5.1 The carbon isotope balance in our experiments
The δ13C signatures of methane vary around a value of
–55‰ (V-PDB), with FT18-4 as the one outlier with a
δ13C of –44‰ (Fig. 2). All methane samples have
depleted δ13C compared to the starting CO (–28‰). The
isotopic difference between CH4 and the starting CO of
~ 25‰ likely reflects13C/12C fractionation during methane
synthesis. Kueter et al., (2019) determined the equilibrium13C/12C fractionations between CO,
CO2 and CH4 at various temperatures
between 1200 °C and 300 °C. At 300 °C, the equilibrium13C/12C fractionation between CO and
CH4 is –3.1‰, meaning that CH4 should
be enriched in 13C relative to CO by 3.1‰. This is at
odds with our observation of a ~25‰ to
~40‰ depletion in13C/12C in CH4relative to the initial reactant CO. Instead methane
δ13C values are broadly consistent with an approach to13C/12C equilibrium between methane
and CO2. This implies that the synthesis of
CO2 from CO oxidation via reaction 4 was nearly
quantitative so that CO2 had nearly the same13C/12C as the initial CO reactant
(–28‰). Subsequent 13C/12C
equilibration between CH4 and this CO2would yield methane with a 25‰ 13C depletion relative
to CO2 at 250 °C, and roughly 40 ‰ at
150 °C (Kueter et al., 2019), corresponding to
predicted methane δ13C values of ~
–53‰ and −68‰ at 250 and 150 °C, respectively. This
data supports near-complete conversion of CO to CO2 and
complete equilibration between CH4 and
CO2. While imperfect, the magnitude of the shift in
δ13C from the initial CO reactant values to the
product methane are generally consistent with these
CH4-CO2 equilibrium values. In this
context, the scatter in the δ13C values, and imperfect
correlation with T (Fig. 2a) would reflect various degrees of departure
from full isotopic equilibrium and/or conversion of CO to
CO2. The FT18-4 outlier would reflect the furthest
removal from CH4 -CO2 equilibrium,
and/or incomplete conversion of CO to CO2, consistent
with this experiment being conducted at the lowest temperature. The
alternative interpretation is that the CH4δ13C values reflect a kinetic carbon isotope effect
involving the reduction of CO. In this case the correlation with
temperature, albeit imperfect, would perhaps reflect a reservoir effect
with a single kinetic fractionation factor, or a temperature-dependent
kinetic fractionation factor.
5.2 The hydrogen isotope balance in our experiments
The δD signatures of methane range from –610 to –580‰ V-SMOW. If
methane is in D/H equilibrium with experimental water (–119‰ V-SMOW),
the methane δD would fall between –250 and –350‰ for experiments
conducted at temperatures between 250 and 130°C (Horibe and Craig,
1995). Thus, the observed δD signatures of methane of roughly –600‰
reflect unambiguous D/H disequilibrium with water. During FTT synthesis,
methane molecules form via a catalytic surface reaction of sequential
hydrogen additions, presumably sourced directly from H2.
It is expected, therefore, that it is the D/H ratio of
H2 that is a dominant factor in controlling the D/H of
methane in our experiments, not that of water. No D/H measurements of
H2 are available in this study, but at equilibrium,
CH4 should be ~ 540‰ higher in δD than
coexisting H2 at ~ 200°C (Horibe and Craig, 1995). In lieu of D/H for
H2 in our experiments, we may make use of three FTT
experimental studies that have reported D/H of both of
CH4 and H2 to infer the likely extent of
isotopic equilibrium between these species. All of these studies report
D/H ratios in CH4 and H2 are within 150
‰ of each other (Fu et al., 2007; McCollom et al., 2010; Taran et al.,
2010), far from the ~ 500‰ difference expected at
equilibrium. This suggests that abiotic methane synthesized in the
laboratory is not in isotopic equilibrium with H2. We
can explore this further if we assume H2 formed in D/H
equilibrium with water in our experiments. In this case δD for
H2 is predicted to be approximately −560‰ at 200oC, in equilibrium with the liquid water δD of -119‰
(Horibe and Craig, 1995; Rolston et al., 1976). Since the product
methane has δD values of approximately –600‰ at this T, it appears that
the CH4 has D/H similar to that of the reactant
H2, as in previous experiments. Under strict isotopic
equilibrium between methane and an infinite pool of H2,
CH4 should be ~ 540‰ higher in δD than
coexisting H2 at ~ 200°C. Similar D/H for CH4 and
H2 would suggest that either the methane consumed most
or all of the H2 available locally, or that there is a
D/H kinetic fractionation that offsets the equilibrium fractionation by
nearly 500‰. Overall, we conclude that a combination of equilibrium
fractionation between H2O and product
H2, followed by synthesis of methane with similar D/H
than reactant H2 explains the methane δD values of
approximately –600‰ obtained here.
The predicted range in δD values relative to VSMOW for
H2 formed in equilibrium with the effectively infinite
reservoir of liquid H2O from ~ 170 to
250 °C is −588 to −513‰, respectively, defining a
slope of 0.44‰/°C over this temperature range. This is
similar to the observed positive correlation between methane δD values
with T in our experiments, with a slope of
~0.38‰/°C, lending further support for
the inheritance of D/H from H2 equilibrated with water,
with the exclusion of the 130°C outlier FT18-4 (Fig. 2). This
interpretation of inheritance of D/H from H2 convolved
with a kinetic isotope effect is consistent with previous similar
published experiments in which the D/H values of CH4 and
H2 are within a few tens of permil of one another (e.g.,
McCollom et al., 2010).