DRIFT PUMICE FROM NEW CALEDONIA:

IMPLICATIONS FOR POLLUTANT DISTRIBUTION IN A REEF SYSTEM

A THESIS

SUBMITTED TO THE GRADUATE SCHOOL

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE

MASTER OF SCIENCE IN GEOLOGY

BY

ARIEL B.E. STEWART

DR. KIRSTEN N. NICHOLSON

BALL STATE UNIVERSITY

MUNCIE, INDIANA

JULY 2012

TABLE OF CONTENTS

1 Introduction ...... 4

2 Methods...... 9

2.1 Sample collection ...... 9

2.2 Sample preparation ...... 10

2.2.1 Geochemistry ...... 10

2.2.2 Petrography ...... 11

3 Study Area ...... 13

3.1 Geology of New Caledonia ...... 13

3.2 Coastal Geomorphology ...... 15

3.3 Economic Geology ...... 17

4 Regional Setting ...... 20

4.1 Tectonic Setting ...... 20

4.2 Ocean Currents ...... 21

5 Geomorphology ...... 23

5.1 Introduction ...... 23

5.2 Data ...... 28

2 5.2.1 Pumice Distribution and Beach Morphology ...... 28

5.2.2 Beach Sediments and Morphology ...... 35

5.3 Results ...... 36

6 Geochemistry ...... 41

6.1 Introduction ...... 41

6.2 Data ...... 43

6.3 Results ...... 49

7 Petrography ...... 56

7.1 Introduction ...... 56

7.2 Data ...... 57

7.3 Results ...... 60

8 Discussion ...... 61

9 Conclusion ...... 63

10 Appendix. Microbiology ...... 65

10.1 Introduction ...... 65

10.2 Methods ...... 66

10.3 Data and Results ...... 66

11 Acknowledgements ...... 71

12 References ...... 72

3 1 INTRODUCTION 1

2

3

4

New Caledonia (35°S 165°W) is an overseas French territory located in the 5

Southwest Pacific Ocean, approximately 1500 km northeast of the Australian coast (Fig. 6

1). The main island, Grande Terre, is approximately 350 km long by 50 km wide, with 7 its long axis oriented N130°. A nearly continuous barrier reef surrounds the island and 8 encloses a lagoon of over 20,000 km2 in area. The barrier reef generally grows 14 to 25 9 km from the coast, but can reach distances of 100 km along the axis of elongation. 10

In 2008 the lagoon was designated a UNESCO World Heritage Site (World 11

Heritage Committee, 2009). This honor served to highlight the ongoing environmental 12 degradation caused by nearly a century of widespread open cast nickel mining on the 13 island. Although nickel mining reached its peak in the 1960s to 1970s, the industry still 14 accounts for a significant portion of the New Caledonian economy. The environmental 15 impacts of open cast mining include acid mine drainage (e.g. Schwartz and Kgomanyane, 16

2008), destruction of littoral (Dumas et al., 2010) and coastal (Marchand et al., 2011) 17 habitats, rapid shallowing of bays (Bird et al., 1984), and reduced coral productivity in 18 the presence of chronically elevated concentrations of suspended sediment (Dumas et al., 19

2010). However, the full extent of the environmental degradation is unknown, as there 20 Figure 1. Combined bathymetric and topographic map of the Southwest Pacific Ocean (modified from CIA, 2009). Black star marks the location of Taupo, New Zealand. Black box marks the boundaries of Fig. 2. Black and grey lines represent modern and relict tectonic margins, respectively (after Bird, 2003). Colored lines represent generalized ocean currents, with arrows pointing toward the flow direction (after Douillet et al., 2001; Davis, 2005; Marchesiello et al., 2010; Gasparin et al., 2011; NOAA, 2011). Ocean currents: EAC, East Australian Current; SEC, South Equatorial Current; STCC, South Tropical Counter Current. has only been one comprehensive catalog of the effects of mining on the rivers and 21 coastline of New Caledonia (Bird et al., 1984). More recent studies of the lagoon 22 ecosystem have focused on the southwestern portion of the lagoon, around the population 23 center of Nouméa (e.g. Ambatsian et al., 1997; Douillet, 1998; Jouon et al., 2008; 24

Ouillon et al., 2010) although smaller scale studies have also been carried out on the east 25

5 coast (Tenório et al., 2005) and in the extreme north (Ambatsian et al., 1997) and south 26

(Neveux et al., 2010) parts of the lagoon. 27

Our study uses land-based data collection methods to map the distributions of 28 suspended sediment and wave energy in the lagoon. The field study included two sample 29 collection periods during which exotic drift pumice was collected from a total of 40 30 distinct locations around the island (Fig. 2). There were 9 beaches sampled in July 2008 31 and 33 sampled in July 2010, with two locations shared between the two periods. 32

Samples were generally taken every 50 to 100 km, but in the Baie de St. Vincent (Fig. 33

2A) and the Boulari-Pirogues Bay adjacent to the Nouméan peninsula (Fig. 2B) samples 34 were taken every 1 to 5 km. 35

Geochemical compositions of select pumice samples were obtained by ICP-MS 36 and ICP-OES for comparison with a database of published geochemical data from the 37

Southwest Pacific region and three potential but unlikely sources outside the region. The 38 geochemical and petrographic signatures were consistent with two recent eruptions in the 39 central Tonga arc. 40

A basic drift trajectory was mapped from central Tonga to New Caledonia 41 following major ocean currents in the Southwest Pacific Ocean (e.g. Richards, 1958; 42

Bryan et al., 2004). Ocean currents potentially rich in pumice were shown to have access 43 to nearly all of the sampled locations, excluding those situated well inside the deepest 44 bays or positioned along a specific stretch of the coast in the northwest that was serviced 45 by the pumice-poor waters coming from the eastern Australian coast. 46

Pumice abundance, a qualitative measure of the volume of pumice observed at 47 each beach, was compared to a published energy distribution map created using a three- 48

6

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, with detailed insets of (A) the the (A) of insets with , detailed

. . Coral reefs (after Andréfouët

DTSI, DTSI, 2009, 2010)

t. t. Dore (after

General Bathymetric Chart of the Oceans, 2010) Oceans, of Chart the Bathymetric General

ters above mean sea level. sea mean above ters

. Map of Grande Terre, New Caledonia (after Caledonia New Terre, Grande of Map .

2

Figure Baie de St. and Vincent (B) the and Nouméan peninsula M in dark pink (shallow), pink (intermediate depth), and light pink (deep). Circles represent sampling locations from 2008 (op me in elevation shows Legend (filled). 2010 49

7 dimensional hydrodynamic box model of the southwestern lagoon developed by the 50

Institut de Recherche pour le Développement (IRD; Douillet, 1998; Jouon et al., 2006; 51

Ouillon et al., 2010; etc.). Pumice abundance showed a direct relationship with the 52 intensity of waves imported from the open ocean (e.g. Brinkman et al., 2001). 53

Wave intensity directly controls the longshore flux, or the volume of sediment 54 moved across a beach by longshore drift per unit time, and subsequently the dimensions 55 of the beach (Davis, 1978; Short, 1999). Therefore pumice abundance directly 56 corresponds to the dimensions of a beach if it were in dynamic equilibrium with 57 longshore flux alone. So if the observed beach dimensions were greater than the 58 dimensions suggested by the pumice abundance, then sediment from additional sources 59 such as fluvial runoff (Davis, 1978; Bird et al., 1984; Davis and Fitzgerald, 2004) must 60 have accumulated on the beach. Conversely, if a beach was already known to have 61 significant fluvial input, but the pumice abundance was consistent with the observed 62 beach dimensions, then any excess sediment must have bypassed the beach to 63 contaminate areas downstream. 64

This type of model using land-based data collection methods would be useful for 65 quickly identifying at-risk coastal areas exhibiting elevated fluvial input in conjunction 66 with reduced marine influence that would benefit from a more detailed site investigation. 67

8 2 METHODS 68

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2.1 Sample collection 72

Pumice samples were collected from 9 sites in 2008 and another 33 sites in 2010 73

(Fig. 2), with samples stored in mesh or plastic bags labeled with the site number. 74

Pumice samples were collected with a bias toward large clasts, so only the sieve 75 diameter, in ϕ, of the largest sample was recorded for each location. 76

Detailed field observations were recorded at each of the 33 locations visited 77 during the 2010 field study. Field observations centered around descriptions of the beach 78 such as the width of the beach along its profile, the size distribution of beach sediments, 79 the amount of subaerial sediments and pumice in the beach backshore, and the landform 80 acting as the landward boundary. GPS coordinates of each location were acquired using 81 an HTC Rhodium 500 running the Windows Mobile 6.1 application DataTap. 82

Beach widths were measured from the low tide line, including exposed bedrock 83 but not sand bars, to the upper limit of wave activity at the base of vegetation, cliffs, or a 84 seawall. The width of the beach was initially described as narrow (<5 m), intermediate (5 85 to 40 m), or wide (>40 m), and later quantified using a combination of site photographs 86 and satellite images. 87 Satellite images were also used to quantify the width of the lagoon from the beach 88 to the outer barrier reef, as well as to verify the presence of fluvial sediments as indicated 89 by brown discoloration of nearshore waters traceable to a nearby fluvial source. 90

Pumice abundance was determined by volume as follows: (1) very abundant 91 meant there was a pumiceous ridge deposit, (2) abundant meant there was a pumiceous 92 strandline or the volumetric equivalent, (3) rare meant a 10 to 30 minute walk was 93 required to collect at least two separate samples, and (4) very rare meant only one sample 94 was encountered. Beaches without any pumice were not encountered. 95

2.2 Sample preparation 96

2.2.1 Geochemistry 97

Geochemical analyses were performed on a total of 34 pumice samples collected 98 from 24 locations. Two samples from each of the 9 locations visited in 2008 and one 99 sample from each of 16 of the 33 locations visited in 2010 were selected for analysis. 100

Only one location, the Baie des Citrons, had samples from both collection periods 101 analyzed. Individual fragments were considered separate samples for analytical 102 purposes. 103

Sample preparation began on site using tap water to remove loose particulate 104 matter. Samples were left to dry outdoors for 24 hours prior to shipment to Ball State 105

University, where the selected samples were scrubbed clean of superficial contamination 106 using a plastic-bristled fingernail brush under de-ionized water (DI water; 18.2 MΩ cm). 107

Weathered surfaces were not removed due to the generally small (1 to 6 cm) diameter of 108 the samples. After scrubbing, samples were soaked in clean DI water for 24-hour 109

10 intervals until the resulting water was visibly clear of soluble contaminants and then dried 110 for 24 hours at 50 to 60°C (modified from Sutton et al., 1995). 111

Clean, dry samples were shipped to the SGS Minerals Services laboratory in 112

Toronto, Ontario for final preparation and testing using methods accredited by the 113

Standards Council of Canada (http://www.scc.ca/) in conformance with ISO/IEC 17025. 114

Samples were pulverized in a standard steel bowl-and-puck mill to pass a 75 µm sieve, 115 which unfortunately introduced significant errors into the Pb, Ni and Cr concentrations. 116

Powdered sample fractions weighing an average of 0.10 g were fused by sodium peroxide 117 digestion in graphite crucibles, dissolved using dilute nitric acid and then split in half for 118 simultaneous analysis by ICP-OES (Perkin Elmer Optima 5300 DV) and ICP-MS (Perkin 119

Elmer Elan 6100). The analytical instruments were calibrated at the beginning of each 120 batch of samples, with method validation achieved through random testing of certified 121 reference materials, duplicates and blanks. 122

2.2.2 Petrography 123

Petrographic analyses were performed on 16 thin sections made from the same 124 samples used for geochemical analyses, although alternate samples had to be used for 7 125 of the locations. For each thin section, the volumetric abundances of minerals and 126 vesicles were visually approximated to the nearest 0.5% by comparison with a published 127 chart of volumetric modal percentages (Philpotts, 1989). However, due to the crystal- 128 poor nature of the samples, only mineral abundances over 1% by volume were reported. 129

Plagioclase compositions were determined using one of two methods depending 130 on whether albite or Carlsbad twins were present in the selected crystal. The 131 compositions of albite-twinned plagioclase were determined using the albite method of 132

11 Lévy (1894; see Fig. 3) whereas the 133 compositions of Carlsbad-twinned 134 plagioclase were determined using a 135 modified version of the Carlsbad-albite Figure 3. Plagioclase with albite twins 136 (sample 10-03) in cross-polarized light; method (after Wright, 1913; Tobi and opposing laminae extinct when rotated 38° 137 counter-clockwise (left) and 40° clockwise Kroll, 1975). (right) from vertical (center). Average 138 rotation angle of 39° corresponds to An64 For Carlsbad-twinned plagioclase (after Lévy, 1894). Scale bar is 0.1 mm. 139 exhibiting a sharply defined twinning 140 plane, the anorthite composition was 141 determined using the empirical Carlsbad- 142 albite curves of Wright (1913). Because 143

Carlsbad-twinned plagioclase rarely had 144 superimposed albite twins, the clockwise 145 and counterclockwise Carlsbad extinction 146 angles were measured in place of the left- 147 and right-hand albite extinction angles 148

(see Fig. 4A). A gypsum plate was used 149 Figure 4. Plagioclase with simple twins under to verify Carlsbad twinning, because only cross-polarized light (sample 10-03B). (A) 150 Carlsbad-twinned phenocryst (upper crystal) this type of twinning displays competing has one twin extinct when rotated (1) 24° 151 counterclockwise from (2) vertical, and the fast and slow elongations at vertical, and other twin extinct when (3) rotated clockwise 152 35°, which corresponds to An76 (after Wright, matching elongation at 45° rotation (see 1913). (B) Insertion of the gypsum plate 153 shows the upper crystal to have (1) competing Fig. 4B; Tobi, 1961; Takahashi, 2002). elongation at vertical and (2) matching 154 elongation at 49° clockwise, confirming Carlsbad twinning (after Tobi, 1961; Takahashi, 2002). Scale bar is 0.1 mm.

12 3 STUDY AREA 155

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3.1 Geology of New Caledonia 159

The geology of Grande Terre, New Caledonia (Fig. 5) is composed of a well- 160 preserved slice of the pre-Cretaceous Eastern Gondwana margin (Cluzel et al., 2010) 161 tectonically overlain by the Ophiolitic Nappe, which continues as the crustal basement of 162 the neighboring Loyalty Basin (Dubois et al., 1974). Sandwiched in between these two 163 units is a complex series of syn-tectonic metamorphic terranes (Cluzel et al., 1994). 164

New Caledonia’s basement terranes were tectonically amalgamated in the Early 165

Cretaceous (Cluzel et al., 2010) along the Eastern Gondwana margin. The basement 166 terranes include the Late Carboniferous Koh ophiolite (Aitchison et al., 1998) and the 167

Central Chain, Boghen, and Téremba terranes, which are a series of arc-derived 168 volcanosedimentary units (Aitchison et al., 1998; Meffre et al., 1996; Campbell, 1984; 169

Cluzel and Meffre, 2002) that were deposited in the Permian to Early Cretaceous (Cluzel 170 and Meffre, 2002; Adams et al., 2009; Cluzel et al., 2010). 171

The basement terranes are cut by an angular unconformity marking the onset of 172 rifting, which lasted from the Late Cretaceous (Cluzel et al., 2010) to the Middle Eocene. 173

A syn-rift sedimentary sequence contains a localized basal conglomerate followed by a 174 Figure 5. Geological map of New Caledonia (after DIMENC/SGNC-BRGM, 2011). Geomorphologic regions are as follows (modified from Bird et al., 1984): southeast (Prony to Hienghène), northeast (Hienghène to Tiabet), northwest (Tiabet to Ouaco), central west (Ouaco to Baie de St. Vincent), and southwest (Baie de St. Vincent to Prony). sequence of terrigenous sediments (Lillie and Brothers, 1970) that transition upward into 175 pelagic sediments (Brothers and Lillie, 1988). Above this is a generally disconformable 176

Late Eocene sediment sequence that begins with a sandy limestone (Lillie and Brothers, 177

1970) followed by a flysch sequence and an olistrosome that reworks all previous 178 sediments (Brothers and Lillie, 1988; Cluzel et al., 2001). 179

In the Late Eocene, Grande Terre began subducting eastward under what are now 180 the Loyalty Islands (Cluzel et al., 2001), resulting in the syn-tectonic amalgamation and 181

14 obduction of a series of metavolcanic terranes capped by the Ophiolitic Nappe. The syn- 182 tectonic block includes the Diahot terrane, which is a Late Cretaceous (Cluzel et al., 183

2011) lawsonite-blueschist to eclogite facies metavolcanosedimentary unit (Fitzherbert et 184 al., 2003); the Poya terrane, which is a Late Cretaceous to Early Eocene (Cluzel et al., 185

2001) massive tholeiitic basalt (Brothers and Lillie, 1988) with some pillows and red 186 argillite (Lillie and Brothers, 1970), which systematically overlies all earlier terranes 187

(Cluzel et al., 1994); and the Pouébo terrane, which is an eclogite facies syn-tectonic 188 mélange of the Diahot, Poya, and other terranes (Cluzel et al., 1994). Overlying all 189 earlier terranes is the Ophiolitic Nappe, which is an incomplete Early Cretaceous 190

(Auzende et al., 2000) ophiolitic sequence primarily composed of peridotite and 191 serpentinite (Lillie and Brothers, 1970). 192

3.2 Coastal Geomorphology 193

The geology exposed along the New Caledonian coastline strongly influences the 194 ruggedness of the coastal topography (Fig. 5; Bird et al., 1984), whereas the shape of the 195 coastline itself is influenced by the intensity of waves entering the lagoon through gaps in 196 the barrier reef (Fig. 2; Jouon et al., 2009). Together, the continental geology and the 197 continuity and proximity of the barrier reef define at least five distinct coastal regions as 198 follows (modified from Bird et al., 1984): the southeast, northeast, northwest, central 199 west, and southwest. 200

