Growth of hematite and boehmite in concretions from ancient karst bauxite: clue for past climate

In the Peri-Adriatic Apulia Carbonate Platform (southern ltaly), late Cretaceous karst bauxites mark an emergence period during a wet tropical climate. Bauxite formed through clay accumulation in the karst, ‘‘in situ’’ bauxitization and late formation of iron-rich concretions in a water-unsaturated pedogenic environment. The concretions, which are geochemical recorders of the environment of formation, have a large core of Al-hematite surrounded by a cortex of alternating Al-hematite and boehmite. Boehmite forms instead of Al-hematite at lower water activity values. Using a model of molecular diffusion and assuming the fluid flow negligible, the time necessary for growth of the concretions has been calculated. The average-sized core grew in f180 ka. The Al-hematite accretionary band grew in f8 ka whereas the boehmite accretionary band grew in f4.5 ka. The average bulk concretions possibly formed in 300–400 ka. The growth of the concretions is assumed to be a two-stage process. In the first stage, the core grew in a relatively long period of wet tropical climate. In the second stage, drier conditions favouring boehmite stability alternated to a wetter climate favouring Al-hematite stability. The growth of the bulk concretions is consistent with the Earth’s long eccentricity cycle.