| Summary: | Nitrous oxide (N2O) is a potent greenhouse gas with a high global-warming potential. N2O is also strongly involved in stratospheric ozone depletion. The Southern Ocean has been considered as one of the dominant oceanic sources of nitrous oxide for the atmosphere, due to the release of the N2O excess accumulated as a result of organic matter remineralization along the deep oceanic pathway and ultimately ventilated in the surface waters of the Southern Ocean. However, actual data are scare and reveal that both undersaturation and oversaturation conditions occur in Southern Ocean surface waters. Undersaturation in N2O of polar surface waters has frequently been ascribed to melting of sea ice that is presumably undersaturated in N2O. During the 2017 PIPERS cruise in the Ross Sea, we carried out the first winter measurements of N2O in winter in both in sea ice and in the water column. Comparison of these new winter surface-water measurements to available summer measurements reveal contrasting results and may challenge the current view of the Southern Ocean being a source of N2O for the atmosphere. In addition, we observed a build-up of N2O in the ice interior, probably being produced by the sympagic microbial community, and a subsequent release to the atmosphere during sea-ice formation. Surprisingly, surface waters, in contrast, appear to act as a sink of N2O for the atmosphere. While we confirm that melting of sea ice decreases the N2O concentration of surface waters, this impact is limited and unable to explain the level of undersaturation previously reported in N2O during summer. Further processes are therefore required to explain this level of undersaturation.
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