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.