“Deep-sea trenches have been viewed as pristine ecosystems unsullied by human activities, 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…,” said Jeffrey Drazen, an oceanographer with the University of Hawaii, about man-made mercury pollution detected in fish and crustaceans at the bottom of the 11,000 meter-deep Marianas Trench in the Pacific Ocean, which has significant implications for how mercury affects the marine environment, and how it may be concentrated in the food chain.
Toxic –2,000 Metric Tons Emitted
Mercury is a naturally occurring element, but more than 2,000 metric tons of it are emitted into the atmosphere each year from human activities, reports the University of Michigan. “This inorganic mercury can travel thousands of miles before being deposited onto land and ocean surfaces, where microorganisms convert some of it to methylmercury, a highly toxic organic form that can accumulate in fish to levels that are harmful to humans and wildlife. Effects on humans can include damage to the central nervous system, the heart and the immune system. The developing brains of fetuses and young children are especially vulnerable.”
At the Goldschmidt Geochemistry Conference in June 2020, scientists revealed that man-made mercury pollution has reached the p Marianas Trench in the Pacific Ocean, which has significant implications for how mercury affects the marine environment, and how it may be concentrated in the food chain.
“Mercury that we believe had once been in the stratosphere is now in the deepest trench on Earth,” said U-M environmental geochemist Joel Blum lead author of the PNAS paper and a professor in the University of Michigan Department of Earth and Environmental Sciences.
At the scientific meeting, Blum’s team and a Chinese-led research group independently reported the detection of human-derived mercury in deep-sea-trench organisms.
Hitching a Ride on Microscopic Particles
The Chinese researchers, who published their findings July 7 in the journal Nature Communications, concluded that the mercury gets to the deep-sea trenches by hitching a ride on microscopic particles of sinking organic matter—including fecal material and dead plankton—that constantly rain down from the upper oceans.
“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.”
Mercury is toxic to humans and other animals, and has been implicated in environmental disasters, observed the Goldschmidt Conference “in the past, most famously at Minamata in Japan in the 1950’s 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, meaning that harmful levels of mercury can be concentrated in animals that sit higher up in natural food webs through the process of bioaccumulation. As an example, this leads to mercury concentrations in swordfish being x40 that of salmon. Mercury is generally poisonous at high levels and can be especially dangerous to the developing fetus.”
“This is a surprise” said researcher Ruoyu Sun. “Previous research had concluded that methlymercury 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”.
“During 2016-2017, we deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches, amongst the most remote and inaccessible locations on Earth, and captured the endemic fauna at 7000-11000 meters and collected sediments at 5500-9200 meters, said co-author Sun..” 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.”
Independently, a group led by Joel Blum with 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, using 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.
Human-Released Mercury Entered Food Webs
“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, said Blum. “We identified this by measuring the mercury isotopic composition, which showed that the ocean floor mercury matched that from fish found at around 400-600 meter depth in the Central Pacific. Some of this mercury is naturally-produced, but it is likely that much of it comes from human activity.
“This work shows that human-released mercury has reached and entered food webs in even the most remote marine ecosystems on earth. This better understanding of the origin of mercury in the deepest reaches of the ocean will aid in modelling the fate of mercury in the atmosphere and oceans”.
“Our findings reveal very little methylmercury is produced in the deep oceans,” said Sun, “and imply that anthropogenic mercury release at the Earth’s surface is much more widespread across deep oceans than was previously though.”
“We know that mercury is introduced into the environment from a variety of natural sources such as volcanic eruptions and forest fires, said Ken Rubin with the University of Hawaii said:. “However, human activities, such as coal and petroleum burning, mining, and manufacturing, are mainly responsible for mercury deposition to marine environments. We are now learning from these two studies 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 profound impact of modern human activities on the planet.”
The Daily Galaxy, Jake Burba, via University of Michigan and The Goldschmidt Conference