6. Conclusion
We performed synthesis of abiogenic methane in the laboratory and
determined δ13C, δD,
Δ13CH3D and
Δ12CH2D2 using a
Panorama gas-source mass spectrometer. We carried out a set of 9
experiments in hydrothermal conditions, at temperatures between 130 and
300 °C. The experiments were performed by heating an aqueous solution in
the presence of Fe0 powder and gaseous CO. We find
δ13C and δD of methane to be isotopically depleted
relative to the starting materials, consistent with equilibrium carbon
isotope fractionation between CO2 produced in the
experiment and the product CH4, and kinetic isotope D/H
fractionations between H2 and product
CH4. We find most of the product methane
Δ13CH3D values trace the temperature
of abiotic methane synthesis within a permil. At the same time, we
observe Δ12CH2D2deficits of up to 40‰ relative to equilibrium attributable to a D/H
combinatorial effect occurring during the methane assembly. The
combinatorial effect results from our inability to determine D/H ratios
of distinctive pools of hydrogen that contribute to individual hydrogens
on the methane molecule. The D/H fractionation associated with the
various steps of hydrogen addition to carbon can explain the direction
and the magnitude of the
Δ12CH2D2 signatures of
methane that would otherwise be at bond-ordering equilibrium. In
instances of some potentially abiotic methane found on Earth,,
particularly under high T,
Δ12CH2D2 deficits may
have been generated during methane synthesis, but were subsequently
erased by D/H reordering. In contrast,
Δ12CH2D2 deficits
generated in our experiments are seen on Earth at sites where the C-H
bond activation was too slow to re-equilibrate
Δ12CH2D2 to
environmental temperatures. As such these dramatic deficits, when
combined with near-equilibrium Δ13CH3D
values, might be an avenue for the identification of abiogenesis on
other worlds.