There were dramatic changes in the amount of radioactive carbon swirling around in the Earth's atmosphere during the last Ice Age, far greater than previously thought, scientists have found.
The discovery was made by studying a half-metre-long stalagmite recovered from a now submerged cave in the Bahamas.
It is also likely to improve the accuracy of archaeological dating, which relies on radiocarbon levels to determine the ages of old bones, wood and cloth.
"We've discovered that beyond about 20,000 years ago there are some dramatic swings in radiocarbon concentration, which means the age offset between the radiocarbon age and true calendar age can be up to 5,000 years," Dr David Richards, of the School of Geographical Sciences, University of Bristol, UK, told the BBC.
Radiocarbon, or carbon-14 as it is also known, is made when high-energy particles called cosmic rays strike the atmosphere.
Earth bombardment
The stalagmite was brought up from the Blue Holes of the Bahamas, the complex of limestone caverns created when sea levels were more than 100 metres lower than they are today.
Because of the way they form, stalagmites can be used to determine past levels of radiocarbon in the atmosphere. They can help establish a timescale for dating, and this information can also be checked against a newer, more accurate method known as uranium dating.
In this case, scientists from the Universities of Bristol, Arizona and Minnesota were able to look back 45,000 years ago - well into the last Ice Age.
Their analysis revealed huge peaks in radiocarbon levels - much greater, the team believes, than can be explained merely by an increase in cosmic ray bombardment on the Earth.
The researchers think something changed in the way carbon was cycled on the planet itself. The best explanation, they argue, would have been a reduction in the amount of carbon dioxide (CO2) dissolved into water - something that could have happened if the oceans began to circulate more slowly in the cold climate of the Ice Age.
Carbon cycle
There is already some evidence for this slow-down from ice-core and ocean-sediment studies. And there are data that indicate this slow-down may soon occur again, although this time it is likely to be driven by a climate that appears to be getting much warmer.
The observation that the carbon cycle was significantly more sluggish in the recent past "may have profound implications regarding the oceans' capacity to take up anthropogenic (human produced) CO2 emissions from fossil fuel burning," said Dr Warren Beck, from the Physics Department at Arizona University, a co-researcher on the stalagmite study, which is published in the journal Science.
Richards, Beck and colleagues also hope their work will help improve the calibration of the radiocarbon timescale used to date archaeological specimens, such as skeletons. Radiocarbon dating, which depends on the steady decay of carbon-14, becomes less and less reliable once the artefact under study gets older than about 16,000 years.
Already, it is clear that some ancient items could have been wrongly dated by several thousand years because of the sharp fluctuations in radiocarbon levels revealed in the stalagmite.
Dr Chris Stringer, from the Natural History Museum, London, UK, said the new study might force a reappraisal of when certain events occurred in early human history.
"It might solve some mysteries," he said. "For example, 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 based on radiocarbon dating. Well, it might turn out that they didn't vanish at all when we recalibrate."