Carbon isotopic characterization for the origin of excess methane in subsurface seawater

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  • Motoki Sasakawa
    Division of Earth and Planetary Sciences, Graduate School of Science Hokkaido University Sapporo Japan
  • Urumu Tsunogai
    Division of Earth and Planetary Sciences, Graduate School of Science Hokkaido University Sapporo Japan
  • Sohiko Kameyama
    Division of Earth and Planetary Sciences, Graduate School of Science Hokkaido University Sapporo Japan
  • Fumiko Nakagawa
    Division of Earth and Planetary Sciences, Graduate School of Science Hokkaido University Sapporo Japan
  • Yukihiro Nojiri
    National Institute for Environmental Studies Ibaraki Japan
  • Atsushi Tsuda
    Ocean Research Institute University of Tokyo Tokyo Japan

書誌事項

公開日
2008-03
権利情報
  • http://onlinelibrary.wiley.com/termsAndConditions#vor
DOI
  • 10.1029/2007jc004217
公開者
American Geophysical Union (AGU)

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説明

<jats:p>We collected samples of seawater, zooplankton, and sinking particles in the northwestern North Pacific to determine the source of excess CH<jats:sub>4</jats:sub> over the saturation value in equilibrium with the atmospheric CH<jats:sub>4</jats:sub> in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface (∼100 m depth) seawater is supersaturated (up to 12%) with <jats:sup>13</jats:sup>C‐enriched CH<jats:sub>4</jats:sub> (up to −33.1‰) relative to surface seawater in equilibrium with the atmosphere (−47‰), suggesting that in situ addition of <jats:sup>13</jats:sup>C‐enriched CH<jats:sub>4</jats:sub> must be responsible for CH<jats:sub>4</jats:sub> enrichment at depth. The <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C of CH<jats:sub>4</jats:sub> emitted from sinking particles (from −36.7 ± 1.2‰ to +5.9 ± 7.5‰) is within the range of that of excess CH<jats:sub>4</jats:sub> in seawater, suggesting that the major source of subsurface excess CH<jats:sub>4</jats:sub> is sinking particles. The unusually <jats:sup>13</jats:sup>C‐enriched <jats:italic>δ</jats:italic><jats:sup>13</jats:sup>C composition of CH<jats:sub>4</jats:sub> emitted from sinking particles suggests that active microbial CH<jats:sub>4</jats:sub> oxidation occurs within the oxic/anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH<jats:sub>4</jats:sub> produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.</jats:p>

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