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Absolute dating

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Title: Absolute dating  
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Subject: Radiometric dating, Dating methodologies in archaeology, Geochronology, Geology, Seriation (archaeology)
Collection: Dating Methodologies in Archaeology, Geochronology
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Absolute dating

Absolute dating is the process of determining an age on a specified time scale in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision.[1][2] Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.

In archeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.[3] Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.

In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young (radiocarbon dating with 14
C
) to systems such as uranium-lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth.

Contents

  • Radiometric techniques 1
    • Radiocarbon dating 1.1
      • Limitations 1.1.1
    • Potassium-argon dating 1.2
  • Luminescence dating 2
    • Thermoluminescence 2.1
    • Limitations 2.2
    • Optically stimulated luminescence (OSL) 2.3
  • Dendrochronology 3
  • Amino acid dating 4
  • See also 5
  • References 6
  • Further reading 7

Radiometric techniques

Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications as a function of the type of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.

Radiocarbon dating

One of the most widely used and well-known absolute dating techniques is carbon-14 (or ecofact.

Limitations

The relatively short half-life of carbon-14, 5730 years, makes the reliable only up to about 75,000 years. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.

An additional problem with carbon-14 dates from archeological sites 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.

Luminescence dating

Thermoluminescence

Thermoluminescence 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 500 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, thermoluminescence 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, Thermoluminescence 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 confirm the antiquity of an item.

Optically stimulated luminescence (OSL)

Optically stimulated luminescence (OSL) dating, a type of optical dating, constrains the time at which sediment was last exposed to light. During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose (De) that the sediment has acquired since deposition, which can be used in combination with the dose rate (Dr) to calculate the age.

Dendrochronology

The growth rings of a tree at Bristol Zoo, England. Each ring represents one year; the outside rings, near the bark, are the youngest.

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This has three main areas of application: paleoecology, where it is used to determine certain aspects of past ecologies (most prominently climate); archaeology, where it is used to date old buildings, etc.; and radiocarbon dating, where it is used to calibrate radiocarbon ages (see below).

In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11,000 years from present.[4]

Amino acid dating

Amino acid dating is a racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.[10]

See also

References

  1. ^ Evans, Susan Toby; David L., Webster, eds. (2001). Archaeology of ancient Mexico and Central America : an encyclopedia. New York [u.a.]: Garland. p. 203.  
  2. ^ Henke, Winfried (2007). Handbook of paleoanthropology. New York: Springer. p. 312.  
  3. ^ Kelly, Robert L.; Thomas, David Hurst (2012). Archaeology: Down to Earth (Fifth edition. ed.).  
  4. ^ McGovern PJ; et al. (1995). "Science in Archaeology: A Review". American Journal of Archaeology 99 (1): 79–142. 
  5. ^ Bada, J. L. (1985). "Amino Acid Racemization Dating of Fossil Bones". Annual Review of Earth and Planetary Sciences 13: 241–268.  
  6. ^ Canoira, L.; Garc�a-Mart�Nez, M. J.; 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 (11): 576.  
  7. ^ Bada, J.; McDonald, G. D. (1995). "Amino Acid Racemization on Mars: Implications for the Preservation of Biomolecules from an Extinct Martian Biota" (PDF). Icarus 114: 139–143.  
  8. ^ Johnson, B. J.; Miller, G. H. (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry 39 (2): 265.  
  9. ^ 2008 [2] 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.
  10. ^ Amino Acid Geochronology Laboratory, Northern Arizona University

Further reading

  • 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. Archived from the original on 2 February 2008. Retrieved 2008-01-13. 
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