The influence of carbon source on abiotic organic synthesis and carbon isotope fractionation under hydrothermal conditions

Abstract

A series of laboratory experiments were performed to investigate the relative contributions of CO and other single-carbon compounds to abiotic synthesis of organic compounds in hydrothermal environments. Experiments were conducted by heating aqueous solutions of CO, CO 2, HCOOH, or CH 4 at 250 °C under reducing conditions, and observing production of CH 4 and other hydrocarbons. Native Fe was included in the experiments as a source of H 2 through reaction with water and as a potential catalyst. Experiments with CO or HCOOH as the carbon source resulted in rapid generation of CH 4 and other hydrocarbons that closely resembled typical products of Fischer–Tropsch organic synthesis. In contrast, experiments using CO 2 or CH 4 as the carbon source yielded no detectable hydrocarbon products. Carbon isotope measurements of reaction products from the CO experiments indicate that the CH 4 and other hydrocarbons were substantially depleted in 13C, with CH 4 δ 13C values 30 to 34‰ lighter than the initial CO. Most of the fractionation apparently occurs during attachment of CO to the catalyst surface and subsequent reduction to surface-bound methylene. The initial step in polymerization of these methylene units to form hydrocarbons involves a small, positive fractionation, so that ethane and ethene are slightly enriched in 13C relative to CH 4. However, subsequent addition of carbon molecules to the growing hydrocarbon chain proceeds with no net observable fractionation, so that the isotopic compositions of the C 3+ light hydrocarbons are controlled by isotopic mass balance. This result is consistent with a previously proposed model for carbon isotopic patterns of light hydrocarbons in natural samples. The abundance and isotopic composition of light hydrocarbons produced with HCOOH as the carbon source were similar to those generated with CO, but the isotopic compositions of non-volatile hydrocarbons diverged, suggesting that the higher hydrocarbons were formed by different mechanisms in the CO and HCOOH experiments. The experiments indicate that CO, and possibly HCOOH, may be critical intermediates in the abiotic formation of organic compounds in geologic environments, and suggest that the low levels of these compounds present in most hydrothermal systems could represent a bottleneck restricting the extent of abiotic organic synthesis in some circumstances.