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Thursday, August 2, 2012

Cave paintings are older than we thought

Cave paintings are notoriously difficult to date. Many don’t contain any organic components that can be subjected to radiocarbon dating, and those that do can’t be processed without damaging the artwork. Alistair Pike of the University of Bristol and his colleagues from universities in Spain and the U.K. used a different approach to date the paintings. They collected minute samples of calcite that had overflowed the artwork and subjected that material to uranium-thorium dating. Using this technique, they gave the oldest cave pictures a minimum age of just under 41,000 years old.

Let me back up a bit and discuss radioactive dating techniques. The best-known radioactive dating method is carbon dating, also called radiocarbon dating. Carbon, like many elements, comes in different isotopes or varieties, depending on how many neutrons are in each atom. While a carbon atom always has six protons, it can have six (12C), seven (13C) or eight (14C) neutrons, with the last of these being radioactive.

Plants accumulate 14C (and the other isotopes of carbon) into their cells during photosynthesis. Animals accumulate 14C from either plants or other animals. As long as an organism is alive, its body will contain the background level of 14C, because it is constantly ingesting new carbon. However, upon its death, the addition of new 14C ceases and the 14C that’s already present begins to decay. By comparing the amount of 14C in an organic sample with the amount in the atmosphere, scientists can determine how long ago the organism that yielded that sample lived. Because 14C has a half-life of 5,730 years, after about 60,000 years there’s too little around to detect. Thus, you can carbon date wooden spear shafts and wooly mammoth bones, but not stone tools (which were never alive) or dinosaur bones (which are far too old).

There are many other radioactive dating methods, each of which has a different half-life, and thus a different useful age range. Uranium-thorium dating relies on the fact that uranium-234 decays to thorium-230 with a half-life of 245,000 years. Unlike with carbon dating, the tested sample does not have to be derived from something that was once alive. Uranium is found in most water sources, and consequently in the rock that precipitates out of that water. Any water that drips across cave paintings over the eons will deposit uranium-containing calcite over the art. It’s this covering that can be dated. Note, that this will give a minimum age for the underlying artwork, which could be much older than its calcite veneer. In this case, the uranium-thorium dating gave the oldest cave paintings a minimum age of just under 41,000 years old.

Why is this significant?  The earliest Homo sapiens were thought to have arrived in the region at about that same time, roughly 40,000 years ago. Neanderthals living in the area were thought to have gone extinct about 42,000 years ago. This means that if the true age of the oldest cave art is actually more than 43,000 years ago, it had to have been done by Neanderthals, not modern humans. On the other hand, if the calcite formed relatively quickly and the cave paintings are in fact just under 41,000 years old, then they were created by Homo sapiens, who either brought their artistry with them when they arrived in Europe, or developed the skill very quickly upon arriving.

You can hear an interview with Pike here.