The rate of creation of carbon 14 is approximately constant. Most of the carbon in the atmosphere, whether carbon 14 or the more usual carbon 12, is chemically combined with oxygen in the form of carbon dioxide. This gas is sucked in by plants, and the carbon atoms used to build their tissues. To plants, carbon 14 and carbon 12 look almost the same (plants are only ‘interested’ in chemistry, not the nuclear properties of atoms). The two varieties of carbon dioxide are imbibed approximately in proportion to their availability. Plants are eaten by animals, which may be eaten by yet other
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