The southeast coast of New Caledonia, from Prony to Hienghène, features cliffs 201 broken by intermittent river deltas and rias (Bird et al., 1984). Sand and gravel beaches 202 become more numerous north of the Ophiolitic Nappe, where the sedimentary basement 203 terranes and low-grade metamorphic rocks of the Diahot terrane become exposed along 204

15 the coast. The barrier reef grows close to shore and is generally linear but very 205 discontinuous (Andréfouët et al., 2006). 206

In the northeast, from Hienghène to Tiabet, the high-grade metamorphic rocks of 207 the Daihot and Pouébo terranes produce cliffs that transition northward into gentle slopes 208 covered by extensive mangrove forests (Bird et al., 1984). The barrier reef grows close 209 to shore in a nearly continuous linear to piecewise arcuate manner, with a second, 210 subparallel fringing reef hugging the coast (Andréfouët et al., 2006). A broad gap in the 211 reef adjacent to the Pam peninsula funnels high-energy waves from the open ocean into 212 the lagoon (Andréfouët et al., 2006; Gasparin et al., 2011). 213

The northwest coast, from Tiabet to Ouaco, is regularly disrupted by steep 214 ultramafic klippes, between which Pleistocene sediments and the volcanic rocks of the 215

Poya terrane form gentle slopes hosting beaches of variable composition (Bird et al., 216

1984). The barrier reef grows an intermediate distance from the coast and is linear and 217 fairly continuous, being intermittently broken by wide channels (Andréfouët et al., 2006). 218

The central west coast, from Ouaco to just south of the Baie de St. Vincent, 219 features numerous small to large bays carved into the Poya and sedimentary basement 220 terranes, with mangrove forests and intermittent beaches of variable composition (Bird et 221 al., 1984). The barrier reef grows close to shore in a piecewise arcuate manner with 222 extensive fringing reefs growing inside the lagoon (Andréfouët et al., 2006). 223

The southwest coast, from the Baie de St. Vincent to Prony, is generally hilly with 224 the coastal morphology being strongly influenced by variations in local geology (Bird et 225 al., 1984). North of Mt. Dore, heterogeneous syn-rift sediments form broad, mangrove- 226 forested bays that are periodically broken by branching peninsulas, which enclose 227

16 numerous small bays lined with sandy beaches. South of Mt. Dore, the Ophiolitic Nappe 228 produces gently sloping gravel beaches. The barrier reef is generally linear and 229 continuous, enclosing a funnel-shaped lagoon that opens to the southwest to allow high- 230 energy waves from the open ocean to enter the lagoon (Andréfouët et al., 2006). These 231 waves are channeled around the Nouméan peninsula (Douillet et al., 2001; Pinazo et al., 232

2004) to bypass the deep bays on either side of the peninsula (Ouillon et al., 2004), but 233 quickly lose energy and transition into the low-energy wind waves characteristic of the 234 lagoon. 235

3.3 Economic Geology 236

In New Caledonia, nickel is mined from the lateritic crusts of weathered 237 peridotites that outcrop in the extensive southern massif and northwestern klippes of the 238

Ophiolitic Nappe. Because nickel accumulates at the base of the laterites (Trescases, 239

1973), a large volume of overburden must be removed to uncover the ore. Waste 240 material is stored on-site in piles specially designed to prevent catastrophic collapse and 241 reduce toxic drainage (Merrien-Soukatchoff et al., 2004). 242

Although the long-term impacts of open cast mining and any associated waste 243 piles are not entirely understood, what is known is that the combined result of extensive 244 deforestation (e.g. Fig. 6), extremely slow rates of revegetation (Jaffré et al., 1977), and 245 the creation of mountainous piles of unconsolidated sediment (Omraci et al., 2003) is 246 accelerated erosion over the long term with strongly elevated rates of sedimentation 247 downstream (Dumas et al., 2010). The high sediment load of affected rivers is evidenced 248 by broad flood plains stained red by lateritic debris (e.g. Fig. 7; Bird et al., 1984). 249

17 Figure 6. View from a mine terrace located just south of Poro, New Caledonia on RPN3.

Figure 7. Rivière des Pirogues where RP1 turns into CR7. 250

18 Sediment transported downstream accumulates in the lowlands, where depositional rates 251

40 times higher than pre-industrial levels have been reported (Stevenson et al., 2001). 252

The majority of this sediment is ultimately destined for river deltas, where it alters 253 coastal morphologies and causes hypersedimentation of bays (Bird et al., 1984; 254

Marchand et al., 2011). Heavy metals like Mn, Co, Cr, and Pb are mostly sequestered in 255 the deposited sediment, but Ni leaches into lagoon waters to contaminate pristine areas 256 far from the initial zone of impact (Ambatsian et al., 1997; Fernandez et al., 2006). 257

Furthermore, chronically elevated concentrations of suspended sediments, such as in the 258 coastal areas adjacent to mines (Jouon et al., 2008), have been shown to reduce coral 259 viability over the long term (Pastorok and Bilyard, 1985), which could have critical 260 implications for the future health of the lagoon ecosystem. 261

19 4 REGIONAL SETTING 262

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4.1 Tectonic Setting 266

Within the Southwest Pacific region (Fig. 1), ongoing subduction of the Pacific 267 plate under the Australian plate produces pumiceous volcanism along the Tonga- 268

Kermadec-Taupo arc, as well as in the Lau-Havre basin due to back-arc extension. This 269 type of volcanism is also common in the Vanuatu arc due to subduction of the Australian 270 plate under the Pacific plate. 271

The Tonga-Kermadec-Taupo arc produces compositionally bimodal (mafic-felsic) 272 volcanic rocks along its length. The Tonga-Kermadec portion of the arc has been active 273 at its present location since the Late Pliocene (Hawkins et al., 1994), while the Taupo arc 274 has only been volcanically active since the Pleistocene (Sutton et al., 1995). Tongan 275 lavas are dominated by basaltic andesite with subordinate dacite and rare rhyolite (Ewart 276 et al., 1977), whereas Kermadec lavas are slightly more mafic, being dominated by basalt 277 with subordinate dacite (Smith et al., 2003) and Taupo volcano currently produces an 278 andesite-rhyolite suite (Sutton et al., 1995; Sutton et al., 2000). The Lau-Havre basin has 279 been volcanically active since the Late Miocene (Parson et al., 1992), and produces 280 basalts similar to mid-ocean ridge basalt (MORB) with a minor trench signature (e.g. 281

Regelous et al., 2008; Haase et al., 2009). 282

Vanuatu has had active volcanism in its southern arc since the Late Miocene 283

(Mitchell and Warden, 1971; Bellon et al., 1984; Greene et al., 1994) and in its central 284 arc since the Early Pliocene (Dugas et al., 1977; Greene et al., 1994). As with the other 285 arcs of this region, Vanuatu arc lavas are compositionally bimodal, being dominated by 286 basalt with subordinate andesite (Colley and Warden, 1974) and rare dacite (e.g. Monzier 287 et al., 1993; Picard et al., 1995). 288

4.2 Ocean Currents 289

Previous studies have shown the importance of ocean currents in the transoceanic 290 distribution of pumice. For example, Richards (1958) mapped the surface currents of the 291

North Pacific Ocean over a period of 18 months using discrete sightings of a pumice raft 292 from the 1952 eruption of Barcéna volcano off the coast of Mexico. A more recent study 293 by Bryan et al. (2004) used a similar technique to determine how the South Equatorial 294

Current (SEC) transported pumice from its source in the central Tonga arc to the eastern 295

Australian coast over a period of about a year. 296

In the Southwest Pacific Ocean, the SEC is the dominant ocean current from the 297 equator to approximately 20°S (Fig. 1). Flowing east to west, the SEC could potentially 298 be enriched in pumice from Tonga, the Lau Basin, or Vanuatu prior to reaching New 299

Caledonia (Ward and Little, 2000; Webb, 2000; Bryan et al., 2004). Upon reaching New 300

Caledonia, the SEC bifurcates and spawns a north jet that flows northwest to recombine 301 with the main body of the SEC and a south jet that wraps around the southern terminus of 302

21 the barrier reef to flow northward up the west coast before continuing west toward 303

Australia (Ganachaud et al., 2008; Gasparin et al., 2011). 304

Upon reaching the Australian coast, the SEC bifurcates again, with the south- 305 flowing arm becoming the East Australian Current (EAC; Schiller et al., 2008). East- 306 flowing offshoots of the EAC combine to form the South Tropical Counter Current 307

(STCC), which breaks against the northwestern coast of New Caledonia near Ouaco. The 308

STCC would likely be poor in pumice until reaching the Kermadec arc or New Zealand 309 to the east (Schiller et al., 2008; Marchesiello et al., 2010). 310

22 5 GEOMORPHOLOGY 311

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5.1 Introduction 315

A beach is a zone of unconsolidated sediment modified by waves (Short, 1999). 316

Although beaches can be defined a number of ways, the classical beach is defined by the 317 tides and extends from the base of the lowest tides to the upper limit of the highest tides, 318 typically at a distinct change in morphology, geology, or at the base of a manmade 319 structure (Fig. 8; Davis, 1978; Hesp and Short, 1999). The terrestrial portion of the beach 320 includes the foreshore and the backshore. The foreshore is the most seaward part of the 321 beach, where sediment is continuously worked by waves (Davis, 1978). This zone is 322 defined by the lower and upper limits of daily wave action and is therefore exposed to 323 both aeolian and subaqueous conditions daily by the combined efforts of waves and tides 324

(Hughes and Turner, 1999). The backshore is located above the foreshore and is where 325 sediment is periodically worked by some combination of tides and wind (Davis, 1978; 326

Hesp and Short, 1999; Davis and FitzGerald, 2004). 327

In the transition zone from foreshore to backshore there are often small sediment 328 deposits called strandlines, which are arcuate to linear ridges of sediment deposited at the 329 landward extent of each wave (Fig. 9; Davis, 1978). Larger deposits are called ridges 330 Figure 8. Profile view of a classically defined beach divided into four zones based on prevalent processes (after Davis, 1978): (1) the nearshore, which is always submerged; (2) the foreshore, which is the active shoreface between mean low tide (MLT) and the highest tide line, a little above mean high tide (MHT); (3) the backshore, represented by the landward face of the berm; (4) the beach boundary, which is typically a hill, cliff, or seawall.

