Preliminary scientific assessment

During the Expedition 325 OSP, emphasis was placed on visual description, measurement of physical properties, and sampling of the cores guided by the preliminary chronology provided by the core catcher samples that were dated by U-Th and 14C by AMS prior to the OSP. Thus, the Expedition Reports section of the volume, scheduled to be published in ~July 2011, will contain a descriptive framework for subsequent postcruise research.

The primary objectives of Expedition 325 require the use of specialized geochemical techniques, paleomagnetic analyses, and detailed investigation of lithological and biological assemblages. These types of analyses were not conducted during the OSP but will be conducted over the coming months at the institutions of the expedition scientists as part of their postcruise research.

Fulfillment of the Expedition 325 scientific objectives is as follows:

1. Establish the course of postglacial sea level change in the Great Barrier Reef (i.e., define the exact shape of the deglaciation curve for the period 20–10 ka).

During the offshore phase, cores were recovered from a succession of fossil reef features from 42 to 127 mbsl. Therefore, most if not all of the postglacial sequence from the LGM to the present day was recovered. Furthermore, the preliminary chronology provided by the core catcher samples confirm that the recovered cores span this period. During the OSP, high-quality coral samples, consistent with shallow, high-energy settings, were taken for dating and sea level change investigations, the results of which are expected to fulfill the first objective.

2. Define sea-surface temperature variations for the region over the period 20–10 ka.

During the offshore phase, massive coral colonies suitable for paleoclimate studies and spanning the LGM and postglacial sequence were recovered in the cores. During the OSP, >200 massive coral colonies, including 17 Porites, were slab-sampled for paleoclimate studies, the results of which are expected to fulfil the second objective.

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

During the offshore phase, cores were recovered from holes in various water depths and situated on four transects in three different geographic areas along the GBR. Therefore, results of analyses of samples taken during the OSP will be interpreted in a broad temporal and spatial context, which will allow better understanding of the development of the GRB in response to environmental changes.

Additional scientific outcomes from Expedition 325

Preliminary dating information and initial observations of the cores at the OSP indicate that several additional scientific outcomes will be achieved:

  1. New sea level and paleoclimate information from recovered corals that likely span the LGM (MIS2), pre-LGM (MIS3), and several earlier Pleistocene periods. This will also provide new information about the evolution of the GBR during these periods.

  2. A 33.3 m record of near continuous sedimentation was recovered in Hole M0058A from the fore reef slope. This will provide a high-resolution record to complement the sea level and paleoclimate records derived from the reef cores collecting on the shelf edge.