Kilometer-scale Δ13CH3D profiles distinguish end-member mixing from methane production in deep marine sediments

Abstract

Methane stored in marine sediments can form by microbial and thermal decomposition of organic matter. Identifying the source of methane allows us to assess the potential of natural gas reservoirs and the limits of subsurface microbial life. However, such assessments are complicated by the burial and transport of methane, which can produce mixtures from multiple sources. We measured the abundances of stable isotopes (13C/12C and D/H), clumped isotopologue 13CH3D, and n-alkanes (C1/C2+3, methane over ethane plus propane) for gas samples collected by mud-logging to investigate how 13CH3D can be used to constrain sources of methane. Two kilometer-scale depth profiles representing the transition between microbial and thermal methanogenic zones were analyzed from the northeastern Gulf of Mexico and the western Black Sea. We found that Δ13CH3D values of methane do not follow conservative two-component mixing between shallow microbial methane and deep thermogenic methane transported by advection. Rather, methane isotopologues indicate re-equilibration along geothermal gradients following burial, which continues up to apparent temperatures of 100 ± 15 °C. The re-equilibration is likely microbially catalyzed, although this apparent temperature is ca. 20 °C higher than the putative upper-temperature limit of microbial methanogenesis in marine sediments. Above 150 °C, methane isotopologues are expected to equilibrate along geothermal gradients because the rate of abiotic D/H exchange becomes comparable to that of temperature increase with burial. This study provides novel kilometer-scale profiles of clumped methane isotopologues as a means to trace the upper-temperature limits of microbial activity in hydrocarbon-rich marine sedimentary environments.