Figure 9. Beach at Kanua Tera Ecolodge (location 10-10) looking northeast. A black strandline composed of organic material in the upper foreshore marks the daily high tide line. Pumice occurs in a supratidal sediment deposit trapped behind driftwood in the lower backshore.

(Fig. 10) or, if the landward swale has been filled in by spring tides, berms (Fig. 11; 331

Hesp, 1999). In the classical literature (e.g. Davis, 1978), the berm is the defining 332 characteristic of the backshore, and any dunes occur above the beach boundary. 333

However, following the classification system of Hesp and Short (1999) it is also possible 334

24 Figure 10. Profile view of a beach in the Port Ouenghi residential area (location 10-30), looking west. A small ridge is visible in the backshore.

Figure 11. Profile view of the headland about 500 m east of the Bouraké Campground (location 10-28), looking north. This is a bedrock beach with a small ridge deposit at the base of a larger berm-like deposit in the backshore, backed by a grassy hill.

335 to treat the berm as a transitional landform that marks the boundary between the intertidal 336 and subaerial portions of the beach (Fig. 12A). As the berm grows, invading the 337

25 Figure 12. Profile view of the different beach morphotypes found in New Caledonia (modified from Hesp and Short, 1999). Key: (1) underlying geology forming a beach platform and boundary landform, (2) sea level at mean low tide, (3) intertidal beach sediments, and (4) subaerial backshore sediment deposit. Grey shading represents pumice deposit, marked by the letter “p”. (A) Beach with ridge or berm in the backshore, and bounded by a hill, cliff, or seawall; pumice found in backshore sediments. (B) Beach with climbing dunes; pumice occurs in a strandline in the lower backshore. (C) Bermless beach bounded by a hill or cliff; pumice occurs in a strandline located either (1) in the backshore or (2) trapped in grass on the boundary hill. (D) Bermless beach bounded by a cliff; pumice found (1) trapped behind an obstacle in the foreshore or (2) perched on a low (<1 m) cliff. (E) Bermless beach with a low (<1 m) seawall; pumice occurs in a perched strandline. subaerial beach, it transitions into a climbing dune (Fig. 12B), with strandlines 338 superimposed on its seaward slope. 339

If the berm atrophies into a strandline (Fig. 12C1), the backshore becomes 340 indiscernible from and is ultimately replaced by the foreshore. If the foreshore lacks a 341 barrier object to trap sediments (Fig. 12D1), strand deposits migrate onto the boundary 342 landform. Strandlines can become trapped in the grass of the boundary hill (Fig. 12C2) 343 or perched on top of a low (<1 m) cliff (Fig. 12D2) or seawall (Fig. 12E). Further 344

26 atrophy of the beach results in a fully submerged beach with cliffs plunging directly into 345 the sea. However, a subaqueous backshore does not necessarily preclude the presence of 346 strandlines in the subaerial portion of the backshore, for example at tidal lagoons where 347 the berm crest emerges as a barrier island but most of the backshore is submerged, and 348 pumice can only be found on the landward side of the lagoon at the base of the subaerial 349 backshore (Fig. 13). 350

In any case, pumice is only found in supratidal strand deposits, as pumice is 351 highly mobile and easily removed from the beach (Bryan, 1971). In general, the location 352 of a strand deposit along the beach profile is directly dependent on the incoming wave 353 intensity, meaning supratidal strand lines are deposited only by unusually strong waves 354 such as those generated in the open ocean during storm events (e.g. Bryan, 1971; 355

Brinkman et al., 2001). As these waves lose energy with distance from the reef, the 356 strand elevation approaches the daily high tide line (Douillet, 1998; Jouon et al., 2006; 357

Figure 13. View of a tidal lagoon located approximately 1.5 km north of the Bouraké campground (location 10-29), looking northeast toward the Baie de St. Vincent. The floor of the lagoon is the landward swale of a berm-like backshore deposit that crests at the outer mangroves, where there is an abbreviated foreshore. There are numerous strandlines of mostly organic detritus in the upper backshore, near the inner mangroves that mark the landward beach boundary.

27 Sahal et al., 2010), making pumice progressively less likely to be preserved and creating 358 a causal relationship between wave energy and pumice abundance. This relationship can 359 be quantified using a map of the energy distribution of surface currents in the lagoon. 360

Wave energies are often modeled in terms of hydrodynamic time parameters, but 361 the parameter most applicable to this study is the surface water export time, θs, which is 362 the number of days needed to replace an initial volume of water in the top 10% of the 363 lagoon water column with new water from the open ocean (Jouon et al., 2006). Pumice 364 abundance will be compared to a map of the θs distribution in the southwestern portion of 365 the New Caledonian lagoon that was created by Jouon et al. (2006) as part of a decade- 366 long study by the Institut de Recherche pour le Développement (IRD). The results will 367 be show that pumice abundance can be used to determine whether a beach is actively 368 experiencing hypdersedimentation from a nearby fluvial point source and would benefit 369 from further study to determine the extent of the problem. 370

5.2 Data 371

5.2.1 Pumice Distribution and Beach Morphology 372

Beach and pumice characteristics are summarized in Table 1. Pumice deposits 373 were universally stranded above the daily high tide line, whose placement along the 374 beach profile depended on the geometry and morphology of the beach (Fig. 12). 375

Abundant pumice most often occurred at traditional berm-type beaches (Figs. 8, 11, 376

12A), which were also the most common beach morphology encountered, probably due 377 to preferential selection for voluminous backshore sediments in the absence of dunes. 378

However, observed “berms” were generally no larger than small ridges (Fig. 10) and 379 sometimes limited to simple strandlines (Fig. 12C1; locations 10-08, 10-18, 10-23). True 380

28 381

29 382

30 383

31 384 dunes were restricted to localized climbing dunes (Figs. 12B, 14; 10-10) whose convex 385 profiles were sometimes imitated by local topography (Fig. 15; 10-06, 10-15, 10-27). 386

Pumice abundances ranged from very abundant to very rare, but pumice was 387 abundant on average. The angularity of collected samples ranged from well rounded to 388 very angular, with an average of slightly rounded. The maximum size of collected 389

Figure 14. Beach with climbing dunes at Kanua Tera Ecolodge (location 10- 10), looking west. The foreshore-backshore boundary occurs at the black strandline, where the poorly cemented foreshore sediments transition into the unconsolidated backshore sediments.

32 Figure 15. Bouraké campground looking west (location 10-27). Profile mimics climbing dunes. samples ranged from medium pebbles (-4 ϕ) to small cobbles (-7 ϕ), with an average of 390 very coarse pebbles (-6 ϕ). Abundant pumice tended to be of average size (-6 ϕ) and 391 angularity (slightly rounded). Rare pumice tended to be slightly smaller than average (-5 392

ϕ), with bimodal angularity (angular or slightly rounded). 393

Abundant to very abundant pumice occurred in every coastal region except the 394 northwest, where pumice was always rare to very rare (10-14 to 16). Otherwise, pumice 395 abundance corresponded to a combination of lagoon width, the size distribution of beach 396 sediments, and supratidal accommodation space. In every region, abundant pumice 397 tended to occur at berm-type beaches on the open coast, where the lagoon was narrower, 398 typically at either slightly embayed sandy to muddy beaches or intermediate width 399 bedrock beaches along bay headlands. Rare pumice tended to occur well inside the 400

33 deepest bays, such as in the Baie de St. Vincent (10-30 to 33), the Dumbéa (10-06) and 401

Boulari-Pirogues (10-07 to 09, 10-11) bays adjacent to the Nouméan peninsula, and the 402 rias (10-21) and estuaries (10-24) of the east coast. However, even in the deepest bays, 403 anomalously abundant pumice occurred at localized sandy beaches (10-08, 10-30). 404

Narrow beaches backed by cliffs had rare pumice regardless of reef proximity, 405 probably because there was limited supratidal accommodation space. Pumice could only 406 be found in perched strand deposits on top of low (<1 m) cliffs (Figs. 12D2, 16; 10-21, 407

10-26), or trapped behind obstructions in the upper foreshore (Figs. 9, 12D1; 10-07, 10- 408

22, 10-24). In contrast, narrow beaches without cliffs tended to have pumice abundances 409 consistent with regional values, as pumice could become stranded at any elevation on 410

Figure 16. Profile view looking northeast along the Plage de Port Bouquet, adjacent to the St. Roche commune (location 10-26). An abbreviated backshore is backed by a low (<1 m) grass-covered cliff, which periodically acts as a perched backshore during high-energy events. Pumice was found in a strandline trapped in the grass on the cliff.

