Formation of Abiogenic Hydrocarbons in Supercritical Fluids under Earth's Upper Mantle Conditions

The formation of hydrocarbons in Earth's interior has traditionally been considered to have biogenic origins; however, growing evidence suggests that some hydrocarbons may instead originate abiotically in the deep carbon cycle. It is widely expected that the Fischer-Tropsch-type (FTT) process, which typically refers to the conversion of inorganic carbon to organic matter in geological settings, may also happen in Earth's interior, but the absence of industrial catalysts and aqueous conditions in deep environments suggest that the FTT process can be very different from that in the chemical industry. Here, we performed extensive ab initio molecular dynamics (AIMD) simulations (>2.4 ns) to investigate the FTT synthesis in dry mixtures and in aqueous solutions at 10-13 GPa and 1000-1400 K. We found that large hydrocarbon-related species containing C, O, and H (>C₂) are abiotically synthesized via the polymerization of CO without any catalyst. Supercritical water, commonly found in the deep Earth, does not prevent organic molecule formation but restricts product size and carbon reduction. Our studies reveal a previously unrecognized abiogenic route for hydrocarbon synthesis in mantle geofluids. These carbon-containing fluids could potentially migrate from depth to shallower crustal reservoirs, thereby influencing Earth's surface carbon budget.