The scientists found high levels of flame retardants in the bodies of similar crustaceans living in the Kermadec Trench, the world's fifth-deepest and more than 4,000 miles from the Mariana.
The small crustaceans that the submarine brought to the surface were inundated with toxic chemicals, writes Damian Carrington at The Guardian, boasting toxin levels 50 times greater than crustaceans that live within China's most heavily polluted rivers.
These industrial chemicals were banned in the late 1970s and do not break down in the environment, therefore, they are known as persistent organic pollutants (POPs). These chemicals have previously been found at high levels in Inuit people in the Canadian Arctic and in killer whales and dolphins in western Europe. They also have a tendency to "bioaccumulate", meaning they can build up in marine organisms over time. A Spam tin was also discovered nearly 5,000m below the surface, with plastic bags adding to the undersea litter.
The pollutants, according to Jamieson, likely sank to the bottom of the ocean through contaminated plastic garbage as well as dead animals that drifted to the bottom and were eaten by the amphipods.
At over six miles deep, the fissures are some of the most remote places on Earth.
Amphipod sea creatures had contamination similar to those in Suruga Bay, Japan that Dr Jamieson described as "one of the most polluted industrial zones of the north-west Pacific".
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Along with the almost 7-mile deep Mariana trench in the North Pacific, Jamieson's team deployed landers to Kermadec, a slightly shallower trench in the South Pacific. The pollution was ubiquitous, found "in all samples across all species at all depths in both trenches", the scientists said. Both chemicals, associated with various health risks, were commonly used for much of the 20th century until they were eventually outlawed. But researchers estimate 1.3 million tonnes of PCBs were produced worldwide.
Each lander was equipped with special traps created to catch tiny shrimp-like crustaceans called amphipods known to inhabit some of the ocean's deepest and most extreme environments. They included a tin of Spam, a can of Budweiser beer and several plastic bags.
Two commensal amphipods on a sponge stem in the Mariana Trench. A commensal ophiuroid is seen at the top of the image.
Though the locations are 10 kilometres (six miles) deep and far from land, researchers from Newcastle University in the United Kingdom identified traces of prohibited chemicals once used as fire retardants in the tiny amphipods.
"A lot of chemicals will have far-reaching impacts that we don't necessarily know about", Dafforn says. 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".
Humans may think that anything dumped in the ocean "magically disappears", Jamieson said.