Mercury pollution detected in the bowels of the Earth

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Human-made mercury (also) pollutes the world’s deepest ocean trenches, two new studies reveal. The human footprint is everywhere, even in the most isolated areas. In September 2019, a team of British researchers claimed to have discovered cases of ingestion of plastic in small shrimps inside six of the deepest oceanic trenches on the planet.

In the Marianas Trough, for example, 100% of the specimens studied, evolving between 6,000 and 11,000 meters deep, had plastic fibers in their digestive tract.

Methylmercury in ocean trenches

But plastic pollution is not our only imprint. Other forms of pollutants from our activities can also seep into the bowels of the Earth. Another example is methylmercury – a toxic form of mercury – as two new studies suggest. These results were presented this week at the 2020 Goldschmidt Geochemistry Conference.

The first team, led by Joel Blum (University of Michigan, United States), detected them in several species of fish and crustaceans more than 7 kilometers deep in the Marianas Trough, near the Philippines. It has also detected it in certain endemic snails from the Kermadec Pit, off the coast of New Zealand.

The second team, led by Dr Ruoyu Sun (Tianjin University, China), also measured the same levels of mercury in crustaceans growing in the Mariana Trench. All were caught between 7,000 and 11,000 meters deep.

Much of anthropogenic origin

It is also important to note that mercury, or its methylated form, can be released to the environment from natural sources. Volcanic eruptions or underwater volcanic conduits are examples. However, a large part of these released pollutants comes from our activities. These include the burning of coal and oil, and the extraction and production of metals.

In these two studies, the researchers suggest that part of the methylmercury identified in the samples studied does indeed come from natural sources. On the other hand, they stress that mercury of human origin is present in greater quantities than that resulting from natural emissions. The isotopic analysis of these pollutants, carried out by these two teams, suggest that this methylmercury was released into the atmosphere before falling back into the upper parts of the ocean via precipitation. Similar readings had in fact already been recorded in several marine organisms operating at depths of around 400 meters in the central Pacific.

On the other hand, in these new cases, a large part of the mercury would have progressively sunk to the bottom of the trenches, entrained by the carcasses of dead fish and marine mammals. These corpses, which then feed the species evolving in great depths, then contaminated them in their turn