Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4H2H2O from 3 to 200 °C

The stable isotopic composition of methane (CH4) is commonly used to fingerprint natural gas origins. Over the past 50 years, there have been numerous proposals that both microbial and thermogenic CH4 can form in or later attain hydrogen equilibrium with water (H2O) carbon dioxide (CO2). Evaluation such requires knowledge fractionation factors between H2O CO2 at temperatures where are found environment, which generally less than 200 °C. Experimental determinations these only available above °C, requiring extrapolation results beyond calibrated range use theoretical calculations lower temperatures. Here, we provide a calibration factor for (H2) (D?CH4(g)–H2(g)) based on experiments using ?-Al2O3 Ni catalysts from 3 Results were regressed as 2nd order polynomial 1000 × lnD?CH4(g)–H2(g) vs. 1/T (K?1) yielding: 1000×lnD?CH4(g)-H2(g)=3.5317×107T2+2.7749×105T-179.48 We combine this previous experimental isotope H2, H2O(g), H2O(l) an interpolatable lnD?CH4(g)–H2O(l) Our resulting 4th following equation: 1000×lnD?CH4(g)-H2Ol=-7.9443×1012T4+8.7772×1010T3-3.4973×108T2+5.4398×105T-382.05 At value our differs by 93‰ relative what would be calculated currently below its (lowest temperature °C; Horibe Craig, 1995). additionally new estimates CH4(g), H2(g), H2O(g) CH4(g) CO2(g) Path Integral Monte Carlo (PIMC) calculations. PIMC H2 agree 1:1 experiments. Finally, compile measurements CH4, CO2, various environmental systems compare observed differences isotopes those expected equilibrium. find compositions some gases marine sedimentary, coalbed, shale environments consistent CH4H2O(l) CH4CO2 In contrast, terrestrial pure culture not These explained qualitatively previously developed conceptual models link free energy gradients microorganisms degree their enzymes promote isotope-exchange reactions H2O.