Surprising Levels of Pollution Found in the Depths of the Mariana Trench

The Mariana Trench is one of the deepest places on the Earth, extending up to seven miles below the sea surface at points. But, according to a new study published in the Journal Nature Ecology & Evolution, even this remote locale doesn’t leave its creatures immune from pollution.

A team of researchers recently sent a remotely operated vehicle into the depths of the trench, reports Christopher Joyce for NPR. Equipped with a camera, water samplers and baited traps, they were planning to poke around and see what was there, Joyce reports.

They didn’t go in search of pollution, but pollution they found.

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.

“You think we're at the Mount Everest of the ocean, the very deepest point, and the levels were coming out orders of magnitude higher than places you would expect for it to be really high,” Alan Jamieson, who led the ROV team, tells Kendra Pierre-Louis at Popular Science.

The pollution levels weren’t the only alarming aspect of the discovery. The types of compounds they found were all considered persistent organic pollutants, meaning they stick around in the environment for a very long time. Two of the most prevalent types, according to the study, are PCBs and PBDEs.

Polychlorinated biphenyls (PCBs) were once used in many industrial applications in coolants and plasticizers, but were banned in the United States in the 1970s, and by a worldwide UN treaty in 2001 due to their toxic effects. Similarly, polybrominated diphenyl ethers (PBDEs) were used in a wide range of products—from electronics to couch cushions—to slow the ignition and spread of fires, but have slowly been phased out around the world over the last several decades.

Their presence in the Mariana Trench is evidence of one of the many concerning traits of POPs: They can travel great distances. These compounds generally don’t dissolve well in water, but favor sticking to the surface of materials like plastic—tiny particles of which rain down onto the deepest parts of the ocean, Carrington writes. Many creatures mistakenly eat this colorful but toxic confetti, causing the POPs to build up in the critter’s body, lurking in their fat tissues.

Whales are a common example of such toxicity from accumulation. These massive creatures become heavily polluted with PCBs because the ocean creatures they eat—fish, shrimp, plankton—contain some level of PCBs and PBDEs. And, as Pierre-Louis reports, when whales and other sea creatures die, their POP-riddled bodies sink to the ocean floor—where deep sea crustaceans scavenge their remains.

Jamieson and his team are now working to understand the effects that these pollutants and toxins have on deep sea marine life. But the discovery of these pollutants is just one more example that our actions can have wide ranging—and very deep—effects.