167

GEOMORPHOLOGY OF REEF ISLANDS AND ATOLL MOTU IN

R.F. McLean

Department of Geography and Oceanography Australian Defence Force Academy Canberra ACT Australia

P.L. Hosking

Department of Geography University of Auckland, Private Bag, Auckland New Zealand

ABSTRACT In Tuvalu reef islands are associated with the smaller table reefs and motu (islets) with larger atol1s. In both cases the basic controls and processes on island development are similar and these include size and geometry of reef platform, sediment production, exposure to prevailing and storm wave action, and stabilization by vegetation and lithification of sediments. Regardless of island size and shape several natural landform units can be distinguished including high gravel ridges on the oce;U;- 'or windward-side, low sand ridges on the lagoon­ or leeward-side, and a central depression or flat between the two. Anthropogenic landforms include the pulaka pit-spoil bank comple,x which provide the greatest relative relief on many islands. Radiometric dating of fossil and contemporary reef flat and island deposits indicates that the islands are geological1y young having accumulated during the last 3000 years. A three stage model relating sea level change rB.·ild reef growth, reef flat development and island formation is proposed. The present stabilit~ . Qf islands is also discussed.

S. Pac, j. Nat. Sci., 1991, 11:167-189

169

Table I : Island. reef and lagoon areas (sq. km)

( I) (2) (3) (4) (5) (6) Island Reef flat Platform Lagoon Reef top Reef type ( I + 2) (3 + 4)

Nanumea 3.9 13.2 17.1 1.3 20.4 Atoll Nanumaga 3.2 1.0 4.2 4.2 Table 2.4 0.7 3. 1 3. 1 Table 3.7 12.3 16.0 3.4 19.4 Atoll 5.5 3.6 9.1 1.1 10.2 Mixed 3.9 21.6 25.5 91.0 116.5 Atoll 3.0 34.0 37.0 205.2 242.2 Atoll 2.2 11.8 14.0 23.8 37.8 Atoll 0.5 0.2 0.7 0.7 Table'

Note: Island area includes land plus beaches ana enclosed lagoons. ponds and swamps.

Table 2 Island, reef and lagoon areas (percent)

Island area Island area Platform area Lagoon area as % of as % of as % of as % of reef reef top reef top reef top platform

Nanumea 23% 19% 84% 16% Nanumaga 76% 76% 100% Niutao 77% 77% 100% Nui 23% 19% 83 % 17% Vaitupu 60% 54% 89% 11 % Nukufetau 15% 3% 22% 78% Funafuti 8% 1% 15% 85% Nukulaelae 15% 6% 37% 63% Niulakita 68% 68% 100% ....o Islands of Tuvalu F lU I . ",Nui \; ~ c l~ !J ~'\)" o g)}~ ~ "C \.: ) , Funafuti ~-=

Nukulaelae

Niulakita ~

Figure I Reef and island outlines of the nine islands of Tuvalu. Note the scale is the same for all islands 171

Table 3 Number and size of islands and islets in year of survey

Area (ha) <1 1-5 5-10 10-10 20-50 50-100 >100 Total

Nanumea 3 2 6 Nanumaga 1 1 NiutaO 1 1 Nui 8 3 2 2 1 18 VaitupU 7 1 1 9 Nukufetau 20 6 2 3 4 2 37 Funafuti 11 14 5 3 34 Nukulaelae 12 4 2 1 2 22 Niulakita 1 1

Total 61 28 11 8 9 5 7 129

GEOMETRY AND LOCATION OF REEF ISLANDS AND ATOLL MOTU

Reef islands and atoll motu are morphologically coherent accumulations of bioclastic reefal materials standing on reef platforms and exposed above the level of the sea at high tide. Given the great differences in the size and shape of the atolls and table reefs in Tuvalu, as illustrated in Figure 1 and quantified iii Tables 2 and 3, it is not surprising that there is equally great variation in the dimensions and geometry of the islands and islets themselves in their location and the proportion of the available reef platform they occupy.

