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But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.

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.

Carbon-14 is present in all living things in minute amounts.

Since it is radioactive, it gradually fades away by radioactive decay until it is all gone.

This leaves the amount in the air relatively constant.

Radiocarbon immediately reacts with oxygen in the air to form carbon dioxide (CO2).

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.

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.

Take the extinction of Neanderthals, which occurred in western Europe less than 30,000 years ago.

Archaeologists vehemently disagree over the effects changing climate and competition from recently arriving humans had on the Neanderthals' demise.

’s free newsletters."data-newsletterpromo-image="https://static.scientificamerican.com/sciam/cache/file/458BF87F-514B-44EE-B87F5D531772CF83_source.png"data-newsletterpromo-button-text="Sign Up"data-newsletterpromo-button-link="https:// origincode=2018_sciam_Article Promo_Newsletter Sign Up"name="article Body" itemprop="article Body" magazine The carbon clock is getting reset.

Climate records from a Japanese lake are set to improve the accuracy of the dating technique, which could help to shed light on archaeological mysteries such as why Neanderthals became extinct.

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