THE OVERLOOKED SOIL-FORMING FACTORS: DISTINGUISHING INFLUENCES OF SOIL AGE AND HYDROLOGY FROM CLIMATE INFLUENCES ON VERTISOLS, AND APPLICATIONS TO INTERPRETING PALEOSOLS

According to the classic model of Jenny, pedogenic processes are driven by interrelationships between five primary soil-forming factors: climate, topography, time, biota, and parent material. Most paleosol studies focus on reconstructions of paleoclimate, emphasizing applications of climate proxies developed from modern soils for estimating paleoprecipitation amount and seasonality, as well as paleotemperature in paleosols. Soil age and soil hydrological state are examined here as important controls on development for Vertisols, which are clay-rich soils formed from shrink-swell processes associated with seasonal moisture deficit and increased evapotranspiration. Trends in macro- and micromorphology and bulk chemistry are used to address three primary topics: 1) rates of Vertisol pedogenesis, 2) characteristics associated with time steps within the pedogenic process, and 3) influences of changes in saturation state in sites where seasonal ponding of water occurs. Soils formed on inset terraces of the Brazos River, TX (<18,000 yrs) and the related San Bernard River floodplain (<6,600 yrs), and on the Coast Prairie climosequence of Texas where soils formed from Beaumont alluvium (< 100,000 yrs: MAP ranging from 700-1450 mm/yr), collectively are used to investigate these three topics. Variations in profile thickness, lack of preservation of relict primary sedimentary bedding, depth of leaching, and depth and degree of development of pedogenic carbonate, slickensides, and clay fabric as well as geochemical mass balance (strain, translocation) are quantified for soils of different ages as well as variable saturation conditions. Results suggest that time-dependent morphological and chemical features may mimic those that are climate-dependent. Soils formed in drier climates may display similar characteristics to those seen in younger profiles, whereas soils formed in wetter climates may exhibit features comparable to older profiles, thus obscuring paleoclimate interpretations obtained from analysis of paleoVertisols. In addition, variable soil saturation state can result in close-spatial variability of parameters used for interpreting time and climate in paleoVertisols, such as depth to Bk and thickness of Bk horizons.