Radiocarbon Dating: How Scientists Read an Object's Age in Carbon

In 1991, hikers in the Alps found a frozen body sticking out of a glacier. Was he a lost mountaineer from last winter? Radiocarbon dating gave the answer: Otzi the Iceman died about 5,300 years ago. No written record, no ID card — just the decay of a single radioactive atom locked inside his tissue told archaeologists his age to within a few centuries.

- What carbon-14 is and how living things absorb it - How half-life turns decay into a clock - How archaeologists use radiocarbon dates to date organic remains - The limits of the method, including contamination and its ~50,000-year range

Cosmic rays constantly strike the upper atmosphere, converting nitrogen-14 into radioactive carbon-14. Plants absorb carbon-14 (along with ordinary carbon-12) during photosynthesis, and animals absorb it by eating plants. While an organism is alive, it keeps replacing carbon-14 at a steady ratio to carbon-12. The moment it dies, that replacement stops — and the carbon-14 already inside begins to decay.

Carbon-14 decays with a half-life of 5,730 years, meaning half of it disappears every 5,730 years. Start with 100% carbon-14 at death; after 5,730 years, 50% remains; after another 5,730 years (11,460 total), 25% remains; after 17,190 years, 12.5% remains. By measuring how much carbon-14 is left in a sample compared to carbon-12, scientists calculate how many half-lives have passed, and therefore how long ago the organism died.

Chemist Willard Libby developed radiocarbon dating in 1949 at the University of Chicago and won the Nobel Prize in Chemistry in 1960 for it. His team tested the method on wood samples from Egyptian tombs with known dates from historical records, and the radiocarbon ages matched closely — proving the technique worked before archaeologists trusted it on unknown finds.

Radiocarbon dating only works on organic material — wood, bone, shell, cloth — never stone or metal, since those never contained living carbon. It also becomes unreliable past about 50,000 years, because so little carbon-14 remains that measurement error swamps the signal. Contamination is a constant danger: a speck of modern glue or oil from a researcher's fingers can throw off a date by centuries, so samples are handled with special tools and cleaned with acid-base washes before testing.