Formation and degradation of immature laterites in response to paleoclimatic changes in the eastern Amazon

Lateritic formations are widespread in eastern Amazon, occurring as bauxite-bearing mature profiles on plateaus or as immature ones in lowlands. Climate and landscape evolution over the Neogene were analyzed via mineralogical (XRD), textural (optical microscopy), and geochemical (ICP-OES, ICP-MS) methods carried out in a selected lateritic profile outcropping in Abel Figueiredo. The profile features columnar and nodular iron crust, overlaid by a spherulitic horizon formed of hard fragments in clayey matrices, covered by yellow topsoil. Major minerals include hematite, goethite, kaolinite, and quartz, with minor anatase and heavy minerals. Hematite and goethite dominate lower horizons, while kaolinite, quartz, Al-goethite, and anatase appear in the matrix and topsoil. Trace elements were immobilized by iron oxyhydroxides and anatase or maintained in zircon and other resistate minerals. Mineralogy and geochemical distribution indicates a transition from iron crusts to fragmented iron crust, followed by physical reworking, leading to the formation of nodules and spherulites, with clay accumulation in between. In a final stage, a topsoil developed over the profile. This set of events requires an alternation between humid tropical climate with more arid conditions, with implications to rainforest and savanna development, resulting in mineral formation and chemical or physical decomposition.