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ABIOTIC METHANE ON EARTH

Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH4), formed by chemical reactions which do not directly involve organic matter, occurs on Earth in several specific geologic environments. It can be produced by either high‐temperature magmatic processes in volcanic and geothermal areas, or via low‐temperature (<100°C) gas‐water‐rock reactions in continental settings, even at shallow depths. The isotopic composition of C and H is a first step in distinguishing abiotic from biotic (including either microbial or thermogenic) CH4. Herein we demonstrate that integrated geochemical diagnostic techniques, based on molecular composition of associated gases, noble gas isotopes, mixing models, and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic CH4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH4 is mainly related to mantle‐derived or magmatic processes, a new generation of data is showing that low‐temperature synthesis related to gas‐water‐rock reactions is more common than previously thought. This paper reviews the major sources of abiotic CH4 and the primary approaches for differentiating abiotic from biotic CH4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.