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    Assessing sulfate reduction and methane cycling in a high salinity pore water system in the northern Gulf of Mexico

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    Pohlman et al.pdf (512.9Kb)
    Date
    2008-05-10
    Author
    Pohlman, John W.  Concept link
    Ruppel, Carolyn D.  Concept link
    Hutchinson, Deborah R.  Concept link
    Downer, R.  Concept link
    Coffin, Richard B.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/2618
    As published
    https://doi.org/10.1016/j.marpetgeo.2008.01.016
    DOI
    10.1016/j.marpetgeo.2008.01.016
    Keyword
     Gas hydrate; Methane; Anaerobic methane oxidation; Sulfate; Brine; Gulf of Mexico 
    Abstract
    Pore waters extracted from 18 piston cores obtained on and near a salt-cored bathymetric high in Keathley Canyon lease block 151 in the northern Gulf of Mexico contain elevated concentrations of chloride (up to 838 mM) and have pore water chemical concentration profiles that exhibit extensive departures (concavity) from steady-state (linear) diffusive equilibrium with depth. Minimum δ13C dissolved inorganic carbon (DIC) values of −55.9‰ to −64.8‰ at the sulfate–methane transition (SMT) strongly suggest active anaerobic oxidation of methane (AOM) throughout the study region. However, the nonlinear pore water chemistry-depth profiles make it impossible to determine the vertical extent of active AOM or the potential role of alternate sulfate reduction pathways. Here we utilize the conservative (non-reactive) nature of dissolved chloride to differentiate the effects of biogeochemical activity (e.g., AOM and/or organoclastic sulfate reduction) relative to physical mixing in high salinity Keathley Canyon sediments. In most cases, the DIC and sulfate concentrations in pore waters are consistent with a conservative mixing model that uses chloride concentrations at the seafloor and the SMT as endmembers. Conservative mixing of pore water constituents implies that an undetermined physical process is primarily responsible for the nonlinearity of the pore water-depth profiles. In limited cases where the sulfate and DIC concentrations deviated from conservative mixing between the seafloor and SMT, the δ13C-DIC mixing diagrams suggest that the excess DIC is produced from a 13C-depleted source that could only be accounted for by microbial methane, the dominant form of methane identified during this study. We conclude that AOM is the most prevalent sink for sulfate and that it occurs primarily at the SMT at this Keathley Canyon site.
    Description
    This paper is not subject to U.S. copyright. The definitive version was published in Marine and Petroleum Geology 25 (2008): 942-951, doi:10.1016/j.marpetgeo.2008.01.016.
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    • Energy and Geohazards
    Suggested Citation
    Marine and Petroleum Geology 25 (2008): 942-951
     

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