Introduction Over the past decade, oceanographic and geophysical surveys along the slope of the Porcupine Seabight off the southwestern continental margin of Ireland have identified upwards of a thousand enigmatic mound-like structures (Figs. 1 and 2). The mounds of the Porcupine Seabight rise from the seafl oor in water depths of 600–900 m and formimpressive conical bodies several kilometers wide and up to 200 m high. Although a few mounds such as Thérèse Mound and Galway Mound are covered by a thriving thicket of coldwater corals, most mound tops and fl anks are covered by dead coral rubble or are entirely buried by sediment (De Mol et al., 2002; Fig. 2, Beyer et al., 2003). Lophelia pertusa (Fig.3) and Madrepora oculata are the most prominent cold-water corals growing without photosynthetic symbionts. The widespread discovery of large and numerous coral-bearing banks and the association of these corals with the mounds have generated signifi cant interest as to the composition, origin and development of these mound structures.Challenger Mound, in the Belgica mound province, has an elongated shape oriented along a north-northeast to south-southwest axis and ispartially buried under Pleistocene drift sediments. In high-resolution seismic profiles the mounds appear to root on an erosion surface (van Rooij et al., 2003). During IODP Expedition307 the Challenger Mound in the Porcupine Seabight was drilled with the goal of unveiling the origin and depositional processes withinthese intriguing sedimentary structures. Challenger Mound, unlike its near neighbors the Thérèse and Galway mounds, has little to no livecoral coverage and, therefore, was chosen as the main target for drilling activities, so that no living ecosystem would be disturbed

No abstract available. <br><br> doi:<a href="http://dx.doi.org/10.22 04/iodp.sd.5.03.2007" target="_blank">10.22 04/iodp.sd.5.03.2007</a>

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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 hy- Proc. IODP | Volume 307 2 Expedition 307 ScientistsExpedition 307 summary drocarbon 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...

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