When living things die, they stop taking in carbon-14, and the radioactive clock is "set"!
Radioactive decay occurs at a constant rate, specific to each radioactive isotope.
Until this century, relative dating was the only technique for identifying the age of a truly ancient object.
By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.
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.
Various geologic, atmospheric and solar processes can influence atmospheric carbon-14 levels.