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Has human-derived mercury reached the deepest oceanic trench on earth?

Research

Over 60 years ago, the outbreak of Minamata disease caused by methylmercury in Japan shocked the whole world. Mercury is a toxic pollutant that spreads globally. As the current emission of mercury is around 3 to 5 times as much as that discharged before the Industrial Revolution, mercury pollution has exacerbated remarkably in the ecosystem.

It’s a popular belief that methylmercury in the ocean mainly originates from the oxygen minimum zone in the central sea(100-1000m). With a wider coverage of aerial survey on mercury data, an increasing number of researches have revealed that there is more methylmercury in the deep ocean (>1000m) than once thought to be.

Recently, the articleMethylmercury produced in upper oceans accumulates in deep Mariana Trench faunapublished inNature Communicationsconfirmed that human-derived mercury is likely to have reached and entered the food webs in even the most remote marine ecosystems on earth, posing potential threats to the fragile deep-sea ecosystem.

This research was jointly conducted by Sun Ruoyu’s and Liu Yu’s teams at Tianjin University and their cooperators from Institute of Deep-sea Science and Engineering in Chinese Academy of Sciences, Institute of Toulouse Earth Environment in French Academy of Sciences, Nankai University and Nanjing University. They collected bio-samples on the seafloor of Mariana trenches with the help of sophisticated deep-sea lander vehicles, thus figuring out the source and transfer route of methylmercury in the deep sea through mercury isotope test.

Since human’s exploration into the deep ocean is limited, many inquiries remain unanswered, such as the exact level of the Methylmercury concentration in the deep sea, the ratio of manmade methylmercury to naturally produced methylmercury, other ways for methylmercury to reach deep sea and the unproven mercury source and transfer route in dozens of oceanic trenches.

Is the deepest ocean a zero-pollution place?

At normal temperature and pressure, mercury is the only metallic element that exists in liquid form with strong volatility.“Mercury could be generated through both natural phenomena and human activities.We know that 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,”said Sun Ruoyu, the lead author of the article and the associate professor of Tianjin University’s School of Earth System Science.

Mercury entering the ocean would be converted into highly toxic methylmercury in a bacterial environment, and the latter tends to concentrate in organisms and then to be absorbed by fishes, mammals and human bodies that are at high nutrition level through food chains. As an example, mercury concentrations in swordfish are 40 times higher than that in salmon.

Accordingly, facing global changes, the study on the origin, transfer and concentration of ocean methylmercury bears great significance for formulating global and regional environmental policies like theMinamata Convention on Mercury.

Previous researches published onNatureand other scientific journals believed that of over 60,000 tons man-made mercury in the ocean, two thirds were in the upper ocean and the rest were in the North Atlantic deep water and Antarctic bottom water locating in the deep water fast-forming zone.

“According to former researches, human-released mercury has increased in North Atlantic due to large-scale industrialization among European and American countries, thus leading to atmospheric mercury growth. When the mercury from atmosphere reaches the upper ocean, it would be influenced by ocean current. The seawater entering into the Arctic Ocean would sink quickly due to temperature drop, which accelerates the manmade mercury sinkage into the deep ocean”, Sun introduced. Moreover, it has been found and confirmed by researchers that there is human-generated methylmercury in the deep ocean in these areas.

“But when researchers estimated mercury pollution in the deep Pacific in the same way, they drew the conclusion that theoretically, there was basically no pollution there, because the Pacific wasn’t located in the deep-water formation zone. It would take a long time for upper polluted seawater to be transported to the deep ocean,”Sun Ruoyu said, noting that it might take thousands of years for the polluted water of Atlantic to be carried by ocean currents and flow to the deep Pacific due to the long distance of more than 20,000 kilometers and low flowing speed.

Based on these previous researches, the academic world used to believe that methylmercury was mostly produced in the top few hundred meters of the ocean and there was barely man-made mercury pollution in the deep ocean of 1,000m or above. Therefore, the deep ocean is regarded as the last“pure land”on earth.

Isotopic test shows man-made mercury pollution in deep sea.

In recent years, the development of deep-sea hydrothermal and deep-diving technology has enabled the homemade bathyscaph and catcher to work in deep sea of 10,000m or above. Sun Ruoyu said: “We chose the Marianas Trench to sample because it was known as the deepest part of the sea. If there exists mercury pollution, so will other places in the world.”

2020-11-03 20:50:57.494000

Mercury Circulation in Sea

In 2016 and 2017, Sun’s research group deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches. They captured the endemic fauna like snailfish and grooved shrimp at 7000-11000 m and collected sediments at 5500-9200 m. Their research found that the concentration of total mercury and methylmercury in grooved shrimp was conspicuously higher than that in the similar amphipod in freshwater and coastal zone.

Sun and other researchers found that fish in this trench at 7000-11000m shared the similar mercury isotopic composition with those at the 0-1000m upper ocean. Taking into consideration other factors like the generative scheme of deep water, the life circle of methylmercury and the transmission route of particles, the research concluded that the methylmercury found in the trench species originates primarily from the upper ocean and that the methylmercury photodegraded in the upper ocean mixed with the non-photodegradable methylmercury in the middle ocean and then together entered into the deep-sea food chain system through sinking particles.

Sun explained: “The mercury derived from the atmosphere entered the ocean in rainfall and was then transported to the deep ocean in the sinking carcasses of whales as well as in small particles like the particle clusters of dead seaweed. During this process, the unique geological structure of the Mariana Trench, and the amplification effect caused by frequent earthquakes all sped up mercury transmission. By measuring the mercury isotopic composition, we found that the ocean floor mercury matched that from fish found at around 400-600m depth in the Pacific. Relevant evidence showed that the mercury there was produced decades ago, that is to say, much of it came from human activity.”

Who takes greater responsibility for deep-sea mercury pollution? Men or Nature?

“To answer this question, currently we can only offer circumstantial evidences by testing the mercury and its isotope within organisms,” Sun said, adding that to test the mercury and its isotope in the seawater would provide a direct answer, which was very difficult to realize, given the extreme low content of mercury in the sea and lack of effective technologies to test accurately the isotopic composition of mercury.

“In addition, the ratio of naturally produced mercury to manmade mercury for the methylmercury found in the Mariana Trench also cannot be identified through isotopic test.”

Marine life cannot survive without food. Three things, namely, the sinking carcasses of whales, hydrothermal water and cold seep are regarded as the “oasis” of deep-sea organisms, together contributing to the diversity of marine life.

“We have proven that the sinking carcasses of whales are one of the carriers for methylmercury’s migration by detecting the mercury isotope in marine life. However, the samples around hydrothermal water and cold seep are yet to be tested to know whether they are the origin of methylmercury too. If the answer is yes, how much did they contribute to the methylmercury absorbed by deep-sea fish?” To answer these questions, Sun suggested that sample collection should be conducted to verify assumption.

Besides, there are dozens of trenches in the world. Does the mercury in these trenches share the same origin and transfer route? “Sample testing in the deep water of these trenches may give us the answer” said Sun. He hoped to produce and analyze multiple samples so as to build a scientific model that would help people better understand where the mercury in the deepest ocean comes from and what will become of it.

“To make a bolder assumption, there may exist unknown undercurrent which flows so fast that carries the upper water to the profundal zone of the Pacific,” Sun Ruoyu said.

In Reference to the Chinese article published byScience and Technology Daily (https://article.xuexi.cn/articles/index.html?art_id=4737766745194864730&item_id=4737766745194864730&study_style_id=feeds_default&pid=&ptype=-1&source=share&share_to=wx_single)

Translated by Chenxu, Zhang Zhuoran

Editor: Eva Yin