Abiogenic methane in volcanic emissions can be non-genetic with the higher hydrocarbons

The emission of methane and the higher hydrocarbons is observed in both mid-ocean ridge and subduction-related hydrothermal systems. It has been recently reported that methane emanating from an extensional ultramafichydrothermal system originates from the abiogenic reduction of (mantle) CO2 (1). The same process is also known to occur in subaerial subduction-related systems (2). These findings implied that abiogenic hydrocarbon production is not restricted to extensional ultramafic-hydrothermal environments, but also occurs under the more oxidizing conditions encountered at convergent plate boundaries. (Submarine) volcanic-hydrothermal systems associated with convergent tectonics might, therefore, have been suitable places for the emergence of life, too. The identification of three subduction-related sources where the vast majority of methane has an abiogenic origin makes these places ideal sites to study the genetic relationship between methane and the higher hydrocarbons. We present hydrocarbon distribution data from one of these places, Nisyros, Greece, for whose hydrothermal system redox and temperature conditions are well constrained. Data indicates that the higher hydrocarbons are non-genetic with the abiogenic methane suggesting two important implications: 1) carbon isotope reversal between methane and the higher hydrocarbons is not a reliable indicator of abiogenic production of hydrocarbons other than methane and 2) abiogenic methane should be more widespread than previously assumed. These two points could also be valid for ultramafic-dominated systems in mid-ocean ridge as well as subduction settings.