Beyond Look-Up Tables: Unveiling the Potential of Bayesian Theory in Strontium Isotope Stratigraphy for Dating Early Angiosperm Appearance in Portugal

Strontium isotope stratigraphy (SIS) relies on comparing the ⁸⁷Sr/⁸⁶Sr isotopic ratios of well-preserved samples to a reference curve that depicts the relationship between the ⁸⁷Sr/⁸⁶Sr isotopic ratios and geological age. The prevalent approach among stratigraphers utilizing SIS involves employing Look-up tables to compare their measurements with available reference curves. This approach is interesting because it is straightforward, easy to use, and widely accepted within the scientific community. However, Look-up tables exhibit limitations in incorporating geological knowledge, such as biostratigraphy, from the studied horizon. Also, they provide only discrete age ranges, neglecting the full probability distributions describing the age of the samples.

In this study, we present a distinctive method based on Bayesian theory to compare measured isotopic ratios and reference isotopic ratios, aiming to address these shortcomings. We subsequently applied this approach to twelve diagenetically screened bivalve shell samples using petrographic thin sections, cathodoluminescence, micro-X-ray fluorescence, and trace elemental analyses. The samples were collected from three sections located in pre-Aptian coastal marine strata from the Lusitanian Basin of Portugal. These sections are of particular significance due to their richness of unique fossil assemblages and excellent preservation of early angiosperm pollen, the timing of whose origin is still controversial. Our study offers an alternative approach for conducting SIS analyses and suggests an Early Barremian age (124.63–126.24 Ma, GTS2020) for pollen showing unequivocal eudicot angiosperm features collected in Portugal.