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Line 1: − + − Due to atmospheric disturbances over the history of the earth, the natural level and distribution of carbon-14 varies over time. For example, extremely recently in the earth's history, testing of [[nuclear bomb|nuclear weapons]] in the atmosphere has increased the concentration of carbon-14 in the Northern Hemisphere. To account for this, standard calibration curves are used to take account of chronological fluctuations, and results can be obtained which are rarely more than 700 years out.+ + + + + + + + + + + + + + + +
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Radiocarbon dating is a method for estimating the age of organic material by measuring the ratio of carbon-14 to carbon-12 present. This technique is based on the measured concentration of carbon-14 in the atmosphere, assuming relative stability, and uses the [[half-life]] of carbon-14, which is 5730 ± 40 years, however a global standard half-life of 5568 ± 30 years is often employed.
'''Radiocarbon dating''' is a method for estimating the age of organic material by measuring the ratio of carbon-14 to carbon-12 present.
The method is based on the fact that the carbon-12 [[isotope]] of carbon is more stable, and therefore more common, than the [[radioactivity | radioactive]] carbon-14 isotope. Most of the carbon in the earth is buried deep underground, where any carbon-14 [[radioactive decay | decays]], leaving behind carbon-12. But carbon in the atmosphere (or in living creatures, which pass the carbon back and forth to the atmosphere relatively rapidly) is often struck by [[cosmic ray]]s, which can convert it from carbon-12 to carbon-14. Therefore the concentration of carbon-14 is higher in living creatures and in the atmosphere than it is in dead or buried forms of carbon.
Thus, archaeologists use radiocarbon dating to measure the age of dead bodies, fossils, and burnt wood. If the concentration of carbon-14 is almost as high as in the atmosphere, then the specimen was recently alive. If it is much lower, then the specimen has been dead a long time.
The mathematical formula that is used to figure the time since death depends on the the measured concentration of carbon-14 in the atmosphere, and also on the [[half-life]] of carbon-14 (the time it takes for half the carbon-14 in a given sample to decay). The half-live of carbon-14 is commonly given as 5730 ± 40 years; however a global standard half-life of 5568 ± 30 years is also often employed.
Obviously, this formula depends on the atmosphere remaining roughly constant in its composition over time. Insofar as the natural level and distribution of carbon-14 varies over time, due to to atmospheric disturbances, the formula will need to be adjusted. For example, in the last few decades, testing of [[nuclear bomb|nuclear weapons]] in the atmosphere has increased the concentration of carbon-14 in the Northern Hemisphere. To account for this, standard calibration curves are used to take account of chronological fluctuations, and results can be obtained which are rarely more than 700 years out.
Other isotopes with longer half-lives can also be used to date objects- however, each method has its own drawbacks. For instance, the decay of potassium-40 is often used to complement the carbon-14 dating of [[dinosaur]] [[fossils]].
Other isotopes with longer half-lives can also be used to date objects- however, each method has its own drawbacks. For instance, the decay of potassium-40 is often used to complement the carbon-14 dating of [[dinosaur]] [[fossils]].
==See also==
[[radiometric dating | Radiometric dating methods]]
==References==
Quarternary Dating Methods, by M. Walker (Wiley & Sons, 2005).
Isotopes: Principles and Applications, by G. Faure and T. Mensing (Wiley & Sons, 2005).