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Scientific objectives

1. Establish the course of postglacial sea level rise at Tahiti (i.e., define the exact shape of the deglaciation curve for the period 20,000–10,000 cal. y BP).

In establishing the deglaciation curve, we hope to assess the validity, timing, and amplitude of the MWP-1A event, assess the maximum sea level drop during the LGM to prove or disprove the sawtooth pattern of sea level rise during the last deglaciation (Locker et al., 1996), and test predictions based on different ice and rheological models.

Reconstruction of sea level curves relies on absolute dating of in situ corals by radiometric methods (230Th/234U by thermal ionization mass spectrometry [TIMS] and 14C by accelerator mass spectrometry [AMS]) and paleobathymetric information deduced from biological communities (corals, algae, and mollusks) that live in a sufficiently narrow or specific depth range to be useful as absolute sea level indicators.

2. Define SST variations for the region over the period 20,000–10,000 cal. y BP.

This information is needed to gain better knowledge of regional variation of SSTs in the South Pacific, climatic variability and identification of specific phenomena such as ENSO, and global variation and relative timing of postglacial climate change in the southern and northern hemispheres.

Methods include stable isotope (δ18O) and trace element (Sr/Ca) analyses on high-resolution (i.e., monthly) sampling of massive coral colonies. Coupled analyses of δ18O and Sr/Ca on the same sample may yield estimates of both temperature and salinity (McCulloch et al., 1996). Stable isotope δ13O measurements, systematically coupled with those of δ18O in coral skeletons, will provide information on other parameters (e.g., solar variations or metabolism processes). Geochemical methods will be coupled with measurements and analyses of the bandwidths and microstructural variations in the coral skeletons.

3. Analyze the impact of sea level changes on reef growth and geometry.

Assessments will be made of the impact of glacial meltwater phases (identification of reef-drowning events) and the morphological and sedimentological evolution of the foreslopes (highstand versus lowstand processes). In addition, modeling of reef building and analyses of environmental change during reef development will be undertaken.

A numerical model simulating reef building will be used to study the effect of a sea level jump on reef geometry and to qualitatively assess the effect of sea level fluctuations on the reef shape and age-depth relationships.

Expedition 310 may provide the opportunity to better constrain the last deglacial history (see Peltier, 1994; Fleming et al., 1998; Okuno and Nakada, 1999) by documenting for the first time the LGM lowstand in well-studied cores in the far field and by comparing the MWP-1A event in the Pacific and Atlantic Oceans. Furthermore, study of coral material from the late glacial to deglacial period should provide the first Sr/Ca SSTs for the LGM in the Pacific, which could then supplement Barbados sample data (Guilderson et al., 1994) and the recent study of Stage 6 corals (McCulloch et al., 1999).