HILLSLOPES IN THE MOJAVE: REGIMES OF WEATHERING, SOIL FORMATION, AND EROSION ALTERED BY A FLIP OF THE PLEISTOCENE-HOLOCENE CLIMATE SWITCH (Invited Presentation)

In upper elevations of the eastern Mojave Desert that receive significant inputs of warm-season precipitation, perennial grasses predominate on hillslopes in fine-grained soils that have developed within relatively thick colluvial mantles. The coarse-grained fraction of these soils is composed of angular clasts derived from metagranite bedrock. The clasts are not significantly weathered. The fine-grained component of these soils is largely derived from eolian sediment inputs that are amenable to OSL dating. The fine fraction of these soils is predominantly coarse silt and fine sand (modes = 75, 200, 500 μm). OSL dates for different sized particles yield two distinct age populations (50-150 μm = early Holocene and 150-240 = latest Pleistocene). Under the current climate regime, regolith production is minimal, but must have been substantially higher during the late Pleistocene for the colluvial mantles to have formed. Lower Pleistocene temperatures were probably not sufficient to increase physical weathering by frost shattering. Enhanced physical weathering and production of regolith may instead be related to increased rates of subcritical cracking caused by wetter conditions. Transition to a drier Holocene triggered a substantial stripping of colluvium and associated soils on more xeric south aspects. This stripping continues to the present because of the limited canopy cover (14.3%) provided by a tall, upright bunch grass (Pleuraphis rigida). Evidence of this stripping of south-facing slopes by overland flow during the Holocene is supported by the presence of thick (up to 5 m) debris aprons that occur predominantly along the base of those slopes. The loss of thicker soils required by grasses has produced a shift from predominance of grasses to desert shrubs. A threshold of minimal vegetation cover required to prevent such erosion has not been crossed on the more mesic north aspects, where the perennial grass canopy cover is substantially higher (36%), and is composed of species (Pleuraphis jamesii and Bouteloua eriopoda) that have low, compact crowns that provide considerably greater protection to the soil surface.