During the 1950s and '60s, the United States and the Soviet Union tested and showed off their shiny new atomic arsenals by detonating hundreds of nuclear weapons at above-ground sites. After each explosion, residual radioactive material, or fallout, was dispersed into the atmosphere and then spread around the world by the wind.
Among these radioactive leftovers is an isotope, or variant, of the element carbon known as carbon-14. This same isotope is generated naturally by cosmic rays and normally occurs in small traces, accounting for just one part per trillion of atmospheric carbon. During the Cold War, though, scientists keeping tabs on the isotope’s concentration found a spike—a near doubling—in carbon-14 levels that coincided with the start of the weapons tests, and a slow, steady decline when the tests were moved underground. It was dubbed the “bomb curve.”
Most carbon-14, whether it’s natural or man-made, American or Soviet, oxidizes into carbon dioxide, and then gets taken in by the oceans and by plants. As animals eat these plants and other animals eat those animals, almost every living thing gets a share of carbon-14 incorporated into its teeth or tusks or hair or horns.
Anyone or anything that was alive during the Cold War got to keep a small souvenir from it inside its body—not enough to do any damage, but enough to date it. If the carbon-14 concentration in some animal or plant tissue is the same as the known level in the atmosphere at a certain date along the bomb curve, that gives you an idea of how old the tissue and the creature it came from is.
In a study led by doctoral student Kevin Uno, a team of researchers from the University of Utah chased down more than two dozen animal tissue samples that had been collected between 1955 and 2008. Previous studies on bomb curve carbon dating had mostly only looked at tree rings and enamel from human teeth, but Uno and company gathered everything from hair from a blue monkey to teeth from hippos and tusks from elephants to stems from various plants. They measured the carbon-14 levels in these samples and then plotted them along the bomb curve to estimate when the sample was collected (which is usually right around when the animal died). For some of the samples, including tusks from elephants that had died in a zoo and in a national park, they knew the animals’ actual ages, and found their estimations were accurate within a year.
The Nuclear Response
That the technique worked so well in a variety of tissues might make it a useful forensic tool to battle poachers.
Every year, an estimated 30,000 African elephants are killed illegally for their ivory tusks. With only some 400,000 animals left in the wild, this kind of slaughter could make the species extinct in just a little over a decade. Poaching and the illegal ivory trade are big business, and those trying to stop it are up against organized and well-armed criminal organizations, corrupt government officials and a quirk in the law.
International treaties have banned the trade of Asian elephant ivory since 1976, and African elephant ivory since 1989, but the laws allow for some loopholes. In some countries, including the United States, any ivory acquired before ’89 is legal to buy and sell. Trying to distinguish legal, pre-ban ivory from poached, post-ban ivory has been incredibly difficult, and ivory traders can move ill-gotten product by claiming that it’s older than it really is. Carbon dating an ivory sample against the bomb curve, though, can date it and reveal how old and how legal it is. It’s science calling BS on poachers and their marketplace enablers.
Uno’s work complements research done at the University of Washington, which uses DNA and isotope analysis to locate the origin point of ivory. Working out the “when and where” of confiscated ivory (and other animal parts, like rhino horns) can help shut down individual dealers but also identify poaching hot zones and guide decisions about where to spend conservation funds or send armed rangers to protect animals, and it’s all thanks to the atomic crumbs left over from the Cold War.