The frigid deep sea is considered Earth’s final frontier.
We know little about life in the deepest parts of the ocean, but new evidence shows we’re already having an impact on it.
Recent tests on shrimp-like crustaceans that live more than six miles below the surface of the Pacific Ocean in the Mariana Trench show high levels of long-banned, cancer-causing pollutants in their bodies.
"We still think of the deep ocean as being this remote and pristine realm, safe from human impact, but our research shows that, sadly, this could not be further from the truth," said Alan Jamieson of Newcastle University, who co-authored a study this week in the journal Nature Ecology & Evolution.
The pollutants likely came from plastic waste and dead animals sinking to the ocean floor, researchers say.
“[These chemicals] don’t like water, and so they will stick to things in the water like plastic, and then that plastic will settle,” says co-author Stuart Piertney of the University of Aberdeen. “Because these deep-sea trenches are the very bottom of the sink for the oceans, there’s a sort of inevitability that they’re going to end up there.”
Jamieson and Piertney’s team used a specially built underwater lander to collect and test bottom-dwelling crustaceans called amphipods from the Pacific Ocean's Mariana Trench and Kermadec Trench.
The tests revealed high levels of pollutants, including polychlorinated biphenyls (PCBs), banned in the US almost 40 years ago for causing cancer and wreaking havoc with hormones.
These are some of the deepest, darkest places on Earth, less well-known to mankind than the surface of the moon. The high pressure there makes life impossible for all but the heartiest creatures.
The researchers used mackerel-baited traps to catch the shrimp-like carrion feeders, then analyzed them for traces of chemicals.
"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," said Jamieson.
The research was carried out in the ocean's hadal zone, between about 3 and 7 miles deep, and comprised of deep trenches in the sea floor caused by tectonic plate activity.
It is believed that some 1.3 million tons of PCBs — which can persist in the environment for decades — were produced from the 1930s to 1970s.
About 65 percent of the total is thought to be in landfills or still in electrical equipment today, and the other 35 percent in coastal sediment and the open ocean.
The scientists also found traces of another long-lived pollutant in the amphipods — polybrominated diphenyl ethers (PBDEs) used in flame retardants.
In the Mariana Trench, the highest PCB levels in samples were 50 times higher than in crabs from paddy fields fed by the Liaohe River, one of China's most polluted.
The team inferred that pollutants must be pervasive "across the world's oceans and to full ocean depth."
Piertney says scientists don’t know enough about the amphipods they tested to tell how the toxins are impacting them.
The same goes for the greater deep-sea ecosystem at the bottom of the Mariana Trench.
“We know absolutely very little about the deep sea, full stop,” Piertney said.