Distribution and Production Mechanisms of N2O in the Western Arctic Ocean

Abstract

AbstractOcean–atmosphere gas exchange in the Arctic Ocean is sensitive to global warming because the decrease of sea‐ice covered area enhances the exchange. Melting sea ice affects the vertical transport of dissolved gases. Few reports of Arctic Ocean studies have described observations of dissolved N2O or temporal variation of sea‐to‐atmosphere N2O flux. Therefore, production mechanisms of this greenhouse gas and the stratospheric ozone depleting gas remain unclear. We measured dissolved N2O and its isotopic signatures in the western Arctic shelf region of the Chukchi Sea and surrounding areas during September in 2014 and 2015. In the Chukchi Sea shelf, the N2O concentration was slightly higher than the value expected under sea–air equilibrium in the surface water. It increased with depth to 23 nmol kg−1 in 2014, although it showed a vertically homogeneous distribution in 2015. Isotopocule ratios of N2O, which include 15N‐site preference in the N2O molecule as well as elemental N and O isotope ratios, indicate that N2O in the Chukchi Sea shelf is a mixture of N2O produced in the bottom water and that of atmospheric origin. The isotopic signature of excess N2O (δ15Nbulk = −3.6‰ to −1.4‰, δ18O = 61.5‰–63.0‰, SP = 38.7‰–49.0‰) suggests that it is produced by archeal or bacterial nitrification and that it is partly reduced by denitrification. Although further quantitative observations are necessary, the results confirmed that the western Arctic Ocean can act as a source of N2O to the atmosphere when sea‐ice cover retreats and the pycnocline is weakened.