QUATERNARY SOILS AND DUST DEPOSITION IN SOUTHERN NEVADA AND CALIFORNIA

Eolian dust constitutes much of the pedogenic material in late Pleisto cene and Holocene soils of many arid regions of the world. Comparison of the compositions and influx rates of modern dust with the eolian co mponent of dated soils at 24 sites in southern Nevada and California y ields information on (1) the composition and influx rate of dust in la te Pleistocene and Holocene soils, (2) paleoclimate and its effects on the genesis of aridic soils, especially with regard to dustfall event s, (3) the timing and relative contribution of dust from playa sources versus alluvial sources, and (4) the effects of accumulation of dust in soil horizons. The <2 mm fractions of A and B horizons of soils for med on gravelly alluvial-fan deposits in the study area are similar to modern dust in grain size, content of CaCO3 and salt, major oxides, a nd clay mineralogy; thus, they are interpreted tb consist largely of d ust. The major-oxide compositions of the shallow soil horizons are nea rly identical to that of the modern dust, but the compositions of prog ressively deeper horizons approach that of the parent material. The cl ay mineralogy of modern dust at a given site is similar to that of the Av horizons of nearby Holocene soils but is commonly different from t he mineralogies of deeper soil horizons and of the Av horizons of near by Pleistocene soils. These results are interpreted to indicate that d ust both accumulates and is transformed in Av horizons with time. Chan ges in soil-accumulation rates provide insights into the interplay of paleoclimate, dust supply, and soil-forming processes. Modern dust-dep osition rates are more than large enough to account for middle and lat e Holocene soil-accumulation rates at nearly all sites. However, the e arly Holocene soil-accumulation rates in areas near late Pleistocene p luvial lakes are much higher than modern rates and clearly indicate a dust-deflation and -deposition event that caused rapid formation of fi ne-grained shallow soil horizons on uppermost Pleistocene and lower Ho locene deposits. We interpret late Pleistocene soil-accumulation rates to indicate that dust-deposition rates were low during this period bu t that increased effective moisture during the late Wisconsinan favore d translocation of clay and CaCO3 from near the surface to deeper in t he soil profile. Pre-late Pleistocene rates are very low in most areas , mainly due to a pedogenic threshold that was crossed when accumulati ons of silt, clay, and CaCO3 began to inhibit the downward transport o f eolian material, but in part due to erosion.