Daniel Obrist reports that geoscientists have long understood that the Arctic is affected by mercury pollution, but know less about how it happens. Remote, cold and seemingly pristine, why is such an idyllic landscape so contaminated with this highly toxic metal? He recently returned from a two-year research project in Alaska, where he led field research into this issue alongside fellow scientists from the University of Colorado; the University of Nevada's Desert Research Institute; the University of Toulouse and the Sorbonne University in France; and the Gas Technology Institute in Illinois.
Their work was the most comprehensive investigation to date of how mercury is deposited to the Arctic tundra, a vast northern ecosystem surrounding the Arctic Ocean. Our findings show that the gaseous form of mercury – emitted by coal-burning, mining and other industrial processes in the industrialized world – is being lofted into the region from thousands of miles away. In the Arctic, it is deposited onto tundra soils and ultimately runs off into ocean waters, threatening the region's wildlife and people.
Industrialized and developing nations emit about 2,000 tons of mercury into the atmosphere every year. Globally, the largest sources include emissions from small-scale and artisanal gold mining and coal-burning power plants. Mercury emissions from human activities take several forms that behave differently in the atmosphere Oxidized mercury, notated as Hg(II), generally settles or is rained out of the atmosphere close to emission sources. In contrast, gaseous elemental mercury, or Hg(0), remains in the atmosphere for a long time and can travel around the globe.
In the Arctic, high levels of mercury are found in beluga whales, polar bears, seals, fish, eagles and other birds. This means that humans are also affected, particularly the Inuit, who rely on traditional hunting and fishing for food. Exposure to high levels of mercury over long periods can lead to neurological and cardiovascular problems. Scientists have been working for more than two decades to determine how mercury makes its way from industrialized countries to the Arctic.
From their observation site north of Alaska's Brooks Range, we determined that gaseous elemental mercury in the atmosphere is the dominant source of Arctic mercury. We calculated that it accounted for 70 percent of the mercury that finds its way into tundra soil. Previous studies focused mainly on deposition of airborne oxidized mercury. However, we estimate that oxidized mercury accounts for less than one-third of mercury deposition, and deposition by rain and snow for only 2 percent.
The next question is how gaseous mercury falls to the ground. We found that plants absorb it from the atmosphere in their leaves, much as they take up carbon dioxide. Then, when the plants shed leaves or die, the mercury they contain is deposited in soil.
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