GEOCHEMICAL THRESHOLDS BRACKETT PH STABILITY FIELDS IN SOILS AND PALEOSOLS

Because of burial compaction the chemical characterization of paleosols is typically performed by the geochemical assay of whole-rock samples. This methodological approach limits pedogenic interpretations because geochemical elements often come from multiple constituent sources. The focus of this research is to test a new geochemical approach for estimating preburial pH of paleosols. Doing so will strengthen interpretations of ancient critical zones because acid-base reactions govern the intensity of weathering, nutrient cycling, and biological activity. To assess a simple geochemical system we first tested the relationship between whole-soil elemental assays and pH of Vertisols queried from the Baylor Soil Informatics Cloud. We discovered geochemical thresholds at pH 6.0 (hydrolysis line) and pH 7.5 (carbonate line) that demarcate three chemical fields. The alkaline chemical field (pH - 7.5 to 8.5) coincides with MAP < 1000 mm. Here, the abundance of Ca-Mg-Na-K oxides is high, and along with Al-Si-Fe oxides, are sourced from an admixture of CaCO₃, small amounts of mafic and felsic minerals, 2:1 clay minerals, and exchangeable cations on clay colloids. When MAP is < 1000 mm, CaCO₃ is depleted and pH falls into the neutral chemical field (pH - 7.5 to 6.0). Here, Al₂O₃ and SiO₂ remain relatively stable, with CaO and MgO dominating the base oxide suite. The acid chemical field (pH < 6) only occurs when MAP is >1200, which exhibits higher Al₂O₃ to SiO₂ ratios from intensifying hydrolysis of primary minerals and the transformation of 2:1 to 1:1 clay minerals. However, the abundance of MgO and K₂O exceeds CaO and Na₂O because Mg and K are stored on both the exchange complexes and in the structure of the remaining 2:1 clay minerals. Results indicate the following: alkaline field – positive reaction to dilute HCl and CaO content > 1.5%; acid field – (CaO + Na₂O)*100 < 1.5%, Na₂O < 0.3%, and CALMAG > 80. The neutral field is identified as nonreactive to dilute HCl but with none of the other acid indicators. Application to paleo-Vertisols of differing weathering intensities from the Devonian, Triassic, and Paleocene shows that the three pH stability fields can be recognized by geochemical assays. Preliminary analyses of a broader spectrum of soil types from the database reveal similar geochemical trends.