Clumped Methane Isotopologue Temperatures of Microbial Methane

Abstract:

We will report the abundance of ¹³CH₃D, a clumped isotopologue of methane, in microbial methane sampled from natural environments. They yield some expected and some unexpected results reflecting both equilibrium and kinetic isotope effects controlling the abundance of ¹³CH₃D in low temperature environments. The four isotopologues of methane (¹²CH₄, ¹³CH₄, ¹²CH₃D and ¹³CH₃D) were measured by a tunable infrared spectroscopy method at a precision of 0.2‰ and accuracy of 0.5‰ (Ono et al., 2014).

Similar to carbonate clumped isotope thermometry, clumped isotopologues of methane become more stable at lower temperatures. The equilibrium constant for the isotope exchange reaction ¹³CH₄ + ¹²CH₃D ⇌ ¹³CH₃D + ¹²CH₄ deviates from unity by +6.3 to +3.5 ‰ for methane equilibrated between 4 and 121 °C, a range expected for microbial methanogenesis. This would be measurably-distinct from a thermogenic methane signal, which typically have apparent ¹³CH₃D-based temperatures ranging from 150 to 220 °C (+3.0 to +2.2 ‰ clumped isotope effect; Ono et al., 2014; Stolper et al. 2014). Marine samples, such as methane clathrates and porewater methane from the Cascadia margin, have ¹³CH₃D-based temperatures that appear to be consistent with isotopic equilibration at in situ temperatures that are reasonable for deep sedimentary environments. In contrast, methane from freshwater environments, such as a lake and a swamp, yield apparent temperatures that are much higher than the known or inferred environmental temperature. Mixing of two or more distinct sources of methane could potentially generate this high temperature bias. We suggest, however, that this high-temperature bias likely reflects a kinetic isotope fractionation intrinsic to methanogenesis in fresh water environments. In contrast, the low-temperature signals from marine methane could be related to the slow metabolic rates and reversibility of microbial methanogenesis and methanotrophy in marine sedimentary environments. The ¹³CH₃D signals do not always correlate with δD and δ¹³C signals, indicating that an understanding of the systematics controlling the abundance of ¹³CH₃D in the environment will add another dimension to methane isotope fingerprinting.

Reference: Ono et al., 2014, Analytical Chemistry, 13, 6487; Stolper et al., 2014, Science, 344, 1500