The Carbon-14 (14C) dating method relies on the radioactive decay of 14C into Nitrogen (N).
Essentially, regular carbon 12C becomes 14C primarily as a result of the bombardment of the earth by cosmic rays. Both forms of carbon combines with oxygen in the atmosphere to become carbon dioxide which is absorbed by plants and eventually eaten by animals and humans. The process keeps the amount of 14C relatively stable within living things until their death. At that time, the 14C will decay (without being replenished) while the stable 12C remains unchanged. Since the ratio 14C / 12C in the atmosphere is known, the ratio in the dead thing can be used to estimate when the dead thing died.
The half-life for C14 is 5568 years.
Current atmospheric ratios
- C12 – 98.89%
- C13 – 1.11%
- C14 – 0.00000000010%
- What if the atmospheric 14C / 12C ratio has changed over time? (e.g., industrial revolution, atomic bomb testing, extreme volcanic activity, changes in earth’s magnetic field) In these cases the estimated date would be off.
From this book chapter.