Variations of depositional redox conditions across the \nCambrian-Ordovician boundary (GSSP, Green Point \nFormation) in western Newfoundland: implications from \nMo-, U-, and S-isotope signatures and I/Ca ratios
Bibliographic record
Abstract
The Green Point Formation in western Newfoundland, Canada, includes the Global \nBoundary Stratotype Section and Point (GSSP) of the Cambrian-Ordovician (Є-O) boundary. \nThis formation consists mainly of slope deposits of alternating lime mudstone and shale interbeds \n(rhythmites). In this study, we conducted an analysis of the isotopic and elemental signals of Mo \nand U in the shale and limestone. Additionally, we examined the textures and bulk δ³⁴Spy signals \nof biological pyrite in the shale and investigated the I/Ca ratios of the limestone. These analyses \naim to provide a more comprehensive understanding of the depositional environments and marine \nredox conditions during the Є-O transition. \nThe analysis revealed broad ranges of the shale authigenic δ⁹⁸ (−0.40 to +3.16‰) \nand the limestone δ⁹⁸Mocarb (−0.30 to +2.34‰) values. These observations, combined with \nsignificantly low Mo concentrations and the lack of fossils and bioturbations, suggest that the \nshale and limestone were both deposited under weakly oxygenated (dysoxic to anoxic) bottom \nconditions. Furthermore, an inverse correlation between the shale δ⁹⁸Moauth and δ²³⁸Uauth values, \nsimilar to those observed in modern semi-restricted basins, indicates that the local continental \nmargin seawater was probably partially isolated from the open ocean. In accordance with this \ninference, significant inverse relationships are observed between the δ³⁴Spy values and the \ncontents of terrigenous elements (Al, Th, and ΣREE), indicating that terrestrial inputs might have \ninfluenced the regional seawater sulfate level. Interestingly, parallel changes are noted between \nthe bulk δ³⁴Spy values and abundances of pyrite precipitated in sedimentary porewaters, \nsuggesting that the δ³⁴Spy variability could also be influenced by varying amounts of pyrite \nformed at different diagenetic stages within the samples. As a result, the fluctuation of bulk δ³⁴Spy \nsignals alone may not necessarily indicate perturbations of the global marine sulfur cycle. \nDespite the evidence for partial basin isolation, the average shallow seawater δ²³⁸U \nsignals (−0.70 ± 0.24‰, 1σ), inferred from the limestone δ²³⁸Ucarb values, were probably close to \ncoeval open oceans. With this estimated mean oceanic δ²³⁸U value, our three-sink U-isotope mass \nbalance model predicts that the Є-O transition was a time of expanded marine anoxia with anoxic \nto euxinic seawater covering 1.0 to 21.1% of the ocean floor. This prediction aligns with the \nobservation of substantially low I/Ca ratios ranging from 0.02 to 0.33 μmol/mol in the limestone. \nThese ratios fall well below the Proterozoic Eon background I/Ca values of ~0.5 to 1 μmol/mol, \nindicating the presence of a shallow marine oxycline along the regional continental margin. \nSimilar observations have been reported in several other age-equivalent sections deposited along \nthe shelf and slope of the ancient Iapetan and Laurentian continental margins. Consequently, it is \nplausible to suggest that seawater surrounding the Iapetan and Laurentian continental margins \nduring the Є-O transition might have been poorly oxygenated, with shallow oxyclines or \nexpanded oxygen minimum zones in the shelf and/or slope areas. These findings agree with \nearlier viewpoints that the middle-late Cambrian to the Early Ordovician marine biodiversity \nplateau was linked to widespread oceanic anoxia.
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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 | 0.000 |
| 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".