Light penetration into Clarens sandstone and implications for deterioration of San rock art

In respect to the weathering of cave art exposed to the sun, cognizance has yet to be taken of the modified thermal conditions and the potential for endolithic biotic activity where the art is located on a light‐transmissive lithology. Where light penetrates rock, the light‐to‐heat transfer is not solely at the surface, and this leads to a thermal gradient that is different from where the paintings are located (and all transfer is at the surface). Light values of up to 200 W/m² were recorded at 0.5 mm depth and up to 100 W/m² at 1mm depth in the dry sandstone; rock moisture data showed that at this site the rock remained dry irrespective of atmospheric conditions. The light penetration means that there can be rapid and large subsurface thermal fluctuations contemporaneous with those at the rock surface, and that the thermal gradient is not as steep (approximately 1°C/mm in the surficial part of the rock) as where light‐to‐heat transfer is solely at the surface. Further, the presence of subsurface photosynthetically active radiation can (potentially) facilitate colonization by endolithic organisms. Here, as part of a study of the weathering of San rock art on sandstone in southern Africa, a first attempt is made to monitor the extent of light penetration and the resulting thermal conditions in the outer few millimeters of the sandstone. © 2009 Wiley Periodicals, Inc.