PRELIMINARY SCIENTIFIC ASSESSMENT
Expedition 307 of the Integrated Ocean Drilling Program successfully completed and surpassed the operations plan set out in the 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 explorationone could only guess what was inside such mounds. Already many results are clear, as detailed elsewhere in this report. The mound is composed of coral (L. pertusa), clay, and coccoliths down to its base at 130155 mbsf, and at least 10 distinct layersactive growth intervals of the coral moundare 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 has been eroded 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 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 directly below the mound a thin layer of Lower Pliocene sediments overlies a thick lower Miocene package of green-gray calcareous siltstones.
For coring operations, we had to be light on our feet. 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 at that time of year. We also made full use of the two extra 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.
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, remarkably little alarm when 1.5 m sections were assigned for microbiological sampling 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 magnetic susceptibility 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 by using wire and saw can result in coral fragmentation, pieces being dragged down the split core surface, and degradation of the sediment structure. To avoid this undesirable outcome, all core sections from Hole U1317C were split by saw after being frozen to 50°C. 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 than conventional methods. Cores from Holes U1317A and U1317D were split by saw in the normal way so that the lithostratigraphers 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 Texas A&M University in College Station, Texas (USA).
The success of Expedition 307 was due to the dedication of IODP staff, Transocean employees, and the members of the science party.
No corals or marine mammals were harmed during the Expedition 307 operations.
Next Section | Table of Contents