What processes control the isotopic composition of microbial methane? Constraints from inorganic experiments and theory
The stable isotopic composition of methane (CH4) is commonly used to fingerprint natural gas origins. Over the past 50 years, there have been numerous proposals that both microbial and thermogenic CH4 can form in or later attain hydrogen isotopic equilibrium with water (H2O) and carbon isotopic equilibrium with carbon dioxide (CO2). Evaluation of such proposals requires knowledge of the equilibrium fractionation factors between CH4 and H2O or CO2 at the temperatures where microbial and thermogenic CH4 form in or are found in the environment, which is generally less than 200°C. Problematically, experimental determinations of the temperature dependence of the equilibrium fractionation factors are only available above 200°C and prior theoretical calculations are highly divergent. To address this, I will describe new experimental results for the calibration of hydrogen isotopic equilibrium in the system CH4-H2-H2O. I will compare these experimental results to new theoretical calculations use advanced techniques and show they agree 1:1. I will then compare this calibration to a compilation of environmental data and discuss where methane samples are found that are consistent with forming or reaching isotopic equilibrium and which are not. Finally, I will close with a proposal that equilibrium hydrogen isotopic compositions between methane and water are a potential biomarker for life on Earth in the present and past and potentially other planetary bodies.