Human-made mercury has polluted bottom of the deepest part of Pacific Ocean in Marianas Trench, studies reveal

This has significant implications for how mercury affects the marine environment, and how it may be concentrated in the food chain, say scientists


                            Human-made mercury has polluted bottom of the deepest part of Pacific Ocean in Marianas Trench, studies reveal
(Getty Images)

Human-made mercury pollution has reached the bottom of the deepest part of the ocean in the Marianas Trench, say scientists. It is the deepest known ocean trench on Earth. According to two independent research teams, both man-made and natural methylmercury, a toxic form of mercury easily accumulated by animals, has been found in fish and crustaceans in the 11,000-meters-deep Marianas Trench in the Pacific Ocean. This has significant implications for how mercury affects the marine environment, and how it may be concentrated in the food chain, say scientists. The findings, being presented at the virtual Goldschmidt geochemistry conference, imply that anthropogenic (human-induced) mercury release at the Earth’s surface is much more widespread and pervasive across deep oceans than was previously thought.

“This is a surprise. Previous research had concluded that methylmercury 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,” says Dr Ruoyu Sun, who is leading a group of researchers from Tianjin University, China, in the analysis. Ocean trenches are long, narrow depressions on the seafloor. Also referred to as the Mariana Trench, it is a crescent-shaped trench in the Western Pacific.

The Challenger Deep is considered to be the deepest point of the Marianas Trench. It was designated a US national monument in 2009. The total area of the Marianas Trench Marine National Monument consists of approximately 95,216 square miles (246,608 square kilometers) of submerged lands and waters of the Mariana Archipelago, east of the Philippines. It is managed by the National Oceanic and Atmospheric Administration (NOAA), the US Fish and Wildlife Service, and the Commonwealth of the Northern Marianas Islands (CNMI), a US territory. 

The submersible deep-sea warrior used by Dr Ruoyu Sun's team for the research 
(Ruoyu Sun and IDSSE-CAS)

During 2016-2017, Dr Sun’s team deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches, among the most remote and inaccessible locations on Earth, and captured the endemic fauna (amphipod and snailfish) at 7,000-11,000 meters and collected sediments at 5,500-9,200 meters. “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,” says Dr Sun.

The second group, 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 (which drops to 10,000 meters) and the Marianas Trench off the Philippines. They 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 work shows that human-released mercury has reached and entered food webs in even the most remote marine ecosystems on earth. According to the researchers, their findings have led to a better understanding of the origin of mercury in the deepest reaches of the ocean and will aid in modeling the fate of mercury in the atmosphere and oceans.

“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 400-600m depth in the Central Pacific. Some of this mercury is naturally-produced, but it is likely that much of it comes from human activity,” says Dr Blum. 

Snailfish at 7000 m in the Kermadec Trench off New Zealand as sampled by Dr Joel Blum's team (Alan Jamieson)

Mercury is introduced into the environment from a variety of natural sources such as volcanic eruptions and forest fires. However, human activities such as coal and petroleum burning, mining, and manufacturing, are mainly responsible for mercury deposition to marine environments, say experts. The two new studies indicate that the effects of this deposition have spread throughout the ocean into the deep sea and the animals that live there, which is yet another indicator of the adverse impact of human activities on the planet.

According to scientists, mercury is toxic to humans and other animals and has been implicated in environmental disasters in the past, most famously at Minamata in Japan in the 1950s where it led to birth defects and severe neurological symptoms. It tends to be concentrated in marine organisms, where small amounts are ingested by some species, which are, in turn, eaten by larger species. This implies that harmful levels of mercury can be concentrated in animals that sit higher up in natural food webs through the process of bioaccumulation. 

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