SEDIMENT PRODUCTION AND TRANSPORT ON HILLY LANDSCAPES

The production and transport of sediment is the focus of extensive study across diverse landscapes. Landscape form has long been recognized to be shaped by processes of erosion, and model quantification of how landscape form may change due to external forcing depends on the laws chosen to characterize the dominant erosion. These laws are rarely based on field data, or direct observation, but are used widely to drive landscape evolution models. Here I focus on hilly soil mantled landscapes to demonstrate how two relatively new tools can be coupled and used to quantify erosion rates and mechanisms.

Soil production rates are quantified using the in situ produced cosmogenic radionuclides, ¹⁰Be and ²⁶Al, extracted from bedrock beneath the soil mantle. Nuclide concentrations are used to infer both local and catchment-averaged (when measured from stream sediments) erosion rates as extensive studies have demonstrated elsewhere. Hillslope sediment transport processes have been quantified by other studies using segmented rods, short-lived isotopes (under very constrained conditions), and fallout ¹⁰Be. I show how both short-lived isotopes and a new method of using optically stimulated luminescence can be used where soil production or erosion rates are known to determine the dominant transport mechanisms. Specific examples from southeastern Australia (using OSL) and the Hubbard Brook experimental watershed in New Hampshire (using the short-lived isotopes ²¹⁰Pb, ⁷Be, and ¹³⁷Cs) show how thoroughly the soil profiles are bioturbated and emphasize the role of overland flow in shaping the landscape.

A Monte Carlo model helps connect the nuclide results with the transport results to suggest that grain movements are independent and random. The model also suggests that 20 % of the transported soil was removed by overland flow over roughly the last 10 ka. This helps explain the divergence of field observations from a prediction of slope morphology and soil depth based on a simple diffusion model. These field results further support a new transport model that combined expressions for simple creep, depth-dependent creep and overland flow to predict soil thickness and suggest how a landscape evolves in response to climatic and tectonic forces.