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Free keywords:
gas hydrate; methane; hydrate ridge; cold seeps; chemosynthesis; anaerobic oxidation of methane; sulfate reduction; archaea; bacteria; beggiatoa
Abstract:
Hydrate Ridge, Cascadia convergent margin, is characterized by abundant methane hydrates at and below the seafloor, active venting of fluids and gases, chemosynthetic communities composed of giant sulfide-oxidizing bacteria and clams, authigenic carbonates, and some of the highest methane oxidation rates ever found in the marine environment. Fluid flow rates vary over six orders of magnitude among closely spaced settings at the crest of Hydrate Ridge. The distribution of benthic communities is mainly related to the sulfide flux from the subsurface sediments produced by anaerobic oxidation of methane (AOM). Even at high flux rates, AOM removes most of the methane seeping from the subsurface. Less than 50% of the methane escapes from bacterial mats, from Calyptogena fields <15%, and from Acharax beds, no methane is emitted to the water column. The precipitation of carbonate derived from AOM is a significant and permanent carbon sink. Hence, compared to the amount of gas hydrates and methane-derived carbonates stored at the summit of Hydrate Ridge, the emission of methane is relatively low due to the efficient filtering capacity of methanotrophic microbial communities. This present review updates current knowledge of the geology of Hydrate Ridge, summarizes recently published data on geomicrobiology and biogeochemistry, and derives a local methane budget.
