Contributions of denitrification and mixing on the distribution of nitrous oxide in the North Pacific

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  • Hiroaki Yamagishi
    Department of Environmental Science and Technology Tokyo Institute of Technology Yokohama Japan
  • Naohiro Yoshida
    Department of Environmental Science and Technology Tokyo Institute of Technology Yokohama Japan
  • Sakae Toyoda
    Solution Oriented Research for Science and Technology (SORST) Project Japan Science and Technology Agency (JST) Kawaguchi, Saitama Japan
  • Brian N. Popp
    Department of Geology and Geophysics University of Hawaii Honolulu Hawaii USA
  • Marian B. Westley
    Department of Oceanography University of Hawaii Honolulu Hawaii USA
  • Shuichi Watanabe
    Department of Environmental Science and Technology Tokyo Institute of Technology Yokohama Japan

説明

<jats:p>We analyzed N<jats:sub>2</jats:sub>O isotopomer ratios (distribution of isotopes within N<jats:sub>2</jats:sub>O molecules) in the eastern tropical North Pacific. The N<jats:sub>2</jats:sub>O isotopomer ratios indicate the contribution of denitrification in the oxygen minimum zone (OMZ, ∼600 m in depth) in the western North Pacific, which is not consistent with the widely accepted nitrification hypothesis. Our models indicate that the N<jats:sub>2</jats:sub>O yield per mole O<jats:sub>2</jats:sub> consumed (dN<jats:sub>2</jats:sub>O/−dO<jats:sub>2</jats:sub>) is 0.008 (0–0.015) nmol/μmol during remineralization and nitrification in the western North Pacific. Nitrification in aerobic deep waters is a minor source of oceanic N<jats:sub>2</jats:sub>O, whereas the N<jats:sub>2</jats:sub>O production in the OMZ is the dominant factor for the oceanic N<jats:sub>2</jats:sub>O distribution. The denitrification in the OMZ is consistent with the correlation between ΔN<jats:sub>2</jats:sub>O (level above atmospheric equilibrium) and AOU (apparent oxygen utilization), and the parallel <jats:sup>18</jats:sup>O‐enrichment of N<jats:sub>2</jats:sub>O and O<jats:sub>2</jats:sub> in the OMZ, which have been believed to support the nitrification hypothesis.</jats:p>

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