Isotopic constraints on the peak of the Early Paleozoic Icehouse
Bibliographic record
Abstract
Abstract While the intensification of the Early Paleozoic Icehouse is commonly cited as the main driver of the Late Ordovician mass extinction, we lack high-resolution, stratigraphically constrained climate records to test this hypothesis. Here, we develop a high-resolution climate record for the Late Ordovician by applying stable isotope geochemistry (δ18O, δ13C; n = 81) and carbonate clumped isotope paleothermometry (Δ47; n = 45) to fossils from the stratigraphically expanded Ellis Bay Formation on Anticosti Island (Canada). Focusing our analysis on fossils that primarily experienced closed-system alteration, we identify two distinct phases of increasing fossil δ18OVPDB values: a moderate increase of 1.0‰–1.5‰ across the Katian/Hirnantian boundary and a larger increase of 2.5‰–4.5‰ in the middle to late Hirnantian. Only the latter is associated with Δ47 evidence for cool tropical sea surface temperatures, and based on its sequence stratigraphic context immediately overlying a regional subaerial unconformity, we interpret this excursion as reflecting the earliest stages of transgression during the waning of the Hirnantian Glacial Maximum. This revises the conclusions of previous paired stable and clumped isotope studies, which argued that the major drop in sea surface temperatures occurred at the Katian/Hirnantian boundary. When integrated with patterns of faunal turnover from Anticosti Island, our conclusion that major tropical cooling and maximum ice volumes did not occur until the middle to late Hirnantian suggests that an apparent pulse of extinction near the Katian/Hirnantian boundary may be in part a stratigraphic artifact generated by widespread glacio-eustically forced unconformities and facies shifts.
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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 | 0.001 | 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) | 0.002 | 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".