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.
- Main article: Radiometric dating
- Main article: 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 is based on 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 fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops. Then this unstable isotope starts to decay 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 the date of death of the organic matter in an artifact or ecofact.
Because the half-life of carbon-14 is 5730 years carbon dating is only reliable about up to 60,000 years, radiocarbon is less useful to date some recent sites. See radiocarbon dating. This technique usually cannot pinpoint the date of a site better than historic records.
A further issue is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
- Main article: 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 buildup of 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).
- 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. http://www.mnsu.edu/emuseum/archaeology/dating/absolutedat.html. Retrieved on 2008-01-13.
- ^ http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.ea.13.050185.001325
- ^ Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth Laureano Canoira, Maria-Jess Garca-Martnez, Juan F. Llamas, Jos E. Ortz, Trinidad De Torres International Journal of Chemical Kinetics 35(11):576-591 (2003)
- ^ Amino Acid Racemization on Mars: Implications for the Preservation of Biomolecules from an Extinct Martian Biota 
- ^ ARCHAEOLOGICAL APPLICATIONS OF AMINO ACID RACEMIZATION 
- ^ 2008  quote: The results provide a compelling case for applicability of amino acid racemization methods as a tool for evaluating changes in depositional dynamics, sedimentation rates, time-averaging, temporal resolution of the fossil record, and taphonomic overprints across sequence stratigraphic cycles.