It is obvious that islands and islets cannot be larger than the reef platform they occupy. On the atolls, islets cover only a small proponion (8 to 23 percent) of the available reef platform exposed at low water. In contrast islands on table reefs cover a much greater proportion, 76 and 77 percent in the case of Nanumaga and Niutao, and 68 percent in the case of Niulakita. Using this criteria, Vaitupu, with 60 percent, is more similar to the table reefs than atolls.

The table reefs have a single island (Table 3) which is compact and centrally located on the reef platform. On the atolls, islets are unevenly distributed around the reef rim to windward and leeward. In most cases islets are concentrated along the windward reef. For instance, long linear islets occupy almost the entire length of the 13 km long reef on the

173 geomorphic processes which have been operating and the characteristics of the sedimentary materials of which the land is made. The natural landforms of the islands are made up exclusively of coralline and associated deposits. predominantly sands and gravels. derived from the adjacent reef or lagoon and emplaced by wave-tide-current action. Storms are of considerable imponance in island building and destruction. and the effects of Hurricane Bebe in 1972 for instance are still recognizable. though in substantially modified form. on the reef flats and islets along the southeastern coasts of Funafuti and Nwrufetau.

In addition to natural landforms several anthropogenic features have been created. most imponantly the excavated depressions and surrounding mounds of the pulaka pit-spoil bank landform unit. (which invariably provide the greatest relative relief on the islands. The pits are utilized for the growing of root crops and fruits. in particular pulaka. taro and banana. More recent anthropogenic changes were associated with construction activities during the Second World War. notably in the provision of airstrips by levelling uneven topography and reclamation with fill obtained by excavating borrow pits through the island deposits down to the reef flat 'hard-pan' beneath several islands. The resulting landforms are a feature of the atolls of Nanumea. Nu!cufetau and especially Funafuti.

ISLAND LANDFORM UNITS A geomorphic investigation of all of the islands in the country was carried out as pan of the recent Land Resources Survey of Tuvalu. During this survey it became apparent that a basic suite- of natural and anthro enic landforms was common to many islands. not discounting the considerable differen~es in individual island size. shape and exposure and intra- and inter-island differences in morphological details. For present purposes the following general landform units can be distinguished:

(I) Oceanside ridge(s) (2) Lagoonside ridge(s) (3) Lagoon margins. sandy inlets and saline flats (4) Central depression or central flat (5) Pulaka pit-spoil bank complex

These landform units are described below and their distribution on one atoll (Nui) and one table reef (Nanumaga) illustrated in Figures 2 and 3. Table 4 compares the terminology used in this paper in relation to that developed by Sollas (1904) and David and Sweet (1904) for Funafuti.

175

NUl ". Landform Units

"" Tokinivae

""\ "" Pongalei . ~ _ " o 1 I. ~ M~tuPUakaka ~ Thipe

".

...... o Ocean ridge complex _ Pulaka pit-spoil bank complex fBi Lagoon ridge complex P:'i!i~·1 Saline flats & sandy inlets

I I Interior flats & ridges 0'-, _ _ __.... ! kilometre

Figure 3 Landform units of Nui atoll 176

Table 4 Island landform terminology

Sollas (1904, p.12) David and Sweet (1904) This paper

Seaward or outer ridge Outer Hurricane Bank (p.65) Oceanside ridge(s) Outer Hurricane Bank (p.105)

Lagoon mound Inner Hurricane Bank (p.65) Lagoonside ridge(s) Lagoon Hurricane Bank (p.lll)

Lagoon margins, s: ndy inlet, saline flat

Central flat Corrosion hollow (p.65) Central depression or flat Central depression or solution area (p .108)

