Calcite formations on prehistoric paintings : the case of the Large cave of Arcy-sur-Cure (28000 - 24500 BP, Yonne, France)

The phenomenon of speleothem formations made of calcium carbonate (CaCO3) in limestone environments is well known. These speleothems can have with various sizes and shapes depending on the geochemical growth parameters. In caves with prehistoric paintings, the formation of subsequent CaCO3 layers can have effects on their preservation and can also partially or totally obstruct them. Thanks to fortunate circumstances, Paleolithic figures were discovered about 20 years ago, hidden behind thick opaque CaCO3 layers, on the walls of the Western gallery of the Large cave of Arcy-sur-Cure. In the frame of an on-going restoration program, a large number of carbonate concretions coming from the Large cave were analyzed in combination with different series of synthetic calcium carbonates. Natural and synthetic samples were studied using complementary microanalytical laboratory techniques. Micro-x-ray diffraction (μXRD), micro-x-ray fluorescence analysis (μXRF) and x-ray absorption near-edge structure spectroscopy (μXANES) based on synchrotron radiation also enabled a more detailed distinction of the different constituents present in these samples at microscopic scale. These investigations allowed gaining information on calcite growth mechanisms, the most important parameters influencing the calcite growth, evaluating the impact of microbial activity, the protective effect of the calcites on prehistoric panels and the proposal of conservation measures.