HR: 08:40h
AN: OS31D-03 INVITED     [Abstracts]
TI: Geochemical Evidence for Gas Hydrates at Atwater Valley and Keathley Canyon, Gulf of Mexico
AU: * Kastner, M
EM: mkastner@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, Ca 92093 United States
AU: Claypool, G
EM: geclaypool@aol.com
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, Ca 92093 United States
AU: Robertson, G
EM:
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, Ca 92093 United States
AU: Schultheiss, P
EM:
AF: Geotek Ltd., 3 Faraday Close Drayton Fields, Daventry, NN11 8RD United Kingdom
AU: Holland, M
EM:
AF: Geotek Ltd., 3 Faraday Close Drayton Fields, Daventry, NN11 8RD United Kingdom
AB: Geochemical analyses of pore fluids of both pressurized and non-pressurized cores provide important constraints on the main objective of the 2005 Gulf of Mexico (GOM) Joint Industry Program (JIP) cruise: to calibrate geophysical estimates of methane hydrate distribution and concentration with downhole logs and measurements on cores. Cores were recovered in two deepwater (1300 meter) lease areas of the GOM (Atwater Valley 13/14 and Keathley Canyon 151). Selected intervals were cored to a maximum depth of 390 meters beneath the seafloor, as indicated in prior logging while drilling boreholes. Methane hydrate is associated with sediment intrusions and seafloor mounds at Atwater Valley 13/14, and with a bottom simulating reflection (BSR) (i.e., possible base of methane hydrate stability) at KC 151. Pore fluid samples were analyzed from both conventional cores and pressure cores; the hydrates in the conventional cores decomposed prior to sampling. The former presence and distribution of methane hydrates in the some of the conventional cores was inferred by indirect methods: from infra-red scanning of cold core temperatures that correlated with pore fluids having lower salinity and chlorinity and mouse-like textures. Cores that were successfully collected under in situ pressure retained methane quantities consistent with 1-5% methane hydrate in the sediment pore volume. Overall, similar proportions were indicated by pore water salinity/chlorinity anomalies, assuming dilution by fresh water released from decompose methane hydrate. In some specific horizons, however, at the Atwater Valley 14 mound sites, the pore fluid salinity/chlorinity data suggest up to 7-9% methane hydrate in the sediment pore volume. Significant variations in sulfate gradients were observed. The steepest gradients, with the sulfate/methane interface (SMI) at or just below the seafloor, were found at the Atwater Valley 14 mound sites. Away from the mound the SMI occurs at a depth of about 7-10 m below the seafloor at both Atwater Valley and Keathley Canyon. The extreme negative delta 13C values (-46 to -50 ppt) of dissolved inorganic carbon at the SMI in both localities clearly indicate that methane advection is intense, and that anaerobic methane oxidation (AMO) is the dominant reaction responsible for sulfate reduction.
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1051 Sedimentary geochemistry
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005