Mercury levels found in remote Pacific trenches are unprecedented

An international team of researchers has discovered amounts of highly toxic mercury in the deepest trenches of the Pacific Ocean that exceed any value ever recorded in remote marine sediments.

The experts report that the mercury levels which have accumulated in the remote Pacific are even higher than in many areas that are directly contaminated by industrial releases.

The researchers have obtained the first-ever direct measurements of mercury deposition in one of the most challenging environments to sample on Earth.

Study lead author Professor Hamed Sanei is the director of the Lithospheric Organic Carbon Laboratory (LOC) in the Department of Geoscience at Aarhus University . 

“The bad news is that these high mercury levels may be representative of the collective increase in anthropogenic emissions of Hg into our oceans,” said Professor Sanei. “But the good news is that ocean trenches act as a permanent dump, and so we can expect the mercury that does end up there will be buried for many millions of years. Plate tectonics will carry these sediments deep into the earth’s upper mantle.”

“But even as mercury is being removed from the biosphere, it remains quite alarming how much mercury has ended up in the ocean trenches. This may be an indicator of the overall health of our oceans.”

Study co-author Dr. Peter Outridge is a research scientist with Natural Resources Canada and the lead author of the United Nations’ Global Mercury Assessment.

“The results of this research help fulfill a key knowledge gap in the mercury cycle, i.e. the true rate of mercury removal from the global environment into deep-ocean sediments,” said Dr. Outridge. “We have shown that sediments in the ocean trenches are mercury accumulation ‘hotspots,’ with mercury accumulation rates many times higher than were previously believed to be present.”

Study co-author Professor Ronnie Glud, the director of the Hadal Centre at the University of Southern Denmark, was the scientific leader of a multi-national expedition to the ocean trenches. 

“This paper calls for extensive additional sampling of the deep-ocean and in particular hadal trenches to support this preliminary work,” said Professor Glud.

“Ultimately this will improve the accuracy of environmental mercury models and the management of global mercury pollution.”

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer