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 Large12CH2Ddeficits 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: