The researchers, from the University of Aberdeen and the James Hutton Institute in the UK, focused on two specific types of chemical pollutants: polychlorinated biphenyls, or PCBs, and polybrominated diphenyl ethers, or PBDEs, both of which may cause a variety of adverse health effects, including neurological, immune and reproductive issues and even cancer (in humans). PCBs were once commonly used in electrical equipment before being banned in the United States over health and environmental concerns in the 1970s. The manufacture and import of PBDEs, which are typically used as flame retardants, has also been restricted in the U.S., although at least one common type of the chemical is still permitted.

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Despite the reductions in their use, both PCBs and and PBDEs can still be detected in marine organisms today. Both have the potential to remain intact for long periods of time, often binding to other particles in the water that can then carry them throughout the ocean. They also have a tendency to “bioaccumulate,” meaning they can build up in marine organisms over time. Just last year, a study conducted by researchers from the Scripps Institution of Oceanography suggested that certain organic pollutants, including PCBs and PBDEs, are widespread in fish throughout the world.

For the new study, the researchers checked for the presence of these chemicals in two of the world’s deepest ocean trenches — the Mariana trench in the Western Pacific, near the Mariana islands, and the Kermadec trench north of New Zealand. To do so, the researchers deployed special devices called “deep-sea landers,” which are small vessels that are released from ships and drop to the bottom of the ocean before floating back up to the surface.

Each lander was equipped with special traps designed to catch tiny shrimp-like crustaceans called amphipods known to inhabit some of the ocean’s deepest and most extreme environments. Afterwards, the researchers tested the amphipods for the presence of PCBs and PBDEs.

They found that both PCBs and PBDEs were present in all species of amphipod in both trenches, and at all depths sampled — up to 10,000 meters deep in both locations. Concentrations tended to be somewhat higher in the Mariana trench, although in both locations they were generally higher than the baseline concentrations typically found in clean coastal areas. In fact, in the Mariana trench, the highest observed concentrations of PCBs were about 50 times greater than the levels that have been found in crabs living near China’s Liaohe River, one of that nation’s most polluted waterways.

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“The only Northwest Pacific location with values comparable to the Mariana Trench is Suruga Bay (Japan),” the researchers note in the paper, “a highly industrialized area with historically heavy usage of organochlorine chemicals.”

These are bombshell results, given that the deep sea is often thought of as one of the world’s last pristine places, mostly out of the reach of human influence. In a comment also published Monday, marine ecologist Katherine Dafforn of the University of New South Wales highlighted the importance of the findings, noting that the authors “have provided clear evidence that the deep ocean, rather than being remote, is highly connected to surface waters and has been exposed to significant concentrations of human-made pollutants.”

What remains unclear is exactly how the contaminants got into the trenches — and why their levels are so high in the Mariana. The authors suspect one of the more likely explanations is that the chemicals in the Mariana trench originated around the “great Pacific garbage patch,” a swirling mass of debris in the northern Pacific. Chemical pollutants in that region could easily cling to plastic waste as it drops through the water column toward the bottom of the ocean.

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More generally, the authors note that large-scale ocean currents can transport chemical-carrying particles over long distances. And previous research has suggested that even contaminants which start out on the surface of the ocean can sink to the deepest places, clinging to garbage or even the bodies of dead animals, within a few months.

All of this is to say that “our proximity to these extreme locations is far from remote, which is why even the deepest chasms of the ocean are no longer pristine,” the authors note in the paper.

Future research may shed more light on how these contaminants may move and magnify throughout the food chain, as well as whether they could be causing any measurable damage to the ecosystems they enter. But for now, the findings serve as a jarring reminder that human activities have consequences all over the planet — and there may nowhere that remains out of our reach.

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