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Radiocarbon dating is achieved by two methods. The traditional " Beta-counting " method is based on the detection of radioactive decay of the radiocarbon 14 C atoms. These techniques are made possible by sensitive electronic instruments developed in the late twentieth century.
Both methods rely on the ongoing production of radiocarbon in the upper atmosphere. Nitrogen atoms high in the atmosphere can be converted to radiocarbon if they are struck by neutrons produced by cosmic ray bombardment. The rate of bombardment is greatest near the poles, where the Earth's magnetic field is dipping into the Earth and therefore does not deflect incoming cosmic rays.
Once the radiocarbon atom is produced, it rapidly combines with oxygen O 2 to produce carbon dioxide CO 2. The carbon dioxide is then incorporated into plant tissues by photosynthesis. Carbon atoms are incorporated into plant tissue by photosynthesis then into animal tissue by ingestion in nearly the same ratio as in the atmosphere.
After that, the ratio of 14 C to 12 C decreases as the radiocarbon decays. The Beta-counting method detects the rate at which purified carbon decays. Libby determined, one gram of pure carbon should produce about 14 However, atmospheric testing of nuclear weapons in the late 's and early 's greatly increased the amount of radiocarbon in the atmosphere, so the decay rate of 14 decays per minute more than doubled.
Therefore, radiocarbon dates are calculated to a "pre-bomb" age of A. This bomb radiocarbon has been gradually removed from the atmosphere by by natural processes, but the "bomb spike" can be shown through the dating by means such as comparing the bottle date and radiocarbon age of wines. The surplus "bomb" radiocarbon is just one of the effects human have had on the ratio of 14 C to 12 C. During the industrial revolution - present increasing amounts of fossil fuels were combusted.
Since the carbon in these fuels was ancient, it contained no radiocarbon. Therefore, prior to atmospheric bomb testing, the proportion of radiocarbon to 12 C was relatively low, giving relatively old ages. In fact, the natural production of radiocarbon has varied as well.
Before the industrial revolution, from - AD, the natural production of radiocarbon was high, so dates are "too young. This natural variation in the ratio of 14 C to 12 C from several factors. The strength of the Earth's field modulates the production of radiocarbon in the Carbon dating after 1950 atmosphere. An strong field sheilds Earth from cosmic rays and reduces the ratio of 14 C to 12 C. The Carbon dating after 1950 produces a powerful solar wind that deflects cosmic rays.
Periods of high solar activity coincide with low 14 C production, and vice versa. Reduced solar activity during the "Little Ice Age" interval from - years ago may be responsible for the "too young" ages during that period. Fluctions in Earth's carbon reservoirs - such as increased burning of fossil fuels - can effect the ratio of 14 C to 12 C in the atmosphere. The ocean circulates high quantities of ancient carbon deep in the ocean. Increased rates of deep-water upwelling may responsible for the "too old" radiocarbon ages during the last glaciation. The production of radiocarbon has not varied wildly through time, but the changes produce consistent differences from calander ages.
This can be overcome by calibration curves calculated by dating materials of precisely known age. The best samples are tree rings, but annually laminated sediments have also produced excellent. Ocean corals, dated by another radiometric method - Uranium-Thorium dating - have also helped to extend the calibration curve beyond the age of the most ancient treering chronologies.
Libby had to determine the rate of radiocarbon production to make his first dates. Anthropogenic 14C variations in atmospheric CO2 and wines. Radiocarbon Kitagawa, H. Atmospheric radiocarbon calibration beyond 11, cal BP from Lake Suigetsu. The Global Carbon Cycle. American Scientist Merrill, R. The Earth's magnetic field, its history, origin and planetary perspective. Academic Press, London. Stuiver, M. A computer program for radiocarbon age calibration. Reimer, P. Radiocarbon Links.Carbon dating after 1950
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