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doi:10.2204/iodp.proc.325.101.2011

Scientific objectives of Expedition 325

1. To establish the course of postglacial sea level rise at the Great Barrier Reef.

The first objective of Expedition 325 was to establish the course of postglacial sea level rise for the GBR—specifically, to define the exact shape of the deglaciation curve for the period from 20 to 10 cal y BP. The expected results will achieve the following:

• Assess the maximum sea level drop during the LGM and establish the timing of its termination;

• Assess the validity, timing, and amplitude of Meltwater Pulse (MWP) events (e.g., 19 ka-MWP, MWP-1A, and MWP-1B);

• Prove or disprove the sawtooth pattern of sea level rise during the last deglaciation (Locker et al., 1996); and

• Test glacio-hydroisostatic modeling–predicted sea level based on different ice and rheological models.

The reconstruction of sea level curves relies on the absolute dating of in situ corals and other reef-building biota provided by radiometric methods (U-series by thermal ionization mass spectrometry and multicollector inductively coupled plasma–mass spectrometry; ¹⁴C by accelerator mass spectrometry) and paleobathymetric information deduced from biological communities (corals, algae, benthic foraminifera, and mollusks) that live in a sufficiently narrow or specific depth range to be useful as absolute sea level indicators.

2. To define sea-surface temperature variations for the region over the period 20 to 10 ka.

The second objective of Expedition 325 was to define sea-surface temperature (SST) variations for the region over the period from 20 to 10 ka to better understand the following:

• The regional variation of SSTs in the southwest Pacific,

• The climatic variability and the identification of specific phenomena such as El Niño Southern Oscillation (ENSO),

• The global variation and relative timing of postglacial climate change in the Southern and Northern Hemispheres,

• The regional variation of SSTs in the tropical southwest Pacific; and

• Climatic variability and specific phenomena such as ENSO.

Methods include stable isotope (δ¹⁸O) and trace element analyses (Sr/Ca ratios by inductively coupled plasma spectroscopy and thermal ionization mass spectrometry) based 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). δ¹³O measurements, coupled with of δ¹⁸O in coral skeletons, will provide information on other parameters (e.g., solar variations or coral metabolic processes). Geochemical methods will be coupled with measurements and analyses of density band widths and microstructural variations in the coral skeletons.

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

The third objective of Expedition 325 was to analyze the impact of sea level change on reef growth and geometry, especially the following:

• Glacial meltwater phases (identification of reef deepening and/or drowning events),

• The morphological and sedimentological evolution of the fore reef slopes (highstand versus lowstand processes),

• The modeling of reef building, and

• Environmental changes during reef development.

Numerical models (e.g., CARB3D, DIONISIS) simulating reef building will be used 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 structure shape and composition, as well as age–depth relationships.

Work done during and after Expedition 325 will provide the opportunity to better constrain the postglacial sea level history (Peltier, 1994; Fleming et al., 1998; Okuno and Nakada, 1999; Lambeck et al., 2002) by documenting the LGM lowstand in well-studied cores in the far-field and by comparing MWP-1A in the Pacific and Atlantic. Furthermore, the study of LGM and early postglacial coral material should allow calculation of the first Sr/Ca-SSTs in the Pacific, which will supplement the Barbados SST history sample (Guilderson et al., 1994) and the results of Expedition 310 for Tahiti (Camoin et al., 2007; Asami et al., 2010; DeLong et al., 2010; Inoue et al., 2010).

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