Single-grain cosmogenic 21Ne concentrations in fluvial sediments reveal spatially variable erosion rates

We evaluated the hypothesis that the spatial variation in erosion in a catchment is reflected in the distribution of the cosmogenic nuclide concentrations in sediments leaving the catchment. Using published data and four new Be-10 measurements in fluvial sediment collected from the outlets of small river catchments, we constrained the spatial variability of erosion rates in the Gaub River catchment in Namibia. We combined these catchment-averaged erosion rates, and the mean slope values with which they are associated, in a digital elevation model (DEM)-based analysis to predict distributions of cosmogenic Ne-21 concentrations in the sediment leaving the Gaub catchment. We compared these synthetic distributions with the distribution of concentrations of cosmogenic Ne-21 (Ne-21(C)) in 32 quartz fluvial pebbles (16-21 mm) collected from the catchment outlet. The Ne-21(C) concentrations span nearly two orders of magnitude (2.6-160 x 10(6) atoms/g) and are highly skewed toward low values. The DEM-based analysis confirms this skew-the measured Ne-21(C) distribution plots within the envelope of distributions predicted for the catchment. This match between measured and synthetic Ne-21 distributions implies that the measured distribution is a signature of the spatial variation in erosion rates.