How old is your mummy?

Necessary background

All living things, including us, contain carbon atoms. As long as we are alive we are constantly exchanging carbon atoms with our environment. We take in carbon atoms in our food and release them by exhaling carbon dioxide into the air. Some of these carbon atoms are radioactive (about one in every million million).

The radioactive carbon is called carbon-14. Some of the carbon-14 atoms decay while they are inside us, but carbon-14 atoms in our food are continually replacing them. This means that as long as we keep eating, the amount of carbon-14 in our bodies remains roughly constant.

Where does carbon-14 come from?

Carbon-14 forms in the upper atmosphere from nitrogen by the action of cosmic rays, mainly from the Sun. The carbon-14 combines with oxygen to form carbon dioxide. This is then taken in by plants to make food, a process called photosynthesis. We eat the plants, or the animals that ate the plants, and that's how carbon-14 gets into us.

When a plant dies, photosynthesis stops. When an animal dies, it stops eating. In both cases the exchange of carbon with the outside world stops. The carbon-14 already inside decays, but is no longer being replaced. This means that the longer something is dead, the less carbon-14 it contains. Every 'half-life' (5,730 years for carbon-14) the amount decreases by half as shown in the graph.

Big breakthrough

In 1947, US scientist Willard F Libby and his team used the amount of carbon-14 in ancient remains to determine their age. At first the technique was used on remains of a known age such as bread that had been buried in Pompeii during the eruption of Mount Vesuvius in AD79, to see if it gave accurate results.

It did. They then carried out tests on older remains, such as wood from the funerary boat of Egyptian Pharaoh Sesostris III, wrappings from a mummy believed to be Cleopatra, and charcoal from a campfire at Stonehenge. Radiocarbon dating was a great success, and in 1960 Libby was awarded the Nobel Prize in Chemistry.

The most famous radiocarbon dating of all was of the Turin Shroud, a religious relic believed to be the cloth that Jesus was wrapped in after he died. It has a ghostly outline of a man with injuries to the head, chest, wrists and ankles. Samples of the shroud, together with samples from other relics were given to three different laboratories, one in Arizona, one in Oxford and one in Zurich. Each lab was given four samples with numbers, but they didn't know which was from the Turin Shroud.

In 1989, their findings were published in the journal Nature. All three labs confirmed the Turin Shroud as being 600-700 years old. It was not the cloth used to wrap Jesus - it was too new by about 1,300 years. When the same method was used on the famous Dead Sea scrolls, they were found to be over 2,000 years old.

Carbon dating method and its limitations

Cleaning the sample and then burning it to produce carbon dioxide is how most radiocarbon dating is done. The amount of radiation coming from the carbon dioxide is then measured with sensitive instruments to determine the amount of carbon-14 present. A new method that uses accelerated mass spectrometry (AMS) makes it possible to count individual atoms of carbon-14, which means that much smaller samples can be used.

Radiocarbon dating cannot be used to date very recent materials, because not enough of the carbon-14 has decayed. In fact, radiocarbon dates are followed by the letter BP ('before present', meaning 1950). Objects about 50,000 years old or more contain so little carbon-14 that it is difficult to measure the radiation. In recent years, AMS has been used to date objects as old as 100,000 years but there is much debate about the accuracy of these dates.

Radiocarbon dating has advanced hugely since Libby pioneered it. One of the biggest problems was the assumption that the amount of carbon-14 in the atmosphere has been roughly constant for the last 50,000 years. We now know it hasn't been, due to changes in solar activity, volcanic eruptions and nuclear weapons testing. However, through comparisons with tree rings (dendrochronology), ice cores and coral samples, corrections can be made so that 'radiocarbon dates' can be converted into 'calendar dates'.

Next time; What exactly is bandwidth, and why is it so important?