The Cold War may be over but its fallout still remains. Thanks to nuclear explosions from these fearful decades, scientists can now work out if elephant tusks were illegally traded, or glean historical droughts from hippo teeth.
In the 1950s and 1960s, America and Russia repeatedly tested their new nuclear arsenals. The fallout from the explosions spread far and wide, blanketing the world in radioactive isotopes, such as carbon-14. This heavy form of carbon is normally a bit-player in the atmosphere, but levels rose so sharply during the Cold War that they created a noticeable spike—the bomb curve.
Some of the carbon-14 was converted into carbon dioxide and taken in by plants. Animals ate the plants and incorporated the carbon-14 into their hair, bones, teeth, and more. In other words, every animal that was around since the Cold War carries traces of its legacy within its body. This radiation is too weak to be harmful but strong enough to act as a timestamp. By measuring the carbon-14 in an animal’s remains and calibrating them against the bomb curve, we can work out when it lived and died.
Scientists have used bomb-curve dating to work out how often the human body makes new tissues, like neurons or heart muscle. Now, Kevin Uno from the University of Utah thinks the same technique can be used to foil poachers and study our wildlife.
Uno’s team analysed 29 samples of hair, horn, teeth, tusks and stems from different animals and plants, whose ages were all known. They used, for example, a discarded oryx horn collected in the field in 1972. They also used tusks from two elephants—Amina, an old female who died in Kenya in 2006, and Misha, who died in Salt Lake City’s Hogle Zoo in 2008.
By measuring the carbon-14 in these samples, and matching them against the bomb curve, Uno managed to estimate their age to within a year or so. Look at graph below, which plots the estimated ages against the actual ones—it’s a nigh-perfect match. The one exception is a blue monkey supposedly collected in the Congo in 1962. That can’t possibly be true. The carbon-14 in its hair is so low that the monkey must have died many years before it found its way into a museum and was tagged.
The technique should work for any date after 1955, when carbon-14 in the atmosphere really started to climb, and around 2030, when levels will fall back to pre-nuke levels. “I am still very amazed at how well it works,” says Uno. “We validated the method using a wide variety of tissues spanning the entire bomb-curve.” The success of the method means that researchers don’t have to be fussy. You can analyse virtually any bit of an elephant’s tusk and get the same age.
Uno think that bomb-curve dating has huge potential in conservation. Consider elephants. There are only around 400,000 African elephants and 50,000 Asian one left in the wild. Poaching is the main threat to their survival, and tens of thousands are killed every year to fuel the illegal trade in ivory. More than 46 tons of the stuff was seized in 2011, with even more expected in 2012. (Read National Geographic’s incredible investigation for more.)
This shouldn’t be happening. The Convention of International Trade of Endangered Species (CITES) banned the trade of ivory form Asian elephants in 1976, and from African elephants in 1989. But loopholes exist. For example, ivory can still be legally traded if it was imported into the US before the ban, allowing sellers to exchange freshly poached ivory by claiming that it’s old. Uno’s technique can help to test these claims by accurately dating a seized piece of ivory. “To my knowledge, it’s the first method that tells us when an elephant died,” he says.
His work complements that of Sam Wasser at the University of Washington. He uses DNA to work out where pieces of ivory came from, and identify Africa’s major poaching hotspots. “It would greatly help to know how recent ivory originated from an area as this helps us determine how recent a hotspot has been active,” he says. And if a seized shipment of ivory includes specimens of varying age, it might mean that people are smuggling ivory from government stockpiles back into the market. “We see great potential in combining our “when” and “where” methods to help address the poaching crisis,” says Uno.
“Ivory aging tools would be most useful for policing large ivory markets such as those in Chinatown in San Francisco, LA and Honolulu,” says Wasser. These sites are selling massive volumes of allegedly legal ivory. “We could randomly test these samples and close these shops down if they are breaking the law.”
The bomb-curve technique has other applications too. By measuring carbon-14 at different parts of hippo teeth and elephant tusks, Uno could work out how fast these tissues grew. And that could unlock a huge treasure trove of information about these animals, and their extinct relatives.
As a graduate student, Uno studied the chemical composition of fossil teeth. The levels of carbon revealed information about their owners’ diet, while the levels of oxygen show how much water there was in their environment. Uno could use these measurements to reconstruct how an animal’s menu and surroundings changed over time.
But over how much time? You can’t tell unless you know how quickly the teeth grow. All you have is a graph without an x-axis—a trend that floats freely in time. With the bomb-curve measurements, that all changes. By combining measurements of carbon-14 and other isotopes, Uno used teeth from two hippos, which died 11 years apart, to construct an 18-year timeline of environmental change in Kenya’s Tsavo National Park. He could even discern the presence of a severe drought in 1995, which forced the hippos to change their diet for years.
“Field observations of animals is expensive and time-consuming,” he says. “Hippos are active mostly at night, making observation impossible, and the habitat of forest elephants makes them hard to see any time of day. So these stable isotope records an efficient, cost effective alternative to that type of monitoring. Isotopes don’t lie.”
Reference: Uno, Quade, Fisher, Wittemyer, Douglas-Hamilton, Andanjeh, Omondi, Litoroh & Cerling. 2013. Bomb-curve radiocarbon measurement of recent biologic tissues and applications to wildlife forensics and stable isotope (paleo)ecology. PNAS http://dx.doi.org/10.1073/pnas.1302226110