QUANTIFYING THE ROLE OF SAPROLITE WEATHERING IN LANDSCAPE EVOLUTION

Much effort focuses on quantifying rates and processes of landscape evolution. Advances in cosmogenic nuclide and thermochronometric methods have enabled us to more effectively examine such rates on timescales relevant to climatic, tectonic and geologic forcings. We focus here on the evolution of upland soil-mantled landscapes where coupling the rates and mechanisms of physical erosion and chemically weathering enables an even more complete understanding. We present an adapted framework for quantifying chemical weathering in shallow soils and deep saprolites in order to extract more meaningful information on the processes and timing of landscape change.

One conventional model for calculating saprolite weathering rates employs a mass balance approach where total denudation rates are derived from cosmogenic nuclides and chemical weathering rates are inferred from immobile element concentrations. We build on this model and present a reevaluation of the approach to quantify saprolite weathering and total weathering rates. Specifically, we suggest that cosmogenic derived denudation rates from saprolite and sediments must be coupled with immobile and mobile element concentrations measured in soil, saprolite, and the unweathered bedrock. We also address previous model limitations and confounding issues such as timing of saprolite weathering, development of deep weathering profiles, potential translocation of secondary weathering products, and collapse of non-isovolumetric soils and saprolites.

Furthermore, we use denudation rates from southeastern Australia and the California Sierra Nevada to validate our model, and propose a new field sampling methodology to more effectively calculate rates of erosion and weathering.