Delta 13C values of methane in sea ice - source or process triggered?

Atmospheric methane concentrations in the Arctic are the highest on earth. Sea ice retreat, thinning, and decreasing multiyear and increasing one-year sea ice will have consequences for the sea-ice-air exchange of that greenhouse gas. A cascade of feedback processes triggered by freezing and melting events may induce seasonally the uptake of methane from sources remote from the locations where methane is released again, either to the atmosphere or the ocean acting as final sink. We investigated sea ice cores from the interior Arctic Ocean, Spitsbergen and the southern Beaufort Sea. We found strong gradients in methane concentrations which correspond to super-saturation related to atmospheric equilibrium. We identified stable carbon isotopic signatures of sea ice-methane in the range of strongly 13C-enriched to 13C-depleted in relation to atmospheric background methane. Measured methane oxidation rates reveal methanotrophic activities, especially at the bottom of the sea ice. We used backward trajectories of sea ice cores to identify the general source regions from the ice cores in the interior Arctic, while the d18O signature of the ice provided information in respect to location on or off the shelf regime and also on specific setting on the inner shelf e.g. on water depth and potential sediment load. Our results highlight sea ice as a potential source of bacterially produced methane but also as a buffer for methane up-taken from terrestrial or fossil methane from degrading permafrost regions on the shelf.