Abstract :
[en] Nitrous oxide (N2O) is a potent greenhouse gas and the main stratospheric ozone-depleting substance, with oceans contributing ~26% of global emissions. Continental shelves account for about a quarter of oceanic emissions, yet the Arctic, where shelves cover nearly half the area and could significantly contribute to emissions, remains understudied, especially in regions like the Kara Sea which receives significant nitrogen (N) inputs from rivers, that could enhance N2O production. This thesis investigates N2O dynamics and fixed N (i.e., bioavailable N, non-N2) sources, using measurements of N2O concentration and N isotopic measurements (i.e., δ15N) of nitrate and total fixed N, across the Kara Sea and Central Arctic, using samples collected during the MOSAiC (2019-2020) and Arctic Century (2021) expeditions. Vertical N2O profiles in the Kara Sea reveal increasing concentrations with depth, which correlate with fixed N deficit, suggesting benthic denitrification as the dominant production pathway. Surface waters of the Kara Sea appeared to be decoupled from benthic processes, nitrous oxide was generally near equilibrium with the atmosphere, and its distribution was largely controlled by sea surface temperature, which determines water solubility. In open ocean area of the northern Kara Sea, near the sea-ice edge, surface waters showed marked undersaturations (down to ~80%), suggesting that sea-ice limits air-sea exchange and promotes undersaturation following surface cooling during transit into the Arctic. During summer, the Kara Sea acted as a net minor N2O sink. Contrary to expectations, river discharge had no discernible impact on nitrous oxide production. Total fixed N δ15N measurements allow us to place an observational constraint on terrestrial fixed N inputs or any additional external inputs such as N2-fixation and atmospheric deposition. We estimate a minimal contribution (up to 10%) of external inputs to the fixed nitrogen pool in the Kara Sea and Central Arctic regions. In addition, nitrate δ15N measurements show that bathymetry exerts a dominant control on the expression of isotopic fractionation related to the internal nitrogen cycling (i.e., assimilation, export, and remineralization), revealing systematic contrasts between shallow and deep shelves. Shallow shelves displayed relatively high nitrate δ15N values, while deeper shelves exhibited lower δ15N values. In both settings, the nitrate δ15N variability reflects an imbalance between ammonium regeneration and nitrification, allowing the expression of the isotopic fractionation of nitrification. Over shallow shelves, the relative contribution of benthic remineralization and nitrification increases, enhancing the benthic imprint on the δ15N signal, whereas in deeper shelves, these processes occur primarily in the water column. To conclude, this thesis supports the view of an enhanced pelagic-benthic coupling on shallow shelves, which likely promotes benthic denitrification, the dominant pathway of fixed N loss in the Arctic Ocean, while simultaneously favoring N2O production and accumulation in bottom shelf water.
