Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues
Sources of methane to sedimentary environments are commonly identified and quantified using the stable-isotopic compositions of methane. The methane "clumped-isotope geothermometer", based on the measurement of multiply substituted methane isotopologues (^(13)CH_3D and ^(12)CH_2D_2), shows promise in adding new constraints to the sources and formational environments of both biogenic and thermogenic methane. However, questions remain about how this geothermometer behaves in systems with mixtures of biogenic and thermogenic gases and different biogenic environments. We have applied the methane clumped-isotope thermometer to a mixed biogenic-thermogenic system (Antrim Shale, USA) and to biogenic gas from gas seeps (Santa Barbara and Santa Monica Basin, USA), a pond on the Caltech campus, and methanogens grown in pure-culture. We demonstrate that clumped-isotope based temperatures add new quantitative constraints to the relative amounts of biogenic vs. thermogenic gases in the Antrim Shale indicating a larger proportion (∼50%) of thermogenic gas in the system than previously thought. Additionally, we find that the clumped-isotope temperature of biogenic methane appears related to the environmental settings in which the gas forms. In systems where methane generation rates appear to be slow (e.g., the Antrim Shale and gas seeps), microbial methane forms in or near both internal isotopic equilibrium and hydrogen-isotope equilibrium with environmental waters. In systems where methane forms rapidly, microbial methane is neither in internal isotopic equilibrium nor hydrogen-isotope equilibrium with environmental waters. A quantitative model of microbial methanogenesis that incorporates isotopes is proposed to explain these results.
© 2015 Elsevier Ltd. Received 19 December 2014; accepted in revised form 8 April 2015; available online 17 April 2015. The idea to measure clumped-isotope compositions of methane from the Antrim shale resulted from a fortuitous meeting of DAS, JME, ALS, and AMM at the 2012 Gordon Research Conference on organic geochemistry. We wish to thank the conveners of that conference, Tim Eglinton and Erdem Idiz (and all other participants) for inviting us to the conference and creating a stimulating atmosphere for scientific discussion and collaboration. We additionally wish to thank Fenfang Wu for help measuring the δD of water from the culturing experiments, Jared Leadbetter for aid in sampling methane from the Caltech pond, and the crew of the R/V Atlantis. Finally, we would like to thank three anonymous reviewers and the associate editor, Ed Hornibrook, for their helpful comments. DAS was supported through the NSF GRFP. Research was supported by Caltech and an NSF EAR grant to JME, ACS-PRF grant to AMM, and NSF grants OCE-1046144, OCE-1155855 and EAR-0950600 to DLV and SSS.
Accepted Version - 1-s2.0-S0016703715002082-main.pdf