Abiotic Methane on Earth

Abstract

[1] Over the last 30 years, geochemical research has demonstrated that abiotic methane (CH 4 ), 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) CH 4 . 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 CH 4 in natural gases, which are frequently mixtures of multiple sources. Although it has been traditionally assumed that abiotic CH 4 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 CH 4 and the primary approaches for differentiating abiotic from biotic CH 4, including novel potential tools such as clumped isotope geochemistry. A diagnostic approach for differentiation is proposed.

Abiotic methane formation during experimental serpentinization of olivine

“…Several subsequent experimental studies confirmed that abiotic reduction of dissolved CO 2 to CH 4 is sluggish at temperatures as low as 300°C, even in the presence of highly elevated H 2 concentrations, except when NiFe alloy was present (16,(18)(19)(20)(21). Additional experiments with 13 C-labeled inorganic carbon compounds also confirmed that background sources are a ubiquitous contributor to CH 4 in experimental fluidrock studies using olivine or other minerals, with abundance that often far exceeds that of bona fide synthesis products (18,19,28).…”

“…Several recent reports have claimed observation of abiotic formation of methane during low-temperature serpentinization of olivine-rich rocks during laboratory experiments. However, using 13 C-labeled carbon sources, this study shows that the methane observed in such experiments is predominantly derived from background sources rather than abiotic synthesis. Conversely, more rapid production of methane is observed when an H 2 -rich vapor phase is present within the reaction vessel.…”

“…Here, the potential for abiotic formation of CH 4 from dissolved inorganic carbon during olivine serpentinization is reevaluated in a series of laboratory experiments conducted at 200°C to 320°C. A 13 C-labeled inorganic carbon source was used to unambiguously determine the origin of CH 4 generated in the experiments. Consistent with previous high-temperature studies, the results indicate that abiotic formation of CH 4 from reduction of dissolved inorganic carbon during the experiments is extremely limited, with nearly all of the observed CH 4 derived from background sources.…”

“…13). Although laboratory experiments can help to constrain the contribution of abiotic synthesis to fluxes of CH 4 in serpentinites, published studies have reported some seemingly conflicting results with regard to the potential for reduction of inorganic carbon to CH 4 during serpentinization (14,15).…”

“…Formation of the CH 4 and other hydrocarbons was attributed to reduction of dissolved CO 2 by H 2 catalyzed by magnetite generated during serpentinization (Reaction 1). However, when this experiment was replicated using a 13 Clabeled inorganic carbon source to trace the origin of organic compounds (i.e., H 13 CO 3 − ), it was found that only a small fraction of the CH 4 and none of the other hydrocarbons observed during the experiment contained the 13 C, showing that synthesis of these compounds from reduction of the inorganic carbon source was very limited under the reaction conditions (17). Instead, the methane in the experiments was composed predominantly of 12 CH 4 , indicating that it was derived from Significance Abiotic methane discharged from serpentinizing rocks supplies metabolic energy to chemosynthetic microbial communities and may have done so since the earliest lifeforms evolved on Earth.…”