A study reveals that a long-banned pollutant is found in the deepest place on earth

I was part of a team recently Discover man-made pollutants In one of the deepest and most remote places on earth – the Atacama Trench, which descends to a depth of 8,000 meters (26,246 feet) into the Pacific Ocean.

The presence of polychlorinated biphenyls (PCBs) in such a remote location confirms a crucial truth: no place on Earth is free from contamination.

PCBs were mass-produced from the 1930s to the 1970s, mostly in the Northern Hemisphere, and were used in electrical equipment, paints, refrigerants, and many other products.

In the sixties, it became clear that they were Damage to marine lifewhich led to an almost universal ban on their use in the mid-1970s.

However, because they take decades to degrade, PCBs can travel long distances and spread to places far from where they were first used, continuing to circulate through ocean currents, winds, and rivers.

our Stady It occurred in the Atacama Trench, which traces the coast of South America for nearly 6,000 km (3,728 mi). Its deepest point is almost as deep as the Himalayas.

We collected sediment from five sites in the trench at different depths ranging from 2,500 to 8,085 m.

We cut each sample into five layers, from the surface sediment to the deeper clay layers, and found PCBs in all of them.

Pollutants stick to dead plankton

In this part of the world, ocean currents bring cold, nutrient-rich water to the surface, which means there are plenty of plankton—the tiny organisms at the bottom of the ocean’s food web.

When plankton die, their cells sink to the bottom, carrying pollutants like PCBs with them. But PCBs do not dissolve well in water and prefer instead to bind to lipid-rich tissues and other pieces of living or dead organisms, such as plankton.

Because seafloor sediments contain so many remains of dead plants and animals, they serve as an important sink for pollutants such as PCBs. About 60 percent of all PCBs were released during the 20th century Stored in the sediments of the deep ocean.

A deep trench like the Atacama acts like a funnel, collecting bits of dead plants and animals (what scientists refer to as “organic carbon”) that fall through the water.

There is plenty of life in the trench, and then microbes decompose the organic carbon in the seafloor mud.

We found that organic carbon in the deepest locations in the Atacama Trench was more degraded than in shallower locations. And at much deeper depths, there were also higher concentrations of PCBs per gram of organic carbon in the sediments.

The organic carbon in the sludge decomposes more easily than the PCBs, which remain and can build up in the ditch.

A look into the past

Storing pollutants means ocean sediments can be used as a rear-view mirror into the past. It is possible to determine when the sediment layer accumulated on the sea floor, and by analyzing pollutants in different layers we can obtain information about their concentrations over time.

The sediment archive in the Atacama Trench surprised us. PCB concentrations were higher in surface sediments, which is in contrast to what we usually find in lakes and seas.

Typically, the highest concentrations are found in the lower layers of sediment deposited in the 1970s through the 1990s, followed by a decrease in concentrations towards the surface, reflecting the ban and reduction of PCB emissions.

At the moment, we still don’t understand why the Atacama is so different. It is possible that we did not look at the sediment closely enough to detect small differences in PCBs, or that concentrations had not yet peaked in this deep trench.

These concentrations are still very low, hundreds of times lower than in areas near sources of human contamination such as Baltic Sea. But the fact that we have found no pollution of any kind is indicative of the scale of humankind’s impact on the environment.

What we can say for sure is that more than 350,000 chemicals It is currently being used globally at the cost of polluting the environment and ourselves. Pollutants have now been found buried beneath the bottom of one of the world’s deepest ocean trenches – and they’re not going anywhere.

Anna SobekProfessor of Environmental Chemistry and Chair of the Department of Environmental Sciences, Stockholm University

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