1. Introduction
Methane (CH4) is observed in a variety of environments on Earth. Abiotic methane (i.e., synthesized by mechanisms that do not involve life or thermal decomposition of organic matter), is suspected to occur in various fluids also containing molecular hydrogen (H2). These fluids are especially prominent in hydrothermal systems where olivine-rich ultramafic rocks are altered via serpentinization reactions. For instance, elevated concentrations of CH4 and H2 are observed in ultramafic-hosted seafloor hydrothermal vent fluids at temperatures typically exceeding 250 °C (Charlou et al., 2010; Welhan and Craig, 1979). In these environments, abiotic methane synthesis is thought to originate from abiotic reactions between CO2 and H2:
CO2+4H2=CH4+2H2O (1)
At temperatures under 340 °C, thermodynamic equilibrium favors carbon reduction in fluids that contain H2 (Klein et al., 2019). This is the Sabatier reaction, often lumped with a class of reactions referred to as Fischer-Tropsch-Type synthesis, or FTT, as reviewed elsewhere (McCollom, 2013; McCollom and Seewald, 2007). Methane is also observed with H2 in seeps and fracture waters from the continental subsurface, at environmental temperatures typically ≤ 100 °C. In fluids from the Kidd Creek mine (Canada), or in hyperalkaline fluids issuing from the underlying ophiolite from the Oman ophiolite, CH4 and H2 are present as major species. The environmental temperatures there allow microbial methane synthesis, but CH4 has been suggested as being dominantly abiotic with only minor mixing with microbial methane on the basis of C and H stable isotopes of methane and associated gases in these sites (Etiope et al., 2015; Fritz et al., 1992; Sherwood Lollar et al., 2008, 2002, 1993). Contributions from microbial methane have also been identified in gas seeps from the sultanate of Oman on the basis of microbiological data (Miller et al., 2016).
The abundances of mass-18 methane isotopologues,13CH3D and12CH2D2, may be tracers of methane origin in nature (Stolper et al., 2014; Wang et al., 2015; Young et al., 2017). The potential for Δ12CH2D2 and Δ13CH3D to demonstrate abiogeneicity of natural methane on Earth and other worlds is illustrated by preliminary experimental work. So far, two Sabatier-type experimental runs have been investigated for Δ13CH3D versus Δ12CH2D2 (Young et al., 2017). Those methane aliquots were generated by CO2reduction (reaction 1) catalyzed by ruthenium at 70 and 90 °C following the method of Etiope and Ionescu (2015). The experiments yielded near-equilibrium Δ13CH3D values of ~+4‰, but large12CH2D2 depletions of approximately 70‰ relative to equilibrium. These empirical values have been thought to reflect a signature acquired by methane during the abiotic reduction of oxidized carbon in the laboratory and potentially in nature (Young et al., 2017).
At Kidd Creek, methane samples have compositions that appear consistent with the Sabatier experimental runs. In the deepest levels of the mine, CH4 shows Δ13CH3D values of 5.2 ± 0.5‰ and Δ12CH2D2 values are 10 to 30‰ below equilibrium, which has been interpreted as potentially direct evidence for an abiotic origin of methane (data in Young et al., 2017; Warr et al. 2021). In contrast, methane from Oman shows negative Δ12CH2D2 value and near-zero Δ13CH3D. The near-zero was considered inconsistent with abiotic signatures (Nothaft et al., 2021). In deep-sea high temperature vents, concordant Δ12CH2D2 and Δ13CH3D data suggested apparent equilibrium temperatures of ~ 340 °C (Labidi et al., 2020). Equilibrium signatures could be acquired during methane synthesis at high temperature, or may reflect bond reordering after synthesis, before or during transport.
The Sabatier experiments reported in Young et al., (2017) were performed at temperatures below 100 °C and in the absence of water. These experimental conditions are challenging to extrapolate to deep-sea hydrothermal systems or continental environments for which Δ12CH2D2 and Δ13CH3D data are available. Consequently, it is not clear how the experimental data should be applied to the interpretation of natural systems. To constrain the Δ12CH2D2 and Δ13CH3D signatures produced by abiotic FTT reactions at variable temperatures, we performed controlled FTT synthesis at hydrothermal conditions in the laboratory at temperatures varying between 130 °C and 300 °C.