Paleosols and Weathering Through Geologic Time: Principles and Applications
Climatic influences on rates and processes of soil development in Quaternary deposits of southern California
Published:January 01, 1988
Leslie D. McFadden, 1988. "Climatic influences on rates and processes of soil development in Quaternary deposits of southern California", Paleosols and Weathering Through Geologic Time: Principles and Applications, Juergen Reinhardt, Wayne R. Sigleo
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Many Quaternary fan and terrace deposits of the arid and semiarid regions of the Mojave Desert and the subhumid (Mediterranean) Transverse Ranges, southern California, have similar parent material and are relatively well dated, thereby enabling evaluation of the influence of changes in climate on the rates and processes of soil development. Within soils of a given chronosequence, soil age and many morphologic, mineralogic, and chemical properties are strongly related, reflecting primarily an evolutionary, time-dependent trend of continuous soil development during Quaternary time. Comparison of soil development in Holocene deposits of similar age of arid, semiarid, and subhumid regions indicates that increases in the amount of effective soil moisture explain observed systematic differences in the rates and processes of soil development. Accordingly, major climatic changes at the end of Pleistocene time have resulted in the development of soil profiles that are, to a certain extent, polygenetic. This change in climate caused a decrease in the depth of leaching, causing accumulation of secondary carbonate and locally gypsum, materials derived from incorportion of eolian fines, in argillic B horizons of Late Pleistocene soils. Increases in dust influx and availability of calcareous dust, caused by the widespread development of playas and decreases in vegetation at the end of the Pleistocene, have probably also influenced rates of soil development.
In subhumid regions, the leaching environment favors development of noncalcic profiles during Quaternary time. This leaching environment may approximate the leaching environment experienced during late Pleistocene time by soils that are forming presently in a semiarid climate that favors accumulation of calcium carbonate. Late Pleistocene and older soils in presently semiarid regions also possess thick, clay-rich B horizons with authigenic clay and iron-oxide minerals, features that indicate soil development during past periods of increased effective moisture. The relatively rapid development of argillic horizons in latest Pleistocene soils of currently subhumid regions is attributed to attainment of a threshold of soil formation, rather than the Pleistocene-to-Holocene climatic change. The threshold is caused by the accumulation of large amounts of silt and organic matter, a process that causes decreases in permeability, increases in available water-holding capacity, and ultimately increases in the rate and magnitude of chemical weathering. Soil chronosequence studies in California demonstrate that rates and processes of soil development may be influenced by changes in climate, in a direct fashion by causing changes in leaching regime, or in an indirect fashion by causing variation in the pattern and intensity of eolian activity.