Abs. No. 50256

EOLIAN DUST IN ARID-LAND SOILS--MAGNETIC PETROLOGY AND NUTRIENT INPUTS

Author(s): REYNOLDS, Richard (rreynolds@usgs.gov), YOUNT, James, and REHEIS, Marith, U.S. Geological Survey, MS 980, Box 25046, Denver, CO 80225 USA; MILLER, David, USGS, 345 Middlefield Rd., Menlo Park, CA 94025; BELNAP, Jayne, USGS, 2280 South W. Resource Blvd., Moab, UT 84532

Keywords: Dust, Soil, Nutrients, Canyonlands, Mojave

Eolian dust furnishes essential plant nutrients to arid-land ecosystems. Measurements of magnetic petrology, complemented by petrographic observations, rapidly establish quantitative mineralogic contrasts between surficial deposits and associated rock substrates and thus enable recognition of eolian dust in desert soils on the scale of ecosystems. The Canyonlands area of the Colorado Plateau is characterized by sedimentary rocks having simple iron-oxide mineralogy dominated by weakly magnetic hematite. In contrast, overlying sandy soils contain highly magnetic, silt-sized magnetite and related Fe-Ti oxides that formed originally in igneous rocks. Magnetic susceptibility (MS) of the soil is about two orders of magnitude greater than that of associated bedrock, yet strongly magnetic pedogenic minerals make up only a small part of the soil. The petrographic observations combined with MS results confirm eolian input of the magnetite in the surficial deposits. Correlation of magnetite content with P, Na, Mg, Zn, Zr, Al, Fe, and Ti reveals eolian input for a large proportion of these elements. Similar studies of fines in surficial deposits from the central Mojave Desert document eolian inputs to alluvial deposits and to a thin silt veneer on nearby peaks. These settings are much more complex in geologic substrate and magnetic mineralogy than those on the Colorado Plateau, so that MS by itself is not always sufficient for inferring the presence of eolian components in some deposits. Other properties, which provide measures of relative amounts of magnetite and hematite, hematite content, and magnetic grain size (reflecting the domain state of magnetite), indicate the presence of wind blown silt. The influence of differing geologic terrains on patterns of eolian input is revealed by consistent differences between magnetic and chemical properties of fines from alluvial surfaces and nearby peaks of the volcanic Greenwater Valley area of Death Valley, and the metamorphic and igneous Valjean Valley area, 75 km to the south.

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