Radiocarbon revolution: the story of an isotope

Sample being removed from bone for carbon dating using accelerator mass spectrometry

A human femur, thought to be from medieval times, being sampled for carbon dating.Credit: James King-Holmes/Science Photo Library

Hot Carbon: Carbon-14 and a Revolution in Science John F. Marra Columbia University Press (2019)

It is nearly 80 years since the discovery of carbon-14, a radioactive isotope of the sixth element. Because its decay can be used to track the passage of time, radiocarbon has made myriad contributions across the Earth, environmental, biological and archaeological sciences. In the wonderfully engaging Hot Carbon, oceanographer John Marra takes this story much further, exploring not just the science, but why we should care about it.

Radiocarbon is scarce in nature, formed in the upper atmosphere through the interaction of cosmic rays with nitrogen. It is rapidly converted to carbon dioxide, and filters into a host of carbon reservoirs in the biosphere and ocean. Living organisms constantly take up 14C, and after they die, the isotope decays at a known rate. By measuring the amount left in a carbon-based sample, it is possible to calculate its age. Since the 1940s, the technique has been used to date materials as much as 60,000 years old, capturing everything from the early migration of modern humans out of Africa, by dating bones and charcoal from ancient hearths, through to the incredibly slow growth rates of mosses living on the fringes of Antarctica. In retelling these facts, Marra offers compelling stories about the great researchers — many long forgotten — whose discoveries made possible the theory, practice and further findings we now take for granted. There’s enough to satisfy the most insatiable informavore.

Hot Carbon starts with the extraordinary story of chemist Martin Kamen, born in Canada to Russian immigrants. In February 1940, Kamen was trying to produce a new isotope of carbon at the Berkeley Radiation Laboratory at the University of California. Sleep-deprived after three nights of collecting sufficient irradiated graphite to measure the hoped-for isotope, he stepped outside. His bedraggled appearance caught the attention of police; worse, he fitted the description of an escaped convict who had gone on a murder spree. Hauled to the police station, Kamen was finally released when a survivor of the bloodbath confirmed he was not the suspect. Kamen returned to the laboratory to find that his colleague Sam Ruben had analysed the carefully gathered sample and found that it was measurably radioactive. The story of 14C thus began with a dose of high drama.

Martin Kamen working in lab on a photosynthesis experiment.

Chemist Martin Kamen was the first to demonstrate the synthesis of carbon-14.Credit: Hansel Mieth/The LIFE Picture Collection/Getty

Originally expected to have a half-life of just minutes or hours, this heavy form of carbon was considered a low research priority. But Kamen and Ruben’s efforts proved that it would be stable over millennia, opening up a breathtaking number of research avenues (its half-life of 5,730 years was determined some years later). Kamen never received the credit he deserved, becoming a victim of the US anti-communist fervour of the 1940s and 1950s. Those who applied his insight, such as chemists Willard Libby and Melvin Calvin, reaped the scientific reward.

We follow the 14C trail through a number of disciplines, learning, for instance, how Calvin and his team used the isotope to trace the way in which plants convert CO2 into sugar, revealing the intricate processes underpinning photosynthesis. We see how radiocarbon was deployed by labs in Britain, Switzerland and the United States to date the flax used to weave the Turin Shroud (believed by some to be the burial cloth of Jesus) to between 1260 and 1390. Radiocarbon dating has shown that Ötzi — the corpse retrieved from melting alpine ice on the Austrian–Italian border in 1991 — is more than 5,000 years old. And we discover how candidate drugs, labelled with 14C at specific parts of the molecule, can be followed through phases of the body’s metabolism to test the drugs’ safety and efficacy. There is so much more. Marra explains, for instance, how, shortly after 14C was discovered, dissolved CO2 in seawater was used to track the movement of currents in the deep ocean, revealing connections around the planet considered unfathomable before.

Carbon-14 may be the star, but scientists, institutions and happenstance have valuable supporting roles. Take Libby, winner of the 1960 Nobel Prize in Chemistry for his work developing radiocarbon dating. At one point, his team waded into the sewers of Baltimore, Maryland, collecting methane produced from human waste to demonstrate unequivocally that it contained considerably more 14C than did archaeological samples and a precisely dated piece of redwood heartwood.

Marra also reveals, in vivid detail, the difficulties faced by early researchers in acquiring precious samples of plankton, which opened up a new perspective on ocean productivity and, ultimately, carbon sequestration. His own experience in this area illuminates the researchers’ pioneering spirit in the face of wild conditions, cramped spaces and sometimes surly ships’ captains. The technological limitations were progressively overcome by dogged perseverance and a belief that the work would help them to understand the oceans’ potential for incorporating inorganic carbon into organic compounds — still the focus of fierce investigation.

Mysteries remain in the Earth sciences, such as the effectiveness of the carbon cycle and the ramifications of human activity, including our seemingly insatiable hunger for fossil fuels. Importantly, Marra shows how 14C can be used to tease out processes across a range of timescales. He explains why the Southern Ocean is the ‘gatekeeper’ to the planet’s ocean circulation, and how abrupt changes in the formation of deep water and the position of the overlying wind belts can drive dramatic shifts in the carbon cycle. Soberingly, a doubling of atmospheric levels of 14C — arising from mid-twentieth-century nuclear-bomb testing — is preserved as a spike in annually formed natural archives, including tree rings. That marker could be chosen to delineate the start of a new geological epoch: the Anthropocene.

Hot Carbon offers a timely perspective on how mind-bogglingly connected our planet is — and how 14C will continue to be important in helping us to understand what lies ahead.

Nature 570, 304-305 (2019)

doi: https://doi.org/10.1038/d41586-019-01895-z


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