Dust Deposition and Pedogenic Modification in an Arid Region: Tracing Soil Development with Strontium, Carbon, and Oxygen Isotopes

Numerous studies have shown that accumulation of dust in desert soils is a primary means of pedogenesis in arid regions. Dust deposition is an important factor significantly impacting, among other processes, pavement development, soil water balance and soil infiltration rates, which, in turn, strongly influence vegetation density and distribution. Establishing linkages between the timing and degree of dust deposition and subsequent soil modification with changes in the local ecosystem aid in our understanding of ecological processes and landscape evolution. The Providence Mountains and Cima Volcanic Field in the Mojave Desert, California have been the sites of several important studies concerning the origins of desert pavements, soils, and landscape evolution. More recently we have conducted strontium, and carbon and oxygen isotopic studies of the carbonate fraction in these soils utilizing contrasting parent materials (mixed plutonic and limestone alluvium, basalt flows) and surface ages (mid to late Pleistocene and Holocene) to better understand the sources, changes in composition, and overall impacts of the incorporation of eolian dust on desert soil development. Sr data documents regional differences in dust delivered to the Eastern Mojave area as compared to Southern New Mexico. We document a strong correlation between dust, Av horizon and soil profile composition. Analysis of fine material from the collected dust samples has constrained Sr compositions of dust (87Sr/86Sr = 0.7101 to 0.7107) delivered to the fans and flow surfaces. Comparisons between older and younger surfaces and with the dust data suggest that de-vegetation and de-stabilization of fan surfaces in the Holocene has caused more local recycling of dust in the region. Carbon and oxygen isotopic compositions of soil carbonate enable us to constrain modes and depth of pedogenic carbonate accumulation, and in some cases distinguish between physically recycled carbonate and re-precipitated carbonate. The greatest variability in C and O isotopic data within a soil profile are in discrete nodules (d13C = -4 to +1 and d18O = -8 to 0) indicating a complex formation history. Av horizons (d13C = 0 to +2.5 and d18O = -4) and surficial dust samples (d13C = -4 to +2 and d18O = -7 to -3) contain physically recycled material as well as re-precipitated carbonate. Some soil samples as well as carbonate collars on surface clasts exhibit anomalously high d13C and d18O values which suggests they are forming under non-equilibrium conditions.