Last used in the 1980s, 'forever chemical' PCB has now been detected in deep ocean trenches

"The Atacama Trench is located in an area with relatively high plankton productivity in the surface water, which leads to the transport of organic material down to the sediment when the plankton dies. When plankton and other organic material sink to the bottom, they carry pollutants that tend to bind to lipids and organic carbon," Anna Sobek, professor of environmental chemistry at Stockholm University and lead author of the study, explained in a statement.

There is little data on pollutants in deep-sea trenches

Many pollutants have a long life cycle, meaning they take a while to degrade in the environment. PCBs are one of the worst, and, being so long-lasting, they can travel great distances and disseminate in remote areas from where they were employed and released. PCBs were extensively used between the 1930s and the 1970s before they were universally outlawed in the middle of that decade. PCBs are nevertheless still in the environment and circulation.

"In our study, we saw that the sediment at the deepest locations in the Atacama Trench had a lower proportion of easily degradable organic carbon. We also found higher concentrations of PCB per gram of organic carbon in [the] sediment deeper in the trench. This is [because] the sediment's organic carbon is degraded, but PCB, which is more long-lived, remains and can therefore accumulate," said Sobek.

According to Professor Ronnie N. Glud, head of the Danish Centre for Hadal Research at the University of Southern Denmark and one of the study's authors, the concentration of PCBs in samples from the Atacama Trench is not alarmingly high yet. However, he points out that areas like the Baltic Sea, North Sea, and Tokyo Bay have seen substantially higher densities.

“These [places have] 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. No one would expect to find pollutants in such a place," said Glud.

However, the researchers point out that there is little data on pollutants in deep-sea trenches. Two previous studies have shown surprisingly high concentrations of PCB and similar contaminants in animals living in the sediment.

"In future studies, we will also study the uptake in bottom-dwelling animals to understand how pollutants spread and can affect the food web in the deep sea trench. We will also study how the microbial community in the deep sea trench may contribute to the degradation of certain pollutants," explained Sobek.

Study Abstract:

"Burial of persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs) in deep-sea sediments contributes to 60% of their historical emissions. Yet, empirical data on their occurrence in the deep-ocean is scarce. Estimates of the deep-ocean POP sink are therefore uncertain. Hadal trenches, representing the deepest part of the ocean, are hotspots for organic carbon burial and decomposition. POPs favorably partition to organic carbon, making trenches likely significant sinks for contaminants. Here we show that PCBs occur in both hadal (7720–8085 m) and non-hadal (2560–4050 m) sediment in the Atacama Trench. PCB concentrations normalized to sediment dry weight were similar across sites while those normalized to sediment organic carbon increased exponentially as the inert organic carbon fraction of the sediment increased in degraded hadal sediments. We suggest that the unique deposition dynamics and elevated turnover of organic carbon in hadal trenches increase POP concentrations in the deepest places on Earth."