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Sea level changes as global climate indicator

Only three deglaciation curves based on coral reef records have been accurately dated for times reaching the Pleistocene/Holocene boundary: in Barbados between 19,000 and 8,000 cal. y BP (Fairbanks, 1989; Bard et al., 1990a, 1990b), in New Guinea between 13,000 and 6,000 cal. y BP (Chappell and Polach, 1991; Edwards et al., 1993), and in Tahiti between 13,750 cal. y BP and 2,380 14C y BP (Bard et al., 1996) (Fig F1). So far, the Barbados curve is the only one to encompass the whole deglaciation because it is based on offshore drilling. However, this site, like New Guinea, is located in an active subduction zone where tectonic movements can be large and discontinuous, so the apparent sea level records may be biased by variations in the rates of tectonic uplift. Hence, there is a clear need to study past sea level changes in tectonically stable regions or in areas where the vertical movements are slow and/or regular.

The Barbados record suggested that the last deglaciation was characterized by two brief periods of accelerated melting superimposed on a smooth and continuous rise of sea level with no reversals (Fig F1). These so-called meltwater pulse (MWP) events (MWP-1A and MWP-1B at ~13,800 and 11,300 cal. y BP, respectively) are thought to correspond to massive inputs of continental ice to the oceans (i.e., ~50–40 mm/y, roughly equivalent to annual discharge rates of 16,000 km3 for MWP-1A). MWP-1A corresponds to a short and intense cooling period between 14,100 and 13,900 cal. y BP in Greenland records (Johnsen et al., 1992; Grootes et al., 1993) and therefore postdates initiation of the Bölling-Alleröd warm period at ~14,900–14,700 cal. y BP (Broecker, 1992). The sea level jump evidenced in New Guinea at 11,000 cal. y BP (Edwards et al., 1993) is delayed by a few centuries when compared to that observed at Barbados. These two meltwater pulses are thought to have induced reef-drowning events (Blanchon and Shaw, 1995). Two “give-up” reef levels have been reported at 90–100 and 55–65 m water depth on the Mayotte foreslopes (Comoro Islands) and have been related to the Bölling and the post-Younger Dryas meltwater pulses (Dullo et al., 1998); similar features are recorded in the southern Great Barrier Reef (GBR) (Troedson and Davies, 2001) and in the Caribbean (MacIntyre et al., 1991; Grammer and Ginsburg, 1992). A third Acropora reef-drowning event at ~7600 cal. y BP has been assumed by Blanchon and Shaw (1995).

However, there are still some doubts concerning the general pattern of sea level rise during the last deglaciation events, including the amplitude of the maximum lowstand during the Last Glacial Maximum (LGM) and the occurrence of increased glacial meltwater with resultant accelerated sea level rise (Broecker, 1990). Furthermore, sawtooth sea level fluctuations between 19,000 and 15,280 cal. y BP (Locker et al., 1996) and a sea level fall coeval with climatic changes at ~11,000 cal. y BP are still controversial topics.

Worldwide sea level compilations indicate that local sea level histories varied considerably around the world in relation to both postglacial redistribution of water masses and a combination of local processes (Lambeck, 1993; Peltier, 1994), although significant deviations between model predictions and field data have been noted in several regions (Camoin et al., 1997). The last deglacial sea level changes at sites located far away from glaciated regions (“far field”) provide basic information regarding the melting history of continental ice sheets and the rheological structure of Earth. The effect of hydroisostasy depends on the size of the islands: for very small islands, the addition of meltwater produces a small differential response between the island and the seafloor, whereas the meltwater load produces significant differential vertical movement between larger islands or continental margins and the seafloor (Lambeck, 1993). There is, therefore, a need to establish the validity of such effects at two ideal sites located at a considerable distance from the major former ice sheets: (1) on a small oceanic island and (2) on a continental margin. In both cases, it is essential for the sites chosen that the tectonic signal is small or regular within the short time period proposed for investigation so that rigorous tests of proposed Northern and Southern Hemisphere deglaciation curves from Barbados and New Guinea can be made. Two such places were proposed: Tahiti and the GBR. This expedition conducted investigations at Tahiti sites only.

Climatic and oceanographic changes during last deglaciation events

The results of the Long-Range Investigation, Mapping, and Prediction (CLIMAP) program suggested that LGM tropical SSTs were similar to modern ones. However, this interpretation is not consistent with snowline reconstructions and paleobotanic data (Rind and Peteet, 1985; Anderson and Webb, 1994).

The available Sr/Ca and U/Ca data from coral reef areas report SSTs 5°C colder than those of today during the LGM and 2°C lower at ~10,000–9,000 cal. y BP at Barbados (Guilderson et al., 1994), whereas studies in the west Pacific indicate that the full amplitude of the glacial–Holocene temperature change may have ranged between 3° and 6°C (McCulloch et al., 1996; Beck et al., 1997; Gagan et al., 1998) (Fig F1). Troedson and Davies (2001) define SSTs immediately south of the GBR some 4.5°C colder during the LGM and 1°C colder at 10,000 cal. y BP. This casts doubt upon the phase shift of 3000 y for climate changes between the two hemispheres that was assumed by Beck et al. (1997), in clear distinction from the apparent synchronism of the last deglaciation, inferred from various sources (i.e., coral records, ice cores, snowline reconstructions, vegetation records, and alkenone palaeothermometry) (Bard et al., 1997).

Coral-based climate studies have successfully been used to document Holocene climatic variations. SSTs warmer by 1°C, monsoonal rainfall, and possibly weaker El NiƱo–Southern Oscillation (ENSO) at ~58,000 cal. y BP in eastern Australia have been deduced from isotopic and Sr/Ca high-resolution measurements on corals from the central GBR (Gagan et al., 1998). An ENSO-like cyclic climatic variation with a return period of 3–5 y has been evidenced in a 4150 cal. y BP coral from the Seychelles, although the intensity of the annual decrease in SSTs caused by monsoonal cooling was lower than that of today (Zinke et al., 2005).

Additional information is required for a better understanding of climatic conditions in tropical regions during the last deglaciation. In these areas, the most debated points are twofold: (1) the quantification of SSTs and the identification of related climatic variations during the last deglaciation events and (2) the timing of the relevant last deglacial warming in the two hemispheres.