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doi:10.2204/iodp.sp.325.2009

Introduction

The timing and course of the last deglaciation is considered an essential component for understanding the dynamics of large ice sheets (Lindstrom and MacAyeal, 1993) and their effects on Earth's isostasy (Lambeck, 1993; Peltier, 1994). Moreover, the disappearance of glacial ice sheets was responsible for dramatic changes in the freshwater fluxes to the oceans that disturbed the general thermohaline circulation and, hence, global climate (e.g., Stocker and Wright, 1991). Coral reefs are excellent sea level indicators, and their accurate dating by mass spectrometry is of prime importance for determining the timing of deglaciation events and, thus, for the understanding of the mechanisms driving the glacial–interglacial cycles. Furthermore, scleractinian coral colonies can monitor sea-surface temperatures (SSTs) as well as other oceanographic parameters (e.g., salinity and sediment run-off) and fossil corals can be used as recorders of past variations in these parameters.

Sea level changes as global climate indicator

Prior to Integrated Ocean Drilling Program (IODP) Expedition 310 (Tahiti Sea Level), only three deglaciation curves based on coral reef records had been accurately dated for times reaching the Pleistocene/Holocene boundary: in Barbados between 19,000 and 8,000 calendar years before present (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). Until recently, the Barbados curve was 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 that 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 vertical movements are slow and/or regular. The expeditions linked to IODP Proposal 519 (Tahiti Sea Level and Great Barrier Reef Environmental Changes [GBREC]) aim to provide new deglaciation curves from tectonically stable regions. Expedition 310 was successfully completed in 2005 and 2006 (Camoin, Iryu, McInroy, et al., 2007).

The Barbados record suggests that the last deglaciation was characterized by three brief periods of accelerated melting superimposed on a smooth and continuous rise of sea level with no reversals (Fig. F1). These so-called 19 ka MWP, MWP-1A, and MWP-1B events (ca. 19,000, 13,800, and 11,300 cal. y BP) are thought to correspond to massive inputs of continental ice (~50–40 mm/y, roughly equivalent to annual discharge rates of 16,000 km³ for MWP-1A). The MWP-1A corresponds to a short and intense cooling between 14,100 and 13,900 cal. y BP in the Greenland records (Johnsen et al., 1992; Grootes et al., 1993) and therefore postdates the 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. Two of these three 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 ca. 7,600 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, saw-tooth sea level fluctuations between 19,000 and 15,280 cal. y BP (Locker et al., 1996; Yokoyama et al., 2000a, 2000b, 2001) and a sea level fall coeval with climatic changes around 11,000 cal. y BP are still controversial topics (Lambeck et al., 2003).

Worldwide sea level compilations indicate that local sea level histories varied considerably around the world in relation to both the postglacial redistribution of water masses and to a combination of local processes (Lambeck, 1993; Peltier, 1994; Lambeck et al., 2003), although significant deviations between model predictions and field data have been noted in several regions (Camoin et al., 1997). The post-LGM 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 the Earth. The effect of hydro-isostasy will depend on the size of the islands: at very small islands, the addition of meltwater will produce a small differential response between the island and the seafloor, whereas the meltwater load will produce 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: on a small oceanic island and 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 are proposed: Tahiti (completed in 2005 and 2006) and the GBR. Expedition 325 will conduct investigations at GBR sites only.

Climatic and oceanographic changes during last deglaciation events

During latest Pleistocene and early Holocene times, climatic variability was primarily related to the effects of seasonality and solar radiation. The results of the Climate: Long range Investigation, Mapping, and Prediction (CLIMAP) Program suggested that the LGM tropical SSTs were similar to the modern ones. However, this interpretation is not consistent with snowline reconstructions and paleobotanic data (Rind and Peteet, 1985; Anderson and Web, 1994).

The available Sr/Ca and U/Ca data from coral reef areas report SSTs 5°C cooler than today during the LGM and 2°C cooler around 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 as 4.5°C cooler during the LGM and 1°C cooler at 10,000 cal. y B.P. 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 contrast to the apparent synchronism of the last deglaciation, inferred from various sources (i.e., coral records, ice cores, snowline reconstructions, vegetation records, and alkenone paleothermometry) (Bard et al., 1997).

Recent studies have documented Holocene climatic variations. 1°C warmer SSTs, monsoonal rainfall, and possibly weaker El Nino-Southern Oscillation (ENSO) around 5,800 y B.P. 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 y old coral from the Seychelles, although the intensity of the annual decrease in SST caused by monsoonal cooling was lower than today (Zinke et al., 2005).

Additional information is required for a better knowledge 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 postglacial warming in the two hemispheres.

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