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Appendix A Radiocarbon Dating The technique of radiocarbon dating was pioneered by Libby in the early 1950s. This technique is based on the half life of 14C of approximately 5,700 years and the fact that the radioisotope is made through cosmic ray interactions in the atmosphere by the reaction of energetic neutrons on 14N to produce 14C and a proton. The 14C mixes in the atmosphere and becomes incorporated into CO2. All living things have ratios of 14C/12C that reflect the value in the atmosphere at the time and place in which they are living. Once a plant or animal dies, it stops incorporating modern CO2 and the 14C slowly decays away. The standard is to reference everything to the ratio of 14C/12C in the atmosphere in 1950. The units are in the change relative to this value as ∆14C in units or ‰ (parts per thousand.) All positive ratios are from after 1950 and all negative ratios correspond to before 1950, due to the loss of 14C because of its radioactive decay. The complication is in determining what the 14C/12C was at the time the animal or plant was living. The historical ratio is influenced by the cosmic ray flux in a given geographical region (it is stronger near the earth’s poles and sensitive to the earth’s magnetic field) and the solar activity that is a dominant source of the earth’s cosmic rays. Tree rings and corals have been used as calibrations to relate the ∆14C to actual calendar ages. Modern humans have modified the purely cosmogenic 14C content of the atmosphere in two ways. Fossil fuel burning has introduced into the atmosphere old (dead) carbon that is so old that it has essentially no 14C left. Therefore, the “modern” 14C/12C ratio was falling until the mid-1950s, when atmospheric nuclear testing created what is now called the “bomb spike.” (see Figure A-1) The testing raised the 14C content of the atmosphere to almost twice the pre- bomb value in the Northern hemisphere at its peak around 1965. With the ces - sation of atmospheric testing, the exchange of the atmosphere with the ocean has gradually reduced the levels to values of ∆14C less than 100‰, down from the peak of >800 ‰. This behavior is clearly seen in the figure below, which shows a modern ∆14C curve using data from Levin and Kromer (2004). This 181
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182 APPENDIX A FIGURE A-1 Atmospheric CO2 (Northern Hemisphere). A-1.eps Change in values of 14C for atmospheric CO2 since 1959. The rise from values near zero bitmap 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.