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How Carbon-14 is Incorporated Into Living Things
Carbon-14 (14C) is produced in the atmosphere by the interaction of neutrons (1n) produced by cosmic rays with the stable isotope of nitrogen, nitrogen-14 (14N):
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 nuclear decay (radioactive decay) of 14C to produce 14N and a beta-particle:
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 only 5730 years.
Because of this 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
(carbon-14 activity is 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, so the initial activity of carbon-14 has NOT been a constant.
The following variations in carbon-14 activity have been noted:
- In 1958 de Vries showed that the carbon-14 activity around 1700 and 1500 A.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:
Possible explanations include:
- 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
Radiocarbon dating, or carbon-14 dating, can be used to date material that had its origins in a living thing as long as the material contains carbon.
Some materials that do not contain carbon, like clay pots, can be dated if they were fired in an oven (burnt) and contain carbon as a result of this.
It should be noted that it is not the artefact that is being dated, it is the soot, ash or charring.
The table below lists some materials that are dated using radiocarbon dating and comments on the reliability of the results:
| Material |
Amount required in grams |
Comments |
|---|
| charcoal and 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 |
