Abiogenic gas: Should the carbon isotope order be reversed?

Biogenic gases were reported to exhibit a “normal” carbon isotope order of δ13C1 < δ13C2 < δ13C3 < δ13C4, while abiogenic gaseous hydrocarbons in igneous rocks and meteorites exhibit a reversed distribution pattern in the order of δ13C1 > δ13C2 > δ13C3 > δ13C4. This reversed order in carbon isotope compositions is commonly thought to be unique to abiogenic gases, thus it can be a criterion for determining their origins. In this study, a closed system Fischer–Tropsch synthesis was conducted at 380 °C under 30 MPa and 390 °C under 30 MPa, 100 MPa, 200 MPa, respectively, with magnetite as a catalyst. Results of carbon isotope compositions of gaseous hydrocarbons were derived, which do not show an expected reversed order in carbon isotope composition from C1 to C3 hydrocarbons, even though features of partially reversed order in carbon isotope values, such as δ13C1 > δ13C2 < δ13C3, can still be commonly observed. This indicates that the reversed order of carbon isotope composition is not a unique criterion that can be typically used to distinguish abiogenic gases.