Biogeochemical signature of elevated methane in water column of the outer East Siberian Arctic Shelf

The East Siberian Arctic Shelf (ESAS) had high methane concentrations in the seawater of the inner shelf over the decades, which was regarded as significant methane source for global warming. The source information of elevated dissolved methane at the inner ESAS has so far been reported, however, the characterizations (i.e., formation and transport) of enriched ones in the outer ESAS remain to date still unclear. To unravel this, we have reported methane properties along south-north transects of the outer ESAS (73.7°-77.1°N and 164.3°-178.0°E, water depths; 41-370m) performed from 2016, 2018 and 2019 ARAON Expeditions. The dissolved methane concentrations in surface seawater were mostly higher than those of the atmospheric equilibrium concentration and its maximum value in the water column of the outer ESAS hotspots had ca. 204 nM. Based on principal component analysis including CTD profiles (i.e., temperature, salinity, dissolved oxygen and fluorescence) and methane concentrations, elevated methane concentrations (88 to 204 nM) were close to fluorescence concentrations (0.1 to 0.4 mg/m³). Furthermore, the isotopic signatures of dissolved methane (δ¹³C; -66.6 to -26.6‰ and δD; -218.8 to -34.0‰) and dissolved inorganic carbon (δ¹³C; -10.1 to -4.4‰) showed large isotopic variations, indicating the methane production in the study area is likely to be complicated using carbon dioxide and methyl substrates. In this regard, organic matter preserved in the submerged permafrost and/or methyl compound produced by phytoplankton might be also potential substrates for elevated methane at some locations. In the near future, mass balance model via end-member approach will be applied for determining the discriminative contributions of possible methane sources in the outer ESAS.