Pulaka pit-spoil bank complex

1. Oceanside ridge(s) The seaward or oceanside perimeter of most of the reef islands and atoll motu in Tuvalu is characterised by a single distinct ridge or multiple ridge-swale complex. Because there appears to be a fairly close relationship between ridge characteristics (height, width, sediment calibre) and exposure (to wind, waves, storms), a useful distinction can be made between the coastal ridge(s)on the windward (eastern and southern sides) and leeward (western and northern) sides of a reef. This distinction is best demonstrated on the larger atolls and islands, especially those that are oriented north-south or northwest-southeast. On the small reef islands and those that are oriented east-west, the distinction between windward and leeward is less clear because refracted waves can wrap around the whole island irrespective of the deep water wave direction, resulting in a more uniform distribution of ~ave energy and mixing of sediment types. For instance on small Niulakita the coastal ridge complex extends around the margin of the island where it is about 70-140 m wide and has a similar general form throughout. On the other hand, on larger Vaitupu, where the distinction between windward and leeward is more in evidence, the coastal ridge complex can be subdivided into four types - northern, eastern, southern and western - each being quite different from the other. 177 Notwithstanding such differences, two features are common. Invariably the highest natural elevation on any islet/island is associated with the coastal ridge crest, generally 3-5 m above the adjacent reef flat level and in some cases higher. Second, the ridge form is usually asym metrical with a steep seaward slope and more gentle backs lope inclining towards a central depression, flat, trough or lagoon.

Three other observations are pertinent: (i) simple single ridges with steep crests are associated with the elongate islets, while dual or multiple ridge-swale complexes are found on the broader more compact islands and the lagoon ward recurving ends on narrower islets; (ii) the most seaward ridge (outermost, most recent) is often the highest, although this is not always the case (e.g. along the western and southern sides of Vaitupu and Fenua Tapu (Nui) respectively, the penultimate ridge is the highest); (iii) dual or mUltiple ridge sequences comprise concentric or parallel ridges indicating the persistence of islet shape with increasing islet size.

In terms of sediment calibre the windward ridges generally consist of coarse coral gravel, cobbles or platy rubble. Clasts are edge rounded and imbricated towards the oceans. The ridge usually surmounts a cemented conglomerate platform or hard-pan which outcrops at the base of the ridge and towards the centre of the active beach. Single ridges with crests immediately behind the beach-vegetation line are well developed along the exposed margins ' of the elongate islets on the eastern sides of the atolls of Funafuti, Nukufetau and Nukulaelae. Such ridges are constructed of reefal materials initially deposited during exceptional storms and reworked during quiet periods. Incremental build-up of a si ngle high ridge may result from washover during several storm-fair weather episodes. Alternatively. each episode may result in the formation of a new ridge to seaward thus enlarging the :sland.

Leeward ridges are generally sandier, broader and have more gentle slopes. They are, however, as distinct as their windward counterparts. Along the western side of Vaitupu the sandy coastal ridge complex occupies a 250-300 m wide strip 4 krn long. It consists of two ridges, the inner one being the highest. Dual ridges are also found in the west and south of Nanumea, Nanumaga and ui. Two main phases of deposition are indicated. Sand is the d"minant sediment, but coarser materials are also deposited and reworked in a downdrift direction to form shingle ridge or complexes. The spit which enclosed the n"nhern lagoon on Vaitupu is an example of the first, while the t.ightly packed sequence of low gravel ridges at the nonhwestern end of Meang (on ui ) provides an example of the second. Westerly storm s in summer are capable of moving the large calibre materials and occasionally coral rubble and blocks are deposited on the leeward sides of atolls forming rubble tracts rather 178

than ridges. These fonn the loci for later islet growth and in some ways are comparable to the lirear gravel ridges which have a similar function on the windward side.

While most of the coastal ridges run parallel with the reef edge or present shorelines, some shore-nonnal or orthogonal ridges do occur, notably on Nui, Vaitupu and Funafuti. These transverse ridges are, usually made up of a hard basal core of conglomerate veneered by coarse rubble and sand on the surface. Successive narrow tongue:like islets trend across the reef flat and are separated from one another by rocky channels or finer deposits of mobile sand and gravel. Such islets occur around the lagoon entrances on Vaitupu and on the eastern reef platfonn of Nui. In time, adjacent islets may be linked by the addition of a shore parallel ridge on the ocean and/or lagoon side. The pattern of transverse ridges and swales along eastern Vaitupu, and on the larger islets of eastern Nui and western Funafuti atolls, suggest that such islet linking and extension has occurred. The result is the fonnation of a larger more compact islet with a more complex pattern of coastal ridges.

