Eolian dust constitutes most of the pedogenic material in latePleistocene and Holocene soils of many arid regions. Comparison ofthe compositions and influx rates of modern dust with the eoliancomponent of dated soils at 24 sites in southern Nevada andCalifornia yields information on: (1) the composition and influxrate of dust in late Pleistocene and Holocene soils, (2) paleoclimateand its effects on the genesis of aridic soils, especially withregard to "dust events", (3) the timing and relative contribution ofdust from playa sources versus alluvial sources, and (4) the effectsof accumulation of fines in soil horizons. The A and B horizons ofsoils formed on gravelly alluvial-fan deposits in the study area aresimilar to modern dust in grain size, content of CaCO3 and salt,major oxides, and clay mineralogy; thus, they are interpreted toconsist largely of eolian dust. The major-oxide compositions of theshallow soil horizons are nearly identical to that of the moderndust, but the compositions of progressively deeper horizons approachthat of the parent material. The clay mineralogy of modern dust at agiven site is similar to that of the Av horizons of nearby Holocenesoils, but is commonly different from the mineralogies of deeper soilhorizons and of the Av horizons of nearby Pleistocene soils. Theseresults are interpreted to indicate that (1) dust both accumulatesand is transformed in Av horizons with time, and (2) that clayminerals can be transformed in only 10,000 years or less. Changes insoil-accumulation rates provide insights into the interplay ofpaleoclimate, dust supply, and soil-forming processes. Moderndust-deposition rates are more than large enough to account formiddle and late Holocene soil-accumulation rates at nearly all sites.However, the early Holocene soil-accumulation rates in areas nearlate Pleistocene pluvial lakes are much higher than modern rates andclearly indicate a dust-deflation and -deposition event that causedrapid formation of fine-grained shallow soil horizons on latePleistocene and early Holocene deposits. We interpret latePleistocene soil-accumulation rates to indicate that dust-depositionrates were low during this period but that increased effectivemoisture during the late Wisconsin favored translocation of clay andCaCO3 from the surface to deeper in the soil profile. Calculatedpre-late Pleistocene rates are very low in most areas, mainly due toa pedogenic threshold that was crossed when accumulations of silt,clay, and CaCO3 began to inhibit the downward transport of eolianmaterial, but in part due to erosion.
