The susceptibility of weathered versus unweathered schist to biological colonization in the Coa Valley Archaeological Park (north-east Portugal)

This study addresses the primary and secondary bioreceptivity of schist used as a support for prehistoric rock art in the Coa Valley Archaeological Park (north-east Portugal) and provides some parameters that can be related to the risk of biologically induced schist weathering. Samples of freshly quarried and naturally weathered schist were characterized in terms of their intrinsic properties and maintained in controlled environmental conditions after inoculation with biofilm-forming cyanobacteria. The physical properties of the studied schist, as well as its abrasion pH, all varied according to the weathering degree of the samples and so did its susceptibility to colonization by biofilm-forming cyanobacteria. Complete separation between weathered and unweathered schist samples in terms of laboratory-induced photosynthetic biomass was obtained by measuring total colour change in the CIE (International Commission on Illumination) L*a*b* colour space. Weathered schist was more bioreceptive than unweathered schist, associated with increased open porosity, water saturation, capillary water and capillarity coefficient and decreased abrasion pH. In the future, it might be possible to determine the susceptibility of schist surfaces to biological colonization through evaluation of colour differences associated with the different weathering degrees presented by those surfaces prior to colonization.