SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

Coral Records of Relative Sea-Level Changes C D Woodroffe, University of Wollongong, Wollongong, NSW, Australia ã 2013 Elsevier B.V. All rights reserv...

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Coral Records of Relative Sea-Level Changes C D Woodroffe, University of Wollongong, Wollongong, NSW, Australia ã 2013 Elsevier B.V. All rights reserved.

Coral and Sea Level Coral and coral reefs have played an invaluable role in the determination of past relative sea-level changes throughout the Quaternary at a range of time scales. Individual corals grow by calcification, their polyps secreting carbonate that is added to the exoskeleton of the colony. Coral polyps maintain a symbiotic relationship with species of dinoflagellate algae, called zooxanthellae. These limit the depth within which corals can survive to the photic zone (typically 0–30 m in clear tropical waters). Corals cannot tolerate more than very brief exposure when emerged above the water surface at lowest tides. Coral reefs are thus restricted to a depth range that varies from around mean low water, to depths at which water clarity limits photosynthesis (excepting cold-water corals found in high latitudes). Coral reefs cover more than 250 000 km2 of the earth’s surface largely within the tropics where sea surface temperatures exceed 18  C (Veron, 1995). The greatest diversity of corals occurs in the Indo-Pacific with a second region centered on the western Atlantic; reefs are absent from the Mediterranean and reef development is limited in the eastern Atlantic and eastern Pacific Oceans (Figure 1). The distribution of species across an individual reef, and of particular growth morphologies, varies according to depth, and in relation to several other environmental parameters such as wave energy, light availability, and sediment loading. Individual coral colonies can continue to grow for several hundreds of years, and after their death may be incorporated into the reef structure, together with other calcareous organisms such as Foraminifera, molluscs, and coralline algae. The reef fabric may be interpreted and the position of the surface of the sea during the time of coral growth can sometimes be inferred on the basis of species or growth form. The growth form of corals varies from delicate branching corals, through encrusting forms, to hemispherical colonies of massive corals. Large colonies of coral contain various geochemical records of conditions reflecting ambient water chemistry experienced during their growth. Such proxy reconstructions recovered from modern living corals have been correlated with, and extend, instrumental and historical records, whereas fossil corals provide an archive of water chemistry conditions at previous times during the Quaternary. Those corals that have been particularly constrained by exposure within the intertidal zone, provide the clearest evidence of former sea levels, particularly if it can be demonstrated that the coral is in growth position. The role that reefs play in reconstructing sea level is comprehensively reviewed by Hopley et al. (2007, chapter 3), Montaggioni and Braithwaite (2009, pp. 405–428), and in numerous entries in the Encyclopedia of Modern Coral Reefs (Hopley, 2011). This article examines the contribution that coral records have made to the reconstruction of Quaternary sea level at four different time scales. First, the value of coral for

the recognition and dating of interglacial sea-level highstands reconstructed from terraces of reef limestone is described, particularly on uplifted shorelines throughout the tropics (Figure 2(a)). Second, the pattern of postglacial sea-level rise determined from drilling the foundations of modern reefs is examined, indicating the broad coincidence of reconstructions from disparate sites. Third, and in contrast, the role of coral in clarifying the variability of relative Holocene sea-level curves along tropical shorelines in the far-field (i.e., remote from former ice sheets), recording variability of record at different sites, is outlined. Finally, the potential of intertidal corals, called microatolls, to track interannual sea-level variations is reviewed.

