The Clumped Isotope Composition of Biogenic Methane.

Abstract:

The excess or lack of ¹³CH₃D, a doubly substituted (“clumped”) isotopologue of methane, relative to that expected for a random distribution of isotopes across molecules, is a function of the processes that generated the methane. For high-temperature thermogenic methane, which typically achieves internal equilibrium, an excess of ¹³CH₃D is expected and the amount of excess can serve as a thermometer. In contrast, biogenic methane often – though not always – has a smaller excess of clumped isotopologues, and sometimes even a deficit of clumped species (“anti-clumped”). The effect presumably arises from kinetic isotope effects accompanying enzymatic reactions in the methanogenic pathway, though the particular reaction(s) has not yet been positively identified. The decrease in clumping is also known to correlate with both the reversibility of the pathway and the methane flux.

In this talk, we will present recent data bearing on the origin and utility of biologic fractionations of clumped isotopologues in methane. Preliminary data suggest that methane deriving from the fermentative pathway is enriched in D-bearing isotopologues, at the same level of clumping, relative to that derived from the CO₂-reductive pathway. This property offers another potential means to distinguish biogenic methane sources in the environment. Recently, we have also begun to measure the ¹²CH₂D₂ isotopologue, for which equilibrium and kinetic isotope effects are predicted to be distinct from ¹³CH₃D. Preliminary data suggest that the combination of both doubly-substituted isotopologues will be especially useful for disentangling mixtures containing biogenic gas.