Potential glacial‐interglacial changes in stable carbon isotope ratios of methane sources and sink fractionation
Bibliographic record
Abstract
Past atmospheric methane emissions can be constrained by δ13CH4 records from ice cores only if changes to source δ13CH4 signatures and sink isotope effects with varying environmental and climatic conditions are accurately known. We present reconstructions of such changes based on paleodata and recent systems observations. The results are specific for budget scenarios and are reported here for two alternative types of budgets, one including aerobic methane emissions (AMP) from plants and the other type without AMP. Shifting atmospheric δ13CO2 potentially led to 13CH4 enrichment by 0.8‰ in the preindustrial Holocene (PIH) (∼150–11,000 years (a) B.P.) and ∼0.3–0.6‰ at the Last Glacial Maximum (LGM) (∼18,000 a B.P.) relative to today. Differing distribution of C3 and C4 plant precursor material may account for 13CH4 enrichment of ∼0.4‰ (PIH) and ∼0.6–1.1‰ (LGM). Temperature‐dependent fractionation and varying methanogenic pathways in wetlands may lead to atmospheric 13CH4 depletion by ∼0.1–1.2‰. Sink fractionation today (7.4‰) is higher than during the PIH (∼7.0‰) and the LGM (∼5.7‰). The cumulative effect of all processes is ∼0.8‰ 13CH4 enrichment in the PIH and ∼1–1.2‰ 13CH4 depletion at the LGM. Budget reconstructions will be inaccurate if these changes are not included.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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 source (direct Gemma or distilled Codex), 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".