Quaternary Sea-Level Highstands The significance of fossil reef limestone, emerged and forming terraces elevated above modern shorelines particularly around the Pacific Ocean, was recognized in the early twentieth century by Reginald Daly who realized that sea-level history provided further support for former Ice Ages that were recorded by moraines marking a series of glacial advances in Europe (Daly, 1915). The advent of suitable dating techniques made it possible to place these shorelines into a sequence based on age, discriminating Holocene from Pleistocene, and more recently, differentiating successive interglacial sea-level highstands. Fossil coral reefs have played an important role over recent decades in refining the history of these sea-level variations. A sequence of reef terraces on Barbados, and a more extensive series on the Huon Peninsula on the north coast of Papua New Guinea, contain corals, now elevated by uplift. Dates on these corals constrain the age of past shorelines. Whereas Holocene corals can be dated using radiocarbon dating techniques (appropriate to determine ages within the past 40 000 years), Pleistocene corals are largely beyond the range of radiocarbon dating, and have been dated primarily by the uranium-series disequilibrium dating methods, supplemented by several other techniques such as electron spin resonance. U-series dating measures the accumulation of 230Th as a daughter isotope in the uranium decay chain over the past 500 000 years, and U-series and 14C date comparisons on corals have extended calibration of the latter technique (Bard et al., 1990). With the initial application of U-series dating, Veeh (1966) showed that there was widespread evidence throughout the tropics for a Last Interglacial shoreline around 120 000 years old at heights averaging 6 m above the present sea level. The islands and continental shorelines, such as Bermuda, Bahamas, Seychelles, and Western Australia, were considered stable. However, several of these island sites, such as Oahu in the Hawaiian Islands, are now considered to have undergone flexural adjustments in response to loading of the ocean floor by adjacent volcanic islands. Mangaia in the Cook Islands for example, together with Mitiaro (Figure 2(b)) and

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Figure 1 Present-day distribution of coral reefs (based on Veron, 1995), showing location of key sites discussed in text: 1, Comoros Islands; 2, Seychelles; 3, Maldives; 4, Cocos (Keeling) Islands; 5, Christmas Island; 6, Abrolhos Islands; 7, Rottnest Island; 8, Ryukyu Islands; 9, Sumba, Indonesia; 10, Huon Peninsula, Papua New Guinea; 11, Guam; 12, Tonga; 13, Hawaiian Islands; 14, Tahiti; 15, Cook Islands; 16, Cayman Islands; 17, Haiti; 18, Bahamas; 19, Bermuda; 20, Barbados.

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Figure 2 Pleistocene reef terraces indicating former highstands of sea level: (a) the northeastern corner of Christmas Island in the Indian Ocean, uplifted by flexure as it migrates over a bulge in the ocean floor prior to subduction into the Java trench, shows a series of terraces that mark former reefal shorelines, the lowermost of which is Last Interglacial in age; (b) the coast of Mitiaro in the southern Cook Islands which is composed of Pleistocene reef limestones that show a pattern of former grooves, mirroring, though at greater spacing to, the modern spur and groove which can be seen seaward of the fringing reef crest; (c) a sequence of two Pleistocene reef limestones on the neighboring makatea island of Mauke, also in the southern Cook Islands, the lower limestone was deposited during marine oxygen isotope stage 7 and the upper (the base of which is marked by the survey staff) is Last Interglacial in age (marine oxygen isotope substage 5e); (d) an erosional notch cut into Tertiary limestone (foreground) and the Last Interglacial reef limestone (indicated by the person in the background) marks a former highstand of the sea on Cayman Brac in the Cayman Islands – similar evidence is widespread throughout many of the islands in the West Indies, especially the Bahamas; (e) a similar reef limestone terrace characterizes the shoreline in Tonga in the Pacific Ocean (here shown on Lifuka in the Ha’apai group in central Tonga); and (f) the southernmost example of such a reef terrace occurs at Fairbridge Bluff on the island of Rottnest off the coast of Western Australia, where a reef existed during the Last Interglacial beyond the latitudinal limit to which present-day corals construct modern reefs.

SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

Mauke (Figure 2(c)), are makatea islands comprising a volcanic interior but with a rim of limestone (makatea) that contains reef units that were formed during marine oxygen isotope stages 7 and 5. These reef units occur at different heights on the different islands as a result of differential rates of flexure in response to loading by the younger volcanic island of Rarotonga (Woodroffe et al., 1991). Particularly useful have been those shorelines that have been uplifted on which there is a flight of raised reefs generally with the higher, and more landward, ones being older. Three such terraces occur above sea level on Barbados; more occur on the rapidly uplifted Huon Peninsula in Papua New Guinea. On the basis of dating of the Huon Peninsula terraces from several sites at which there have been different rates of uplift, a history of sea-level positions has been developed, and it has been possible to link this record with that of oxygen isotopes from deep-sea cores (Chappell and Shackleton, 1986). Figure 3 shows the sea-level curve derived from the Huon Peninsula and deep-sea core evidence. More terraces are found on those coasts that are uplifting rapidly, such as at active plate margins, including islands in Indonesia such as Sumba, along the coast of Japan including the Ryukyu Islands, and in the Caribbean, for example on the island of Haiti. With refinements in dating it has been possible to extend the chronology back to identify older interglacial reefs. Reefs associated with oxygen isotope stage 7 are now widely recognized (Figure 2(c)), and reef limestones associated with stages 9 and 11 have also been dated (Siddall et al., 2006). Diagenetic changes, particularly recrystallization of coral mineralogy (aragonite to calcite), impose constraints, but the advent of thermal ionization mass spectrometry (TIMS) U-series dating, and modeling of open system behavior based on isotope ratios, have enabled greater precision for age determinations on increasingly small samples (Thompson and Goldstein, 2005). The record of sea-level highstands derived from radiometric dating of corals from Pleistocene reef limestones provides chronological support for control on the timing of ice-sheet growth and ocean volume variations through the superimposition of variations in the eccentricity, obliquity, and precession of the earth’s orbit. More recently, it offers the prospect of resolving millennial-scale, suborbital oscillations of amplitude

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10–15 m that appear overprinted on the longer-term oscillations (Figure 3). Such variations have been observed during glacial periods, 55–30 ka (Yokoyama et al., 2001), and appear within interglacial reef terraces such as those on Huon Peninsula, Oahu, and Barbados during oxygen isotope stage 5 (Thompson and Goldstein, 2005). Figure 4 illustrates how the reef limestone terraces are superimposed in different ways on shorelines that are experiencing different rates and patterns of vertical movement. Whereas fossil shorelines are stretched out on uplifting coasts, they are superimposed on coasts that are stable, such that the Last Interglacial is generally the most prominent terrace (Figures 2(d), 2(e), and 2(f)). A one-dimensional forward computer spreadsheet model of reef growth based on a sea-level compilation like that shown in Figures 3 and 4 has been developed (Koelling et al., 2009). This enables simulation of the anticipated sequence of reefs in different tectonic settings. It produces stepped reef terraces that resemble those observed on Huon Peninsula and in other settings (Figure 5). It is also possible to identify sequences of Pleistocene reefs on shorelines that have been subsiding. Coral atolls provide one example of such a setting and here carbonate layers have formed like icing on a cake, at periods when the sea has been high. Dating of successive highstands, separated by solutional discontinuities, constrains the pattern of subsidence (Figure 4). On the one hand, these sequences provide a record of sea-level stands, on the other hand, there is a need to establish the rate of coastal uplift or subsidence, and whether it has been constant. Last Interglacial reef limestones are prominent from many sites in the tropics and there is an emerging consensus that they formed during a relatively stable period of sea level 2–6 m above present between 128 and 116 ka (Siddall et al., 2006). However, there is less agreement about the details of smaller oscillations, as indicated by reef growth, within that highstand. Several researchers have identified what appear to be multiple reef units within the Last Interglacial. Others, through comparison of constructional reef with erosional notches, have inferred that the interglacial culminated with a rapid rise of several meters such as could occur if the West Antarctic ice sheet collapsed (Blanchon et al., 2009).

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Figure 3 A curve of sea level over the past 240 000 years derived from dating of raised reef terraces on Huon Peninsula correlated with the oxygen isotope record from deep-sea cores. Reproduced from Chappell J and Shackleton NJ (1986) Oxygen isotopes and sea level. Nature 324: 137–140. The curve has been derived by correcting the observed elevation of fossil corals for uplift using an uplift rate inferred from the height of the Last Interglacial shoreline. Evidence for a more detailed sequence of oscillations has been proposed on the basis of further dating of corals from Huon (large dots show samples used by Chappell et al., 1996, oscillations during glacial times, green line, is after Yokoyama et al., 2001). Recently, closer scrutiny of U-series ages and their correction for open system behavior has also implied oscillations during interglacial times (red lines). Reproduced from Thompson WG and Goldstein SL (2005) Open-system coral ages reveal persistent suborbital sea-level cycles. Science 308: 401–404.

