During the late Austral summer of 2023, we carried out three surveys in the West Antarctic Peninsula (WAP) from Horseshoe Island (67° 514 south) to the Northern tip of the Peninsula to document the distribution of CH4 in surface waters. We observed a striking feature in Dodman Island in the Grand Didier Channel with a marked supersaturation of methane (up to 400%) in the bay of the island, whereas saturation (maximum of 260%) was observed elsewhere. Our main hypothesis is that this supersaturation is linked to meltwater from the glacier on the island, which acts as a source of methane in the water column. This hypothesis is supported by vertical profiles of CH4 concentration, field observations of sub-glacial water flowing to the surface of the water column, as well as by variations in salinity showing a freshwater inflow. This phenomenon has already been suggested in the Arctic (Lamarche-Gagnon et al., 2019) but does not yet seem to have been demonstrated in the Antarctic. These data show that it would be worthwhile investigating areas with active glaciers to determine whether melting glaciers can be a source of methane for the Antarctic water column.

Nitrous oxide (N2O) distribution in the North Kara Sea during summer 2021 shows elevated N2O concentrations in shelf water masses, highlighting the significance of coupled benthic-pelagic and nitrification-denitrification processes. In surface waters, temperature emerges as the primary driver of N2O concentrations, with a clear negative correlation between both parameters. Most surface waters are near saturation for N2O, and the saturation deficit exhibits an inverse relationship with temperature. Under-ice water influx originating from the open ocean exhibits strong under-saturation (80%), attributed to the limited air-sea exchange in sea ice covered waters. Contrary to expectations, river supply does not exert a discernable influence on N2O concentrations in the studied area. This study reveals the potential of the Arctic Siberian shelves for the uptake of atmospheric N2O during summer.

Previous studies have reported an accumulation of nitrous oxide (N2O) on shallow continental shelves of the western Arctic Ocean. In this study, we sampled seawater profiles for N2O measurements in the eastern Arctic shelves, in the North Kara Sea, in the context of the Arctic Century Expedition. Despite some variability in the vertical distribution, we typically observe an accumulation of N2O in shelf bottom waters, which correlates with a fixed nitrogen (N) deficit. Longer residence times on the shelf promote greater N2O enrichment and a larger fixed N deficit. These observations point towards N2O production at depth, linked to benthic denitrification processes that are intensified on productive shelve areas. However, in surface waters, physical processes – i.e. temperature-dependent solubility and air-sea exchange – emerge as the main factor controlling N2O concentrations. We observe low saturations of 80% at the surface of open ocean stations influenced by water that has previously flowed beneath sea ice. Arctic surface water becomes undersaturated due to cooling and remains undersaturated due to limited air-sea exchange. River supply does not exert a discernable influence on N2O concentrations of the studied area. This study reveals the potential of the Arctic Siberian shelves as a sink of atmospheric N2O during the summer.

The rapid melt of snow and sea ice during the Arctic summer provides a significant source of low-salinity meltwater to the surface ocean on the local scale. The accumulation of this meltwater on, under, and around sea ice floes can result in relatively thin meltwater layers in the upper ocean. Due to the small-scale nature of these upper-ocean features, typically on the order of 1 m thick or less, they are rarely detected by standard methods, but are nevertheless pervasive and critically important in Arctic summer. Observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in summer 2020 focused on the evolution of such layers and made significant advancements in understanding their role in the coupled Arctic system. Here we provide a review of thin meltwater layers in the Arctic, with emphasis on the new findings from MOSAiC. Both prior and recent observational datasets indicate an intermittent yet long-lasting (weeks to months) meltwater layer in the upper ocean on the order of 0.1 m to 1.0 m in thickness, with a large spatial range. The presence of meltwater layers impacts the physical system by reducing bottom ice melt and allowing new ice formation via false bottom growth. Collectively, the meltwater layer and false bottoms reduce atmosphere-ocean exchanges of momentum, energy, and material. The impacts on the coupled Arctic system are far-reaching, including acting as a barrier for nutrient and gas exchange and impacting ecosystem diversity and productivity.