Radiocarbon dating
Dating once-living material by measuring how much radioactive carbon-14 is left; half of it decays every 5,730 years.
High in the atmosphere, cosmic rays knock neutrons loose, and these turn a few nitrogen atoms into carbon-14, a radioactive form of carbon. It mixes into the air as carbon dioxide, plants take it in and animals eat the plants, so living things carry a small, steady proportion of carbon-14.
When an organism dies, or when a piece of tissue stops renewing itself, no fresh carbon comes in, and the carbon-14 already there decays steadily back into nitrogen. Half of it is gone after about 5,730 years, half of what remains after another 5,730, and so on, so measuring what is left tells you how long ago the carbon was locked in. Willard Libby developed the method in 1949 and won the 1960 Nobel Prize in Chemistry for it. It reaches back about 60,000 years, roughly ten half-lives, after which too little is left to measure.
Because the amount of carbon-14 in the air has varied, raw dates are calibrated against tree rings.
Sometimes the variation helps. In 2012 Fusa Miyake reported a sudden jump in carbon-14 in tree rings for AD 774 to 775, and a similar spike was found for 993 to 994. These Miyake events are thought to be caused by extreme bursts of particles from the Sun. Every tree alive at the time recorded them, so they act as precise markers: counting rings from the 993 spike showed that Vikings in Newfoundland were cutting wood in 1021.
Coming up in future stories
- A 2,000-Year-Old Seed From Masada Grew Into a Tree 2005–2021 · Kibbutz Ketura, Israel (seeds from Masada and Qumran)
- The Monument Built Before Farming c. 9500 BC · Göbekli Tepe, Şanlıurfa, Turkey
- The Nuclear Tests That Unmasked a Fake Painting 2019 study · ETH Zurich; forgery made in the USA
- The Oldest Known Tree Was Cut Down for Science Summer 1964 · Wheeler Peak, Nevada
- There Are Sharks Older Than the United States 2016 · North Atlantic, Greenland