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At the rim of a jungle pond near the

Sudanese border of the Congo, a dozen silent soldiers—personal guerrilla warriors of the war criminal Joseph Kony—hide in the shadows with AK-47s. When an unaware forest elephant slips through the foliage for a drink, they shoot it in an instant. They will strip the twin tusks and trade them to middlemen for weapons and supplies. The middlemen will then covertly move the ivory through container ships at one of several East African ports and onto the black market in the United States, China, or one of several Southeast Asian nations.

Those seeking to protect these elephants and slow the illegal ivory trade face a formidable task: distinguishing the black-market ivory from the lawfully traded stuff. But now, researchers are working to stem the flow of illegal ivory by using, of all things, nuclear fallout.

Most international laws ban the trade of ivory procured in the last few decades, so distinguishing the outlawed from the legal ivory is often a case of figuring when the elephant died. But until now, there's been no test for establishing this. Columbia University geologist Kevin Uno and a team of researchers published a paper today in Proceedings of the National Academy of Sciences outlining how the residual radiation of Cold War nuclear weapons testing— in the form of the radioactive isotope carbon-14—can put a time stamp on ivory. "These tusks are tracking the atmospheric concentrations of carbon-14," Uno says. "In the ivory trade, the age of the ivory really counts."

You Are What You Eat

In the late 1950s and early 1960s the Soviet Union and the United States detonated hundreds of atomic weapons aboveground, from the freezing winterscapes of Siberia, the Nevada deserts, and the island paradise of the Bikini Atolls. "It was mostly the USSR and USA flexing their muscles," Uno says, "but we injected a lot of carbon-14 into the atmosphere through aboveground nuclear testing."

Here's how the warheads left their mark: Each nuclear blast released streams of neutrons into the sky, transforming some of the sky's nitrogen into radioactive carbon. Although carbon-14 occurs naturally in the air, by 1963 nuclear testing—the peak of these blasts—had effectively doubled the natural amount. Then, in October 1963, the two powers agreed to stop open-air tests.

Carbon-14 takes a few thousand years to decay back into nitrogen. (That's why scientists use the isotope for radiocarbon dating of ancient objects—they know the rate ofdecay.) However, the atmospheric levels of carbon-14 have been diminishing at a faster pace since the open-air ban, and the reason is that it doesn't stay suspended above the earth. Most of the element winds up in the oceans, and it's also inhaled by plant life across the globe. "And whatever comes along and eats a leaf, or a blade of grass, is now eating that carbon-14 and using it to build their tissues," Uno says.

Because elephants eat plants, he says, they grow tusks with radioactive levels that mirror the levels of the atmosphere. And by charting the tusks' carbon-14 levels against the atmospheric records, he can date pre-2000 ivory accurately to within a year and younger ivory to within a range of two years.

"People have tried to use methods like this before, but it actually is quite a complicated problem," says Greg Hodgins, a radioactive-dating expert at the University of Arizona who was not involved in the study. "They've demonstrated that there is a lot of temporal information locked in these tissues." Uno's team gets a far more accurate reading using an accelerator mass spectrometer, a device that speeds and separates the carbon-14 atoms from the regular carbon atoms in a sample so both can be directly counted.

"We're certainly not the first to think of this," Uno says. But he and his team are the first to do it accurately, cheaply, and without sacrificing a large sample. "We use less than the weight of a pinch of salt," he says, "and right now it costs $500 a sample."

Uno's team also showed that the carbon-14 count in an elephant's tusk can reveal more than just a single date. "Elephant tusks are continually growing," Hodgins says, "and they have these lines, sort of the equivalent of growth rings in trees or shells." Even over the period of a few years, the carbon-14 levels in the atmosphere change enough to give different measurements at separate points on the elephant's tusks. "So you can sample different spots along the tusks and get the rate of growth," Hodgins says. This helps researchers get a better date-of-death estimate, and Uno says that even with carved ivory figurines (a common end product of the poached ivory), you can determine the growth pattern of the ivory's grain.

Time Slipping Away

Over the last five years, the amount of ivory illegally exported from Africa has more than doubled, chiefly to China (which accounts for 70 percent of black-market sales) but also to Southeast Asian countries such as Thailand and Vietnam. For a trade already steeped in violence, the rising demand for ivory is luring even more sinister characters, such as Kony's Lord's Resistance Army, toward elephant poaching.

"There was already a perception that when you get ivory it's worth a lot of money, but it was never a deliberate strategy," says Kasper Agger, a field researcher who tracks the activities of the Lord's Resistance Army for the nonprofit Enough Project, "but it's become part of a survival tactic to resource their operations."

"It's easy game, and it's easy to sell. It has a high value, and you don't have a lot of risks taking down elephants," Agger says. Agger sees the ivory trade as tied to the political instability of several central African regions. Uno agrees: "It's a ripple effect that's basically turning ivory into guns. The ivory trade is funding guerrilla armies and organized crime." Uno hopes that his new test—by helping overseas authorities identify black market ivory—will help to stem the flow of funds to Central African warlords. "We want to help the antipoaching communities battle this problem," Uno says.

But the clock is ticking. With each passing year, Uno's carbon-14 test loses its efficacy. "The earth's atmosphere and environment is turning back to natural level of carbon-14," Hodgins says. And as time goes by, "there's less and less difference [between] the natural levels and how much there is in the atmosphere today." Old ivory samples will still get the same readings, but in 15 years from now, researchers will no longer be able to date newly poached ivory this way.

Uno readily admits that while his test might help, the problem is far bigger than just identifying black-market ivory. "We need to stop demand for ivory, and that will stop the poaching of elephants," he says. "This is not something that should only fall on the shoulders of the 15 or so African nations. It's an international problem."

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