Methane sources in gas hydrate-bearing cold seeps: Evidence from radiocarbon and stable isotopes |
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| Authors: | JW Pohlman JE Bauer EA Canuel KS Grabowski DL Knies CS Mitchell MJ Whiticar RB Coffin |
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| Institution: | 1. Department of Earth, Ocean, and Atmospheric Science, University of British Columbia, Canadar/>;2. Department of Microbiology and Immunology, University of British Columbia, Vancouver, BC, Canada;3. Graduate Program in Bioinformatics, University of British Columbia, Vancouver, BC, Canada V6T 1Z4;4. Genome Science and Technology Program, University of British Columbia, Vancouver, BC, Canada V6T 1Z4;5. ECOSCOPE Training Program, University of British Columbia, Vancouver, BC, Canada V6T 1Z3;6. Department of Botany, University of British Columbia, Canada |
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| 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% 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 m 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. |
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| Keywords: | Hydrate Gas hydrate Methane Radiocarbon Stable isotope Cold seep Methanogenesis |
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