A Two-Dimensional Perspective on CH4 Isotope Clumping

Isotope ratios have been used extensively to trace the origins of methane gases (e.g. Schoell 1980). For this purpose, the stable isotope ratios C∕C and D/H have been paramount. These ratios refer to the atomic abundances of the rare isotopes of carbon and hydrogen relative to the more abundant isotopes, in aggregate, and inclusive of all of the methane isotopic molecular species in a sample of gas. We therefore refer to these ratios as “bulk” isotope ratios. The term “isotopologue” refers to specific isotopic versions of the molecules. For example, the “CH3D isotopologue” refers to the CH3D + CH2DH + CHDH2 + CDH3 permutations of the D-substituted isotopic species of CH4 collectively. In the geosciences, the term “clumping” denotes more than one heavy isotope in a single molecule or molecular unit (e.g. COOO + COOO + COOO in the CO3 2– moiety within the CaCO3 crystalline structure). In this chapter, the results of recent studies of the relative abundances of the clumped methane species CH3D and CH2D2 measured at the University of California, Los Angeles (UCLA) are summarized. We begin with a description of the goals of this research program. The original excitement about making use of the C–O multiply substituted isotopologue of CO2, COO + COO, derived from acid digestion of carbonate was due to the prospect of removing the various logical degeneracies that have historically plagued our interpretations of the significance of O/O (usually expressed as δO values, the per mil differences in O/O from a standard material) in carbonates (Eiler et al. 2005). Marine carbonate oxygen isotope ratios can vary in response to temperature, ice volume, or secular variations in the δO of the oceans. By using the temperature-dependent propensity of C and O to form bonds as a homogeneous (as in a single-phase as opposed to heterogeneous fractionation between two separate phases, in this case carbonate and water) thermometer, the relationship between bulk δO in the carbonate and that in the water becomes irrelevant if the goal is to deduce temperature of formation. The goal, therefore, was to develop an isotopic tracer in which the bulk isotope ratios are normalized out. The CH4 clumping project at UCLA, in collaboration with the Carnegie Institution of Science, began in 2008 with the prospects for funding by the Deep Carbon Observatory and an eye toward replicating the powerful aspects of carbonate clumping for methane.