(Image: Everett Collection/Rex)
Cold war nuclear bomb tests let us pinpoint the age of murder victims and individual brain cells – but thanks to the test ban, we’re running out of fallout
JUDITH Bartlett was 28 when she became a missing person. She vanished on her way to work one day in 1964, in the country town of Bathurst, New South Wales. There were several investigations, but no leads. Her three young children grew up confused and hurt that she would abandon them. “I had a mother who I thought loved me,” Bartlett’s daughter Frances Ryan, who was 10 at the time, recently told reporters. “Then I was told she’d gone away.”
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In Australia alone, each year about 1600 people are listed as missing long term. Too often, families never find out what happened. But in 2009, human remains were found in remote bushland, more than 200 kilometres outside Bathurst. There was little to work with, but police sent bone fragments to archaeologist at Waikato University in New Zealand for analysis. An expert in radiocarbon dating, Petchey was accustomed to working with pieces of bone left behind by the first Pacific Islanders around 3000 years ago. But dating these much younger bones would require different tools.
Rather than base her analysis on the decay rate of naturally occurring carbon isotopes – the norm in radiocarbon dating – Petchey used another source: fallout from nuclear bombs that has lingered in the atmosphere since the peak of cold war weapons testing in the early 1960s. Petchey was able to confirm that the police were now dealing with a murder case.
Since the first demonstration of the forensic technique in 2005, a number of labs around the world have dated bodies this way. It is the only accurate method to set a time of death when you have nothing else to go on. But it isn’t just a crucial new tool for criminal forensics. This kind of carbon dating can also allow investigators to date ivory and rhino horn to work out if the source is illegal. Other researchers are using the technique to study the renewal of human brain cells, finding that our brains turn over cells at a greater rate and in more regions than we thought, giving insights into disease and the effects of ageing.
“With nothing else to go on, the bomb curve is the only accurate way to set a time of death”
One of the darkest legacies of the 20th century is proving to be the key to a great many secrets. But there is a catch – time is running out. As the artificial spike in radiocarbon produced by the bomb tests is absorbed by the biosphere, the atmospheric concentration is slowly returning to its natural levels. Bit by bit, accuracy will slip. There may only be a decade or two left.
Before the invention of thermonuclear weapons in the 1950s, natural levels of radiocarbon were produced by cosmic rays. Highly energetic neutrons zipping through the atmosphere collide with nitrogen atoms, creating a radioactive isotope of carbon known as carbon-14 by knocking out a nitrogen proton and taking its place. Carbon-14 is absorbed by plants during photosynthesis and passed up the food chain, ending up inside the cells of all living things.
Carbon-14 decays to nitrogen-14 with a half-life of about 5700 years. So, from the moment an organism dies – and stops taking on new carbon – the amount of carbon-14 in its cells will decrease. By measuring the ratio of carbon-14 to the more naturally abundant carbon-12, radiocarbon dating can give the rough age of organic tissue going back 40,000 years. This is how Petchey dates ancient tissue, for example.
Standard carbon dating is great for studying Palaeolithic diets or identifying the skeleton of Richard III, but when it comes to more recent material you hit a problem. For the past 200 years, carbon-12 emissions from fossil fuels have been diluting the amount of carbon-14 in the atmosphere, making accurate dating of recent material impossible.
Until we learned to stop worrying and use the bomb data, that is. Between 1952 and 1963, when above-ground testing was banned, the US and the Soviet Union doubled the amount of carbon-14 in the atmosphere by detonating thermonuclear weapons. As the two nations rushed to complete their testing before the ban came into effect, levels rose sharply and then steadily fell (see diagram).FIG-mg29720801.jpg
This gives us what is known as the bomb curve. By measuring the amount of carbon-14 in organic material and matching it to records of levels of radiocarbon in the atmosphere going back to the 1950s, we can tell when it formed. And for tissue that renews itself, such as bone collagen, it also gives a good estimate of the date of death. The amount of carbon-14 in Bartlett’s bones, for example, mirrored the amount in the atmosphere when she died. DNA tests confirmed that it was her.
When no other tissue is available, bone collagen will do. But it is than other tissue, because it is replaced every 10 to 20 years during a person’s lifetime. Carbon-14 could have been taken up at any point in that period. Hair or fingernails are far more useful because they are faster growing, taking up carbon more frequently as they make new cells. The radiocarbon level is therefore more up to date. Using the bomb curve, fast-growing tissue dates the time of death to within months. And where cells are not replaced, such as in tooth enamel, .
Tooth enamel acts like a forensic clock, says , a physicist at Lawrence Livermore National Laboratory in California, who recently helped Canadian police work out the age of two people from their remains. One, a 4-and-a-half-year-old child from British Columbia, was initially thought to be around 7. Only a skull was left to identify the man unearthed in Newfoundland. His tooth enamel confirmed he’d died in his fifties and his hair gave the date.
Samples are analysed using a mass spectrometer. “It only takes five minutes per sample,” says Buchholz. “And it’s incredibly precise.” Although researchers like Petchey and Buchholz have had great success with the technique, not many people know about it. “There are thousands of John and Jane Does that this tool could help to identify, but many forensics units aren’t even aware of it,” he says.
“There are thousands of John and Jane Does this tool could help to identify”
As Buchholz cleans up his latest scrap of tissue, another physicist, over on the east coast of the US, is working with very different remains.
Up to the tusk
at the Lamont-Doherty Earth Observatory in Palisades, New York, is using the bomb curve in the fight against poaching. Elephant tusks grow throughout the life of the individual, so . This is a vital tool in determining whether an ivory artefact is made from a tusk from an elephant that was alive after the Convention on International Trade in Endangered Species banned trade in ivory in 1989.
