CLIMATIC EFFECTS on CATCHMENT-AVERAGED 10BE EROSION RATES FROM THE ARID PROVIDENCE MOUNTAINS, SOUTHERN CALIFORNIA

We use cosmogenic ¹⁰Be concentrations in alluvial sediments to determine the catchment-averaged erosion rates of watersheds draining Proterozoic gneisses and Mesozoic granites of the Providence Mountains, eastern Mojave Desert, California. The region lacks Quaternary faulting and includes pre-Pleistocene alluvial fan complexes, which suggest gradual decay of this range since at least Pliocene time. However, surficial mapping on 1 m LiDAR indicate that many different fill terraces exist, which could be evidence for fluctuations in sediment supply from the source region. To investigate whether these changes are modulated by climate change, we sampled active washes to estimate recent catchment-averaged erosion rates, and stratigraphic sections to estimate paleoerosion rates using ¹⁰Be, ²⁶Al, and OSL.

Here, we focus on initial results from wash sediments. Erosion rates vary from 33.27 ± 1.20 to 62.40 ± 1.40 m/My and show no apparent relation to grain size. These rates are similar to cosmogenic nuclide-determined erosion rates elsewhere in the Mojave Desert (~35 to 40 m/My). However, they exceed those estimated for granitic catchments in the humid Appalachian Mountains (17 to 37 m/My), another landscape lacking recent tectonism.

We hypothesize that higher rates in the Mojave reflect more frequent geomorphically effective rainfall compared to the humid Appalachians. For example, in the Providence Mountains, precipitation is characterized by high intensity, short duration convective storms, and there is less vegetation to provide cohesive strength to soils. This translates to a higher frequency of intense precipitation that might evacuate sediment from colluvial hollows and initiate stream or debris flow incision in steep valleys, resulting in relatively fast hillslope erosion rates. Conversely, in the Appalachians, where there is more root cohesion, the frequency between debris flow-initiating events is likely longer, resulting in relatively slow erosion rates. The difference in erosion rates between arid landscapes in California and humid landscapes in the Appalachians could thus be explained by the frequency of hillslope boundary lowering events such as landslides and debris flows.