Using environmental magnetism and radionuclide analysis to deconstruct landscape structure in a small semi-arid catchment, South Africa

Ganora reservoir (c.4ha), constructed in 1910, is fed by intermittent streams draining a small (c.4km2), steep upland catchment in the Sneeuberg Mountains of South Africa's Eastern Cape. Located in a semi-arid environment, this catchment has been subjected to heavy grazing pressures since the mid nineteenth century and has also experienced a highly variable annual, monthly and daily rainfall regime. The present day landscape is composed of steep backslopes, colluvial footslopes with badland erosion and an incised channel network that transports sediment to the reservoir. A 2.8m sediment core was retrieved from the reservoir in 2007. It was dated using two gamma-emitting radionuclides, 210Pb and 137Cs. Reconstruction of sedimentation rates using the 210Pb ‘crs’ model suggested that rates increased after 1965, with peaks in the 1970s, coincident with heavy and widespread rainfall in March 1974, and again in the early 1990s. The core was also analysed for a number of magnetic susceptibility and remanance characteristics and several gamma-emitting radionuclides that either have long half lives or are direct daughters of isotopes that have long half lives (e.g 40K, 235U, 226Ra, 228Ac). Properties of the core sediment samples were compared with a range of potential sediment sources collected from the catchment on which the same measurements were made. Changes in sediment characteristics indicate that, after the mid 1960s, the dominant erosion source shifted from fan sediments at the top of the main incised channel to colluvial areas dominated by badland erosion. As the badlands have been present in the catchment since at least 1945 (from aerial photograph evidence) this suggests that the drainage network has expanded, thereby connecting the reservoir to new sediment sources. Thus the analysis has demonstrated possible changes in landscape structure at the scale of the channel network over the late 20th Century