It is 80 years since the first nuclear weapon test – codenamed Trinity – detonated above the desert in New Mexico. Today the hidden legacy of nuclear bomb tests can still be found in our cells – and is proving surprisingly useful to scientists.
It’s in your teeth. Your eyes and your brain too. Scientists call it the “bomb spike” (or “bomb pulse”) – and for more than half a century its signature has been present inside the human body.
On 16 July 1945, scientists of the Manhattan Project detonated the first nuclear weapon, known as the Trinity test, in New Mexico. The 18.6kt explosion lit up the sky and sent a blast of searing heat across the desert as a fireball lofted high into the sky. In the days that followed, white flakes and dust rained down on areas downwind. A now de-classified report from the time warned that radioactive particles spread over an area of more than 2,700sq miles (6,993sq km). And this test was just the start of the atomic era.
In the 1950s, there were so many nuclear bomb explosions above ground that they transformed the chemical make-up of the atmosphere – altering the carbon composition of life on Earth ever since, along with oceans, sediments, stalactites and more. Unlike the direct radioactive fallout from the explosions, the bomb spike is not harmful. In fact, it’s proven surprisingly helpful for scientists in recent years. Some have even gone so far as to describe it as the “mushroom cloud’s silver lining”.
Why? Evidence of the pulse is so ubiquitous that it can, among many other insights, tell forensic scientists when a person was born (or died), provide discoveries about the age of neurons in our brains, reveal the origin of poached wildlife, determine red wine vintage and even unlock the true age of centuries-old sharks (see box: “The bomb spike’s multiple uses”). And now it may also help to define a new geological era. In July 2023, a group of earth scientists recommended that its presence in a Canadian lake – along with other human-made markers from the mid-20th Century – should represent the official start of the Anthropocene. So, what exactly is the bomb spike, and what can it reveal about us and the world?
Before the 1963 Nuclear Test Ban Treaty obligated signatory nations to test nuclear bombs underground, governments exploded hundreds of atomic weapons out in the open air. More than 500 of these blasts – mainly conducted by the US and Russia – spewed their contents into the atmosphere.
It’s well-established that these tests spread radioactive material far and wide, harming humans and wildlife and rendering whole regions uninhabitable. Perhaps lesser known outside the scientific laboratory is that the bombs also reacted with natural nitrogen to form new isotopes – particularly carbon-14.
By the 1960s, overground bomb testing had produced almost twice the amount of carbon-14 in the atmosphere compared with previous levels. First the isotope entered water, sediments and vegetation, and then it passed along the food chain to humans. It has even reached organisms in the deepest ocean trench.
“In essence, every carbon pool on Earth which was in exchange with atmospheric CO2 since the late 1950s has been labelled by bomb carbon-14,” writes Walter Kutschera of the University of Vienna, who published a review of the scientific applications of the spike in the journal Radiocarbon in 2022. Back in the mid-20th Century, scientists noted the carbon-14 spike when atmospheric testing stopped, but it took decades for them to realise that the elevated levels might be useful. From the 1950s onwards, they had been using carbon-14 to date paleolithic remains or ancient texts, but that was based on its radioactive decay – known as radiocarbon dating. The isotope is unstable: it decays slowly into nitrogen with a half-life of 5,730 years. So, when a Neanderthal died, for instance, the quantity of carbon-14 in their bones and teeth would have started to gradually decline. Measure the extent of the decline, and you have a Neanderthal date of death. (BBC)
Radiocarbon dating, however, tends to be limited to samples that are more than 300 years old, because of the isotope’s slow decay rate. Any younger, and it hasn’t decayed enough for an accurate date. Muddying recent dating further is humanity’s introduction of additional carbon dioxide into the atmosphere since the Industrial Revolution – the so-called Suess effect.
Around the turn of the century, however, researchers realised that the bomb spike could help them use carbon-14 in a different way – and crucially it allows for dating within the past 70-80 years.