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Figure 4 Schematic representation of reef terraces on shorelines that have different histories of vertical movement. The inset shows a simplified version of the sea-level curve shown in Figure 3, highstands identified with marine oxygen isotope stage 7 and substages 5e, 5c, and 5a are marked. The relative location of terraces representing those highstands on shorelines that are uplifting at an average rate of 0.5 m a 1, stable, and subsiding at 0.1 m a 1 is shown, and can be compared with the modern reef that has developed during the Holocene.

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Distance (m) Figure 5 A one-dimensional forward computer spreadsheet model of reef growth based on a sea-level compilation like that shown in Figures 3 and 4 has been developed. This enables simulation of the anticipated sequence of reefs in different tectonic settings. It produces stepped reef terraces that resemble those observed on Huon Peninsula (illustrated) and in other settings (based on Koelling et al., 2009).

Postglacial Sea-Level Rise At the Last Glacial Maximum, around 20 000 years ago, sea level was of the order of 120–130 m lower than present as a result of the volume of water that was locked up in ice sheets. Reconstruction of the shoreline at this time is largely derived from morphological evidence such as submerged cliffs and notches, and deposits of shallow-water shells. Reefs occurred along what were to become the foreslopes of modern reefs and these deposits are now largely unknown or inaccessible.

The pattern of ice melt and concomitant sea-level rise during the termination, however, has been reconstructed primarily from the vertical growth of reefs that tracked the rising sea (Figure 6). The initial reconstruction was based on drilling of offshore reefs around Barbados (Fairbanks, 1989). An extended set of analyses of the deeper corals from this site, combined with a model of continental deglaciation, indicated that the Last Glacial Maximum may have experienced this period of low sea level from as early as 26 000 years ago (Peltier and Fairbanks, 2006). The transgression is recorded at several

SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

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rise (Montaggioni and Braithwaite, 2009). One consequence of phases of rapid sea-level rise may be the drowning of reefs. Recently, extensive relict reefs have been described from water depths of around 30 m, both in the muddy waters of the southern Gulf of Carpentaria, Australia (Harris et al., 2008), and at the site of the southernmost Pacific reefs at Lord Howe Island (Woodroffe et al., 2010).

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Figure 6 Record of postglacial sea-level rise from the peak of the glacial until the apparent cessation of ice melt around 6000 years ago (data from Barbados, Fairbanks, 1989; Huon Peninsula, Chappell and Polach, 1991; Tahiti, Montaggioni et al., 1997; and Mayotte, Comoro Islands, Camoin et al., 2004).

other sites. The latter part of this sea-level rise was determined simultaneously by drilling through the Holocene reef on the Huon Peninsula in New Guinea on the other side of the world, which showed a similar pattern of rise (Chappell and Polach, 1991). Subsequently, comparable long records of postglacial sea-level rise have been derived from Mayotte in the Comoros Islands and Tahiti in the Society Islands (Bard et al., 2010), with only a slightly shorter record from the Abrolhos Islands off the coast of western Australia (Eisenhauer et al., 1993). Independent support for the reconstruction of this pattern of sea-level rise comes from dating of muddy shoreline deposits such as mangrove wood and peat on the continental shelf off northwestern Australia and on the Sunda Shelf. The curves derived from these disparate sites are remarkably similar and have been used in attempts to interpret phases of ice melt (meltwater pulses) and rapid sea-level rise (Figure 6). However, there is a limit to the rate at which reefs can accrete vertically. Although individual colonies of coral can grow rapidly, with massive corals averaging an extension of around 10 mm a 1 and branching corals up to 100 mm a 1, the reef itself is a porous aggregation of coral containing calcareous algae and loose calcareous sediment, incompletely lithified into a structure and partially eroded by physical and biological erosion processes. Reefs rarely accrete vertically at rates of more than 10 mm a 1, and in most cases at rates considerably less than this. Consequently, reefs have been unable to keep pace with the most rapid rates of sea-level rise that have been driven by pulses of meltwater, except on those coasts that are uplifting (Montaggioni, 2005). The peak of the last glaciation appears to have been terminated by rapid polar melting around 19 ka with little evidence that reefs in existence at that time were able to keep pace with the rising sea. Several phases of reef generation have been suggested separated by meltwater pulses. There is general acceptance of a rapid rise in sea level associated with meltwater pulse 1A (MWP-1A) around 14 ka when a rise of about 20 m occurred in less than 500 years, but there is less consensus in relation to subsequent periods of rapid sea-level

