Carbon dating is used to deduce the age of organic material–i.e., formerly living thing. The technique hinges on Carbon 14, a radioactive isotope of Carbon that decays at a steady rate. Organisms capture a certain amount of Carbon 14 from the atmosphere when they are alive. By measuring the ratio of the Carbon 14 to non-radioactive Carbon, the amount of Carbon 14 decay can be determined, giving an age for the specimen in question.
But all of that assumes that the amount of Carbon 14 in the atmosphere has been constant. Any variation would speed up or slow down the “clock.” for example, the atmospheric testing of nuclear bombs added a great deal more Carbon 14 to the atmosphere.
In archaeology, 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). Unfortunately, various geologic, atmospheric and solar processes can influence atmospheric Carbon 14 levels.
To adjust, scientists have accounted for the variations by calibrating the archaeological Carbon clock against the known ages of tree rings. As a rule, Carbon dates are younger than calendar dates: a bone dated to 10,000 years is actually around 11,000 years old.
But 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.