Changes in the Isotopic Signature of Atmospheric Nitrous Oxide and Its Global Average Source During the Last Three Millennia
Bibliographic record
Abstract
Abstract Nitrous oxide (N2O) is a strong greenhouse gas whose mole fraction in the atmosphere has increased over the industrial period. We present a new set of isotope measurements of N2O in air extracted from ice cores covering the last 3,000 years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (267 ± 1) nmol/mol and average tropospheric N2O isotopic values of δ15NavPI = (9.5 ± 0.1)‰, δ18OPI = (47.1 ± 0.2)‰, δ15NαPI = (17.8 ± 0.4)‰, and δ15ΝβPI = (1.2 ± 0.4)‰. From PI to modern times all isotope signatures decreased with a total change of δ15Nav = (−2.7 ± 0.2)‰, δ18O = (−2.5 ± 0.4)‰, δ15Nα = (−2.0 ± 0.7)‰, and δ15Νβ (−3.5 ± 0.7)‰. Interestingly, the temporal evolution is not the same for δ15Nav and δ18O. δ18O trends are relatively larger during the early part, and δ15Nav trends are larger during the late part of the industrial period, implying a decoupling of sources over the industrial period. Using a mass balance model, we determined the isotopic composition of the total average N2O source. Assuming that the total present source is the sum of a constant natural source and an increasing anthropogenic source, this anthropogenic source has an isotopic signature of δ15Navsource,anthrop = (−15.0 ± 2.6)‰, δ18Osource,anthrop = (30.0 ± 2.6)‰, δ15Nαsource,anthrop = (−4.5 ± 1.7)‰, and δ15Nβsource,anthrop = (−24.0 ± 8.4)‰. The 15N site preference of the source has increased since PI times, which is indicative of a relative shift from denitrification to nitrification sources, consistent with agricultural emissions playing a major role in the N2O increase.
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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 | 0.000 |
| 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 | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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 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".