Methane is a potent greenhouse gas, and there are concerns that its natural emissions from the Arctic could act as a substantial positive feedback to anthropogenic global warming. Determining the sources of methane and the biogeochemical processes controlling them is important for understanding present and future Arctic emissions. Here we apply multiply substituted isotopologues, or clumped isotopes, of methane as a new tool to identify the origins of ebullitive fluxes in Alaska, Sweden and the Arctic Ocean. When methane forms in isotopic equilibrium, clumped isotope measurements indicate the formation temperature. In some microbial methane, however, non-equilibrium isotope effects, which are probably related to the kinetics of methanogenesis, lead to low proportions of clumped isotopes. We identify four categories of emissions in the studied samples: thermogenic methane, deep subsurface or marine microbial methane formed in isotopic equilibrium, freshwater microbial methane with non-equilibrium clumped isotope values, and mixtures of microbial and thermogenic methane (i.e., combinations of the first three end members). Mixing between thermogenic and microbial methane produces a non-linear variation in clumped isotope values with mixing proportion that provides new constraints for the formation environment of the mixing end-members. Analyses of microbial methane emitted from lakes, as well as a methanol-consuming methanogen pure culture, support the hypothesis that non-equilibrium clumped isotope values are controlled, at least in part, by kinetic isotope effects related to the reversibility of the enzymatic reactions involved in methanogenesis. In addition, we observe a positive correlation between clumped isotope values and flux in lacustrine microbial methane samples, suggesting a link between the kinetics of methanogenesis and the rate of emissions from lakes.

Diverse origins of Arctic and subarctic methane point source emissions identified with multiply substituted isotopologues

251st National Meeting (2016)

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