Radiocarbon Dating
The method of dating organic material by measuring remaining carbon-14, exploiting its 5730-year half-life and continuous atmospheric production.
Radiocarbon dating measures the age of organic material — wood, bone, charcoal, seeds, cloth — by the amount of radioactive carbon-14 (¹⁴C) remaining in it. It was developed by Willard Libby at the University of Chicago in 1949 and earned him the 1960 Nobel Prize in Chemistry. The method transformed archaeology and Quaternary science by providing, for the first time, a widely applicable absolute chronology for the last ~50,000 years.
The principle exploits the continuous production of ¹⁴C in the upper atmosphere: cosmic rays create neutrons that convert nitrogen-14 into carbon-14, which oxidizes to carbon dioxide and enters the global carbon cycle. Living organisms continuously exchange carbon with the atmosphere, so their tissues maintain an equilibrium ¹⁴C concentration. At death, the exchange stops, and the ¹⁴C decays back to nitrogen-14 with a half-life of 5730 years — the same radioactive decay physics that governs all nuclear clocks. Measuring the remaining fraction tells how long ago the organism died.
Practice is more subtle than principle. The decay rate means a sample loses half its ¹⁴C every 5730 years, so beyond about ten half-lives the remaining signal is too weak to measure — the practical limit is roughly 50,000 years. The atmospheric ¹⁴C concentration is not constant over time: it fluctuates with solar activity and the carbon cycle, and since the mid-20th century nuclear testing has added a large spike. Radiocarbon ages are therefore reported in radiocarbon years and converted to calendar dates by calibration curves built from absolutely dated tree rings (dendrochronology), corals, and lake varves; the calibration curve for the Northern Hemisphere is IntCal, now extended back beyond 50,000 years.
Contamination is the archaeologist's enemy: even a few percent of modern carbon makes an ancient sample appear thousands of years younger, and pretreatment (acid–base–acid washing, collagen extraction) is essential. Sample size requirements have fallen dramatically with accelerator mass spectrometry (AMS), which counts ¹⁴C atoms directly and can date a few milligrams of material. Radiocarbon dating combines naturally with archaeological stratigraphy: stratigraphy establishes relative order, radiocarbon anchors it in absolute time, and together they build secure chronologies for sites around the world.
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archaeology dating isotopes radiocarbon dating