HR: 1340h
AN: OS23A-1063    [Abstracts]
TI: Geologic Control on Porewater Geochemistry in Gas Hydrate Bearing Sediments in the Gulf of Mexico
AU: * Smith, J P
EM: joseph.smith@nrl.navy.mil
AF: Naval Research Laboratory (NRL), Marine Biogeochemistry (Code 6114) 4555 Overlook Ave, SW, Washington, DC 20375, United States
AU: Hamdan, L J
EM: leila.hamdan@nrl.navy.mil
AF: Naval Research Laboratory (NRL), Marine Biogeochemistry (Code 6114) 4555 Overlook Ave, SW, Washington, DC 20375, United States
AU: Wood, W T
EM: warren.wood@nrlssc.navy.mil
AF: Naval Research Laboratory (NRL), Geology-Geophysics (Code 7432) Stennis Space Center, Stennis Space Center, MS 39529, United States
AU: Coffin, R B
EM: rick.coffin@nrl.navy.mil
AF: Naval Research Laboratory (NRL), Marine Biogeochemistry (Code 6114) 4555 Overlook Ave, SW, Washington, DC 20375, United States
AB: Sedimentary geologic features such as faults, fissures and salt diapirs impact vertical and lateral fluid flow, and hence, methane advection in gas hydrate bearing marine sediments. High resolution, geochemical characterization, (piston cores) and geophysical (seismic) surveys were conducted in the Atwater Valley and Alaminos Canyon regions of the Gulf of Mexico in 2005 and 2007 respectively, in order to constrain fluid and gas flux in these areas. Porewater sulfate, chloride, methane and DIC concentrations as well as stable carbon isotope ratios of DIC confirm that at both sites geochemistry is largely influenced by local geology. Specifically, at locations in Alaminos Canyon where faults or fissures are evident in seismic data, porewater geochemistry appears to be controlled by advective processes. In the case of Atwater Valley, high methane vertical flux rates calculated from concentration data plotted against depth are significantly correlated to increasing porewater salinity likely resulting from salt diapirs which reduce gas hydrate stability. By contrast to these examples, sedimentary geochemical properties (for example) of cores collected distal to the faults, fissures or diapirs described above appear to be diffusion-dominated. Seismic data from the two study areas shows a large difference the scale, frequency and occurrence of geologic features that could influence fluid flow and gas fluxes. Therefore, accurate, spatially-normalized methane flux estimates for comparison between the sites depend largely on adequate representation of local geology. Results from these studies suggest that targeted, high- resolution geophysical and geochemical characterization of the local geologic environment is critical to understanding fluid and methane flux and estimating methane hydrate distributions in gas hydrate bearing marine sediments.
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4852 Photochemistry
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting