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Serpentinization potentially contributes to the origin and evolution of life during early history of the Earth. Serpentinization produces molecular hydrogen (H 2 ) that can be utilized by microorganisms to gain metabolic energy. Methane can be formed through reactions between molecular hydrogen and oxidized carbon ( e.g. , carbon dioxide) or through biotic processes. A simple criterion, the H 2 /CH 4  ratio, has been proposed to differentiate abiotic from biotic methane, with values approximately larger than 40 for abiotic methane and values of <40 for biotic methane. The definition of the criterion was based on two serpentinization experiments at 200 °C and 0.3 kbar. However, it is not clear whether the criterion is applicable at a wider range of temperatures. In this study, we performed sixteen experiments at 311–500 °C and 3.0 kbar using natural ground peridotite. Our results demonstrate that the H 2 /CH 4  ratios strongly depend on temperature. At 311 °C and 3.0 kbar, the H 2 /CH 4  ratios ranged from 58 to 2,120, much greater than the critical value of 40. By contrast, at 400–500 °C, the H 2 /CH 4  ratios were much lower, ranging from 0.1 to 8.2. The results of this study suggest that the H 2 /CH 4  ratios cannot reliably discriminate abiotic from biotic methane.

The H2/CH4 ratio during serpentinization cannot reliably identify biological signatures