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

Abstract

We analyzed N 2 O isotopomer ratios (distribution of isotopes within N 2 O molecules) in the eastern tropical North Pacific. The N 2 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 2 O yield per mole O 2 consumed (dN 2 O/−dO 2 ) 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 2 O, whereas the N 2 O production in the OMZ is the dominant factor for the oceanic N 2 O distribution. The denitrification in the OMZ is consistent with the correlation between ΔN 2 O (level above atmospheric equilibrium) and AOU (apparent oxygen utilization), and the parallel 18 O‐enrichment of N 2 O and O 2 in the OMZ, which have been believed to support the nitrification hypothesis.