Microanalysis and synthesis of calcite. Growth mechanisms on prehistoric paintings in the Large Cave, Arcy-sur-Cure (Yonne, France)

摘要

The formation of subsequent layers of calcium carbonate (CaCO3) in karst caves is a well-known phenomenon observed especially on the prehistoric paintings in the Large Cave (Grande Grotte) of Arcy-sur-Cure, in France. About 40 natural samples coming from the Large Cave were already investigated in a previous work and classified as a function of their crystal morphology, texture, composition and appearance. The presence of at least two calcite types was revealed on the walls: translucent and opaque white or grey calcites. The latter opaque calcites completely obstruct the paintings. The formation of these different calcites depends on various geochemical and geophysical parameters and could also be influenced by biotic processes. Questions remain about formation mechanisms leading to different calcite types and, in particular, about the role of micro-organisms as well as interactions between prehistoric paint layers and calcite layers. Additional natural calcites were studied in order to confirm the first classification. Various syntheses of abiotic calcite were performed to study the influence of environmental parameters on calcite growth. Moreover, several series of biotic calcite were synthesised in a specific medium with two bacteria isolated from the cave in order to test their calcifying properties. All natural and synthetic samples were analysed using complementary microanalytical laboratory techniques. Micro-x-ray fluorescence analysis (micro-XRF) and x-ray absorption near-edge structure spectroscopy (XANES) based on synchrotron radiation also enabled a more detailed distinguishing of different minerals within the complex biotic and abiotic mixtures and layered synthetic samples at microscopic scale. Comparison of natural and synthetic samples gives more detailed insights into the formation mechanisms of the different calcite types found in the Grande Grotte and allows proposing an adapted conservation strategy of its prehistoric paintings. Copyright (C) 2008 John Wiley & Sons, Ltd.