How Does Radiocarbon Dating Work? Counting the Clock in Dead Wood and Bone

Almost all carbon is carbon-12, stable, with six protons and six neutrons. About one atom in a trillion is carbon-14, with two extra neutrons, which is made in the upper atmosphere when cosmic rays strike nitrogen and which is radioactive, turning back into nitrogen with a half-life of 5,730 years. The carbon-14 mixes into the carbon dioxide of the air, plants take it in by photosynthesis, animals take it in by eating plants, and so every living organism holds carbon-14 in the same proportion as the atmosphere, topped up as fast as it decays. At death the intake stops and the decay continues. After 5,730 years half the carbon-14 is gone; after 11,460 a quarter remains; and measuring the fraction left gives the time since death.

Libby's original method in 1949 counted the decays directly, waiting for the rare clicks of a Geiger counter from a large sample, which took days and needed grams of material. Since the 1980s the standard method has been accelerator mass spectrometry, which does not wait for atoms to decay but counts them, sorting the carbon atoms in a sample by mass in a particle accelerator and tallying the carbon-14 directly. It needs only a milligram, a flake of charcoal, a thread of cloth or a splinter of bone, and gives a result in hours with a precision of a few decades for material a few thousand years old. What can be dated:

• •Anything that was once living and still holds its carbon: wood, charcoal, seeds, bone, shell, leather, cloth, paper, peat
• •Not stone, metal or pottery, though the soot on a pot or the straw in a brick can be
• •Material up to about 50,000 years old; beyond that too little carbon-14 remains to measure
• •Not the age of an object but the death of the organism, so a beam cut from the heart of an old oak dates centuries before the house it was built into

The method assumes the atmosphere's carbon-14 has always been the same, and it has not. Cosmic rays vary with the sun's activity and the Earth's magnetic field, so the starting level has drifted over the millennia, and a raw radiocarbon age can be off by centuries. The fix came from trees. A tree ring is a year, its wood holds the carbon-14 of that year, and by measuring rings from overlapping timbers in a sequence that now runs back more than twelve thousand years, scientists built a calibration curve that converts a radiocarbon measurement into a calendar date. It has been extended to fifty thousand years with corals, cave formations and lake sediments, and a radiocarbon date is now quoted as a calendar range with a probability attached, such as 3350 to 3100 BC at 95 percent confidence, which is how the iceman got his date.

Humans have moved the level too. Coal and oil burning since the industrial revolution added old, carbon-14-free carbon to the air and made recent samples look older, and nuclear bomb tests in the 1950s and 1960s nearly doubled the carbon-14 in the atmosphere, a spike that is now used in reverse to date anything that grew since, from wine vintages to the cells of a human brain.

The Shroud of Turin, a linen cloth bearing the image of a crucified man and venerated as Christ's burial cloth, was dated by three laboratories in 1988 to between 1260 and 1390, which is when it first appears in the historical record. The Dead Sea Scrolls dated to the last centuries BC and the first AD, matching their script. The Vinland Map, supposedly evidence of Norse knowledge of America, had parchment from the 1430s and ink from the twentieth century. And the method reshaped prehistory as a whole: before it, the ages of European monuments were guessed by supposed influence from Egypt, and radiocarbon showed that Stonehenge and the megalithic tombs of Brittany were older than the pyramids, which forced the whole story of who taught whom to be rewritten.

The method assumes an organism took its carbon from the air, and some did not. The deep ocean holds carbon that has been out of contact with the atmosphere for centuries, so its carbon-14 has partly decayed, and a fish, a shellfish or a seal looks several hundred years older than a land animal that died on the same day. The same reservoir effect appears in lakes fed by limestone springs, which carry ancient carbon dissolved from the rock. People who ate a great deal of fish carry a smaller version of the offset in their bones.

Contamination is the other hazard, and it matters most for old samples. A bone buried for forty thousand years has very little carbon-14 left, so a trace of modern carbon from roots, groundwater or a museum conservator's glue can make it look thousands of years younger. Laboratories therefore clean samples hard before measuring: charcoal is washed in alternating acid and alkali to strip out soil compounds, and bone is dissolved to extract its collagen, the protein most likely to hold only original carbon. Better cleaning methods have made a number of Stone Age dates older than first measured.

Radiocarbon results come with conventions that are easy to misread. A raw result is reported in years BP, meaning before present, but present here is fixed at 1950, the date chosen so that results would not shift every year and before bomb testing distorted the atmosphere. So 3,000 BP means roughly 3,000 years before 1950, not before today. By long-standing convention laboratories also calculate these raw ages using Libby's original half-life estimate of 5,568 years rather than the more accurate 5,730, because the calibration curve absorbs the difference when the result is converted to a calendar date.

You can see the underlying arithmetic with a worked example. Suppose a sample retains 10 percent of the carbon-14 a living organism would have. Every half-life halves the amount, so you ask how many halvings turn 1 into 0.1. That is the logarithm of 10 to base 2, about 3.32 half-lives, which at 5,730 years each comes to roughly 19,000 years. In practice the measurement has an uncertainty, the calibration curve wiggles, and a single raw result can map to more than one possible calendar range, which is why published dates are given as ranges rather than single years.

In 2012 a team led by the Japanese researcher Fusa Miyake reported something nobody expected in the rings of an old cedar: a sudden jump in carbon-14 in the wood of a single year, AD 774 to 775, far sharper than the gentle drifts the calibration curve normally shows. The same spike was soon found in trees around the world, and a second, smaller one in 993 to 994. These Miyake events are thought to record bursts of high-energy particles, most likely from exceptionally powerful solar storms, which briefly raised carbon-14 production everywhere at once.

Because each spike sits in one known year, it works as a timestamp. If a timber still has its outermost ring, the one that grew just under the bark, you can measure carbon-14 ring by ring until you find the jump, then count rings outward to the bark and get the exact year the tree was felled rather than a range. In 2021 researchers did this with wood cut by metal tools at L'Anse aux Meadows, the Norse site in Newfoundland. They found the 993 signal and counted out to 1021, the first exact year fixed for Europeans in the Americas.

When Libby and his colleagues worked out the theory in the 1940s, nobody had yet detected natural carbon-14 in living things, and the predicted amount was so small it might have been impossible to measure. The first proof came from an unlikely place: methane collected at a sewage works in Baltimore. Sewage methane is made by microbes breaking down fresh waste, so its carbon had recently been part of living things. The team concentrated the heavier isotopes in it and compared it with methane from petroleum, whose carbon is millions of years old. The sewage gas was radioactive and the petroleum gas was not, just as the theory predicted.

The next question was whether the clock gave the right answers. In 1949 Libby and James Arnold measured samples whose ages were already known, including wood from the tombs of the Egyptian pharaohs Djoser and Sneferu, dated from lists of kings, and timber whose tree rings had been counted. Plotted against their expected ages, in what became known as the curve of knowns, the results fell close to the predicted line. That agreement with history, more than the physics alone, persuaded archaeologists to trust the method. Only later, as measurements grew more precise, did small mismatches with tree-ring samples reveal the need for calibration.

Radiocarbon dating works because living things take in carbon-14 from the air until they die, after which it decays with a half-life of 5,730 years; measuring how much remains, now by counting the atoms directly in a mass spectrometer, gives the time since death for anything organic up to about fifty thousand years old. Because the atmosphere's carbon-14 has varied, raw results are converted to calendar dates with a curve built from tree rings, and the method has settled arguments from the Shroud of Turin to the age of Stonehenge.