34 grass-covered hills above the backshore (Fig. 12C2; 10-04, 10-09). 411

5.2.2 Beach Sediments and Morphology 412

The size distribution of beach sediments was somewhat indicative of the amount 413 of wave energy available to a beach, e.g. gravel tended to occur on the open coast or 414 along bay headlands, sand tended to accumulate in shallow bays on the open coast, and 415 mud tended to occur deep inside bays. However, the presence of fluvial sediments 416 tended to increase the proportion of either gravel or mud, such as at tidal lagoons hosting 417 mangrove forests (Fig. 13) or estuarine mud flats (Fig. 17). 418

Bedrock beaches occurred anywhere the sediment outflow exceeded sediment 419 inflow, such as on bay headlands experiencing sediment bypass or deep inside bays, 420

Figure 17. Magenta Beach, looking northeast (location 10-05). The mudflats act as an intertidal backshore. An abbreviated foreshore is exposed at low tide. The low (<1 m) seawall creates a perched subaerial backshore where pumice can be found.

35 away from any significant sediment sources. However, some of the beaches listed as 421 being predominantly composed of bedrock were actually fronted by beachrock (Fig. 18) 422 or augmented by artificial boulder gravel (Fig. 19). This distinction is important because, 423 although artificial boulder gravel occurred without regard to wave energy, it had the 424 potential to reduce the supratidal accommodation space, so that at some beaches, pumice 425 only occurred in localized gullies (locations 10-02, 10-16), on the leeward side of jetties 426

(Fig. 19; 10-32), or on top of short seawalls (Figs. 12E, 17; 10-05). 427

5.3 Results 428

On the macro scale, pumice abundance was directly related to the width of the 429 lagoon adjacent to the beach. However, this relationship did not continue into the meso 430 scale. For example, in the deeper bays there were localized sandy beaches with 431

Figure 18. View looking southeast from the pier in Poum village (location 10-14). There is beachrock in the foreshore and unconsolidated sediments in the backshore.

36 Figure 19. Campground at Tomo Wharf, looking south (location 10-32). The beach lacks a backshore due to the placement of the seawall, but there is a low-energy pocket next to the jetty where a small berm-like deposit has formed. Pumice was found in this deposit. anomalously abundant pumice, and on the open coast there were narrow, cliffed beaches 432 with anomalously rare pumice. Furthermore, pumice was always rare to very rare in the 433 northwest, regardless of other factors. 434

The simplest explanation is that on the macro scale, lagoon width was directly 435 proportional to longshore wave energy, which was in turn inversely proportional to the 436 distance from the barrier reef (Douillet, 1998; Jouon et al., 2006). However, on the meso 437 scale, bathymetric and topographic irregularities created fluctuations in the wave energy 438 distribution (e.g. Douillet et al., 2001; Fernandez et al., 2006; Jouon et al., 2006) that 439 were reflected by the pumice distribution. Specifically, pumice was abundant wherever 440 wave energy, in terms of θs, was below a regional threshold value (see Fig. 20), which 441

37 Figure 20. Surface sigma layer water export time, θs, in days, calculated for the entire southwestern New Caledonian lagoon (after Jouon et al., 2006). Black line marks threshold value of θs between abundant (marked “A”) and rare (“R”) pumice. Threshold value is (A) 8 days in the Baie de St. Vincent and (B) 15 days in the Boulari-Pirogues Bay (see Fig. 2 for locations). was 8 days in the Baie de St. Vincent, and 15 days in the bays surrounding the Nouméan 442 peninsula (see Table 1). In Boulari Bay there was a sandy beach that was previously 443 described as having anomalously abundant pumice (10-08) that actually corresponded to 444 a localized energy high, where θs dropped below the regional threshold value. In this 445 same area, there was a narrow beach backed by cliffs with rare pumice (10-07), 446 supporting the idea that pumice was rare at this type of beach because there was no 447 supratidal accommodation space. Three beaches did not conform to this model (10-06, 448

10-29, 10-30), possibly due to unmodeled complexities in water flow on the meso to 449 micro scale in structurally controlled bays (e.g. Short and Masselink, 1999). 450

38 Extrapolating this relationship to the rest of the island perimeter, pumice should 451 have been abundant wherever θs was below the regional threshold value, provided there 452 were no topographical restrictions to pumice deposition. Therefore, since the beach 453 characteristics observed in the northwest did not deviate significantly from other beaches 454 around the island, there must have been some other factor involved in pumice 455 distribution. The most obvious difference between the northwest and the other regions is 456 that in the northwest, longshore waves were derived from the east-flowing STCC (see 457

Fig. 1), which was unlikely to be rich in pumice until well after passing New Caledonia, 458 whereas in the other regions, longshore waves were derived from the pumice-rich SEC. 459

Longshore waves were not only critical to supplying pumice to beaches, they also 460 provided the dominant means of removing sediment from beaches as well as supplying 461 sand produced by sorting during longshore drift (e.g. Davis, 1978). Sandy beaches were 462 typically of intermediate width (5 to 40 m) and had abundant pumice, suggesting that 463 longshore sediments only accumulated where θs was below the regional threshold, which 464 was typically along the open coast. Where longshore input lagged behind sediment 465 removal, either due to low θs, such as in deep bays, or because of headland bypass, 466 beaches were composed of either bare bedrock or locally-derived gravels. Conversely, 467 beaches near fluvial point sources became dominated by fluvial mud or gravel that could 468 potentially increase beach widths to well above the 40 m attained by sand beaches. 469

Taking narrow (0 to 5 m) to intermediate (5 to 40 m) width beaches as “healthy”, 470 it becomes apparent that of the beaches with fluvial sediments, “healthy” beaches tended 471 to be located on the open coast (10-12, 10-17, 10-19, 10-23, 10-27 to 28), or, if embayed, 472 were either somewhat removed from the nearest fluvial source (10-11, 10-32 to 33), or 473

39 situated within a localized energy high (10-07, 10-24). Conversely, “unhealthy” beaches, 474 or those wider than 40 m, tended to be situated very near a fluvial point source (10-21) or 475 in a secondary depositional area downstream, such as a mangrove swamp (not sampled), 476 tidal lagoon (10-29), or mudflat (10-05). However, for “healthy” beaches where θs was 477 below the threshold value, further research would be needed to determine the volume of 478 sediment bypassing each beach as well as to determine the ultimate destination of the 479 sediment. 480

40 6 GEOCHEMISTRY 481

482

483

484

6.1 Introduction 485

The bulk geochemical compositions of the samples, including major oxide and 486 trace element concentrations, were compared to published geochemical data from five 487 volcanic regions in the Southwest Pacific Ocean (Fig. 1) including the Tonga arc (Ewart 488 et al., 1973; Ewart et al., 1977; Turner et al., 1997; Wendt et al., 1997; Ewart et al., 489

1998; Bryan et al., 2004; Caulfield et al., 2008; Hebden, 2009), Lau Basin (Turner et al., 490

1997; Ewart et al., 1998; Regelous et al., 2008; Haase et al., 2009), Vanuatu arc (Eissen 491 et al., 1992; Monzier et al., 1993; Robin et al., 1994; Maillet et al., 1995; Peate et al., 492

1997; Handley et al., 2008), Kermadec arc (Ewart et al., 1977; Turner et al., 1997; Ewart 493 et al., 1998; Worthington et al., 1999; Wright and Gamble, 1999; Smith et al., 2003; 494

Haase et al., 2006; Wright et al., 2006; Graham et al., 2008), and Taupo, New Zealand 495

(Sutton et al., 1995; Turner et al., 1997). Samples from three unlikely sources outside the 496

Southwest Pacific region (Fig. 21) were also included to diversify the data set and 497 validate the methods used for comparison. These included the 1962 eruption of Protector 498

Shoal in the South Sandwich Islands (Frick and Kent, 1984; Jokiel and Cox, 2003; Leat 499 et al., 2007); the 1883 eruption of Krakatao, Indonesia (Frick and Kent, 1984; Jokiel and 500 Figure 21. World map (modified from Kious and Tilling, 1996; Gaba, 2006). Red circles mark locations of Krakatao, Indonesia; Bárcena, Mexico; and the South Sandwich Islands.

Cox, 2003); and the 1952 eruption of Bárcena Volcano of San Benedicto Island, Mexico 501

(Jokiel and Cox, 2003). The first two eruptions were chosen because of similarities to the 502 tectonic settings and bulk geochemical compositions of volcanic rocks from the Tonga 503 arc and Taupo, New Zealand, respectively. The third eruption was chosen for its unique 504 tectonic setting on an ocean ridge that has been inactive for c. 3.5 Ma (Taran et al., 2002), 505 and for its unusually high-K felsic eruptives (Richards, 1958). 506

Potential sources were systematically excluded from the data set using graphical 507 methods, including bivariate plots of major oxides and multiple element discrimination 508 diagrams. Bivariate plots were used to magnify differences between tectonic settings in 509 terms of fractional crystallization (Harker, 1909; Winter, 2001) or pressure during initial 510 melt generation (Kuno, 1959; Jakeš and White, 1972). Multiple element diagrams were 511

42 used to expose subtle differences in trace element concentrations caused by variations in 512 melt hydration (Pearce, 1983; McCullough and Gamble, 1991) or partial melting (Winter, 513

2001; Pearce and Stern, 2006). 514

6.2 Data 515

The collected pumice samples were generally light in color, indicating a high 516

SiO2 content. Pumice fragments collected during the 2008 field period ranged in color 517 from medium to light grey (Fig. 22A), whereas pumice collected in 2010 came in a wider 518 variety of colors such as light tan (Figs. 22B, C) or grey, dark reddish brown (Fig. 22D), 519 and banded grey and black (Fig. 22E). Weathering-related alteration included reddish- 520 brown colored stains (Fig. 22C), superficial bleaching (Fig. 22F), and extensive internal 521

Figure 22. Examples of pumice characteristics: (A) grey color with attached biological matter, including tubular serpulid shells (sample from 2008); (B) well-rounded, tan color (sample 10-01B); (C) very angular, tan color with reddish-brown stains (10-16A); (D) dark red color with pore-space contamination by particulate matter (10-05-3A); (E) black and grey banded color (10-01A); (F) bleached white color with superficial contamination by brown algae (10-10-1A).

43 fractures, none of which are expected to have significantly altered the geochemistry of 522 the samples. Other forms of contamination included superficial colonization by serpulids 523

(Fig. 22A) or algae (Fig. 22F), and pore space contamination (Fig. 22D) by particulate or 524 biological matter (see Appendix). Biological and pore space contaminants were carefully 525 removed prior to analysis. 526

The analyzed samples were of fairly homogenous geochemistry (Table 2), being 527 generally low in mafic elements (e.g. <1.9 weight% MgO, <35 ppm Co) although some 528

T of the less compatible elements varied widely (e.g. 2.4 to 9.9 wt% total Fe as Fe2O3 ). 529

The samples were generally low in incompatible elements (e.g. <1.1 wt% K2O, <13.5 530 ppm Rb, <1.4 ppm Cs) although on the multiple trace element discrimination diagram 531

(Fig. 23) the large ion lithophile elements (LILE: Cs, Ba, Rb, K, Sr) were obviously 532

Figure 23. Trace element discrimination diagram of samples in Table 2. Elements ordered by decreasing incompatibility (after Sun and McDonough, 1989), with elements of equivalent mobility reordered to account for the presence of an aqueous fluid (Ba > Rb after Pearce, 1983; K > U, Sm > Zr after McCullough and Gamble, 1991). Depleted mid-ocean ridge basalt (N- MORB) concentrations from Sun and McDonough (1989). Labeled samples discussed in text.

44 533

45 534

46 535

47 536

537

48 enriched compared to the high 538

field strength elements (HFSE: Th, 539

U, Zr, Ti, Y, REE) for all but four 540

samples (10-01A, 10-05-03A, 10- 541

20A, 10-25A). Two of the HFSE, 542

Zr and Ti, tended to be especially 543

depleted, displaying distinctly 544

negative anomalies. 545

As for the lanthanides, or 546

rare earth elements (REE), the light 547

elements (LREE) were generally 548

neither enriched nor depleted 549

compared to the heavy elements 550

(HREE; Fig. 24). The four outliers 551

from Fig. 23 were enriched in 552

LREE over HREE, and a fifth 553

outlier (10-10-01A) was enriched 554

in HREE over LREE. All of the 555

samples had a slightly negative Eu 556

anomaly. 557

6.3 Results 558

The major element oxide 559

49 Figure 24. Rare earth element (REE) diagram of samples in Table 2. Element concentrations of C1 chondrite from Sun and McDonough (1989). Labeled samples discussed in text. chemistry of the pumice samples shows a low MgO accompanied by rapid depletion of 560

T Fe2O3 (Fig. 25A). This pattern implies a well-differentiated melt depleted in early- 561 forming minerals such as Mg-olivine, followed by late titanomagnetite removal (Winter, 562

2001). The samples are clearly more differentiated than the basalts and andesites of the 563

Lau Basin. Furthermore, the low K2O (Fig. 25B) content of the samples implies melt 564 generation under low-pressure conditions (Kuno, 1959; Winter, 2001), and the low K/Rb 565 ratios (Fig. 25C) are consistent with the lack of any notable K-phases (e.g. biotite, 566 muscovite, or K-feldspar), as Rb substitutes for K in these minerals (Jakeš and White, 567

1972). Therefore the samples are unlikely to be from any of the calc-alkaline regions 568

(Taupo, Krakatao, or Bárcena volcano), leaving only the island arcs as potential sources. 569

Origin in a subduction zone is confirmed by the trace element signatures (Fig. 570

23), which exhibit preferential enrichment of LILE over HFSE due to interaction with an 571

50 T Figure 25. Fenner diagrams of (A) total Fe as Fe2O3 , (B) K2O, and (C) K/Rb. Fields represent published data ranges for labeled regions (see text for references). New Caledonian drift pumice represented by open (2008) and filled (2010) circles. aqueous fluid (Pearce, 1983; McCullough and Gamble, 1991). In most of the samples, 572 the more mobile HFSE (Th, La, Ce, Pr) were depleted nearly to N-MORB concentrations 573 while the least mobile LILE (Sr) were at least somewhat enriched, suggesting low- 574 pressure devolatilization of the subducting slab (Pearce and Stern, 2006). 575

Comparing the trace element distributions of the pumice samples to the ranges 576

T displayed by siliceous eruptives (MgO < 2.5%, Fe2O3 < 11.0%) from the four island arcs 577

51 in the data set shows the majority of the samples plotting within the Tonga arc field (Fig. 578

26A). Some of the outliers could be from the Kermadec arc (Fig. 26B), whose trace 579

element signature displays a similar pattern of differential enrichment compared to the 580

New Caledonian pumice samples, but has excessive enrichment of all of the trace 581

elements, especially those with the lowest mobility (e.g. Sm, Zr, Eu, Dy, Y, Yb). The 582

New Caledonian samples are generally inconsistent with the Vanuatu (Fig. 26C) and 583

South Sandwich (Fig. 26D) arcs, which display less enrichment of the least mobile LILE 584

(e.g. Sr) than the medium compatibility HFSE (e.g. Pr, Nd, Zr). However, similar to the 585

Kermadec arc, the wide range of concentrations in the Vanuatu arc trace element 586

signature make it a potential source of at least some of the outlying samples. 587

Figure 26. See Fig. 23 for details. Grey fields represent concentration ranges of felsic eruptives from (A) Tonga arc, (B) Kermadec arc, (C) Vanuatu arc, and (D) South Sandwich arc.

52 On the REE discrimination diagram (Fig. 24), most of the New Caledonian 588 pumice samples displayed neither enrichment nor depletion of LREE compared to the 589

HREE, as well as a slightly negative Eu anomaly indicating plagioclase removal. 590

Comparing the REE distribution of the samples to the ranges displayed by siliceous 591

T (MgO < 2.5%, Fe2O3 < 11.0%; see Fig. 25A) eruptives from the three Southwest Pacific 592 island arcs included in the data set shows the majority of the New Caledonian samples to 593 be consistent with an origin in the Tonga arc (Fig. 27A). The samples are generally 594 inconsistent with the Kermadec arc (Fig. 27B), whose REE distribution is generally 595 enriched compared to the New Caledonian samples, especially for the less mobile 596 elements (e.g. Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). The samples are also generally 597

Figure 27. See Fig. 24 for details. Grey fields represent concentration ranges of felsic eruptives from (A) Tonga arc, (B) Kermadec arc, and (C) Vanuatu arc.

53 inconsistent with the Vanuatu arc (Fig. 27C), whose REE distribution exhibits excessive 598 enrichment of the LREE compared to the New Caledonian samples. However, as with 599 the trace element distribution, the wide range of REE values displayed by both the 600

Kermadec and Vanuatu arcs make these potential sources of at least some of the outliers. 601

Both the trace element and REE diagrams show a strong correlation between the 602

New Caledonian samples and the felsic eruptives of the Tonga arc, even though there are 603 only two eruptions actually represented by the Tonga arc field in both diagrams: the 2001 604 eruption of an unnamed volcano (0403-091) in central Tonga (Bryan et al., 2004) and the 605

2006 eruption of Metis Shoal, which is also in central Tonga (Hebden, 2009). The 606 geochemical compositions of pumice from both of these eruptions are fairly homogenous, 607 but there is some clustering evident in a bivariate plot of Al2O3 versus MgO (Fig. 28). 608

Although Al2O3 can vary widely with MgO, even for just one volcano (e.g. 609

Wright et al., 2006), some authors have recently remarked on the fairly tight clustering 610 between samples of different eruptive events in the Southwest Pacific (e.g. Regelous et 611 al., 2008; Haase et al., 2009) and even between different types of rock from a single 612

Figure 28. Bivariate plot of Al2O3 versus MgO. Oxide concentrations expressed as weight percent. Fields represent published data ranges for the 2001 eruption of an unnamed volcano (0403-091) in the central Tonga arc (Bryan et al., 2004) and the 2006 eruption of Metis Shoal (Hebden, 2009). New Caledonian drift pumice represented by open (2008) and filled (2010) circles. Labeled samples discussed in text.

