Fast Abiotic Production of Methane at Temperatures Below 100°C

Abstract:

Fischer-Tropsch Type (FTT) reactions, e.g., the Sabatier synthesis between H₂ and CO₂, are considered a main source of abiotic methane on Earth and likely on other planets. Several laboratory FTT experiments demonstrated abiotic CH₄ production at temperatures above 200°C, by using Fe, Ni or Cr catalysts, simulating hydrothermal conditions in peridotite-hosted systems in mid-ocean ridges. Nevertheless, at least on laboratory experiment time-scale, Fe-Ni-Cr catalysts do not support CH₄ generation at T<100°C, such as those of land-based serpentinization systems. We have recently reported rapid production of considerable amounts of CH₄ (>800 ppmv in 155 mL bottles after 1 day) via Sabatier reaction at 90, 50 and 25°C, using small concentrations of non-pretreated ruthenium (Ru) equivalent to those occurring in chromitites in continental ultramafic rocks (Etiope & Ionescu, 2014; Geofluids, doi:10.1111/gfl.12106). We have repeated the experiments by using ¹³C-enriched CO₂ and we confirm fast production of CH₄at percentage levels. The experiments performed so far show that:

1. considerable amounts of CH₄can be produced in dry conditions below 100°C with small quantities of Ru;

2. under the same experimental conditions (<100°C), Fe, Ni and Cr oxides do not produce CH₄;

3. low T Sabatier reaction can produce CH₄ with a large C isotope fractionation between CO₂ and CH₄, leading to relatively “ light” (¹³C-depleted) CH₄, resembling microbial gas;

4. the CO₂-CH₄isotope separation decreases over time and by increasing the temperature;

5. minor amounts of C₂-C₆hydrocarbons are also generated.

Our laboratory data are compatible with the isotopic patterns of CH₄ naturally occurring in land-based seeps and springs. Our experiments suggest that Ru-enriched chromitites could potentially generate CH₄ at low T. Since Ru is reported in Martian meteorites, low T abiotic CH₄ production on Mars via Sabatier reaction cannot be excluded (Etiope et al. 2013, Icarus, 224, 276-285).