Light in the cave: Opal coating detection by UV-light illumination and fluorescence in a rock art context

Abstract

The formation of silica coatings on the cave walls of the Points cave raises questions about the analytical access to the specificities of the pictorial material (geochemistry and petrography); and about the state of conservation of the rock art. Conventional in situ spectroscopic techniques (pXRF, pRaman) are ineffective to identify and characterize these coatings. In this study, we propose to use a UV fluorescence method for the detection and recognition of opaline coatings, based on the fluorescence specificities of the uranyl-silica complexes composing these deposits. Spectral identification using UV laser-induced fluorescence spectroscopy coupled with UV illumination was performed on samples, µ-samples and on the walls of the Grotte aux Points rock art site. The well-defined peaks observed in the fluorescence emission spectra due to uranyl ions validate the detection of the complex opal-uranyl and its correspondence with the green fluorescence observed under UV light at micro and macroscopic scales. In situ optical measurements under UV illumination reveal the presence of an opal layer, in particular on the rock art walls of the cave. Observations on the occurrence and distribution of opal provide the first insights into the evolution of the walls and the chronological constraints on the development of the opal layer. regarding the interactions between the silica coating and the pigment suggested by the multi-scale observations of the µ-samples, it opens the question of rock art conservation. Thus, by developing a specific method of non-destructive characterization of opal coatings, this study starts a new approach for the study of the taphonomy of decorated walls and proposes to use siliceous mineralization both as a marker of the natural history of caves and as an index of their use by ancient human groups.