Global inventory of doubly substituted isotopologues of methane (Δ <sup>13</sup> CH <sub>3</sub> D and Δ <sup>12</sup> CH <sub>2</sub> D <sub>2</sub> )
Bibliographic record
Abstract
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 investigate the mechanisms underlying global atmospheric trends in CH4. 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://doi.org/10.5285/51ae627da5fb41b8a767ee6c653f83e6, Defratyka et al., 2025). 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, agricultural practices and landfills. 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, with only single study of photochemical oxidation via OH and Cl, which constitutes 5 % of the laboratory experiments entries. 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 impact on understanding global CH4.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.003 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.003 |
| Science and technology studies | 0.001 | 0.006 |
| Scholarly communication | 0.000 | 0.002 |
| Open science | 0.004 | 0.004 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".