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- Jonathan Butler
Current Research
Quantifying erosion rates and erosion history in semiarid South Africa using cosmogenic nuclide inventories
The main focus of my PhD research involves using the cosmogenic nuclides Berylium-10 and Aluminium-26 to constrain erosion rates at different spatial scales in the semiarid Orange River valley of South Africa. Cosmogenic nuclides are produced by interactions between cosmic radiation from outer space and atoms in minerals in the upper few meters of the earth's surface. Measuring the concentration of these nuclides in surface materials allows determination of both the exposure age of a landsurface and also the rate at which a surface has eroded over thousands to hundreds of thousands of years.
At the scale of small mountain catchments in the Great Karoo, soil erosion and associated landscape degradation is a significant environmental problem. Although recent rates of soil erosion are well-documented, in order to understand whether this land degradation is caused by natural climatic change or human impacts, a longer term history of erosion is needed. Cosmogenic nuclide inventories offer a means of generating such longer term data. This element of the study involves collaboration with the Universities of Cape Town, Oxford, Leicester and Coventry.
Cosmogenic nuclide analysis of quartz grains in river-borne sediment allows the average erosion rate at the scale of an entire drainage basin to be quantified. This 'catchment-averaged' approach assumes that the sediment in a basin is well-mixed and that the residence time in the basin is insignificant compared with the time the quartz grain spent at the surface in the catchment. It is proposed to use this approach to quantify the erosion rate for the entire Orange River catchment by sampling at different locations along the 7,000km course of the river. By comparing the erosion rates at these different locations, the robustness of the catchment-averaged approach may be assessed.
By quantifying the erosion rate of bedrock outcrops at various locations in the Orange basin, and comparing this rate to the catchment-averaged rate at different spatial scales, the overall development of the western margin of the South Africa can be constrained. In addition, role of the Augrabies Falls, the principal knickpoint on the Orange River, is being investigated by measuring the retreat rate using cosmogenic nuclides. The history and evolution of the Orange River basin may be better understood by quantifying the rate and style of retreat of the Falls.