Nitrogen 15–enriched Precambrian kerogen and hydrothermal systems
Bibliographic record
Abstract
Nitrogen isotope data are reported for kerogen from Precambrian black shales at prehnite‐pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems. Nitrogen in micas was acquired from N‐bearing aqueous metamorphic hydrothermal fluids generated, from breakdown of sedimentary kerogen and dehydration of K‐silicates, at deeper crustal levels. The 2.7 Ga kerogens and hydrothermal K‐micas yield δ15N values of 15–24‰, compared to existing data of 2–6‰ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate δ15N of 7–12‰, implying a secular decrease in δ15N of shale kerogen. The 15N‐enriched nitrogen in Archean shales and hydrothermal vein systems cannot be caused either by N isotopic shifts accompanying metamorphism or Rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of δ15N with N, C/N ratios, or metamorphic grade. The magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1‰ to greenschist and about 3‰ to amphibolite facies. Furthermore, 15N‐enriched values cannot stem from long‐term preferential diffusional loss of 14N as samples were selected from terranes where 40Ar/39Ar ages are within a few million years of concordant U‐Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15N‐enriched values stem from a different N cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from −6 to 30‰. Given the temporal association of chert‐banded iron formation with mantle plumes, the range is consistent with mixing between mantle N2 of −5‰ and the enriched marine reservoir identified in this study. We attribute the secular trend from Archean kerogen of 15–24‰ to Phanerozoic kerogen averaging 3–4‰ to a secondary atmosphere derived from CI‐chondrite‐like material and comets with δ15N of 30–42‰. Shifts of δ15N to its present values of 0‰ can be accounted for by a combination of sequestration of atmospheric N2 into sedimentary rocks and mantle degassing of −5‰ mantle N2.
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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) | 0.001 | 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".