Uranium-series dating of carbonate formations overlying Paleolithic art : interest and limitations

Abstract : The Uranium-Thorium (U/ Th) series dating method, developed 50 years ago, has proven its usefulness and reliability for the dating of marine (corals) and continental (speleothems) secondary carbonates deposits. Recently, improvements of the analytical techniques (TIMS – Thermo-Ionization Mass Spectrometry, and then MC-ICPMS – Multicollector-Inductively Coupled Plasma Mass Spectroscopy and laser ablation) allow the dating of very small samples and increase the potential of this method. Given the difficulties of dating cave art (other than drawings created with charcoal, which can be directly dated by 14C), indirect dating methods have been sought. During the last decade, several publications have reported the dating by the U/ Th method of thin layers of calcite overlying Paleolithic paintings and engravings or the support of these representations. In these cases, the age of calcite formation is assumed to provide a minimum age (terminus ante quem) for the underlying paintings or engravings or a maximum age (terminus post quem) when it is the support that is dated. The current article describes the relevance and potential of this method when applied to the dating of calcitic layers deposited above or below prehistoric drawings, together with the specific difficulties encountered in U/ Th dating of such thin deposits. An initial difficulty is that thorium may be present in the calcite from the beginning (detritic thorium), making age corrections necessary. Another difficulty is that in the humid conditions prevalent in caves, the walls may have been subject to runoff over time. In this case, thin calcite layers covering paintings or engravings may have been altered, with possible chemical exchange between the water and the calcite. The most probable effect of this ‘open system’ behavior is the leaching of uranium, leading to an overestimation of the age of the calcite. Recent applications of the U/ Th method to the dating of rock art have shown that this phenomenon, if not correctly identified by means of independent methods, may become a significant source of error. For this reason, it is important to know the concentrations of uranium in each calcitic sample, as this makes it possible to detect local anomalies that have led to a substantial loss of this element. In a recent paper concerning the U/ Th dating of eleven Paleolithic decorated caves in the Cantabrian Region (Spain), extremely early dates were determined (more than 41,000 years in one case) and the authors speculated that certain representations could have been produced by Neanderthals. However as detailed analytical data (uranium content) have not been published one cannot appreciate the reliability of the ages obtained. Then, in the absence of confirmation by an independent dating method, it is premature to base an archaeological reasoning on these dates. This article emphasizes the necessity of carrying out several analyses on the same sample, and when possible on several layers from its thickness. Moreover it is important to perform cross dating using U/ Th and 14C (or even using other elements such as 226Ra or 231Pa) in order to verify the consistency of the results. Several recent examples will illustrate this necessity. It should be also recalled that the calibration curves used to correct radiocarbon ages are largely based on the simultaneous dating by U/ Th and 14C of the same samples of speleothems and corals. Finally, it has to be mentioned that when the deposits underlying paintings or engravings are studied, the data obtained could be distant in time from the creative act. In fact, the growth of calcite is controlled by environmental factors and is favored during temperate and humid periods. Thus, a large number of calcitic layers overlying paintings could have been deposited during the Holocene. They could also have grown during a brief warming of the last glacial period, or represent a mean age between several growing periods. Nevertheless, these chronological data could bring relevant information, once their validity has been verified.