OSL DATES RECORD ASPECT-RELATED DIFFERENCES IN SOURCE AND TIMING OF SEDIMENTS IN SOILS OF MOJAVE DESERT HILLSLOPES

Portions of the eastern Mojave Desert receive significant warm-season (monsoonal) precipitation that is capable of supporting native perennial grasses. These grasses form the dominant cover on mesic northern aspects where relatively thick, extensive soils have developed within stony colluvium. Similar soils occur on more xeric, southern aspects, but are restricted to small, isolated remnants of colluvial deposits. The existence of these remnants indicate the southern aspects were at one time more extensively mantled by soils. Hillslope soils contain blocky, coarse-crystalline clasts and sandy loam matrices. The fine fractions are consistently bimodal on all slopes (modes = 75 and 400-600 μm). The coarse silt mode apparently represents additions of aeolian sediments. The medium sand mode is too coarse to be of eolian origin, but was probably derived from the weathering of bedrock outcrops or clasts within colluvium. Coarser material generated by weathering on the surface would have been exposed to light before translocation into the colluvium. The timing of deposition of both eolian silt and medium sand fractions were determined using OSL. On north-facing slopes, ages of both silt and sand indicate steady accretionary and inflationary soil profile development through the late Pleistocene and into the mid Holocene (~16-5 ka). On southern aspects, medium sand deposition occurred 18-23 ka and eolian deposition occurred later (13-16 ka). Little in the way of eolian sediments have accumulated in the remnant soils on south-facing slopes during the Holocene. The difference in behavior between south and north slopes may result from the effectively dryer climate after the Pleistocene-Holocene transition. During the Holocene, the more xeric environment of southern aspects has supported only patchy, incomplete cover of perennial grasses. The lack of dense grass cover limits the capacity of those surfaces to accumulate aeolian sediments. Sparser perennial grass cover and greater bedrock exposure has generated greater runoff and associated erosion of soils on south-facing slopes. In contrast, the considerably denser cover of perennial grass has continued to function as an effective dust trap north-facing slopes throughout the Holocene.