Radiocarbon dating is achieved by two methods. The traditional "Beta-counting" method is based on the detection of radioactive decay of the radiocarbon (14C) atoms. The AMS (Accelerator Mass Spectrometry) method is based on the detection of mass of 14C atoms in the sample (and therefore its ratio of 14C to 12C). 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.
14N + n => 14C + p
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 (O2) to produce carbon dioxide (CO2). The carbon dioxide is then incorporated into plant tissues by photosynthesis.
The ratio is 14C / 12C is maintained through growth and repair until the tissue dies. After that, the ratio of 14C to 12C) decreases as the radiocarbon decays. The Beta-counting method detects the rate at which purified carbon decays. As W.F. Libby determined, one gram of pure carbon should produce about 14 (13.56) radioactive decays per minute.
14C => 14N + β
A rate of 7 decays/gram/minute would indicate an age of one half-life, or 5730 years old. 3.5 decays/gram/minute of carbon would be produced by a sample 11,460 years old.
However, atmospheric testing of nuclear weapons in the late 1950's and early 1960'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 1950 A.D. Material which died after 1950 has such high amounts of radiocarbon its age is reported as "percent modern (1950)" (example 180% modern). 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 14C to 12C. During the industrial revolution (1850 - 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 12C was relatively low, giving relatively old ages.
In fact, the natural production of radiocarbon has varied as well. Before the industrial revolution, from 1800 - 1400 AD, the natural production of radiocarbon was high, so dates are "too young." From 1400 AD to 300 BC they are "too old," and prior to 300 BC , they are too young.
This natural variation in the ratio of 14C to 12C results from several factors
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 results. 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.
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Kitagawa, H. and van der Plicht, J. 2000. Atmospheric radiocarbon calibration beyond 11,900 cal BP from Lake Suigetsu. Radiocarbon 42: 369-382.
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