Numerical and analytical models of cosmogenic radionuclide dynamics in landslide-dominated drainage basins

In many tectonically active regions on Earth, landslides dominate sediment delivery to channels. While cosmogenic radionuclides (CRN) represent a valuable tool to document basin-averaged erosion rates, a better understanding of CRN dynamics in these systems is required before erosion rates can be reliably inferred from CRN concentrations. The stochastic nature of landslides results in variability of CRN concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood. To guide sampling strategies, we developed a 2-D plan view model to simulate the influence of landslides on CRN dynamics. We employ a sediment mixing model, in which concentrations of CRNs from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from CRN concentrations. This model is further used to test a new analytical solution for the distribution of CRN concentrations on hillslopes in a landslide-prone system. Results suggest that for landscapes dominated by deep (> 5 m) landslides, only large drainage basins (>= 100 km(2)) provide consistently reliable estimates of erosion rates from CRN concentrations. For landscapes dominated by shallow landslides (0.7-2 m), smaller basins can produce reliable estimates. Results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes. These findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing CRNs; if properly done, the variability can reveal useful information about geomorphic processes in landslide-prone areas.