Testing grain size dependent variability of cosmogenic nuclide concentrations for isochron burial dating of fluvial sediment (Pannonian Basin, Hungary)

In this study a new approach of cosmogenic radionuclide (CRN) isochron burial dating was developed for locations where the maximum clast size of the sediment does not reach the size of using single cobble samples. This modification makes use of the presumption that different grain size fractions have diverse pre-burial histories and thus are deposited with diverse amount of CRNs. Accordingly, a bulk sample is taken at a certain sediment depth, from which several grain size fractions are selected. These are treated as individual samples for isochron burial age determination. To test our hypothesis, a fluvial sediment succession of the Paleo-Danube River in the Western Pannonian Basin, with an assumed Pliocene age was sampled at two sample depths. The different grain size fractions (0.25-0.5; 1-2; 4-8; 10-20; and 40-60 mm; n=7) provided variable ²⁶Al and ¹⁰Be concentrations, supporting the validity of our assumption. The difference between the lowest and largest concentrations was limited making age determination and outlier identification more challenging compared to an isochron with more variable CRN concentrations. After a careful outlier identification, χ² minimisation inverse modelling yielded 3.9 ± 0.7 Ma as the age of fluvial sedimentation. The offered sampling strategy opens a window towards further application of the CRN isochron burial dating. The burial age of fluvial sediments that were hitherto undatable due to their limited thickness and small clast size can be determined. After the presented adjustment of the sampling strategy the age of deposition can be calculated by the standard ways of isochron burial dating.