Clumped Isotope Thermometry of Geologic Methane (13CH3D) using Tunable Laser Mid-Infrared Spectroscopy

Methane is both an alternative energy source as well as a significant greenhouse gas, and holds the potential for rapid release to the atmosphere, possibly triggering abrupt climate change in the past and in the future. The majority of methane on the Earth is biogenic, originating from microbial methanogenesis, or thermogenic sourced from previously formed biogenic organic materials. Methane can be also produced abiogenically during serpentinization and even mantle-sourced methane has been implicated. Carbon (¹³C/¹²C) and hydrogen (D/H) isotope ratios of methane and associated short chain hydrocarbons provide critical information about the abiogenic/biogenic origin of methane but data can be inconclusive. We have developed and tested a Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) Instrument to be used for precise measurements of the abundance of doubly isotope-substituted methane (¹³CH₃D). The TILDAS instrument measures direct absorption in the mid-infrared (~ 8 μm) region using continuous wave quantum cascade laser combined with a 76 m pathlength astigmatic absorption cell. Initial test result indicates the precision for ¹³CH₄, ¹²CH₃D and ¹³CH₃D for 0.2 ‰ or better for comparison between two reference gases. Accuracy of the methods for δ¹³C and δD is evaluated by comparing measurements by conventional isotope ratio mass spectrometry. Calibration of clumped isotope scale (δ¹³CH₃D) is underway using methane produced at various temperatures. Following an isotope exchange reaction (¹³CH₄ + ¹²CH₃D ↔ ¹³CH₃D + ¹²CH₄), precise measurements of ¹³CH₃D abundance is expected to provide new and critical information about the temperature at which methane was formed (or thermally equilibrated). Biogenic origin becomes highly unlikely, for example, if the estimated temperature is higher than 120°C, i.e., current high-temperature limit of microbial methanogenesis. Although significant questions remain regarding isotope exchange kinetics, and clumped isotope systematics during oxidation of methane, the development of TILDAS instrument, along with high resolution-isotope ratio mass spectrometry instrument, is the first step towards answering these questions and establish clumped methane isotopologue as a robust diagnostic tool for biogenecity of methane. Relatively small footprint of the TILDAS instrument may offer unique opportunities for future flight missions.