Investigating water-column and sediment N2O cycling in the Western Arctic using stable isotopes and isotopomers.

Given its large expanse of continental shelves, the Artic Ocean may be a significant net source to the atmosphere of N₂O which is both a greenhouse and ozone destroying gas. N₂O is produced as a by-product or intermediate during aerobic nitrification and denitrification either in the water-column or the sediments. We measured water-column N₂O concentrations, N and O isotopes and isotopomers at several water-column stations as well as in 10 sediment cores on the productive Chukchi Sea continental shelf and at offshore stations in western flank of the Beaufort Sea during 3 cruises in fall 2015 (U.S Arctic GEOTRACES), fall 2016 (CHINARE), and fall 2017. At shallow shelf stations, N₂O concentrations and saturation generally increased with depth to up to ~140% of saturation. Offshore, N₂O maxima co-occurred with NO₃^- concentration maxima, originating from remineralized organic material in Pacific Winter Water (PWW) advected from the shelf. Overall, we estimated an annual sea-air flux of 0.02 Tg N yr^-1 for the Western Arctic, which accounts only up to 1% of the current global oceanic N₂O source estimates. Isotopic fractionation was seen to occur during N₂O production as salinities corresponding to PWW and overlying Pacific Summer Water (PSW) were associated with relatively low δ¹⁵N-N₂O values. A significant positive linear correlation between N₂O concentrations and N deficit at both shelf and offshore stations suggested an important role for denitrification in the sediments as a source of N₂O. SP also increased with δ¹⁸O-N₂O in the PWW and PSW, suggesting co-occurring N₂O consumption in the sediments. We will discuss the first comprehensive sedimentary N₂O stable isotope and isotopomer profiles to elucidate N₂O transformations in Arctic sediments.