Absolute dating is the process of determining a specific archaeological date. Such dating is opposed to relative dating based on stratigraphy, which is the realization 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 techniques usually involve the physical or chemical properties of the raw materials used to create 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. Bold textSUCK ON THAT MUTHA FUCKA!!!!!!!!!!!!!!!!!!!!!!!!!!! Archaeology, archeology, or archæology (from the Greek words αρχαίος = ancient and λόγος = word/speech/discourse) is the study of human cultures through the recovery, documentation and analysis of material remains and environmental data, including architecture, artifacts, biofacts, human remains, and landscapes. ... Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ...

Methods of absolute dating

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. Radiocarbon dating is a radiometric dating method that uses the naturally occurring isotope carbon-14 to determine the age of carbonaceous materials up to ca 60,000 years. ... Organic material or organic matter is informally used to denote a material that originated as a living organism; most such materials contain carbon and are capable of decay. ...

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. Particularly in dry, desert climates, it is possible 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 AMS (accelerator mass spectrometry) dating, which allows a date to be derived from a very small sample, has been very useful in this regard. Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. ...

Potassium-argon dating

Other radiometric dating techniques are available for earlier periods. One of the most widely used is the Potassium-argon_dating (K/A) technique. Potassium–40 is a radioactive isotope of potassium that breaks down into argon–40, a gas. The half–life of potassium–40 is far longer than that of carbon–14—1.3 billion years. 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/A dating can be used only for inorganic substances: rocks and minerals. Potassium–40 in rocks gradually breaks down into argon–40. That 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. Potassium-argon or K-Ar dating is a geochronological method used in many geoscience disciplines. ...

See also

• Dendrochronology (tree ring-dating)
• Incremental dating
• Radiometric dating

Pinus taeda Cross section showing annual rings, Cheraw, South Carolina Pine stump showing growth rings Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree ring patterns. ... Incremental dating techniques allow the construction of year-by-year annual chronologies, which can be fixed ( linked to the present day and thus calendar or sidereal time) or floating. ... Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. ...

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.