Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation
Bibliographic record
Abstract
Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (y13Corg) and whole-rock carbonate (y 13Ccarb) fractions. The sections are well constrained biostratigra-phically using graptolites, lithostratigraphically and palaeogeographically. y13Corg data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the y13Ccarb data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive y13Corg excursion of 3–6x begins just below the base of the Hirnantian Stage and peaks in the lower part of the Normalograptus extraordinarius biozone of lower Hirnantian. This is followed by an interval of reduced y13C values and a second peak of similar magnitude, which occurs in the lower Normalograptus persculptus biozone (upper Hirnantian). These peaks appear to correlate well with episodes of glacial expansion described from West Africa. Global correlation between y13C curves suggests that the timing of peak positive excursions is not completely synchronous between different regions. In particular, the lower Hirnantian peak seen in Arctic Canada and some other areas appears to be suppressed in sedimentary successions from the circum-Iapetus region, where peak values occur in later Hirnantian time. Thus, no single, regional y13C curve can reliably serve as a benchmark for high-resolution, global correlation. These data provide support for the hypothesis that the positive y13C shifts seen in these sections and many others worldwide are
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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 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".