A group of limestone stalagmites from a submerged cave beneath the Bahamas has thrown doubt on conclusions about human history drawn from carbon dating. The scientists who found it say that there are also important implications for climate change.
The formations, recovered from a cavern which was created when sea levels were about 100m (330ft) lower than today, showed that more than 20,000 years ago there were dramatic shifts in the amount of radioactive carbon – often known as "carbon-14" – in the atmosphere.
Because the ratio of carbon-14 to its stable cousin, carbon-12, is used as the basis of carbon dating of fossils, any widespread variation in that balance would confuse the dating of items such as plants or animals which existed around those times.
"It means we have tended to underestimate the true age of objects from 20,000 to 40,000 years ago by up to 8,000 years," said Dr David Richards, of the School of Geographical Sciences at the University of Bristol. However, he added, "this may change the timings, but it won't change the order of events."
Carbon-14 is formed in the upper atmosphere by high-energy particles striking nitrogen atoms. These are dispersed through the atmosphere and taken up by living objects. The ratio of carbon-14 to carbon-12 in fossils indicates how long ago the organism lived.
The stalagmites, from the Blue Caves beneath the Bahamas, were formed about 45,000 years ago. Scientists from the universities of Bristol, Arizona and Minnesota found that there were peaks in radiocarbon levels which could not be explained by any increase in atmospheric radiation. Instead, they think that the take-up of carbon in the planet changed – possibly by a fall in the amount of carbon dioxide (which would contain radiocarbon) dissolved in water.
Carbon dating is known to be unreliable for objects which are more than 16,000 years old; the new discovery suggests that some ancient items may be several thousand years out.
"It might solve some mysteries," suggested Dr Chris Stringer, an anthropologist at the Natural History Museum. "For example, based on radiocarbon dating, the modern humans in Europe seem to vanish for about 5,000 years from many parts of the continent between 30,000 and 40,000 years ago. Well, it might turn out that they didn't vanish at all when we recalibrate."
However it would not change the dating of items such as the Turin Shroud, which was made within the past 1,000 years.
Climate-change scientists say that the findings may show that there is more carbon in the atmosphere when ocean circulation slows, as happened in the Ice Age. That would warm the planet. There are signs that such slowing is happening now – which could accelerate the global warming scientists have already found.
