The Carbon-14 cycle

Radiocarbon dating (usually referred to simply as carbon-14 dating) is a radiometric dating method. It uses the naturally occurring radioisotope carbon-14 (14C) to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years old. Carbon has two stable, nonradioactive isotopes: carbon-12 (¹²C) and carbon-13 (¹³C). There are also trace amounts of the unstable radioisotope carbon-14 (¹⁴C) on Earth. Carbon-14 has a relatively short half-life of 5,730 years, meaning that the fraction of carbon-14 in a sample is halved over the course of 5,730 years due to radioactive decay to nitrogen-14. The carbon-14 isotope would vanish from Earth's atmosphere in less than a million years were it not for the constant influx of cosmic rays interacting with molecules of nitrogen (N₂) and single nitrogen atoms (N) in the stratosphere. Both processes of formation and decay of carbon-14 are shown in Figure 1.

Figure 1: Diagram of the formation of carbon-14 (forward), the decay of carbon-14 (reverse). Carbon-14 is constantly be generated in the atmosphere and cycled through the carbon and nitrogen cycles. Once an organism is decoupled from these cycles (i.e., death), then the carbon-14 decays until essentially gone.

When plants fix atmospheric carbon dioxide (CO₂) into organic compounds during photosynthesis, the resulting fraction of the isotope ¹⁴C in the plant tissue will match the fraction of the isotope in the atmosphere (and biosphere since they are coupled). After a plants die, the incorporation of all carbon isotopes, including ¹⁴C, stops and the concentration of ¹⁴C declines due to the radioactive decay of ¹⁴C following.