Carbon-14 is normally produced in the atmosphere through cosmic ray interaction with nitrogen. As shown in the figure below, atmospheric testing of nuclear weapons in the early 1960s caused a major disturbance in the steady state natural distribution of this radioisotope. After the peak in 1963, 14C levels began decreasing exponentially to the contemporary level of 110% that of 1950 and should reach pre-bomb levels within 20 years. The observed decrease is due to sequestration of CO2 in environmental reservoirs and the introduction of 14C depleted CO2 from the burning of fossil fuels. As has been demonstrated in analyses of gallstones, the elevation in 14C above the contemporary level can be used to date biological samples that are not actively turning over carbon. The 14C content of any tissue is a function of dietary 14C levels and the residence time of carbon in that tissue. Several studies of 14C in human tissues during the late 1960s and early 1970s demonstrated incorporation of bomb-pulse 14C into a variety of organs with variable turnover rates; the slowest turnover rates are observed in collagen and bone while brain exhibits the fastest. We hypothesized that, because NFT and SP are essentially insoluble, they do not turn over carbon at a significant rate and that the 14C levels in these structures would be indicative of their average age of formation.
