Abstract Cyclostratigraphy is an important tool for understanding astronomical climate forcing and for reconstructing geological time in sedimentary sequences, provided that an imprint of insolation variations caused by Earth's orbital eccentricity, obliquity and precession is preserved (Milankovitch forcing). Understanding astronomical climate forcing has proven fundamental for the study of Cenozoic climate systems and the construction of high-resolution continuous time scales (astrochronologies). Pre-Cenozoic astrochronologies face several challenges: (1) uncertainties in the deep-time astronomical solutions and parameters; (2) less complete and less well preserved strata; and (3) the sparsity of geochronologic anchor points. Consequently, Paleozoic astrochronologies are typically based on identification of the stable 405 kyr eccentricity cycle instead of shorter astronomical cycles. Here, a state-of-the-art review of Ordovician cyclostratigraphy and astrochronology is presented, as well as suggestions on their robust application in an Ordovician context. Ordovician astronomically driven climate dynamics are suggested to have influenced processes like glacial dynamics, sea-level variations and changes in biodiversity. Ordovician cyclostratigraphic studies can help to construct high-resolution numerical time scales, ideally in combination with high-quality radio-isotopic dating. As such, cyclostratigraphy is becoming an increasingly important part of an integrated stratigraphic approach to help disentangle Ordovician stratigraphy and palaeoenviromental changes.

Abstract: Radioisotopic age determinations targeted at key stratigraphic successions worldwide continue to refine the geological timescale with increasing precision and accuracy and to unravel the tempo of global geological, palaeoclimatic and palaeobiotic processes that have shaped our planet. The last decade has witnessed significant progress in the calibration of the Permian Period through integrated stratigraphic, palaeontological and high-precision geochronological investigations. These studies have largely focused on the Cisuralian and Lopingian stages, particularly the end-Permian mass extinction, whereas much of the Guadalupian and its associated events remain inadequately calibrated. A compilation of the high-precision U–Pb geochronology generated in the past ten years yields ages of 298.92±0.19 Ma for the onset of the Permian, 293.52±0.17 Ma for the base-Sakmarian, 290.10±0.14 Ma for the base-Artinskian, 272.95±0.11 Ma for the Cisuralian–Guadalupian boundary, 265.22±0.34 Ma for the base-Capitanian, 254.14±0.12 Ma for the base-Changhsingian and 251.90±0.10 Ma for the Permian–Triassic boundary. Extension of modern astrochronological methods to the Palaeozoic Era presents new opportunities for broader stratigraphic correlations and enhanced calibration of the Permian timescale. Supplementary material: Table S1 (U–Pb data) is available at https://doi.org/10.6084/m9.figshare.c.3917425

We studied a high-resolution multiproxy data set, including magnetic susceptibility (MS), CaCO 3 content, and stable isotopes (δ 18 O and δ 13 C), from the stratigraphic interval covering the uppermost Maastrichtian and the lower Danian, represented by the pelagic limestones of the Scaglia Rossa Formation continuously exposed in the classic sections of the Bottaccione Gorge and the Contessa Highway near Gubbio, Italy. Variations in all the proxy series are periodic and reflect astronomically forced climate changes (i.e., Milankovitch cycles). In particular, the MS proxy reflects variations in the terrigenous dust input in this pelagic, deep-marine environment. We speculate that the dust is mainly eolian in origin and that the availability and transport of dust are influenced by variations in the vegetation cover on the Maastrichtian-Paleocene African or Asian zone, which were respectively located at tropical to subtropical latitudes to the south or far to the east of the western Tethyan Umbria-Marche Basin, and were characterized by monsoonal circulation. The dynamics of monsoonal circulation are known to be strongly dependent on precession-driven and obliquity-driven changes in insolation. We propose that a threshold mechanism in the vegetation coverage may explain eccentricity-related periodicities in the terrigenous eolian dust input. Other mechanisms, both oceanic and terrestrial, that depend on the precession amplitude modulated by eccentricity, can be evoked together with the variation of dust influx in the western Tethys to explain the detected eccentricity periodicity in the δ 13 C record. Our interpretations of the δ 18 O and MS records suggest a warming event ~400 k.y. prior to the Cretaceous-Paleogene (K-Pg) boundary, and a period of climatic and environmental instability in the earliest Danian. Based on these multiproxy phase relationships, we propose an astronomical tuning for these sections; this leads us to an estimate of the timing and duration of several late Maastrichtian and Danian biostratigraphic and magnetostratigraphic events.