NATURAL ABUNDANCE 14C AS A TOOL TO CONSTRAIN SUBSURFACE MICROBIAL METHANOGENSIS AND METHANOTROPHY

Constraining the sources and cycling of light hydrocarbon gases, particularly methane, in subsurface systems is challenging and requires the use of multiple approaches. Methane production can occur via thermogenic, microbial or water-rock interaction processes. The relative contribution of these processes has long been investigated using gas component ratios and isotopic compositions. However, these parameters can be subsequently altered by the metabolic activities of other components of the subsurface microbial community, such as methane consumption by methanotrophs. Further, these isotopic ratios cannot differentiate whether methane is being produced from ancient, recalcitrant carbon sources or from modern carbon sources being transported into the system via groundwater flow.

Natural abundance ¹⁴C analysis is a potential means to constrain these processes and their timing. Because sedimentary carbon is geologically old, it contains no significant ¹⁴C (d¹⁴C = -1000‰). In contrast, carbon transported into a subsurface system via groundwater flow from the surface may have more modern d¹⁴C values (circa +100‰).

Comparison of the d¹⁴C values of dissolved inorganic carbon (DIC) and methane in deep ground water in the Witwatersrand Basin in South Africa was able to resolve recent/ongoing microbial methanogenesis contributing to dissolved methane pools. At two sites, current/ongoing methanogenesis was contributing up to 24% and 40% of the observed methane from DIC containing significant amounts of ¹⁴C. At one site methane was likely being transported into the system from another source due to ¹⁴C contents that were higher than those present in the DIC.

¹⁴C analysis was also applied to understand controls on coal bed methane (CBM) fluxes at a site in the San Juan Basin, CO. d¹⁴C values of soil gas CH₄ (-930‰) indicated that while the primary methane source was ancient, a resolvable contribution of current methanogenesis from a more modern carbon source was also occurring. Methane fluxes to the surface were primarily controlled by microbial oxidation. The d¹⁴C of soil gas CO₂ indicated that it was derived from methane oxidation and the d¹⁴C of microbial phospholipids fatty acids (PLFA) were also ¹⁴C depleted indicating incorporation of CBM carbon into the microbial biomass.