A new reverse subchron (C33n.1r) in the Campanian: astronomical duration estimate and geomagnetic/chronostratigraphic implications
Notice bibliographique
Résumé
The Cretaceous Normal Polarity Superchron (CNPS, chron C34n, Aptian–Santonian, ~83–118 Ma in CK95 GPTS) is followed in the Campanian by two relatively long chrons (chron C33r, 3.925 My duration and then the normal chron C33n, 5.456 My duration) straddling most of the Campanian stage. The analysis of the geomagnetic reversal history has classically determined two nearly linear segments for the late Cretaceous–Cenozoic interval divided at chron C12r. The length of chrons in the younger interval has no systematic trend and henceforth is considered stationary for statistical analysis with a mean chron length of 0.248–0.219 My while the older segment has 0.749 My mean chron length. The stationarity for this latter interval is attained, however, when the two long polarity chrons C33r and C33n adjacent to the CNPS are omitted. Studies in the weakly magnetized southern England chalk succession and marine Bearpaw Shale in the Canadian Rockies from Alberta have argued about the presence of a number of reversals within C33r and C33n (and C34n). However, all these remain ambiguously established and not incorporated in the standard GPTS despite their significance for theories of geodynamo behavior and potentiality for high-resolution stratigraphic correlations that could notably impact, for instance, the chronostratigraphy of dinosaur-bearing terrestrial Upper Cretaceous of the Western Interior of North America. In any case, no polarity subchrons within C33r or C33n have been reported in any deep-sea record or in the landmark pelagic “scaglia” sections from the Gubbio area in the central Italian Apennines, for which a good integrated biostratigraphy and a thorough paleomagnetic record exists. Here, we report on a new reverse subchron in the lower part of C33n, informally named the Postalm Fall Subchron (PFS), retrieved in the Postalm section (Gosau Group, Northern Calcareous Alps of Austria). The Postalm section shows a deepening trend from upper Santonian conglomerates and grey shelf marls to pelagic bathyal red marly limestones of Campanian age. The section has previously been studied in the frame of an integrated multi-proxy stratigraphic study that includes high-resolution calcareous plankton biostratigraphy, magnetostratigraphy, stable isotopes, strontium stratigraphy and Fe content. Robust paleomagnetic data has pinpointed the top of C34n that defines the Santonian-Campanian boundary together with key biostratigraphic markers in the lower part of the red unit. The integrated study extents upwards for about 170 m up to calcareous nannofossil zone UC16 in chron C32 in the late Campanian. The cyclic nature of the pelagic sequence has been studied by means of spectral analysis on the limestone/marl couplet thickness data and geochemical proxies that allows identifying the short and long eccentricity cycles and to establish a cyclostratigraphic framework. From 33 new tightly collected samples in this study, 23 display unambiguous reverse polarity and conform the PFS subchron that straddles 3–4 precession cycles (~70 ky duration) within the UC15b calcareous nannofossil biozone. The average sediment accumulation rate at Postalm (~2 cm/ky compared to ~0.6–1 cm/ky at Gubbio) and the high-quality paleomagnetic signal have favored this discovery. The absolute age calibration and related geomagnetic and chronostratigraphic implications would be discussed.
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 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,002 | 0,002 |
| 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) | 0,005 | 0,001 |
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 ».