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TitleCH4 isotopic ordering records ultra-slow hydrocarbon biodegradation in the deep subsurface
AuthorJautzy, J JORCID logo; Douglas, P M J; Xie, H; Eiler, J M; Clark, I D
SourceEarth and Planetary Science Letters vol. 562, 116841, 2021 p. 1-12,
Alt SeriesNatural Resources Canada, Contribution Series 20200045
Mediapaper; on-line; digital
File formatpdf
Subjectsenvironmental geology; isotopes; hydrocarbons; methane; Paleozoic
Illustrationstables; profiles; models; plots
Released2021 05 15
AbstractInvestigation of the biology and biochemistry of the deep subsurface provides invaluable information regarding the limit of life in extreme environments and its role in the global carbon cycle. It has been observed that subsurface microbial CH4can form in apparent isotopic equilibrium, both with respect to methane clumped isotopic species and D/H fractionation with respect to coexisting water. This observation fostered the suggestion that methanogenic metabolisms in energy-starved environments can operate through slow and reversible enzymatic reactions. Here we present isotopic data including a vertical profile of clumped isotopic indices of methane from Paleozoic-aged pore waters in an aquiclude system from the Michigan Basin. We show evidence of both internal isotopic equilibrium of methane and intermolecular H-isotopic equilibrium between methane and co-occurring non-gaseous n-alkanes. Various mixing and microbial metabolic models were tested and allowed us to identify the possibility of the production of methane at thermodynamic equilibrium from the syntrophic degradation of sedimentary n-alkanes at ultra-slow rates. Significance Statement: The recent ability to measure the clumped isotopic composition of methane has fostered new ways of observing the deep subsurface biogeochemistry and has been proposed as a new independent geothermometer when methane is formed at or near isotopic equilibrium. In this manuscript, we present the first continuous Paleozoic sedimentary profile of clumped isotopes in pore methane (i.e.directly sampled from the pores of tight sedimentary rocks) from a paleo-bioreactor in the subsurface and show that a syntrophic metabolic network between fermentative bacteria and mainly acetotrophic methanogens can thrive in an energy limited environment.
Summary(Plain Language Summary, not published)
Recent analytical developments allowing measurement of the clumped isotope composition of methane have led to new ways of observing deep subsurface biogeochemistry. In addition, this type of measurement has been proposed as a new geothermometer to assess the formation temperature of methane formed at isotope equilibrium. In this manuscript, we present the first continuous sedimentary profile of clumped isotopes of methane collected directly from the pores of subsurface sedimentary rocks. Our results suggest that fermentative bacteria and methanogens can grow as a syntrophic community at a very slow rate, at the limit of life.

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