Centennial Museum gecko logo

Desert Diary


The Native American City of Paquimé was abandoned about 1350 A.D. Oh, really? How do we know that?

This is a legitimate question—there are no written records. For dating the last 40,000 or so years, carbon-14 is invaluable. Carbon comes in three different varieties, or isotopes: 12carbon (the plentiful kind), 13carbon, and 14carbon. 14Carbon is a radioactive form created by cosmic-ray bombardment of nitrogen in the atmosphere. All varieties of carbon act the same chemically, and all organisms contain carbon.

Plants, incorporating the products of photosynthesis into their bodies, end up with the three carbon isotopes nearly proportional to their presence in the air—as do animals that eat plants or other animals. But after death, no new carbon is added, and the radioactive carbon slowly reverts to nitrogen at a fixed rate. By measuring the amount of radioactivity left in plant or animal remains, we can tell how long it's been since new carbon was added—though long dead, a sandal or fragment of rawhide tells us when it lived.
pen and ink


Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.

Prairie Dog Mummy

A natural mummy of a prairie dog recovered from a dry cave. Both the soft tissues of the mummy and the associated plant matter supply suitable material for 14C dating.



Libby, W. F. 1955. Radiocarbon dating. Second ed. University of Chicago Press, Chicago. 175 pp.

Web Resources

Radiocarbon Laboratory, University of Waikato, New Zealand. A wealth of information. It includes a K - 12 section for school children and lay people who want an easily understandable introduction.