Rock Art uranium-thorium and radiocarbon cross-dating at Altamira cave, Spain

For the first time, systematic coupling of 230 Th/ 234 U and 14 C dating methods was performed on secondary carbonate thin layers covering or underlying rock art from Altamira cave, Spain. Analyses were performed on the same carbonate samples weighing only a few milligrams, in order to compare the dates obtained on the same fraction by the two methods. Depending on the location of the carbonate samples above or under the representations, minimum or maximum ages were respectively obtained. This intercomparison was undertaken as previous results showed that 230 Th/ 234 U ages obtained on calcitic parietal deposits in other caves were in some cases older than the 14 C ages obtained on the same samples. This was confirmed in the present study, where we furthermore found that the oldest 230 Th/ 234 U ages were those of the samples with the lowest uranium content, suggesting that uranium loss is the mechanism responsible for age bias. Thus, we suggest that the 230 Th/ 234 U ages obtained on secondary carbonate deposits covering Altamira cave paintings are in some cases biased by the mobilization of radionuclides, giving apparent ages that are too old. Consequently, the agreement between 230 Th/ 234 U and 14 C ages is a prerequisite to obtaining meaningful ages. The oldest validated ages are 20-23 ka Cal BP for CaCO 3 samples covering the cave paintings, in agreement with known archaeological remains for Altamira cave prehistoric occupations, dating back to the late Gravettian/early Solutrean periods, but not earlier. This timing is however older than the 14 C ages obtained for the direct dating of charcoal from the bisons of the polychrome ceiling, confirming the existence of several periods of cave art for this famous ceiling.