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.

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.