Cosmogenic Neon-21 Analysis of Individual Detrital Grains: Opportunities and Limitations

Cosmogenic nuclide analyses of amalgamated sediment samples collected at basin outlets are widely used to determine basin-averaged denudation rates. However each grain leaving a basin has a potentially unique history of erosion, transport and storage, and so the frequency distribution of cosmogenic nuclide concentrations in large numbers of individual grains can provide an integrated signature of the basin's geomorphic history. The need for relatively large amounts of pure quartz (~20-30 g) and a lengthy and expensive sample preparation process generally limits the application of in situ cosmogenic 10Be and 26Al to amalgamated sediment samples. The extraction and determination of cosmogenic 21Ne, however, is relatively straightforward and inexpensive allowing for a large number of individual measurements. Moreover, the relatively small sample size (0.1-1 g) required allows cosmogenic 21Ne to be measured in individual quartz pebbles in the range. We have combined measurement of cosmogenic 21Ne in individual pebbles and 10Be and 26Al in sand and pebbles from the Gaub River in central-western Namibia with numerical modelling to examine opportunities in and limitations of the analysis of cosmogenic 21Ne concentrations in individual grains in a sedimentary deposit as a method for unravelling landscape evolution. 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 × 106 atoms.g-1) and are highly skewed toward low values. These results are corroborated by (1) cosmogenic 10Be in amalgamated sediment samples from 11 of the Gaub's tributaries and cosmogenic 10Be and 26Al in 15 of the individual quartz pebbles, and (2) by a numerical model assuming a linear correlation between denudation rate and slope. (1) and (2) confirm that the measured 21Ne distribution is a signature of the slope dependence of, and hence spatial variation in, denudation rates in the study basin. 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 pebbles can be used to infer aspects of source area geomorphology. The results of the sensitivity analyses also show that lithology can affect the shape of the cosmogenic 21Ne concentration distribution indirectly by exerting control on the spatial pattern of denudation in a drainage basin. Model results further indicate that the shape of the distribution of cosmogenic 21Ne is sensitive to the range of erosion rates that are present in a basin and this sensitivity is sufficient to allow the use of the cosmogenic 21Ne concentration distribution for accurately constraining the range of process rates in the basin.