Chapter 17 · Radioactivity and Nuclear Chemistry
17.6Radiocarbon Dating: Using Radioactivity to Measure the Age of Fossils
10 min · two checks
Predict
Why does carbon-14 dating work on a wooden tool but not on a rock that never contained living carbon?
The idea
- Explain the carbon-14 clock.
- State the age range where it is useful.
Cosmic rays keep producing ¹⁴C in the atmosphere, and living tissue holds a known ratio of ¹⁴C to ¹²C while it is exchanging carbon. After death, the ¹⁴C decays and is not replaced. Measuring how much is left, compared with the living ratio, gives the time since death through the half-life relation. The half-life is 5730 years, so the method is useful from a few hundred years to roughly 50,000 years. Beyond that, too little ¹⁴C remains to measure well.
The clock assumes you know the starting ratio. That ratio has wiggled slightly over millennia, and calibration against tree rings and other records corrects it. The method dates once-living material: wood, charcoal, bone collagen, cloth. It does not date dinosaur bones directly; those fossils are far too old and the original organic carbon is gone. A famous artifact can be dated only if a sample of its organic material is sacrificed to the measurement.
Keep these
- ¹⁴C intake stops at death, then the isotope decays.
- Half-life 5730 years; useful to about 50,000 years.
- Only once-living carbon is a candidate.