5.5 The temperature relationship with Δ12CH2D2
Experiments with data at the highest temperatures, 210 and 250 °C, produced methane with Δ12CH2D2 values of between −3 and −12‰. This is substantially more positive than in experiments done at 180, 170 and 130 °C, which produced methane with Δ12CH2D2 values of between −32 and −27‰ (Fig. 3b). The Sabatier experiments at T ≤ 90°C (Young et al., 2017) yielded methane with Δ12CH2D2 values of roughly −55‰. Taken together, the Δ12CH2D2 data show a clear positive correlation with temperature (Fig. 3b). This may reflect the possibility that different D/H ratios among pools of hydrogen at each reaction step are dependent on temperature, with greater differences at lower temperatures. At > 200°C, the observed Δ12CH2D2 values require pools of hydrogen atoms that comprise methane to be at most 200 ‰ different from one another. At < 100°C, the 70‰ Δ12CH2D2 deficits of the Sabatier experiments require at least 700‰ D/H difference between pools of hydrogen atoms.
The effect of bond reordering is unclear in this set of experiments. One might imagine that irrespective of experimental temperatures, all abiotic methane synthesized during FTT experiments is associated with Δ12CH2D2 deficits > 70‰, as the natural consequence of extreme and variable D/H fractionation factors among the four hydrogen addition steps. This would be the case if kinetic isotope effects for each step in the CH4-building reaction sequence were fully independent of temperature. After methane synthesis, Δ12CH2D2 values would approach equilibrium values > 0‰ due to bond reordering. Bond reordering – here, homogenization of the D/H among the hydrogens comprising CH4 molecules – would generate ana-posteriori correlation between temperature and Δ12CH2D2 as observed on Fig. 3b. This scenario may not be excluded. Nonetheless, we favor a temperature-dependent fractionation in D/H between H-addition steps during CH4 construction as the source of negative Δ12CH2D2 values since these effects can be seen in the theoretical treatments in Cao et al., (2019) and Young (2019).
To summarize, the Δ12CH2D2 relationship with experimental temperature (Fig. 3b) may be explained by two different scenarios. First, kinetic isotope effects for different H-addition steps may vary as a function of temperature. This would cause the resulting combinatorial Δ12CH2D2 values to show a variability naturally correlated to temperature. Alternatively, an a-posteriori bond reordering may occur in the hydrothermal cells, from an initial Δ12CH2D2 value that is always —70‰. If experiments at higher temperature allow further re-ordering than those at lower temperatures, temperature and Δ12CH2D2 will display a positively correlation.