Interplay between geological materials and the environment at the Dazu Rock Carvings, China

The study of the historical built environment is of great significance to the heritage conservation and the reconstruction of the level of anthropogenic pollution in the past. On the Dazu Rock Carvings, China, a black to reddish-brown, relatively dense mineral film with a thickness of approximately 150 μm was observed on the surface of the sandstone. The chemical composition of the mineral film studied by Scanning Electron Microscopy- Energy Dispersive Spectroscopy (SEM-EDS), Electron Probe Micro- Analyz (EPMA), X-ray Absorption Spectroscopy (XAS), Total Organic Carbon (TOC), and Ion Chromatography (IC), revealed that its development was comprehensively controlled by environmental conditions, especially by environmental pollution. The mineral film consisted primarily of clay minerals and iron oxide. It also contained a large amount of detrital quartz and feldspar, with a clear boundary with the underlying mineral matrix. The bacterial community was also investigated by next generation sequencing and its potential metabolisms through PICRUSt2. The mineral film on the surface of sandstone and its microbial community may provide a certain degree of protection for the internal structure. As a passive sampler, the variation of the concentration of pollution elements in the mineral film is helpful to understand the past environmental pollution of the city. Besides the relevance for stone heritage conservation, this study provides insights into the interplay between geological materials and the environment.