8 8 § S 3 s 38 s 5 E ij a 83883583I 88 aa 33838?§338?5?? I -_1_ |„1 ! i j i 1 Hi gli S 1 i 1 ill 91 ills 1 I 1 a i i i 1 § i § § §!i i i a 111 298

APPENDIX D aHLC TIME-SERIES FROM SAMPLED MICROATOLLS. 299

Site I

co E o

1995

CO E o

1970 1975 1980 1985 1990 1995

-15

-20-

-25-

co -30- E 3" /3-^L?' o -35- ,0-°""" / UCT0 /3* 94a ^ X -40- iCTC-D-D-V O 94b -45 —I— T 1970 1975 1980 1985 1990 1995 Year 300

-15 92a 93b 94b -20 92b 94a Index 93a -25 co -30- E o -35-

-40 3-D-O-D-D

-45 I— 1970 1975 1980 1985 1990 1995

u./ -

0.8- 0.9- /^? X TJ I - ^M^ C w 1.1 - \£&CF Q5n .

1 2- Q9h Qda

o 93a 1.3-j i 1 1 T 1970 1975 1980 1985 1990 1995 Year 301

Site II

- R1(W) ° index Master -20- - R21(E) -25-, **\ S^\ co -30- :> E o -35-

-40-

-45- I 1 1 1 1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year CO -30- E o -35-

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year 303

-15 11 (S), 73T -20 T1(SW), Index Master

-25 T1(SSW). .j co y^ti-c-o-crcr ^ - 30 *\s-e,~tr*'- ' ' E o 35 - -40-

-45 1970 1975 1980 1985 1990 1995

0.7 T1(S). -. 73T

0.8 A T1(SW). -D Index Master

Ti(SSW) 0.9

X -a 1 - c tra-d1 1.1 -

1.2-

1.3 1970 1975 1980 1985 1990 1995 Year 304

Site IV

CO E -30- o

1970 1975 1980 1985 1990 1995

-iu-

-20-

-25- co IS -30-

E S~ O-CK0 o -35-

-40- o E12(S) • E12(N)

-4b- I 1 i • 1970 1975 1980 1985 1990 1995

-15

-20-

-25- co •J-a-a^ -30- •k E lK o ^«igg*O-05?B^>»tK: -35-

-40- 811a 811b -45 T T 1970 1975 1980 1985 1990 1995 Year CO E o

1970 1975 1980 1985 1990 1995

0.7 -* E11(W) 811a -*• E11(E) 811b 0.8- 812 -° E12(S) Index Master -* E12(N) 0.9- CTQK X T3 1 - C

1.1 -

1.2-

1.3 1 I I I— 1970 1975 1980 1985 1990 1995 Year -25-

-30- E o -35-

1970 1975 1980 1985 1990 1995

-15

x\ **-**-*-** -20- ^cr-cr^ cro-o^ '^oo-oo-o-c/ -25- co -30 E o -35-

-40- B2(NW) B2(SE) -45 T —I— 1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 -15

-20-

-25- co -30- E o -35-

-40- B4(S) B4(N) -45 1 1970 1975 1980 1985 1990 1995 -15

-20-

-25- co -30- E -* B1(SE) B3(S) o -35 -* B1(NW) B4(S)

-o B2(NW) B4(N) -40-I -• B2(SE) Index Master -45 —I— 1970 1975 1980 1985 1990 1995 0.7

0.8-

0.9-

X TJ 1 - C 1.1- B1(SE) -=> B3(S) 1.2- B1(NW) -» B4(S) B2(NW) -* B4(N) B2(SE) —— Index Master 1.3 T —I— —I— 1970 1975 1980 1985 1990 1995 Year -15

-20-

-25-

CO -30- E o -35-

-40- B5(N) B5(S)

-45 —I— —I— 1970 1975 1980 1985 1990 1995

co E o

1970 1975 1980 1985 1990 1995

-15

D-C -20- g^H-cr ^ c CKl-d^-D-r/ff -cfL -25-

CO -30- E u -35-

-40- B7(N) B7(S) -45 I— T 1995 1970 1975 1980 1985 1990 Year 1970 1975 1980 1985 1990 1995

-15

-20

-25-

co• B5(N) B7(S) -30- E B5(S) B8(SE) u -35- B6(SE) B8(NW)

