A new chronostratigraphic anchor for the early Cambrian: Integrating δ¹³C chemostratigraphy and Bayesian astrochronology in West Gondwana (Anti-Atlas, Morocco)

Damien Pas, Matthias Sinnesael, Valentin Jamart, Kamal Mghazli, Gerd Geyer, Ed Landing, Nasrrddine Youbi, Jihane Ounar, Hamdi Omar, Moulay Ahmed Boumehdi, Jarno J.C. Huygh, Anne-Christine Da Silva, Allison C. Daley

The early Cambrian represents a critical interval in Earth’s history characterized by rapid biological innovation and significant perturbations in global carbon cycling. However, a comprehensive understanding of the processes at play and their spatial and temporal relationships is hindered by fossil provincialism, diachronous bioevents, and inadequate high-precision geochronology. The Tiout section (Anti-Atlas, Morocco) offers a unique opportunity to address these challenges as it is a continuous 1205 m-thick succession of sedimentary rocks with multiple volcanic ash beds, together with the lowest occurrence (LO) of determinable, mineralized trilobites and a refined biostratigraphy in West Gondwana. In this study, we present new high-resolution δ¹³Ccarb stratigraphy, coupled with portable XRF elemental geochemistry and Bayesian astrochronology, to refine the temporal framework of this lower Cambrian succession. The Tiout δ¹³Ccarb record captures multiple global excursions, including the termination of the Shiyantou Carbon Isotope Excursion (SHICE), excursions II and III, and the early Atdabanian/Repinaella Zone Excursion (EAREZE or excursion IV), which has been commonly confused with the Cambrian Arthropod Radiation Excursion (CARE). Cyclostratigraphic analysis of the detrital SiO₂ proxy 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. The EAREZE peak below the LO of determinable trilobites is dated at 520.046 ± 0.097 Ma. The Tiout section provides a robust, multi-proxy reference for a potential Global Stratotype Section and Point (GSSP) for Cambrian Series 2.