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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.
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.
Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
Those laid down during the fall and winter have a dark color because of the presence of dead vegetation; those deposited during the rest of the year have a light color.
The stratigraphy may also reflect seasonal variation in the velocity of stream flow.
The varved-clay method is applied with fair accuracy on deposits up to 12,000 years old.
Streams flowing into still bodies commonly deposit layers (varves) of summer silt and winter clay through the year.