A GEOLOGIC RECORD OF COMPETING SULFATE-DEPLETION PROCESSES WITHIN CONTINENTAL-RISE SEDIMENTS OVERLYING METHANE GAS HYDRATES OF THE BLAKE RIDGE REGION (OFFSHORE SOUTHEASTERN UNITED STATES)
Marine sediments of the Blake Ridge (offshore South Carolina and Georgia) contain sulfide minerals that point to AMO as an important diagenetic process both today and in the recent geological past. At the present-day methane-sulfate interface, upward-diffusing methane is consumed by reaction with downward-diffusing sulfate, producing a geochemical environment that promotes the authigenic precipitation of sulfide minerals. These sulfide minerals, mainly pyrite, are enriched in the heavy isotope of sulfur (34S), whereas solid-phase sulfide higher in the sulfate reduction zone contains more 32S. This result is consistent with larger fluxes of methane in the region derived from underlying methane gas hydrate deposits.
The sedimentary record of a portion of the Blake Ridge (ODP Site 995) back to the Late Miocene (~6.2 Ma) shows that changing depositional conditions seem to emphasize sulfate reduction over AMO in progressively older sediments. Sulfide mineral concentration changes from low baseline values (0.2 weight percent) in youthful sediments to higher values (0.4 to 0.6 wt %) in older sediments. Baseline values of δ34S also increase from -45 to -30 with increasing depth and sediment age. Geochemical conditions today favor more sulfide mineralization in association with AMO, whereas conditions in the past likely responded to higher delivery rates of sedimentary organic matter conditions necessary to ultimately produce the amount of methane gas hydrates occurring within the Blake Ridge region.