47 Causal Connections amongst Chemical/Physical Weathering, Geomorphology, and Biologic Processes I

Thursday, 11 August 2005
9:00 AM - 12:00 PM

INVITED: HILLSLOPE WEATHERING DYNAMICS IN A SOUTH AFRICAN SEMI-ARID SAVANNA

Paper number: 47-5
Program: THEME SESSIONS
Time: 10:40 AM – 11:00 AM

Authors

Abstract

Kruger National Park occupies roughly two million hectares of the South African Lowveld, a gently rolling terrain bounded to the west by the Drakensburg escarpment and to the east by the volcanic Lebombo hills. We are studying weathering and landscape development on the western side of the park, which is underlain by 3.1 Ga granitic gneisses of the Kaapvaal craton. Hillslopes in the Kruger landscape are often described as catenary sequences, where 0.5 to 0.7 meters of rainfall/year drive downslope transport and deposition of weathering products, both clays and solutes, forming a predictable sequence of soil types. Hillcrest soils are typically porous and sandy, grading downslope into clay-rich soils that force water to the surface, forming a seep line in the rainy season. Footslope soils contain abundant salts, which may represent accumulations of soluble weathering products from upslope. In a geochemical sense these hillslopes can be thought of chromatographic columns that accentuate differential solute mobility along extended (~1km) flowpaths. Using an immobile index element (Nb) approach, we are evaluating downslope elemental mobility on a series of granitic catenas with differing effective precipitations. We will further quantify rates of elemental transfers and landscape evolution via a combination of U-Th isotopic disequilibrium and cosmogenic radionuclide (26Al/10Be) work. These rates will be used to parameterize a dynamic landscape model for the region, using LIDAR-derived digital elevation models to fine tune our understanding of the routing of water and weathering products across these hillslopes.