The bomb pulse, though, isn’t constant.
“In the 13 years that above-ground [nuclear bomb] tests were conducted, the relative amount of 14C in the atmosphere almost doubled the normal level,” Holland explains. “Viewed on a longitudinal chart, the total amount of 14C present in the atmosphere shows a dramatic pulse in late 1963, followed by a gradual decline of approximately 4 percent a year as the ‘artificial’ bomb-created 14C is absorbed out of the atmosphere by living things and the ocean.”
That means if that skeleton found in the woods doesn’t show the presence of large amounts of carbon-14, that’s a pretty good indication that it dates before 1950. But if there’s evidence of the bomb pulse’s effects, it’s possible for forensic scientists to determine that the bones belong to a person who died sometime between 1950 and the present.
“Additionally, because the graph of atmospheric 14C shows a sharp, upward slope, beginning with the first [nuclear bomb] test in 1950, peaking in late 1963 (when the test ban treaty was signed), and then a more gradual absorption curve continuing into the present, it may be possible to ‘fine-tune’ the date of death for individuals who died post-1950,” Holland says.
By examining organic tissues with different growth and “turnover” rates, such as fingernails and hair relative to bone, scientists can estimate where on the bomb pulse downward slope the sample falls and speculate an age.
“Tissues that remodel slowly, such as bone, which may take years to turnover, will reflect a higher concentration of bomb 14C than will tissues, such as hair, which will reflect 14C absorbed closer to the time of death, and therefore lower on the declining slope,” Holland says. “This can result in estimates of year-of-death within one to five years.”
Carbon-14 levels in fingernails, for example, can help scientists determine the year of death with 91 percent accuracy, according to a 2022 study published in Journal of Forensic and Legal Medicine.