Lamont-Doherty Earth Observatory, United States

Challenger Mound, a putative carbonate mound structure covered with dead deepwater coral rubble and located in Porcupine Seabight on the southwest Irish continental margin, was the focal point of twelve days of scientific drilling aboard the JOIDES Resolution during Integrated Ocean Drilling Program Expedition 307. Specific drilling objectives included the following: 1. Establish whether the mound base rested on a carbonate hardground of microbial origin and whether past geofluid migration events acted as a prime trigger for mound genesis. 2. Define the relationship, if any, between mound initiation, mound growth phases, and global oceanographic events. 3. Analyze geochemical and microbiological profiles that define the sequence of microbial communities and geomicrobial reactions throughout the drilled sections. 4. Examine high-resolution paleoclimatic records from the mound section using a wide range of geochemical and isotopic proxies. 5. Describe the stratigraphic, lithologic, and diagenetic characteristics, including timing of key mound-building phases, for establishing a depositional model of deepwater carbonate mounds and for investigating how they resemble ancient mud mounds. In addition to the mound, one site immediately downslope of Challenger Mound and an upslope site were drilled to (1) constrain the stratigraphic framework of the slope/mound system, (2) identify and correlate erosional surfaces observed in slope sediment seismics, and (3) investigate potential gas accumulation in the sediments underlying the mound. Drilling revealed that the mound rests on a sharp erosion boundary. Sediments below this erosion surface consist of glauconitic and silty sandstone drift deposits of middle Miocene age that grade upward toward more clay rich intervals. The latter are tentatively interpreted to represent relatively low energy environments in the late Miocene-Pliocene succession. The Pliocene strata end abruptly in a firmground that is overlain by the Pleistocene mound succession. Biostratigraphic results suggest that the hiatus between the two successions spans at least 1.65 m.y. The mound flanks are draped by late Pleistocene (<0.26 Ma) silty clay deposits that frequently contain dropstones. The mound succession just above the firmground is represented by interbedded grainstone, floatstone, rudstone, packstone, and wackestone in decimeter thicknesses, all reflecting relatively rapidly changing depositional realms. Above this lower level, the mound succession shows pronounced recurring cycles of Pleistocene coral floatstone, rudstone, wackestone, and packstone on a several meter scale that are well represented in the carbonate content change and are most probably associated with Pleistocene glacial-interglacial cycles. A role for hydrocarbon fluid flow in the initial growth phase of Challenger Mound is not obvious either from the lithostratigraphy or from initial geochemistry and microbiology results. We found no significant quantities of gas in the mound or in the subbasal mound sediments, nor were carbonate hardgrounds observed at the mound base. Microbial effects on mound and submound diagenesis are more subtle. We detected the methane-sulfate transition only in the deeper-lying Miocene silt and sandstones underlying the mound, where methane concentrations and prokaryotic cell abundances increase with increasing depth. In the mound itself, interstitial water profiles of sulfate, alkalinity, Mg, and Sr suggest a tight coupling between carbonate diagenesis and microbial sulfate reduction. Decomposition of organic matter (organoclastic) by sulfate reduction may drive the biogeochemical processes of mineralogical transformation by (1) producing CO (sub 2) , which enhances aragonite dissolution and (2) increasing overall dissolved inorganic carbon concentration, which allows dolomite or high-Mg calcite to precipitate. Furthermore, periods of rapid sedimentation overlying hiatuses apparently left distinct signals in the interstitial water chemistry of the Pleistocene sediments that surround and partially bury the carbonate mounds of Porcupine Seabight.