The relative abundances of resolved l2CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases

Abstract

We report measurements of resolved 12 CH 2 D 2 and 13 CH 3 D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12 CH 2 D 2 from 13 CH 3 D provides unprecedented insights into the origin and evolution of CH 4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ 12 CH 2 D 2 and Δ 13 CH 3 D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH 4/ H 2 D/H exchange, underscoring the importance of reliable thermometry based on the CH 4 molecules themselves. Where Δ 12 CH 2 D 2 and Δ 13 CH 3 D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis versus biological processes. Deficits in 12 CH 2 D 2 compared with equilibrium values in CH 4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13 CH 3 D that accompany the low 12 CH 2 D 2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ 12 CH 2 D 2 values are a key tracer of microbial recycling.

Methane clumped isotopes: Progress and potential for a new isotopic tracer

“…Given the low natural abundance of D, the combined mass-18 ion current is primarily (~98%) determined by the abundance of 13 CH 3 D (Stolper et al, 2014a). More recently, a prototype version of a larger-radius highresolution isotope ratio mass spectrometer, the Nu Instruments Panorama that can routinely resolve 13 CH 3 D and 12 CH 2 D 2 was developed (Young et al, 2016;Young et al, 2017). A newer, production version of the Thermo Ultra employs an improved beamfocusing and detection design that also allows it to resolve the two mass-18 isotopologues .…”

The geochemistry of methane isotopologues

“…The ∆ 13 CH 3 D data indicated apparent equilibrium temperatures averaging 153°C. These temperatures are much higher than the <50°C temperatures at which abiotic methane was inferred by Etiope et al (2011b) (Young et al, 2017). These data indicate that methane carrying clumped isotopologue abundances similar to those of typical thermogenic gases probably comprises a large fraction of the Chimaera seep gas and that such gases may contribute more to the seepage flux than was previously suggested.…”

“…5.1) that shows several insights into the origin and fate of the methane stable isotopologues in the environment. As of the time of writing, methane clumped isotoplogue data have appeared in at least a dozen articles Stolper et al, 2014a;Stolper et al, 2014b;Inagaki et al, 2015;Stolper et al, 2015;Wang et al, 2015;Douglas et al, 2016;Lopes et al, 2016;Wang et al, 2016;Young et al, 2016;Whitehill et al, 2017;Young et al, 2017). The ∆ 13 CH 3 D data were shown to be independent of and complementary to δ 13 C and δD.…”

“…Figure 5.5 | Evolution of CH 4 isotopologue ratios in closed-system, unidirectional bond-breaking processes. Predictions are derived from models and/or data presented by the MIT and UCLA teams (Wang et al, 2016;Whitehill et al, 2017;Young et al, 2017). Calculations used the estimated weighted average of modern sources of atmospheric methane as the starting point.…”

“…There are now several studies which have investigated D/H ratios of methane and other hydrocarbons as a function of maturity (Sackett, 1978;Berner et al, 1995;Sackett and Conkright, 1997;Tang et al, 2005;Ni et al, 2011). However, kinetic isotope effects involving hydrogen addition or abstraction are often large and by themselves do not explain the geologically-reasonable apparent equilibrium temperatures of~150 to 220°C obtained for reservoir gases that have been studied for their clumped isotopologue compositions (Stolper et al, 2014a;Stolper et al, 2015;Wang et al, 2015;Young et al, 2017). There is also evidence that δD values of CH 4 approach values expected for isotopic equilibrium between CH 4 and H 2 O in formation waters at temperatures characterizing reservoirs and/or source rocks (~150 to 250°C) (Clayton, 2003;Wang et al, 2015), although findings of insignificant hydrogen exchange occurring under these conditions also exist (Yeh and Epstein, 1981).…”