(CN) — The decaying flesh of fish that lived in bodies of water tainted with mercury — an element that is harmful to the human nervous system — has sank and introduced toxic pollutants to the most remote ocean trenches on Earth, according to a study published Monday.
Mercury is a naturally occurring metallic element that is liquid at room temperature. More than 2,000 metric tons of it are emitted each year from human activity such as the burning of fossil fuels, leading to its presence in soil, air and water.
Microorganisms convert some of the inorganic mercury that falls from the atmosphere into methylmercury, a highly toxic organic form that builds up in fish and is dangerous to humans and wildlife.
High exposure to mercury in all its forms can cause neurological disturbances, lung damage, skin rashes and kidney abnormalities in addition to nerve damage, according to the U.S. Centers for Disease Control and Prevention.
How did this neurotoxin make its way to the most inaccessible caverns of the deep sea — including the 36,000-foot-deep Mariana Trench in the northwest Pacific?
It started with coal-fired power plants, cement factories, incinerators, mines and other operations spewing toxins into the atmosphere, according to the study published Monday in Proceedings of the National Academy of Sciences.
An analysis of fish carcasses — also called carrion — and crustaceans called amphipods collected in two Pacific Ocean trenches shows that while some mercury can be traced to “natural” sources, the majority comes from human activity, University of Michigan researcher Joel Blum said in a statement released with the study.
"Mercury that we believe had once been in the stratosphere is now in the deepest trench on Earth," said Blum, environmental geochemist and lead author of the study. "It was widely thought that anthropogenic mercury was mainly restricted to the upper 1,000 meters of the oceans, but we found that while some of the mercury in these deep-sea trenches has a natural origin, it is likely that most of it comes from human activity."
Researchers were able to identify mercury samples from different origins based on the ratio of their isotopes, which carry a unique chemical signature. Mercury isotopes found in amphipods from the deep-sea caverns matched the isotopes of fish that feed near the ocean surface at depths of around 500 meters, the study found.
"We studied the trench biota because they live in the deepest and most remote place on Earth, and we expected the mercury there to be almost exclusively of geologic origin — that is, from deep-sea volcanic sources," Blum said. "Our most surprising finding was that we found mercury in organisms from deep-sea trenches that shows evidence for originating in the sunlit surface zone of the ocean."
Samples were collected in the Mariana Trench southwest of Guam and at depths of up to 32,800 feet in the Kermadec Trench in the Pacific northeast of New Zealand, said study co-author and University of Hawaii oceanographer Jeffrey Drazen.
"These samples were challenging to acquire, given the trenches' great depths and high pressures," Drazen said. "The trenches are some of the least studied ecosystems on Earth, and the Mariana snailfish was only just discovered in 2014."
At a meeting this past June coordinated by both Blum’s team and researchers led by Chinese scientists, independent findings showed that human, or anthropogenic, activity drove the presence of mercury in deep-sea organisms.
The Chinese-led study, published July 7 in the journal Nature Communications, found mercury latches onto organic matter — including fecal material and dead plankton — that continuously sinks from the surface to the bottom of the sea.
The study led by Blum found that the sinking flesh of fish who lived near the mercury-tainted ocean surface delivered most of the toxin detected in the deep-sea trenches.
“The importance of carrion to trench food webs is consistent with observations of seafloor communities of fish and amphipods that depend upon carrion for nutrition in other deep-sea regions,” the study says. “Sinking carcasses as a delivery mechanism provides a means for increasing the efficiency of the biological pump for Hg (mercury) to the deep-seafloor in hadal (deep-sea) environments and possibly also in abyssal settings.”
The finding is critical information for policymakers looking to understand the correlation between mercury emission on a global scale and mercury found in seafood, said Blum.
"Yes, we eat fish caught in shallower waters, not from deep-sea trenches," Blum said. "However, we need to understand the cycling of mercury through the entire ocean to be able to model future changes in the near-surface ocean."
In the massive, emerging economies of China and India, new coal-fired power plants are brought online every week, belching high levels of mercury into the atmosphere and emitting unknown quantities of mercury into the ocean.
North America and Europe have seen decreases in toxic emissions, the study said.
Drazen said that as long as mercury is emitted from anthropogenic sources, the element will continue to be brought into oceans by rainfall, runoff from rivers and windblown dust.
"Deep-sea trenches have been viewed as pristine ecosystems unsullied by human activities,” Drazen said. “But recent studies have found traces of anthropogenic lead, carbon-14 from nuclear weapons testing, and persistent organic pollutants such as PCBs in organisms living in even the deepest part of the ocean, which is known as the hadal zone."
Researchers did not immediately respond to a request for further comment on the study, which is titled "Mercury isotopes identify near-surface marine mercury in deep sea trench biota."
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