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

The source of carbon-14 used in radiocarbon dating is **carbon dioxide in the atmosphere**. Carbon-14 is produced in the upper atmosphere through the interaction of cosmic rays with nitrogen-14. This carbon-14 becomes incorporated into carbon dioxide, which is then absorbed by plants during photosynthesis. Animals consume the plants, and thus carbon-14 becomes part of their biological systems. When the organism dies, it stops taking in carbon-14, and the existing carbon-14 in its tissues begins to decay, allowing scientists to determine the age of the organism through radiocarbon dating.