‘Perhaps the most important isotope’: how carbon-14 revolutionised science

On February 27, 1940, Martin Kamen and Sam Ruben discovered carbon-14 at the University of California Radiation Laboratory in Berkeley, California.

"‘Perhaps the most important isotope’: how carbon-14 revolutionised science

The discovery that carbon atoms act as a marker of time of death transformed everything from biochemistry to oceanography – but the breakthrough nearly didn’t happen

Martin Kamen had worked for three days and three nights without sleep. The US chemist was finishing off a project in which he and a colleague, Sam Ruben, had bombarded a piece of graphite with subatomic particles. The aim of their work was to create new forms of carbon, ones that might have practical uses.

Exhausted, Kamen staggered out of his laboratory at Berkeley in California, having finished off the project in the early hours of 27 February 1940. He desperately needed a break. Rumpled, red eyed and with a three-day growth of beard, he looked a mess.

And that was unfortunate. Berkeley police were then searching for an escaped convict who had just committed several murders. So when they saw the unkempt Kamen they promptly picked him up, bundled him into the back of their patrol car and interrogated him as a suspected killer.

Thus one of most revolutionary pieces of research undertaken in the past century was nearly terminated at birth when one of its lead scientists was accused of murder. It was only when witnesses made it clear that Kamen was not the man the police were after that he was released and allowed to go back to the University of California Radiation Laboratory to look at the lump of graphite that he and Ruben had been irradiating.

It did not take the pair long to realise they had produced a substance with remarkable properties, one that has since transformed a host of different scientific fields and continues to help scientists make major discoveries. By irradiating graphite, they had created carbon-14.

“That gloomy night and morning of 27 February 1940 began a revolution in physiology, biochemistry, archaeology, geology, biomedicine, oceanography, palaeoclimatology and anthropology as well as nuclear chemistry,” says environment researcher John Marra, author of the newly published Hot Carbon: Carbon-14 and a Revolution in Science. “Carbon-14, perhaps the most important isotope to life on Earth, was ‘born’.”

Carbon-14 has six protons and eight neutrons in its nucleus. By contrast, most of the carbon in our bodies and in the outside world, known as carbon-12, has six protons and six neutrons. Crucially, those two extra neutrons make the nucleus of a carbon-14 atom unstable so that it decays radioactively into an atom of nitrogen. More importantly, these decays are relatively infrequent so that it is possible to measure changes in a carbon sample over tens of thousands of years. (See box below.)

What are carbon dating, isotopes and half-lives?

The nucleus of an element is made up of subatomic particles: protons and neutrons. The number of protons in the nucleus of an element defines its chemical behaviour. But atoms of the same element can possess different numbers of neutrons in their nuclei. These different forms are known as isotopes.

Carbon has three main isotopes: carbon-12, carbon-13 and carbon-14. The first two are stable but the last decays radioactively.

In any sample, carbon-14 atoms will take around 5,730 years to lose half their number. Thus carbon-14 is said to have a half-life of 5,730 years.

Robin McKie

“Carbon is what we are made of,” says Marra, who is professor of earth and environmental sciences at Brooklyn College, New York. “Carbon is life. It is fundamental also to how we live, how the Earth is habitable – pretty much everything. And since the discovery of a long-lived radioisotope of carbon, we have an amazing tool to delve into almost every aspect of existence on Earth – and perhaps the universe.”

As Marra reveals in this remarkable history of carbon-14, scientists quickly realised the isotope must affect living beings today. Cosmic rays batter the upper atmosphere and send cascades of neutrons through the air, they calculated. These neutrons strike atoms of nitrogen, the main component of Earth’s atmosphere, and transform some into atoms of carbon-14. In turn, these atoms combine with oxygen to create radioactive carbon dioxide that is absorbed by plants, which are then eaten by animals. “Every living thing on Earth thus becomes radioactive, albeit slightly,” says Marra.

And it dawned on Willard Libby of Chicago University that the radioactivity generated by carbon-14 could be exploited to tremendous advantage. A chemist who had worked on the Manhattan Project to build the first atom bomb, Libby realised that when an organism dies, it will stop absorbing carbon, including carbon-14, and its existing store of the latter will slowly decay. So, by measuring the radioactivity of a sample taken from the organism, its carbon-14 content could be estimated and the date of its time of death could be measured. The sciences of archaeology and palaeontology were about to be revolutionised.

A major problem had to be overcome, however. Carbon-14 exists in only very low levels in the tissue of recently deceased animals and plants: about one in a trillion of their carbon atoms are carbon-14. By contrast, natural background radiation – from thorium and uranium in rocks and other sources – is much, much higher. How could researchers separate carbon-14’s weak signal from this overwhelming background noise?

