Carbon reacts with oxygen in the atmosphere and forms carbon dioxide. As plants absorb carbon dioxide during photosynthesis, they maintain the same ratio of carbon-14 to carbon-12 as in the atmosphere. Likewise, animals have the same ratio of carbon isotopes as the plants they eat. When a plant or animal dies, however, it can no longer absorb carbon. From this point on, the organism's carbon-14 levels decrease as the radioactive carbon decays. In radiocarbon dating, the age of an object is determined by comparing the object's carbon-14 levels with carbon-14 levels in the atmosphere. For example, if the ratio of carbon-14 to carbon-12 in a fossil is half the ratio in the atmosphere, then the organism lived about 5730 years ago.

Because atmospheric carbon-14 levels can change over time, the calculated age of the fossil is not totally accurate. To get a more accurate radiocarbon date, scientists compare the carbon-14 levels in a sample to carbon-14 levels in objects of known age. Such objects might include trees (which can be dated by counting tree rings) or artifacts from a specific historical period.

Radiocarbon dating can be used to date any carbon-containing object less than 50,000 years old, such as the artifact in Figure 11. Objects older than 50,000 years contain too little carbon-14 to be measurable. To date objects thought to be older than 50,000 years, scientists measure the amounts of radioisotopes with longer half-lives than carbon-14. Geologists, for instance, use the half-lives of potassium-40, uranium-235, and uranium-238 to date rock formations. The older the rock, the lower are the levels of the radioisotope present.

Figure 11 Radiocarbon dating has helped archaeologists learn more about ancient civilizations. Excavations in Abydos, a major archaeological site of ancient Egypt, have unearthed fascinating artifacts. This mummy case, containing the remains of a cat, is 1900 years old.

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