Industrial pollution has reached even the most remote corners of Earth: the deepest part of the sea.
Scientists have discovered "extraordinary levels" of polychlorinated biphenyls, or PCBs, in the Mariana and Kermadec trenches, two of the deepest ocean chasms on the planet.
"Trenches have been considered as pristine environments, but also (given their locations and topography) as likely sinks for contaminants that enter the marine environment," Alan Jamieson, a marine ecologist at Newcastle University in England, wrote this week in the journal Nature Ecology & Evolution.
Banned in the 1970s, PCBs were once widely used as dielectric and coolant fluids in transformers, capacitors and other electrical devices. About 1.3 million tons of the chemical was produced, and an estimated 35% of that is now residing in coastal sediments, according to the study.
Known as persistent organic pollutants, these contaminants do not degrade naturally and can stick around in the environment for decades. The chemicals bond with bits of plastic and other kinds of organic debris, and are transported in air, soil and coastal waters. They've even been found in the Arctic, far from industrialized areas.
While exploring life in the ocean's hadal zone, a region 3.7 to 6.8 miles deep, Jamieson and colleagues measured contaminants in tiny shrimp-like scavengers called amphipods.
These crustaceans feed at the bottom of ocean trenches, ingesting any contaminants found in the environment. The pollutants are then stored in their in fat tissue. The contaminants spread through the food chain when the amphipods are eaten by bigger animals, such as fish.
Using deep-sea landing vehicles equipped with baited traps, the scientists collected samples of three species of amphipod from the Mariana Trench, in the western Pacific, and the Kermadec Trench, off New Zealand. Each of the 12 animals they collected was contaminated.
"Regardless of depth, regardless of trench, regardless of species," Jamieson said, pollutants "were present in all of them."
The study found elevated concentrations of contaminants, including flame retardants, in the upper portions of the trenches, about 4.5 miles deep. But amphipods with the most pollutants came from the Mariana Trench, possibly because the chasm is located closer to industrialized regions and beneath the Great Pacific Garbage Patch. Plastic debris and dead animals contaminated with industrial chemicals sink into the ocean, where they're eaten by the amphipods.
In parts of the world considered relatively pollution-free, the concentration of PCBs is less than one nanogram per gram of material sampled. (There are 1 billion nanograms in a gram.) In polluted areas, that level can rise to several hundred nanograms per gram.
PCB levels in the crustaceans from the Mariana Trench were up to 50 times greater than in crabs from the Liaohe River, one of the most polluted waterways in China, the study found.
While not as common as PCB pollution, the level of flame retardants in the deep ocean was comparable to or higher than levels in coastal waters off New Zealand.
The "disturbing" results show the deep ocean is much more connected to the surface than most people think, Dafforn wrote.
Jamieson added that the deepest point in the ocean, Challenger Deep in the Mariana Trench, is only about seven miles deep — about half the length of Manhattan. That's far from remote, he said.
In future studies, Jamieson said he hopes to determine how these pollutants affect the physiology of deep-sea creatures and their surrounding ecosystem.