About carbon dating
About carbon dating
Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings.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.
“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 the extinction was climate driven or competition with modern humans,” says Paula Reimer, a geochronologist at Queen’s University in Belfast, UK.
Common materials for radiocarbon dating are: The radiocarbon formed in the upper atmosphere is mostly in the form of carbon dioxide. Because the carbon present in a plant comes from the atmosphere in this way, the radio of radiocarbon to stable carbon in the plant is virtually the same as that in the atmosphere.
Plant eating animals (herbivores and omnivores) get their carbon by eating plants.
The problem, says Bronk Ramsey, is that tree rings provide a direct record that only goes as far back as about 14,000 years.
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.
As explained below, the radiocarbon date tells us when the organism was alive (not when the material was used).
This fact should always be remembered when using radiocarbon dates.Bronk Ramsey’s team aimed to fill this gap by using sediment from 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.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.Various geologic, atmospheric and solar processes can influence atmospheric carbon-14 levels.But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.