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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.

Contents

Radiometric techniques

Radiometric dating is based on the constant rate of decay of radioactive isotopes. Given an initial and a present quantity of such an isotope and its half-life, the time elapsed may be calculated. Various methods apply to different materials and timescales. If a very short period of time has passed, as measured in number of half-lives, a particular technique will be less accurate and more susceptible to statistical fluctuations in the inherently random decay events. If many half lives of the isotope of interest have passed, too much of the sample may have decayed to provide an accurate reading.

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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.

Limitations

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.

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 decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K-Ar dating was used to calibrate the geomagnetic polarity time scale.

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.

Limitations

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.

Other

See also

References

  1. ^ Bada, J. L. (1985). "Amino Acid Racemization Dating of Fossil Bones". Annual Review of Earth and Planetary Sciences 13: 241–268. doi:10.1146/annurev.ea.13.050185.001325.   edit
  2. ^ Canoira, L.; Garc�a-mart�nez, M. J. ��S.; Llamas, J. F.; Ort�z, J. �� E.; Torres, T. D. (2003). "Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth". International Journal of Chemical Kinetics 35: 576. doi:10.1002/kin.10153.   edit
  3. ^ Bada, J. (1995). "Amino Acid Racemization on Mars: Implications for the Preservation of Biomolecules from an Extinct Martian Biota". Icarus 114: 139–143. doi:10.1006/icar.1995.1049.   edit
  4. ^ Johnson, B. J.; Miller (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry 39: 265. doi:10.1111/j.1475-4754.1997.tb00806.x.   edit
  5. ^ 2008 [1] 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.

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