Figure 1.

Description of ¹⁴C Bomb-Pulse Dating

(A) Atmospheric testing of nuclear weapons during the late 1950s and early 1960s generated large amounts of ¹⁴C, which was spread throughout the atmosphere as CO₂. Bomb ¹⁴C mainly enters the human body through the diet. (B) The highest ¹⁴C level in atmospheric CO₂ was observed in 1963, when the specific activity of ¹⁴C in carbon in the atmosphere was approximately twice that of the natural level (expressed in units of fraction modern [F¹⁴C], approximately equal to 1.0 before the nuclear weapon testing). Since the testing ban in 1963, atmospheric ¹⁴C-specific activity has decreased because of the CO₂ uptake by the oceans and biosphere and because of the fossil fuel ¹⁴C-free CO₂ input. The ¹⁴C activity concentration in the human diet approximately follows the atmospheric curve with some delay (36). The average age of carbon in the tissue can be estimated by analyzing its ¹⁴C content, as visualized in the Figure. The uncertainty of the ¹⁴C measurement leads to a corresponding uncertainty in the age calibration. The typical uncertainty is 0.5% to 1% and is mainly based on the number of atoms that have been counted. We define the physical age of a tissue sample as the difference in years between the calendar date of surgery and the calendar date obtained from applying the F¹⁴C value of the tissue sample to the atmospheric bomb-pulse curve. The physical age is thus an estimate of the average time since metabolism ceased, or the turnover rate of carbon in the tissue, or a combination of both these effects. Furthermore, the physical age is the average for the whole sample fraction analyzed, which may consist of several different building parts. Data of atmospheric CO₂ are from the CaliBomb homepage (http://calib.org/CALIBomb/, accessed November 3, 2020) with the Levin data set (15, 16, 17). This curve is valid for the northern hemisphere, with insignificant regional variations (37).