The Mariana Trench, in the western Pacific Ocean between Japan and Papua New Guinea, plunges nearly seven miles below the surface at its deepest point. It is one of the most inaccessible environments on Earth, but it has not escaped the impact of humanity’s violence.
A group of scientists have now found radioactive carbon-14—at levels high enough to indicate it originated from the detonation of nuclear bombs—in the flesh of shrimplike crustaceans living in the trench. “Typically, we say the trenches are far away from us; they’re very deep and they’re pristine. But actually, they’re not,” says earth scientist Jiasong Fang of Shanghai Ocean University, who worked on the new study. “Everything can get into the trenches.”
Scientists can trace the reach of aboveground nuclear detonations, the first of which was conducted in 1945, by measuring levels of carbon-14—a radioactive isotope of carbon produced when neutrons from nuclear reactions collide with nitrogen atoms in the atmosphere. (It is also produced naturally, at lower levels, by cosmic rays bombarding the atmosphere.) Atmospheric carbon-14 levels doubled in the 1950s and 1960s as scores of hydrogen bombs were tested. A very small amount of the “bomb carbon” from these gigantic explosions has decayed, but the rest has spread around the world and been taken up via carbon dioxide by plants, which are then eaten by animals—including humans.
Until now, it has been unclear whether bomb carbon has managed to spread into the farthest crevices of the world, especially the deepest seas. It would have taken natural oceanic circulation about 1,000 years to carry it to the depths of the Mariana Trench. And in fact, testing for the new study showed that the waters of the trench did have low levels of carbon-14—which is what the researchers expected, given the long travel time from atmosphere to deep ocean.
But when they used traps to catch and test crustaceans living at these depths, they detected much higher levels of the isotope in these animals’ tissues and gut contents than in the surrounding waters. The bomb carbon had to be arriving another way that brought it there faster, and the researchers surmised it was taking a shortcut via the food chain. Organic matter—including the poop and carcasses of surface-dwelling life—falls through the water column in just weeks or months. When crustaceans on the seabed munch these morsels, they absorb the signature of nuclear tests into their bodies, the researchers say in their study, published online in April 2019 in Geophysical Research Letters [pdf].
Other studies conducted around the world have also recently identified the residue of the weapons tests of the mid-20th century—as well as the Chernobyl and Fukushima nuclear disasters—in mountain glaciers, another landscape often considered pristine and remote. Together with the results from the Mariana Trench, these findings “prove that atmospheric and oceanic circulation distribute bomb-derived radioactivity globally, even to the most remote sites,” says Edyta Lokas of the Institute of Nuclear Physics PAS in Krakow, Poland, who worked on the glacier research, which was presented at a meeting of the European Geoscience Union (EGU) in April. Worse still, the fallout locked in glaciers includes more worrisome radioactive elements (such as americium-241, a product of the decay of plutonium)—and could be released as the world warms and the ice thaws. “The legacy of radioactive contamination will be felt by many generations ahead,” Lokas says.
Such clear, long-lasting imprints of nuclear testing are among the markers proposed by scientists who suggest humans have changed the planet so much that we are now living in a distinct geological epoch, often called the “Anthropocene.” Proponents of this idea say the fact that remnants of these tests are reaching glaciers and deep oceans indicates humans have begun fundamentally altering the geology of the entire planet. “It just shows that the human signal is getting even to places that were thought to be distant or remote from human influence,” says Jan Zalasiewicz, a paleobiologist at the University of Leicester in the U.K. who researches the Anthropocene.
But it is not just relatively rare impacts, like nuclear tests, that have reached these remote environments; more mundane human contamination can be found there too. Researchers recently announced they had found microplastics—degraded pieces of larger plastics, as well as microbeads and synthetic fibers—in every crustacean they tested from the Mariana Trench. “It was disheartening but not unexpected,” says William Reid, an ecologist at Newcastle University who worked on the study, published in Royal Society Open Science in February. “It is probably the saddest piece of research I have ever been involved with.” Microplastics have also been detected in glaciers, according to results presented at the April EGU meeting.
In fact, humanity’s actions are so far-reaching that a 2018 study in Current Biology suggested just 13 percent of Earth’s oceans can still be classed as wilderness. “I very much doubt,” Reid says, “there are many places left on the surface or seafloor of this planet that we have not impacted.”