Integrated astrochronology and geochronology for the Terreneuvian–Cambrian Series 2 Boundary: Insights from the Tiout Section, Morocco

The early Cambrian represents a critical interval in Earth’s history, characterized by rapid biological innovation and significant perturbations in global carbon cycling. However, complete understanding of the processes at play and their spatial and temporal relationships are hindered by fossil provincialism, diachronous bioevents, and limited high-precision geochronology. The Tiout section (Anti-Atlas, Morocco) offers a unique opportunity to address these challenges, preserving a continuous 1205 m-thick succession across the Lie-de-Vin, Igoudine, and Amouslek formations, with multiple volcanic ash beds, together with the lowest occurrence (LO) of trilobites in West Gondwana.

Here we present new high-resolution δ¹³Ccarb stratigraphy, coupled with portable XRF elemental geochemistry and Bayesian astrochronology, to refine the temporal framework of this early Cambrian succession. The Tiout δ¹³Ccarb record captures multiple global excursions, including the termination of the Shiyantou Carbon Isotope Excursion (SHICE), the excursions II and III, and the Cambrian Arthropod Radiation Excursion (CARE or excursion IV). The CARE peaks below the LO of trilobites and is temporally constrained between U–Pb-dated ash beds at 521.06 ± 0.12 Ma and 519.23 ± 0.14 Ma. Cyclostratigraphic analysis of detrital SiO₂ proxies, combined with U–Pb-dates, confirms previous interpretation of an eccentricity-precession origin for the decameter-scale rhythmic alternations between lighter-colored marlstones and darker limestones expressed in the weathering profile. By interpreting a short-eccentricity cycle of ~110 ky, we built a floating astronomical timescale spanning 4.87 Myr.

Our data support the reproducibility and global significance of carbon isotope excursion CARE/IV as a chronostratigraphic anchor near the projected base of Cambrian Series 2. The diachronous nature of the trilobite LO across paleocontinents, confirmed here through integrated astrochronology and chemostratigraphy, underscores the limitations of biostratigraphy alone for global correlation. The Tiout section provides a robust, multi-proxy reference for evaluating alternative boundary criteria, contributing critical data towards the eventual selection of a Global Stratotype Section and Point (GSSP) for Cambrian Series 2.