Uno has been working with Environment Canada, which can confiscate and destructively sample items from auction houses if it suspects they are made from contraband ivory. Canada outlawed the trade in ivory in 1975, so any younger items are illegal there. And the technique can also be used to date other wildlife products, like rhino horns, tiger pelts and pangolin scales, as well as illegally logged hardwoods.
Uno hopes bomb curve dating will give him a handle on the wildlife trade on a global scale. Every year, on the basis of large seizures of elephant tusks, conservationists calculate how many elephants have been killed and how many remain in the wild. “But what if some of these tusks in fact come from leaks in the vast stocks that governments hold?” says Uno.
Corrupt officials have a lot to gain from exploiting those ivory stores. “The illegal wildlife trade is worth some $20 billion a year,” he says. By using the bomb curve to date a random sample of seized tusks, it would be possible to work out how many come from government stores.
While Buchholz and Uno use the bomb curve to uncover the secrets of the dead, others are chasing the secrets of the living. at the Karolinska Institute in Stockholm, Sweden, was one of the first to use the technique in forensics. But he soon realised that radiocarbon isotopes not only tell us when a person died, they also date the death of individual cells. This has enabled Frisén’s team to gain insights into how organs grow and develop, the rate cells in different parts of the body are renewed, and which bits of us remain forever young.
The lining of the gut, for example, is replaced every five days. Rib muscles, on the other hand, last about 16 years. Almost the entire body is replaced during a person’s lifetime, including the skeleton. But Frisén’s most remarkable revelations involve the organs that do not fully renew themselves: the brain and heart.
Using the bomb curve to date brain cells in human cadavers last year, Frisén found evidence of substantial neurogenesis in the dentate gyrus. These neurons are important for cognitive function and memory sorting, helping us tell similar things apart. “The Rolling Stones and The Beatles both played rock music, but the dentate gyrus helps you distinguish between them,” says Frisén.
His team also looked at the human olfactory bulb, the smell centre of the brain, and found that unlike in other mammals the cells in this part of the brain do not renew themselves. In other mammals, the nearby lateral ventricular wall produces stem cells that then migrate as new neurons to the olfactory bulb. Humans also produce neural stem cells in the ventricular wall, but they don’t make it to the olfactory bulb. Instead, they migrate to the central striatum. Again, this is unique to us. “We’ve found quite pronounced neurogenesis in the human striatum, even though there is little or none in any other animal,” says Frisén. “It’s very exciting.”
The striata, organs the size of a golf ball on each side of the brain, are involved in motor control and have cognitive functions to do with learning and the development of addiction, for example. They are often affected by stroke and other diseases, including Parkinson’s and Huntington’s, which are incurable at the moment. Frisén thinks the new discoveries could change this. “We could harvest stem cells from the ventricular walls in these patients and perhaps implant the differentiated cells into their striata,” he says.
Fundamental mysteries
Meanwhile, Frisén’s lab is also exploring the turnover of human heart cells. They found that heart cells are indeed replaced, just very slowly. In young people, about 1 per cent are replaced each year, dropping to 0.5 per as we age. “It means that even in very old people, half of their heart cells are the ones they were born with,” he says. “The majority of cells in the heart and cortex are not exchanged. They function decade after decade.”
There are certainly dangers to frequent cell turnover, such as cancers. But here, too, there is uncertainty. “Colon cancer is very common, whereas small intestinal cancer is almost unheard of,” he says. “Why? Cell division occurs at the same rate in both.”
Frisén feels that with the help of the bomb curve he is on the cusp of unlocking fundamental mysteries about how the human body endures, heals and ages – something that could revolutionise the treatment of disease and slow our decline into old age. But there is still much to find out. Is replacing brain cells even viable, for example? “Maybe replacing them would be like swapping a hard drive,” he says. “You’d lose long-term memories and new ones wouldn’t be integrated.”
Bomb-curve dating is so far the only way to get an accurate picture of human cell renewal, especially in the brain. The window of opportunity is closing, however. There will soon be so little of the bomb test carbon-14 left in the atmosphere it will be impossible to distinguish samples from different years. “As the bomb curve becomes flatter, there is more uncertainty in the dating,” says Uno.
So far Frisén has relied on improvements in mass spectrometry to offset the effect of the flattening bomb curve. He also has tissue stored in biobanks, in which the clock has essentially been stopped. But there is not enough to cover the multiple avenues people might want to explore. Researchers like Uno and Frisén are racing to do as much as possible before time runs out.
Eventually, other dating techniques are likely to emerge, although there is nothing on the horizon. “It would be useful to have another carbon-14 pulse,” says Buchholz. “But no one wants another nuclear bomb.” Perhaps another isotope thrown up by past nuclear testing might be used, one that hasn’t been as diluted as carbon-14? There is no obvious contender. Strontium, for example, doesn’t hang around in the air long enough to be absorbed by living tissue. There is also a way to date organic material that relies on the fact that amino acids in cells contain a type of carbon that can be in two configurations. After death, it flips from one to the other. But the rate of flipping is sensitive to environmental influences like temperature and humidity, making it unsuitable for accurate dating.
In New South Wales, the terrible truth about their mother brought comfort to Bartlett’s children. “My brothers and I have found out that our mother didn’t leave us,” Ryan said. Without the bomb curve, it is unlikely her mother would have been identified. But we have perhaps only another decade for date of death analyses, says Buchholz. For now, everything hangs on the bomb curve. “We need to hurry, hurry, hurry,” says Frisén.
This article appeared in print under the headline “Explosive revelations”
Article amended on 1 January 1970
When this article was first published, it misstated the daughter nuclide of carbon-14.