LATE ARCHEAN GAS FORMED BY MICROBIAL HYDROGENOLYSIS OF ORGANIC MATTER

Natural gases in Archean rocks have been discovered in Canadian, South African and Scandinavian shields. Due to their unique isotopic pattern of a broad δD variation accompanied by a narrow δ¹³C one between alkanes and the coexistence of intrusive igneous rocks and serpentinization, these gases have been thought to be unique cases of abiotically generated natural gases. However, our new investigation on isotope signatures of these gases suggests that they are formed by microbial activities.

The asynchronous hydrogen and carbon isotopic fractionations in the Archean gases suggest that hydrogen atoms are from two sources with distinctively different δD values. In each produced alkane molecule, two hydrogen atoms are from the serpentinization derived H₂ with remarkably depleted deuterium, while the rest hydrogen atoms are inherited from organic matter with normal δD. This mechanism of hydrogen biodegradation successfully explains δD and δ¹³C variations among the alkanes discovered in Kidd Creek Mines in the Canadian Shield.

Based on numerical simulations of bulk and clumped isotopic fractionations of methane generation, an inverse primary deuterium kinetic isotope effect (DKIE) is proved to exist in the hydrogenolysis process. The inverse primary DKIE and the inverse ¹³C KIE suggest that the Kidd Creek gases were generated by microbial hydrogenolysis of organic matter. This newly recognized geochemical pathway might play an important role in the carbon and hydrogen geochemical cycles of the late Archean Eon.