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

Annie Bourbonnais, University of South Carolina, School of the Earth, Ocean & Environment, Columbia, United States, Mark A Altabet, Univ Massachusetts Darmouth, New Bedford, United States and Julie Granger, University of Connecticut, Marine Sciences, Groton, United States
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.