A modern soil characterization approach to reconstructing physical and chemical properties of paleo-Vertisols

Abstract

Because of burial compaction of sediments, elemental oxides of bulk samples are commonly used by paleopedologists to infer physical and chemical properties of paleosols (fossil soils) for interpreting pedogenic pathways, fertility levels, taxonomic classification, and climate conditions. As a consequence, the interpretive value of lithified paleosols is limited. Here we establish an array of transfer functions along a modern Vertisol climosequence for converting bulk soil elemental oxides to physical and chemical properties as determined by the USDA-NRCS characterization program. These transfer functions permit, for the first time, the reconstruction of colloidally-based physical and chemical properties of paleosols. We discovered, for example, that bulk soil CaO+MgO is a powerful predictor of fine clay, pH, base saturation, coefficient of linear extensibility (COLE), and cation exchange capacity (CEC). Al~2~O~3~ is important for estimating total clay, Fe~2~O~3~ for crystalline pedogenic iron oxides (Fe~d~), Ni for amorphous pedogenic iron oxides (Fe~o~), and Na~2~O/Al~2~O~3~ for exchangeable sodium percentage (sodicity) and electrical conductivity (salinity). SiO~2~ in turn relates moderately well with bulk density and with COLE in calcareous horizons, whereas CaO and CaCO~3~ content can be used interchangeably. Both detrital and pedogenic carbonate have an important influence on many accessory physical and chemical soil properties and on the selection of the appropriate transfer functions. Consistent with modern arid to semiarid soils, a reconstructed late Triassic desert shrubland paleo-Vertisol has high pH, high pedogenic iron oxide content, and intermediate levels of CEC, shrink-swell potential, and calcium carbonate content. In contrast, a forested late Cretaceous paleosol with Vertisol properties is more weathered with slight acidity, moderate base saturation, and relatively high amorphous iron oxides due to periodic anaerobiosis. The third paleosol, a Vertisol, formed under a Paleocene woodland and has intermediate properties. None of the paleosols appear to have saline or sodic conditions with the late Cretaceous and early Paleocene paleosols having greater plant nutrient availability because of neutral pH and greater water holding capacity related to clay content.