Chronological Methods 8 - Radiocarbon Dating

Radiocarbon, or Carbon-14, dating is probably one of the most widely used and best known absolute dating methods. It was developed by J. R. Arnold and W. F. Libby in 1949, and has become an indispensable part of the archaeologist's tool kit since. Its development revolutionized archaeology by providing a means of dating deposits independent of artifacts and local stratigraphic sequences. This allowed for the establishment of world-wide chronologies.


  
Where does C -14 Come From?

Radiocarbon dating relies on a simple natural phenomenon. As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).

Bombardment Reactions

The unstable isotope is brought to Earth by atmospheric activity, such as storms, and becomes fixed in the biosphere. Because it reacts identically to C-12 and C-13, C-14 becomes attached to complex organic molecules through photosynthesis in plants and becomes part of their molecular makeup. Animals eating those plants in turn absorb Carbon-14 as well as the stable isotopes. This process of ingesting C-14 continues as long as the plant or animal remains alive.

Diffusion Ingestion

  


C-14 Decay Profile

The C-14 within an organism is continually decaying into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere. When the organism dies, the ratio of C-14 within its carcass begins to gradually decrease. The rate of decrease is 1/2 the quantity at death every 5,730 years. That is the half-life of C-14. The animation provides an example of how this logarithmic decay occurs. Click on the "Show Movie" button below to view this animation.

C-14 Decay Profile

  


How is a C-14 Sample Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a C-14 sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

C-14 Processing



The Limitations of Carbon 14 Dating

Using this technique, almost any sample of organic material can be directly dated. There are a number of limitations, however.



Links

Radiocarbon Journal



References

 



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