The distribution and isotopomeric characterization of nitrous oxide in the Eastern Gotland Basin (central Baltic Sea)

Abstract. Nitrous oxide (N2O) is a greenhouse gas with a global warming potential ∼ 300 times that of carbon dioxide. Coastal areas are important sources of N2O to the atmosphere but the biogeochemical pathways of N2O production and consumption are not well understood. We measured the concentrations and nitrogen (N) and oxygen (O) stable isotopes (δ15N and δ18O) of N2O in the Baltic Sea to constrain the sources and sinks of N2O in this system. Further, we used the intramolecular 15N variation or the site preference (SP) as additional tracer. Samples were taken at 7 stations during a cruise with R/V Elisabeth Mann Borgese to the Eastern Gotland Basin (central Baltic Sea) in May/June 2019. The isotope signatures of N2O accumulation in the oxycline reflected production predominantly via ammonia oxidation. In the waters where hydrogen sulfide (H2S) was detected, active N2O consumption was implied by pronounced decrease in N2O levels in tandem with enrichments in δ15Nbulk, δ18O and SP. High site preference values of N2O observed in suboxic waters of the stations where H2S buildup was minimal to non-detectable point to the possibility of non-canonical denitrification pathways mediated by fungi or abiotically. A sedimentary source of N2O was observed in those stations, which resulted in a decoupling of the δ15Nbulk and δ18O of N2O. Our results reveal that transient oxygenation events have the potential to modify the N cycling within the oxic-anoxic interface even if for shorter time scales.