dsolread.doc 5/1/95 Quaternary Soils and Dust Deposition in Southern Nevada and California Marith C. Reheis, Jonathan C. Goodmacher, Jennifer W. Harden, Leslie D. McFadden, Thomas K. Rockwell, Ralph R. Shroba, Janet M. Sowers, and Emily M. Taylor Abstract Eolian dust constitutes most of the pedogenic material in late Pleistocene and Holocene soils of many arid regions. Comparison of the compositions and influx rates of modern dust with the eolian component of dated soils at 24 sites in southern Nevada and California yields information on: (1) the composition and influx rate of dust in late Pleistocene and Holocene soils, (2) paleoclimate and its effects on the genesis of aridic soils, especially with regard to "dust events", (3) the timing and relative contribution of dust from playa sources versus alluvial sources, and (4) the effects of accumulation of fines in soil horizons. The A and B horizons of soils formed on gravelly alluvial-fan deposits in the study area are similar to modern dust in grain size, content of CaCO3 and salt, major oxides, and clay mineralogy; thus, they are interpreted to consist largely of eolian 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 approach that of the parent material. The clay 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 the mineralogies of deeper soil horizons and of the Av horizons of nearby Pleistocene soils. These results are interpreted to indicate that (1) dust both accumulates and is transformed in Av horizons with time, and (2) that clay minerals can be transformed in only 10,000 years or less. Changes in soil-accumulation rates provide insights into the interplay of paleoclimate, dust supply, and soil-forming processes. 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 of fine-grained shallow soil horizons on late Pleistocene and early 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 Wisconsin favored translocation of clay and CaCO3 from the surface to deeper in the soil profile. Calculated pre-late Pleistocene rates 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. INTRODUCTION The presence of eolian dust in soils and the relative contribution of dust to soil formation in both arid and humid areas has been debated for decades. Most researchers now agree that dust is a ubiquitous component of soils formed in arid areas, although some argue that calcareous dust does not contribute significantly to the content of pedogenic calcium carbonate in some localities. Many studies have also shown that eolian dust is an important component of soils in subhumid to humid areas. In arid and semiarid areas, eolian dust also plays a significant role in geomorphic processes. For example, desert pavements, ubiquitous features in arid regions, form when eolian dust accumulates beneath a surface layer of stones; this process acts to smooth initially rough surfaces. Once formed, however, the reduced porosities of dust-rich horizons lead to increased runoff and erosion, which increases surface roughness. Detailed studies of dust influx facilitate studies of paleoclimate based on modelling of soil-forming processes such as translocation of pedogenic carbonate. Most research on the eolian component of soils has focused on identifying the presence of dust and estimating its proportion relative to soil parent materials and in-situ weathering products. Approaches have included (1) assuming that most or all of a specific pedogenic component is dust- derived because it is not present in the parent material and is not likely to be formed in large quantities by chemical weathering of the parent material (such as CaCO3 and gypsum); (2) estimating the content of dust in soils versus the parent material using trace elements or isotopes; (3) comparing the mineralogy of dust and soils with parent material; and (4) using micromorphological techniques to identify foreign components or characteristic shapes of grains. Despite general agreement on the importance of dust to soil genesis, few studies have compared modern rates of dust deposition to estimated amounts of dust in soils of known age to compare the compositions and deposition rates of modern dust to dust in soils. Quantitative comparisons are important to studies of soil genesis, paleoclimatic reconstruction from soil properties, and soil chronosequences used to estimate the ages of surfaces and deposits. For example, soils that formed downwind of a large dust source may be significantly better developed than soils of the same age that formed in sheltered areas. A project to study modern dust deposition in southern Nevada and California was initiated in 1984 to provide data on modern dust composition and influx rates for use in a numerical model relating soil carbonate to paleoclimate and in soil-chronosequence studies in the southern Great Basin and Mojave Desert (fig. 1) in support of tectonic and stratigraphic investigations for the Yucca Mountain Project. In this paper, we relate the composition of modern dust to soil properties and compare modern rates of dust influx with late Pleistocene and Holocene rates estimated from soils at 24 sites in southern Nevada and California. A companion paper relates modern dust data from these and an additional 31 sites to climate, source area, and source lithology. List of files in the database and general contents: All *.doc files are in Word for Windows 1.0, all *.xls files are in Excel 3.0. dsolread.doc Modified abstract and introduction for soil-dust paper (in review) and list of files. dsolproc.doc Sampling and laboratory procedures for soil datafiles; equations; references for procedures and for some data from published sources dsolfoot.doc Footnotes, comments, and definitions of abbreviations and acronyms in soil files. dsolloc.xls Site data for soil profiles: soil locations, type and lithology of parent material, corresponding dust-trap number, and estimated climate data. dsoldes.xls Field descriptions of soils. dsindprn.xls Calculated soil-development-index values of soils. dsolab.xls Laboratory data for soil horizons. dsolpw.xls Calculations of soil profile weights of silt, clay, CaCO3, and salt. dsolox.xls Major-oxide data for Kyle Canyon, Silver Lake, and Wilson Creek soils. dsolmin.xls Clay mineralogy of soils (mostly culled from cited publications). averate.xls Average soil accumulation rates of silt, clay, CaCO3, and salt. intrate.xls Interval soil accumulation rates of silt, clay, CaCO3, and salt. intrtdev.xls Averages and standard deviations of interval soil accumulation rates.