KarstBase a bibliography database in karst and cave science.
Featured articles from Cave & Karst Science Journals
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Catena, 2002, Vol 50, Issue 2, p. 43-51
Growth of hematite and boehmite in concretions from ancient karst bauxite: clue for past climate
Mongelli G. ,
Abstract:
In the Peri-Adriatic Apulia Carbonate Platform (southern Italy), 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 M-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 similar to180 ka. The Al-hematite accretionary band grew in similar to180 ka whereas the boehmite accretionary band grew in similar to4.5 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. (C) 2002 Elsevier Science B.V. All rights reserved
In the Peri-Adriatic Apulia Carbonate Platform (southern Italy), 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 M-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 similar to180 ka. The Al-hematite accretionary band grew in similar to180 ka whereas the boehmite accretionary band grew in similar to4.5 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. (C) 2002 Elsevier Science B.V. All rights reserved
Keywords: accumulation, activity, ancient, apulia, bauxite, bauxites, c, carbonate, carbonate platform, clay, climate, cretaceous, diffusion, environment, flow, fluid, fluid flow, fluid-flow, form, growth, italy, karst, karst bauxites, late cretaceous, model, platform, science, southern, stability, time, times, values, water, wet,