Radiocarbon dating

Radiocarbon dating is a technique for determining the age of very old objects consisting of organic (carbon-based) materials, such as wood, paper, cloth, and bone. The technique is based on the fact that both stable and radioactive isotopes of carbon exist. These isotopes behave almost identically in biological, chemical, and physical processes.

Carbon-12, a stable isotope , makes up about 99% of all carbon found in nature . Radioactive carbon-14 is formed in the atmosphere when neutrons produced in cosmic ray showers react with nitrogen atoms.

Despite the fact that it makes up no more than 0.08% of the earth's crust, carbon is an exceedingly important element. It occurs in all living materials and is found in many important rocks and minerals, including limestone and marble, as well as in carbon dioxide . Carbon moves through the atmosphere, hydrosphere, lithosphere, and biosphere in a series of reactions known as the carbon cycle . Stable and radioactive isotopes of the element take part in identical reactions in the cycle. Thus, when green plants convert carbon dioxide to carbohydrates through the process of photosynthesis , they use both stable carbon-12 and radioactive carbon-14 in exactly the same way. Any living material consists, therefore, of a constant ration of carbon-14 to carbon-12.

In the mid-1940s, Willard F. Libby realized that this fact could be used to date organic material. As long as that material was alive, he pointed out, it should continue to take in both carbon-12 and carbon-14 in a constant ratio. At its death, the material would no longer incorporate carbon in any form into its structure. From that point on, the amount of stable carbon-12 would remain constant. The amount of carbon-14, however, would continuously decrease as it decayed by beta emission to form nitrogen. Over time, the ratio of carbon-14 to carbon-12 would grow smaller and smaller. That ratio would provide an indication of the length of time since the material had ceased being alive.

Radiocarbon dating has been used to estimate the age of a wide variety of objects ranging from charcoal taken from tombs to wood found in Egyptian and Roman ships. One of its most famous applications was in the dating of the Shroud of Turin. Some religious leaders had claimed that the Shroud was the burial cloth in which Jesus was wrapped after his crucifixion. If so, the material of which it was made would have to be nearly 2,000 years old. Radiocarbon dating of the material showed, however, that the cloth could not be more than about 700 years old.

Radiocarbon dating can be used for objects up to 30,000 years of age, but it is highly reliable only for objects less than 7,000 years old. These limits result from the fact that eventually carbon-14 has decayed to such an extent that it can no longer be detected well or, eventually, at all in a sample. For older specimens, radioisotopes with longer half-lives can be used for age determination.

See also Half-life; Radioactive decay

[David E. Newton ]

RESOURCES

BOOKS

Taylor, R. E., et al., eds. Radiocarbon Dating: An Archaeological Perspective. Orlando: Academic Press, 1987.

——. Radiocarbon After Four Decades: An Interdisciplinary Perspective. New York: Springer Verlag, 1992.

RADIOCARBON DATING

RADIOCARBON DATING is the measurement of the age of dead matter by comparing the radiocarbon content with that in living matter. The method was discovered at the University of Chicago in the 1940s, but

further research had to wait until the end of World War II. Radiocarbon, or radioactive carbon (C-14), is produced by the cosmic rays in the atmosphere and is assimilated only by living beings. At death, the assimilation process stops. Living matter, wherever found on earth, always has the same ratio of radioactive carbon to ordinary carbon. This ratio is enough to be measured by sensitive instruments to about 1 percent accuracy.

The bold assumption that the concentration of radiocarbon in living matter remains constant over all of time appears to be nearly correct, although deviations of a few percentage points do occur. It has been possible to determine the accuracy of the basic assumption back some 8,000 years, and a correction curve has been produced that allows absolute dating by radiocarbon back 8,000 years. The deviation is about 8 percent, at maximum.

The discovery of the radiocarbon dating method has given a much firmer base to archaeology and anthropology. For example, human settlers, such as the big-game hunting Clovis peoples of the American High Plains and the Southwest, first came to the Americas in substantial numbers at least 12,000 years ago. On the other hand, the magnificent color paintings of the Lascaux Cave in France are 16,000 years old, made 4,000 years before the first substantial number of human beings came to the Americas. By the end of the twentieth century, firm radiocarbon dates for human occupation of North America had never exceeded 12,000 years—the famous Kennewick Man, discovered in Oregon in 1996, was determined to be 9,300 years old—whereas in Europe and Asia Minor these dates reached back to the limits of the radiocarbon method and well beyond, according to other dating methods.

