Dates and rates : an examination of hillslope sediment ages and the rates of controlling geomorphic processes using cosmogenic nuclides

Bedrock weathering on hillslopes is responsible for the majority of sediment located in drainages in dryland environments. Weathered bedrock is stored on slopes as colluvium before it is eroded from hillslopes and introduced to the fluvial system. The balance between the rate of bedrock weathering and erosion govern the amount of material that is supplied by hillslopes. Hillslope process literature suggests that colluvial thickness over bedrock controls the rate of bedrock weathering and that slope controls the rate of erosion where diffusive transport is the principal transport mechanism. Furthermore, climate has been shown to control the effectiveness of various weathering and erosional processes, and therefore may also influence colluvial thickness on slopes. In this study, we use concentrations of the terrestrial in-situ produced cosmogenic (TCN) isotope 10Be, which forms when quartz minerals interact with cosmic radiation near the surface of the Earth, to explore the rates of dryland hillslope processes and exposure histories of colluvial material. We measured 10Be concentrations in twenty bedrock and colluvium samples from two hillslope study areas; the Sandia Mountains, and the eastern Mojave Desert. At our study areas, significant variation in isotopic concentrations (3.06 - 14.03 x105 atoms10Be g-1) arising from differential rates of bedrock weathering, downslope transport, and residence time of material in thick colluvial mantles. Bedrock weathering rates increase when bedrock is mantled by colluvium, and that most colluvial material is produced from mantled bedrock. Furthermore, slope aspect likely influences rates of soil production. Rates of downslope transport can be constrained using TCN concentrations, and that assumptions of steady-state transport rates are not applicable for sediment at or below B-horizons. TCN measurements collected through a soil profile may be used to understand the ages and depositional conditions for hillslope colluvium. Colluvial mantles are dynamic landscape feature and climate plays an important role in influencing hillslope process. As a result, Quaternary climate fluctuations limit the applicability of steady-state conditions to the evolution of dryland hillslopes.