54 event, even if those rocks are of uniform mineralogy (e.g. Smith et al., 2003). Minute 613 variations in Al2O3 could be related to variations in plagioclase removal with progressive 614 differentiation (Winter, 2001), which would be consistent with the switch from a positive 615

Eu anomaly in the REE of the more mafic Tongan lavas (not shown; Ewart and 616

Hawkesworth, 1987) to the negative Eu anomaly displayed by the pumice samples (Fig. 617

24). 618

On the plot of Al2O3 versus MgO (Fig. 28), the New Caledonian samples are 619 clearly split by year of collection, with most of the samples from 2008 plotting within the 620 field representing the Metis Shoal eruption of 2006 (Hebden, 2009), and most of the 621 samples from 2010 consistent with pumice erupted from the unnamed volcano (0403- 622

091) that erupted in 2001 (Bryan et al., 2004). Samples 10-25A and 10-20A, which were 623 previously identified as outliers, plot within the Metis Shoal and 0403-091 fields, 624 respectively. Considering that the trace element and REE distributions of these two 625 outliers displayed the most divergence from the rest of the samples, it is possible that the 626 reference data did not cover the full range of compositions produced by the two 627 eruptions. Therefore, although the three remaining outliers (10-01A, 10-10-1A, 10-05- 628

3A) could have been from either the Vanuatu or Kermadec arc, it is also possible that 629 sample 10-05-3A was erupted from Metis Shoal in 2006, and the other two samples from 630 the unnamed volcano in 2001. 631

55 7 PETROGRAPHY 632

633

634

635

7.1 Introduction 636

The results of the bulk geochemical analyses suggested that the collected pumice 637 was dominantly erupted from the Tonga arc. The typical mineral suite reported for felsic 638 pumice from the Tonga arc (Ewart et al., 1973) includes, in order of decreasing 639 volumetric abundance, calcic plagioclase (An72-88; 4-7% modal), clinopyroxene (augite; 640

0.5-1.9% modal), orthopyroxene (0.3-0.5% modal), and opaque minerals 641

(titanomagnetite; 0.3-0.5% modal). The groundmass is reported to be a highly vesicular 642 brown glass littered with microlites dominantly composed of plagioclase and pyroxene 643 with subordinate opaque minerals and apatite (Ewart et al., 1973). Although plagioclase 644 is dominantly calcic, more sodic plagioclase has been reported on an older seamount 645

(An45-55; Hawkins, 1985). Orthopyroxene is reported to exhibit progressive iron 646 enrichment over time, ranging from bronzite in the pre-historic deposits (Hawkins, 1985) 647 to hypersthene in an eruption from 1969 (Bryan, 1971) and pigeonite in an eruption from 648

2001 (Bryan et al., 2004). Hydrous minerals (mica, amphibole) and K-feldspar have not 649 been reported. 650 7.2 Data 651

In hand sample there were scattered eye-visible (0.5 to 2 mm diameter) 652 phenocrysts of plagioclase feldspar (3 to 5% by volume) and pyroxene (1 to 3%), with 653 rare dark minerals visible under a 10-power hand lens. Samples analyzed 654 petrographically (Table 3) contained, in order of decreasing volumetric abundance, 655 vesicles (55 to 90%), interstitial glass (5 to 44%), plagioclase feldspar (1-3%), 656 clinopyroxene (≤2.5%), orthopyroxene (≤1%), and opaque minerals (≤1%). The samples 657 were generally highly vesicular (70% modal; Figs. 29, 30) with vesicles spherical to 658 elongate parallel to flow. In unwashed samples the outermost vesicles had rims of red 659 discoloration or microscopic detritus such as sparry calcite grains (Fig. 29) and 660 foraminifera (see Appendix). 661

57 Figure 29. Large vesicle lined with microscopic detritus and surrounded by smaller vesicles (sample 10-08B; 70% vesicles). (A) Plane polarized light. (B) Cross-polarized light. Scale bar is 0.1 mm.

Figure 30. Flow-parallel needles and microlites in cross-polarized light (sample 10-20A). Displayed minerals are mostly plagioclase with some pyroxene. Scale bar is 0.1 mm.

Interstitial glass (25% modal) was generally light brown in plane polarized light 662 and littered with up to 1% plagioclase microlites and rare clino- or orthopyroxine 663 microlites (Fig. 30). All samples were crystal poor (≤5% modal). Minerals generally 664 displayed pyroclastic texture (Fig. 31) and occurred as either individual phenocrysts (0.01 665 to 2 mm diameter) or glomerocrysts (1-3 mm diameter) of plagioclase with (Fig. 32A) or 666 without (Fig. 32B) other minerals. 667

Fifty-eight plagioclase crystals were found to be suitable for visual estimation of 668 composition. Plagioclase was generally calcic, ranging from labradorite (An55) to 669 anorthite (An100) with a modal composition of bytownite (An85). One andesine (An47) 670 crystal was found. Plagioclase was generally seriate with microlites, and occurred as eu- 671

58 Figure 31. Prismatic plagioclase (plag), clinopyroxene (cpx), and opaque (opq) minerals (sample 10- 01B). Some cleavage planes have been traced on the clinopyroxene crystals to highlight the 90° cleavage angles hidden in the pyroclastic texture. Scale bar is 0.1 mm.

Figure 32. Glomerocrysts in cross-polarized light. (A) Plagioclase, clinopyroxene, and opaque minerals (sample 10-28B). (B) Plagioclase with compositional zoning (lower left) and twinning with tilted twin planes (upper right), which appear as thick, dark black lines (sample 10-25A). Scale bars are 0.1 mm. to anhedral prisms (Fig. 31) or laths oriented parallel to the flow direction (Figs. 4, 30). 672

Some crystals displayed compositional zoning under cross-polarized light (Fig. 32B). 673

The two pyroxenes displayed markedly different textures in thin section. 674

Clinopyroxene generally occurred as eu- to anhedral prisms (Figs. 31, 32A) with some 675 simple twinning, whereas orthopyroxene generally occurred as anhedral laths (Fig. 33) 676 oriented parallel to flow. Clinopyroxene was commonly intergrown with plagioclase and 677 opaque minerals, either as mineral aggregates or poikilitic overgrowths. Resorption 678 textures were evident in clinopyroxene but more common in orthopyroxene. 679

59 Figure 33. Partially resorbed orthopyroxene (sample 10-05-3A) lath with small anhedral opaque inclusions. Phenocryst shown (A) in plane polarized light and cross- polarized light at (B) vertical extinction and (C) 45° clockwise rotation. Scale bar is 0.1 mm.

Opaque minerals generally occurred as anhedral prismatic inclusions in other 680 minerals (Fig. 33) or as interstitial growths in glomerocrysts (Figs. 31, 32A). Individual 681 phenocrysts were exceedingly rare. 682

7.3 Results 683

The minerals and textures observed in thin section were generally consistent with 684 those reported for similar pumice from the Tonga arc. Volumetric abundances were 685 consistent for all minerals except plagioclase, which was less abundant than previously 686 reported (1-3% observed versus 4-7% expected; Ewart et al., 1973). However, the total 687 mineral abundances (≤5% modal) were identical to those reported from a recent pumice 688 eruption in central Tonga (Bryan et al., 2004). Plagioclase removal would explain both 689 the declining plagioclase abundances and the negative Eu anomaly observed in the REE 690 distribution (Fig. 24). Furthermore, variations in plagioclase abundances between 691 eruptions could account for the slight, but measurable differences in Al2O3 with 692 decreasing MgO (Fig. 28). 693

60 8 DISCUSSION 694

695

696

697

The pumice samples collected in 2008 and 2010 from beaches around New 698

Caledonia are of fairly uniform geochemistry. Barring five outliers, the samples are low 699 in both mafic and alkali elements, with the LILE enriched compared to the HFSE and the 700

LREE neither enriched nor depleted compared to the HREE. This geochemical signature 701 is consistent with published geochemical data from the central Tonga arc. A Tongan 702 provenance is further supported by the mineralogy, as petrographic slides from the 2010 703 samples have a mineral assemblage (i.e. calcic plagioclase, clinopyroxene, 704 orthopyroxene, and opaque minerals) similar to that reported from the Tonga arc. 705

However, the samples are depleted in plagioclase compared to the regional mineral 706 assemblage, suggesting a recent plagioclase removal phase. This idea is supported by a 707 negative Eu anomaly in the REE diagram as well as by a slight, but noticeable decline in 708

