In this month’s ‘Science Notes’, we are discussing yet another form of dating: uranium-thorium (U-Th) dating, also known as uranium-series dating. Readers may already be aware of the technique, as it has featured a few times in research covered by CA over the years (see CA 83, 93, and 259), but recently it made international headlines for its use in determining that cave paintings in Iberia pre-date the presence of modern humans. The methodology that led to such an unexpected and ground-breaking discovery seemed worthy of being highlighted. (This may also be a cheeky attempt to sneak in remarkable archaeological research from outside our usual remit of Great Britain and Ireland.) Until recently, most cave art was roughly dated by grouping examples based on style, an approach with many problems and constraints. But by applying U-Th dating to cave art, we could be seeing a revolution in cave-art chronologies in the next few years.
Over the past decade, there has been considerable debate among archaeological scientists over the best way to date Palaeolithic cave art: radiocarbon or U-Th dating. Recently, U-Th dating appears to be winning the battle. While radiocarbon dating requires the partial destruction of the art, can only be used on organic pigments, and at present cannot be used to date material more than 50,000 years old, U-Th does not require any removal of the art, can be used regardless of the colouring material used, and is able to extend dating by up to 450,000 years.
The method is based on the decay series that begins with uranium isotopes and ends with a stable lead isotope. In the beginning of this process, uranium-238 (238U), with a half-life of 4.5 billion years, decays into daughter isotopes, one of which is thorium-230 (230Th), which has a half-life of 76,000 years. As both isotopes decay at these different rates, they approach an equilibrium. If this equilibrium is disrupted, for instance through the removal of thorium, it will re-establish itself at a predictable and measurable rate. As uranium is soluble in groundwater and thorium is not, minerals that precipitate out of water (such as calcium carbonate) will initially only contain uranium and not thorium, and the decay process will start anew. By calculating the degree of disequilibrium between uranium and 230Th still present in a sample, a formation date is obtained. In archaeology, this technique is often used to date speleothems, or cave deposits made of calcium carbonate formed from groundwater (for example, stalagmites and stalactites), to achieve a chronology of artefacts embedded in the carbonate or, most commonly, from cave art drawn on the deposits.
It is this methodology that was applied to calcium carbonate crusts overlying paintings from three sites in Spain: La Pasiega (Cantabria), Maltravieso (Extremadura), and Ardales (Andalucía). As the dating technique has become more precise, the team was able to remove only a small fragment of crust (in most cases <10mg), meaning that very little had to be destroyed and, most importantly, leaving the art intact. The results demonstrated that the oldest piece, from Maltravieso, had a minimum age of 66,700 years and that the other two caves had artworks that were at least 64,800 years old. This significantly predates the arrival of modern humans into Europe by approximately 20,000 years and implies that the most likely artists were Neanderthals, who were quite active in the region at that time.
Usually, so as not to affect the artwork, only the carbonate directly overlying the painting is sampled, but this means that only a minimum date can be achieved – and the actual date of the artwork could be many thousands of years earlier. One of the sites the team analysed, however, provided a unique way of circumventing this problem. At Ardales Cave, three of the samples came from free-standing stalagmites, which had broken at some point in the more recent past, revealing the pigmented art in section. This allowed the team to sample carbonate from both before the art was painted and after, without destroying any of the artwork and thereby achieving both minimum and maximum dates. Through this, the team found that the paintings had been done over distinct periods: they were able to ascertain that one piece of art was painted between 48,700 and 45,300 years ago, while another was done prior to 65,500 years ago. Both were on the same cave formation and both pre-dated modern humans in the region, who appeared approximately 42,000 years ago.
This is a significant discovery, as previously – although Neanderthal origins for some examples of cave art had been posited by a few scholars – it was elsewhere assumed that Neanderthals were not capable of symbolism or abstract thought. The sophistication that would have gone into making some of these paintings, including use of a light source and the preparation of pigments for colouring, shows evidence of clear intention and forethought. With similarly styled art found in other parts of Western Europe – opening up the possibility that this art movement was not isolated to Iberia – it seems that the Neanderthals may have been much more advanced than we have previously given them credit.
This article appeared in CA 339.
