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 is to establish the course of postglacial sea level rise in the GBR—in other words, to define the exact shape of the deglaciation curve for the period from 20 to 10 ka. The expected results will achieve the following:

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

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

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

  • Test Glacio-hydro-isostatic modeling–predicted sea level 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 (U-Th by thermal ionization mass spectrometry [TIMS] and multicollector inductively coupled plasma–mass spectrometry [MC-ICP-MS]; 14C by accelerator mass spectrometry [AMS]) and paleobathymetric information deduced from biological communities (corals, algae, benthic foraminifers, 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 is to define 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 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 inductively coupled plasma spectroscopy [ICP] and 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 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 is to analyze the impact of sea level changes 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 shape and composition, as well as age–depth relationships.

Expedition 325 will provide the opportunity to better constrain the postglacial history (Lambeck et al., 2002; 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 MWP-1A in the Pacific and the Atlantic. Furthermore, the study of LGM and very early postglacial coral material should allow calculation of the first Sr/Ca SSTs 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; Asami et al., 2010; DeLong et al., 2010; Inoue et al., 2010).