Radiocarbon dating of prehistoric rock paintings by selective oxidation of organic carbon

Abstract

DATING of prehistoric rock paintings (pictographs) has traditionally relied on indirect evidence. This includes inferences based on the archaeological context, such as superpositions of pictorial styles¹ and the depiction of images that constrain their ages^1,2, as well as dating of deposits that either cover the art in situ^3,4 or contain separated fragments of the painted surface⁵. Migration of ions between the bulk rock and the natural coatings that form on a newly exposed surface has also been exploited to date petroglyphs (rock carvings) in desert regions^6–13. Until recently^14–17, however, direct dating (by radiocarbon techniques) of pictographs has not been possible^18,19, mainly because of the problem of separating inorganic carbon from the organic material in the pigments. Here we report on a new technique which allows this separation to be effected by using a low-temperature, low-pressure oxygen plasma to oxidize selectively the organic component; this may then be analysed using standard¹⁴C methods. We have applied this technique to a portion of a pictograph from the Lower Pecos region of southwest Texas (Fig. 1). The date obtained, 3,865±100 yr BP (before present) is consistent with that expected on the basis of archaeological inference²⁰. As organic carbon is a ubiquitous component of pictograph paints, this technique should be applicable to rock paintings throughout the world.