Al2O3 with decreasing MgO. 709

The results of the bulk geochemical analyses show that the 2008 samples were 710 dominantly from a 2006 eruption of Metis Shoal in central Tonga and the 2010 samples 711 were dominantly from a 2001 eruption of an unnamed volcano in central Tonga. Some of 712 the outlying samples could have come from the Vanuatu or Kermadec arcs, although 713 Vanuatu would be the most likely source of any outliers due to the directionality of 714 currents in the Southwest Pacific Ocean. 715

Between 2008 and 2010, the younger pumice was likely removed by unusually 716 high tides or storm tides, exposing older pumice stranded at a higher elevation as the 717 dominant deposit on the beach face. This natural attenuation of pumice by high-energy 718 waves caused pumice abundances to naturally mimic wave energies, providing a land- 719 based method of measuring wave intensities. This would explain why pumice abundance 720 corresponded inversely to the width of the lagoon on the macro scale, but not on the meso 721 scale where bathymetric and topographic irregularities would begin to affect the wave 722 energy distribution. So although pumice abundance did not always correspond to the 723 width of the lagoon, it directly corresponded to the incoming wave energy well into the 724 micro scale. 725

62 9 CONCLUSION 726

727

728

729

By using beach width as an indicator of the presence and extent of 730 hypersedimentation in the nearshore area, pumice abundance can be used to determine 731 whether sedimentation is restricted to the area immediately surrounding the beach or 732 bypassing the beach to contaminate secondary depositional areas downstream. In this 733 way, pumice abundance could be useful for determining which places have been most 734 affected by mining or urban waste. 735

Future research of this type should begin with obtaining a higher resolution map 736 of the θs distribution in the southwest portion of the New Caledonian lagoon, modeled 737 with the latest iteration of the IRD hydrodynamic box model. The map used for this 738 study was published using an early version of the model (Jouon et al., 2006), but, as the 739 project was only recently finalized (Ouillon et al., 2010), a more recent version of the 740 model may display a stronger relationship with pumice abundance. 741

Future field studies should focus on obtaining higher resolution data in the 742 southwestern lagoon, especially at locations not previously sampled (e.g. the mangrove 743 swamps of Dumbéa Bay). Resampling of locations visited during this study could verify 744 the proposed theory of pumice preservation. However, due to the rapid attenuation of 745 samples erupted in 2006 and the ongoing attenuation of earlier deposits it may be 746 necessary to wait for another pumice eruption before continuing with this type of 747 research. 748

64 10 APPENDIX. MICROBIOLOGY 749

750

751

752

10.1 Introduction 753

Biological specimens observed on the pumice samples included abandoned shells 754 of serpulid worms (Fig. 22A), remnants of coral colonies, living green and brown algae 755

(Fig. 22F), and tests of benthic foraminifera. Any of these contaminants could have 756 attached to the pumice outside of the New Caledonian lagoon (e.g. Jokiel and Cox, 2003), 757 potentially recording information about any intermediate stops along the route, or 758 providing a minimum drift time (e.g. Bryan et al., 2004). However, the lack of intact 759 soft-bodied organisms suggests the pumice had been stranded for a substantial period of 760 time prior to collection (Bryan et al., 2004), and any or all of the collected biological 761 specimens could be local species that attached in situ. 762

Of the attached organisms, serpulid shells and foraminiferal tests were the most 763 readily collected for analysis. Serpulid worms secrete calcium carbonate to form tube- 764 shaped shells that are continually enlarged through the lengthening and widening of the 765 anterior opening (Hedley, 1958). For at least one species of serpulid, shell length has 766 been shown to correspond to the age of a specimen (e.g. Qian and Pechenik, 1998), 767 which could be used to determine the minimum age of the pumice. 768

Foraminifera are single-celled organisms whose tests are enlarged through the 769 successive addition of new chambers. Specific test morphologies are unique to each 770 species. Foraminifera are sensitive to oceanographic parameters (e.g. temperature, 771 salinity, depth) so the presence of species inconsistent with the low-latitude shelf 772 environment of the New Caledonian lagoon could indicate that the pumice had visited an 773 exotic location prior to stranding (Phleger, 1960). 774

10.2 Methods 775

The Scanning Electron Microscope or SEM (Hitachi TM-1000) at the Department 776 of Geological Sciences, Ball State University was used to take high-power photographs 777 of biological contaminants collected from pumice fragments and accompanying 778 sediments collected in July 2008. Serpulids were cut from sample surfaces as both whole 779 and crushed specimens. Foraminifera were recovered either directly from sample 780 surfaces or hand-picked under a sedimentological microscope with a size 00 paintbrush 781 from the 150-250 µm fraction of sediment emptied from the collection bags. Additional 782 foraminifera were discovered in the thin sections created from the July 2010 samples. 783

Biological specimens were mounted on the SEM stage under atmospheric 784 pressure using double-sided adhesive stickers. After evacuating the chamber, the SEM 785 software was used to adjust the stage and lens for the best presentation of the sample. 786

The SEM was recalibrated no more than 7 days prior to each use. 787

10.3 Data and Results 788

Serpulid shells were fairly common biological contaminants on pumice collected 789 in 2008, and although they were rare on pumice from 2010, the shells were still easily 790 discernable due to their large size and stark white color (Fig. 22A). The longest serpulid 791

66

tubes were commonly arranged in sinuous masses (e.g. Fig. A1A), with later shell growth 792 overlapping or enveloping earlier growth, making it difficult to determine the total shell 793 length and, consequently, the specimen age. 794

Serpulid tube walls exhibited multiple layers, each with a distinct texture (Figs. 795

A2, A3). The innermost layer exhibited a basketweave-like texture (Fig. A1B to D) that 796 would probably not produce distinct, individual layers that could, for example, be 797 counted to determine the age of the specimen. However, obvious scavenging of the inner 798 walls (Fig. A1B) served to highlight the fact that even if the ages of the serpulids were 799 known, an indeterminate amount of time had passed between death and collection. 800

Figure A1. SEM photos showing (A) a serpulid shell on pumice from location 08-04. Scale bar is 1 mm. (B) Zoomed view of scavenging trails on the inner wall. (C) View of inner wall with (D) close-up showing basketweave-like texture. Scale bars are 0.1 mm.

67

Figure A2. SEM photos showing (A) cross-sectional and (B) edge views of a serpulid from location 08-03. Some layering is apparent where it has become slightly separated, but as seen in the close up view, the texture is more massive than layered. Scale bars are 0.1 mm.

Figure A3. SEM photos showing (A) a serpulid shell segment from location 08-03 with (B) distinct layers are apparent along a broken edge. (C) Zoomed view shows the different texture of each layer. Scale bars are 0.1 mm. 801

68

Foraminiferal tests collected from the 2008 pumice samples and associated 802 sediments included three specimens from location 08-01 identified as Elphidium 803 advenum (Fig. A4A), Peneroplis pertusus (Fig. A4B), and Quinqueloculina seminulum 804

(Fig. A4C), and one unidentified, partially dissolved benthic species from location 08-06 805

(Fig. A5). Two species of Cibicides (Fig. A6A, location 10-01; Fig. A6B, location 10- 806

28) were observed in thin section. All of these foraminifera were consistent with the 807

New Caledonian foraminiferal assemblage (Debeney et al., 2011) and most likely 808 originated within the lagoon. Furthermore, the presence of a partially dissolved specimen 809

Figure A4. SEM photos showing (A) foraminifera and two shells from location 08-01. Foraminifera are identified as Elphidium advenum (upper right), Peneroplis pertusus (lower left), and Quinqueloculina seminulum (lower middle). Scale bar is 1 mm. Zoomed views of (B) Peneroplis pertusus and (C) Quinqueloculina seminulum. Scale bars are 0.1 mm.

69

Figure A5. SEM photo of a partially dissolved benthic foraminiferal specimen from location 08-06. Scale bar is 0.1 mm.

Figure A6. Microphotographs of two unidentified species of Cibicides from locations (A) 10-01 and (B) 10-28. Scale bar is 0.1 mm.

suggests that any exotic foraminifera would not have been preserved in the stranded 810

pumice, making further study of this nature counterproductive. 811

812

70 11 ACKNOWLEDGEMENTS 813

814

815

816

This study was funded by the National Science Foundation (OISE IRES Grant 817

0727323 to Dr. K.N. Nicholson) and Ball State University (ASPiRE Research Grant 818

I215-10 to Ariel Stewart). 819

Special thanks to Dr. Kirsten Nicholson, Dr. Richard Fluegeman, Dr. Jeffry 820

Grigsby, and Dr. Rice-Snow for providing invaluable help and advice; Sherry Stewart, 821 for being a great help in the field and taking many of the photographs used in this paper; 822 and the undergraduate students Rachel Grande, Aaron Alexander, and Chris Rickey for 823 their work collecting samples and taking field notes in July 2008. 824

Maps were made using GRASS GIS (http://grass.osgeo.org/), QGIS 825

(http://www.qgis.org/) and Inkscape (http://www.inkscape.org/). Geochemical data were 826 modeled using R (http://www.R-project.org/) and Ggobi (http://www.ggobi.org/). 827

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