Toxic mercury, accumulating in the arctic, springs from a hiddensource - Muslim Azan Clock

Mercury is a naturally occurring element that has been enriched inthe environment by human activities such as coal combustion andmining. When converted to methylmercury by microbial processes inthe ocean, it can accumulate in fish and wildlife at concentrationsup to a million times higher than the levels found in theenvironment. "In humans, mercury is a potent neurotoxin," explainsco-principal investigator Elsie M. Sunderland, Mark and CatherineWinkler Assistant Professor of Aquatic Science at HSPH.

"Itcan cause long-term developmental delays in exposed children andimpair cardiovascular health in adults." Mercury is considered a persistent bioaccumulative toxin because itremains in the environment without breaking down; as it travels upthe food chain, from plankton to fish, to marine mammals andhumans, it becomes more concentrated and more dangerous. "Indigenous people in the Arctic are particularly susceptibleto the effects of methylmercury exposure because they consume largeamounts of fish and marine mammals as part of their traditionaldiet," Sunderland says. "Understanding the sources ofmercury to the Arctic Ocean and how these levels are expected tochange in the future is therefore key to protecting the health ofnorthern populations." Sunderland supervised the study with Daniel Jacob, Vasco McCoyFamily Professor of Atmospheric Chemistry and EnvironmentalEngineering at SEAS, where Sunderland is also an affiliate. Mercury enters Earth's atmosphere through emissions from coalcombustion, waste incineration, and mining.

Once airborne, it candrift in the atmosphere for up to a year, until chemical processesmake it soluble and it falls back to the ground in rain or snow.This deposition is spread worldwide, and much of the mercurydeposited to Arctic snow and ice is re-emitted to the atmosphere,which limits the impact on the Arctic Ocean. "That's why these river sources are so important," saysFisher. "The mercury is going straight into the ocean." The most important rivers flowing to the Arctic Ocean are inSiberia: the Lena, the Ob, and the Yenisei. These are three of the10 largest rivers in the world, and together they account for 10%of all freshwater discharge to the world's oceans.

The Arctic Oceanis shallow and stratified, which increases its sensitivity to inputfrom rivers. Previous measurements had shown that the levels of mercury in theArctic lower atmosphere fluctuate over the course of a year,increasing sharply from spring to summer. Jacob, Sunderland, andtheir team used a sophisticated model (GEOS-Chem) of the conditionsin the Arctic Ocean and atmosphere to investigate whether variableslike melting ice, interactions with microbes, or the amount ofsunlight (which affects chemical reactions) could account for thedifference. Incorporating those variables, however, was not enough. The GEOS-Chem model, which is backed by rigorous environmentalobservations and more than a decade of scientific review,quantifies the complex nuances of the ocean-ice-atmosphereenvironment.

It takes into account, for example, ocean mixing atvarious depths, the chemistry of mercury in the ocean and theatmosphere, and the mechanisms of atmospheric deposition andre-emission. When the Harvard team adapted it for their Arctic mercurysimulations, the only adjustment that could explain the spike insummertime concentrations was the incorporation of a large sourceto the Arctic Ocean from circumpolar rivers. This source had notbeen recognized previously. As it turns out, approximately twice as much mercury in the ArcticOcean originates from the rivers as from the atmosphere.

"At this point we can only speculate as to how the mercuryenters the river systems, but it appears that climate change mayplay a large role," says Jacob. "As global temperaturesrise, we begin to see areas of permafrost thawing and releasingmercury that was locked in the soil; we also see the hydrologicalcycle changing, increasing the amount of runoff from precipitationthat enters the rivers." "Another contributing factor," he adds, "could berunoff from gold, silver, and mercury mines in Siberia, which maybe polluting the water nearby. We know next to nothing about thesepollution sources." As the contaminated river water flows into the Arctic Ocean, Jacobsays, the surface layer of the ocean becomes supersaturated,leading to what scientists call an "evasion" of mercuryfrom the ocean into the lower atmosphere. "Observing that telltale supersaturation, and wanting toexplain it, is what initially motivated this study," saysFisher. "Relating it to Arctic rivers was detective work.

Theenvironmental implications of this finding are huge. It means, forexample, that climate change could have a very large impact onArctic mercury, larger than the impact of controlling emissions tothe atmosphere. More work is needed now to measure the mercurydischarged by rivers and to determine its origin." Fisher, Jacob, and Sunderland were joined on this work byco-authors Anne L. Soerensen, a research fellow at SEAS and HSPH;Helen M.

Amos, a graduate student in EPS; and Alexandra Steffen, anatmospheric mercury specialist at Environment Canada. The work was supported by the National Science Foundation's ArcticSystem Science Program.

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