| High interstitial methane concentrations, gas hydrates, and interstitial fluid chemistry may have a profound influence on sediment diagenesis. Both pore fluids and their associated sediments from gas hydrate bearing sections on the Blake Ridge were studied. The objective was to evaluate relationships between the ephemeral gas hydrates and gases, pore fluid movement, and mineral phases that remain in the sedimentary record.; Pore water geochemical data suggest that diffusive exchange of pore fluid constituents influence diagenesis between the seafloor and the top of the gas hydrate zone. The onset of significant carbonate diagenesis occurs at ∼20 mbsf (the sulfate-methane interface SMI) where microbial-mediated anaerobic methane oxidation (AMO) is inferred to be active. Authigenic calcite (δ 13C values as low as –7.0‰), and dolomite (δ 13C values between –3.6 and –13.7‰) forms at, or just beneath (between 20 and 80 mbsf) this boundary. Changes in pore water chemistry with depth lead to formation of authigenic siderite (δ 13C and δ18O values from +5.0 to +10.9‰ and +2.9 to +7.6‰, respectively) just above and within the gas hydrate zone (150–450 mbsf). Authigenic minerals are primarily disseminated within sediments.; Sites over the Blake Ridge Diapir, where upward advection of methane-rich fluids along fault conduits support chemosynthetic communities, maintains a shallowing of the SMI, and stimulates precipitation of microcrystalline interstitial and cavity-filling radial-fibrous methane-derived carbonate cements were studied. Authigenic calcite and aragonite nodules with depleted δ 13C values (as low as –48‰), and δ18O values in isotopic equilibrium with bottom waters, form at or very near the seafloor.; The Cape Fear Slide is a massive submarine landslide linked to gas hydrate-induced sediment instability. Within the slide scar, 14C-age dating of the uppermost slide surface indicate that a major event is coincident with the last sea level lowstand (∼18,000 years B.P.). Sediments beneath this slide surface are geochemically distinct from the overlying sediment drape (dominated by their previous diagenetic history), but do not exhibit evidence for post-slide diagenesis. Methane-derived authigenic dolomite occurring as localized nodules, apparently formed along faults or failure planes associated with the Cape Fear Slide, and are not in close proximity to the sea floor. |