-40- B6(NW) Index Master B7(N) -45-- —I— —I— 1970 1975 1980 1985 1990 1995

0.7 B5(N) B7(S) B5(S) B8(SE) 0.8- B6(SE) B8(NW) B6(NW) Index Master 0.9- B7(N)

X CD . TJ 1 C 1.1 ~ ^v^z

1.2-

1.3 1970 1975 1980 1985 1990 1995 Year 310

-25- co E o -35-

1970 1975 1980 1985 1990 1995

-15

-20-

-25- co -30- E o -35- B10a(S) B10b(S) -40- B10a(N) B10b(N)

-45 i^ —I^— 1970 1975 1980 1985 1990 1995 -15

-20-

-25- co -30-

-35-

-40- B11(NW) B11(SE)

-45 T T 1970 1975 1980 1985 1990 1995 Year 311

-15

-20-

-25- co B9(S) B10b(N) :> 30- E B9(N) B11(NW) o 35- o B10a(S) B11(SE)

40- B10a(N) Index Master

D B10b(S) -45 —I— —I— —I— 1970 1975 1980 1985 1990 1995

0.7 B9(S) -m B10b(N) B9(N) B11(NW) 0.8- B10a(S) -« B11(SE) B10a(N) —— Index Master 0.9- B10b(S)

X TJ 1 c /^J*^S 1.1 -

1.2

1.3 1970 1975 1980 1985 1990 1995

-15

-20-

-25 H _i CO 2-30- E u -35 H -40- B12(SE) B12(NW) -45 —I^— T 1970 1975 1980 1985 1990 1995 Year 312

-15

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year -15

-20

-25- f\ CO -30- E o -35- B12(SE) B14(S) B12(NW) B14(N) -40- B13(S) Index Master

-45 I— —I— 1970 1975 1980 1985 1990 1995

0.7

0.8-

0.9-

x TJ 1 - C 1.1 "

1.2-

1.3 1970 1975 1980 1985 1990 1995 Year - io -

-20-

-25- ^^&trtr&**^^ _i CO 2 -30- E o -35-

-40- : A i or Ml , Amrci -45- 1 i i i 1970 1975 1980 1985 1990 1995

co :> E o

1970 1975 1980 1985 1990 1995

co E o

1970 1980 1985 1995 Year 315

-15

-20-

-25- co -30- E o -35- A10(N) -40- A10(S) • A12(SE) A11(N) -, A12(NW) A11(S) —— Index Master -45 —r —I^— 1970 1975 1980 1985 1990 1995 0.7 A12(SE)

0.8- A12(NW) Index Master 0.9-

X 03 y T) 1 - c 1.1 -

1.2-

1.3 —I— 1970 1975 1980 1985 1990 1995

-15

-a Al3a(SE) Al3b(SE) -20- -+ A13a(NW) A13b(NW) -25 H —i CO 2-30- E o -35 -i -40

-45 T T 1970 1975 1980 1985 1990 1995 Year 1970 1975 1980 1985 1990 1995

1995

-20-

-25- co :> -30- E o -35- A13a(SE) A13a(NW) A15(NW) -40- A13b(SE) A16(NW) A13b(NW) A16(SE) A15(SE) Index Master -45 —r T 1990 1995 1970 1975 1980 1985 Year 0.7

0.8-

0.9-

X CD . TJ 1 C 1.1 - ^ A13a(SE) -» A13a(NW) 1.2- -o A13b(SE) A16(NW) * A13b(NW) A16(SE) -a A15(SE) Index Master 1.3 I T 1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995

-15

-20-

-25- co -30- E o -35-

A20a(SE) A20b(SE) -40- A20a(NW) A20b(NW)

-45 —1^— T T 1970 1975 1980 1985 1990 1995 Year 318

1975 1980 1985 1990 1995

-15

-20-

-25- co :> -30- E -a A19(SE) A20b(SE) o -35- -* A19(NW) A20b(NW)

A22b(N) -40- -o A20a(SE) -. A20a(NW) Index Master -45 I —I— 1970 1975 1980 1985 1990 1995

0.7

0.8-

0.9-J X CO

1.1 -a A19(SE) -o A20b(SE) * A19(NW) -• A20b(NW) 1.2-i o A20a(SE) -v A22b(N) -, A20a(NW) ——— Index Master 1.3 1— T T 1970 1975 1980 1985 1990 1995 Year -20-

-25-

-30-

-35-

-40- 51a * 51b

-45-- 1 1 I T 1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 -15

-20-

-30-

-35-

-40- • 53a » 53b "45 -J 1 , 1 1 1970 1975 1980 1985 1990 1995 Year -15

-20-

-25- ^WTj-o-On,*2^m&* -30- 51a 53a -35- 51b 53b

-40- 52a Index Master 52b -45 1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year VII.

co -30- E o -35-

1970 1975 1980 1985 1990 1995

co E o

1970 1975 1980 1985 1990 1995

co E o

1970 1975 1980 1985 1990 1995 Year 1970 1975 1980 1985 1990 1995

0.7

1970 1975 1980 1985 1990 1995 Year VIII.

