Persistent pollutants found in one of Earth’s deepest ocean trenches

Samples from the bottom of the Atacama trench were taken with this "multiple core sampler". Photo: Anni Glud/SDU.

In a recent study, published in Nature Communications, researchers from Stockholm University and several other institutions analyzed sediment from different depths in the Atacama Trench, a deep sea trench that extends approximately 4,200 kilometers along the coasts of Peru and Chile, and reaches a maximum depth of just over 8,000 meters. The researchers found PCB in all of the samples they analyzed, including those taken at depths of up to 8,085 meters.

"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," said Anna Sobek, professor of environmental chemistry at Stockholm University and lead author of the study.

Many pollutants are long-lived, meaning that they take a long time to break down in the environment. This also means that they can be transported over long distances and spread to places far from where they were used and released. PCB is one such pollutant. PCBs were used in large quantities from the 1930s to the 1970s and were globally banned in the mid-1970s. Nevertheless, PCBs are still present and circulating in the environment.

PCBs can accumulate in the deep sea

The environment in a deep sea trench is unique. The trench acts as a sink for dead organic material, both from the water column and from landslides on the seafloor. As a result, a lot of organic carbon accumulates in the deep sea trench, consisting of dead plankton, among other things, and there is an active microbial community that breaks down the organic material.
 

Anna Sobek.
Photo: Rickard Kihlström

"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 that there were higher concentrations of PCB per gram of organic carbon in sediment deeper in the trench. This is due to the fact that the sediment's organic carbon is degraded, but PCB, which is more long-lived, remains and can therefore accumulate," said Anna Sobek.

The concentration of PCBs in samples from the Atacama Trench is not alarmingly high, according to Professor Ronnie N. Glud, director of the Danish Center for Hadal Research at the University of Southern Denmark and one of the authors behind the study He points out that much higher concentrations have been found in places like the Baltic Sea, North Sea, and Tokyo Bay.

“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. No one would expect to find pollutants in such a place”, said Ronnie N. Glud.

Insufficient data from ocean trenches

There is currently not much data on pollutants in deep sea trenches. Two previous studies have shown surprisingly high concentrations of PCB and similar pollutants in animals living in the sediment.

"In future studies, we will also study the uptake in bottom-dwelling animals to try 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," said Anna Sobek.

As part of this research project, researcher Sebastian Abel from Stockholm University will soon be traveling to Japan. There, he will embark a ship to place passive samplers in the depths of the Japan Trench. With these samplers, the researchers will measure environmental pollutants in the water of the trench at depths of 5,000-9,000 meters, something that has never been done before.

The study was conducted by researchers from Stockholm University, University of Gothenburg, University of Southern Denmark, Aarhus University, Japan Agency for Marine-Earth Science and Technology, and Tokyo University of Marine Science and Technology.

Read article in Nature Communications.