Distribution and Production Mechanisms of N<sub>2</sub>O in the Western Arctic Ocean
Sakae Toyoda, Takahito Kakimoto, Kushi Kudo, Naohiro Yoshida, Daisuke Sasano, Naohiro Kosugi, Masao Ishii, Sohiko Kameyama, Mahomi Inagawa, Hisayuki Yoshikawa‐Inoue, Shigeto Nishino, Akihiko Murata, Shigeyuki Ishidoya, Shinji Morimoto
Abstract
Abstract Ocean–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 N 2 O or temporal variation of sea‐to‐atmosphere N 2 O flux. Therefore, production mechanisms of this greenhouse gas and the stratospheric ozone depleting gas remain unclear. We measured dissolved N 2 O 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 N 2 O 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 N 2 O, which include 15 N‐site preference in the N 2 O molecule as well as elemental N and O isotope ratios, indicate that N 2 O in the Chukchi Sea shelf is a mixture of N 2 O produced in the bottom water and that of atmospheric origin. The isotopic signature of excess N 2 O ( δ 15 N bulk = −3.6‰ to −1.4‰, δ 18 O = 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 N 2 O to the atmosphere when sea‐ice cover retreats and the pycnocline is weakened.