Operational strategy

The Barbados offshore drilling (Fairbanks, 1989) demonstrated that the reef sequences corresponding to the last deglaciation developed on slopes forming discontinuous successive terraces of various lateral extent and stratigraphic thickness. Therefore, to recover the entire postglacial reef sequence, successive reef terraces that occur seaward of the living barrier reef must be drilled.

Our detailed analyses of the combined GBR site survey data sets have demonstrated the occurrence of successive reef features at various depths between 130 and 25 mbsl, which correspond to potential drilling targets. Thus, at each geographic location, we cored a transect of several holes in order to recover the entire postglacial reef sequence. Initial results obtained from Expedition 310 confirm that this drilling strategy is sound (Camoin et al., 2007).

Based on the results of seismic, bathymetric, and sample data acquired during the Southern Surveyor cruise in September–October 2007, four transects located within three geographical areas (Fig. F2) were drilled during Expedition 325: Hydrographer's Passage (2 transects), Noggin Pass (1 transect), and Ribbon Reef 3 (1 transect). Depths at these locations ranged from 47.27 to 167.14 mbsl (LAT taken from corrected EM300 data).

The exact location of drill holes was determined during the expedition by checking the nature and morphology of the seafloor with a through-pipe camera system. This enabled the protection of benthic biota from any disturbance by drilling. Following the completion of each hole, the cored lithologies were assessed by the Co-Chief Scientists and onboard sedimentologists to refine the position of the next hole. All holes were sited within a 125 m radius around the proposed drilling sites approved by the EPSP and within the transect boundaries given in the research permit issued by the Great Barrier Reef Marine Park Authority (GBRMPA). Figures F3 and F11, F5 and F12, F7 and F13, and F9 and F14 show the general locations and enlargements of groupings of sites for the holes at HYD-01C, HYD-02A, RIB-02A, and NOG-01B, respectively.