Iterative outlier identification for robust cosmogenic ²⁶Al/¹⁰Be burial dating of fluvial terraces: A case study from the Danube River (Vienna Basin, Austria)

The burial age of a fluvial terrace of the Danube River in the central Vienna Basin (Austria) was determined using the cosmogenic radionuclides (CRNs) 26Al and 10Be. The terrace is located in an uplifted block on the eastern side of the Vienna Basin Transform Fault. The influence of potential outliers on the burial age and the robustness of their identification was tested. For this purpose, a previously published dataset from two different subsurface depth levels of a gravel pit was used. These two independent datasets share the same post-burial history (burial age and post-burial denudation rate) and differ only in their sampling depths. Their burial ages are determined via the joint application of two widely applied burial age calculation approaches: the classic isochron (ISO) and χ2 fitting inverse modelling (INV) methods. The ratio of the pre- and post-burial CRN inventories and effect of inclusion and exclusion of data points (bootstrapping) were estimated and introduced as a new method for outlier identification. Results show that a single method may overlook the real outliers; therefore the use of diverse ways of outlier identification is strongly recommended. Samples of very low concentrations are prone to be dominated by CRNs produced after burial, which may lead to a bias in the CRN ratio and thus deviation of the calculated burial age. Outlier identification and age calculations are iterated until the model results converge into a coherent dataset yielding a good model fit. The burial age of the terrace was determined to be 1.2 ± 0.4 Ma. This is the first numerical terrace age from the intrabasinal hills located to the south of the Danube within the Vienna Basin and is considerably older than the previously assumed Marine Isotope Stage 12 (MIS 12) age of this landform. Quantification of the terrace age enabled us to estimate an uplift rate of 21-56 m Myr-1 in this area, which is similar to published uplift rates along the Danube in the Vienna and Danube basins despite their different tectonic settings. The estimated post-burial denudation rate for the upper level (16.1 ± 11.7 m Myr-1) was higher than that for the lower level (6.3 ± 3.7 m Myr-1); however, it is in agreement within uncertainties. This might indicate a mean denudation rate of 11.6 ± 9.0 m Myr-1 or a recent increase in the surface denudation rate.