Libby solved the problem by carefully shielding his detectors and developing ways to tune out any radiation that made it through to the walls of his device. Then he turned to the gas methane, which contains carbon, to provide final validation of his technique, comparing samples from two very different sources. One sample was extracted from natural gas, a fossil fuel whose carbon-14 should have decayed long ago. The second came from the city of Baltimore sewerage system and was extracted from human excrement. It should be rich in carbon-14, having just been produced by humans, Libby reasoned.

And that is exactly what he found. Ancient methane had no carbon-14. By contrast, methane newly excreted by humans was relatively rich in the isotope. As Marra says: “Human waste from sewer lines sent science onward.”

Libby then provided final proof of his dating technology by measuring the radioactivity – and, by inference, the age – of a series of organic samples of known antiquity: wood from the Egyptian funeral ship of Sesostris III, linen that had wrapped a Dead Sea scroll and a bread roll that had been “cooked” in the volcanic eruption that buried Pompeii. His results perfectly matched the known dates of the items he had scanned.

It was a brilliant undertaking for which Libby was awarded the Nobel prize for chemistry in 1960, though he was lucky in one sense. Libby assumed that the rate of carbon-14 production in the atmosphere had been constant for the past few tens of thousands of years. In fact, it has varied fairly widely, thanks to changes in sunspot activity, atmospheric nuclear bomb tests and rising emissions of carbon dioxide from fossil fuels. These have to be taken carefully into account when estimating ages, scientists now realise, though the underlying basis of radiocarbon dating remains sound.

More recently, radiocarbon dating has changed from simply measuring the radioactivity emitted by carbon-14 nuclei to directly counting numbers of atoms of the isotope in a sample. This is done using a technique called accelerator mass spectrometry (AMS), which has allowed scientists to date bones, artefacts and other carbon-based items from the tiniest sample. “This was a huge advance,” says Marra. “Instead of grams of material to analyse, AMS requires only milligrams.”

In this way, the developers of AMS triggered a dating revolution that began in the 60s and has since “ushered in a ‘new archaeology’ revolution”, says Marra. One example involving the use of carbon-14 resulted in the overturning of the idea that past western European cultures had depended on practices and ideas that began in the Middle East and slowly disseminated westwards with the spread of farming. Radiocarbon dating revealed a very different picture and showed that the neolithic cultures of Britain, France and central Europe must have evolved independently.

Later, the technique was used by laboratories in Britain, Switzerland and the United States to date the flax used to weave the Turin shroud. This cloth, marked with the negative image of a bearded man, was believed by some to be the burial shroud in which Jesus was wrapped after crucifixion. Using only a few fragments of cloth, scientists dated it to 1260-1390AD.

Over the years, uses of carbon-14 have spread well beyond dating ancient artefacts. Drugs can be labelled with carbon-14 and followed as they pass through the body in order to test their safety and efficacy. Other researchers have used the isotope to trace the way in which plants convert carbon dioxide into sugar, revealing the intricate processes underpinning photosynthesis.

In addition, carbon-14 has been exploited to study plankton and other forms of sea life, revealing how the waters of the oceans circulate in a great interconnected web of currents that sweep round the planet. “The carbon content of a fish will register what it has been eating, which in turn will reflect the chemistry of the surrounding water, which will be influenced by how the ocean has mixed,” says Marra. For good measure, carbon-14 is now playing a major role in uncovering how climates have changed on Earth over tens of thousands of years, work of immense importance as scientists struggle to understand how rising carbon emissions are now triggering dangerous global heating.

“We have gained substantial understanding about the natural world over the past 60 to 70 years, in no small part because of carbon-14,” says Marra. Certainly, it is hard to exaggerate the impact it has had on science. Yet its discoverers, Kamen and Ruben, both fared badly in the wake of their breakthrough.

Kamen, who came from a family of Lithuanian and Belarusian émigrés, aroused the suspicion of US security forces after the US entered the second world war and he was observed dining with Soviet consular officials. He was summarily sacked from his laboratory and his passport was impounded. Kamen was later brought before the House Un-American Activities Committee in 1948, accused of passing secrets to the Soviets. It was not until the end of the century that his reputation was rehabilitated.

Ruben had even worse luck. After Pearl Harbor, he began research on the physiological effects of phosgene gas, a chemical weapon. During one test, an ampoule of the gas broke and he was sprayed with phosgene. He died a few hours later.

If the story of carbon-14 is one of the remarkable examples of scientific progress in the 20th century, the sad fates of two of its main players is a sign of the turbulent times in which they lived."