Clumped Isotopologues of Methane: Measurements and Modelling

Abstract

Methane (CH4) is one of the most potent greenhouse gases, significantly contributing to global climate change. Although its atmospheric concentration is much lower than that of carbon dioxide, the higher global warming potential of methane makes it a critical component in climate system studies. Accurate identification and quantification of its sources are crucial for understanding the methane cycle and mitigating its impact on the atmosphere.

Clumped isotopologues of methane have emerged as additional tracers for studying the atmospheric methane budget. This thesis provides novel insights into the clumped isotope composition of methane (CH4), emphasising method development, current and historical atmospheric CH4 composition and biogenic CH4 production pathways. A robust analytical system for extracting and measuring the complete set of single and double substituted isotopologues of CH4 was successfully developed, ensuring high precision and accuracy across diverse sample types. From our first direct measurements of the atmospheric 13CH3D and 12CH2D2, we conclude that the previously known source composition and sink fractionations must be revised.

Following this, our examination of firn air samples from Greenland revealed a significant change in clumped isotopes over the past three decades, offering new constraints on the Kinetic Isotope Effects (KIEs) of atmospheric sinks and insight into the effect of source-sink disequilibrium on the temporal evolution of Δ12CH2D2, in particular. Additionally, our investigation of biogenic CH4 production in marine sediments unveils distinct isotopic signatures associated with different methanogenic pathways, enhancing our understanding of microbial contributions to the isotopic composition of atmospheric CH4. Overall, this thesis contributes to the clumped isotope measurements for source signatures and variability in the atmosphere.