But the almost tenfold increase in atmospheric C14 that peaked around the mid-1960s has been followed by a rapid decline since the nuclear test ban treaties and the cessation of high-yield, above-ground nuclear tests.
In fact, C14 is assimilated so rapidly that from about 1963, its half-life in the atmosphere has only been about 11 years.
To tackle this question, the authors isolated neuronal and non-neuronal nuclei from cerebral cortex tissue samples.
Current atmospheric C14 is about twice the level it was before the 1950s.
First author Kristy Spalding and colleagues capitalized on this relatively rapid decline in C14 to develop their dating method.
The authors first established that there is a relationship between the C14 content of DNA and the atmospheric C14 in the local area when that DNA was made.
Unlike many other macromolecules in a cell, DNA is chemically stable once laid down, so its C14 levels are not expected to change even if the DNA ages.