5.3 Methane clumped signatures: does13CH3D reflect peak temperature of
methane synthesis?
We find Δ13CH3D values to roughly
track with the expected equilibrium values within 1‰ for all experiments
except FT18-3 (170 °C - see below). Data from the Sabatier experiments
reported in Young et al., (2017) are also included in Figure 3. Although
run under different conditions, the Sabatier experiments also show
equilibrium signatures for Δ13CH3D.
Overall, the data suggest that FTT processes generate
Δ13CH3D values that are near
equilibrium at the temperature of synthesis. The equilibrium signatures
suggest that despite D/H kinetic isotope effects associated with
hydrogen addition during the assembly of methane molecules, the13CH3D bond-ordering remains
controlled by synthesis temperature. This is an important perspective
for the interpretation of natural methane where abiogenesis is
suspected: unless substantial mixing is involved, one may use
Δ13CH3D values to constrain the
approximate synthesis temperature of abiotic methane.
For FT18-3, the measured Δ13CH3D of
5.8±0.4‰ (2 s.e.) reflects a 3‰ deviation from an equilibrium value of
2.8‰ at 170 °C (Fig. 3a). The equilibrium temperature corresponding to
the measured Δ13CH3D value for FT18-3
is \(25_{-8}^{+5}\) °C, far from the peak experimental temperatures,
but similar to room temperature (Fig. 3a). We argue against the
possibility of Δ13CH3D
re-equilibration towards low temperature in FT18-3 since there is no
physical reason why Δ13CH3D reordering
to room temperature would occur only in this experiment. Like every
other experiment, gases were extracted from the gold reaction cells
within 24 hours of cooling the reactors, stored in stainless steel
containers, and measured for Δ13CH3D
and Δ12CH2D2 at UCLA
within two weeks upon synthesis. On this basis, we argue that the
elevated Δ13CH3D value for FT 18-3
must reflect an unidentified mass-dependent fractionation of up to 3‰,
rather than re-equilibration.
5.4 Large12CH2D2 deficits in abiotic methane
The Δ12CH2D2 values
here exhibit pronounced disequilibrium, in sharp contrast with those of
Δ13CH3D. The
Δ12CH2D2 values are
exclusively of negative sign, ranging from −3.0 ± 2.5‰ (1σ) at 210 °C to
−32.0 ± 1.6‰ at 183 °C. These values are substantially lower than the
expected equilibrium values of 9.2‰ and 4.3‰ at 130 and 250 °C,
respectively. We argue that the
Δ12CH2D2 deficits are
apparent disequilibrium signatures resulting from combinatorial effects.
In general, combinatorial effects arise when a molecule contains
indistinguishable atoms of the same element, and that these atoms come
from pools with distinct isotope ratios, as has been predicted and shown
for methane previously both from theory and in the laboratory (Röckmann
et al., 2016; Taenzer et al., 2020; Yeung, 2016). Among the two mass-18
isotopologues of methane, only
Δ12CH2D2 can be
affected by combinatorial effects, because it is the isotopologue with
two indistinguishable deuterium substitutions for hydrogen.
The root of the combinatorial effect comes from the notation convention
used with clumped isotopes. As an example, consider a sample of methane
gas with δD = –100 ‰ relative to SMOW and δ13C = -10
‰ relative to PDB, corresponding to a measured bulk D/H ratio of
1.40184⋅10-4 and a13C/12C ratio of 0.01112483. In this
example we will use a measured12CH2D2/12CH4ratio of 1.11600×10-7. To calculate the value for
Δ12CH2D2, we compute
the stochastic ratio from the measured bulk carbon and hydrogen isotope
ratios. Isotope-specific mole fractions for singly-substituted
isotopologues are closely approximated as: