Clumped isotopes tracing methane cycle and budget: a review

Methane (CH₄) is both a major energy resource and a key greenhouse gas in the Earth's carbon cycle. The clumped isotope geochemistry of methane (Δ¹³CH₃D and Δ¹²CH₂D₂) offers a unique and effective tool for understanding methane sources and sinks. In this work, we comprehensively compiled a global methane clumped isotopes dataset (n = 1061). Combined with our own measured data (n = 26) and machine learning predictions, we discuss the efficacy of clumped isotope analyses of CH₄ to infer methane origin mechanisms and to constrain post-generation processes. Insights gleaned through field observations, laboratory-controlled experiments and geochemical modelling allow an isotopologue-scale reconstruction of the biogeochemical methane cycle and its evolution through transformation (or gas migration) fractionation. The most up-to-date evidence suggests that equilibrated and disequilibrium clumped isotope compositions record reservoir geothermal events and subsequent microbial alteration, as well as kinetically-driven methane metabolism, in the low-temperature surface Earth environment. Modern estimates of the atmospheric methane budget, incorporating source and sink contributions through both forward and reverse constraints, are essential for leveraging clumped isotopes to gain a deeper understanding of Earth system dynamics. Future research that expands the observation of methane clumped isotope data on a broader scale and integrates these findings into methane cycle modelling could provide crucial insights into the potential impacts of anthropogenic influences on the global carbon cycle and climate change.