Mismatches Between Predicted and Observed Mineral Associations in Soils: Keys to Future Paleosol Interpretations

Interpretation of paleosols rests on our theoretical and empirical understanding of mineral distributions within soils. Paleoenvironmental interpretation of paleosols rests on knowledge of environmental controls on soil-mineral formation. Our understanding of complex interactions of climate and field-based mineral thermodynamics and kinetics is still limited. Recent studies from intensive research sites allows us to identify mismatches between predicted and observed pedogenic mineralogy. As an example, nearly all textbooks show smectite as the dominant product of basalt weathering in arid and semiarid environments, but smectite is not found in field observations of lava weathering where the impact of mixed mineralogy dust is minimized. Evidence from Hawaii suggests that smectite is not an important weathering product in soil formation of dry region soils anywhere on the islands. Whereas, the concentrations of ions in soil solution fall into a “smectite” field on a mineral prevalence diagram, no smectite is identified. Instead, allophane and halloysite have formed, and their isotopic properties can be tied to prevalent long-term weathering environments. Although smectite is favored thermodynamically, it is not formed because rapid drying produces an unfavorable kinetic environment. This result cascades into other interesting mineral synthesis issues such as formation of pedogenic dolomite - Mg released during weathering ends up in dolomite rather than smectite. Why do desert soils in some regions have smectite and low Mg calcite? – Possibly because they contain dust rich in mica that can weather through solid-phase transformation to smectite while also providing a sink for Mg. Another result of kinetically driven controls on soil genesis is that Oxisols form more rapidly in semi-arid environments than in humid environments because in the former oxides crystallize more rapidly. Research in Hawaii and other intensive field sites provide evidence that will enhance future soil-genesis predictions and our ability to interpret paleosol sequences.