Scientists Find Human-Made Mercury Pollution In World's Deepest Ocean Trenches

The scientists found toxic mercury in fish and crustaceans living more than 11,000 metres below the surface of the ocean in the Mariana Trench. The findings were presented at the Goldschmidt Conference, annual, international conference on geochemistry and related subjects, organised by the European Association of Geochemistry and the Geochemical Society.

With the use of submarine robots, the scientists identified human-derived methylmercury - a toxic form of mercury that easily accumulates in some animals in the fish and crustaceans living in the deepest part of the western Pacific Ocean.

The study has significant implications on the impact of mercury on the marine environment and how it may be concentrated in the food chain.

"This is a surprise. Previous research had concluded that methlymercury was mostly produced in the top few hundred meters of the ocean. This would have limited mercury bioaccumulation by ensuring that fish which forage deeper than this would have had limited opportunity to ingest the methylmercury. With this work, we now believe that isn't true," researcher Ruoyu Sun, who led a group of researchers from Tianjin University in China, was quoted by Phys.org.

As small amounts of mercury are ingested by some species which in turn are eaten by larger species, the liquid metal can become concentrated in marine organisms. This leads to more harmful concentrations of mercury accumulating in animals higher up the food chain.

At high levels, mercury is poisonous to animals and humans, especially to the developing fetus.

In 2016-2017, Dr Sun's team used deep-sea vehicle to capture animals from between 7,000-11,000m and take sediment samples at 5,500-9,200m from the seafloor of Mariana and Yap trenches, which are amongst the most remote and inaccessible locations on earth.

"We are able to present unequivocal mercury isotope evidence that the mercury in the trench fauna originates exclusively from methylmercury from the upper ocean. We can tell this because of the distinctive isotopic fingerprint which stamps it as coming from the upper ocean," said Dr Sun.

Meanwhile, a separate team led by Dr Joel Blum from the University of Michigan, sampled fish and crustaceans from two of the deepest Pacific Trenches – the Kermadec trench near New Zealand and the Mariana Trench of the Philippines.

The team found that mercury found in the species in both locations is largely derived from the atmosphere and had entered the ocean through rainfall.

"We know that this mercury is deposited from the atmosphere to the surface ocean and is then transported to the deep ocean in the sinking carcasses of fish and marine mammals as well as in small particles," Dr Blum said.

"This work shows that human-released mercury has reached and entered foodwebs in even the most remote marine ecosystems on earth. This better understanding of the origin of mercury in the deepest reaches of the ocean will aid in modeling the fate of mercury in the atmosphere and oceans," Dr Blum added.

"Our findings reveal very little methylmercury is produced in the deep oceans, and imply that anthropogenic mercury release at the Earth's surface is much more widespread across deep oceans than was previously thought," Dr Sun said.