Quantifying Processes Governing Soil-Mantled Hillslope Evolution

This chapter presents an overview of how field-based methods quantify the processes shaping upland, soil-mantled landscapes. These methods have been applied across diverse field areas, ranging from the tropical sandstones of northern Australia to the alpine granites of the Sierra Nevada in California. In all cases, the landscapes examined through such work are relatively gently sloping with a generally continuous soil mantle. Soil on such upland landscapes is distinctly defined to be the physically mobile layer derived primarily from the underlying parent material with organic content from native flora and fauna. These upland soils are distinguished from agriculture or lowland soils by the convex-up, hilly topographies that are the focus of this study. Parent material is generally saprolite, i.e. weathered bedrock that retains relict rock structure and is physically immobile. Processes shaping such landscapes include the physical and chemical processes that weather the parent material, and the processes moving the soil downslope. These processes are quantified using several different field-based methods. In situ produced cosmogenic nuclides (¹⁰Be and ²⁶Al) are measured in the parent material directly beneath the mobile soil mantle and define the relationship between soil production rates and the overlying soil thickness. The same cosmogenic nuclides are measured in detrital sediments sampled from local channels to quantify basin-averaged erosion rates. Mobile and trace elements are measured in both the parent material and the soils to define chemical weathering rates and processes.