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Formation of abiogenic hydrocarbons in supercritical fluids under Earth's upper mantle conditions

Nore Stolte, Tao Li, Ding Pan

TL;DR

This study investigates abiogenic hydrocarbon formation in Earth's upper mantle by simulating uncatalyzed Fischer-Tropsch-type chemistry from CO and $H_2$ under high-pressure, high-temperature conditions with and without water, using extensive ab initio molecular dynamics. It finds that CO can polymerize into larger hydrocarbon-containing species without catalysts, with higher pressures promoting larger molecules, while aqueous environments limit product size and alter oxidation states. The results reveal a distinct abiogenic pathway for hydrocarbon synthesis in mantle geofluids and imply a potential mechanism for deep carbon transport from subduction zones to shallower reservoirs, influencing Earth's surface carbon budget. The work provides mechanistic insight into how extreme P–T conditions and water content shape the formation and redox state of hydrocarbons in the deep carbon cycle.

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 Fisher-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 \textit{ab initio} molecular dynamics (AIMD) simulations ($>$ 2.4 ns) to investigate the FTT synthesis in dry mixture and in aqueous solutions at 10-13 GPa and 1000-1400 K.We found that large hydrocarbon-related species containing C, O, and H($>$C$_2$) 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.

Formation of abiogenic hydrocarbons in supercritical fluids under Earth's upper mantle conditions

TL;DR

This study investigates abiogenic hydrocarbon formation in Earth's upper mantle by simulating uncatalyzed Fischer-Tropsch-type chemistry from CO and under high-pressure, high-temperature conditions with and without water, using extensive ab initio molecular dynamics. It finds that CO can polymerize into larger hydrocarbon-containing species without catalysts, with higher pressures promoting larger molecules, while aqueous environments limit product size and alter oxidation states. The results reveal a distinct abiogenic pathway for hydrocarbon synthesis in mantle geofluids and imply a potential mechanism for deep carbon transport from subduction zones to shallower reservoirs, influencing Earth's surface carbon budget. The work provides mechanistic insight into how extreme P–T conditions and water content shape the formation and redox state of hydrocarbons in the deep carbon cycle.

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 Fisher-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 \textit{ab initio} molecular dynamics (AIMD) simulations ( 2.4 ns) to investigate the FTT synthesis in dry mixture and in aqueous solutions at 10-13 GPa and 1000-1400 K.We found that large hydrocarbon-related species containing C, O, and H(C) 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.

Paper Structure

This paper contains 6 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Results
  3. Discussion
  4. Conclusion
  5. Ab initio molecular dynamics (AIMD) simulations
  6. Oxidation state

Figures (5)

  • Figure 1: Carbon-carbon (a) and carbon-hydrogen (b) radial distribution functions for mixtures of CO, H$_2$ and H$_2$O.
  • Figure 2: Reaction products in mixtures of CO, H$_2$ and H$_2$O at different P-T conditions. In addition to these products, a substantial amount of CO, H$_2$, and H$_2$O remained.
  • Figure 3: Fraction of total carbon in C$_n$ molecules produced in mixtures of CO, H$_2$ and H$_2$O. $n$ is the number of carbon atoms in the molecule.
  • Figure 4: Formation of 2,3-hydroxy-4-oxobut-2-enoic acid from CO, H$_2$ and H$_2$O at 13 GPa and 1400 K.
  • Figure 5: The mean oxidation state of carbon atoms in the different mixtures of CO, H$_2$ and H$_2$O. The hatched bars show data at 13 GPa, and the solid bars show data at 10 GPa. Temperature is indicated above the bars, and composition is indicated below the bars. The local charge on C atoms and their bonded neighbors was determined using the maximally localized Wannier function centers to localize electrons Marzari2012Maximally, so that the oxidation state of carbon could be deduced. The error bars show the standard deviation of the mean oxidation state at each time step in simulations.