Quantifying sediment transport on desert piedmonts using 10Be and 26Al

In situ produced 10Be and 26Al, measured in 40 sediment samples collected from the Iron and Granite Mountain piedmonts, eastern Mojave Desert, provide a unique view of piedmont modification processes and process rates over the 103 to 105 year time scale. Cosmogenic nuclide-based models suggest that the Iron and Granite Mountains generate 0.11–0.13 and 0.082–0.097 m3 of sediment per year per meter of rangefront, respectively. The sediment moves down the piedmont in an active transport layer (ATL), which is 20 to 30 cm thick (based on visual observations, measurements of depth to a buried B-horizon, cosmogenic nuclide data, and maximum ephemeral channel depths). Sediment in this layer is well-mixed vertically and horizontally on the 102 year time scale, indicating that the small ephemeral channels, which dominate the piedmont surface migrate quickly. Interpretive models of increasing nuclide activities at depth in two pits suggest steady sediment deposition on the piedmont (at rates between 17–21 and 38–45 m Ma−1) until the late Pleistocene epoch, when a discontinuity to markedly lower nuclide activities in the isotopically well-mixed active transport layer suggests that deposition stopped, a significant change in piedmont behavior. Nuclide activities in 10 amalgamated surface samples, each collected along a different 4-km-long transect, increase steadily away from the mountain front. Thus, we infer that sediment is uniformly dosed by cosmic rays as it is transported down the Iron and Granite Mountain piedmonts. Interpretive models suggest that long-term average sediment speeds down the Iron and Granite Mountain piedmonts are a few decimeters to a meter per year.