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

TL;DR

This study shows that hydrocarbons can form without life in Earth's mantle under high pressure and temperature, challenging traditional views of their origin.

Contribution

The study reveals a new abiogenic pathway for hydrocarbon synthesis in Earth's mantle using supercritical fluids.

Findings

Large hydrocarbon species form abiotically via CO polymerization without catalysts.

Supercritical water limits product size and carbon reduction in hydrocarbon synthesis.

Abiogenic hydrocarbons may migrate to the crust, affecting Earth's surface carbon budget.

Abstract

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.

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