BIBLIOGRAPHY

Libby, Willard F. Radiocarbon Dating. Chicago: University of Chicago Press, 1965.

Renfrew, Colin. Before Civilization: The Radiocarbon Revolution and Prehistoric Europe. Cambridge, U.K.: Cambridge University Press, 1979.

Willard F.Libby/a. r.

See alsoArchaeology ; Chemistry ; Indian Mounds .

radiocarbon dating(¹⁴C dating) A dating method for organic material that is applicable to about the last 70 000 years. It relies on the assumed constancy over time of atmospheric ¹⁴C:¹²C ratios (now known not to be valid), and the known rate of decay of radioactive carbon, of which half is lost in a period (the ‘half-life’) of every 5730 years ± 30 years. The earlier ‘Libby standard’, 5568 years, is still widely used. In principle, since plants and animals exchange carbon dioxide with the atmosphere constantly, the ¹⁴C content of their bodies when alive is a function of the radiocarbon content of the atmosphere. When an organism dies, this exchange ceases and the radiocarbon fixed in the organism decays at the known half-life rate. Comparison of residual ¹⁴C activity in fossil organic material with modern standards enables the calculation of the age of the samples. Since the method was first devised it has been realized that the atmospheric ¹⁴C content varies, as the cosmic-ray bombardment of the outer atmosphere that generates the ¹⁴C varies. Correction for these fluctuations is possible for about the last 8000 years by reference to the ¹⁴C contents of long tree-ring series (e.g. those for Pinus longaeva, bristlecone pine).

radiocarbon dating (¹⁴C dating) A dating method for organic material that is applicable to about the last 70 000 years. It relies on the assumed constancy over time of atmospheric ¹⁴C: ¹²C ratios (now known not to be valid), and the known rate of decay of radioactive carbon, of which half is lost in a period (the ‘half-life’) of every 5730 years ± 30 years. The earlier ‘Libby standard’, 5568 years, is still widely used. In principle, since plants and animals exchange carbon dioxide with the atmosphere constantly, the ¹⁴C content of their bodies when alive is a function of the radiocarbon content of the atmosphere. When an organism dies, this exchange ceases and the radiocarbon fixed in the organism decays at the known half-life rate. Comparison of residual ¹⁴C activity in fossil organic material with modern standards enables the calculation of the age of the samples. Since the method was first devised, it has been realized that the atmospheric ¹⁴C content varies as the cosmic-ray bombardment of the outer atmosphere that generates the ¹⁴C varies. Correction for these fluctuations is possible for about the last 8000 years by reference to the ¹⁴C contents of long tree-ring series, e.g. those for bristlecone pine.

radiocarbon dating (⁴C dating) A dating method for organic material that is applicable to about the last 70 000 years. It relies on the assumed constancy over time of atmospheric ¹⁴C: ¹²C ratios (now known not to be valid), and the known rate of decay of radioactive carbon, of which half is lost in a period (the ‘half-life’) of every 5730 ± 30 years. (The earlier ‘Libby standard’, 5568 years, is still widely used.) In principle, since plants and animals exchange carbon dioxide with the atmosphere constantly, the ¹⁴C content of their bodies when alive is a function of the radiocarbon content of the atmosphere. When an organism dies, this exchange ceases and the radiocarbon fixed in the organism decays at the known half-life rate. Comparison of residual ₁₄C activity in fossil organic material with modern standards enables the age of the samples to be calculated. Since the method was first devised it has been realized that the atmospheric ¹⁴C content varies, as the cosmic-ray bombardment of the outer atmosphere that generates the ¹⁴C varies. Correction for these fluctuations is possible for about the last 8000 years by reference to the ¹⁴C contents of long tree-ring series, e.g. those for bristlecone pine (Pinus longaeva).