Hydrothermal simulation of abiogenic hydrocarbon formation from formate: implications for C₂-unit chain growth and isotopic signatures

Abiogenic hydrocarbons in geological settings, particularly in mid-ocean-ridge hydrothermal systems, have been proposed as potential precursors to life and as sources of primitive nutrients for surrounding biomes. Formate (HCOO⁻), an abundant and relatively stable organic anion in such systems, is regarded as an intermediate in abiotic reaction pathways. In this study, we performed two sets of hydrothermal simulation experiments using sodium formate as the carbon source and magnetite or olivine as catalysts. Both experiments produced methane-dominated gaseous hydrocarbons, primarily through hydrogenation of formate and methanation of its decomposition products (e.g., CO and CO₂). Notably, the elevated yield of i-C₄ results in total C₄ abundances exceeding those of C₃. In addition, the gaseous products deviate from the typical Anderson–Schulz–Flory distribution, which may reflect a possible contribution from C₂-unit coupling, although this interpretation remains speculative and is not directly supported by the present experimental data. Hydrocarbon gases are enriched in deuterium relative to the coexisting H₂ and exhibit a systematic trend of δD_C2 < δD_C3 < δD_C4. This pattern may be consistent with kinetic isotopic effects associated with deuterated surface intermediates. Based on these observations and the alkylene growth mechanism, we suggest a carbon isotopic fractionation model characterized by a monotonic increase and convergence with increasing carbon number. Our results further indicate that, in the presence of magnetite or olivine, aqueous sodium formate can generate detectable amounts of gaseous hydrocarbons under hydrothermal conditions without the addition of elemental iron. This finding suggests the potential significance of formate-driven hydrocarbon formation in alkaline hydrothermal systems.