GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION FAROUK EL-BAZ AWARD FOR DESERT RESEARCH: MODERN DUST IN THE SOUTHWESTERN U.S.: COMPARISON TO SOILS AND DESERT LOESS

Research during the past several decades has demonstrated that dust is incorporated into soils as they develop, forms a dominant component of surface soil horizons in arid regions, and provides plant nutrients and increased water-holding capacity. Modern dust samples collected in the Mojave and southern Great Basin deserts over the past 30 years show close similarities in particle size and composition to the dust component in soil horizons at paired sites, especially Av (vesicular, dust-dominated) horizons, suggesting that modern dust may also serve as an indicator of loess sources. However, modern dusts in both these deserts and the Canyonlands area of Utah (CANY) contain significant larger amounts of anthropogenically generated trace elements (Cd, Cu, Ni, Pb, Sb, and Zn) than do the Av and shallow soil horizons. Dust deposits (loess) retained in potholes weathered into high bedrock surfaces are analogous to young loess and show spatial trends in chemistry and magnetic properties that suggest mixing of local dust sources with Mojave Desert sources. Holocene loess deposits dated by OSL at two sites in the Mojave Desert and one site in CANY can be compared with modern dust from nearby dust samplers. At the Mojave loess sites, there are distinct trends toward bimodal distribution and finer grain sizes upward in the sections, suggesting changes in dust sources through time. The youngest Mojave loess samples exhibit nearly the same particle-size distributions as the modern dust, whereas the youngest CANY loess is somewhat coarser and better sorted than are most of the nearby modern dust samples. Modern deposition rates measured in the dust traps are roughly 20%-30% slower than geologic rates suggested by available loess ages. These results suggest that modern dust samples can provide valuable insights into the provenance of loess and dust-rich soils provided that the influence of anthropogenic emissions on trace-element chemistry is considered. Modern dust deposition rates are short-term measurements, and can be minimum estimates of total deposition rates unless dust traps are modified using wind baffles or results adjusted using calibration equations. Thus, caution is required to compare these rates to loess deposition rates.