Estimates of biogenic methane production rates in deep marine sediments at Hydrate Ridge, Cascadia Margin

Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is\ud considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is\ud biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ\ud methanogenesis. We estimated the in situ methanogenesis rates in Hydrate Ridge (HR) sediments by coupling\ud experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete\ud locations in the sediment column. When starved in a biomass recycle reactor, Methanoculleus submarinus\ud produced ca. 0.017 fmol methane/cell/day. Quantitative PCR (QPCR) directed at the methyl coenzyme M\ud reductase subunit A gene (mcrA) indicated that 75% of the HR sediments analyzed contained <1,000 methanogens/\ud g. The highest numbers of methanogens were found mostly from sediments <10 m below seafloor. By\ud considering methanogenesis rates for starved methanogens (adjusted to account for in situ temperatures) and\ud the numbers of methanogens at selected depths, we derived an upper estimate of