It took nearly 30 years and a lot of heated debate, but a team of researchers has finally produced what archaeologists, geologists, and other scientists have long been waiting for: a calibration curve that allows radiocarbon dating to achieve its full potential. The new curve, which now extends back 50,000 years, could help researchers work out key questions in human evolution, such as the effect of climate change on human adaptation and migrations.
The basic principle of radiocarbon dating is fairly simple. Plants and animals absorb trace amounts of radioactive carbon-14 from carbon dioxide (CO2) in the atmosphere while they are alive but stop doing so when they die. The steady decay of carbon-14 from archaeological and geological samples ticks away like a clock, and the amount of radioactive carbon left in the sample gives a reproducible indication of how old it is. Most experts consider the technical limit of radiocarbon dating to be about 50,000 years, after which there is too little carbon-14 left to measure accurately.
There is one major glitch in the approach, however: The amount of carbon-14 in the atmosphere varies with fluctuations in solar activity and Earth's magnetic field, and "raw" radiocarbon dates have to be corrected with a calibration curve that takes these fluctuations into account.
Since the early 1980s, an international working group called INTCAL has been developing and perfecting just such a curve, a process that has unfolded in several stages. To calibrate the period extending from the present to about 12,000 years ago, the team has used thousands of overlapping tree-ring segments from the Northern Hemisphere, which provide a very accurate check of raw radiocarbon dates and how much they must be corrected. But for dates older than the available tree-ring record, the researchers had to turn to several other, less-precise data sets on ancient CO2 levels, including fossil foraminifers (single-celled organisms that secrete calcium carbonate) and corals.
By 2004, the INTCAL group was able to agree on a curve that stretched to 26,000 years ago, because the foraminifer and coral data were in reasonably close agreement up to that point. That curve, called INTCAL04, was published the same year. But hopes to extend the curve all the way to 50,000 years ago were dashed. The data sets diverged from each other by up to several thousand years after 26,000 years ago, and researchers could not agree on which ones were most accurate and how to combine the several data sets.
More recently, however, thanks to new and more accurate data from foraminifers, corals, and other sources--plus some fancy statistical treatments that help predict which way data gaps bend the curve--the INTCAL group has been able to resolve most of the discrepancies. "It took the group quite a while to come together and agree," says INTCAL team leader Paula Reimer, a geochronologist at Queen's University Belfast in Northern Ireland. But the new data, combined with what Reimer calls a "real sense of necessity" among team members to resolve the debates, won the day.
The new curve, called INTCAL09 and published this week in the journal Radiocarbon, not only extends radiocarbon calibration to 50,000 years ago but also considerably improves the earlier parts of the curve, researchers say.
Getting those dates right is critical to understanding such questions as whether humans began painting caves when the climate was colder or warmer, says Clive Gamble, an archaeologist at the University of London, Royal Holloway. For example, the raw radiocarbon dates for the spectacular paintings of horses, lions, bison, and other animals at Chauvet Cave in southern France, the oldest known cave art, come out at 32,000 years ago, right after a major cold spell hit Europe; but the new calibration curve makes the earliest paintings at Chauvet 36,500 years old, a period of relative warmth.
And John Hoffecker, an archaeologist at the University of Colorado, Boulder, says that the data sets behind the new curve will allow a more-precise correlation between radiocarbon dates and prehistoric climate reconstructions based on Greenland ice cores and other proxy indicators of ancient weather. Even before the adoption of the new curve, Hoffecker says, those data sets were suggesting that modern humans had moved into Europe about 45,000 calibrated years ago, much earlier than previously thought--and early enough for them to have had substantial contact with Neandertals over thousands of years.
Although the new curve is a major landmark, it is "definitely not the last word" in radiocarbon calibration, Reimer says. Her team is already planning an update for 2011, "as we learn more about the Earth's carbon reservoirs and how they changed over time."