The Aging Element

Much headline-making northern science relies in part on a scientifically popular technique for telling how old something is. Thanks to carbon dating, we now know the dwarf mammoths of Wrangell Island lasted thousands of years longer than their big mainland-dwelling brethren. The Mesa archaeological find in northern Alaska establishes that hunters roamed the region at least 9700 years ago, a number we can pretty well fix because of carbon dating. And so it goes, for just about every bit of northern research not related to the Exxon Valdez oil spill.

Generally, the stories under the headlines don't explain what "carbon dating" means. That hasn't bothered me much, because I've got a working understanding of the principles involved---or at least so I thought, until a determined reader asked me to clarify how the process worked. Urrgh. With a new appreciation of what I didn't know, I checked out how a good writer on science explains carbon dating. Thus, with thanks to persistent readers and to author E.C. Pielou, here's why carbon dating works.

All carbon is not created equal. The same element can have different isotopes or atomic numbers---in effect, the weights of their nuclei differ. Carbon has three isotopes, known as carbon 12, carbon 13, and carbon 14, often called simply C-12, C-13, and C-14. (As they usually appear in scientific papers, the numbers appear as a superscript in front of the element symbol, the upper-case C, but no newspaper typesetter is going to put up with that level of nuisance.)

The carbon in the carbon dioxide we breathe, for example, is about 99 percent C-12. The remaining one percent is virtually all C-13. That's fine, because the two isotopes are the same element. The very rare carbon 14 is another matter, though, because it is radioactive. By losing an electron, every atom of C-14 eventually will decay into an atom of N-14; that is, it will turn into ordinary nitrogen.

As radioactive substances go, C-14 doesn't last long. Half of any number of atoms of carbon 14 will have become nitrogen 14 after 5730 years have passed. That's simply another way of saying that C-14 has a half-life of 5730 years.

Since the earth is more than four billion years old, all C-14 would have decayed into nitrogen long ago if new C-14 atoms weren't being produced. Cosmic radiation zaps enough atoms of ordinary carbon into the radioactive form to keep the planetary supply of carbon 14 at near 60 metric tons. That sounds enormous, but it's puny next to the worldwide total of all forms of carbon in the atmosphere, biosphere, and fresh and salt waters, which stands at about 40 trillion metric tons (that's 40 followed by twelve more zeros).

Growing plants make withdrawals from that limited supply of radioactive carbon. They must assimilate carbon from the atmosphere's carbon dioxide, and they take in atoms of C-14 right along with atoms of stable carbon isotopes. The carbon gets built into the plants, and then into any animal that eats the plants.

Right---that means every living thing is slightly, but measurably, radioactive. However, the radioactive material is being built into the organism only while it lives. Dead plants and animals don't breathe or build up their mass. From the moment of death on, all their C-14 is on a one-way trip toward becoming nitrogen.

Knowing how much radioactive carbon a mammoth tooth would have contained when its owner lived, a researcher with the right equipment can measure how much it now has and then calculate how long ago the mammoth died. Charcoal from a hunter's campfire offers a similar kind of radiocarbon clock. Thus, carbon dating---more precisely, radiocarbon dating---lets us measure temporal distances into the past.