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How does Radiocarbon

Dating Work?

Radiocarbon dating depends on the fact that there are three naturally occurring isotopes of carbon found on earth.  Two of these isotopes, C-12 and C-13 are stable, but the third, C-14 is not stable.  The unstable isotope, C-14, is created by the breakdown of nitrogen in our atmosphere when it is struck by cosmic radiation.  This unstable isotope travels to earth by atmospheric activity, such as storms, and becomes fastened in the biosphere.  The carbon found in the earth's atmosphere is made up of 99.89% C-12, 1.11% C-13 and 0.00000000010% C-14.  That is, there is 1 C-14 isotope for every 1,000,000,000,000 C-12 isotopes.

The C-14 isotope reacts exactly like C-12 and C-13, therefore it bonds to complex organic molecules through photosynthesis in plants.  It then becomes absorbed by animals who eat these plants.  Once absorbed, the unstable isotope will decay and become a stable C-12 molecule.  The unstable C-14, however, is continually being replenished as long as the animal (or plant) continues to live and eat.  The ratio of C-14 to C-12 inside the animal will therefore remain approximately constant, and equal to that of the earth's atmosphere.  When the animal or plant dies the C-14 ceases to be replenished, and thus only decays.  The amount of C-14 in the dead animal relative to the amount in the atmosphere slowly decreases. This decrease is of exponential decay as opposed to constant decay.

In 1949 Libby, Anderson and Arnold measured the decay of C-14 and found that in 5568 years the amount of this unstable isotope was half that of the original sample.  After another 5568 years the amount of C-14 had again halved.  From this they concluded that the half life of the C-14 isotope was 5568 years.  It was later found that the true half life of C-14 is actually 5730 years.  Using this fact any organism that was once alive can be dated. By testing the ratio of C-14 to C-12 in the organism, and comparing this to the original ratio found in the atmosphere (the ratio that the organism would have had while alive), the number of half-lives of C-14 that had occurred can be calculated. From this, the amount of years since the organism had died can easily be found.

 This method of dating can be used on an impressive list of materials which include;

• Charcoal, wood, twigs and seeds.
• Bone.
• Marine, estuaries and riverine shell.
• Leather.
• Lake muds (gyttja) and sediments.
• Soil.
• Pollen.
• Hair.
• Pottery.
• Wall paintings and rock art works.
• Iron and meteorites.
• Corals and foraminifera.
• Blood residues.
• Textiles and fabrics.
• Paper and parchment.
• Fish remains.
• Insect remains.
• Water.

This is only a sample of materials which have been dated by radiocarbon dating.  The vastness of this technology undoubtedly shows why it has become an indispensable tool in several fields.