2. Lagoonside ridge(s) The lagoonside ridges are less pronounced than the seaward coastal ridges which predate them and protect them from ocean swell and stonn waves. The ridges are lower, typically extending 1 to 2 m above lagoon reef flat level, the absolute elevation varying dependent on exposure and lagoon fetch length. Whether bank- or mound-like, the ridges are typically built of foraminiferal or algal sand or molluscs derived from the adjacent lagoon, though occasionally spits of coral gravel recurve around the ends of islets from seaward to fonn a lagoonside ridge. In such cases several closely spaced ridges may be found. Mostly, however, the lagoon ridges are single features possessing an asymmetric morphology, steeper to the lagoon.

Lagoonside ridges have their most obvious expression along the western shores of the elongate atoll islets on the eastern sides of Nukulaelae, Funafuti and Nukufetau where they form linear features several kilometres in length. They are less well developed, although ,present, on NuL Cemented beachrock is commonly developed on the lagoon ~hores of the ridges.

3. Lagoon margins, sandy inlets and saline flats In several areas the ocean or lagoon ridges fonn spits or banks which create shallow re­ entrants around the lagoon margins. These inlets may be invaded by water at high tide or only occasionally during the very highest tides. Sedimentation within the inlets, typically by . fme grained muds and san~y sediments and orga~ic debris, takes place through the building of low gradient peripheral beaches and by the trapping of sediment by vegetation, milngroves, 179

Pemphis, algae and salt grasses. As a result the inlets get converted to 'dry' land saline flats, the conversion process being hastened in instances where a barrier develops right across the inlet entrance.

Virtually all stages in the conversion process from inlet to flat can be identified on Nui and Vaitupu where the largest and most advanced stages are in the Pongalei and Tengiemate areas of eastern Nui and eastern Vaitupu respectively. On both these and other islands (e.g. Nukulaelae) natural sedimentation around the lagoon margins is being aided by shore protection and reclamation works.

4. Central depression and central flat Enclosed within the encircling oceanside ridges or situated between the seaward and lagoonward ridges, the land surface may be flat or form a shallow depression or trough. While there are considerable inter-island differences in detail, two features of the central depression are common. First, it typically has the lowest land elevations on any island, and is the area most likely to be inundated by a rise in the water table or washover from storm wave action. Second, the surface deposits, generally sands on the flats and gravels in troughs, are quite thin, typically 0.5-2.0 m thick, and these form a veneer over a 'hard pan' of cemented conglomerate or former reef flat. In places ' the basal surface is exposed at the surface, enclosing a lagoonlet, tidal pool, pond or basin.

On long islets the interior low area primarily results from the spatial separation of the higher seaward and lagoonward ridges, rather than from post-accumulation solution as suggested by David and Sweed (1904) for Funafuti. Formation of the adjacent ridges precludes the supply of sediment to fill the intervening gap, although in exceptional storms where the ridges are narrow or low, breaching or overtopping may occur and lobes of sediment can be deposited in the gap. This occurred on Funafuti during Hurricane Bebe in 1972. However, the effects are local and on the elongate atoll motu on the eastern side of Funafuti, Nukufetau and Nukulaelae the central depression occupies an almost continuous trough several kilometres long and rarely more than 100 m wide separating the seaward and lagoon ridges.

Wider low areas occur elsewhere. On the two main islets ' of Nanumea the central depression reaches 300 m wide on the-northern islet () and over 500 m wide on the southern islet (Nanumea). In both cases the surface is flat, comprised of sand and has an elevation of approximately 2 m above reef flat level. Similar broad features occur on Vaitupu and Nui although in these instances topographic surveys show that the surface consists of broad undulations that are barely recognizable on the ground. Deposition of these flats 180 appears to have resulted from a combination of lagoonal inlet sedimentation and coastal accretion when the islands were at an incipient stage of development. Frequently these areas are associated with the presence of phosphatic soils.

On the table reef islands of Nanumaga, Niutao and Niulakita the central depression appears to have been formed by coastal ridge occlusion, the basin shape of the interior resulting from backslopes of the innermost ridges which enclosed a former shallow lagoon and its rocky surrounds. Non-cemented deposits, where they occur on the floor of the basin comprise thin scattered coral gravels; mangrove and Pemphis vegetation is common in these low and inhospitable areas.

