Clumped 13C–13C isotopologue signatures of thermogenic ethane

Abstract

The origin of hydrocarbons is a central question in climate science, biogeochemistry, and astrobiology. Carbon and hydrogen isotopic ratios have been used to constrain hydrocarbon sources; however, studies based on these parameters alone are lacking. The recent development of a doubly substituted (“clumped”) ethane isotopologue (13C2H6) is expected to be useful in distinguishing between thermogenic and abiotic ethane owing to the specific signature of the former. However, the mechanistic determinants of clumped 13C–13C isotopologue signatures in thermogenic ethane remain insufficiently understood. In this study, we conducted pyrolysis experiments over ∼ 2 years under controlled temperatures and pressures, with lignin, docosane (C22H46), and marine sediments used as reagents. All ethane samples exhibited clumped 13C–13C isotopologue signatures with similarly nearly absent or slight 13C–13C enrichment against a stochastic distribution. The 13C–13C clumping of thermally produced ethane from organic matter was similar to that of natural thermogenic ethane and different from that of putatively abiotic ethane of Kidd Creek and the Dingo gas field in the Amadeus Basin, which showed a significantly lower abundance of 13C–13C relative to stochastic distribution. The abundances of 13C–13C in ethane suggest that the signature of thermogenic ethane is inherited from a C–C bond in the precursor molecule and altered by a combinatorial effect during thermal cracking. Post-genetic ethane decomposition via heating might alter the 13C–13C signature in ethane, although the effects are difficult to observe. Thermogenic ethane should exhibit a positive clumped isotopologue signature regardless of thermal maturity; therefore, clumped isotopologue signatures may serve as key indicators to distinguish between thermogenic and abiotic ethane.