Tracing sources of atmospheric methane using clumped isotopes

Abstract

We apply a recently developed measurement technique for methane (CH₄) <b>isotopologues</b>^* (isotopic variants of CH₄-¹³CH₄, ¹²CH₃D, ¹³CH₃D, and ¹²CH₂D₂) to identify contributions to the atmospheric burden from fossil fuel and microbial sources. The aim of this study is to constrain factors that ultimately control the concentration of this potent greenhouse gas on global, regional, and local levels. While predictions of atmospheric methane isotopologues have been modeled, we present direct measurements that point to a different atmospheric methane composition and to a microbial flux with less <b>clumping</b> (greater deficits relative to stochastic) in both ¹³CH₃D and ¹²CH₂D₂ than had been previously assigned. These differences make atmospheric isotopologue data sufficiently sensitive to variations in microbial to fossil fuel fluxes to distinguish between emissions scenarios such as those generated by different versions of EDGAR (the Emissions Database for Global Atmospheric Research), even when existing constraints on the atmospheric CH₄ concentration profile as well as traditional isotopes are kept constant.