From carbon dating
Carbon 14 is thought to be mainly a product of bombardment of the atmosphere by cosmic rays, so cosmic ray intensity would affect the amount of carbon 14 in the environment at any given time.
Radiocarbon’s entry into the global carbon cycle starts in the atmosphere where it is formed by the interaction of neutrons produced by cosmic rays with nitrogen atoms.
But when a plant or animal dies, it can no longer accumulate fresh carbon 14, and the supply in the organism at the time of death is gradually depleted.
Since the rate of depletion has been accurately determined (half of any given amount of carbon 14 decays in 5,730 years), scientists can calculate the time elapsed since something died from its residual carbon 14.
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