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.