The Quantification of Mass Loss via Geochemical Weathering in Hillslope Processes

Mass loss from hillslopes occurs by both solute removal and physical erosion, and generally it is felt that, for a given rock type, the proportion of total mass loss by solution increases as the physical erosion rate diminishes. This study focuses on the chemical weathering losses on a hillslope located at the base of the Great Escarpment, in southern NSW, Australia where extensive soil production, transport and erosion has been previously documented by Heimsath and his collaborators. The hillslope is bounded on one side by a fault that has been deeply incised by a creek. Soil pits and a trench were dug across the hillslope in order to map the soil-saprolite interface and quantify solution mass losses from both the saprolite and soil as a function of depth and transport distance. Results show that soil thickness varies fairly linearly with transport distance. The chemical data show that up to 30% of the mass of the original material is lost in solution from the saprolite, and another 30% from soils transported downslope. The distribution of major element concentrations (Si, Ca, Mg, etc.) changes systematically as a function of soil thickness and transport distance. On the ridge crest, where soil particle movements are the result of diffusion-like processes, the soils are geochemically stratified. These soils have higher concentrations of almost every major element closer to the soil/saprolite boundary, and these concentrations decrease as one moves further into the soil (as soil thickness increases) and further from the ridge (as transport distance increases). In the concave regions of the hillslope, distributions of elements are much more homogeneous. The results differ from those obtained previously at the top of the escarpment, where long-term erosion rates are slower and solute loss can account for up to 80% of landscape lowering.