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).