Rock varnish as a natural canvas for rock art in La Proveedora, northwestern Sonoran Desert (Mexico): Integrating archaeological and geological evidences.

Rock surfaces covered with desert varnish frequently feature petroglyphs and thus comprise an important resource for the artistic expression of ancient societies. Knowledge about the processes of varnish formation would be helpful to assess the production, regeneration and durability of this "natural canvas" and related rock art. We performed a multidisciplinary study of rock varnish associated with petroglyphs at the archaeological site of La Proveedora (Sonora, Mexico) that has been occupied by humans for at least 4500 years. An inventory and mapping of these ancient engravings revealed 5873 individual images on 1331 rock surfaces. Classification of these motifs showed that geometric forms were the most commonly featured, followed by zoomorphic and anthropomorphic designs. Colorimetric measurements according to the L*a*b* system confirmed that the carvings have a strong color contrast with the background patina. Petrographic observations revealed the variable thickness of the varnish coating, which penetrates deeper into the granitic rock at contacts with plagioclases and ferromagnesian minerals, forming a "rooted" configuration of the lower boundary. Under SEM thin globular laminae were observed; plagioclase and biotite adjacent to the varnish showed signs of alteration. EDS microanalysis, LIBS and X-ray diffraction demonstrated that the patina consists of Fe and Mn oxihydroxides and clay minerals with differing amounts of Fe and Mn within the coating. These results support the polygenetic model of rock varnish development. Weathering of granite and eolian particles give rise to clay mineral formation, whereas Fe and Mn mobilization and reprecipitation is controlled by microorganisms. Varnish formation was more intensive under moister conditions which prevailed in Sonora during the terminal Pleistocene. Thus, rock varnish comprised a non-renewable resource for ancient artists, being a relict feature. The Holocene rates of patina development were too slow to re-pigment engravings, so the petroglyphs represent a persistent element of a cultural landscape. We attribute the surprising durability of the dark coatings to the "rooted" configuration of their lower boundaries that could be important for petroglyph preservation. [ABSTRACT FROM AUTHOR]