TORONTO -- An international team of scientists have discovered unprecedented amounts of highly toxic mercury in Pacific Ocean trenches that exceed any prior record, and are higher than many areas that are directly contaminated by industrial waste.

The study, published in the journal Scientific Reports on Wednesday, was a multi-national effort with scientists from Canada, Denmark, Germany and Japan. Researchers recorded the first-ever direct measurements of mercury deposits up to 10 kilometres below the surface of the Pacific.

Ocean sediments are the largest repository or "sink" for mercury, the study states, but measurements have never been taken from greater than six kilometres below. The team took samples from the Atacama Trench, off the coast of Peru and Chile, and Kermadec Trench off the coast of New Zealand.

Researchers found mercury concentrations of up to 400 nanograms (ng), whereas measurements in other parts of the world’s oceans have been recorded at less than 80 ng, on average. The study found sediment from the Atacama Trench contained significantly higher mercury concentrations than the Kermadec Trench.

Lead author of the study Hamed Sanei stated in a release that “the bad news is that these high mercury levels may be representative of the collective increase in emissions of mercury into our oceans.”

"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,” he said. “Plate tectonics will carry these sediments deep into the earth's upper mantle".

The deep ocean samples had mercury concentrations that approach the level of some of the most contaminated shelf sites in the world such as the Bohai Sea in China (574 ng) and the Laurentian Trough in Canada (520 ng).

Sanei said that the mercury found in the areas studied by researchers exceed any value ever recorded in remote marine sediments, and is even higher than many areas “directly contaminated” by industrial waste.

“It remains quite alarming how much mercury has ended up in the ocean trenches,” he said. “This may be an indicator of the overall health of our oceans."

Research scientist with Natural Resources Canada and study co-author Peter Outridge said the study will “help fulfill a key knowledge gap in the mercury cycle – the true rate of mercury removal from the global environment into deep-ocean sediments.”

Researchers say the study shows that sediment in Pacific Ocean trenches are mercury accumulation “hot spots” and that the prevalence of the contaminant was “many times higher” than previously thought.

The study calls for extensive additional sampling of the deep-ocean to collect more data and to improve global mercury contamination modelling.


This story has been corrected to include the correct spelling of Hamed Sanei and Peter Outridge's names.