From the La Brea tar pits museum in L.A. -
Radiocarbon Dating
At this moment, as in the past, a relatively constant amount of radioactive carbon, or carbon-14, is formed in the upper atmosphere by cosmic ray bombardment of nitrogen (14N). As the carbon-14 spreads through the atmosphere, it combines with oxygen (O), forming carbon dioxide (CO2) which is eventually absorbed by plants. Plant eaters ingest the carbon-14 and it is eventually passed up the food chain to the carnivores. When they die, no new carbon-14 is added to their tissues, and the carbon-14 present gradually changes back to nitrogen at a fixed rate. Therefore, the amount of carbon-14 relative to normal carbon tells scientists how long ago an organism died. After 5,730 years, only one-half of the original carbon-14 is present and after about 45,000 years essentially all carbon-14 has reverted back to nitrogen.
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