Nonequilibrium clumped isotope signals in microbial methane

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of AAAS for personal use, not for redistribution. The definitive version was published in Science 348 (2015): 428-431, doi:10.1126/science.aaa4326.Methane is a key component in the global carbon cycle with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source identification, the abundance of its multiply-substituted “clumped” isotopologues, e.g., 13CH3D, has recently emerged as a proxy for determining methane-formation temperatures; however, the impact of biological processes on methane’s clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters.Grants from the National Science Foundation (EAR- 1250394 to S.O. and EAR-1322805 to J.C.M.), N. Braunsdorf and D. Smit of Shell PTI/EG (to S.O.), the Deep Carbon Observatory (to S.O., B.S.L., M.K., and K.-U.H.), the Natural Sciences and Engineering Research Council of Canada (to B.S.L.), and the Gottfried Wilhelm Leibniz Program of the Deutsche Forschungsgemeinschaft (HI 616-14-1 to K.-U.H. and M.K.) supported this study. D.T.W. was supported by a National Defense Science and Engineering Graduate Fellowship. D.S.G. was supported by the Neil and Anna Rasmussen Foundation Fund, the Grayce B. Kerr Fellowship, and a Shell-MITEI Graduate Fellowship