Summer N₂O dynamics in the western Arctic Ocean : Distributions, Processes, and Fluxes

Abstract

<p>We collect seawater samples from 32 stations for N<sub>2</sub>O analysis between August 6 and August 25, during 2017 ARA08B cruise in western Arctic Ocean (WAO), covering from Southern Chukchi Sea (SC) to Northern Chukchi Sea (NC). At surface depth (~50 m), N<sub>2</sub>O concentrations were 10.9‒19.4 nmol L<sup>-1</sup>, and distinct pattern was observed between SC and NC. N<sub>2</sub>O concentrations were increased from surface to bottom (~50 m) at SC, corresponding to positive relationship of ∆N<sub>2</sub>O (N<sub>2</sub>O<sub>measured </sub>- N<sub>2</sub>O<sub>equilibrium</sub>) with DIN (NO<sub>3­</sub><sup>-</sup> + NO<sub>2</sub><sup>-</sup>) and negative relationship between ∆N<sub>2</sub>O and N<sup>*</sup>. It suggests that nitrification and denitrification are the main processes to produce N<sub>2</sub>O at SC. On the other hand, N<sub>2</sub>O concentration at NC increased from the south to north, and remained vertically constant. It may be the result of physical processes such as dilution by sea ice melting water, and high solubility that affected by low temperature and low salinity. The highest N<sub>2</sub>O concentrations were observed at intermediate depth (50‒200 m), ranging 13.4‒21.9 nmol L<sup>-1</sup>. It would be determined by high solubility and active biogeochemical processes synthetically. Concentrations of N<sub>2</sub>O were rapidly diminished to 400 m, ranging 10.2‒14.1 nmol L<sup>-1</sup>, and did not be remarkably altered under 400 m, ranging 11.3‒13.7 nmol L<sup>-1</sup>. It might be affected by advection of Atlantic Water (AW) and existence of Arctic Bottom Water (ABW), and influence of biogeochemical processes was negligible at deep and bottom depth (below 200 m). N<sub>2</sub>O flux was calculated to determine that the WAO is sources or sinks region for atmospheric N<sub>2</sub>O. Positive N<sub>2</sub>O flux was observed at SC, and it indicate that N<sub>2</sub>O gas is released to atmosphere at SC. Negative value of N<sub>2</sub>O flux at NC suggest that atmospheric N<sub>2</sub>O is absorbed into NC. Furthermore, positive relationship of N<sub>2</sub>O flux with environmental parameters (temperature, salinity, and ∆N<sub>2</sub>O) also observed in WAO. These results provide comprehensive information of the spatial N<sub>2</sub>O distribution and main processes which decide N<sub>2</sub>O distribution in WAO, and also suggest that air-sea N<sub>2</sub>O flux could be affected by changing environments of the Arctic Ocean.</p>