The H(2)/CH(4) ratio during serpentinization cannot reliably identify biological signatures

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.