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Basic Information on Absolute Dating

Chronometric or Absolute Dating

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Absolute dating helps in identying what age items actually are at an archaeological site.

Chronometric dating, commonly called absolute dating, gives age in actual years. There is usually a margin of error with absolute dating techniques. The two most commonly used are dendrochronology and radiocarbon dating. Archaeomagnetism, thermoluminescence, fission track, paleomagnetism, and potassium-argon dating are other methods.

Basic Principles

There are three major divisions when dating material: chronometric or absolute dating, relative dating, and experimental.
   Absolute dating gives age in actual years, usually with a margin of error. There are several methods of absolute dating which are currently used.
    Dendrochronology is tree ring dating. Dendrochronology is currently the only method that is truly absolute and can give a date to the exact year, often times even the season. The growth ring patterns in trees differ with each passing year, depending upon climatic conditions. A plot must exist for the general area of the site. Naturally, trees in various locations will have different climates and therefore growth. If you have two samples that are both 750 years old, the tree rings from a sample near a very arid region will be far different from a sample that came from a lush tropical area.
   Radiocarbon dating, C14 dating, is the standard in North America. C14 dating can be used on any organic material. Perishable materials, residues in ceramic vessels, charcoal from firepits, and carbonized plant elements can be dated with radiocarbon.
   All living material incorporates C14 into their tissues, but as soon as something dies, it can no longer take it in. The amount within the tissue then begins to decrease through radioactive decay at a known rate. The time required for 50% of a radioactive material to decay is known as the half-life. The half-life of C14 is currently known to be approximately 5,700 years. Radiocarbon dating has a maximum range of about 50,000 years.
When giving a date, there is an error factor, a plus or minus range to dating known as a standard deviation. From there it goes to probability! If a standard deviation of 100 is given...there is only a statistical probability of 67% that the organism lived in that 200 year period. To increase chances of being correct, the deviation must be increased to two, or a 400 year time period. At two standard deviations, there is a 97% probability that the specimen died (and possibly lived for that matter!) within the 400 years. Multiple radiocarbon assay dates can improve date sampling of a site.

Archaeomagnetism can be used to date ceramics or other fired clays. Dating is based upon thermoremnant magnetism of ferromagnetic minerals that usually occur in clays or clay type soils. In unfired clays, the direction of these minerals is relatively random, but once heated, some of the grains become aligned with the earth's magnetic field. Upon cooling, these objects retain this magnetic orientation. Features such as kilns, pits, and surface firing areas can be dated using this method. One is then able to compare these specimen with dated secular variation curves that document the long-term changes in declination and inclination of magnetic north for different regions. This gives a relatively accurate date with a standard deviation of 5-15%
Thermoluminescence, or TL, is based upon the elements from which ceramic fabrics are made, which contain certain amounts of radioactive impurities that emit alpha, beta, and gamma radiation. Part of the energy from this radioactive decay accumulates and is stored as trapped electrons and electron "holes" in the ceramic material. The age of a ceramic object is then determined by quickly heating a sample to a temperature in excess of 500 degrees C, at which the trapped electrons and holes are let free.
The thermoluminescence, or light, that is emitted is recorded with a photomultiplier tube. The glow recorded is then measured with an electrometer, which then graphs a glow curve of light intensity versus temperature. The amount of light emitted is proportional to the age of the specimen since its last heating episode, which is presumed to be its original firing. The standard deviation for TL is about 15%. It is important to note that ceramics are highly susceptible to external cosmic and gamma rays. Since each sample must be independently calibrated for its own sensitivity and radiation rate, it is not widely used.