IODP Expedition 325 Scientific Prospectus

doi:10.2204/iodp.sp.325.2009

Scientific objectives

The objectives of IODP Proposal 519 and Expedition 325 are well aligned with the high-priority objectives of the theme "Environmental Change, Processes and Effects" from the IODP Long-Range Plan. A fundamental research initiative outlined for the IODP Long-Range Plan concerns the record and causes of rapid climate changes that are the basic objectives of our proposal for the late Pleistocene and the Holocene. Such records of natural climate change provide a framework for evaluating the possible effects of anthropogenic change on the environment.

The scientific objectives of Expedition 325 are

1. To establish the course of postglacial sea level rise at the GBR (i.e., to define the exact shape of the deglaciation curve for the period 20,000 to 10,000 cal. y BP). The expected results are the following:

To assess the validity, timing, and amplitude of MWP events (e.g., 19,000 cal. y BP event, MWP-1A, and MWP-1B);
To assess the maximum sea level drop during the LGM and establish the timing of its termination;
To prove or disprove saw-tooth pattern of sea level rise during the last deglaciation (Locker et al., 1996); and
To test predictions based on different ice and rheological models.

The reconstruction of sea level curves will rely on the absolute dating of in situ corals and other reef building biota provided by radiometric methods (²³⁰Th/²³⁴U by thermal ionization mass spectrometry [TIMS]; ¹⁴C by accelerator mass spectrometry [AMS]) and paleobathymetric information deduced from biological communities (corals, algae, benthic foraminifers, and molluscs) that live in a sufficiently narrow or specific depth range to be useful as absolute sea level indicators.

2. To define SST variations for the region over the period 20,000 to 10,000 cal. y BP in order to get a better knowledge of

The regional variation of SSTs in the southwest Pacific;
The climatic variability and the identification of specific phenomena such as ENSO; and
The 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 ratios by TIMS) analyses on high-resolution (i.e., at the monthly scale) 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). δ¹³C 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 band widths and micro-structural variations in the coral skeletons.

3. To analyze the impact of sea level changes on reef growth and geometry, especially

The impact of glacial meltwater phases (identification of reef drowning events);
The morphological and sedimentological evolution of the foreslopes (highstand versus lowstand processes);
The modeling of reef building; and
Environmental changes during reef development.

A numerical model simulating reef building will be used in order to study the effect of abrupt sea level rise events on reef geometry and to assess qualitatively the effect of sea level fluctuations on reef shape and composition as well as age–depth relationships.

The present proposal may provide the opportunity to better constrain the deglacial history (Peltier, 1994; Fleming et al., 1998; Okuno and Nakada, 1999) by documenting the LGM lowstand in well-studied cores in the far-field and by comparing the MWP-1A in the Pacific and the Atlantic. Furthermore, the study of very early deglacial coral material should allow the first Sr/Ca SSTs for the LGM in the Pacific, which could then supplement the Barbados sample data (Guilderson et al., 1994), the study of Papua New Guinea marine isotope Stage 6 corals (McCulloch et al., 1999) and the results of Expedition 310 (Camoin, Iryu, McInroy, et al., 2007).

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