THE IMPORTANCE OF CLIMATE IN MODULATING HILLSLOPE WEATHERING ENVIRONMENTS AND LANDSCAPE EVOLUTION IN THE MOJAVE DESERT

Climate change exerts an important control on weathering processes, vegetation cover, soil thickness, and landscape evolution. Hillslopes at a site in the eastern Mojave Desert are mantled by soils derived from weathering of meta-granitoid bedrock combined with eolian inputs of fine sand and silt. Sediment from both sources was deposited on the soil surface and incorporated into colluvium, allowing both processes to be dated with optically stimulated luminescence (OSL). OSL ages indicate a period of increased colluviation and soil development in the late Pleistocene facilitated by dust deposition and increased rates of physical weathering. The wetter late Pleistocene was conducive to accelerated mechanical weathering by climate-dependent subcritical cracking processes. Topoclimate provides another important control on weathering environments, soil morphology, and associated vegetation. Soils with dense grass cover extensively mantle the mesic north-aspect hillslopes, while more xeric south-aspect hillslopes are dominated by desertscrub, thin soils and more extensive bedrock exposure. Cosmogenic radionuclide abundances and rock hardness measurements in exposed bedrock and bedrock buried by colluvium indicate higher rates of erosion under colluvium. Higher erosion rates in these environments are potentially related to moisture-driven vapor pressures that increase subcritical cracking rates relative to drier surface environments. Remnants of older colluvium with moderately developed soils on south aspects indicate they were once more extensively mantled by thicker colluvial deposits. The transition to drier conditions in the Holocene diminished vegetation cover on more xeric south aspects, triggering widespread erosion, whereas the more mesic north aspects retained more grass cover that minimized erosion.