Carbon dating
Carbon dating, also known as radiocarbon dating, is a scientific method developed by Professor Willard F. Libby in the late 1940s for determining the age of carbon-containing materials. This technique is based on measuring the ratio of carbon-14, a radioactive isotope, to carbon-12, a stable isotope, in a sample. Carbon-14 is produced in the atmosphere when cosmic rays interact with nitrogen-14 and is incorporated into living organisms through processes like photosynthesis. Once an organism dies, it no longer absorbs carbon-14, and the existing carbon-14 begins to decay at a known rate, with a half-life of approximately 5,730 years.
By analyzing the remaining carbon-14 in a sample, scientists can estimate the time since the organism's death, making this method particularly valuable in archaeology for dating ancient artifacts such as bones, wood, and shells. Although carbon dating has made significant contributions to our understanding of historical timelines, it has faced criticism for potential inaccuracies due to factors like nuclear testing and the assumption of a constant carbon exchange ratio globally. Despite these challenges, the technique remains widely used, and Libby was awarded the Nobel Prize in 1960 for his groundbreaking work in this field.
Carbon dating
Technique for dating ancient organic materials by measuring their carbon-14 content
Also Known As Carbon-14 dating; radiocarbon dating
Date Discovery published on March 4, 1949
Also known as carbon-14 dating and radiocarbon dating, this technique has revolutionized archaeological research by making it possible to assign highly accurate dates to artifacts for which no other precise form of dating is possible.
During the late 1940’s, Professor Willard F. Libby developed carbon dating at the University of Chicago. After Libby had received his bachelor and doctoral degrees from the University of California at Berkeley, he became a lecturer and then an assistant professor spent his research time during the early 1930’s developing the Geiger counter—a device still use to detect and measure weak natural and artificial radioactivity.
During the early 1940’s Libby was awarded a Guggenheim Fellowship and went to Princeton University to continue his research in radiochemistry. That work was interrupted by World War II, during which he worked on the Manhattan Project at Columbia University. There he developed a technique for gaseous diffusion separation and enrichment of Uranium-235 that was used in the atomic bomb that was dropped on Hiroshima. After the war ended, Libby accepted a position as professor of chemistry at the University of Chicago, where he continued his research in radiochemistry, which included applications involving isotope tracers, tritium for uses in hydrology and geophysics, and radiocarbon. His research involving radiocarbon led to the application of carbon-14 dating to determine the age of carbon-containing materials up to about 50,000 years old. It would become an important dating tool for archaeology.
How Carbon Dating Works
The stable and most abundant form of carbon is carbon-12, which contains six protons and six neutrons, giving it a total of twelve subatomic particles. Carbon-12 atoms are ubiquitous in all living organisms. However, another, less abundant, form of carbon, carbon-14, also contains six protons. However, it has eight neutrons, instead of six, causing it to be radioactive when it is also present in biological organisms. Carbon-14 atoms produced when energetic cosmic rays collide with nitrogen-14 atoms which are present in the nitrogen gas that constitutes 78 percent of Earth’s atmosphere. Carbon-14 atoms then react with oxygen in the air to form carbon dioxide. During photosynthesis plants take up this carbon dioxide and incorporate it into their plant fibers, which are eventually eaten by animals and humans. Thus, all biological organisms contain small amounts of carbon-14.
After a biological organism dies, it stops taking up carbon dioxide, and the carbon-14 it already has is not replaced. Its carbon-14 continues to decay by giving off energy in the form of electrons that can be measured by radiation counters. The counters thus measure the amount of carbon-14, which is usually expressed as a ratio against the amount of carbon-12. A ratio in a sample from an old biological artifact, such as a piece of word or a bone, can be compared to the ratio in a living organism to determine the age of the artifact that used to be alive
Half-life is the time required for half of the number of radioactive atoms in a given sample to decay. The half-life for carbon-14 has been determined to be 5,730 years. These means that half of the carbon-14 in a given sample decays every 5,730 years. Therefore, the age of an organism that died many years ago can be calculated by determining how much of its carbon-14 has been lost over time. For example, an object that has lost one-half its carbon-14 would be about 5,730 years old. One that has lost three-quarters of its carbon-14 would be about twice that old.
Impact
In 1960, Willard Libby won the Nobel Prize for his development of the carbon-14 dating technique. Since his time, his technique has been used to determine the age of a wide variety of materials, ranging from bones and antlers to charcoal, wood, and various marine and freshwater shells. However, the technique has received some criticism for being a less than perfect method. For example, the presence of nuclear reactors and open-air testing of nuclear bombs may influence any organisms that die after the 1940’s. In addition, Libby built the technique on an exchange reservoir hypothesis based on the assumption that the exchange of carbon-14 for carbon-12 would continue to be constant all over the world. There has been disagreement regarding the validity of this premise.
Bibliography
Deevey, Edward S. “Radiocarbon Dating.” Scientific American 186 (February, 1952): 24-28.
Hedges, Robert E., and John A. J. Gowlett. “Radiocarbon Dating by Accelerator Mass Spectrometry.” Scientific American 254 (January, 1986): 100-107.
Libby, Willard F. Radiocarbon Dating. 2d ed. Chicago: University of Chicago Press, 1965.
‗‗‗‗‗‗‗. “Radiocarbon Dating.” In The Frontiers of Knowledge. Garden City, N.Y.: Doubleday, 1975.
Maschner, Herbert D. G., and Christopher Chippindale, eds. Handbook of Archaeological Methods. 2 vols. Lanham, Md.: AltaMira Press, 2005.
Taylor, Royal E. Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press, 1987.