Bayesian astrochronology and radioisotope geochronology reveal contrasting carbon-isotope and biotic turnovers across the Jurassic–Cretaceous boundary in Tethys and Boreal realms

The Jurassic–Cretaceous boundary, conventionally placed at the base of the Berriasian, remains one of the most contentious horizons in the Phanerozoic timescale because the basal Berriasian is difficult to correlate consistently across Austral, Tethyan, and Boreal realms. Recent refinements in calcareous nannofossil biostratigraphy, notably the zonation proposed by Casellato and Erba (2021), together with emerging high-resolution carbon-isotope records from multiple basins, promise more robust global constraints on this transition. Although not tied to a single global event, the Jurassic–Cretaceous interval registers evolutionary turnover and reorganization of marine ecosystems, including shifts in carbonate production, ocean circulation, and floral assemblages, yet efforts to resolve their timing and drivers are hampered by fossil preservation/provincialism, stratigraphic discontinuities, and limited high‑precision geochronology.

The BH-02 well (207 m thick, Tithonian–Berriasian, Manifa and Sulaiy formations) in central Saudi Arabia offers a suitable archive to address these issues. High‑resolution calcareous nannofossil biostratigraphy/counting, integrated with correlations to calpionellid and calcareous nannofossil biozonations in Kuwait, enables recognition of key bioevents across the Jurassic–Cretaceous transition, while complementary strontium isotope geochronology, detailed cyclostratigraphy, and Bayesian astrochronology refine the temporal resolution to less than 100 kyr. Within this integrated scheme, the δ¹³Ccarb record captures both the early Tithonian and Late Berriasian carbon‑isotope excursions, which are placed in a high‑resolution age model together with Nannofossil Calcification Events I and II and the Late Berriasian Nannoconus Event, thereby constraining the coupling of biotic and isotopic change along the southern Tethyan margin.

Cyclostratigraphic analysis of high-resolution gamma-ray and potassium logs (~20,000 data points) using multitaper spectral methods, evolutive harmonic analysis, correlation coefficient spectra, band-pass filtering, and wavelet analysis reveals a pervasive ~7 m cycle interpreted as long eccentricity (405 kyr). Extraction of 30 E405 cycles implies a duration of ~12.1 Myr for the studied interval, in close agreement with independent Sr-isotope estimates of ~11.9 Myr (137.9–149.8 Ma). Age–depth modelling is achieved using astroBayes, a Bayesian inversion framework that jointly assimilates astrochronologic and radioisotopic constraints to reduce interpolation uncertainties between dated horizons and to resolve subtle changes in sedimentation rate while considering prior information on sedimentation and potential hiatuses.

This integrated stratigraphic, geochemical, and astrochronologic framework provides a precisely constrained, orbitally calibrated reference section for the Jurassic–Cretaceous boundary on the Arabian Plate. Comparison with coeval successions reveals contrasting carbon-isotope trends between the Tethyan and Boreal realms, reflecting a decoupling of oceanographic conditions through the J/K transition with recoupling during the Weissert Event, signaling a renewed phase of oceanic connectivity.