5. Pulaka pit-spoil bank complei In contrast' to the previous landforms which have resulted from natural geomorphic processes, the pulaka pit-spoil bank complex is an anthropogenic landform comprising an artificially constructed flat-floored depression (pit) surrounded by an elevated rim (spoil bank). The pit is excavated to the level of the water table and spoil from the diggings is dumped around the edges building a spoil bank above the level of the surrounding land. The resulting topography is frequently chaotic, especially where groups of pits and banks occur in close proximity. Invariably the highest relative relief on any island is associated with this landform unit.

Pulaka pits occur on each of the islands in Tuvalu with the exception of Niulakita. Generally the pits are concentrated in particular locations. On the reef islands of Niutao and Nanumaga there are three and four main areas of pits respectively. On the atolls, pits are concentrated on one islet at Nanumea, two islets at Nukulaelae and three islets at Nui and Nukufetau. On Vaitupu, the distribution is much more widespread with over 100 separate areas of pits scattered throughout the island. While there is great variety in the number, size and distribution of pits qn each island (which is a function of population size, tenure, communal organisation, accessibility and ground water quality amongst other things) it appears that most pits are located in marginal positions, coincident with the junction between the inner backslope of the coastal ridge and the c~ntral depression.

The foregoing sketch of the basic landform units does not do justice to the variations in Tuvalu's island morphology, either in plan or in profile. Details of such variations are evident on the magnificent coloured geological maps and sections of Funafuti which accompany David and Sweet's (1904) text in the Report of the Coral Reef Boring Committee of the Royal Society on the Atoll of Funafuti; and for the other islands, on the landform maps and cross-sections included in the individual Island Reports of the Tuvalu Land Resources 181 survey. Nevertheless, to a greater or lesser degree, the basic landfonn units are present on most islands.

CONTROLS AND PROCESSES OF ISLAND BUILDING AND srABIL IZA TION The present distribution and characteristics of island types and landform units has resulted from the interplay of a complex set of controls and of processes both ancient and modem, natural and anthropogenic. Size and geometry of the reef top, sediment production and availability from adjacent reefs and lagoons, and relative exposure are important controls. A combination of local wind-driven waves, including the easterly trades and summer westerlies as well as long ocean swells of distant origin (Mclean, 1980), are the primary processes in moving sediments and constructing islands. although catastrophic storms may be of crucial importance in the initial delivery of a large mass of coralline debris to the reef flat. This material can be later reworked into island deposits by normal wave action. The massive storm rubble banks formed during Hurricane Bebe in 1972 along the eastern sides of Nukufetau and panicularly Funafuti provide local examples. The initial Funafuti deposit was described in detail by Maragos, Baines and Beveridge (1973) and the first stages of its subsequent landward migration by Baines and Mclean (1 976). Analogous but older storm deposits comprising the ocean ridge(s) landform unit on Funafuti have been described by Hedley (1 898). Sollas (1904), David and Sweet (1904) and McLean (1974). CopectivelY these references document the important role of storms in island building and extension.

Once formed, emergent island deposits are stabilized by different processes including colonization by terrestrial vegetation and lithification of sediments. Vegetation, through root binding and the accumulation of humus stabilizes the surficial materials and enables soil formation to proceed. Lithification of unconsolidated island deposits also takes place through several processes including phosphatization. Details of thi s process and on the resulting crustose phosphates, which occur on all nine islands in Tuvalu, are given by Rodgers (1989). The degree of soil formation (depth, horizonation, colour) and phosphatization are important indicators of the relative age of the deposits in which they occur and thus can be used as aids in deciphering the chronology of island landform development. Older surfaces are occupied by deeper, darker soils with some horizonation or by crustose phosphates, while younger surfaces possess thin, light and immature soils and show no obvious signs of horizonation.

