Methane oxidation and formation of metastable organics under hydrothermal and supercritical water conditions

Dr. Dionysis Foustoukos

Geophysical Lab, Carnegie Institution of Washington, USA

A series of experiments have been conducted to investigate the kinetic rate of abiotic methane oxidation in the CH4-O2-H2O system at subseafloor hydrothermal conditions (200-350oC, 30 MPa). Results indicate that complete methane oxidation to CO2 proceeds in significantly lower kinetic rate than the decomposition rates of indermetiate carbon species (e.g. HCHO, CH3OH, HCOOH). However, this might not be the case at supercritical water conditions (T>500oC), where for example, decarboxylation of HCOOH has been shown to be slower than CH4(aq) oxidation. Most importantly, our data suggest that the rates of oxidation process approximate the extent of CO2(aq) conversion to CH4(aq) even when it’s Fischer-Tropsch catalyzed at an ultimate rate by FeNi under highly reducing conditions. This has important implications on carbon cycling and the habitability of hydrothermal environments associated with the anoxic/oxic boundaries established during seawater circulation in the upper crust. Experimental results also reveal formation of metastable carbon species. For example, trace concentrations of dissolved CO(aq) and H2(aq) were observed at 350oC-30MPa. Thus, it appears that partial CH4(aq) oxidation might occurred leading to release of H2(aq) which is incorporated into the water-gas-shift reaction to produce metastable CO(aq). Both species are present only at the very early stages of the experiments suggesting that strongly disequilibria conditions might be responsible for triggering partial CH4(aq) oxidation. Furthermore, partial oxidation of CH4(aq) and formation of methylene radicals could play a key role in polymerization reactions towards synthesis of higher chain alkanes at elevated temperatures and pressures (500-800oC; 1 GPa). This might also have an important effect on 13C/12C and D/H fractionation patterns. Polymerisation reactions enhance elimination of the light 1H and heavy 13C isotopes, producing hydrocarbons that exhibit an inverse isotopic trend in 13C while being D-enriched relative to the isotopic composition of the precursor CH4. Accordingly, results from a series of piston-cylinder experiments will also be presented, accessing the evolution of dissolved CH4 in the presence of Fe-bearing oxides to impose better constrains on the extent of partial methane oxidation and polymerization to complex organics under supercritical water and oxidizing redox conditions.