HR: 08:30h
AN: OS11C-03    [Abstracts]
TI: Geochemical Evidence for Gas Hydrates in the Indian Ocean
AU: * Kastner, M
EM: mkastner@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AU: Solomon, E
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AU: Torres, M
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, United States
AU: Spivack, A J
AF: Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, United States
AU: Borole, D V
AF: National Institute of Oceanography, Dona Paula, Goa, 4003004, India
AU: Hangsterfer, A
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AU: Das, H C
AF: Oil India Ltd, R/D Department, Duliajan, As 786602, India
AU: Robertson, G A
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AB: Geochemical analyses of pore fluids collected from both non-pressurized and pressurized cores provide important constraints on the presence and distribution of gas hydrates in the Indian Ocean. Cores were recovered from two deepwater (900-1170 meter) basins offshore southeast India, the Krishna-Godavari (KG) (10 sites) and Mahanadi Basins (2 sites), and from one site in the Andaman Sea (1600 meters water depth). A bottom simulating reflector (BSR) (i.e., possible base of methane hydrate stability) was present at most of the sites cored and evidence for methane hydrate was obtained at each of the sites. The gas hydrates in most of the conventional cores decomposed prior to sampling. The former presence and distribution of methane hydrate in some of the conventional cores was inferred by indirect methods: from infra-red imaging of cold core temperatures that correlated with pore fluids having lower salinity and chloride concentrations and occasionally mousse-like textures. Chloride concentrations and salinity anomalies, assuming dilution by water released from decomposed methane hydrate, suggest pore volume occupancies on the order of, <1% to a maximum of ~61% at two of the sites in the KG Basin, and <1% to a maximum of ~76% at the site in the Andaman Sea. In the KG Basin, the highest methane hydrate concentrations were associated with fracture zones in clay/silt sediments or in some coarser grained horizons, although gas hydrate was not present in all coarse- grained sediment horizons. Overall, the % occupancies based on pressure core methane concentrations and the chloride concentrations in conventional cores were similar. The pressure core samples used for these determinations were 1 m long, and the conventional core samples were 10-30 cm whole round core samples, chosen via IR imaging. Variations in sulfate gradients were observed; the steepest gradient with the sulfate/methane interface (SMI) at 8 mbsf was observed in the KG Basin. The deepest SMI obtained, at 25 mbsf, was documented at the Andaman Sea site. The shallow depths of the SMI at all sites indicate that methane advection is intense. The extreme negative δ ¹³C values of the dissolved inorganic carbon, ranging from -38‰ to -46‰ at the SMI, coincide with the depths of maximum alkalinity values. These δ ¹³C-DIC values indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction. The percent of sulfate reduction by organic matter oxidation relative to sulfate reduction by methane oxidation is slightly higher at the KG Basin sites than at the Mahanadi Basin or Andaman Sea sites. Authigenic carbonate formation is widespread, in particular at the Andaman Sea site. Bromide concentrations provide insight on both organic matter diagenesis and the type of organic matter. The Br/Cl ratios are distinct in the three regions cored: they are highest at the Andaman Sea Site, intermediate in the KG Basin and lowest in the Mahanadi Basin.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting