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Preliminary scientific assessment

Expedition 307 successfully completed and surpassed the operations plan set out in the Expedition 307 Scientific Prospectus. Drilling at all three planned sites reached target depths, sediment upper sections were double- to quadruple-cored with the advanced piston corer (APC), and each site was wireline logged. We now have the core material that, with postcruise analysis, will be used to meet the expedition objectives and confirm or disprove many hypotheses about carbonate mound initiation and growth.

Challenger Mound is one of thousands of mound structures in Porcupine Seabight and is the first of these to be cored deeper than 12 m, so coring this structure was a true exploration. Already many results are clear, as detailed elsewhere in this report. The mound is composed of cold-water coral fragments (L. pertusa), clay, and coccoliths down to its base at 130–155 mbsf, and at least 10 distinct layers—active growth intervals of the coral mound—are evident in lithology and physical properties. Sea bottom temperature in Porcupine Seabight during the last glacial period was too cold for L. pertusa, suggesting that the growth intervals most probably correspond to Pleistocene interglacials. Much of the late Pleistocene material is absent from the top of the mound, while at the same time siliciclastic sediment is building up in drifts both upslope and downslope; the mound is slowly being buried. By positioning holes along an upslope transect across the mound site (U1317), we have exposed a short stratigraphic cross section that will be useful for reconstructing the growth of Challenger Mound and similar structures.

The theory that this mound is built from carbonate precipitated by microbes fed by methane seeps has been disproved, although the role of prokaryotes in both carbonate dissolution and secondary cementation reactions may be important. The lithology and age of the enigmatic sedimentary packages that underlie the mound, known previously only from seismic lines, have been identified. The mound is rooted on an erosional unconformity that was identified at all three sites, and a thick early–middle Miocene package of green-gray calcareous siltstones lie beneath the mound.

The expedition was planned for only 10 days of science operations with a short transit from Dublin, Ireland, to the sites, which left little room for technical delays. We were fortunate to have mild sea conditions; no time was lost to weather, which is certainly not guaranteed west of Ireland in early May. We also made full use of the extra 2 days of operations that resulted from the early departure from Dublin at the start of the expedition.

The holes were originally planned to be cored with the APC to refusal and then deepened to target depth with XCB. However, we found in the first hole at Site U1316 that the deeper sediments were more lithified than expected; thus XCB coring was frustratingly slow. We therefore decided to core these sediments using rotary core barrel (RCB), which proved to be faster (penetration rate = 6.3 m/h compared to 2.7 m/h), had similar recovery (average = 80%), caused less biscuiting than XCB coring, and provided cores more suitable for geochemistry, microbiology, and fine-scale stratigraphy (Table T1).

Downhole logging initially proved to be a challenge in these relatively shallow holes. For the first logging attempt in Hole U1316A, we set the pipe at the optimistically shallow depth of 30 mbsf to maximize the logged interval, but the sediment at this depth was too soft to retain integrity and the logging tools were blocked from passing downhole. However, the three subsequent downhole logging operations were successful, with all logging tools reaching total depth, including the check shot survey at Site U1317.

The expedition employed an integrated sedimentological/​geochemical/​microbiological approach. Each laboratory group was aware of the value of their analyses to the other groups and the connections between, for example, microbial action, interstitial water chemistry, and diagenetic alteration of the sediments. There was, therefore, support for the extensive microbiological sampling—even when 1.5 m sections were assigned and disappeared from the core receiving area to the hold for sampling for deoxyribonucleic acid (DNA), lipids, cell enumeration, and experimental work. Additionally, this expedition was the first to operate the Fast Track multisensor core logger (MSCL) without problems for all cores, including the sections sampled for microbiology.

In the core laboratory, a new technique was developed for splitting cores that contain coral in an unlithified matrix. Conventional methods of core splitting using wire and saw can result in coral fragmentation and pieces being dragged down the split core surface, destroying the sediment structure. To avoid this undesirable outcome, all core sections from Hole U1317C were split by saw after being frozen to –50°C (Fig. F17). Some short freeze-cracks in the split core face were apparent, but the coral structure was preserved and it was generally agreed that this method produced superior results compared to conventional methods. Cores from Holes U1317A and U1317D were split by saw in the normal way so that the sedimentologists could describe at least one full stratigraphic section immediately; Holes U1317B and U1317E remained unopened for potential whole-round CT scanning and splitting while frozen at the IODP Gulf Coast Core Repository, College Station, Texas (USA).