Absolute dating
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Absolute dating is the process of determining a specific date for an archaeological or palaeontological site or artifact. 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.
Absolute dating contrasts with the relative dating techniques employed, such as stratigraphy. Absolute dating provides a numerical age for the material tested, while relative dating can only provide a sequence of age.
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[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 an organic artifact's or ecofact's date of death.
[edit] Disadvantages
Because the half-life of carbon-14 is short, the older a specimen is, the greater the margin of error becomes. Carbon dating is only reliable within the past 40,000 years. Radiocarbon is also less useful for historic sites or recent sites. The standard margin of error is plus or minus 50 years. Because of this, the technique usually cannot pinpoint the date of a site better than historic records and previous knowledge of the site.
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. The date received from this test is for the last time that the object was heated. Common dates tested are the firing of ceramics (archaeology), and the setting of rocks (geology).
[edit] Thermoluminescence
Thermoluminesence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to 350 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.
[edit] Disadvantages
Radiation levels do not remain constant over time. Fluctuating levels can skew results - for example, if an item went through several high radiation eras, thermoluminesence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. Because of these and other factors, Thermoluminesence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to authenticate an item as antiquity.
[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.
- Dating Exhibit - Absolute Dating. Minnesota State University. Retrieved on 2008-01-13.
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