Researchers recently discovered anthropogenic pollutants in one of the deepest and most remote places on Earth, the Atacama Trench, which plunges 8,000 meters (26,246 feet) deep into the Pacific Ocean.
The presence of PCBs in such a remote location confirms an important fact: there is no place on Earth that is free from pollution.
PCBs were produced in large quantities from the 1930s to the 1970s, mainly in the Northern Hemisphere, and were used in electrical equipment, paints, coolants, and many other products.
In the 1960s, it became clear that they were harmful to marine life, leading 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 with ocean currents, winds and rivers.
Our study took place in the Atacama Trench, which runs along the coast of South America for almost 6,000 kilometers (3,728 miles).
The researchers collected sediment at five locations in the trench at varying depths from 2,500 to 8,085 meters.
Each sample was divided into five layers, ranging from surface sediments to deeper clayey layers, and PCBs were found in each.
And in this part of the world, ocean currents bring cold, nutrient-rich water to the surface, which means plankton, tiny organisms, are abundant at the bottom of the ocean food web.
When plankton dies, their cells sink to the bottom, taking pollutants such as PCBs with them. But PCBs do not dissolve well in water and instead prefer to bind to lipid-rich tissues and other parts of living or dead organisms such as plankton.
Because bottom sediments contain so much dead plant and animal remains, they serve as an important sink for pollutants such as PCBs. About 60% of PCBs released in the 20th century are stored in deep ocean sediments.
A deep trench like the Atacama acts like a funnel, picking up bits of dead plants and animals (which scientists call “organic carbon”) that fall into the water.
The trench is full of life, so the microbes are working to decompose the organic carbon in the silt of the seabed.
Organic carbon in the deepest places of the Atacama Basin was found to be more decomposed than in shallower places. At deeper depths, there were also higher concentrations of PCBs per gram of organic carbon in sediments.
The organic carbon in the sludge decomposes more easily than the PCBs that remain and can accumulate in the trench.
And the storage of pollutants means that ocean sediments can be used as a rear-view mirror to the past. We can determine when a layer of sediment has accumulated on the seafloor, and by analyzing pollutants in different layers, we can get information about their concentrations over time.
In the Atacama Trench, PCB concentrations were higher in surface sediments, in contrast to what we normally find in lakes and seas.
As a rule, the highest concentrations are found in the lower layers of sediments deposited in the 1970s-1990s, after which concentrations decrease towards the surface, reflecting the ban and reduction of PCB releases.
And right now, we still don’t understand why the Atacama was so different. It is possible that we did not examine the sediment closely enough to detect small differences in PCB levels, or that the concentrations in this deep trench have not yet peaked.
These concentrations are still very low, hundreds of times lower than in areas close to sources of anthropogenic pollution, such as the Baltic Sea. But the fact that we haven’t found any pollution is a testament to the magnitude of human impact on the environment.
What is certain is that over 350,000 chemicals currently in use around the world are polluting the environment and ourselves. Pollutants have been found buried under one of the world’s deepest ocean trenches – and they’re not going anywhere.
The report was prepared by Anna Sobek, Professor of Environmental Chemistry and Head of the Department of Environmental Sciences at Stockholm University.
Source: Science Alert
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