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Methane and nitrous oxide distributions across the North American Arctic Ocean during summer, 2015 Fenwick, Lindsay Alexandra


We collected Arctic Ocean water column samples for methane (CH₄) and nitrous oxide (N₂O) analysis on three separate cruises in the summer and fall of 2015, covering a ~10,000 km transect from the Bering Sea to Baffin Bay. Our sampling program provides a large-scale, three-dimensional view of methane and nitrous oxide concentrations across contrasting hydrographic environments, from the deep oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive continental shelf regions of the Bering and Chukchi Shelves. Percent saturation relative to atmospheric equilibrium ranged from 30-800% and 75-145% for CH₄ and N₂O, respectively, with the highest concentrations of both gases occurring in waters overlying the continental shelf in the northern Chukchi Sea. Nitrification and denitrification in the sediments of the Bering and Chukchi Shelves constituted a major source of N₂O to the water column, and the resulting high N₂O concentrations were transported across the entire North American Arctic. Methane sources to the water column were more spatially heterogeneous, reflecting a greater variety of hydrographic influences, including likely inputs from sediments, rivers, and sea ice processes. Localized regions of high CH₄ concentrations were observed at various locations across our sampling transect, but unlike N₂O, CH₄ was rapidly consumed through microbial oxidation, as shown by the ¹³C enrichment of CH₄ at low concentrations. High CH₄ signatures were thus more localized across our sampling region. For both CH₄ and N₂O, surface super-saturation and sea-air fluxes were generally low across the region, with valuess of 1.3 ± 1.2 mol m-² d-¹ and -0.52 ± 1.0 µmol m-² d-¹, for CH₄ and N₂O, respectively. Low surface water concentrations were at least partially attributable to dilution by low CH₄ and N₂O fresh water. Our results provide insight into the factors controlling the distribution of CH₄ and N₂O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed.

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