Integrated stratigraphy and Bayesian age modelling at Tiout (Anti-Atlas, Morocco) constrain early Cambrian δ¹³C excursions and trilobite radiation

Abstract

The early Cambrian represents a critical interval in Earth’s history, marked by rapid biological innovation and major perturbations in global carbon cycle. However, the timing and relationships among these processes remain poorly constrained due to fossil provincialism, diachronous bioevents, and inadequate high-precision geochronology. The Tiout section (Anti-Atlas, Morocco) provides a unique opportunity to address these challenges, preserving a continuous 1205-m-thick succession with multiple volcanic ash beds, the lowest occurrence (LO) of trilobites, and a refined biostratigraphy in West Gondwana. We integrate high-resolution δ¹³Ccarb stratigraphy, elemental geochemistry, U–Pb geochronology and biostratigrahy within a Bayes­ian astrochronological age-depth model to provide numerical ages for carbon isotope excursions and biotic events. The new δ¹³Ccarb record from Tiout documents multiple global excursions, including the termination of the Shiyantou Carbon Isotope Excursion (SHICE), the excursions II and III, and the Early Atdabanian/Repinaella Zone Excursion (EAREZE or excursion IV). The EAREZE peaks below the LO of trilobites and is dated at 520.046 ± 0.097 Ma, providing the first direct numerical calibration of this excursion within a single stratigraphic record bracketed by two ash beds. These results establish a robust, multi-proxy framework that synchronizes global carbon cycle records with the onset of trilobite radiation, and positions Tiout as a candidate reference section for the Global Stratotype Section and Point (GSSP) of Cambrian Series 2.