-13"

-20-

_^^wy^ -25- s

co -30- E o -35-

-40-

is 133a * 133b -45- I |— r- I 1970 1975 1980 1985 1990 1995

-20-

co E o -35-

1970 1975 1980 1985 1990 1995 Year -15

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year 1970 1975 1980 1985 1990 1995

-5

-10-

-15- A^V ^\ ^ Vx'^O.^d^^'r 0''5 OCT co :> -20- E o -25-

-30- 131 -35 T T T 1965 1970 1975 1980 1985 1990 1995

co E

T r 1970 1975 1980 1985 1990 1995 Year 326

1 r 1965 1970 1975 1980 1985 1990 1995

1995 XI.

co E CJ

1970 1975 1980 1985 1990 1995

co :> E CJ

-45-| 1 1 1 i 1 i 1 \ i r 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

co E CJ

1970 1975 1980 1985 1990 1995 Year T i i i 1 I 1 I 1 r 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

1 1 1 r 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year 329

Site XII.

-15

co E o -35-

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year 330

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year 331

Site XIII.

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year -15

-20-

-25-

CO :s-30 - E o -35-

-40- D1(E) D1(W)

-45 I 1 1 I 1 1965 1970 1975 1980 1985 1990 1995

- 13 ~

-20-

-25- co -30- E CJ -35- ^o-o.^^^^0*^^

-40- irvFi • n^fWi

APL — -4D —1 1 1 1 i 1970 1975 1980 1985 1990 1995

-io -

-20-

-25-

-30-

-35-

40- -. DSfWt • D3fE^

45- i i i i i i 1960 1965 1970 1975 1980 1985 1990 1995 Year 1970 1975 1980 1985 1990 1995

-15

-20-

-25- co 2-30-J E o -t, D5(SE) D8(W) -35 H -* D5(NW) D14(E) -o D7(E) -40- D14(W) -. D7(W) Index Master -• D8(E) -45 T T T~ 1960 1965 1970 1975 1980 1985 1990 1995

0.7

0.8-

0.9-

x CD , TJ 1 C 1.1- D5(SE) D5(NW) D8(W) 1.2- D7(E) 014(E) D14(W) D7(W) D8(E) Index Master 1.3 T T T T —, , 1960 1965 1970 1975 1980 1985 1990 1995 Year 334

-25- co -30- E o

1970 1975 1980 1985 1990 1995

1960 1965 1970 1975 1980 1985 1990 1995

-15

-20-

-25- c-o-, co -30- D-C E o -35-

-40- D8(E) D8(W)

-45 . , 1 1970 1975 1980 1985 1990 1995 Year 335

1970 1975 1980 1985 1990 1995

1960 1965 1970 1975 1980 1985 1990 1995 0.7

0.8-

0.9- x TJ 1 C 1.1 ~ -ft D5(SE) -* D5(NW) D8(W) 1.2 -o D7(E) D14(E) -. D7(W) D14(W) -cr- D8(E) Index Master 1.3-| 1 r —I 1— —I 1 1960 1965 1970 1975 1980 1985 1990 1995 Year -15

-20-

S, -25- ^-•^'Wrt,, 'W*4** co -30- E o -35-

-40- D15(E)

-45 T T T i 1965 1970 1975 1980 1985 1990 1995

-15

-20

-25 co -30- E ^V^ o -35-

-40- D16(E) D16(W) -45 1970 1975 1980 1985 1990 1995 Year

1970 1975 1980 1985 1990 1995 Year 337

1985 1995

-20-

co H E CJ

-40-|

-45 1970 1975 1980 1985 1990 1995 Year CO E o

-45 1960 1965 1970 1975 1980 1985 1990 1995

1960 1965 1970 1975 1980 1985 1990 1995 Year 339

Site XV.

