Abstract
Methane clumped isotope signatures of abiogenesis may be diagnostic of
the origin of methane on Earth and other planetary bodies. On Earth,
identifying methane synthesis pathways has proven challenging because of
the lack of unambiguous signatures of the provenance of methane
molecules. The abundances of mass-18 isotopologues of methane,13CH3D and12CH2D2, could help
identify occurrences of abiotic methane in nature. We performed
synthesis of abiogenic methane in the laboratory and determined
δ13C, δD, Δ13CH3D
and Δ12CH2D2. We
carried out a set of experiments in hydrothermal conditions between 130
and 300 °C. The experiments were performed by heating water in the
presence of Fe0 powder and CO. The reduction of water
on metallic iron led to the formation of H2. CO was
reacted with both H2 and H2O, generating
both CH4 and CO2. We systematically
extracted and quantified methane, and determined δ13C,
δD Δ12CH2D2, and
Δ13CH3D of the methane using a
Panorama gas-source mass spectrometer (Nu, Ametek). We observe
δ13C and δD of methane to be isotopically depleted
relative to the starting materials. The δ13C data
indirectly suggest isotopic equilibrium may have been reached for carbon
isotopes between methane, carbon monoxide and carbon dioxide in our
experiments. In contrast, D/H ratios are inconsistent with equilibrium
isotopic fractionation. This suggests that under our experimental
conditions, hydrogen additions to carbon are governed by kinetics, and
that subsequent D/H re-equilibration was limited. While
Δ13CH3D values approximately track
experimental temperature,
Δ12CH2D2 values are
displaced far from equilibrium. We find exclusively negative
Δ12CH2D2 values,
showing deficits down to 40‰ relative to thermodynamic equilibrium. We
interpret the data as evidence for distinct, kinetically induced D/H
pools contributing to methane assembly (a combinatorial effect). The
cumulative D/H fractionations associated with CO (or CO2in nature) hydrogenation likely explain the direction and magnitude of
Δ12CH2D2 values during
abiotic methane formation. We suggest that near equilibrium
Δ13CH3D with negative
Δ12CH2D2 signatures
will help identify methane formed abiotically in nature.