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Marine records, such as corals, have been used to push farther back in time, but these are less robust because levels of carbon-14 in the atmosphere and the ocean are not identical and tend shift with changes in ocean circulation.Bronk Ramsey’s team aimed to fill this gap by using sediment from bed of Lake Suigetsu, west of Tokyo.The technique hinges on carbon-14, a radioactive isotope of the element that, unlike other more stable forms of carbon, decays away at a steady rate.
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 Bronk Ramsey, is that tree rings provide a direct record that only goes as far back as about 14,000 years.
“If you’re trying to look at archaeological sites at the order of 30,000 or 40,000 years ago, the ages may shift by only 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.
Living organisms are constantly incorporating this C-14 into their bodies along with other carbon isotopes.