-15

-20

-25 H _i co 2 -30- E o -35-I -40- P1(E) -* P1(W) ^^i^ -45 1965 1970 1975 1980 1985 1990 1995

-15 81 83

-20- 82

-25- co -30- E kp-o-cx oo-. o -35- a01(TO -40-

-45 I I I T T 1965 1970 1975 1980 1985 1990 1995

1965 1970 1975 1980 1985 1990 1995 Year 0.7 P1(E) 82

0.8- P1(W) 83

81 Index Master (Lagoon) 0.9-

X ® y -o 1-1 1.1-

1.2-

1.3 T ,, , x ! 1965 1970 1975 1980 1985 1990 1995

co E o

1970 1975 1980 1985 1990 1995

co E c

1980 1985 1995 Year -20-

-25-

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995

<***^a^^X^^ D-D-C-CT P7{E) P7(W) P16(W) P11(E) PP4(W) P11(W) PP4(E) P16(E) Index Master (Interisland Passage) I T T —I— 1970 1975 1980 1985 1990 1995 Year 0.7

0.8-

0.9-

X TJ 1 C 1.1-

P16(W) 1.2- PP4(W) PP4(E) Index Master (Interisland Passage) 1.3 —I— —I— 1970 1975 1980 1985 1990 1995

1995

-15

-20-

-25 -| _i co 2-30- E <\o-o-cro.. , o ^cr-o-o-o-o-o-o-o-o-oo-o- -35 H -40- PP13(E) PP13(W) -45 1970 1975 1980 1985 1990 1995 Year -15

-20-

-25- co -30- E ^^-D-O-O-D-CX o a o-a-o-D-D-' ^. -35- ^o-^m^^°

-40- 84 85

-45 —I— 1970 1975 1980 1985 1990 1995

-15

-25- co E o

1970 1975 1980 1985 1990 1995 Year -15 7*

-20-

-25-I co -30

-35

-L PP9(W) -40- o PP13(E) -* PP13(W) 86b. ~a 84 Index Master (Interisland Passage) -45 , , ,— 197 0 1975 1980 1985 1990 1995 Year 0.7

0.8-

0.9-

X a> . TJ 1 C 1.1 " PP9(E) PP9(W) 85 1.2- PP13(E) 86a PP13(W) 86b. 84 Index Master (Interisland Passage) 1.3 1 I— I 1970 1975 1980 1985 1990 1995 -15

-20-

-25- co -30- E o -35-

-40- PP5(E) PP5(W) -45 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

-15

-20-

-25- co -30- E o -35-

-40- PP30(W) PP30(E)

-45 —' i i'" T T T T T T 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year -15

-20-

-25-

CO -30- E o -35-

-40- PP30(E) PP30(W) Index Master (Reef Flat) -45 T ~ ii i 1 i "~ii ir r 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

0.7 -* PP5(E) a PP30(W)

0.8 -* PP5(W) * PP30(E) Index Master (Reef Flat) 0.9n

X CD 1 TJ 1 - C 1.1 -

1.2-

1.3 H 1 \ 1 1 1 1 f | 1 r 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 Year 1970 1975 1980 1985 1990 1995

-15

-20-

Q -25- H>D^rcr tK, / ^^-cr^oo-c

-30-

-35-

-40- 162 -45 —I— 1970 1975 1980 1985 1990 1995

-10-

-20-

-25-

•30-

35-

40-

0 163a — -• 163b

45-| r • i I • " I I I 1960 1965 1970 1975 1980 1985 1990 1995 Year 1965 1970 1975 1980 1985 1990 1995 -15

-20

DQ -25-| ^D^OC^DQ / D0OQO.Q| _J co Q S-30-I C7 000-Q E o -35 -I -40-

-45 I 1— I 1960 1965 1970 1975 1980 1985 1990 1995

T T 1960 1965 1970 1975 1980 1985 1990 1995 Year -15

-20-

-25- co E CJ

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year 349

1970 1975 1980 1985 1990 1995

co E o -35-

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year XIX.

1995

1970 1975 1980 1985 1990 1995

1970 1975 1980 1985 1990 1995 Year -15

w 171 -20-

-25- co -30- E o -35- ~—~v^""^ -40- V~.^— -45 - 4a ^ 1 1 1 II 1 1995

1995

1995 352

APPENDIX E CORRELATION MATRICES FOR HABITAT SUB-SAMPLES. 353

APPENDIX E.

CORRELATION MATRICES FOR HABITAT SUB-SAMPLES.

The following matrices contain the correlation coefficients calculated between the raw and standardised aHLC time-series compiled from the microatolls growing within discrete habitats at each sample site. Correlation coefficients calculated between the raw aHLC-time- series are presented in non-italicised text, those calculated between the standardised time- series are italicised. Bold r-values are significant at the 5% confidence level. Slight differences in the r-values calculated for the raw and standardised time-series arise because the standardised time-series is normalised against the mean calculated for the entire radial profile, and not just the portion in common with the time-series with which it is being compared.

