Sediment tracing and environmental history for two small catchments, Karoo Uplands, South Africa

Abstract

As part of an investigation of land degradation in the South African Karoo, three types of fine sediment signatures (radionuclides, mineral magnetism and sediment geochemistry) were used to trace the origins of fine particulate sediment accumulating in two reservoirs that were constructed around the mid 1930s. Both reservoirs are fed by gullies that are known to have been in existence before their construction. Robust chronologies were derived from the 137Cs profiles in the reservoir sediments, from aerial photographic records, and from an analysis of extreme storm events that have left stratigraphic markers in the reservoir sediments. Extreme daily rainfalls have become more frequent in the last 30 years but sediment accumulation rates in the two reservoirs differ significantly. One reservoir has high sediment accumulation rates between 1958 and 1964 that correlates with an expansion in the area of cereal cultivation. The second catchment has no cultivated land, but sedimentation rates in the reservoir declined after 1941 when livestock grazing densities decreased by a factor of around three. Despite the increase in the frequency of high magnitude daily rainfalls after 1970, sediment availability, as controlled by land use, appears to be the most important limit on changes in sedimentation rates. Differences in lithology and land use provided an opportunity to evaluate the number of sources that could successfully be discriminated in each catchment at different timescales. 137Cs and unsupported 210Pb can be used to trace sources of actively transported or very recently deposited sediment and to discriminate sources of reservoir sediments between topsoils (cultivated and uncultivated) and subsoils or gully sidewalls. Lack of current 137Cs inputs means that it will become less useful as a future discriminator of topsoil/subsoil sources because it only has a half-life of ca. 30 years and will eventually be undetectable. The potential for unsupported 210Pb to provide an alternative method for discriminating these sources is demonstrated. Using Principal Component and Discriminant Function Analysis, and the three groups of sediment fingerprint signatures, it was possible to discriminate doleritic soils from other potential sources in the catchments, including valley fill and soils underlying vegetation that had been recently burned as part of the rangeland management system. Results suggest that fire was an important contributor to destabilising soil, which was subsequently delivered to the reservoir during high magnitude rainfalls. It is concluded that: (1) changes in the extent of cereal cultivation and livestock grazing density are most closely related to temporal changes in sedimentation rates in each of the two reservoirs; (2) temporal changes in the magnitude and frequency of extreme daily rainfalls do not correlate with reconstructed patterns of sediment accumulation through time; (3) the gully systems provide connectivity between the dominant hillslope sources and the reservoirs but have not contributed significantly to the sediment deposited in either reservoir; and (4) there is no evidence to suggest that sources of sediment accumulating in the reservoirs have changed significantly over the last ca. 70 years.