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Shocking: Deepest parts of the ocean polluted with PCB chemicals

Poly-Chlorinated Biphenyls (PCBs) continue to persist in the environment, despite most countries banning them since the 1970s. They pose a significant threat to both human and marine life. 

In a groundbreaking study, deep-sea researchers have recently discovered traces of PCBs at the bottom of the Atacama Trench in the Pacific Ocean. This startling discovery highlights the far-reaching effects of human activities on even the most remote and isolated ecosystems.

The research team, consisting of scientists from the Department of Environmental Science at Stockholm University and the Danish Center for Hadal Research at the University of Southern Denmark, conducted an expedition to the Atacama Trench to study its sediment layers.

After retrieving sediment cores from five different locations within the trench, they found that all of the surface sediment samples analyzed contained PCBs.

The study, led by Professor Anna Sobek and Professor Ronnie N. Glud, has been published in the prestigious scientific journal Nature Communications

What are PCBs?

In the 1930s, chemists initially introduced PCBs primarily in building materials and technical components. These substances comprise 209 different components. Most countries have banned PCBs and classified them as a highly persistent environmental toxin. This is due to their potential to cause cancer and reproductive harm.

Although the world’s production of PCBs significantly declined in the 1970s, these substances still pose an ongoing environmental threat. In 2018, researchers found that half of the world’s killer whale populations were weakened by PCB exposure. Moreover, another study reported that deep-sea scavenging amphipods contained large amounts of PCBs.

Professor Glud has participated in more than ten expeditions to deep-sea trenches worldwide. He commented on the significance of their findings: “It is thought-provoking that we find traces of human activity at the bottom of a deep-sea trench; a place that most people probably perceive as distant and isolated from our society.”

What the researchers have learned

Surface-level human activities can affect deep-sea trenches, as these expeditions have shown by dispelling the misconception. They have also revealed a surprisingly rich, active, and diverse ecosystem at the ocean’s greatest depths. 

Moreover, the studies have demonstrated that deep-sea trenches accumulate large amounts of organic material. This contributes to the oceans’ capacity to absorb atmospheric carbon released through fossil fuel burning.

However, not only organic material accumulates in these hadal trenches. In 2021, the Danish Center for Hadal Research reported that mercury accumulates in the sediments of these trenches. In 2022, researchers announced that black carbon also accumulates in these deep-sea environments, similarly to PCBs. The combustion of fossil fuels primarily forms black carbon.

A sediment core containing PCBs from the Atacama trench. Credit SDU
A sediment core containing PCBs from the Atacama trench. Credit SDU

Not yet time to panic

Although the recent discovery of Poly-Chlorinated Biphenyls (PCBs) in the Atacama Trench has raised concerns, Professor Glud assures that the concentration of these pollutants in the trench’s samples is not alarmingly high. He points out that researchers have found much higher concentrations of PCBs in places like the Baltic Sea, North Sea, and Tokyo Bay. Concentrations there range from 300 to 1500 times higher than those found in the Atacama Trench.

“These are places with a lot of human activity, so one would expect that. The Atacama samples do not show very high concentrations, but considering that they were retrieved from the bottom of a deep-sea trench, they are relatively high. A priori no one would expect to find pollutants in such a place,” said Professor Glud.

PCBs are hydrophobic, meaning they are not very soluble in water. They tend to bind to organic material that sinks to the bottom. Professor Anna Sobek explains that the Atacama Trench is located in an area with relatively high production of plankton in surface waters. As the plankton dies, it sinks to the ocean floor. Large amounts of material descend down the steep slopes. They then deposit in the trench’s deepest areas.

Microorganisms eventually decompose the organic material that reaches the bottom of the Atacama Trench. This causes PCBs to accumulate in the sediment. Over time, PCBs slowly redeposit as persistent compounds. This is why researchers can find increasing concentrations of PCBs in remote areas like the hadal trenches, even though most countries largely banned them worldwide in the 1970s.

“Unlike coastal areas where PCB concentrations are typically higher in deeper sediment layers deposited 50 years ago, PCB concentrations in hadal sediments are highest in the upper sediment layers, indicating that PCBs have only recently reached the deeper trenches and that concentrations have not yet peaked: We may see higher concentrations in a few years,” said Professor Glud.

About the deep-sea trenches where PCBs were found

The deep-sea trenches are home to various microorganisms and animals that have adapted to the extreme living conditions. It is possible that these trenches also harbor organisms capable of metabolizing the pollutants deposited there. This is one of the focus points of the Danish Center for Hadal Research. Researchers there have a stock of frozen sediment samples collected from expeditions to different deep-sea trenches in 2021 and 2022.

Different layers of ocean depths where PCBs were found
Different layers of ocean depths where PCBs were found

“We are interested in finding out if PCBs are also present in other deep-sea trenches or if they are unique to the Atacama Trench. We also want to investigate the bacteria that live down there and learn more about their function,” said Professor Glud.

The deep-sea trenches are located in the hadal zone of the ocean, which lies at depths of 6-11 km. There are about 27 hadal trenches, named after the Greek god Hades, who ruled the underworld. As researchers continue to explore these remote and mysterious environments, they aim to better understand the impact of pollutants like PCBs on these ecosystems and the organisms that inhabit them.

The discovery of PCBs in the Atacama Trench serves as a stark reminder of the pervasive and long-lasting consequences of human activities on even the most remote ecosystems. This research underscores the importance of continued monitoring and mitigation efforts. With consistent effort, they hope to reduce the impact of environmental toxins on the planet’s most vulnerable habitats.

More about PCBs

Poly-Chlorinated Biphenyls (PCBs) are a group of man-made organic chemicals consisting of 209 different substances, also known as congeners. They were first produced in the 1930s and used in a variety of industrial and commercial applications. PCBs were popular due to their chemical stability, low flammability, and insulating properties. 

Common uses included electrical transformers and capacitors, coolants, flame retardants, plasticizers, adhesives, and as additives in paints, sealants, and caulking.

PCBs are harmful to both humans and the environment for several reasons:


PCBs are extremely stable and resistant to breaking down in the environment. As a result, they persist for long periods, allowing them to bioaccumulate in plants, animals, and humans. Even though many countries have banned PCB production since the 1970s, they still pose a threat to ecosystems and human health due to their persistence.


PCBs accumulate in the fatty tissues of living organisms, a process called bioaccumulation. As these chemicals move up the food chain, they become more concentrated, posing a greater risk to top predators and humans who consume contaminated food.


Researchers have shown that PCBs can cause a range of adverse health effects in both humans and animals. PCBs are carcinogenic (cancer-causing). They can also affect the immune, reproductive, nervous, and endocrine systems. Long-term exposure to PCBs may lead to developmental issues in children, memory and learning problems, and other neurological disorders.

Environmental impact

Due to their persistence and bioaccumulation, PCBs can cause significant harm to ecosystems, impacting the health of various plant and animal species. The contamination of water bodies, soil, and air can lead to the death of aquatic life, reduced reproductive success in birds and mammals, and disruption of entire ecosystems.

The discovery of PCBs’ harmful effects on human health and the environment led to a widespread ban on their production and use in many countries. However, the long-lasting nature of these chemicals means that they continue to pose a threat, and ongoing research and monitoring efforts are necessary to assess and mitigate their impacts.


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