DISTINGUISHING BIOGENIC VERSUS ABIOGENIC SOURCES OF METHANE FROM C ISOTOPE SIGNATURES: APPLICATION TO INTERPRETING POTENTIAL MARTIAN DATA

The origin of methane in continental hydrothermal spring systems has wide ranging implications for the study of hydrothermal systems, and detecting life on other planets. The recent detection of methane occurring in the Martian atmosphere and the possibility that its origin could be attributed to biological activity, have highlighted the importance of understanding the mechanisms that distinguish biogenic from abiogenic sources. A number of recent studies have demonstrated that methane produced by purely abiological processes (>120ºC) can be difficult to distinguish from methane produced biologically at low temperature (<120ºC). The d13C of the methane has traditionally been used as the critical factor in distinguishing between different methane sources, where it was thought that biological processes were responsible for methane with very light carbon isotope ratios (~ -60‰) while methane with d13C values closer to 0‰ were formed abiogenically. It has been now shown that biological and non-biological methane can have variable d13C values making d13C alone an unreliable identifier for non-biological methane provenance. Clearly, the difficulty in distinguishing biogenic from abiogenic methane in terrestrial continental aqueous systems makes it all the more difficult to ascertain methane origin on the surface of Mars and therefore draw conclusions on the existence of a Martian biosphere.

In this work we outline a novel technique for determining methane biogenicity. We report the d13C of CO2 and CH4 of gases collected from two terrestrial hot springs: Great Boiling Springs, NV (GBS) and Surprise Valley Hot Springs, CA (SVHS). The d13C values for CO2 range from -12.0 to -8.5‰ for GBS, and from -24.8 to -12.8‰ for SVHS. The d13C values for CH4 range from -48.3 to -24.9‰ for GBS, and from -51.2 to -50.0‰ for SVHS. Our data indicate that methane is in isotopic equilibrium with CO2 at depth and therefore provides a means to calculate formation temperature. All samples indicate temperatures above 155°C using calculated carbon isotope equilibrium fractionation factors. This minimum formation temperature excludes the possibility that CH4 formed directly from the action of microorganisms, and is consistent with the lack of detection of methanogens from most of these springs using culture-independent molecular techniques.