Variation of authigenic 10Be/9Be in spatial and temporal scale and its implications for applicability of dating method for river dominated deltas

The authigenic ¹⁰Be/⁹Be dating method has emerged as a novel chronometer in the last few decades. Initially introduced by Bourles et al. (1989: Geochim. Cosmochim. Acta) for dating of oceanic sediments the method has been successfully applied to date continental sediments in some recent studies. The method utilizes decay of cosmogenic radionuclide ¹⁰Be to date the deposition of sediment in the range of 0.2 to ~14 Ma. Different source of the two isotopes ⁹Be and ¹⁰Be results in changes in the authigenic ¹⁰Be/⁹Be ratio (R) as the sediment gets transported from deltaic to offshore settings through shelfal environment. Accurate determination of initial ¹⁰Be/⁹Be ratio (R₀) is of paramount importance in this dating method to calculate the correct age of the deposited sediment. It is therefore necessary to quantify the variability of authigenic initial ratio for the effective application of the method.

This study aims to describe the variability of authigenic ¹⁰Be/⁹Be ratio in temporal as well as spatial scale. We analyzed the samples for authigenic ¹⁰Be/⁹Be ratio from two different study sites, the Late Miocene to Quaternary Slanicul de Buzau section in Romania and the Holocene Po River delta plain in Italy. 109 samples were sampled from Slanicul de Buzau section with a sampling step resolution of ~20–30 ka aiming to interpret ratio variability in temporal scale. Po coastal plain yielded 50 samples from 4 different parasequnces (5,6,7 and 8) (Amorosi et al., 2017: Mar. Pet. Geol.) to cover the variability in spatial scale. The analysis of samples from these study sites allowed us to understand the variability of the ratio in terms of sediment source proximity and change in sediment provenance.

Back calculation of initial ratio R₀ in 109 samples from Slanicul de Buzau section with the help of measured authigenic R and interpolated magnetostratigraphic age of samples reveals low variability in R₀ in deltaic and shallow water sediments, while the offshore facies exhibit significantly higher variability.

Results from measured ratio R in 50 samples of Po Plain does not show a drastic variability, slightly higher R in the samples from parasequence 6 can be attributed to the different provenance of this parasequence in contrast to the sediment source of the latter parasequences (Amorosi et al., 2020: Basin res.).

This observed low variability of authigenic ¹⁰Be/⁹Be reveals the good suitability of the dating method for river dominated deltas. We have also attempted to propose a minimum number of sample count necessary to be analyzed when using this dating method. The approach aims to estimate the sample size dependency of the ratio by running 1000 simulations and bootstrapping to estimate the 95% CI for variability of ratio, taking into consideration the three statistics namely, range, IQR, and SD. Bootstrapping performed in our data, in a particular group of n samples (e.g., samples from a specific facies type), suggests that at least nearly half the sample size of n is necessary to represent the initial ratio variability and to avoid the potential bias caused by the initial ratio variability.