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Toxic man-made mercury pollution has reached the deepest point in the world’s oceans, the Mariana Trench.
The heavy metal is toxic to humans and animals and was found seven miles beneath the surface in the trench in the Pacific Ocean.
Deep-sea landers captured sea creatures up to 11,000 metres below the surface, and found “unequivocal” evidence that mercury in their bodies came from the upper ocean.
In sea animals, the metal is often concentrated in larger organisms, when small amounts are ingested by smaller species, which are in turn eaten by larger ones.
Mercury has been implicated in environmental disasters in the past, including at Minamata in Japan in the 1950s when pollution from a factory caused thousands of deaths.
Two teams of scientists reported that man-made methylmercury has been found in fish and crustaceans in the Mariana Trench.
The work was reported at the Goldschmidt Geochemistry conference.
Dr Ruoyu Sun, leading a group of researchers from Tianjin University in China, said that the results came as a surprise to his team.
He said: “Previous research had concluded that methlymercury was mostly produced in the top few hundred metres 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.”
Dr Sun explained how his team employed sophisticated deep-sea lander vehicles on the floor of the Mariana and Yap trenches, among the most inaccessible places on Earth.
The scientists captured local fauna and sediment samples – and the distinctive radioactive “fingerprint” of the mercury in the samples shows it came from the surface, he said.
Dr Sun added: “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.”
A second group led by Dr Joel Blum of the University of Michigan sampled fish and crustaceans from the Kermadec trench near New Zealand (which drops to 10,000 metres) and the Mariana trench.
Dr Blum 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.
“We identified this by measuring the mercury isotopic composition, which showed that the ocean floor mercury matched that from fish found at around 400 to 600-metre depth in the central Pacific.
“Some of this mercury is naturally-produced, but it is likely that much of it comes from human activity.”