Person: Edward D. Young

Abstract

Stable isotope compositions of methane (δ¹³C and δD) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane ‘clumped’ isotopologues Δ¹³CH₃D and Δ¹²CH₂D₂ are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis.

In this study, we report the first systematic use of methane Δ¹³CH₃D and Δ¹²CH₂D₂ in the context of continental sedimentary basins. We investigated sedimentary formations from the Southwest Ontario and Michigan Basins, where the presence of both microbial and thermogenic methane was previously proposed. Methane from the Silurian strata coexist with highly saline brines, and clumped isotopologues exhibit large offsets from thermodynamic equilibrium, with Δ¹²CH₂D₂ values as low as −23‰. Together with conventional δ¹³C and δD values, the variability in Δ¹³CH₃D and Δ¹²CH₂D₂ to first order reflects a mixing relationship between near-equilibrated thermogenic methane similar to gases from deeper Cambrian and Middle Ordovician units, and a source characterized by a substantial departure from equilibrium that could be associated with microbial methanogenesis. In contrast, methane from the Devonian-age Antrim Shale, associated with less saline porewaters, reveals Δ¹³CH₃D and Δ¹²CH₂D₂ values that are approaching low temperature thermodynamic equilibrium. While microbial methanogenesis remains an important contributor to the methane budget in the Antrim Shale, it is suggested that Anaerobic Oxidation of Methane (AOM) could contribute to reprocessing methane isotopologues, yielding Δ¹³CH₃D and Δ¹²CH₂D₂ signatures approaching thermodynamic equilibrium.

Abstract

We report measurements of resolved ¹²CH₂D₂ and ¹³CH₃D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve ¹²CH₂D₂ from ¹³CH₃D provides unprecedented insights into the origin and evolution of CH₄. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ¹²CH₂D₂ and Δ¹³CH₃D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH₄/H₂ D/H exchange, underscoring the importance of reliable thermometry based on the CH₄ molecules themselves. Where Δ¹²CH₂D₂ and Δ¹³CH₃D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in ¹²CH₂D₂ compared with equilibrium values in CH₄ gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in ¹³CH₃D that accompany the low ¹²CH₂D₂ abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ¹²CH₂D₂ values are a key tracer of microbial recycling.