Radiocarbon Dating

Every living organism continuously exchanges carbon with its environment -- plants absorb it directly from the atmosphere during photosynthesis, and animals absorb it indirectly by eating plants or other animals -- and a tiny, consistent fraction of that carbon is a radioactive isotope called carbon-14, formed continuously in the upper atmosphere when cosmic rays interact with nitrogen. While an organism is alive, this constant exchange keeps the ratio of carbon-14 to the far more common, stable carbon-12 roughly constant within its tissues, matching the ratio present in the atmosphere at large. The moment an organism dies, that exchange stops entirely, and the carbon-14 already present in its remains begins decaying back into nitrogen at a fixed, predictable rate, without being replenished by any new intake. Carbon-14 decays with a half-life of about 5,730 years, meaning that after that period, exactly half of the original carbon-14 in a sample will have decayed away, and after two half-lives, only a quarter remains, and so on. By precisely measuring the current ratio of carbon-14 to carbon-12 remaining in an organic sample and comparing it to the known ratio in living organisms, scientists can calculate how many half-lives have elapsed since the organism died, and therefore estimate its age. Because carbon-14 decays at such a specific, unchanging rate, this method is reliable for dating organic material up to roughly 50,000 years old; beyond that point, so little carbon-14 remains in a sample that the measurement becomes too imprecise to be useful, and other dating methods relying on different isotopes with longer half-lives are used instead.