4. Results
For nine experiments conducted at temperatures between 130 and 300°C, between ~5 and ~80 micromoles of abiotic CH4 were obtained. All results may be found in Table 1. Previous experimental studies of abiotic synthesis have shown that background sources can contribute significant amounts of methane, which can be misleading if not carefully quantified (McCollom et al., 2016 and references therein). In previous experiments employing the same methodology as used here but without added CO indicated that background sources contribute <3 μmoles CH4 (McCollom et al., 2010). Our experiments for which we have isotope results have resulted in the accumulation of 35 to 80 micromoles of methane, suggesting our sample are dominated by genuinely abiotic CH4, synthesized by CO reduction. A simple mass balance suggests that the near-complete conversion of the Fe0powder to magnetite (reaction 2) produced the H2-rich vapor phase, as shown in McCollom et al., (2010). This is an essential condition to ensure reaction 3 proceeds efficiently, allowing the abiotic synthesis of methane (McCollom, 2016).
An inverse correlation between CH4 quantities and temperatures is observed (Fig. 1), indicating that experiments at lowest temperatures allowed the highest accumulations of methane from CO reduction. The experiments at 275 °C and 300°C yielded 5 and 10 micromoles of CH4 respectively, which was insufficient to characterize isotopologue abundances; the low yields at these temperatures likely reflect rapid conversion of CO to CO2 by the water-gas shift reaction (reaction 4), short-circuiting reduction to methane. Seven experiments conducted at temperatures between 130 and 250 °C yielded at least 35 micromoles of CH4, allowing measurements of rare isotopologue ratios. We interpret the relationship between methane amount and experimental temperatures (Fig. 1) as a reflection of the lifetime of the injected CO: upon CO injection, conversion to CO2 through reaction 4 begins, precluding methane synthesis. Equilibrium thermodynamics suggest that reaction 4 goes to completion, hence CH4 synthesis occurs in a short time window between injection of CO and conversion to CO2.  Reaction 4 is faster at higher temperatures. Evidently, in our experiments conducted at T > 250°C, CO conversion to CO2 happens so rapidly that very little FTT-derived methane is generated.
The δD of product methane in our experiments vary between –583‰ and –608‰ versus V-SMOW (Fig. 2), much lower than the starting δD of water (–119‰). Hydrogen isotope ratios of methane are positively correlated with temperatures (Fig. 2). At the highest temperatures, the hydrogen isotope composition of methane tends to be the most elevated. A clear outlier to this trend is FT18-4, the experiment conducted at the lowest temperature (130°C), which shows the highest δD of the series (–582‰). The δD values for the methane synthesized in this study are different from those obtained in McCollom et al., (2010) (with values of ~ –550‰) although experimental conditions except for temperature were similar. We attribute the small differences as the simple reflection of variable starting water for the experiments: in the study of McCollom et al., (2010), the reactant water was purchased from Fischer®. The measured δ13C of methane varies between –44 and –62‰ versus V-PDB (Fig. 2). The carbon isotope values obtained here are similar to previous results obtained using the same experimental procedures and starting CO (McCollom et al., 2010). The δ13C values are weakly correlated with temperature but interpretations of the correlation should be regarded with caution, considering the importance of FT18-4, the one outlier on Fig. 2b.
We find Δ13CH3D values between 1.7±0.4‰ (1σ, at 250 °C) and 4.8±0.2‰ (1σ, at 130 °C) for all experiments except one outlier, FT18-3, with a Δ13CH3D of 5.8±0.2‰ (1σ, at 170 °C). Most of the Δ13CH3D values track the temperature of abiotic methane synthesis within 1 ‰ (Fig. 3a). In contrast with Δ13CH3D, Δ12CH2D2 values are exclusively negative, ranging from −3.0 ± 2.5‰ (1σ) at 210 °C to −32.0±1.6‰ (1σ) at 183 °C (Fig. 3b). The negative Δ12CH2D2 values indicate that the relative amount of12CH2D2 among methane isotopologues is lower than predicted for a stochastic gas equilibrated at T > 1000 °C. The Δ12CH2D2 values are positively correlated with peak experimental temperatures (Fig. 3b); the magnitude of the disequilibrium relative to stochastic is greater at the lowest temperatures. The data are shown in the Δ13CH3D - Δ12CH2D2 space on Fig. 4, with natural samples from deep-sea vents (Labidi et al., 2020), methane from the Kidd Creek mine (Young et al., 2017) and Oman fluids (Nothaft et al., 2021), for which methane is potentially abiotic in origin.