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Absolute dating can be achieved through the use of historical records and through the analysis of biological and geological patterns resulting from annual climatic variations, such as tree rings (dendrochronology) and varve analysis.After 1950, the physical sciences contributed a number of absolute dating techniques that had a revolutionary effect on archaeology and geology.
By matching the tree rings on an archaeological sample to the master sequence of tree ring patterns, the absolute age of a sample is established.
The best known dendrochronological sequences are those of the American Southwest, where wood is preserved by aridity, and Central Europe, where wood is often preserved by waterlogging.
In dendrochronology, the age of wood can be determined through the counting of the number of annual rings in its cross section.
Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life.
The carbon-14, along with nonradioactive carbon-13 and carbon-12, is converted to carbon dioxide and assimilated by plants and organisms; when the plant or animal dies, it no longer acquires carbon, and the carbon-14 begins to decay.
The conventional method of measuring the amount of radioactive carbon-14 in a sample involved the detection of individual carbon-14 decay events. This technique involves the direct counting of carbon-14 atoms through the use of the accelerator mass spectrometer and has the advantage of being able to use sample sizes up to 1,000 times smaller than those used by conventional radiocarbon dating.
Thus it is possible to measure the time that has elapsed since the material solidified.
Thermoluminescence, used in dating archaeological material such as pottery, is based on the luminescence produced when a solid is heated; that is, electrons freed during radioactive decay and trapped in the crystal lattice are released by heating, resulting in luminescence.
The radioactive carbon-14 method of dating is used to determine the age of organic matter that is several hundred years to approximately 50,000 years old.