Enhanced Transport of Dissolved Methane From the Chukchi Sea to the Central Arctic

Rising temperatures in the Arctic Ocean can cause considerable changes, such as decreased ice cover and increased water inflow from the Pacific/Atlantic sector, which may alter dissolved methane (CH4) cycles over the Arctic Ocean. However, the fate of dissolved CH4 in the Arctic remains uncertain. Here, we show that CH4 in the Chukchi Sea is enhanced in the shelf/slope areas, stored in the Upper Halocline (UHC), and transported to the central Arctic, contributing to the CH4 excess (Delta CH4) in the basins. The concentration of Delta CH4 in the UHC was increasing (0.1 nM per year) and the Delta CH4 has been distributed deeper and farther in the last decade than in the 1990s because of the intensification of Pacific water inflow due to oceanographic (currents) and atmospheric forcings (winds). We found heterogeneous CH4 (208.4% +/- 131.7%) in the Polar Mixed Layer and CH4 supersaturation (1,100.9%-1,245.4%) in the below-ice seawater in the basins, which may indicate the effect of sea ice cycles with the support of sediment-origin CH4. We estimate the sea-to-air flux to be 1.1-2.4 mu mol CH4 m(-2) day(-1) during the ice-free period in the Chukchi Sea, which suggests that the Chukchi Sea is currently a minor source (0.003 Tg in summer) of atmospheric CH4. Taken together, we propose a bottom-up mechanism for CH4 transport and emission and are concerned that the increases in the concentration of Delta CH4 and the transport distance/rate of Delta CH4 plume are occurring, with the potential to affect CH4 emissions in the Pacific sector of the Arctic Ocean. Plain Language Summary The Arctic Ocean's continental shelves hold a large amount of methane beneath the sea, and warming-induced decreases in sea ice coverage and freshwater inputs have had a significant impact on this methane's fate. An important gap in our current knowledge concerns the interactions between the ocean surface and bottom, as well as those between the shelf and basins. This knowledge provides direct observations of enhanced methane transport from shelf regions to the central Arctic in the last decade compared to that in the 1990s, demonstrating the effect of a changing Arctic on methane emissions. Shelf-to-basin methane not only points to a rapid change in ice-free shallow areas but also the consequences in ice-covered open oceans. The Arctic Ocean is currently a minor source of atmospheric methane, but the increasing methane excess in the water column, the enhanced methane transport to the central Arctic, and the decreasing sea ice cover imply potential methane emissions in the near future.