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Research title Single grain detrital cosmogenic 21Ne analysis: A new tool to study long-term landscape evolution. Summary of research PhD thesis submitted on the 6th of February, 2008.
Abstract of thesis:
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 10Be and 21Ne analyses and numerical modelling.
The Gaub River study catchment is a tributary of the ~15,500 km2 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.
SRTM DEM of the field site (Gaub river, Namibia)
Denudation rates in the steeper escarpment sub-catchments of the upper Gaub, based on analysis of 10Be 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 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. The analysis of 10Be 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 10Be 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.
3D View - ASTER DEM (Gaub river, Namibia)
Analyses of 21Ne in 32 individual quartz pebbles [16-21 mm] collected from the outlet of the Gaub yield cosmogenic 21Ne [21NeC] concentrations that span nearly two orders of magnitude [2.6-160 x 10e6 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.
Landsat ETM+ image of the field site (Gaub river, Namibia)
Sensitivity analyses conducted using a GIS-based numerical model of cosmogenic nuclide acquisition show that the shape of the frequency distribution of 21NeC 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 21NeC 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 21NeC is sensitive to the range of erosion rates that are present, this sensitivity is not sufficient to allow the use of the 21NeC concentration distribution for accurately constraining the range of process rates in a catchment. Supervisors Professor Trevor Hoey
Professor Paul Bishop
Recent publications Jansen, J.D., Codilean, A.T., Fabel, D. 2009. Recent rock uplift across the path of Cooper Creek? Some inferences from cosmogenic nuclides and the long profile. In: G.C. Nanson. The Geomorphology & Quaternary Environments of Eastern Central Australia. International Conference on Geomorphology Field Guide. Melbourne, July 2009. 8 pp. View full text >> Recent research grants 2008. “Single grain 21Ne/10Be ratios in fluvial quartz pebbles: Testing the reliability of cosmogenic 21Ne in sediment”, £15,010 (NERC Cosmogenic Isotope Analysis Facility award 9054.0408) |
