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How does radiocarbon dating determine the approximate age of organic materials?
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Archaeology
Archaeologists mapping their finds at Pachacamac, Peru, an Indigenous town occupied from approximately 200 bce to 1532 ce, when it was sacked by conquistadors under the command of Francisco Pizarro.
Radiocarbon dating determines the age of organic materials by measuring the decay of the radioactive isotope carbon-14.12
The Process
- Formation and Absorption: Carbon-14 is created in the upper atmosphere by cosmic rays and enters the carbon cycle as carbon dioxide. Living organisms, such as plants and the animals that consume them, continuously absorb this carbon, maintaining a stable ratio of carbon-14 to stable carbon isotopes.123
- Death and Decay: Upon death, an organism ceases to take in new carbon. The stable carbon isotopes remain constant, but the carbon-14 begins to decrease at a known, constant rate through radioactive decay.12
- Measurement: Carbon-14 has a half-life of 5,730 ± 40 years. By using an accelerator mass spectrometer, researchers measure the remaining proportion of carbon-14 in a sample to calculate how much time has elapsed since the organism died.123
Calibration
Because atmospheric carbon-14 levels have fluctuated over time, raw dates are calibrated using tools like dendrochronology (tree-ring analysis) to ensure accuracy. This method is generally effective for organic remains between 500 and 50,000 years old.231