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.