Global inventory of doubly substituted isotopologues of methane (Δ¹³CH₃D and Δ¹²CH₂D₂)

Abstract. Measurements of methane (CH₄) molecules containing two rare isotopes (¹³CH₃D and ¹²CH₂D₂), also termed doubly substituted or ‘clumped’ isotopologues, have the potential to provide two additional isotopic dimensions to help investigating mechanisms producing the recent global trends and CH₄ budget over decadal timescale. In this work, we summarise the current state of research on doubly substituted CH₄ isotopologues, with an emphasis on compiling results of all relevant work. The database comprises 1475 records compiled from the literature published until April 2025 (https://dx.doi.org/10.5285/51ae627da5fb41b8a767ee6c653f83e6). For field samples, 40 % of records were sourced from natural gas reservoirs, while microbial terrestrial (e.g., agriculture, lake, wetland) samples account only for 12.5 %. Lakes samples contribute 75 % to collected microbial terrestrial samples. There is limited or no representation of samples coming from significant microbial CH₄ sources to the atmosphere, like wetlands, agriculture and landfill. To date, laboratory experiments were mostly focused on microbial (28 % of samples from laboratory experiments) and pyrogenic (15 %) methanogenesis or anaerobic (16 %), and aerobic (8 %) CH₄ oxidation, and only a single contribution to studies of photochemical oxidation via OH and Cl (5 %). The distinct ranges of Δ¹³CH₃D and Δ¹²CH₂D₂ values measured in these studies suggests their potential to improve our understanding of atmospheric CH₄. This work provides an overview of the major gaps in measurements and identifies where further studies should be focussed to enable the highest immediate impact on understanding global CH₄.