Abs. No. 52937

SR ISOTOPE AND MINERALOGIC STUDIES OF SOILS IN THE MOJAVE DESERT, CA: IMPLICATIONS FOR THE RELATIVE ROLES OF CHEMICAL WEATHERING AND ATMOSPHERIC DUST DEPOSITION

Author(s): MCLAIN, A. A., MCFADDEN, L.D., and ASMEROM, Y.A, Earth and Planetary Sci., Univ. of New Mexico, Albuquerque, NM, 87131; MCDONALD, E.V., Desert Res. Inst., Reno, NV

Keywords: DESERT, WEATHERING, STRONTIUM, MINERALOGY, DUST

The Providence Mountains (PM) and Cima Volcanic Field (CVF) in the Mojave Desert, CA have been the sites of several important studies concerning the origins of desert pavements, soils, and landscape evolution. More recently we have conducted strontium (Sr) isotopic studies of pedogenic carbonate in these soils utilizing contrasting parent materials (PM: mixed plutonic and limestone; CVF: basalt) 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. Previous Sr isotope data confirm that eolian dust is the primary source of calcium (Ca) in the pedogenic carbonate of these soils, but each soil profile has its own distinct carbonate Sr signature which varies little from surface to the lower horizons. The Sr isotope ratios of vesicular horizons (Av) and carbonate rinds at depth in the Holocene PM soils are shifted closer to the ratios of the underlying parent material than are the late Pleistocene PM soils. The purpose of this study is to examine the mineralogic reasons for these differences in the carbonate Sr composition of the Av horizons between soils located within kilometers of each other. X-ray diffraction results show variation in the mineralogy of the Av horizons, particularly in types of feldpars and relative abundance of carbonate minerals, suggesting that in the young soils, more of the alluvial material present in the Av horizon is being chemically weathered to produce the observed Sr signatures. Pedogenic calcite at depth also records this mixture of weathered dust and weathered fine alluvial material. We propose that the incorporation of silt and clay rapidly modifies the upper part of the soil to a far less permeable state and increases it's water-holding capacity, which is arguably conducive to chemical weathering. Previous studies of the mineralogy of the Av suggested, but could not confirm some contribution of chemical weathering to soil development. This study more convincingly clarifies the role of chemical weathering in these desert soils.

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