Background Essay: The Dating Game: Radioactive Carbon
Scientists choose from two primary techniques to determine the age of artifacts and other objects found in the earth. Radiocarbon dating is used for dating once-living matter less than 40,000 to 60,000 years old, like fossilized bones and teeth. Radiometric dating is used for dating objects millions and even billions of years old. Both techniques rely on the properties of radioactive isotopes, which are unstable elements that decay into stable ones over time.
In the early decades of the twentieth century, scientists first developed an understanding that certain elements are radioactive and that these unstable isotopes decayed -- or lost particles from their nuclei, thus becoming different elements -- at a constant rate over time. Knowing a radioactive isotope's decay rate, a scientist can say that after a given amount of time, half of the atoms in a radioactive "parent" sample will be transformed into its stable "daughter" product. After another equal amount of time, half of the remaining radioactive atoms will decay. And so on. This is what is meant by "half-life."
Living plants exchange their carbon with the carbon in the air. The air contains mostly carbon-12, the most common carbon isotope, but it also contains a small amount of carbon-14, a radioactive isotope produced in the atmosphere when cosmic rays bombard nitrogen atoms. All living plants -- and all living things that depend on plants -- contain these two isotopes in the same ratio as the air does. When a plant or other living thing dies, it stops taking in fresh carbon from the air. While its carbon-12 content remains constant, its carbon-14 content decreases because carbon-14 is radioactive and slowly decays into nitrogen. Hence, the ratio of the two isotopes changes over time.
The half-life of carbon-14 is 5,730 years, which means that in 5,730 years, half the carbon-14 atoms will have changed to nitrogen. Assuming carbon-14 has always been present in the atmosphere and in all living things in the same concentration, the ratio of carbon-14 to carbon-12 in a sample of a dead plant and the ratio in living plants can be measured and then compared to help determine when the plant died.
Dating beyond the 40,000 to 60,000 year threshold using radioactive carbon is problematic -- even with the most advanced detection instrumentation -- because so little carbon-14 remains in a sample after that time. After 5,730 years, a decaying piece of organic matter will contain half of the carbon-14 it died with. After 11,460 years, it will contain one-fourth, and after 57,300 years, it will contain only one-tenth of one percent.