In living things, what is the source of the carbon-14 that is used in radiocarbon dating?

The source of the carbon-14 used in radiocarbon dating is **carbon dioxide in the atmosphere**. Carbon-14 is produced in the upper atmosphere when cosmic rays interact with nitrogen-14, and this carbon-14 then combines with oxygen to form carbon dioxide. This carbon dioxide is subsequently absorbed by plants during photosynthesis and enters the food chain, becoming part of living organisms.