The clock was initially calibrated by dating objects of known age such as Egyptian mummies and bread from Pompeii; work that won Willard Libby the 1960 Nobel Prize in Chemistry.
But even he “realized that there probably would be variation”, says Christopher Bronk Ramsey, a geochronologist at the University of Oxford, UK, who led the latest work, published today in Science.
“It also confirms, as had previously been surmised, that as the last ice age ended changes in the carbon cycle in the ocean, associated with climate change, caused changes in the radiocarbon levels,” says Edwards.
Timothy Jull, of the Department of Geosciences at the University of Arizona, says that the study helps refine the carbon-14 record and adds a lot of information about the Laschamp event by improving the record for that period.
“That’s its main advantage to me, it adds clarity to what’s happening 35,000–45,000 years ago.” He cautions, however, that at other points there are large levels of uncertainty in the data.
According to Tom Higham, of the School of Archaeology at the University of Oxford, this work is an exciting and important contribution to radiocarbon data as it is “a very detailed and high coverage record”.
The pair of stalagmites used in the latest research are from the Hulu Cave in Jiangsu Province, Eastern China.According to the team, the carbon-14 data has similarities to the geomagnetic record and indicates an abrupt increase in radiocarbon around the Laschamp excursion.This, they say, suggest that changes in the Earth’s magnetic field could be responsible for much of the historic fluctuations in atmosphere radiocarbon.His colleague, Lawrence Edwards at the University of Minnesota, adds, “They include very little carbon derived from the limestone around the cave, most of the carbon came directly from the atmosphere.” To date the carbon-14 in the stalagmites, the researchers compared it with adjacent levels of thorium-230, a radioactive isotope with a date record that goes back further than that for atmospheric radiocarbon.The team was able to align atmospheric carbon-14 levels with around 300 new dates, ranging from 54,000 to 18,000 years ago.
These do not always provide a direct record of atmospheric carbon levels, however, as the carbon-14 has often passed through other systems that add more carbon to the mix.