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
Notice bibliographique
Résumé
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.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,001 | 0,001 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».