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doi:10.2204/iodp.proc.307.101.2006

Preliminary conclusions

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 major 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 nor 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 a Miocene firmground whose origin appears to be an erosional unconformity. The mechanism for the initiation of mound growth (i.e., colonization by corals) awaits closer examination and analysis of the core sections containing the mound base boundary at Sites U1316 and 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 late 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/bivalve bed hiatus at the top of Unit 3 at Site U1318. Development of phosphorite nodules on a fine sand basement is suggestive of an upwelling regime. 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 Deep Sea Drilling Project, 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 cold-water corals (L. pertusa), clay, and coccoliths down to its base at 130–155 mbsf. These represent intervals of mound development. The identified growth intervals therefore most probably correspond to Pleistocene environmental changes. 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 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 the 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 microbially formed Paleozoic–Mesozoic mounds. Rather, Challenger Mound is in many ways more reminiscent of the Cenozoic bryozoan mounds located at the shelf edge margin of 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 little evidence of microbial automicrite, which has been suggested for many ancient mud mounds. The prokaryote abundances in the deeper Miocene sediments are actually significantly greater than in the overlying Pleistocene sediments and relative to the global coverage for this depth. The process that maintains the conical mound geometry with steep flanks is probably related to the sediment-trapping ability and structural network 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 (Fig. F16).

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. Sulfate and alkalinity profiles in the mound reflect zones of microbially mediated organic mineralization through sulfate reduction between 0 and 50 mbsf. Magnesium also shows a loss, as is evident in the decreased Mg/Ca ratio at these depths (Fig. F15). With the distinct increase in Sr in this interval, we propose that aragonite dissolution is releasing Sr to the interstitial pore fluids. Concurrently, dolomite or some other Ca-Mg carbonate mineral (e.g., low-Mg calcite, calcian dolomite, or dolomite) is precipitating and removing Mg. Decomposition of organic matter by sulfate reduction (organoclastic) may be driving this process by (1) producing CO₂, which enhances aragonite dissolution, and (2) increasing the overall dissolved inorganic carbon concentration. The absence of dissolved sulfide further suggests that the sulfide produced through microbial sulfate reduction may react with detrital iron minerals and be precipitated as iron sulfide minerals, principally pyrite. The reduction of ferric iron and precipitation of pyrite drives carbonate alkalinity and pH up and favors carbonate precipitation.

These reactions may be described by the following overall equation:

Lower sulfate, Ca, and Mg and higher alkalinity and Sr concentrations in Hole U1317E suggest that microbially mediated organic mineralization is more intense at the mound top (Fig. F15), where sediments remain in situ, relative to the mound flanks penetrated in Holes U1317A through U1317D, where the sediment is more likely to have been transported downslope.

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