THE IMPACTS OF TEMPORAL AND SPATIAL CLIMATIC CHANGES ON ALLUVIAL SOILS GENESIS IN SOUTHERN CALIFORNIA.
AuthorMCFADDEN, LESLIE DAVID.
KeywordsSoil formation -- California, Southern
Paleopedology -- California, Southern
Climatic geomorphology -- California, Southern
Soils and climate -- California, Southern
Committee ChairBull, William B.
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractSeveral 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.
Degree GrantorUniversity of Arizona
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Botanical reflections of the encuentro and the Contact Period in southern Marin County, California.Duncan, Faith Louise.; Longacre, William A.; Reid, J. Jefferson; Martin, Paul S. (The University of Arizona., 1992)Plant indicator species and longitudinal paleobotanical data were used as independent measures to document the human ecological record of the contact period in southern Marin County, California. These data suggest that archaeological and documentary records are insufficient for examining changes in land management and use during the contact period. Prior to A.D. 1579, Western Miwok peoples had not encountered Europeans face to face. This early phase of the contact period is marked the possible introduction of New World species through passive cultural vectors. Two brief encounters occurred between the Miwok and Europeans between A.D. 1579 and 1775. Introduced and weedy plant species from fossil samples appear to confirm these encuentros and confirm the archeological evidence for intermittent contact during the second phase of the contact period. Modern and fossil pollen samples suggest that the intensity of human disturbance is geographically stratified and related to exploration, procurement, and management of specific resources. Coastal prairie, the redwood forest, and Bay salt marshes were the most affected by the second phase of the contact period. Shifts in vegetation diversity and increases in the numbers of introduced and weedy species were compared between ruderal and undisturbed contexts. These data were used as analogs to monitor the final phase of contact between A.D. 1775 to 1817. Hypotheses derived from ethnohistoric and ethnographic sources that suggest rapid shifts in land management practices and changes in plant representation were corroborated by some pollen data. Specifically, the ecological responses to the suppression of anthropogenic burning, changes in land tenure and parcelization, and the initiation of grazing and logging practices were examined. The cumulative impacts of introduced plants, shifts in land management from Miwok to Euroamerican-dominated resource procurement and subsistence practices, and ecological responses of plant species suggests that the contact period might better be defined on ecological terms rather than by purely material cultural or ethnographic definitions. In southern Marin, paleobotanical data provide a measurable indication of the ecological character of the pre-contact landscape and the cultural processes that effectively altered its character during the contact period.