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Microbial methane generation and gas transport in shallow sediments of an accretionary complex, southern hydrate ridge (ODP Leg 204), offshore Oregon, USA  [2006]

Claypool, George E. Milkov, Alexei V. Lee, Young-Joo Torres, Marta E. et al.

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Sediments at the southern summit of Hydrate Ridge display two distinctmodes of gas hydrate occurrence. The dominant mode is associatedwith active venting of gas exsolved from the accretionary prismand leads to high concentrations (15%–40% of pore space) of gas hydratein seafloor or near-surface sediments at and around the topographicsummit of southern Hydrate Ridge. These near-surface gashydrates are mainly composed of previously buried microbial methanebut also contain a significant (10%–15%) component of thermogenichydrocarbons and are overprinted with microbial methane currentlybeing generated in shallow sediments. Focused migration pathwayswith high gas saturation (>65%) abutting the base of gas hydrate stabilitycreate phase equilibrium conditions that permit the flow of a gasphase through the gas hydrate stability zone. Gas seepage at the summitsupports rapid growth of gas hydrates and vigorous anaerobicmethane oxidation.The other mode of gas hydrate occurs in slope basins and on the saddlenorth of the southern summit and consists of lower average concentrations(0.5%–5%) at greater depths (30–200 meters below seafloor[mbsf]) resulting from the buildup of in situ–generated dissolved microbial methane that reaches saturation levels with respect to gas hydratestability at 30–50 mbsf. Net rates of sulfate reduction in the slope basinand ridge saddle sites estimated from curve fitting of concentration gradientsare 2–4 mmol/m³/yr, and integrated net rates a
re 20–50 mmol/m²/yr. Modeled microbial methane production rates are initially 1.5mmol/m³/yr in sediments just beneath the sulfate reduction zone butrapidly decrease to rates of <0.1 mmol/m³/yr at depths >100 mbsf. Integratednet rates of methane production in sediments away from thesouthern summit of Hydrate Ridge are 25–80 mmol/m²/yr. Anaerobicmethane oxidation is minor or absent in cored sediments away fromthe summit of southern Hydrate Ridge.Ethane-enriched Structure I gas hydrate solids are buried more rapidlythan ethane-depleted dissolved gas in the pore water because of advectionfrom compaction. With subsidence beneath the gas hydratestability zone, the ethane (mainly of low-temperature thermogenic origin)is released back to the dissolved gas-free gas phases and produces adiscontinuous decrease in the C₁/C₂ vs. depth trend. These ethane fractionationeffects may be useful to recognize and estimate levels of gashydrate occurrence in marine sediments.