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Excess methane in continental hydrothermal emissions is abiogenic

¹ Institut für Geowissenschaften, Goethe-Universität, Altenhöferallee 1, 60438 Frankfurt am Main, Germany
² Istituto Nazionale di Geofisica e Vulcanologia, Via Ugo la Malfa 153, 90146 Palermo, Italy
³ Institut für Atmosphäre und Umwelt, Goethe-Universität, Altenhöferallee 1, 60438 Frankfurt am Main, Germany

Correspondence: *E-mail: Jens.Fiebig{at}em.uni-frankfurt.de.

Thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100. We present hydrocarbon distribution data of continental hydrothermal gases, whose methane has been independently identified to derive from the abiogenic reduction of CO₂. We find that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases. A similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis. These findings strongly suggest that abiogenic methane production in continental-hydrothermal systems is a more widespread process than previously assumed. The maximum contribution of such emissions to the modern atmospheric CH₄ budget is estimated at ~1%.