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Use of Carbon-14 in Radiocarbon Dating Chemistry Tutorial

Key Concepts

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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):

1 n
14 N
14 C
1 H

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:

14 C
14 N
0 e

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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:

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:

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

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.

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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

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A Brief History of Carbon-14 Dating Methods

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