SITE I (REEF FLAT).

92a 92b 93a 93b 94a 94b Index 92a . 0.44 0.61 0.72 0.76 0.54 0.75 92b 0.45 . 0.03 0.26 0.26 -0.08 0.14 93a 0.62 0.05 - 0.89 0.85 0.91 0.91 93b 0.71 0.26 0.92 - 0.90 0.86 0.93 94a 0.77 0.26 0.88 0.91 - 0.91 0.98 94b 0.54 -0.07 0.94 0.88 0.91 - 0.95 Index 0.67 0.22 0.89 0.88 0.94 0.85 •

SITE II (REEF FLAT).

R1(W) R2KE) Index R1(W) -0.14 0.27 R21(E) -0.13 0.92 Index 0.31 0.89

SITE III (REEF FLAT).

TKS) T1(SW) TKSSW, 73T Index T1(S) 0.90 0.72 0.53 0.91 T1(SW) 0.89 0.77 0.45 0.95 T1(SSW) 0.76 0.78 0.75 0.95 0.75 73T 0.57 0.44 0.75 0.92 0.74 Index 0.93 0.94 SITE IV (REEF FLAT).

E11(W) El 1(E) E12(S) E12(N) 811a 811b 812 Index E11(W) - 0.25 0.83 0.54 -0.52 0.34 -0.02 0.93 E11(E) 0.41 - -0.14 0.81 -0.61 0.51 0.56 0.51 E12(S) 0.83 -0.15 - 0.28 0.41 0.40 -0.41 0.62 E12(N) 0.57 0.85 0.25 - -0.47 0.50 0.56 0.84 811a -0.77 -0.73 0.31 -0.61 - 0.60 -0.49 0.83 811b 0.40 0.48 0.46 0.50 0.58 - -0.05 0.79 812 -0.16 0.63 -0.43 0.55 -0.57 0.01 - 0.51 Index 0.32 0.86 -0.14 0.80 0.75 0.44 0.51 -

SITE VII (LAGOON).

87 88 89a 89b Index 87 . -0.04 0.57 0.81 0.65 88 -0.06 - 0.23 -0.41 0.70 89a 0.64 0.26 - 0.56 0.83 89b 0.81 -0.38 0.58 - 0.77 Index 0.68 0.67 0.86 0.85 -

SITE VIII (LAGOON).

133a 133b 134a 134b Index 133a . 0.60 0.66 0.71 0.94 133b 0.60 • 0.19 0.31 0.70 134a 0.66 0.17 - 0.47 0.76 134b 0.77 0.34 0.50 - 0.73 Index 0.96 0.69 0.69 0.76 -

SITE X (REEF FLAT).

M13(S) M13(N) 131 132a 132b Index M13(S) . 0.52 0.31 0.44 -0.05 0.87 M13(N) 0.49 - -0.25 0.15 -0.59 0.45 131 0.31 -0.25 - 0.27 -0.27 0.58 132a 0.43 0.13 0.27 - 0.54 0.77 132b -0.05 -0.58 -0.27 0.56 - 0.28 Index 0.46 -0.03 0.06 0.55 0.37 -

S1TF XI (REEF FLAT).

FKN) F1(S) F2(S) F2(N) F3(N) F3(S) Index 0.55 F1(N) -0.54 0.05 0.55 0.81 0.61 -0.45 -0.45 -0.37 F1(S) -0.58 0.02 -0.01 - -0.15 -0.04 0.81 F2(S) 0.11 0.01 0.37 - 0.66 0.15 0.83 F2(N) 0.55 0.05 0.37 0.62 - 0.82 0.89 F3(N) 0.83 -0.53 -0.13 0.11 0.83 - 0.64 F3(S) 0.55 -0.48 -0.04 0.77 0.80 0.89 0.61 Index 0.59 -0.37 SITE XII (REEF FLAT).

H1(E) H1(W) H7(E) H7(W) Index H1(E) - -0.54 0.75 0.03 0.85 H1(W) -0.52 - -0.80 -0.11 -0.46 H7(E) 0.75 -0.80 - 0.17 0.78 H7(W) 0.02 -0.16 0.18 - 0.63 Index 0.81 -0.49 0.76 0.58 -

SITE XIV(LAGOON).

