Microbial Methane Generation and Gas Transport in Shallow Sediments of an Accretionary Complex, Southern Hydrate Ridge (ODP Leg 204), offshore Oregon, USA

Abstract

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%–40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%–15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north 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 micro1Claypool, G.E., Milkov, A.V., Lee, Y.J., Torres, M.E., Borowski, W.S., and Tomaru, H., 2006. Microbial methane generation and gas transport in shallow sediments of an accretionary complex, southern Hydrate Ridge (ODP Leg 204), offshore Oregon, USA. In Trehu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proc. ODP, Sci. Results, 204, 1–52 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 28910 West Second Avenue, Lakewood CO 80226, USA. geclaypool@aol.com 3BP America, Exploration and Production Technology Group, Houston TX 77079, USA. 4Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea. 5College of Atmospheric and Ocean Science, Oregon State University, 104 Ocean Administration Building, Corvallis OR 97331-5503, USA. 6Department of Earth Sciences, Eastern Kentucky University, 512 Lancaster Avenue, Richmond KY 40475-3102, USA. 7Department of Earth and Environmental Sciences, 227 Hutchison Hall, University of Rochester, Rochester NY 14627, USA. Initial receipt: 1 February 2005 Acceptance: 1 March 2006 Web publication: 18 August 2006 Ms 204SR-113 G.E. CLAYPOOL ET AL. MICROBIAL METHANE GENERATION AND GAS TRANSPORT 2 bial methane that reaches saturation levels with respect to gas hydrate stability at 30–50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2–4 mmol/m3/yr, and integrated net rates are 20–50 mmol/ m2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of 100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25–80 mmol/m2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.