Absolute dating
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Absolute dating is the process of determining a specific archaeological date. It contrasts with the relative dating of stratigraphy, which is based on the fact that deposits closer to the surface of the ground are generally younger than more deeply buried deposits. Some archaeologists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies a certainty and precision that is rarely possible in archaeology. Absolute dating is usually based on the physical or chemical properties of the materials of artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us when a particular cultural event happened, but when taken as part of the overall archaeological record they are invaluable in constructing a more specific sequence of events.
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[edit] Methods
[edit] Radiocarbon dating
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it measures radioactive decay. Carbon-14 is an unstable isotope of normal carbon, carbon-12. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they absorb carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the absorption of carbon-14 stops. This unstable isotope starts to break down into nitrogen-14. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the proportion of carbon-14 in organic material, scientists can determine a fossil’s date of death.
However, because the half-life of carbon-14 is short, this dating technique is less dependable for specimens older than 40,000 years than it is for more recent remains. A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as dead trees to remain in their natural state for hundreds of years before people use them as firewood, after which they become part of the archaeological record. Dating when that particular tree died does not necessarily indicate when the fire burned. This is also true of the heartwood of a tree, which will appear younger than the outer rings of the same tree because it has had less time to incorporate carbon-14 into its makeup. For this reason, many archaeologists prefer to use samples from short-lived plants (such as weeds or crops) for radiocarbon dates. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be derived from a very small sample, has been very useful in this regard.
[edit] Potassium-argon dating
Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium-argon dating (K-Ar dating). Potassium-40 is a radioactive isotope of potassium that breaks down into argon-40, a gas. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14. With this method, the older the specimen, the more reliable the dating. Furthermore, whereas carbon-14 dating can be done only on organic remains, K-Ar dating can be used only for inorganic substances: rocks and minerals. As potassium-40 in rocks gradually breaks down into argon-40, the gas is trapped in the rock until the rock is heated intensely (as with volcanic activity), at which point it may escape. When the rock cools, the breakdown of potassium into argon resumes. Dating is done by reheating the rock and measuring the escaping gas.
[edit] See also
- Dendrochronology (tree ring-dating)
- Incremental dating
- Radiometric dating
- Dating methodology (archaeology)
[edit] References
- Timing is Everything, a short course in archaeological dating techniques, by Kris Hirst at About.com
- Chronometric dating in archaeology, edited by R.E. Taylor and Martin J. Aitken. New York: Plenum Press (in cooperation with the Society for Archaeological Sciences). 1997.
