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Uses of Radioisotopes: Carbon-14 Dating

Key Concepts

Carbon has 3 isotopes, 2 of which are stable (carbon-12 and carbon-13) and one which is radioactive (carbon-14). Of these isotopes, the most common in nature is carbon-12.

Production of Carbon-14

Carbon-14 is produced in the atmosphere by the interaction of neutrons produced by cosmic rays with the stable isotope of nitrogen, nitrogen-14:

1         14         14         1  
  n   +     N       C   +     H
0         7         6         1  

The carbon-14 atoms produced are then incorporated into carbon dioxide (CO2) molecules to produce 14CO2 molecules which mix with the most common 12CO2 molecules in the atmosphere.

The 14CO2 enters plant tissue as a result of photosynthesis or absorption through the roots.

14C enters animal tissue when animals eat plants containing 14C.

The amount of 14C produced in the atmosphere is balanced by the continual decay of 14C to produce 14N and a beta-particle:

14         14         0  
  C       N   +     e
6         7         1-  

Radiocarbon dating

When a plant or animal dies it stops taking in carbon-14 and radioactive decay begins to decrease the amount of carbon-14 in the tissues.

The age of the plant or animal specimen containing carbon, such as wood, bones, plant remains, is determined by measuring the ratio of carbon-12 to carbon-14.

The half-life of carbon-14 is 5730 years, because of its relatively short half-life, carbon-14 can only be used to date specimens up to about 45,000 years old. After this the amount of carbon-14 present in the sample is too small to be measured precisely. Carbon-14 can not be used to measure the age of very young specimens as the difference between the amount of carbon-12 and carbon-14 will not be sufficient to be detected.

Carbon-14 dating relies on the following assumptions:

  • the initial activity of carbon-14 in plant and animal tissues is a constant,
        independent of time and geographical location

  • the specimen has not been contaminated with modern carbon-14

It is known that the radiocarbon content of the atmosphere has varied in the past.

  • In 1958 de Vries showed that the carbon-14 activity around 1700 and 1500A.D. was up to 2% greater than in the 1900's (this is known as the de Vries Effect)

  • In 1955 Suess found that the activity of twentieth century wood is almost 2% lower than that of nineteenth century wood (this is known as the Suess Effect)

There are a number of possible reasons for the variation in radiocarbon content of the atmosphere:

  • changes in the cosmic ray flux due to the activity of the sun

  • changes in the Earth's magnetic field which modulates the proton flux which in turn affects the rate of production of carbon-14

  • increased CO2 in the atmosphere due to the combustion of fossil fuels since the beginning of the Industrial Revolution

  • the explosion of nuclear devices and the operation of nuclear reactors since 1945 has greatly increased the level of carbon-14 activity

Calibration curves have been produced by comparing radiocarbon dates with other dating methods such as dendrochronology (a dating method using the tree's growth rings). This allows corrections to be made on radiocarbon dates in order to produce more accurate dates.

Materials that can be Dated using the Carbon-14 Method

Material Amount required in grams Comments
charcoal & wood 25 Usually reliable

grains, seeds, nutshells, grasses, twigs, cloth, paper, hide, burnt bones 25 Usually reliable

organic material mixed with soil 50-300 As much soil as possible must be removed from the specimen for the date to be reliable

peat 50-200 Reliable if roots of modern plants are removed

ivory 50 Usually reliable, but the interior of tusks is younger than the exterior

charred bones 300 Heavily charred bones give reliable dates but lightly charred bones give unreliable dates because of carbon exchange with modern carbon-14

inorganic carbon in shells 100 not very reliable due to carbon exchange with carbon-14 in waters containing carbonate

pottery and iron 2000 - 5000 Often reliable. carbon-14 is incorporated into the pottery or iron at the time it was made

A Brief History of Carbon-14 Dating Methods

  • 1934, A.V. Grosse published a paper reporting that the mineral eudialyte contained radioactivity in excess of that expected. He suggested that this was due to the presence of isotopes produced by the interaction of cosmic rays with elements such as oxygen, silicon, iron, and zirconium. (cosmic ray neutrons were discovered by Locker in 1933)

  • 1934, F.N.D. Kurie exposed nitrogen to fast neutrons and observed some nitrogen nuclei emit a particle that made a long, thin track in a cloud chamber

  • 1936, Burcham and Goldhaber showed this to be a reaction of nitrogen-14 with a neutron producing carbon-14 and a proton.

  • 1939, C.G. and D.D. Montgomery suggested carbon-14 could be produced in the atmosphere by the interaction of cosmic ray neutrons with nitrogen-14

  • 1940, Kamen prepared a measurable quantity of carbon-14

  • 1946, W.F. Libby estimated that the average global production rate of carbon-14 was sufficient to detect it in natural samples

  • 1947, Anderson demonstrated the presence of carbon-14 in methane extracted from sewage gas and suggested its use for dating biological samples

  • 1949, Arnold and Libby demonstrated the feasibility of the carbon-14 method of dating by analysing archeological samples of known age

  • 1952, Libby published his famous book "Radiocarbon Dating", and was awarded the Nobel Prize for Chemistry in 1960


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