SILICA AND CARBONATE SOURCES FOR ARIDISOLS ON A GRANITIC PEDIMENT, WESTERN MOJAVE DESERT

Three pedons on a granitic pediment in the western Mojave Desert, a Typic Durorthid, a noncalcareous Typic Haplargid, and a Typic Haplargid with carbonates below 80 cm, were studied to evaluate the potential of mineral weathering as a source of Si and Ca for the duripan and Bk horizons. Calcareous and noncalcareous soils occur as contiguous polypedons on the pediment surface, with calcareous soils overlying calcareous saprolite, and noncalcareous soils overlying noncalcareous saprolite. The irregular distribution of opaline silica and CaCO3 in these pedons cannot be attributed to a uniform input of eolian deposits across the pediment. Micromorphological and chemical evidence indicates that primary crystalline silicates in the calcareous horizons of these Aridisols have been weathered and replaced by opaline silica and calcite. Dissolution of plagioclase is a probable source of Ca in soil and saprolite carbonates. The lack of silt- and volcanic-glass-enriched vesicular A horizons suggests that eolian deposits of carbonates and volcanic ash are less significant in the formation of these soils than the contributions of weathering primary crystalline silicates in the granitic saprolite. The contribution of the weathering of crystalline silicates to soil formation in arid regions should not be overlooked.