Holocene Sea-Level Variations Whereas the early stages of postglacial sea-level rise appear to be generally similar at points around the tropics, or discrepancies can be modeled in terms of mantle viscosity, the pattern of mid- and late Holocene sea-level change varies geographically. As the rate of sea-level rise has slowed, regional disparities in the pattern of relative sea-level history have become increasingly apparent (Lambeck et al., 2010). Corals have played an important role in deciphering these sea-level changes. A significant difference between the pattern of sea-level change in the Pacific and that in the West Indies was first recognized by Fairbridge (1961) who plotted radiocarbon ages for coral and other shoreline indicators from around the tropics onto one age–depth diagram. He interpreted the evidence to indicate a series of oscillations in sea level over the past few millennia, but the evidence for periods of sea level above its present level came primarily from Australia, whereas that for sea level remaining below present level came from Florida and the Caribbean. Since the 1970s, it has been increasingly accepted that the sea-level envelope which best describes the Pacific region reached a level close to present around 6000 years ago with evidence of a mid-Holocene sealevel highstand, whereas in the Atlantic region, sea level has been continuously rising up until present but at a decelerating rate (Adey, 1978). It is now recognized that the earth’s crust has undergone vertical movements in response to the loading of water in the ocean basins and across continental shelves. This is termed hydroisostasy. Rapid uplift of those areas that were covered by ice sheets has occurred, and intermediate field areas in what is termed the forebulge have undergone submergence as a result of compensation within the mantle (Lambeck et al., 2010). Those parts of the tropics that are in the far-field and therefore not influenced directly by the isostatic rebound that has accompanied the melting of the ice sheets, appear to have experienced a slight lowering of sea level over recent millennia. This process has been termed ocean siphoning; as the volume of water in the ocean is spread throughout the ocean basins that have increased slightly in size as a result of adjustments in the forebulge region (Mitrovica and Milne, 2002). This is most clearly seen by the contrast in the elevation of sea-level evidence between the western Atlantic and the Indo-Pacific region. In the western Atlantic, there is no evidence for the sea level having been above its present level during the Holocene. Reefs contain fewer coral species in the West Indies than in the Pacific region. One species of staghorn coral, Acropora palmata, dominates the reef crest and grows most prolifically within the upper 5 m on the reef front. It has been widely used to reconstruct sea level because it is considered an indicator of shallow water (Lighty et al., 1982;

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Toscano and Macintyre, 2003). Although the depth of living A. palmata has frequently been observed to be concentrated in this depth range, it is important to note that individual colonies of the coral are also present in deeper water. Use of material of this species recovered in cores has been further criticized because it is rarely possible to determine whether the dated material was actually in its growth position, and there are situations where fragments of this branching coral have been transported either to greater depths or onto land and deposited in storm ridges, implying that uncertainties of sea-level position could range over 10 m (Blanchon, 2005; Gischler, 2006). Figure 7 summarizes radiocarbon dating results, extending an earlier sea-level reconstruction for the West Indies and indicating sea-level rise at a decelerating rate up until present based on ages determined on A. palmata. In contrast, there is widespread evidence from the IndoPacific region that the sea reached a level close to present at least 6000 years ago and that it has been slightly higher since then in much of the region. Figure 7 also shows radiocarbon dates from coral, and intertidal indicators such as mangrove sediments, from the northern Great Barrier Reef (Larcombe et al., 1995). The clearest evidence comes from regions where massive corals, particularly microatolls (described below), are found in their position of growth (Chappell, 1983). Whereas there are emergent reef limestones of Last Interglacial age on many islands in the Pacific, as described by Daly, it is also clear that some of the emergent reef limestones are Holocene in age and record a time of higher sea level. Interpretation of the details of sea-level position requires a careful consideration of the evidence. On many island shorelines, there is a conglomerate of coral boulders, such as seen in Figure 8(a). However, storms are very effective at dislodging corals and transporting them, and depositing an accumulation of rubble. There has been an ongoing controversy as to the

