The Homerian (late Wenlock, Silurian) carbon isotope excursion from Perunica: Does dolomite control the magnitude of the carbon isotope excursion?
Bibliographic record
Abstract
The δ13Ccarb records from two geographically close sections of the shallow-water Kozel Limestone Member (late Wenlock Motol Formation; Perunica microplate) significantly differ in the magnitude of the Homerian carbon isotope excursion as well as in their dolomite content. The present paper tests a hypothesis as to whether a difference of about 2‰ in the magnitude of the δ13Ccarb anomaly is caused by the different content of dolomite, which could be enriched in both 13C and 18O relative to coexisting calcite, as has been suggested by experimental data. The new data obtained reveal that the δ18O composition of calcite and dolomite was probably controlled by the pore-fluid composition during limestone diagenesis and that both carbonates seem to be close to equilibrium in oxygen isotope composition. On the other hand, the δ13C values of dolomite are similar to those of calcite, and thus the carbon isotope composition of both carbonates was probably determined by the precursor carbonate composition. Moreover, the values of δ13Cdolomite/δ13Ccalcite ratios as well as their variability suggest that both calcite and dolomite did not reach equilibrium in their carbon isotope composition. Whole-rock, mineralogical, and C and O isotope data clearly show that dolomite is not the cause for the differences in magnitudes of the δ13C records observed between dolomite-bearing and dolomite-lacking shallow-water limestone successions. The question as to whether the observed differences in the δ13C records of the studied sections across the Homerian carbon isotope excursion were controlled by the dependence of seawater composition on water depth and (or) proximity to shoreline or if the δ13C values were later affected by secondary processes during limestone diagenesis is still unsolved.
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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 | 0.001 |
| Scholarly communication | 0.001 | 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".