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A Journey to a New Land

Mammoths and Tundra
Multimedia Library

Dr. Erle Nelson

Department of Archaeology, Simon Fraser University

How Radiocarbon Dating Works

Radiocarbon dating works because in the upper atmosphere a very strange event takes place; cosmic rays from outer space come in and interact with the earth’s atmosphere and create a special form of carbon called radiocarbon. It’s a radioactive isotope of carbon which eventually decays. We know what its half-life is. We know that it has a regular decay rate – one half of it disappears every 5, 568 years. This stuff is created in the upper atmosphere and is quickly converted into ordinary carbon dioxide, except that this carbon dioxide has radiocarbon in it instead of ordinary carbon. That carbon dioxide is used by plants in their normal process of photosynthesis and therefore gets incorporated into the tissue of all of the plants on earth, and then gets further incorporated into the tissue of all of the animals that eat the plants, and so on. Radiocarbon dating is based on the fact that when these plants and animals stop taking up radiocarbon from the atmosphere, either directly in the case of plants, or indirectly in the case of animals, the concentration of radiocarbon in those tissues begins to decrease. If we find some old tissue lying around in an archaeological site, an old piece of charcoal or an old animal bone or something of that nature, we can measure the radiocarbon concentration in that tissue and from that we can determine the time elapsed since that particular plant or animal stopped incorporating carbon from its environment. That’s what a radiocarbon age is. It doesn’t give an age of an artefact, for example, directly, but it can tell us when a plant died or an animal died. In that sense, a charcoal date on charcoal found in a fireplace, for example, does not give an age for when that fire was lit, it tells you when the tree from which the charcoal was made died. And one has to keep these things in perspective when analyzing radiocarbon information.