Not only are emergent land surfaces stabilized through vegetation and lithification but so are the island margins. Intertidal and supratidal vegetation, such as mangrove and Pemphis and other coastal shrubs and trees serve to slow down panicle movement and entrap sediment. Unconsolidated coastal sands and gravels become lithified, and in Tuvalu two types of rock 182

are commonly fonned: beachrock and beach conglomerate. Both give an island some resistance to erosion. Beachrock is an intertidally cemented sandstone which slopes at the same angle as the beach. Its. presence marks the location of the contemporary or previous island beachline. Beachrock is common along the sandy lagoon shores of the atoll islets and along the leeward beaches on the reef islands. Beach conglomerate is composed of coarser rubble and sand and is firmly indurated. It forms roughly horiwntal platfonns ,and was initially described as the 'breccia sheet' or 'breccia rampan' by David and Sweet (1904) on Funafuti. Conglomerate or breccia platfonns commonly occur in two locations in Tuvalu; as shore-parallel bands along the ocean shore on the eastern sides of Nukulaelae, Funafuti, Nuktifetau and elsewhere; and, less commonly as shore-normal strips that run perpendicular to the shore e.g. on the eastern side of Nui and Vaitupu. Both beachrock and conglomerate :fonn natural seawalls, and groynes, and serve to protect an island from erosion and to slow down and trap sediment moving alongshore.

ORIGIN AND EVOLUTION OF ATOLLS AND REEFS The foregoing processes of island deposits and stabilization have not been going on for long; and, in geological terms the islands themselves are extraordinarily young (see next section). In contrast, the basic structural features of the atolls and table reefs are very much older geologically speaking. Atolls and reefs have been in the Tuvalu area for millions of years, and like those elsewhere in the world, they consist of a thick pile of limestone overlying a volcanic basement at depth. The origin of such features, lying hundreds of kilometres from the nearest volcanic or continental land mass, and surrounded by water 3-5 -km deep has been widely debated though Darwin's ideas are still the most widely accepted. Darwin (1842) distinguished three types of reefs: fringing reefs, barrier reefs and coral atolls and suggested that these evolved through an ordered sequence. With subsidence of the volcanic basement and upward reef growth the initial fringing reef becomes a barrier reef which in tum evolves into an atoll with a reef rim surrounding a lagoon. The resulting atolls ta1ce on various shapes and sizes dependent on the nature and size of the original volcano and the processes of coral growth. In time smaller atolls can become table reefs.

'TIiis simple subsidence model has been conflpned through drilling; initially at Funafuti in 1896-98 where the main driIl hole on was still in limestone at a depth of 330 m (Bonney, 1904). More recent geophysical evidence from Funafuti and Nukufetau suggests that the total thickness of limestone over basic volcanic material is about 550 m at Funafuti and 770 m at Nukufetau (Gaskell and Swallow, 1953). It is important to recognize that Darwinian subsidence takes place slowiy over very long periods of time, millions of years. For instance, Schofield (1977a) calculated that in Tuvalu and Kiribati during the Cenowic the average IJIte of sinking has been about 0.025 m per 1000 years. 183

In addition to slow subsidence the developing limestone piles have been subjected to fluctuations in ocean water level as a result of the waxing and waning of Ice sheets, notably throughout the Quaternary. During the last two millions years sea level has risen and fallen many times. Over the last glacial cycle the magnitude of sea level rise and fall has been much more substantial than that of subsidence. Evidence from elsewhere in the central Pacific, including Tarawa in Kiribati, suggests that the last time sea level was around its present position (during the last inter-glacial) was about 125,00Q years ago. Since then sea level has fallen reaching its lowest level (>-100 m) some 20,000 years ago, at which time the atolls and patch reefs of Tuvalu would have been emergent and similar in appearance to present day Nauru and Banaba. Subsequently, with the melting of the ice sheets in the northern hemisphere, sea level rose and flooded the palaeo-islands and permitted the renewed growth of coral over the last inter-glacial reef surface. Although the presence of this older reef has not been confirmed by radiometric dating in Tuvalu, it has been in Tarawa where it occurs at a depth of 11-17 m below the islands and indicates ~hat the base of the modem -sequence rests on reefs of last inter-glacial age (Marshall and Jacobsen, 1985).

