Carbon Dating

views updated May 09 2018

Carbon dating

Carbon dating is a technique used to determine the approximate age of once-living materials. It is based on the decay rate of the radioactive carbon isotope 14C, a form of carbon taken in by all living organisms while they are alive.

Before the twentieth century, determining the age of ancient fossils or artifacts was considered the job of paleontologists or paleontologists, not nuclear physicists. By comparing the placement of objects with the age of the rock and silt layers in which they were found, scientists could usually make a general estimate of their age. However, many objects were found in caves, frozen in ice , or in other areas whose ages were not known; in these cases, it was clear that a method for dating the actual object was necessary.

In 1907, the American chemist Bertram Boltwood (18701927) proposed that rocks containing radioactive uranium could be dated by measuring the amount of lead in the sample. This was because uranium, as it underwent radioactive decay, would transmute into lead over a long span of time. Thus, the greater the amount of lead, the older the rock. Boltwood used this method, called radioactive dating, to obtain a very accurate measurement of the age of Earth. While the uranium-lead dating method was limited (being only applicable to samples containing uranium), it was proved to scientists that radioactive dating was both possible and reliable.

The first method for dating organic objects (such as the remains of plants and animals) was developed by another American chemist, Willard Libby (19081980). He became intrigued by carbon14, a radioactive isotope of carbon. Carbon has isotopes with atomic weights between 9 and 15. The most abundant isotope in nature is carbon12, followed in abundance by carbon13. Together carbon12 and carbon13 make up 99% of all naturally occurring carbon. Among the less abundant isotopes is carbon14, which is produced in small quantities in the earth's atmosphere through interactions involving cosmic rays. In any living organism, the relative concentration of carbon14 is the same as it is in the atmosphere because of the interchange of this isotope between the organism and the air. This carbon14 cycles through an organism while it is alive, but once it dies, the organism accumulates no additional carbon14. Whatever carbon14 was present at the time of the organism's death begins to decay to nitrogen14 by emitting radiation in a process known as beta decay. The difference between the concentration of carbon14 in the material to be dated and the concentration in the atmosphere provides a basis for estimating the age of a specimen, given that the rate of decay of carbon14 is well known. The length of time required for one-half of the unstable carbon14 nuclei to decay (i.e., the half-life ) is 5,730 years.

Libby began testing his carbon14 dating procedure by dating objects whose ages were already known, such as samples from Egyptian tombs. He found that his methods, while not as accurate as he had hoped, were fairly reliable. He continued his research and, through improvements in his equipment and procedures, was eventually able to determine the age of an object up to 50,000 years old with a precision of plus-or-minus 10%. Libby's method, called radiocarbon or carbon14 dating, gave new impetus to the science of radioactive dating. Using the carbon14 method, scientists determined the ages of artifacts from many ancient civilizations. Still, even with the help of laboratories worldwide, radiocarbon dating was only accurate up to 70,000 years old, since objects older than this contained far too little carbon14 for the equipment to detect.

Starting where Boltwood and Libby left off, scientists began to search for other long-lived isotopes. They developed the uranium-thorium method, the potassium-argon method, and the rubidium-strontium method, all of which are based on the transformation of one element into another. They also improved the equipment used to detect these elements, and in 1939, scientists first used a cyclotron particle accelerator as a mass spectrometer. Using the cyclotron, carbon14 dating could be used for objects as old as 100,000 years, while samples containing radioactive beryllium could be dated as far back as 1030 million years. A newer method of radioactive tracing involves the use of a new clock, based on the radioactive decay of 235uranium to 231protactinium.

See also Fossils and fossilization; Geochemistry

carbon dating

views updated May 18 2018

carbon dating (radiocarbon dating) A method of estimating the ages of archaeological specimens of biological origin. As a result of cosmic radiation a small number of atmospheric nitrogen nuclei are continuously being transformed by neutron bombardment into radioactive nuclei of carbon–14. Some of these radiocarbon atoms find their way into living trees and other plants in the form of carbon dioxide, as a result of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stable carbon atoms begins to fall as the radiocarbon decays. The ratio 14C/12C in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40 000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908–80) and his coworkers in 1946–47.

carbon dating

views updated May 29 2018

carbon dating (radiocarbon dating) Method of determining the age of organic materials by measuring the amount of radioactive decay of an isotope of carbon, carbon-14 (C14). This radio-isotope decays to form nitrogen, with a half-life of 5730 years. When a living organism dies, it ceases to take carbon dioxide into its body, so that the amount of C14 it contains is fixed relative to its total weight. Over the centuries, this quantity steadily diminishes. Refined chemical and physical analysis is used to determine the exact amount remaining, and from this the age of a specimen is deduced.

carbon dating

views updated May 21 2018

car·bon dat·ing • n. the determination of the age of an organic object from the relative proportions of the carbon isotopes carbon-12 and carbon-14 that it contains. The ratio between them changes as radioactive carbon-14 decays and is not replaced by exchange with the atmosphere.

carbon dating

views updated May 11 2018

carbon dating the determination of the age of an organic object from the relative proportions of the carbon isotopes carbon-12 and carbon-14 that it contains. The ratio between them changes as radioactive carbon-14 decays and is not replaced by exchange with the atmosphere.

carbon dating

views updated May 08 2018

carbon dating See RADIOCARBON DATING.

carbon dating

views updated May 29 2018

carbon dating See radiocarbon dating.