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doi:10.2204/iodp.pr.307.2005
Preliminary conclusions and observations can be framed within the set of hypotheses generated prior to the expedition.
1. Gas seeps act as a prime trigger for mound genesis—a case for geosphere–biosphere coupling.
Drilling to the base of Challenger Mound and deeper suggested that geofluid (i.e., hydrocarbons) did not play a role in mound genesis and growth. A role for hydrocarbon fluid flow in the initial growth phase of Challenger Mound is not obvious from either lithostratigraphy no the 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. The mound rests on an apparent Pliocene firmground whose origin does not appear to be microbial. The mechanism for the initiation of mound growth (i.e., colonization by corals) awaits closer examination of the core sections from the five holes at Site U1317.
2. Prominent erosional surfaces reflect global oceanographic events.
Holes penetrating erosional unconformities at all three sites were drilled and the lithology was linked to the interpreted seismic facies (Fig. F8). An important erosional surface is the Miocene/Pliocene hiatus, which correlates to the firmground under the mound itself at Site U1317, the unconformity under the coral-bearing Unit 2 at Site U1316 and the phosphorite/oyster bed hiatus at the top of Unit 3 at Site U1318. Development of phosphorite nodules on a fine sand basement is suggestive of an oceanographic change. Furthermore, linkage of the seismic stratigraphy and the core lithology at Site U1318, particularly in Subunits 3A and 3B, will provide key information on sediments that have eroded at the deeper Sites U1317 and U1316. We expect that we will be able to link all of the sites from Porcupine Seabight with biostratigraphy from Neogene marine sections from other DSDP, ODP, and IODP sites of the eastern North Atlantic. This will support interpretations of the timing of the unconformities.
3. The mound may be a high-resolution paleoenvironmental recorder because of its high depositional rate and abundance of micro- and macrofossils.
The mound is composed of at least 10 distinct layers of coral (L. pertusa), clay, and coccoliths down to its base at 130–155 mbsf. These represent intervals of active growth of the coral mound. L. pertusa is known to have migrated from midlatitudes to the north during the last glacial–interglacial transitional period, and it began recolonizing the coral mounds in Porcupine Seabight. The identified growth intervals therefore most probably correspond to Pleistocene interglacials. A series of well-established proxies will be used to study paleoenvironmental change including response to Pleistocene glacial–interglacial cycles. Challenger Mound is also partially buried in recent drift deposits that contain indications of rapid deposition rates (based on interstitial water chemistry) and evidence for change on glacial–interglacial time periods (distinct intervals of dropstone occurrence).
4. The Porcupine mounds are present-day analogs for Phanerozoic reef mounds and mud mounds.
There are still debates on depositional processes that formed ancient carbonate mud mounds that occur ubiquitously in Paleozoic–Mesozoic strata worldwide. Nevertheless, it is clear that Challenger Mound is not a present-day analog for these microbially formed Paleozoic–Mesozoic mounds. Rather, Challenger Mound is in many ways more reminiscent of the Cenozoic bryozoan mounds buried at Great Australian Bight (James et al., 2000). A significant difference from Great Australian Bight mounds, however, is the preservation of carbonate mound or reef structures in an essentially siliciclastic environment. The mound section shows no evidence of microbial roles in stabilizing sediment and forming automicrite, which have been suggested for many ancient mud mounds. The process that maintains the conical mound geometry with steep franks is still unclear. This could be related to the sediment-buffering ability of branching colonies of L. pertusa. The presence/absence of coral frameworks is the key feature to answer this question and will be approached by assessing the 3-D distribution of corals using computerized tomography (CT) scanning.
Microbial effects on mound and submound diagenesis may play a subtle role in degrading and stabilizing carbonate fractions both within the mound and within the deeper and older Miocene silty clays. Profiles of sulfate, alkalinity, Mg, and Sr in the mound interstitial water suggest a tight coupling between carbonate diagenesis and microbial sulfate reduction. Mineralization of organic matter by sulfate reduction (organoclastic) may drive the process of aragonite weathering followed by dolomite or high-Mg calcite precipitation within mound sequences by (1) producing CO2, which enhances aragonite weathering and (2) increasing the overall dissolved inorganic carbon concentration, which allows dolomite to precipitate. In the deeper Miocene silt and sandstones underlying the mound, we detect the methane–sulfate transition, where methane concentrations and prokaryotic cell abundances increase with depth.