Japanese lake records will yield an even better carbon clock, helping to pin a cause on the extinction of Neanderthals
The carbon clock is getting a makeover. Climate records from a Japanese lake are set to improve the accuracy of the dating technique, helping to explain archaeological mysteries such as the extinction of Neanderthals.The carbon clock is based on the fact that radioactive carbon-14 decays at a steady rate, whereas other forms of carbon in bones, plants and other once-living things do not vanish with time.Willard Libby won the 1960 Nobel Prize in Chemistry for carbon dating Egyptian mummy tombs, burnt Pompeian bread, and other objects of known age. In setting the carbon clock, though, Libby had to assume that radioactive carbon levels in Earth’s atmosphere had stayed the same for tens of thousands of years. Otherwise, the clock would speed up and slow down as carbon-14 levels in the atmosphere waxed and waned.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 work, published today in Science1. Various geologic, atmospheric and solar processes can influence carbon-14 levels on Earth.Beginning in the 1960s, scientists started to gain an appreciation of these carbon-14 shifts and began calibrating the carbon clock by comparing the known ages of tree rings to their carbon ages. As a rule, carbon dates are younger than calendar dates: a bone carbon-dated to 10,000 years is around 11,000 years old, and 20,000 carbon years roughly equates to 24,000 calendar years.The problem, says Ramsey, is that tree rings push the carbon record back to only about 14,000 years. Marine coral records have been used to fill in the gap, but scientists know that the levels of carbon-14 in the atmosphere and ocean are not identical and tend shift with changes in ocean circulation.In 2006, Ramsey’s team collected a roughly 70-metre core sample from the bed of Lake Suigetsu, west of Tokyo. Two distinct sediment layers have formed in the lake every summer and winter over tens of thousands of years. Ramsey’s team painstakingly counted these layers to come up with their actual dates stretching back 52,000 years. Preserved leaves -- “they look fresh as if they’ve fallen very recently,” Ramsey says -- yielded 651 carbon dates that could be compared to the calendar dates.The new dates won’t force archaeologists to abandon old measurements wholesale, says Ramsey, but they could help narrow the window of key events in human history. “If you’re trying to look at archaeological sites, at the order of 30 or 40 thousand years ago, the ages may only shift by a few hundred years but that may be significant in putting them before or after changes in climate,” he says.Take the extinction of Neanderthals, which occurred in western Europe less than 30,000 years ago. Archaeologists vehemently disagree over the roles of climate and competition from recently arriving humans. A more accurate carbon clock will yield better dates for any overlap of humans and Neanderthals, as well as determine how climatic changes influenced their movements.“If you have a better estimate of when the last Neanderthals lived to compare to climate records in Greenland or elsewhere, then you’ll have a better idea of whether it was climate driven or competition with modern humans,” says Paula Reimer, a geochronologist at Queen’s University in Belfast, UK. She will lead efforts to combine the new Lake Suigetsu measurements with marine and cave records to come up with the new standard for carbon dating.
Bronk Ramsey, C. et al. Science 338, 370-374 (2012).
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Callaway, E. Carbon clock gets more accurate [back by 3pm pls]. Nature (2012). https://doi.org/10.1038/nature.2012.11622