61a 61b 62a 62b 63a 63b Index 61a . 0.90 0.94 0.84 0.88 0.86 0.95 61b 0.92 . 0.90 0.87 0.88 0.90 0.96 62a 0.94 0.92 - 0.91 0.96 0.89 0.98 62b 0.87 0.89 0.95 • 0.97 0.97 0.96 63a 0.89 0.89 0.98 0.97 - 0.98 0.97 63b 0.87 0.90 0.89 0.97 0.98 - 0.98 Index 0.96 0.96 0.99 0.95 0.97 0.97 -

SITE XV(REEF FLAT).

PP5(E) PP5(W) PP30(W) PP30(E) Index PP5(E) . 0.91 0.79 0.60 0.93 PP5(W) 0.90 • 0.83 0.74 0.96 PP30(W) 0.80 0.83 - 0.78 0.92 PP30(E) 0.60 0.74 0.78 - 0.83 Index 0.93 0.96 0.90 0.89 "

SITF XV (LAGOON).

PKE) P1(W) 81 82 83 Index 0.82 P1(E) 0.90 0.07 -0.45 0.33 P1(W) 0.91 . 0.02 -0.48 0.22 0.91 81 0.08 0.06 - 0.23 0.69 0.79 82 -0.42 -0.47 0.25 - 0.13 0.12 83 0.36 0.29 0.69 0.15 - 0.91 Index 0.71 0.78 0.52 0.55 0.49 "

SITE XVI (LAGOON).

161a 161b 163a 163b Index 161a m 0.88 0.31 0.43 0.92 161b 0.91 - -0.75 0.86 0.96 163a 0.31 -0.69 - 0.61 0.83 163b 0.44 0.85 0.62 • 0.86 Index 0.91 0.96 0.79 0.79 •

SITE XVI (INTERISLAND PASSAGE).

162 CSa CSb Index 162 -0.28 -0.09 0.31 CSa -0.26 0.80 0.89 CSb -0.11 0.81 0.97 Index 0.14 0.82 0.98 SITE XVII (LAttOQM)

121 122a 122b Index 121 0.19 0.24 0.23 122a 0.21 - 0.97 0.99 122b 0.23 0.98 - 0.99 Index 0.22 0.99 0.99

SITE XVII (REEF FLAT).

K1(E) K1(W) Index K1(E) — — 0 g7 Q gg K1(W) 0.87 - 0.93 Index 0.97 0 -93

SITE XIX (REEF FLAT).

S5(N) S5(S) 171a 171b 172a 172b 173 Index S5(N) - 0.91 -0.17 0.38 0.77 0.47 0.49 0.77 S5(S) 0.91 - -0.15 0.29 0.75 0.50 0.51 0.79 171a -0.18 -0.15 - 0.10 0.37 -0.41 -0.68 -0.20 171b 0.38 0.30 0.05 - 0.66 0.91 0.81 0.83 172a 0.76 0.73 0.34 0.67 - 0.81 0.65 0.93 172b 0.45 0.48 -0.39 0.90 0.83 - 0.85 0.89 173 0.49 0.51 -0.67 0.93 0.69 0.84 - 0.89 Index 0.73 0.76 -0.23 0.84 0.95 0.91 0.84 - ^«r%,»»C»»^l««»«0<0»^»«0.a>CO^«0»0.8i5CO<» 357

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APPENDIX F

MATRICES OF RAW AND NORMALISED R-VALUES FOR CORRELATIONS BETWEEN ALL MICROATOLLS. 361

APPENDIX F.

MATRICES OF RAW AND NORMALISED R-VALUES FOR CORRELATIONS BETWEEN ALL MICROATOLLS.

Matrices of the raw and normalised r-values for correlations between all microatolls are included on the disks below as Microsoft Excel (version 5.0a) workbook files, which should be readable using either PC or Macintosh versions. The disks are PC formated, and the files are:

Disk One; Raw Data Matrices.

Rmatraw.xls Matrix of R-values for correlations between separate raw aHLC time-series.

Pmatraw.xls P-values (a=0.05) for correlations between separate raw aHLC time-

series.

PrawT/F.xIs Significance of correlations between separate raw aHLC time-series. T

denotes significant at the 5% level. Disk Two: Normalised Data Matrices.

Rmatind.xls Matrix of R-values for correlations between separate normalised aHLC time-series.

Pmatind.xls P-values (a=0.05) for correlations between separate normalised aHLC time-series.

Pind17F.xls Significance of correlations between separate normalised aHLC time- series. T denotes significant at the 5% level.

d^nlogM • Cnc^S