Scientists alarmed by mercury levels found at bottom of Pacific Ocean

The record-setting measurements -- detailed Wednesday in the journal Scientific Reports -- revealed unprecedented levels of the heavy, silvery element at the bottom of Pacific.

Mercury, called Hg on the periodic table, is a toxic element that occurs naturally, but is also released by human activities like mining and fossil fuel combustion.

Until now, scientists had never measured such tremendous mercury concentration in marine sediments, naturally occurring or otherwise.

"The bad news is that these high mercury levels may be representative of the collective increase in anthropogenic emissions of Hg into our oceans," lead study author Hamed Sanei said in a press release.

"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," said Sanei, a professor of geosciences and director of the Lithospheric Organic Carbon Laboratory at Aarhus University in Denmark.

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

Scientists suggest the surprise discovery is a reminder of how little is known about the global mercury cycle, such as the rates at which mercury is carried from terrestrial environs into the world's oceans.

"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," said co-author Peter Outridge, a research scientist with Natural Resources Canada and lead author of the United Nations' Global Mercury Assessment.

The study's authors hope the surprise findings will motivate other scientists and research groups to expand the scope of mercury measurements surveys, especially in the deep ocean.

"Ultimately this will improve the accuracy of environmental mercury models and the management of global mercury pollution," said co-author Ronnie Glud, professor and director of the Hadal Center at the University of Southern Denmark.