The very first analysis of mercury in sediments from the deepest parts of the Pacific Ocean showed that they are hotspots for the toxic element and contain up to 56 times more mercury than other deep-sea areas.

Since 2017, the Minamata Convention on Mercury has been working to reduce anthropogenic mercury emissions by phasing out mercury-containing products and to regulate the use of the metal in gold mining. But whether released through human activity or from natural sources, the often poisonous metal eventually finds its way into soils and marine sediments in the form of inorganic salts, organic compounds or even particle-bound and free elemental metal.

But how much mercury really gets into the sediments of the deep sea, which make up 91% of the sea floor, has so far only been estimated from analyzes of fine dust in open water. Researchers have now for the first time analyzed the mercury content of 12 sediment cores of the Pacific Ocean that were recovered from a depth of six to ten kilometers.

An image showing detailed bathymetric data with specific sampling points in the Kermadec Trench

The team found values ‚Äč‚Äčthat were 22 to 56 times higher than expected. In particular, the Atacama Trench, off the coast of Peru, contained most of the mercury ever reported from a remote location and more than some parts of the Mediterranean basin directly affected by industrial mercury releases. Overall, sea trenches only make up 1% of all deep-sea areas, but can accumulate 12 to 30% of all marine mercury.

The study’s lead researcher, Hamed Sanei from Aarhus University in Denmark, called the results of his team “alarming”. “This can be an indicator of the overall health of our oceans,” he suggested. But while the deposits may be representative of human use of mercury, the deep ocean floors are likely to become a permanent dump for the toxic metal. Eventually it is carried into the earth’s mantle by plate tectonics.

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