## 3 Results
### Digital Optical Microscopy  
The samples collected from the petroglyph sites were first studied under the microscope without any preparation to gather an overview of their physical characteristics (microscope images of the samples are reported in Supplementary Figure S4), while microscope observation of thin sections allowed us to evaluate different aspects such as the thickness of the varnish, the possible presence of weathered areas in the bedrock, and where visible, the penetration of microorganisms in the stone. Six of the rock samples (AV1, AV2, AV3, EZ1, EZ2, EZ3) collected from the petroglyph sites are coated with a black crust (Supplementary Figure S4), the so-called desert varnish. It has been observed that underneath the thin black outer layer, a bright reddish layer is present, which is intermixed with the stone grains (Supplementary Figure S4). The black varnish has a compact, shiny appearance, but when observed under microscope, the surface is not homogeneous and presents reddish and white patches. Indeed, in the highest points, the black layer is missing and the orange layer or the white rock are exposed. In the lowest points (depressions) an accretion of brownish material is visible, with an accumulation of dust grains in the micro-basins. The appearance of the black crust can vary from sample to sample and be thinner and/or more intermixed with the reddish layer (Supplementary Figure S4). In addition, the black crust does not occur homogeneously throughout the same sample, and different stages of weathering of the outer black layer can be observed. Sample EZ4 differs from the others and is covered by an orange crust (Supplementary Figure S4). In some areas of sample EZ4, it appears that traces of the black varnish are present. Finally, the presence of cyanobacterial aggregates was extensively detected in the samples (Supplementary Figure S4,f). The thin sections showed that the limestone bedrock of all the samples mainly consists of microfossils of foraminifera, bryozoans, diatoms and turritella. Samples AV1, AV2, AV3, EZ1, EZ2 and EZ3, which are characterized by the black coating, presented similar features. The black varnish has a variable width, ranging from about 20 um in the thinnest areas to about 130 um in the thickest parts (Figure 5), but never exceeding 150 um. The reddish layer is not homogeneously distributed and is present only in some areas. What is clearly visible in all samples is a weathered area underneath the varnish. In Figure 5, examples of this phenomenon are shown. The zones below the black varnish appear to be moderately to heavily weathered, and grains of the bedrock have been leached, leaving behind a very porous area with an average depth of about 800 um but that can also reach about 1.8 mm, as observed in sample EZ3 (Figure 5), which shows different layers of weathering. In the porous weathered areas, it can be noticed that the cavities and pores close to the surface are coated by the dark crust (see also Supplementary Figure S5). In other areas, beneath the black coating, the bedrock is heavily weathered, and the stone appears completely fragmented into small particles with loose arrangement (Figure 5). In these areas it can be observed that the shells of the fossils seem to be more resistant to the weathering processes and remain almost intact, while the softer stone matrix is leached. In sample AV1, mixed with the loose grains of the weathered stone, we observed a granular brown material. Aggregates of cyanobacteria colonized this area, as is well-illustrated in Figure 5e. It can be noticed that in this zone, the black varnish appears completely fragmented and detached from the bedrock, and in some areas is no longer present. A different situation was observed in the thin section of sample EZ4. The orange crust is very porous and reaches a thickness greater than 1 mm. Interestingly, underneath the orange layer, no weathered area was observed. The bedrock looks undamaged till the boundary with the crust.