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extent to which radiocarbon dating of corals from within a boulder conglomerate provides evidence for a higher sea level. Some boulder conglomerates, containing disoriented fragments of coral, appear to be storm deposits (Figure 8(b)). Clearer evidence for the position of the sea can be discriminated where coral colonies can be unequivocally shown to be in the position in which they grew. For example, if corals and associated organisms, including Tridacna clams, occur in their position of growth and at an elevation above where they can now grow, the conglomerate can be inferred to have formed as part of a former reef that grew to a higher sea level (Figure 8(c)). Where fossil coral is found in its position of growth, there remains the need to establish the relationship to former sea level, often the coral is an indicative marker, indicating, as in the case of A. palmata that the sea surface was above the coral by anything up to about 5 m. In the Indo-Pacific, there are several corals that are considered to indicate water depths of less than 10 m; these include A. robusta, A. humilis, and Pocillopora verrucosa (Montaggioni and Braithwaite, 2009). Sometimes, fossil coral is found in growth position, but cannot be clearly related to a modern equivalent to verify the elevation to which it would have grown (Figure 8(d)). The vertical errors introduce uncertainties into the interpretation of former sea levels together with error terms associated with the dating techniques. Elsewhere, it may be possible to detect topographical features, such as the algal rim, or spur and groove features that characterize modern reef crests (e.g., the emergent Holocene limestone on Guam, Figure 9(a)), emerged above their modern equivalents. Encrusting vermetid gastropods, oysters, barnacles, and tubeworms have each been used as shallow-water indicators, as has the occurrence of cement types within reef deposits (Montaggioni and Braithwaite, 2009). One of the most convincing lines of evidence comes from regions where massive corals can be shown to have been constrained by former low tide levels.

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Figure 7 Holocene sea level as indicated by radiocarbon dates on coral in the West Indies and northern Australia. An Atlantic sea-level curve (green line) is shown based on ages determined on Acropora palmata from Florida and the West Indies (after Toscano and Macintyre, 2003). Dates on corals from the Great Barrier Reef include fossil samples that grew in shallow water as well as indicators of intertidal elevations (after Larcombe et al., 1995), and imply a sea-level curve for northeastern Australia (purple line) that reached present sea level around 6000 years ago and has been higher than present for several millennia. It is important to note that there are vertical error terms (which are not shown) associated with each sample based on uncertainty as to the extent to which it indicates the elevation of former sea level, and age errors associated with the dating techniques (error terms are generally comparable to the size of the symbol).

SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

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Figure 8 Mid-to-late Holocene sea-level evidence at, and above, modern sea level from the Indo-Pacific region. (a) Many reef rims are characterized by a conglomerate, such as the coral conglomerate, that outcrops along the seaward shore of this reef island on the Cocos (Keeling) Islands in the eastern Indian Ocean; (b) a boulder conglomerate on Suwarrow Atoll in the northern Cook Islands in which the coral boulders are disoriented and appear to have been emplaced by storm activity; (c) a conglomerate immediately adjacent to that shown above, on Suwarrow, in which the corals are in growth position, and are accompanied by other biota (such as the Tridacna clam immediately behind the machete), which indicates a growing mid-Holocene reef at an elevation above that to which these organisms are presently able to grow; and (d) in situ branches of the blue octocoral Heliopora, exposed on Addu Atoll in the Maldives, which indicate a sea level close to, and possibly slightly above, modern.

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Figure 9 Microatolls: (a) the large flat-topped corals in the foreground are microatolls on Guam, they occur in an embayment into Holocene limestone (background) in which a spur and groove pattern (like that seen on the reef front in Figures 2(b) and 8(a)) is preserved; (b) microatolls of a massive species of Porites within a field of branching Acropora on the reef flat in the Cocos (Keeling) Islands; (c) large specimens of Porites microatolls on Cocos from which it has been possible to extract a record for most of the twentieth century; and (d) uplifted corals from the southwest coast of Simeulue, off the western Sumatra, Indonesia. The upper surface of the coral has experienced a series of changes in water level related to the Indian Ocean dipole and coseismic uplift, culminating in the entire coral being lifted out of the water in the 2004 Aceh-Andaman earthquake (see Meltzner et al., 2006; photo courtesy of Kerry Sieh).

