Using the analysis of iron and iron oxides in paleosols (TEM, geochemistry and iron forms) for the assessment of present and past pedogenesis

This study investigates the iron oxide chemistry and mineralogy of two relict paleosols, a Ferri-Endostagnic Lixisol (Bathiplinthic) and a Profondi-Endostagnic Luvisol (Chromic). The estimated inception of their pedogenesis is Early Pleistocene. The two paleosols belong to the same geomorphic body, but are found at different elevations and have experienced different hydrology expressed as either dark red or mottled dominant colors throughout their profile. We hypothesized that the difference in color reflected a recent transformation of iron oxides that were mainly formed in the past. The aim of the research was to discriminate recent from relict soil forming processes by evaluating iron oxide composition, soluble forms, and geochemistry.In the studied profiles hematite and goethite (αFeOOH) are the main iron oxides. Their fate depends on the present pedogenic conditions. Both hematite and goethite are a source of iron for recently synthesized iron oxides (feroxyhyte (δ′FeOOH) and Fe-protophases), but hematite is preferentially dissolved in reducing conditions. In the red soil profile, hematite content still masks the presence of the other iron oxides, while the mottled color of the topographically lower profile, with poorer internal drainage, is a consequence of a more advanced hematite dissolution.The iron oxides (hematite, goethite and recently synthesized iron oxides) are the result of different pedoclimatic conditions in former and present time. Hematite is inherited from past pedogenesis, while feroxyhyte and Fe-protophases have been recently formed. Goethite can be of either current (transformed from feroxyhyte) or inherited origin.These results indicate that the soils studied are polygenetic; the red color and hematite content are the results of past soil forming processes acting in different climatic and drainage conditions.