LATE QUATERNARY GEOCHRONOLOGIC RECORD OF SOIL FORMATION AND EROSION: THE IMPORTANCE OF VEGETATION IN MODULATING HILLSLOPE RESPONSE TO CLIMATE CHANGE

Periods of increased alluvial fan aggradation around the late Pleistocene-Holocene transition (LPH) and again in the middle- to late Holocene have been reported for sites throughout the Mojave Desert. Aggradational events have long been attributed to increasing aridity that diminished hillslope vegetation and increased erosion. Recent dating of alluvial fan sediments, however, suggests that some fan aggradation may not be so tightly linked to major vegetation changes, but instead driven by changes in storm type and frequency. Variability in the timing of aggradation throughout the region may also be related to contrasts in vegetation as a function of varying elevation and seasonality of precipitation. Hillslopes at a site in the eastern Mojave Desert (~1,500 asl) are mantled by various thicknesses of soil and where sufficiently thick, perennial C₄ grasses are sustained by warm-season precipitation. Well-developed soils with dense perennial grass cover extensively mantle mesic north-aspect hillslopes, while more xeric south-aspect hillslopes are dominated by thin colluvium, extensive bedrock exposure, and desertscrub vegetation. Remnants of older colluvium on south aspects indicate those hillslopes were once extensively mantled by thicker colluvial deposits. OSL ages indicate a period of increased colluviation in the late Pleistocene facilitated by enhanced bedrock weathering; incorporation of eolian sediments generated fine-grained B horizons in those deposits. The transition to drier conditions in the Holocene diminished vegetation cover on more xeric south aspects, but not until the middle Holocene was that cover apparently reduced below a threshold that led to a major episode of soil erosion and sediment production. North aspect hillslopes experienced little erosion during the LPH transition. We attribute variable hillslope response to the strong role played by grass-dominated vegetation at this eastern Mojave Desert site, which receives substantial inputs of warm-season precipitation. The history of sediment production differs from sites to the west, where summer precipitation markedly diminishes, and at lower elevations, where smaller precipitation inputs have long been insufficient to support vegetation capable of protecting underlying soils during times of climatic transitions.