Plunging more than 10 km (6 mi) down, you might think the the oceans' deepest trenches would be a relatively safe haven from human-made pollutants that are causing problems closer to the surface – but you'd be wrong. UK researchers have found extremely high levels of chemical pollutants in deep-sea scavengers.

The team of scientists from Newcastle University, the University of Aberdeen and the James Hutton Institute went to the deepest areas of the ocean, specifically the Pacific's Marianna and Kermadec trenches, and used deep-sea landers to bring back amphipods – shrimp-like crustaceans that live at the bottom of the deep sea and feast on particles that drift down.

The fatty tissue of the sea creatures was examined and found to have high levels of persistent organic pollutants (POPs), including polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). In fact, PCB levels were 50 times greater than amounts found in crabs from paddy fields fed by the Liaohe River, one of China's most polluted rivers.

"The amphipods we sampled contained levels of contamination similar to that found in Suruga Bay, one of the most polluted industrial zones of the northwest Pacific," says lead author Alan Jamieson. "The fact that we found such extraordinary levels of these pollutants in one of the most remote and inaccessible habitats on Earth really brings home the long term, devastating impact that mankind is having on the planet."

PCBs were once used in a number of industrial processes, but were discovered to be a carcinogen and endocrine disruptor and banned in the U.S. in 1979. Roughly 1.3 tons of the chemical were produced over its lifetime, and are known to persist for decades. PBDEs are used as flame retardants and electrical insulators and are found in a wide range of products and materials. Structurally similar to PCBs, PBDEs are restricted under the Stockholm Convention treaty due to their toxicity and persistence in the environment.

The study suggests the chemicals filtered down from surface plastics and dead animals before being eaten by the amphipods, which would in turn become food for larger animals. Deep ocean trenches act as sinks for pollutants that accumulate through the food chain and increasing concentrations of such chemicals at the bottom of the oceans.

"This research shows that far from being remote, the deep ocean is highly connected to the surface waters," said Jamieson. "We're very good at taking an 'out of sight out of mind' approach when it comes to the deep ocean, but we can't afford to be complacent."

The researchers will next attempt to understand the effects of such contamination and the potential knock-on effects for the wider ecosystem.

The findings were published in Nature Ecology & Evolution.