Quaternary soils and dust deposition in southern Nevada and California

Abstract

Eoliandustconstitutesmuchofthepedogenic material in late Pleistocene and Holocene soils of many arid regions of the world.Comparisonofthecompositionsand influx rates of modern dust with the eolian component of dated soils at 24 sites in southern Nevada and California yields informationon(1)thecompositionandinflux rate of dust in late Pleistocene and Holocene soils, (2) paleoclimate and its effects on the genesis of aridic soils, especially with regard to dustfall events, (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<2mmfractionsofAandBhorizons of soils formed on gravelly alluvial-fan deposits in the study area are similar to moderndustingrainsize,contentofCaCO3and salt,majoroxides,andclaymineralogy;thus, they are interpreted to consist largely of dust. The major-oxide compositions of the shallow soil horizons are nearly identical to that of the modern dust, but the compositions of progressively deeper horizons approachthatoftheparentmaterial.Theclay mineralogyofmoderndustatagivensiteis similar to that of the Av horizons of nearby Holocene soils but is commonly different from the mineralogies of deeper soil horizonsandoftheAvhorizonsofnearbyPleistocenesoils.Theseresultsareinterpretedto indicate that dust both accumulates and is transformed in Av horizons with time. Changes in soil-accumulation rates provide insights into the interplay of paleoclimate,dustsupply,andsoil-formingprocesses. Modern dust-deposition rates are more than large enough to account for middle and late Holocene soil-accumulation rates at nearly all sites. However, the early Holocene soil-accumulation rates in areas near late Pleistocene pluvial lakes are much higher than modern rates and clearly indicate a dust-deflation and -deposition event that caused rapid formation offine-grained shallow soil horizons on uppermost Pleistocene and lower Holocene deposits. We interpret late Pleistocene soil-accumulation rates to indicate that dust-deposition rates were low during this period but that increased effective moisture during the late Wisconsinan favored translocation of clay andCaCO3fromnearthesurfacetodeeper inthesoilprofile.Pre‐latePleistocenerates are very low in most areas, mainly due to a pedogenic threshold that was crossed when accumulations of silt, clay, and CaCO3 began to inhibit the downward transport of eolian material, but in part due to erosion.