Aeolian dust in Colorado Plateau soils: nutrient inputs and recent change in source

Abstract

Aeolian dust (windblown silt and clay) is an important component in arid-land ecosystems because it may contribute to soil formation and furnish essential nutrients. Few geologic surfaces, however, have been characterized with respect to dust-accumulation history and resultant nutrient enrichment. We have developed a combination of methods to identify the presence of aeolian dust in arid regions and to evaluate the roles of this dust in ecosystem processes. Unconsolidated sandy sediment on isolated surfaces in the Canyonlands region of the Colorado Plateau differs greatly in mineralogical and chemical composition from associated bedrock, mainly aeolian sandstone. Detrital magnetite in the surficial deposits produces moderately high values of magnetic susceptibility, but magnetite is absent in nearby bedrock. A component of the surficial deposits must be aeolian to account for the abundance of magnetite, which formed originally in far-distant igneous rocks. Particle-size analysis suggests that the aeolian dust component is typically as much as 20-30%. Dust inputs have enriched the sediments in many elements, including P, Mg, Na, K, and Mo, as well as Ca, at sites where bedrock lacks calcite cement. Soil-surface biologic crusts are effective dust traps that apparently record a change in dust sources over the past several decades. Some of the recently fallen dust may result from human disturbance of land surfaces that are far from the Canyonlands, such as the Mojave Desert. Some land-use practices in the study area have the potential to deplete soil fertility by means of wind-erosion removal of aeolian silt.

Figure 1

Location map of sampling sites and inset map of western U.S. Boundary of the Colorado Plateau surrounds rectangle indicating the area of the site-location map. GB, Great Basin; Mo; Mojave Desert; S, Sonoran Desert; M, Marysvale volcanic field; SJ, San Juan volcanic field.

Figure 2

Plots of MS (in 10⁻⁷ m³ kg⁻¹) (a) and hard isothermal remanent magnetization (in 10⁻⁴ Am² kg⁻¹) (b). For each site or average for a group of sites (site nos. 1–3 and 4–6), the left-hand bar represents BSC (0–0.5 cm in depth); the middle bar represents underlying surficial sediment (0.5–5 cm in depth); and the right-hand bar represents bedrock. At site UES, the middle bar represents a depth of 1–10 cm. For both parameters, 46 samples from the BSC, 45 from underlying sediment, and 16 from bedrock are represented.

Figure 3

Plots of elemental contents as well as Zr/Ti and K/Mg, normalized to highest value, shown at the top of bar sets. Plots represent averaged values from all sites, except the Ca plot, which shows results from site 12 where bedrock contains little calcite cement. Values are in ppm (p) or wt %. Bars represent depths and bedrock as in Fig. 2. Results are from energy-dispersive x-ray fluorescence for Ti, Fe, Zr, Mn, Cu, Mo, and Zn, and are from inductively coupled plasma–atomic emission spectroscopy for the others.