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By counting each pair of varves the age of the deposit can be determined.
The absolute dating methods most widely used and accepted are based on the natural radioactivity of certain minerals found in rocks.
These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, uranium-thorium, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism).
Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
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.
Because rainfall patterns vary annually, any given set of tree ring patterns in a region will form a relatively distinct pattern, identifiable with a particular set of years.
Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life.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.