MULTI-PROXY APPROACHES TO INTERPRETING CLIMATE AND SOIL DURATION IN THE DEEP-TIME GEOLOGIC RECORD USING VERTISOLS

Vertisol-like paleosols, which are clay-rich and exhibit abundant evidence for extensive shrink-swell processes, contain important paleoclimate information that is generally under-interpreted by geologists. Paleoclimatic studies based on paleosols are hindered by a lack of diagnostic climate indicators determined from thorough studies of climosequences in modern soils. NSF-funded research examined Vertisols formed on late Pleistocene (<35 ka) Beaumont alluvium in the Coastal Prairie region of Texas along a climatic transect. These studies have characterized distinctive morphological and chemical features that correlate well with climate and identified paleoprecipitation proxies that include total Fe content of soil Fe-Mn nodules, depth to pedogenic carbonate (DTC), Ti:Zr content vs. depth, total element mass-flux, and the CALMAG geochemical climofunction, which are all robust and calibrated specifically for Vertisols. Interpretation of paleosols is confounded, however, by the development of common soil features in response to different soil-forming factors. Vertisol studies that included a chronosequence from the Brazos Valley, the lower Rio Grande Valley, and a hydrosequence from the San Bernard floodplain in Texas included pedons sampled as part of the previous climosequence study. Initial findings suggested that vertic properties are acquired in as little as a few hundred years, and that steady-state conditions are achieved in only a few thousand years. Poor soil drainage can retard soil development but is generally recognizable due to redoximorphic features. However, there is great potential for confusion between immature soil development due to young soil age versus that attributed to reduced MAP. The diagnostic morphological and chemical characteristics identified in Vertisols can be used to interpret MAP and duration of pedogenesis for Vertisol-like paleosols occurring within deep-time stratigraphic successions in order to reevaluate their paleoclimatic and time significance.