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However, atmospheric measurements from the last 50 years show varying carbon 14 levels throughout.

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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.To test this oversight, the researchers measured a series of carbon 14 ages in southern Jordan tree rings calculated as being from between 16.Sure enough, it showed that plant material in the southern Levant showed an average carbon offset of about 19 years compared with the current northern hemisphere standard calibration curve.Preserved leaves in the cores — “they look fresh as if they’ve fallen very recently”, Bronk Ramsey says — yielded 651 carbon dates that could be compared to the calendar dates of the sediment they were found in.

The recalibrated clock won’t force archaeologists to abandon old measurements wholesale, says Bronk Ramsey, but it could help to narrow the window of key events in human history.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.“There has been much debate for several decades among scholars arguing for different chronologies sometimes only decades to a century apart, each with major historical implications.And yet these studies […] may all be inaccurate since they are using the wrong radiocarbon information,” Manning said.The possible reason for this, the team believes, could be due to climatic conditions in our distant past.