Rajeev Lal
Final PhD seminar

Nuclear Physics Department

The northern Australian river basins are largely unexploited for intensive agriculture.  Climate change, however, is predicted to make the traditional Australian agricultural areas in the south drier, and to cause an increase in the frequency of extreme events. Hence, coupled with the demands from an increasing population, it is likely that the northern river basins will increasingly be targeted for agricultural development. If the development of these catchments is to be sustainable, then the rates of soil loss and soil production from the underlying bedrock must be comparable.  There are, however, very few measurements of soil erosion and production rates in northern Australia. In this study the technique of accelerator mass spectroscopy (AMS) has been employed to determine soil erosion and soil formation rates using the analysis of plutonium-239 (²³⁹Pu) and beryllium-10 (¹⁰Be) tracers in soil and sediment cores and surface rocks from the Daly River Basin in the wet-dry tropics of northern Australia. ²³⁹Pu, released in the 1950s and 1960s by atmospheric nuclear weapons tests, was used to obtain a quantitative assessment of recent rates of soil loss. Soil cores 40-50 cm deep were collected from fields broadly classified into forest, grazing and cultivated areas. Cores taken from undisturbed and unburnt areas in open eucalypt woodland forest were used as reference sites. The soil loss rates have been established by comparing the excess or deficiency of the ²³⁹Pu tracer over that of the reference sites. Due to agricultural practices, modern soil loss rates can be significantly higher than long-term soil formation rates, but establishing soil formation rates has proved to be a difficult problem. At long-term equilibrium, however, soil formation from the underlying rock is balanced by soil loss from the surface. In this study two independent methods were applied for determining soil production rates to the carbonate plains of the Daly Basin. First, the geological denudation rates were determined from measurements of in situ produced cosmogenic ¹⁰Be concentrations in surface rocks sampled across the catchment. Second, the soil production rates were determined from measurements of meteoric ¹⁰Be inventories in soil cores.  Results from both methods agree well thus providing evidence that this landscape is in morphological equilibrium over geological timescales and are comparable to rates measured in other parts of monsoonal Northern Australia. However modern soil loss rates due to current agricultural practices were found to be up to two orders of magnitude higher than these equilibrium rates causing unsustainable soil loss and thus soil conservation strategies have to be prioritised in the future development of these catchments.