Edward Winter, of Heaton Normal Intermediate School, asks :-

I have been reading about the amazing discovery of a 5000 year-old-man, the `ice mummy'. He was dated by carbon-14 dating. What is this and how does it work?

Tom Higham, a radiochemist then with the Radiocarbon Dating Laboratory at Waikato University, responded.

The `Iceman' was discovered in 1991, preserved in ice in the alpine region between Austria and Italy. To find out when the Iceman died, radiocarbon dating was applied. This is a scientific dating method which is used in over 120 laboratories around the world.

A living organism is constantly incorporating carbon into its body through food uptake, which builds bones, skin and hair. A small part of the carbon we consume is called `radioactive' carbon or simply radiocarbon. Radioactive means that it has an unstable atomic structure. This means that after a certain period of time, carbon-14 decays or disappears. As long as a living organism is taking up carbon, it is keeping the carbon-14 in its body at a constant level, but when death occurs, the carbon-14 begins to disappear and is not replaced. In the 1950s, scientists discovered that carbon-14 disappears at a known rate. They found that every 5568 years, half the carbon-14 left in the remains of an organism has gone (the radiocarbon `half-life'), so by measuring the carbon-14 remaining, they were able to calculate independent ages for carbon samples from archaeological sites. We can date carbon samples from today, back to about 60 000 years ago using this method.

Tiny fragments of the Iceman's bone, skin and grass from his boots have been dated in two radiocarbon laboratories, in Oxford and Zurich. (A piece of grass or skin about the size of one grain of rice is needed for a date). The dates placed the age of the Iceman between 3400-3100 BC, or about 5500 years ago, the oldest example of a well-preserved human body ever found.