When scientists first started removing amphipods from the Mariana Trench, which are crustaceans that scavenge the hadal zone at depths of up to eleven kilometers, and performing chemical analyzes on their tissue, the results were not what they had anticipated at the planet’s most remote and inaccessible biological environment. Mercury. Not trace amounts that fit the background of the natural geology. concentrations that are much higher than what the surrounding geology would generate on its own, in species that live in total darkness under pressures that crush almost everything that has ever been constructed by mankind, at a depth that has never seen sunshine since the formation of the ocean.
Mercury travels a lengthy, roundabout, and mostly undetectable route from a coal-fired power plant in China or a gold mining operation in the Amazon basin to the bottom of the Mariana Trench until you start looking for it. Mercury vapor from burning coal is released into the atmosphere, where it can travel thousands of kilometers on air currents before being deposited into the ocean’s surface by dry deposition and rainfall.
Inorganic mercury binds to organic particles in the surface ocean, which is biological material that is continuously moving downward from the productive sunlight zone. Mercury is carried by marine snow, which is a constant stream of dead organisms, feces, and organic aggregates that carry nutrients and carbon downward. It builds up in the particles as it descends, and those particles concentrate in the deepest topographic trap that is accessible.
The ocean’s deepest traps are called hadal trenches. Sediment and biological matter are drawn inward from the surrounding abyssal plain by its V-shaped geometry, which consists of steep walls that narrow toward a floor that is sometimes more than ten kilometers below sea level. The hadal zone is finally reached by material that settles on the level ocean floor at 4,000 meters and then moves in the direction of the trench edge. The chemical traces of everything that was carried in the organic matter above, even the mercury that traveled from an industrial atmosphere decades or centuries ago, are accumulated on the trench floor together with the silt.
This is a biological issue rather than merely a geological one because of the transformation that takes place in the sediment. Inorganic mercury is transformed into methylmercury, the organic form of the metal that biological systems absorb and hold onto, by sulfate-reducing bacteria that are active in the oxygen-depleted sediments of the deep trench floor. The Minamata illness disaster in Japan during the 1950s, when industrial wastewater contaminated coastal seafood and severely damaged the neurological systems of the communities that consumed them, was caused by methylmercury. The conversion is taking place in the hadal zone in sediments that no one is keeping an eye on and in animals that no one is eating, but which are a part of a food web that extends upward through the water column.
It has been discovered that amphipods at the deepest hadal depths contain tissue amounts of mercury that show significant accumulation over the course of their lives. The organic debris that settles through the trench, including the particles that brought the mercury to depth, is what they eat. Because biomagnification functions in the same manner at the bottom of the ocean as it does in any other food web, predators that eat amphipods in the deepest zones—snailfish, which have been found at depths of almost 8,000 meters—accumulate even higher amounts. Since these creatures don’t reach commercial fisheries, the concentrations don’t necessarily pose a direct hazard to human health. However, they reveal a pattern of contamination that reaches the world’s most isolated marine ecosystem.
As expected, the governance condition is insufficient. Reducing mercury emissions from industrial sources, such as power stations, artisanal mining, and cement production, is the goal of the Minamata Convention on Mercury, which came into effect in 2017. The rate at which additional mercury enters the ocean will eventually decrease when it is implemented, as it is a significant tool for managing atmospheric loading. It does not, however, address the assessment of methylmercury production in hadal sediments, the monitoring of deep-ocean accumulation, or any binding criterion for what quantities in deep-sea biota would be considered an unacceptable level of contamination. Both literally and conceptually, the regulatory architecture ends well above the hadal zone.
