Methane sources and sinks in continental sedimentary systems: New insights from paired clumped isotopologues 13CH3D and 12CH2D2

Identifying thermogenic and microbial methane in deep water Gulf of Mexico Reservoirs

“…Interestingly, our model does not agree with an equilibrated biogenic  12 CH2D2 endmember but rather indicates the microbial endmember must be unequilibrated at -20‰. A unequilibrated microbial signature is not surprising and has been seen before in natural and laboratory microbial cultures (Young et al, 2017;Giunta et al, 2019). Future work is required to characterize the  12 CH2D2 signature of this disequilibrated microbial  12 CH2D2 endmember.…”

“…The sample from Galveston that appears based on context and other geochemical properties to be primarily thermogenic in origin (sample Galveston B7D) has a  12 CH2D2 value of 2.5‰, consistent with thermodynamic equilibrium at temperatures consistent with thermogenic gas production. Thus, it meets the criteria of an equilibrated thermogenic gas whose clumped isotope apparent temperature reflects the geophysical temperature of gas generation (a common finding for analyses of ∆ 12 CH2D2 and ∆ 13 CH3D from conventional thermogenic gases; (Young et al, 2017;Giunta et al, 2019)). The other three samples selected for ∆ 12 CH2D2 measurement were chosen from suites of gases having geochemical properties suggesting the presence of a component of microbial gas, including relatively low  13 C and/or D and elevated gas dryness, and ∆18 values suggesting apparent temperatures of less than 100 ˚C.…”

“…Initial work on the characterization of thermogenic and microbial methane from a range of natural environments indicated that clumped methane isotopologues could be used to determine formation or re-equilibration temperatures of methane Inagaki et al, 2015). Later, kinetic isotope effects in clumped isotopologues were noted both in microbial and thermogenic systems (Wang et al, 2015;Young et al, 2017;Douglas et al, 2017;Shuai et al, 2018a;Gruen et al, 2018;Ash et al, 2019;Giunta et al, 2019). Laboratory cultured methanogens and microbial methane from certain natural environments have been reported to have a kinetic isotope effect (KIE) (Wang et al, 2015;Stolper et al, 2015;Young et al, 2017;Gruen et al, 2018;Giunta et al, 2019).…”

“…Later, kinetic isotope effects in clumped isotopologues were noted both in microbial and thermogenic systems (Wang et al, 2015;Young et al, 2017;Douglas et al, 2017;Shuai et al, 2018a;Gruen et al, 2018;Ash et al, 2019;Giunta et al, 2019). Laboratory cultured methanogens and microbial methane from certain natural environments have been reported to have a kinetic isotope effect (KIE) (Wang et al, 2015;Stolper et al, 2015;Young et al, 2017;Gruen et al, 2018;Giunta et al, 2019). These KIEs tend to be correlated with hydrogen isotope disequilibrium between methane and water (in the direction of strong D depletion in product methane) and have been interpreted to be a result of a low degree of reversibility in the enzymatic reactions that allow methanogens to produce methane (Wang et al, 2015;Stolper et al, 2015), or as an artifact due to residues of anaerobic oxidation of methane (Giunta et al, 2019).…”

Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes

“…CH 4 sampled from freshwater ecosystems and from methanogen culture experiments exhibits low or negative Δ 18 values that indicate strong kinetic isotope effects (Douglas et al, 2016;Giunta et al, 2019;Gruen et al, 2018;Stolper et al, 2015;Wang et al, 2015;Young et al, 2017). Isotopic models of hydrogenotrophic methanogenesis predict that this kinetic isotope effect is controlled by the reversibility of the enzymatic reactions of methanogenesis (Stolper et al, 2015;Valentine et al, 2004;Wang et al, 2015).…”