New dating method sheds light on cave art
5 October 2008 by Tom Marshall
Scientists are revolutionising our understanding of early human societies with a more precise way of dating cave art.
Bison painting in the Altamira cave, Spain
Instead of trying to date the paintings and engravings themselves, they are analysing carbonate deposits like stalactites and stalagmites that have formed over them.
This means they don't risk harming irreplaceable art, and provides a more detailed view of prehistoric cultures.
The researchers spent two weeks in Spain last year testing the new method in caves, and have just returned from another fortnight's expedition to sample nine more caves, including the so called 'Sistine Chapel of the Palaeolithic', Altamira cave.
"This lets us challenge assumptions about the age of cave art," says Alistair Pike, senior lecturer in archaeological sciences at Bristol University, who has helped develop the technique.
When combined with evidence from archaeology and other disciplines, it promises to let researchers create a more robust and detailed chronology of how humans spread across Europe at the end of the last ice age.
"It's a big step in understanding the timing of how cave art was produced," adds Pike. "It is also shedding light on the reasons for its production - why do you get a sudden flourishing of cave art at certain periods?"
Ice age migrations
The results so far are in line with archaeologists' hypothesis that sudden flowerings of cave art came as rapid climate change was causing Palaeolithic cultures to move quickly about Europe, first as the coldest period of the ice age approached, and then as the ice age drew to a close and inhabitable areas expanded.
There have been surprises, though - in several caves whose art had previously been assumed to date from the same period, the new dating technique has revealed that the paintings were done in several phases, possibly over 15,000 years (25,000 years ago to just 10,000.)
The dating method involves a technique called uranium series dating. It works on any carbonate substance, such as coral or limestone, and involves measuring the balance between a uranium isotope and the form of thorium that it decays into.
The technology isn't new - it was first developed in the mid-twentieth century, and is often used in areas like geology and geochemistry. But successfully applying it to date cave art is a big leap in our understanding of human prehistory.
Results proved that the art was made at least 12,000 years ago.
Pike has extensive experience using the technique to date ancient bones. He had the idea of using it on Palaeolithic art during an expedition to Cresswell Crags in Derbyshire, the site of rock engravings thought to be Britain's only Palaeolithic cave art.
Pike noticed that calcite deposits had formed over the engravings, and realised he could use uranium series dating to pinpoint when. The results proved that the art was made at least 12,000 years ago, and so did indeed date from the Palaeolithic.
Before this, the only tool for dating the engravings was stylistic comparison with similar sites on the continent - a far less precise business.
Scraping at stalagmites
It soon became clear that uranium series techniques could be useful at many other cave art sites. The process is lengthy and painstaking - researchers must scrape off enough of the calcite crust for an accurate dating, while taking care not to harm the art underneath, or even contaminate the sample with the older limestone behind the art.
Black and violet painted horses overlying red figures at Tito Bustillo, Asturias. Areas of thin calcite coatings can be seen at the top left and bottom middle, and small stalactites following cracks.
"It takes ages," Pike says. "To get enough samples for three diagnostic dates at Cresswell Crags, two people had to scrape away for four days. It's often difficult to get to the art, and we frequently find ourselves crawling through tiny fissures. It seems some of the decoration was deliberately done in the least accessible parts of caves."
Last year the team visited around ten Spanish caves and took samples from four, ending up with 20 usable dates; a three-year research grant from NERC has now provided enough funding to get around 130 more dates. This would more than double the number of dates for cave art of this period for the whole of Europe.
Better-known techniques like radiocarbon dating are of limited use in dating cave art.
On their recent trip to Spain the team encountered some exceptional opportunities for dating, in the form of paint layers within stalagmites. These allow the age of the painting to be bracketed by both maximum and minimum dates.
Better-known techniques like radiocarbon dating are of limited use in dating cave art. Much of it isn't made with organic materials, but is simply carved into the rock, or painted with non-organic pigments like ochre. Even art that does use organic materials like charcoal, which can be dated using radiocarbon, presents problems.
For one thing, there's no way of telling whether these materials are contemporary with the art or much older - ancient artists could have drawn with charcoal that was already in the cave when they arrived.
Bristol researcher removing sample of thin calcite overlying red painted deer in La Pasiaga cave, Cantabria
Furthermore, cave paintings are delicate and precious - scraping bits off them to send to the dating lab is rarely welcome. And there's a serious risk of contaminating the samples being analysed with more modern or older carbon.
"The cave art chronology based on radiocarbon has been the subject of much heated debate and controversy in the past few years, so by using this new method we aim to solve the problem of when the art was made," Pike explains. "This will allow us to tackle the perhaps more difficult question of why it was made."