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Published: 28 August, 2003

Carbon dating useful, but applications are limited

Is carbon dating an effective way of finding how old something is?

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Cosmic rays from outer space bombard nitrogen in the earth's atmosphere to produce a form (isotope) of the element carbon that is radioactive. This is called carbon 14, and it provides a very small portion of the carbon that becomes part of plants when they use the carbon dioxide from air for photosynthesis. Animals obtain this same carbon 14 when they eat plants or each other. In short, carbon 14 is a basic building block of living plants and animals, including people.

As long as you are alive, you will have the same proportion of carbon 14 to other carbon as exists in other living organisms, but when you die, the radioactive carbon in your body begins to decay at a known rate. After about 5730 years, only half of the original radioactive carbon is left. Practically no carbon 14 remains after 50,000 years have gone by.

This means that carbon dating can only be used to date things that existed 50,000 years ago or less. It wouldn't be of any help in dating a dinosaur, for instance, because the last dinosaurs went extinct about 65 million years ago. Carbon dating was useful, though, to Cornell researchers and students who excavated some extinct woolly mammoths near Watkins Glen in central New York State; they determined that the mammoths lived at the end of the last Ice Age, some 12,000 years ago.

Although carbon dating can be done on a variety of materials, including bones of ancient people and other animals, it often works best on items that are essentially pure carbon, such as charcoal from wood fires. Radiocarbon dating of burnt plants has helped archaeologists trace the origins and spread of agriculture in many parts of the world.

In the past, scientists used equipment, such as Geiger counters, to detect and count the decay of individual atoms of carbon 14. Now they can also use atomic accelerators to find the amount of carbon 14 in a sample without having to wait for decay events. This allows the scientists to quickly date very small amounts of material, such as individual seeds.

More information on the history, uses, methods and problems of carbon dating can be found on the web at: www.c14dating.com