Tracing recent environmental changes and pedogenesis using geochemistry and micromorphology of alluvial soils, Sabie-Sand River Basin, South Africa

• We studied alluvial soils in the Sabie-Sand River Basin.
• Soils are weakly developed with lithological discontinuities.
• Soil geochemistry and micromorphology provided evidence of environmental change.
• Flood events affected soil development in the Sabie-Sand River Basin.

Three pedons on the alluvial terraces of the Sabie-Sand River Basin within Kruger National Park, South Africa, were studied to improve our understanding of recent environmental changes, and assess degree of chemical weathering and pedogenesis in the area using geochemical and micromorphology proxies. Particle-size distributions were obtained using Malvern Mastersizer; soil geochemistry was determined by XRF and thin sections by routine laboratory procedures. The soils are predominantly sandy (> 94% sand in all samples). The mean phi-values of the soils had little variation suggesting that reworking of sediments upwards in individual profiles produced a more uniform pedogenesis rather than coming from different physical sources. Calcification is the dominant pedogenic process in these alluvial soils. The Chemical Index of Alteration (CIA) proved a more suitable index than Chemical Index of Weathering (CIW) for evaluating weathering in the terraces. The micromass and b-fabrics are mostly granostriated and partly brown mosaic speckled. MISECA values for the degree of soil development range from 4 to 9, which mean weakly to moderately-developed soils. Coarse secondary calcite nodules and coatings are responsible for cementation as observed in pedon 2, which suggests calcium carbonate precipitation from periodical flooding and evaporating groundwater events. The features and diagnostic properties of the soils on the alluvial terraces along the Sabie-Sand River provide evidence for land surface impacts of recent environmental changes in this internationally important conservation area. Precise dating of calcium carbonate precipitates is, however, needed to put the observed evidence into a wider geochronological perspective.