HR: 09:00h
AN: OS31D-04    [Abstracts]
TI: The effects of mass deposition, gas flux and salt diapirs on surface sediment geochemistry in Keathley Canyon (northern Gulf of Mexico)
AU: * Pohlman, J W
EM: john.pohlman@nrl.navy.mil
AF: Virginia Insitute of Marine Science, Department of Physical Sciences, Gloucester, VA 23062 United States
AU: * Pohlman, J W
EM: john.pohlman@nrl.navy.mil
AF: Geo-Centers, Inc, Gas Hydrate Research Group, Washington, DC 20375 United States
AU: * Pohlman, J W
EM: john.pohlman@nrl.navy.mil
AF: Naval Research Lab, Marine Biogeochemistry, Washington, DC 20375 United States
AU: Ruppel, C
OS31D-04 AF: Georgia Tech, Department of Earth and Atmospheric Sciences, Atlanta, GA 30332 United States
AU: Hutchinson, D R
OS31D-04 AF: USGS, Coastal & Marine Geology, Woods Hole, MA 02543 United States
AU: Hart, P E
OS31D-04 AF: USGS, Coastal & Marine Geology, Menlo Park, CA 94025 United States
AU: Plummer, R
OS31D-04 AF: Geo-Centers, Inc, Gas Hydrate Research Group, Washington, DC 20375 United States
AU: Knies, D
OS31D-04 AF: Naval Research Lab, Marine Biogeochemistry, Washington, DC 20375 United States
AU: Grabowski, K
OS31D-04 AF: Naval Research Lab, Marine Biogeochemistry, Washington, DC 20375 United States
AU: Coffin, R B
OS31D-04 AF: Naval Research Lab, Marine Biogeochemistry, Washington, DC 20375 United States
AB: The geochemical signature of the shallow (<10 m) sediments and pore waters reflects both the flux of methane from deep hydrate and gas accumulations and near-surface geologic processes such as mass wasting and erosion. Consumption of methane and sulfate by the anaerobic oxidation of methane (AOM) at the sulfate methane interface (SMI) broadly affects the profiles of dissolved and solid state constituents. The dynamic pore water system provides a short-term record of the gas flux, while the solid state phases record the long-term patterns. In the northwest Gulf of Mexico - where salt diapirism is prevalent - it is necessary to evaluate both the dissolved and solid state constituents of the shallow sediments in order differentiate the effects of gas flux and the complex geology. Detailed pore water and solid state geochemical analyses were performed on 18 piston cores collected near the edge of a salt withdrawal minibasin in Keathley Canyon, a few kilometers from the site of JIP drilling. The dataset includes: measurements of dissolved sulfate, sulfide, chloride and methane concentrations; carbon isotopes of methane (constrains the origin of the gas); grain size distributions; percent stable carbon isotopic composition of total organic carbon (TOC) and carbonate; and radiocarbon dating of TOC. This dataset is likely one of the most exhaustive ever compiled on shallow cores from a methane hydrate province. One key result is the observation of widespread nonuniformity in sulfate and chloride pore water profiles, with evidence indicating the persistence of seawater values sometimes extending to several meters depth. Numerical modeling of the pore water data suggests that a physical perturbation associated with mass wasting (rapid sedimentation) is the most likely explanation for these observations. This interpretation is supported by the several other lines of evidence both from our datasets and from geophysical observations in the area. In one case, a radiocarbon age-inversion (older sediments lying above younger sediments) provides unequivocal evidence for mass wasting. In other cases, the evidence for mass deposition is inferred from comparing depositional patterns in the Keathley Canyon cores with isotopic data from DSDP Core 619 collected at nearby Pygmy Basin. By constraining the factors that control the surface sediment geochemistry, we are able to provide a more realistic description of the relationship between the complex near surface sediments and the deeper hydrate and gas systems.
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3004 Gas and hydrate systems
DE: 4273 Physical and biogeochemical interactions
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005