An Exceptionally Cold Case

by Nicholas Mee on September 19, 2020

Ötzi emerging from the ice. Credit: Helmut Simon.

On 19 September 1991 a startling discovery was made by two German tourists hiking in the Alps close to the Austrian-Italian border. They came across a body emerging from the ice of a melting glacier.

Investigations by the local authorities soon revealed that this was no ordinary corpse. It was the body of a man who had been dressed in animal hides, a cloak of woven grasses, and a bear skin cap. He had suffered a traumatic death. Lodged deep within his shoulder was a flint arrowhead. And associated with the body were many other remarkable artefacts including an axe with a copper blade. 

Reconstruction of Ötzi’s copper axe.

Clearly these were not the remains of a recently deceased mountaineer. They were much more mysterious and intriguing than that. The murder victim was nicknamed Ötzi after the Ötztal Alps where he was found. But just how old was the body?

Before the nuclear era there was no way to answer such a question. Archaeologists had no option but to rely on literary clues in the historical record to assign dates to their discoveries. Written records were, of course, available for classical Greece and Rome, and the king lists of Ancient Egypt established a time-line that was believed to provide reliable dates back as far as five thousand years before the present. But even in Egypt dating was only possible for finds such as those bearing the cartouche of a pharaoh or those buried within a dateable tomb. Any dates assigned to pre-literate cultures were essentially guesswork. Cultures might be classified as belonging to the Stone Age, the Bronze Age or the Iron Age. But how long ago was the Bronze Age? No one really knew.

Willard Libby

William Libby on the cover of Time magazine in August 1955.

The 1950s saw a revolution in archaeology due to a technique for dating organic materials invented by Willard Libby (1908-1980) at the University of Chicago. The nucleus of the carbon atom was the key to Libby’s breakthrough. 

The carbon nucleus contains six protons. It has two stable isotopes—carbon-12 and carbon-13—with six and seven neutrons in the nucleus, respectively. In 1940, Martin Kamen and Sam Ruben created a third isotope using Ernest Lawrence’s cyclotron at Berkeley. This isotope, carbon-14, was relatively long lived, but unstable with a half-life of 5730 years and this would prove crucial for archaeologists.

Carbon-14 is also created naturally. The Earth is bathed in a flux of high energy cosmic rays arriving from the Sun and elsewhere in the universe. High in the atmosphere, the impact of a cosmic ray on an atomic nucleus may release a neutron that subsequently merges with a nitrogen-14 nucleus knocking out a proton and transforming the nucleus into carbon-14. (This carbon-14 will eventually decay back into nitrogen-14.) The level of carbon-14 in the atmosphere is in equilibrium—the rate of its creation matches the rate at which it decays—so to a good approximation this level remains constant. In the region of one in a trillion carbon atoms in the atmosphere has a carbon-14 nucleus. 

In animals and plants there is a natural turn-over of carbon as it is metabolised. All this carbon ultimately derives from the processing of atmospheric carbon dioxide by plants. So in living organisms the carbon-14 to carbon-12 ratio matches the atmospheric ratio. When an organism dies, however, its metabolism ceases, carbon-14 continues to undergo radioactive decay, but it is no longer replenished. With the passage of time, carbon-14 steadily transforms into nitrogen-14, and the ratio of carbon-14 to carbon-12 gradually decreases, ticking away like a slow ponderous grandfather clock, and this was the basis for Libby’s method.

After 5730 years half the carbon-14 nuclei will have decayed, so the ratio is half the atmospheric ratio and half the ratio currently in your body tissue. After twice this time the proportion of carbon-14 nuclei will have halved again. By measuring the carbon-14 to carbon-12 ratio in ancient organic remains the time that has elapsed since it was alive can be calculated. By 1949 Libby and his colleagues showed that this process could be harnessed into a practical technology.

The Curve of Knowns

To test and calibrate his dating procedure Libby needed a range of artefacts with known ages. He identified two main sources of this material. There were samples from the wood at the heart of very ancient tree trunks. Although such trees might have been alive until relatively recently the carbon within their core would have been deposited long ago, and just how long ago could be determined quite precisely by the number of tree rings. There were also wooden artefacts from Egypt whose age could be deduced from the Egyptian king lists and the context in which they were discovered.

Solar barque of Sesostris III.

Libby analysed the carbon from several ancient trees including a sample from the heartwood of a giant redwood known as the Centennia Stump felled in 1874. The tree ring count showed that the wood in the core of the stump was three thousand years old. The Egyptian artefacts analysed by Libby included the solar barque of the Twelfth Dynasty pharaoh Sesostris III dating to around 1850 BC, cedar wood from the Upper Chamber of the Southern Pyramid of the Fourth Dynasty pharaoh Sneferu dating to around 2600 BC, and wood found within the brickwork of the tomb of the First Dynasty pharaoh King Djet, or Zet as he used to be known, dating to around 3000 BC. 

Libby plotted his data on a chart that he referred to as The Curve of Knowns. The radiocarbon date of each sample showed a good match to the known date of the sample. It was clear that the technique was going to work and that archaeologists could have confidence that the radiocarbon method could be used to provide dates for organic artefacts where an age could not be inferred by any other method.

Libby’s radiocarbon technique has transformed archaeology. It has become so routine that it no longer seems miraculous. It is reliable for dates ranging up to about ten carbon-14 half-lives or 55,000 years before the present. In 1960 Libby won the Nobel Prize in Chemistry for this epoch-making innovation. The Curve of Knowns chart shown above is taken from the lecture notes for his Nobel Prize Acceptance Speech. (The half-life of carbon-14 quoted on the chart has now been superseded by a more accurate value.)

Bog Oaks

Radiocarbon dating was established on the assumption that the atmospheric concentration of carbon-14 has remained fixed. Although this is true to a reasonable approximation, the world is not quite so neat and orderly. The rate at which carbon-14 forms in the atmosphere changes due to variations in solar activity and factors related to the Earth’s magnetic field. To account for this, dendro-chronology, or tree-ring data, has been used to calibrate and refine radiocarbon dating by revealing historical variations in the atmospheric carbon-14 to carbon-12 ratio.

Continuous sequences of tree rings have been compiled by analysing preserved ancient tree trunks such as bog oaks found in the peat bogs of Ireland. Dendro-chronologists now have reliable and continuous northern hemisphere tree-ring data stretching back over twelve thousand years. This ancient wood shows how the natural carbon-14 to carbon-12 ratio has varied over the centuries and this has improved the accuracy and precision of radiocarbon dating.

So How Old is Ötzi?

Much has been learnt about Ötzi the Iceman. High levels of copper and arsenic were found in his body, which suggests that his work may have involved the smelting of copper. He was about 45 years old when he died and he was suffering from a number of ailments. He had parasitic worms in his gut, arthritic joints, gallstones, hardened arteries and rotting teeth. He also had numerous tattoos produced by rubbing charcoal into lesions in his skin. From the contents of Ötzi’s stomach it seems that his last meal included fatty ibex meat and einkorn grain.

The radiocarbon dating of Ötzi’s remains reveal that he lived some time around 5300 years ago.

Further Information

There is more about the connection between nuclear physics and archaeological dating here: Nuclear Time Shift.

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