FIGURE A-1 Atmospheric CO2 (Northern Hemisphere).
Change in values of 14C for atmospheric CO2 since 1959. The rise from values near zero results from atmospheric testing of nuclear weapons. The decline from peak values reached in late 1963 results from the exchange of CO2 between the atmosphere and ocean. As a result of this exchange, levels of 14C in the ocean have slowly risen. The gap in the curve between 1973 and 1976 is due to a lack of atmospheric data for 1974 and 1975.
SOURCE: Courtesy of Alice Mignerey.
rapid rise and fall enables the dating of modern (younger than 1950) samples to within a few years in some cases.
Radiocarbon dating has advanced tremendously with the advent of the technique of accelerator mass spectrometry (AMS) to identify individual 14C atoms. This is a direct counting method and does not rely on detecting the radiation that is emitted when the 14C atoms decay. This improvement has led to the capability to radiocarbon date samples of less than 1 mg in mass. This new technique enabled the anthrax samples to be analyzed.