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Fixed Nitrogen Cycling and Nitrous Oxide Production in the Arctic Ocean: Shelf transformation processes and implications for Arctic biogeochemistry

par Muller, Sofia
Président du jury Arndt, Sandra
Promoteur Fripiat, François ;Delille, Bruno
Publication Non publié, 2026-01-12

Thèse de doctorat

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Résumé :

Nitrous oxide (N2O) is a potent greenhouse gas and the main stratospheric ozone-depletingsubstance, with oceans contributing ~26% of global emissions. Continental shelves account forabout a quarter of oceanic emissions, yet the Arctic, where shelves cover nearly half the areaand could significantly contribute to emissions, remains understudied, especially in regions likethe Kara Sea which receives significant nitrogen (N) inputs from rivers, that could enhance N2Oproduction. 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) ofnitrate and total fixed N, across the Kara Sea and Central Arctic, using samples collected duringthe MOSAiC (2019-2020) and Arctic Century (2021) expeditions. Vertical N2O profiles in theKara Sea reveal increasing concentrations with depth, which correlate with fixed N deficit,suggesting benthic denitrification as the dominant production pathway. Surface waters of theKara Sea appeared to be decoupled from benthic processes, nitrous oxide was generally nearequilibrium with the atmosphere, and its distribution was largely controlled by sea surfacetemperature, 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 surfacecooling during transit into the Arctic. During summer, the Kara Sea acted as a net minor N2Osink. Contrary to expectations, river discharge had no discernible impact on nitrous oxideproduction. Total fixed N δ15N measurements allow us to place an observational constraint onterrestrial fixed N inputs or any additional external inputs such as N2-fixation and atmosphericdeposition. We estimate a minimal contribution (up to 10%) of external inputs to the fixednitrogen pool in the Kara Sea and Central Arctic regions. In addition, nitrate δ15N measurementsshow that bathymetry exerts a dominant control on the expression of isotopic fractionationrelated to the internal nitrogen cycling (i.e., assimilation, export, and remineralization),revealing systematic contrasts between shallow and deep shelves. Shallow shelves displayedrelatively high nitrate δ15N values, while deeper shelves exhibited lower δ15N values. In bothsettings, the nitrate δ15N variability reflects an imbalance between ammonium regeneration andnitrification, allowing the expression of the isotopic fractionation of nitrification. Over shallowshelves, the relative contribution of benthic remineralization and nitrification increases,enhancing the benthic imprint on the δ15N signal, whereas in deeper shelves, these processesoccur primarily in the water column. To conclude, this thesis supports the view of an enhancedpelagic-benthic coupling on shallow shelves, which likely promotes benthic denitrification, thedominant pathway of fixed N loss in the Arctic Ocean, while simultaneously favoring N2Oproduction and accumulation in bottom shelf water.