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 (Be and Al) 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. Short-lived, fallout-derived isotopes (Pb, Cs, and Be) are measured in soil profiles to quantify sedimenttransport processes and short-term erosion rates. Parent material strength (competence), or resistance to shear, is measured using a hand-held shear vane as well as a cone penetrometer. The methodology for some results and the connections drawn from this diverse toolbox are summarized in this chapter. An important conclusion connecting parent material strength to the physical processes transporting soil and the chemical processes weathering the parent material emerges with the observation that parent material strength increases with overlying soil thickness and, therefore, the weathered extent of the saprolite. This observation highlights the importance of quantifying School of Earth and Space Exploration, Arizona State University, AZ, USA. Email: [email protected] Hydropedology, Edited by H. Lin. DOI: 10.1016/B978-0-12-386941-8.00007-1 Copyright 2012 Elsevier B.V. All rights reserved. 205 Hydropedology, First Edition, 2012, 205–242 Author's personal copy