Dating wood and bone

Section 4 of 5

When a plant or animal dies, it stops taking in fresh carbon. From that moment, its carbon-14 runs down like a clock.

A living tree holds carbon-14 in almost the same share as the air. Once it is cut down, no new carbon dioxide comes in. The carbon-12 in its wood stays as it is, and the carbon-14 decays, halving every 5,700 years. So the share of carbon-14 left tells how long ago the tree died. The same goes for anything made from a living thing: wood and charcoal, bone, linen and wool, seeds, paper and leather.

The carbon-14 clock starts when a living thing dies A line chart of the share of carbon-14 left against time. While the living thing is alive, the line stays level at 100%, close to the share in the air. At a dashed line marked dies, the line starts to fall, halving every 5,700 years. A dashed guide from 25% on the side meets the curve at 11,400 years after death. 0% 25% 50% 75% 100% carbon-14 left alive dies 0 11,400 22,800 years after it died 25% left: 11,400 years
While it is alive, a plant or animal keeps almost the same share of carbon-14 as the air. After it dies, the share halves every 5,700 years. Wood with a quarter of the living share left died about 11,400 years ago.

Counting the atoms

Willard Libby and his team in Chicago worked out carbon dating in the late 1940s. They counted carbon-14 atoms as they decayed, which took several grams of sample. To test the method, they dated wood from ancient Egyptian tombs whose age was known from written records, and the results came out close to the known ages. Libby won the Nobel Prize in Chemistry in 1960.

Most labs now use an accelerator mass spectrometer, a much bigger relative of the mass spectrometer that weighs molecules. It sorts atoms by weight and counts the carbon-14 atoms themselves, so less than a milligram of carbon is enough: a single seed will do.

  1. The lab cleans the sample, washing out carbon from the soil it lay in. For bone, it dissolves away the mineral and keeps the protein, collagen.
  2. It burns the sample to carbon dioxide, then turns the carbon dioxide into a speck of graphite, the same layered carbon as in a pencil.
  3. The spectrometer counts the carbon-12, carbon-13 and carbon-14 atoms in the graphite. Living things take in the heavier atoms a little more slowly, and the carbon-13 count lets the lab correct for that. The share of carbon-14 then gives the age.

How far back

After 50,000 years, close to nine half-lives, less than 0.3% of the carbon-14 is left. A speck of modern carbon in the sample would then swamp it, so about 50,000 years is as far back as carbon dating reaches. Dinosaur bones, at least 66 million years old, are dated by other radioactive atoms in the rocks around them.

Coal, oil and natural gas formed from living things millions of years ago, so their carbon-14 has all decayed. Burning them adds carbon dioxide with no carbon-14 to the air. In 1955 Hans Suess found that wood grown in the 20th century held a little less carbon-14 than older wood, and that is one of the ways scientists know the extra carbon dioxide in the air comes from fossil fuels.

Quick check

Why does coal hold no carbon-14?