Lamont-Doherty Earth Observatory, United States

Challenger Mound is a prominent mound structure covered with dead cold-water coral rubble on the southwest Irish continental margin and was the focus of 12 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 roots 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 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. Obtain 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 cold-water carbonate mounds and for investigating how they resemble ancient mud mounds. Two further sites, located down- and upslope of Challenger Mound, completed a transect to (1) constrain the stratigraphic framework of the slope/mound system, (2) identify and correlate erosional surfaces observed in seismic sections, and (3) investigate potential gas accumulation in the sediments underlying the mound. Drilling revealed that the mound rests on a sharp erosional boundary. Drift sediments below this erosion surface consist of glauconitic and silty sandstone of early-middle Miocene age. The Miocene strata end abruptly in a firmground that is overlain by the late Pliocene-Pleistocene mound succession. The mound flanks are draped by late Pleistocene (<0.26 Ma) silty clay deposits that frequently contain dropstones. The mound succession mainly consists of floatstone and rudstone formed of fine sediments and cold-water branching corals. Pronounced recurring cycles of several meter scales were recognized in carbonate content and color changes 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 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 subtle. We detected the methane-sulfate transition only in the Miocene silt and sandstones underlying the mound, where methane concentrations and prokaryotic cell abundances increase with increasing depth. In the mound succession, interstitial water profiles of sulfate, alkalinity, Mg, and Sr suggest a tight coupling between carbonate diagenesis and microbial sulfate reduction. Decomposition of organic matter by sulfate reduction (organoclastic) 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 (calcium-rich) precipitation. Furthermore, periods of rapid sedimentation overlying hiatuses left distinct signals in the interstitial water chemistry of the Pleistocene sediments that surround and partially bury the mounds of Porcupine Seabight.