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    Methane sources in gas hydrate-bearing cold-seeps : evidence from radiocarbon and stable isotopes

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    Pohlman et al_Fossil Methane_Marine Chemistry.pdf (242.7Kb)
    Date
    2009-06-16
    Author
    Pohlman, John W.  Concept link
    Bauer, James E.  Concept link
    Canuel, Elizabeth A.  Concept link
    Grabowski, Kenneth S.  Concept link
    Knies, D. L.  Concept link
    Mitchell, C. S.  Concept link
    Whiticar, M. J.  Concept link
    Coffin, Richard B.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/2903
    As published
    https://doi.org/10.1016/j.marchem.2009.07.001
    Keyword
     Hydrate; Gas hydrate; Methane; Radiocarbon; Stable isotope; Cold seep; Methanogenesis 
    Abstract
    Fossil methane from the large and dynamic marine gas hydrate reservoir has the potential to influence oceanic and atmospheric carbon pools. However, natural radiocarbon (14C) measurements of gas hydrate methane have been extremely limited, and their use as a source and process indicator has not yet been systematically established. In this study, gas hydrate-bound and dissolved methane recovered from six geologically and geographically distinct high-gas-flux cold seeps was found to be 98 to 100% fossil based on its 14C content. Given this prevalence of fossil methane and the small contribution of gas hydrate (≤1%) to the present-day atmospheric methane flux, non-fossil contributions of gas hydrate methane to the atmosphere are not likely to be quantitatively significant. This conclusion is consistent with contemporary atmospheric methane budget calculations. In combination with δ13C- and δD-methane measurements, we also determine the extent to which the low, but detectable, amounts of 14C (~ 1-2 percent modern carbon, pMC) in methane from two cold seeps might reflect in situ production from near-seafloor sediment organic carbon (SOC). A 14C mass balance approach using fossil methane and 14C-enriched SOC suggests that as much as 8 to 29% of hydrate-associated methane carbon may originate from SOC contained within the upper 6 meters of sediment. These findings validate the assumption of a predominantly fossil carbon source for marine gas hydrate, but also indicate that structural gas hydrate from at least certain cold seeps contains a component of methane produced during decomposition of non-fossil organic matter in near-surface sediment.
    Description
    Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 115 (2009): 102-109, doi:10.1016/j.marchem.2009.07.001.
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    • Energy and Geohazards
    Suggested Citation
    Preprint: Pohlman, John W., Bauer, James E., Canuel, Elizabeth A., Grabowski, Kenneth S., Knies, D. L., Mitchell, C. S., Whiticar, M. J., Coffin, Richard B., "Methane sources in gas hydrate-bearing cold-seeps : evidence from radiocarbon and stable isotopes", 2009-06-16, https://doi.org/10.1016/j.marchem.2009.07.001, https://hdl.handle.net/1912/2903
     

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