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Such large flat-topped corals are called microatolls and are described in the following section. Not all corals within a reef, however, reflect the precise level of the sea. Most corals grow in shallow water and radiocarbon dates, even if they are on corals that are in their growth position, are directional, rather than indicative, of sea-level position (Montaggioni and Braithwaite, 2009). For example, it can be seen that most ages of fossil coral recorded from drilling studies on reefs in the Great Barrier Reef, and shown in Figure 7, fall below the curve interpreted to represent sea-level position. Reef growth can adopt one of several strategies: keep up, catch up, or give up, depending on the reef’s relationship to sea level and the rate and direction of sea-level change (Neumann and Macintyre, 1985). Reef growth involves the consolidation of reefal material into a structure; where rates of sea-level rise are too rapid, typically greater than 10 mm a 1, the reef is drowned. At slightly slower rates of rise, the reef is displaced upward and landward, termed backstepping. As rates of sealevel rise decelerate, reef growth may catch up with sea level as appears to have happened on many atolls and on outer reefs on the Great Barrier Reef (Hopley, 1982). Where the rate of rise is similar to, or less than, the rate of reef growth, reefs have the potential to keep up with sea level, as has occurred with the reefs around much of the Caribbean. If the sea falls, as has happened across much of the Indo-Pacific region, then a reef may become emergent, with a relatively bare reef flat replacing formerly diverse coral communities, as shown by the emergent Holocene conglomerate platform that underlies many reef islands on the margins of atolls (Figure 8(a)). Many Holocene reefs established over Pleistocene substrates as the rise in sea level slowed around 7000 years BP. Initial coring and dating of fringing reefs indicated similarity between reefs in Hanauma Bay in Oahu, Hawaii, and those at Galeta Point in Panama. In each case, the dating implied a reef growth history in which the reef grew up and then prograded out. The interpretation that this supported a similar decelerating sea-level curve in both locations went counter to evidence elsewhere in the Pacific that there had been a mid-Holocene sea level slightly above present. This discrepancy can be resolved if the Hawaiian reef is interpreted as a catch-up reef (Fletcher and Jones, 1996). If sea level is stable, a reef that has reached sea level generally builds seaward, called progradation. The way in which it builds seaward varies in different settings, in some cases it may involve stepped build-out, and in others it may build uniformly (Kennedy and Woodroffe, 2002). Each of these sealevel and reef-growth scenarios can be illustrated by different reefs from around the world, illustrating the range of responses that Holocene reef morphology can show to variations in the rate of sea-level change. These growth strategies have been comprehensively described by Montaggioni (2005).

Microatolls and Interannual Variations in Sea Level Individual colonies of massive coral can grow up to a level close to mean low tide and then continue to grow outward, forming flat-topped discoid corals, termed microatolls (Figure 9(a)). Microatolls initially termed miniature atolls, are living on the outer margin but are predominantly dead and

limited by water level on their upper surface (Figure 9(b)). Microatolls in their growth position are fixed biological sealevel indicators and can be useful indicators of the level that previously limited growth, with the larger specimens reaching several meters across and containing a growth record of tens to hundreds of years (Figure 9(c)). Fossil microatolls have been used extensively to reconstruct Holocene sea-level variations on reef tops. For example, the significance of microatolls was recognized during the 1973 Royal Society expedition to the Great Barrier Reef, northeastern Australia, when it was realized that those found above sea level indicated a higher stand of the sea (Scoffin and Stoddart, 1978). Further study showed that sea level has been decreasing over the past 6000 years with respect to the inner Great Barrier Reef (Chappell, 1983). Much of the Indo-Pacific reef province has experienced relative sea levels in the mid- and late Holocene that have been slightly above present. Microatolls have sometimes been preserved at a height presently above that of their living counterparts on reef flats in the eastern Indian Ocean, Southeast Asia, northern Australia, and across much of the equatorial Pacific Ocean (Smithers and Woodroffe, 2000). Typically, microatolls comprise a single colony of massive Porites, though several other genera can also adopt the microatoll form and grow to several meters in diameter. Microatolls grow upward until constrained by a water level close to mean low tide level, after which they continue lateral growth and become disk-shaped corals (Figure 10(a)), contrasting with dome-shaped colonies that are in water deep enough not to be constrained by sea level (Figure 10(b)). On coasts that experience seismic uplift, a coral previously not limited by sea level may be elevated above the water level, and the exposed upper surface will die, but with continued lateral growth at a lower elevation (Figure 10(c)). A spectacular example of this occurred in association with the 2004 Aceh-Andaman earthquake with uplift on the island of Simeulue, off the western coast of Sumatra (Meltzner et al., 2006; see Figure 9(d)). Where a microatoll, previously limited by water level, experiences an increase in the level of the sea, it can resume vertical growth and begin to overgrow the formerly dead upper surface (Figure 10(d)). Successive falls in water level result in exposure of the living rim, and a series of terracettes develop, recording the fall in water level (Figures 10(e)). Fluctuations of water level with a periodicity of several years are recorded on the upper surface of a microatoll as a series of concentric undulations or annuli (Figure 10(f)). This pattern has been described on microatolls from reef flats in the central Pacific tracking, but lagging behind, sea-level fluctuations that are linked to the atmospheric–oceanic El Nin˜o phenomenon that is a major control on the ocean surface in the central Pacific Ocean (Woodroffe and McLean, 1990). Banding within the coral provides insight into events in the life history of the coral. It can reveal times at which growth has been lowered by changes in the water surface. In areas where storms are experienced, overturning of the colony can occur during individual storms, or microatolls may have responded to the moating of water that could have occurred behind boulder ramparts formed as a result of storms. In these cases, their upper surface may record elevation of water level within impounded moats above that of regional sea level. Although in

SEA LEVEL STUDIES | Coral Records of Relative Sea-Level Changes

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Figure 10 A schematic illustration of the response of the upper surface of microatolls to changes of sea level or uplift of the land, with annual bands (laid down by the living tissue that is shown in red) within the coral skeleton recording the pattern of coral growth; (a) if sea level remains constant from year to year, a massive coral that has grown up to sea level continues to grow outward but with its upper surface constrained at water level; (b) if the coral does not reach water level, it adopts a domed growth form and is not constrained by water level; (c) if the coast undergoes uplift, then a coral previously not limited by sea level may be raised, and the exposed upper surface will die, but with continued lateral growth at a lower elevation (as in Figure 9(d)); (d) if the water level increases, a coral previously constrained by exposure at low tides can resume vertical growth and begin to overgrow the formerly dead upper surface; (e) if water level decreases, then the microatoll adopts a series of terracettes; and (f) if there are fluctuations of water level with a periodicity of several years, then the upper surface of the microatoll consists of a series of concentric undulations – such a pattern can be seen on microatolls from reef flats in the central Pacific where El Nin˜o results in interannual variations in sea level (based on Woodroffe and McLean, 1990; Lambeck et al., 2010).

open-water situations microatolls can enable centimeter-scale reconstructions of former sea level, they are subject to misinterpretation if moating has occurred, and it is therefore important to assess the geomorphological setting within which these corals grew before using fossil specimens to draw conclusions about past sea levels. In addition to tracking sea level, emerged microatolls have been used to record uplift associated with earthquakes, as for example, along the Sumatran coast (Sieh et al., 2008), and may contain evidence of fluctuations in sea level at decadal to century time scales (Yu et al., 2009).

See also: U-Series Dating. Paleoceanography, Biological Proxies: Corals, Sclerosponges and Mollusks. Radiocarbon Dating: AMS Radiocarbon Dating; Conventional Method. Sea Level Studies: Eustatic Sea-Level Changes – Glacial–Interglacial Cycles; Eustatic Sea-Level Changes Since the Last Glacial Maximum; Geomorphological Indicators; Isostasy: Glaciation-Induced Sea-Level Change. Sea-Levels, Late Quaternary: Late Quaternary Relative Sea-Level Changes in the Tropics.

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