Low variability of authigenic 10Be/9Be across the Holocene Po plain parasequences reveals suitability of dating method for highstand deltaic deposits

The authigenic ¹⁰Be/⁹Be dating method presents a valuable tool for reconstructing depositional chronologies in sedimentary environments, requiring only ubiquitous mud for sampling. Nevertheless, studies elucidating the variability of the ¹⁰Be/⁹Be record preserved in epicontinental successions are lacking, despite the essential nature of such knowledge for the application of authigenic ¹⁰Be/⁹Be in geochronology. In this study, we investigate the variability of measured natural ¹⁰Be/⁹Be ratios in sediment cores recovered from the Holocene Po River delta plain in Northern Italy, aiming to unveil the influence of changing sedimentary environments and provenance on the beryllium isotopic signature. We identified significant variations in the authigenic ¹⁰Be/⁹Be ratios across parasequences, which correlates with a provenance change from the Eastern Alps to the Po River. The observed variation would cause an age offset of ∼1 Myr if unrecognized in a dated succession. Our analysis revealed consistent ratios between the delta plain (primarily represented by swamp) and delta front consisting of proximal prodelta facies, suggesting a prevalent riverine signature in the proximal prodelta, likely maintained by hyperpycnal flow deposition. Statistical assessments based on random sampling and bootstrapping highlighted the importance of a sample size of n > 10. Furthermore, a standard deviation of the observed variability indicates a necessity of an additional 9% uncertainty in authigenic ¹⁰Be/⁹Be dating studies if the sample size is smaller. Overall, our findings emphasize that the normal regressive highstand settings of a deltaic system maintain relatively stable beryllium isotopic fluxes, which are favourable for the authigenic ¹⁰Be/⁹Be dating application, if provenance changes are known.