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.
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.
As a chronology tool, C dating can provide ages for samples as old as 50,000 years.
The small sample size capability of AMS radiocarbon dating greatly expands the potential for dating geologic material previously undateable using older proportional counting methods.
New methods are developed and tested as necessary to meet specific dating needs and to improve the overall accuracy and precision of the lab.
It is too soon to know whether the discovery will seriously upset the estimated dates of events like the arrival of human beings in the Western Hemisphere, scientists said.
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.
The more accurate carbon clock should yield better dates for any overlap of humans and Neanderthals, as well as for determining how climate changes influenced the extinction of Neanderthals.
“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.