Growth of the modem reefs and development of the present detailed morphology of the atolls and table reefs has taken place more recently, in -the last few thousand years. Radiometric dating of corals from boreholes on Tarawa atoll, reported by Marshall and Jacobsen (1985), show that Holocene reef growth began about 8000 years ago, which is consistent with the time of initiation of reef growth on other Pacific atolls (e.g. Enewetak, Mururoa). A minimum sea level curve based on dated corals from Tarawa (Figure 4A) indicates a rate of sea level rise of 5-8 m per 1000 years from 8000 to 6000 years BP, and by projecting this trend it can be seen that sea level fIrst reached around its present position no later than 4500 years BP. Until local borehole data and dates become available, this general model of Holocene sea level rise and reef growth can ~.adopted for Tuvalu.

FORMATION, DEVELOPMENT AND AGE OF ISLANDS Oi ven this perspective it is clear that the surficial features of the present reefs and islands are young, that is their formation and development must post-date c.4500 years B.P. Equa};y clear is the fact that a pre-condition for island formation is the provision of an intertidal reef platform on which subaerial deposits can accumulate. Radiometric dating of in-situ materials from fossil and modem reef flats indicate that in Tuvalu the reef platform fIrst reached sea level about 4000 years ago, although the spatial coverage and number of dated samples is insufficient to assert this with certainty _ Nevertheless results in Table 5 do show that reef flat sUrfaces have been continuously developing since that date . During this period, and with a relatively stable sea level, islands began to develop. An abundant supply of reef and lagoon 184

A +2 Surface samples. various islands. Thvalu 0 O"'Q'Q'Q'vv Q '0'

2

, , o 2 4 6 8 10 Time (thousands of years before present)

B +2 Q) Island formation 0 ~ Q) ~flat formation

14

o 2 (4 6 8 10 Time (thousands of years before present)

Figure 4 Reef and island response to sea level change over the last 8000 years. (A) Time­ depth plot showing minimum sea level curve (dashed line) and corals from the boreholes on Tarawa atoll surface (after Marshall and Jacobsen, 1985). Surface reef flat and island samples from Tuvalu (mainly Funafuti, see text). Note this plot is schematic on the vertical (height) axis. (B) Model of reef growth and island formation for Tuvalu showing three stages of development. 185

Table 5 Radiocarbon ages of in-situ reefal materials

Lab Location Material Conventional Ref number C14 age (BP)

GX 4610 Niulakita - fossil reef flat Coral 3905±150 (I) exposed in pool surface

GX 4613 Funafuti atoll - Calcareous 291O±140 (I) modern lagoonside reef flat algae

GX 4545 Funafuti atoll - Tengako Coral 2525±135 (I) fossil reef flat exposed in borrow pit

GX 4615 Funafuti atoll - Funafara Coral 2160±135 (I) Heliopora modern lagoonside reef flat

MG 191 Funafuti atoll - Coral 19OO±IOO (2) Porites lagoonside reef flat

NZ 1601 Funafuti atoll - Luarnotu Coral 1615±60 (3) Heliasrraea fossil lagoonside reef

NZ 1602 Funafuti atoll - Funafara, Coral 1335±60 (3) Porires lagoonal coast

GX 4543 Funafuti atoll - Fongafale Calcareous 1245±125 (I) modern oceanside reef flat algae

OX 4527 Funafuti atoll - Funangongo Coral 1185±125 (I) Heliopora modern lagoonside reef flat

GX4609 Nukulaelae atoll - Fangaua Coral 1015±130 (I) Heliopora modern oceanside reef flat

(I) This study (2) Kaplin (1981, Table 2) (3) Schofield (1977a, Table I) 186

sediment became available to be shifted by stonns, fair weather waves and swell, and accumulate on reef flats. Early emergent deposits created sheltered environments which induced sedimentation. This in turn encouraged further expansion of island land masses.

Eight radiocarbon dates on coral boulders and other materials from coastal sites on Funafuti, published by Kaplin (1981) and Schofield (1977a) together with 20 unpublished dates on stonn blocks, coral gravels and island sands collected by one of the authors (RFM) on Funafuti, indicate that at least on this atoll islet growth took place very rapidly during the period from approximately 2000 to 1000 radiocarbon years ago. Whether or not this rapid phase of development occurred on other islands in the group is not known. Only one radiocarbon date from another island is presently available: that from a transported coral incorporated within a phosphatized rubble deposit in the centre of Niulakita. This sample (aX 4610) had a conventional radiocarbon age of 4040 ± 165 years, the oldest surficial date from Tuvalu.

