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Carbon dating is used to work out the age of organic material — in effect, any living thing.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.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.The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.

The ratio of carbon-12 to carbon-14 is the same in all living things.

Age determinations can also be obtained from carbonate deposits such as calcite, dissolved carbon dioxide, and carbonates in ocean, lake, and groundwater sources.

Cosmic rays enter the earth's atmosphere in large numbers every day and when one collides with an atom in the atmosphere, it can create a secondary cosmic ray in the form of an energetic neutron.

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.

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