Types of absolute dating methods
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.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.Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide.Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14.After another 5,730 years only one-quarter of the original carbon-14 will remain.After yet another 5,730 years only one-eighth will be left.By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.
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Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built.