HR: 14:15h
AN: OS52C-03 INVITED     [PDF]
TI: Controls on Regional Gas Hydrate Occurrence at Southern Hydrate Ridge (ODP Leg 204)-Geochemical Evidence
AU: * Claypool, G E
EM: geclaypool@aol.com
AF: Consulting geochemist, 8910 West 2nd Avenue, Lakewood, CO 80226 United States
AU: Milkov, A V
EM: Milkav@BP.com
AF: BP America, P.O. Box 3092, Houston, TX 77079 United States
AU: Lee, Y
EM: yjl@rock25t.kigam.re.kr
AF: Korea Institute of Geoscience and Mineral Resources, 30 Kajung-Dong, Yusong-Gu, Daejon, 305-350 Korea, Republic of
AU: Torres, M E
EM: mtorres@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, OR 97331-5503 United States
AU: Borowski, W S
EM: W.Borowski@eku.edu
AF: Department of Earth Sciences, Eastern Kentucky University, Richmond, KY 40475 United States
AU: Tomaru, H
EM: tomaru@gbs.eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, Tokyo, 113-0033 Japan
AU: Leg 204 Science Party, .
EM:
AF: Ocean Drilling Program, Texas A&M University, College Station, TX 77845-9547 United States
AB: Ocean Drilling Program Leg 204 on southern Hydrate Ridge found concentrated gas hydrate (20-40% of pore volume) only in shallow (0-30 m) sediments over a limited area (0.3 x 0.5 km) at the summit of the Ridge, just above seismically imaged gas migration pathways. Although summit gas hydrates are dominated by microbial methane ($\delta^{13}$C = -65 permil), the hydrates contain thermogenic ethane ($\delta^{13}$C = -34 permil) and sediments cored within the migration conduits contain elevated amounts of C2-C6 hydrocarbons having carbon isotopic compositions indicating hydrocarbon origin at temperatures on the order of 120-140 deg C. At sites away from the summit of southern Hydrate Ridge, gas hydrate occurrence is more limited, with average contents on the order 1-3% of pore volume. Non-summit sites have no significant input of migrated thermogenic hydrocarbons to the hydrate stability zone. However, intense microbial activity results in relatively steep (1.6 to 3.6 mM/m) methane concentration gradients, and a top of gas hydrate occurrence marked by distinctive hydrocarbon fractionation (ethane depletion, propane enrichment) in the dissolved gas geochemistry. Ethane in shallow sediments is isotopically light ($\delta^{13}$C = -52 permil) and probably related to microbial acetogenesis and carboxyl group reduction. Formation of Structure I methane hydrate involves a fourfold ethane-enrichment in the hydrate relative to the dissolved gas phase, and results in a net transfer of ethane from the pore water to the sediment. Steep porosity gradients result in the burial velocity of sediment being greater than that of pore water with respect to the sediment-seawater interface. Downward sediment velocity is even greater relative to the base of gas hydrate stability, which is shoaling due to tectonic uplift concurrent with sedimentation. Decomposition of ethane-enriched gas hydrate upon subsidence beneath the stability zone results in transfer of ethane from the sediment back to dissolved gas/free gas phases, and produces large shifts in C1/C2 ratio. Carbon isotopic composition of dissolved inorganic carbon indicates that anaerobic methane oxidation is an important process mainly in the summit region. At most non-summit sites, rapid burial apparently limits the amount of methane diffusing upward to the sulfate reduction zone.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting