5.6 Hydrogen isotope homogenization in nature: the link with the laboratory
The data collected for this study are plotted on figure 4 with published methane isotopologue data from a variety of geological settings, including sedimentary basins, deep crystalline environments and serpentinization sites (Ash et al., 2019, Giunta et al 2019 2021, Young 2017 2019, warr et al 2021, Lin et al 2023). Extreme signatures associated with active anaerobic oxidation of methane (Giunta et al 2022, Liu et al 2023) were omitted for clarity. Signatures observed in abiotic methane synthesized in the laboratory are rare in nature (Fig. 4). This might suggest that abiotic methane has been under-sampled so far, and/or that bond reordering of once-abiotic methane is at play in natural environments.
Equilibrium values for Δ12CH2D2 (and Δ13CH3D) are seen in most high-temperature hydrothermal vents with temperatures ≥ 250°C to date (Fig. 4). Based on experiments, we would anticipate natural abiotic methane synthesized at > 250°C to have equilibrated Δ13CH3D values associated with ~10‰ Δ12CH2D2 deficits relative to equilibrium (Fig. 3b). Based on the experiments, subsequent bond reordering would seem to be required in order to erase the low Δ12CH2D2 values associated with constructing CH4 molecules and replace them with the equilibrium values exhibited by the natural high-temperature hydrothermal samples (vents) shown in Fig. 4. High-temperature hydrothermal systems may provide conditions conducive to bond reordering. Methane is thought to be formed abiotically in fluid inclusions of basement rocks at temperatures of 300 °C or higher (Früh-Green et al., 2022; Klein et al., 2019). Those fluid inclusions might be maintained at temperatures of 300 °C or above for durations that may be thousands of years or even far longer, allowing bond reordering to take place. Because homogenization evidently occurs, the deficits associated with initial methane synthesis are “cryptic”, i.e., they are not observable in nature, only in the laboratory where experimental durations are short, and reordering does not take place.
Abiotic methane formed and kept at low temperature might have a chance to preserve deficits in Δ12CH2D2, should it be restricted to environments where the kinetics of D/H homogenization within methane molecules are unfavorable or residence times are short. This offers a chance to use the combination of Δ12CH2D2 and Δ13CH3D to identify abiotic methane in nature, as it has been done with microbial methane (Ash et al., 2018; Giunta et al., 2022, 2019; Young et al., 2017). Abiogenesis may notably account for signatures observed in methane from ~25°C fluids from the Kidd Creek mine. There, methane samples have Δ13CH3D values of 5.2 ± 0.5‰, translating to temperatures of ~45 °C. The methane Δ12CH2D2 values from the deepest level in the mine are 10 to 30‰ below equilibrium (data in Young et al., 2017, also shown here on Fig. 4). These deficits are not as low as those seen in experiments, and would be compatible with FTT signatures affected by later bond reordering resulting from processing by anaerobic methane oxidation (Warr et al., 2021; Young et al., 2017).
Abiogenesis alone is more challenging to reconcile with the observations from serpentinizing systems in Oman. There, methane is vented to the surface in ~35°C hyperalkaline fluids issuing from the underlying ophiolite. It carries some of the highest δ13C yet measured in natural methane (Miller et al., 2016). Whether microbial methanogenesis can explain the isotopic features of Oman methane is debated (Etiope, 2017; Miller et al., 2016). Recent measurements of Δ12CH2D2 and Δ13CH3D were made available on new gas aliquots from Oman, in Nothaft et al., (2021). The one sample with the highest δ13C has a negative Δ12CH2D2 value and near-zero Δ13CH3D (Fig. 4). This is inconsistent with abiotic signatures reported here: no abiotic methane made in the laboratory, at any of the investigated temperatures, has yielded a near-zero Δ13CH3D (Fig. 3a). Conversely, microbial methane does exhibit near-zero or negative Δ13CH3D values (Young et al., 2017, Giunta et al., 2019) and the Oman data appear consistent with microbial signatures (Fig. 4). The simplest explanation for the data in Nothaft et al., (2021) remains that despite the elevated δ13C, the Δ12CH2D2 - Δ13CH3D signature reflect substantial contributions of microbial methane in the Oman ophiolite (details in Nothaft et al., 2021).