Single grain detrital cosmogenic Ne-21 analysis: a new tool to study long-term landscape evolution

Cosmogenic nuclide concentrations in alluvial sediment are now routinely used to estimate time and space-averaged catchment-wide denudation rates, but have the potential to offer considerably more. This is because each grain leaving a catchment has a potentially unique history of erosion, transport and storage, and so the frequency distribution of cosmogenic nuclide concentrations in large numbers of grains can provide an integrated signature of the catchment's geomorphic history. This thesis evaluates the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of single grain detrital cosmogenic nuclide concentrations through the use of a combination of cosmogenic Be-10 and Ne-21 analyses and numerical modelling. The Gaub River study catchment is a tributary of the ~15,500 square km 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 sub-catchments of the upper Gaub, based on analysis of Be-10 concentrations in 6 amalgamated sediment samples, range from 12.5±0.8 to 14.1±0.9 m/Myr. These rates are twice those obtained for 5 upland plateau sub-catchments: 3.4±0.2 to 8.0±0.5 m/Myr. The Be-10 analyses show that there is a very strong linear relationship between measured denudation rate and mean slope of a sediment sample's source catchment. The analysis of Be-10 in a sample from the outlet of the Gaub and in the 11 sub-catchment samples confirms that, despite the strong geomorphic asymmetry that characterises this catchment, sediment leaving the upper Gaub is well mixed. The denudation rates based on Be-10 are consistent with published cosmogenic nuclide data and confirm the finding that the steeper escarpment zone is eroding more rapidly than both the more gently sloping coastal plain and the upland plateau. Analyses of Ne-21 in 32 individual quartz pebbles (16 - 21 mm) collected from the outlet of the Gaub yield cosmogenic Ne-21 [(Ne-21)c] concentrations that span nearly two orders of magnitude (2.6 – 161.2 million atoms/g) and are highly skewed toward low values. These results are corroborated by a DEM-based analysis assuming a linear correlation between denudation rate and slope, inferring 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 conducted using a GIS-based numerical model of cosmogenic nuclide acquisition show that the shape of the frequency distribution of (Ne-21)c concentrations in sediment leaving a catchment is sensitive to the range of processes operating in the sediment's source areas and that this distribution can be used to infer aspects of source area geomorphology and geomorphological processes. The results also show that lithology can affect the shape of the (Ne-21)c concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment. The results of the sensitivity analyses further indicate that although the shape of the distribution of (Ne-21)c is sensitive to the range of erosion rates that are present, this sensitivity is not sufficient to allow the use of the (Ne-21)c concentration distribution for accurately constraining the range of process rates in a catchment