Constraining the aggradation mode of Pleistocene river deposits based on cosmogenic radionuclide depth profiles and numerical modelling

Pleistocene braided river deposits commonly represent long periods of non-deposition or erosion that are interrupted by rapid and short aggradation phases. When dating these sedimentary sequences with in-situ produced cosmic radionuclides (CRN), simple concentration-depth profiling approaches fall often short as they assume that the alluvial sedimentary sequence has been deposited with a constant and rapid aggradation rate and been exposed to cosmic radiations afterwards. Numerical modelling of the evolution of CRNs in alluvial sequences permits to account for aggradation, non-deposition and erosion phases, and can simulate which scenarios of aggradation and preservation are most likely representing the river dynamics. In this study, such a model was developed and applied to a Middle Pleistocene gravel sheet (Zutendaal gravels) exposed in NE Belgium. The model parameters were optimized to the observed 10Be and 26Al concentrations of 17 sediment samples taken over a depth interval of 7 m. In the sedimentary sequence, (at least) three individual aggradation phases can be distinguished that were interrupted by non-deposition or erosion lasting each ~40 kyr. The age for the onset of aggradation was further constrained to kyr, and further narrows down the anticipated age window [500;1000] Ma of the terrace gravels. This age, within error limits, does not invalidate previous correlations of this gravel sheet with the Cromerian Glacial B, and marine isotope stage (MIS) 16. The deposition of the entire sedimentary sequence likely represents more than one climatic cycle, and demonstrates the importance of accounting for the depositional modes of braided rivers when applying in-situ cosmogenic radionuclide techniques