Pore-water sulfate concentration gradients, isotopic compositions, and diagenetic processes overlying continental margin, methane-rich sediments associated with gas hydrates

Geochemical investigations of pore waters and sediments from a well-studied gas hydrate setting (Carolina Rise-Blake Ridge, offshore southeastern United States) reveal that interstitial sulfate concentration gradients are predominantly linear, suggesting a localized sink at the sulfate-methane interface (SMI). Microbially-mediated co-consumption of sulfate and methane (anaerobic methane oxidation, AMO) provides a biogeochemical link between the interstitial sulfate and methane pools. Methane (in equilibrium with gas hydrate) diffuses upward to the SMI, where it is consumed by AMO. This additional sink for sulfate acts to modify pore-water sulfate concentrations, producing steep (and perhaps linear) sulfate profiles and shallow SMIs.; The evidence for AMO is substantial, including {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm Sigma CO2{rcub}{dollar} values as depleted as {dollar}-{dollar}37{dollar}perthous{dollar} PDB, indicating significant carbon contribution from methane. Carbon-cycling between the dissolved CO{dollar}sb2{dollar} and methane pools via AMO explains their extreme {dollar}sp{lcub}13{rcub}{dollar}C-depletion within continental rise sediments worldwide. Diagenetic modeling based on methane concentrations (ODP Site 995) shows that co-consumption of sulfate and methane is responsible for at least: 35% of total sulfate depletion! Sites with larger sulfate gradients should have even larger proportions of sulfate consumed by AMO.; Geochemical data provided by worldwide DSDP-ODP drilling indicate that sites associated with gas hydrates are overwhelmingly characterized by shallow SMI depths ({dollar}<{dollar}50 meters) and steeper (linear) sulfate gradients. Patchy distribution of bottom-simulating reflectors (BSRs) at the study area and elsewhere, and other mounting geophysical evidence suggest that BSRs indicate trapped accumulations of gaseous methane rather than gas-hydrate-containing sediments. In contrast, shallow SMI depths on continental rises uniformly indicate co-consumption of sulfate and methane, significant subsurface methane storage, and the presence of gas hydrate. Contour maps of SMI depth strongly suggest that the Blake Ridge is a vast storage area for interstitial methane and gas hydrate. In deep-water settings, large sulfate gradients (and shallow SMIs) can be reliably used as potential gas hydrate indicators. Addition of DSDP-ODP sites with shallow SMIs lacking BSRs to the known occurrences of gas hydrate significantly increases the number of deep-water, marine gas-hydrate localities.