Speaker: Daniel Stolper (Caltech)
Methane is a critical greenhouse gas, energy resource, and product of microbial metabolisms. Despite its importance in the carbon cycle, distinguishing the sources of methane in natural systems is challenging due to gas migration and potentially overlapping chemical and isotopic characteristics of biogenic and thermogenic sources. Knowledge of the formation temperatures of methane would help distinguish biogenic and thermogenic methane in nature; however, formation temperatures of naturally occurring methane are generally unknown. I will present results of a new geothermometer that can be used to measure methane formation temperatures. It is based on the measurement of rare, multiply substituted (‘clumped’) methane isotopologues (13CH3D and 12CH2D2), the abundances of which are functions of temperature for isotopically equilibrated systems. I will describe the calibration of this clumped isotope geothermometer and present measurements from a range of experimental and natural thermogenic and biogenic gases. These results suggest that clumped-isotope-based temperatures generally represent formation temperatures for natural samples. This was not necessarily an expected result: It has implications for our understanding of the chemistry of how methane forms in nature and the sources of methane in various environments, and it can be used to test various methane formation models.