Uranium Isotope Evidence for Pervasive Marine Anoxia During the Late Ordovician Mass Extinction.
Bibliographic record
Abstract
The Ordovician witnessed an explosion in marine biodiversity followed by the first of the 'big-five' Phanerozoic mass extinctions, the Late Ordovician mass extinction (LOME). The LOME consists of two discrete faunal turnovers; the first coincident with the onset of Hirnantian glaciation and the second with end-Hirnantian deglaciation. Lithologic and geochemical evidence suggests widespread marine anoxia triggered the second faunal turnover; however, these redox proxies only speak to the bottom water or porewater conditions present at the site of deposition. Uranium isotopic ratios preserved in marine carbonates record global seawater conditions due to the fact that the ocean residence time for uranium (~500 ky) is significantly longer than ocean mixing times. Bulk carbonate samples from the Upper Ordovician of Anticosti Island, Canada were analyzed to evaluate global marine redox patterns across the LOME. The Anticosti section was chosen because of its well-studied sequence and biostratigraphy and the fact that it has not been subjected to deep burial or tectonic processes. Isotopic analysis of bulk carbonates record relatively uniform values of ~0.1 per mil across the Katian-Hirnantian boundary and into the mid-Hirnantian. The values then exhibit an abrupt negative shift to ~-0.4 per mil in the Late-Hirnantian, followed by a return to values of ~0.1 per mil in the early Rhuddanian. This negative shift is roughly coincident with the second faunal turnover and is similar in magnitude to the shift reported across the end-Permian extinction. These results support earlier interpretations of widespread marine anoxia associated with the second faunal turnover; however they are at odds with recent interpretations of an intense anoxic event in the late Hirnantian-early Rhuddanian.
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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 | 0.000 |
| 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".