THE IMPACTS OF TEMPORAL AND SPATIAL CLIMATIC CHANGES ON ALLUVIAL SOILS GENESIS IN SOUTHERN CALIFORNIA.

Abstract

Several soil chronosequences were studied in southern California to determine the relative impacts of time and climatic change on soil genesis. Studying soil development in climatic regimes that vary from the moist, xeric climate of the coastal basins and Transverse Ranges to the hot, arid climate of the interior deserts of southern California provide data useful for evaluation of the impact of climatic change as well as time on pedogenesis. Seven distinctive stages of soil development are recognized in the study area. The first three occur in Holocene soils, and the last four are associated with late to mid-Pleistocene geomorphic surfaces. A distinct pattern of secondary soil mineral authigenesis is observed in increasingly older soils. The rapid formation of vermiculite and iron oxyhydroxides in xeric climates is attributed to rapid alteration of unstable Fe-bearing aluminosilicates. Continuous weathering of abundant feldspars results in a predominance of neogenetic kaolinite in mid-Pleistocene soils. Slightly acidic to mildly alkaline soil pH, rapid hydrolysis, and availablity of organic complexes result in formation of significant amounts of metastable ferrihydrite in young Holocene and late Pleistocene soils. Ferrihydrite dehydration and crystal aggregation result in hematite formation and increasingly lower Fe(,2)O(,3)o:Fe(,2)O(,3)d ratios. Arid climatic regimes are conductive to minimal chemical weathering. Clay/iron oxhydroxide regression analyses and mass balance calculations show that much of the silicate clay and secondary carbonate have been derived from external sources rather than by chemical weathering. Clay mineral authigenesis is characterized primarily by conversion of montmorillonite to palygorskite. A compartmental model developed in this study accurately predicts calcic horizon development under Holocene soil water balance characteristics. Results of model predictions indicate that the distribution of carbonate observed in latest Pleistocene soils is related to past changes in climate. In addition, mass balance calculations suggest that large decreases in chemical reaction rates in soils due to soil temperature decreases may well be offset by increases in the magnitude of weathering. However, the results of this study indicate that calcium carbonate provides the most sensitive index of past climates when compared to other indices and that temporal change in climate has significantly influenced soil development in southern California.