Constraining methane formation/removal pathways with stable isotopes in different aquatic environments throughout the summer season in the Kolyma Region, North-East Siberia

Large uncertainties exist on the past, current, and future atmospheric methane budget, especially on the role of Northern Arctic wetlands, which are important methane sources. CH4 emissions from the large Arctic carbon reservoirs are predicted to increase when Arctic temperatures rise and permafrost thaws. Organic carbon released by Arctic permafrost degradation increases methanogenesis, enhancing the flux of methane to the atmosphere. Emissions of methane from wetlands are difficult to quantify because of the complex mechanisms involved in methane production and removal and their heterogeneity within different aquatic environments. During the relatively short summer season the change of many environmental parameters (e.g. temperature, active layer depth, pH, water depth, substrate biolability) will affect methanogenesis. The different production and removal mechanisms are associated with different isotope effects, and therefore isotope measurements may help quantify the relative importance of the various processes. In this study, we investigate the isotopic signature (δD and δ13C) of methane in Siberian lakes, ponds, wetlands, streams, and rivers to better understand the seasonal variations in methane mixing ratios and stable isotopes. Sampling of ebullition released by sediment stirring was conducted in different aquatic environments at locations with different underlying permafrost type (Pleistocene-aged Yedoma or Holocene-aged floodplain), and at different times during the summer season. In addition, air samples were taken above each aquatic sampling site. Preliminary results showed a wide range of δD values and δ13C values, varying from about -200‰ to -400‰, and -80‰ to -40‰, respectively. Our final methane isotope results allow identifying seasonal variations in: (1) methane formation (acetate fermentation or CO2 reduction) and removal pathways (oxidation) and (2) type of substrates. Mixing ratios and the isotopic signature of methane may allow us to better understand how the methane formation/removal pathways vary during the summer season. Understanding seasonal variability of methane release from various source environments will help to model current and future changes in the global methane budget.