Reliability of Radiocarbon Dating

History

• Radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949. Libby was able to prove the accuracy of radiocarbon dating by correctly estimating the age of wood from an ancient Egyptian barge for which the age was known from historical records. In 1960 he was awarded the Nobel Prize in chemistry for his work. There are now over one 130 radiocarbon dating laboratories around the world.

How It Works

• Carbon is the principal component of all plants and animals. Plants absorb carbon through photosynthesis, and animals absorb the carbon through either eating the plants themselves or other animals. A small part of that carbon is radiocarbon (carbon-14 or C14). This radioactive compound is unstable and eventually degrades completely. Through his work as an atomic scientist during the 1940s, Libby determined that the "half-life" of C14 was 5,568 years (half the amount of time it takes to decay). By measuring this rate of decay, scientists are able to determine the age of an organic sample.

Things That Can Be Radiocarbon Dated

• Carbon is the fourth-most-common element in the universe, behind hydrogen, helium and oxygen, so there are plenty of things that can be dated using radiocarbon dating. Materials like bone, wood, shells, seeds, charcoal, soil, sediments, pottery, hair, resins, paper and fabric can be dated. Because of improvements to the technology in the 1970s, very small amounts of an item can now be dated, down to about the size of a grain of rice.

Controversy

• Hoping to prove that the Shroud of Turin (a sacred religious artifact that believers say is the burial cloth of Jesus) was authentic, the Archbishop of Turin agreed to have a sample of the fabric subjected to radiocarbon dating in the 1980s. Three separate laboratories, in the U.S., England and Switzerland, came up with very similar dates--1260 to1390 AD--and concluded that the cloth was a medieval fake. Since then, many people--especially Christians--have questioned the reliability of radiocarbon dating. To add to the controversy, one of the scientists who examined the cloth, Ray Rogers said--shortly before his death in 2006--that he believed samples may have been inadvertently taken from a piece of the cloth that had been repaired in the 16th century.

Reliability

• Several methods are used to test the accuracy of radiocarbon dating. One way, as Libby did, is to test it on items already in the historical record. Other ways are by using it on wood collected from trees where the rings can be counted, or by comparing it to other dating technologies like uranium/thorium dating, thermoluminescence and obsidian hydration. Calibrating for inconsistencies in rates of decay, radiocarbon dating is accurate to plus or minus 163 years over a 26,000-year period.