Several speculative hypotheses stem from this discussion and limited data. First, that the basic outlines of the reef islands and atoll motu were well established by about 1000 years ago. Although they have since become modified to a lesser or greater degree through subsequent erosion and redistribution of island materials and additions of reef flat, reef edge and lagoon detritus, notably by stonn wave action~ the islands and islets. have persisted since that time. Second, this persistence has been aided by the stabilizing processes discussed earlier, both on the land surfaces and around the periphery particularly through the fonnation of beachrock and beach conglomerate. Third, the initial and older island nuclei, are indicated by areas of crustose phosphate and darker, deeper, more mature soils. These are invariably associated with the interior flats and inner backslopes of the coastal ridges. Fourth, the reef islands are older than the atoll motu. This is a consequence of two factors: (1) the earlier provision of a suitable reef platfonn for island accumulation - implying that upward coral growth with the rise in sea level was faster on the smaller table reefs than larger atolls; and (2) the table reefs became more rapidly and completely covered with emergent sedimentary deposits - because the reef platforms were broader and more compact, thereby having a more readily available immediate source of reefal materials for emplacement.

DISCUSSION AND REVIEW Based on the foregoing data a three stage model relating sea level change to reef growth, reef flat fannation and island formation has been developed (Figure 4B). This model is believed to be applicable to all of the islands in Tuvalu; reef islands and atoll motu. What is clearly shown is that reef flats and islands have developed very recently. Implied is the 187 fact that the basic suite of landforms found on the islands also developed very rapidly. Also implied, but not clearly shown, is that the absolute timing of stages 2 and 3 can vary between islands, as well as within islands, the latter particularly on the larger atolls. Thus, reef flats and islands can be at different stages of development: at the present time some are at an early stage (e.g. Funafuti); others, at a more mature stage (e.g. Niulakita) as evidenced by the data in Table 2.

Th~re is a second point. Island fonnation in this model is clearly associated with a first order still-stand of sea level roughly around its present position. The question of whether or not there have been minor variations in sea level over the last 4000 years, as suggested by Schofield (1977a) is not addressed. Nor is the question, raised initially by David and Sweet (1904) and supponed by Schofield (1977b) that a recent fall in sea level has been the dominant cause of the fonnation of atoll islets. While there is clear evidence for recent emergence elsewhere in the South Pacific, the evi<,lence is more ambiguous in Tuvalu. Cenainly no clear indisputable evidence of emergence is discernible from island morphology, at least on the atolls. Many more radiocarbon dates from both the reef islands and atoll islets are required to test the hypotheses proposed in the previous section,;llI1d before the sequential development of the island landfonn units, described earlier in this' paper, can be fitted into an absolute chronology.

While there is abundant evidence of contemporary erosion and accretion around many of the islands in Tuvalu, it is believed that most have persisted in their general location for a relatively long period of time. Cenainly all of the islands sketched, mapped or charted by the explorers, traders, missionaries and visitors in the 19th Century are easily recognizable today. And a comparison of the detailed maps surveyed during the 1896-98 scientific expeditions to Funafuti, and those of today show little change, notwithstanding the impact of Hurricane Bebe in 1972.

For the future only minor changes in the reef island outlines of Nanumaga, Niutao and Niulakita are forecast because they already occupy over two-thirds of the available reef platform. Similarly with Vaitupu. But on the larger atolls reef platform space is still available for island accretion and stonn waves are expected to playa significant role in future islet development. In all cases human activities will be imponant in causing detailed landfonn changes on the islands and around their shorelines. The question of possible landform changes ~sulting from the projected rise in sea level ,consequent upon the greenhouse effect is not considered here. 188 REFERENCES Baines, G.B.K. and McLean, RF. 1976 Sequential studies of hurricane bank evolution at Funafuti atoll. Marine Geology 21, Ml·M8.

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