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Deepest Oceanic Trenches

Scientists Find Anthropogenic Mercury in Earth’s Deepest Oceanic Trenches

Two teams of marine biologists have found methylmercury, a potent toxin that bioaccumulates in marine food webs, in sediments and endemic fauna from Mariana, Yap and Kermadec trenches in the Pacific Ocean.

A snailfish (bottom left) and small amphipods (center) at 8,145 meters in the Mariana Trench, the deepest stretch of ocean in the world that is located in the western Pacific Ocean. Image credit: University of Aberdeen.

A snailfish (bottom left) and small amphipods (center) at 8,145 meters in the Mariana Trench, the deepest stretch of ocean in the world that is located in the western Pacific Ocean. Image credit: University of Aberdeen.

One of the research teams, led by Tianjin University’s Dr. Ruoyu Sun, measured mercury concentrations and isotope compositions in snailfish and crustaceans captured at 7,000-11,000 m and sediments collected at 5,500-9,200 m in Mariana and Yap trenches.

“During 2016-2017, we deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches, amongst the most remote and inaccessible locations on Earth, and captured the endemic fauna and collected sediments,” Dr. Sun said.

“We were 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.”

“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.”

The second team, led by University of Michigan’s Dr. Joel Blum, sampled fish and crustaceans from Mariana and Kermadec trenches.

“We used mercury isotopic signatures at both locations to show that mercury found in trench species is largely derived from the atmosphere and enters the ocean in rainfall,” the scientists said.

“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.

“We identified this by measuring the mercury isotopic composition, which showed that the ocean floor mercury matched that from fish found at around 400-600 m depth in the Central Pacific.”

“Some of this mercury is naturally-produced, but it is likely that much of it comes from human activity.”

“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.

The scientists will present their results this week at the 2020 Goldschmidt Geochemistry Conference.

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R. Sun et al. Marinana Trench Fauna Accumulate Methylmercury Produced in Upper Oceans. Goldschmidt Abstracts 2020: 2504

J. Blum et al. Contrasting Hg Isotope Ratios from the Kermadec and Mariana Trenches. Goldschmidt Abstracts 2020: 207

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