Radiocarbon Dating

Radiocarbon dating is often used to obtain an absolute date for recent organic materials.

1. Carbon-12 (¹²C) is a stable carbon isotope.
2. Cosmic rays convert ¹²C to carbon-14. ¹⁴C is radioactive.
3. The carbon in atmospheric carbon dioxide (CO₂) can be ¹⁴C or ¹²C.
4. Living plants take up CO₂. Plants are eaten by animals. When either die, the uptake of ¹⁴C ceases.
5. We can estimate the age of the organism's death by the ¹⁴C / ¹²C ratio.

Use the graph to help answer these questions:

1. Estimate how many years does it take for 50% of the ¹⁴C to change to ¹²C.
2. How many half-lives is this ?
3. How many years until only 10% of the ¹⁴C is left ?
4. If you have a plant specimen with 25% of its ¹⁴C left, how old in years is that specimen ?
5. If a bone artifact is thought to be 35,000 years old, what percentage of the ¹⁴C would be left ?
6. Explain why is the practical limit for dating with ¹⁴C about 40,000 years.