Environmental Science and Engineering Seminar
Methane is a potent greenhouse gas and an important energy source, and it is important to understand how it is formed, transported, and consumed in the Earth system. In particular, there are concerns that its natural emissions from the Arctic could act as a substantial positive feedback to anthropogenic global warming. Recently, the analysis of multiply substituted isotopologues, or clumped isotopes, of methane has been developed as a new tool to identify methane formation environments. When methane forms in isotopic equilibrium, its clumped isotope composition is controlled by the formation temperature, and can be used as a geothermometer. In some microbial methane, however, non-equilibrium isotope effects, probably related to the kinetics of methanogenesis, lead to low clumped isotope values. Following an overview of methane clumped isotope geochemistry, I will discuss applications of this measurement to identify the origins of methane bubble fluxes in Alaska, Sweden and the Arctic Ocean. Based on clumped isotope measurements and other geochemical indicators we find evidence for four categories of emissions from Arctic methane seeps: 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, in part, by kinetic isotope effects induced during enzymatic reactions involved in methanogenesis. These results indicate that these kinetic isotope effects vary widely in microbial methane produced in Arctic lake sediments, and may provide new insights into the biogeochemistry of microbial methanogenesis.