Abstract: Carbon isotope chemostratigraphy in the Late Ordovician in Arctic Canada: the signal of the Hirnantian glaciation
Bibliographic record
Abstract
Four sections through late Ordovician to earliest Silurian strata in the central Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (δCorg) and whole-rock carbonate (δCcarb) fractions. The lithologies and graptolite faunas provide good constraints on the age and depositional environment of these strata. δCorg data appear to provide a signal that mainly refl ects chemical changes in the seawater. However, sediment reworking and diagenesis appears to have had a signifi cant infl uence on the δ Ccarb signal. Results show that a positive δCorg excursion of 3–6 ‰ begins just below the base of the Hirnantian Stage and peaks in the lower part of the N. extraordinarius Biozone of lower Hirnantian. This is followed by an interval of reduced δ13C values and a second peak of similar magnitude, which occurs in the lower N. persculptus Biozone (upper Hirnantian). These peaks appear to correlate well with episodes of glacial expansion described from West Africa and this is supported locally by the relationship between the isotope shifts and lithologic evidence for sea level fall. Global correlation of δ13C curves suggests that the timing of positive excursions is not synchronous in all regions of the world. In particular, the lower Hirnantian peak seen in Arctic Canada and some other areas appears to be reduced in some circum-Iapetus regions, where peak values occur in later Hirnantian time. The hypothesis that one regional δ13C curve can reliably serve as a benchmark for high-resolution, global correlation, is not supported by these observations. These data suggest that an important factor in controling the δ13C values in Late Ordovician epeiric and continental margin seas was the variation in rates of weathering of carbonate platforms, exposed during the glacio-eustatically controled sealevel fall. This caused the isotope value of the C-weathering fl ux to shift towards the 13C-enriched carbonate end-member, increasing the δ13C value of carbon transported by rivers to both epeiric seas and the oceans. Differences in magnitude between Hirnantian δ13C excursions in shallowerand deeper water parts of epeiric sea basins, as well as between different regions, may be explained by water mass differentiation between those regions and differences in regional sea level histories.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.002 |
| Science and technology studies | 0.002 | 0.000 |
| 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".