Single Grain Detrital Cosmogenic Nuclide Analysis. A New Tool to Study Long-Term Landscape Evolution

We use the Gaub River study catchment in western Namibia to examine opportunities in and limitations of the analysis of terrestrial cosmogenic nuclide concentrations in individual grains in a sedimentary deposit as a method for unravelling landscape evolution.

The Gaub is a tributary of Kuiseb, one of the major ephemeral river systems draining western Namibia. The geomorphology of the upper Gaub is that of a high elevation passive margin and is characterised by an extensive low-relief upland region and a highly dissected, high-relief zone marking the Great Escarpment.

Denudation rates in the steeper escarpment part of the Gaub, based on the analysis of ¹⁰Be in six sediment samples, average around 13 m/Myr. This rate is twice that obtained for the upland plateau, where denudation rates from five sediment samples average around 5.5 m/Myr. The ¹⁰Be analyses show that there is a very strong linear relationship between measured denudation rate and mean slope of a sediment sample's source catchment.

Analyses of ²¹Ne in 32 individual quartz pebbles collected from the outlet of the Gaub yield cosmogenic ²¹Ne concentrations that span nearly two orders of magnitude (2.6 – 161.2 x 10⁶ atoms/g) and are highly skewed toward low values. These results are corroborated by a numerical model assuming a linear correlation between denudation rate and slope, that the measured distribution is a signature of the slope dependence of, and hence spatial variation in, denudation rates in the study catchment.

Sensitivity analyses using the numerical model show that the shape of the frequency distribution of ²¹Ne concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment's source areas and that this frequency distribution of cosmogenic nuclide concentrations in the clasts can be used to infer aspects of source area geomorphology.