Geochemical Trends along a Modern Vertisol Climosequence Applied to Vertic-Paleosols as Proxy for Physical-Chemical Colloidal Properties and Pedogenic Interpretations

Paleosols are subjected to a variety of diagenetic processes that irreversibly alter important physical-chemical colloidal properties (e.g. pH, base saturation, cation exchange capacity or CEC). Diagenesis precludes the use of modern soil characterization techniques to quantify paleosol properties, limiting interpretations of ancient pedogenic processes required for taxonomic classification, ecosystem reconstruction, and climate inferences. As a consequence, we compare bulk soil molecular oxide ratios to standard soil laboratory characterization properties along a Vertisol climosequence spanning 200 to 1450 mm of mean annual precipitation, and use these relationships to reconstruct physical-chemical colloidal properties of paleosols. CIA-K (Al/Al+Ca+Na) is a strong predictor of CaCO3 content and also of base saturation, pH, and CEC/clay. For noncalcareous horizons, CIA-K correlates best with CEC/clay, but also with CEC, Fed (free Fe oxides), clay content, and COLE (coefficient of linear extensibility). The base loss ratio (Al/Ca+Mg+K+Na) correlates strongly with pH and base saturation and secondarily with CaCO3 content. In noncalcareous horizons the correlation is with CEC and CEC/clay. The calcification ratio (Ca+Mg/Al) is less robust only moderately correlating with CaCO3 content and pH. The clayeyness ratio (Al/Si) relates to Fed, and in noncalcareous horizons with clay content, COLE, CEC, base saturation, and CEC/clay. The salinization ratio (Na/K) correlates with exchangeable Na and electrical conductivity. Further, many of these molecular oxide ratios relate strongly and positively to mean annual precipitation. Results indicate that Vertisols weather by removal of exchangeable base cations rather than by authigenic clay formation and translocation. Application to two Cretaceous paleosols yields reasonable estimates of physical-chemical colloidal properties that existed prior to diagenesis, enhancing pedogenic interpretations and taxonomic classification.