Title: Abiotic methane in continental serpentinization sites: present status and a strategy for the future

Speaker: Martin Schoell

Abstract:
Abiotic formation of methane and higher hydrocarbons is one of the most pivotal topics in regard to the origin of life, because inorganic reactions in rocks have been active since billions of years on Earth and possibly on Mars and could have provided the energy source as well as chemical substrates that could have led to more complex organic molecules.

With systematic studies in the last 10 years we know of more than 80 sites of abiotic methane occurring in 17 countries all over the world. The geologic settings of abiotic methane occurrences are ophiolites, peridotites and/or ultramafic rocks. Temperatures in continental settings are relatively low and range between 100 and 130˚C and at these temperatures, reaction rates are slow and classical catalysts like Fe, Ni and Cr are ineffective. But Etiope found that Platinum group elements like Ruthenium and Rhenium are efficient catalysts for FTT/Sabatier reaction CO2 +H2→ CH4 + H2O at temperatures as low as 20˚C. Yet direct analyses that can measure formation temperatures like position-specific isotopes and clumped isotopes are rare but, if available, could give information on the depth of formation and together with heat flow models would provide information on the time of methane formation.

Methane carbon and hydrogen isotopes in abiotic methanes range from -70 to +10+‰ and -350 to -100‰, respectively. This range exceeds, and overlaps with the methane isotope variations of all biotic natural gases. The primary driver of isotope variations in biotic methanes is temperature, however, because formation temperatures of continental abiotic methanes are low, we must invoke completely different processes to explain the isotope variations in abiotic methanes, both for carbon and hydrogen isotopes. Until we do not understand these isotope variations we cannot develop a unified genetic model for abiotic methane formation, and until we do not have it, the role of abiotic methane in regard to the origin of life is questionable. The same applies to the coeval formation of traces of higher hydrocarbons together with methane.

In this regard the ELSI study of Suda et al. GCA 206, (2017 201-215 of the Hakuba Happo abiotic methane may serve as an exemplary study of methane and C2+ hydrocarbons as it addresses the origin of abiotic methane and C2+ HC with new analytical procedures that have so far not been applied to other abiotic methane occurrences. I feel strongly that it would be an excellent strategy of ELSI to make the effort and expand the Suda et al. analyses to many abiotic methane occurrences. The existing methane carbon and hydrogen data and other findings could serve as a basis to select critical occurrences. This world-wide expansion with more comprehensive analyses and their interpretation in the geologic context would be a strategy to develop a unified genetic model for abiotic methane in continental settings.