Concentrations of mercury in the environment have been increased by human activities such as mining, the use of coal as fuel, and incineration of waste but, as well as the previously recognised atmospheric sources of the element, scientists have now found that rivers around the North Pole contribute to the high levels of mercury in the Arctic Ocean.
A new study – published in Nature Geoscience
and led by researchers from the Harvard School of Engineering and Applied Sciences (SEAS) and the Harvard School of Public Health (HSPH) – has shown that rivers flowing into the circumpolar region are responsible for twice the amount of mercury that the atmosphere is.
"In humans, mercury is a potent neurotoxin," explained co-lead author Elsie M. Sunderland, Assistant Professor of Aquatic Science at HSPH. "It can cause long-term developmental delays in exposed children and impair cardiovascular health in adults."
Mercury is classified as a persistent biocumulative toxin. When microbial processes in the sea convert it to methylmercury, it can accumulate in the bodies of fish, mammals and birds at levels a million times higher than in the environment. The further up the food chain it goes, the more concentrated it becomes.
"The Arctic is a unique environment because it's so remote from most anthropogenic (human-influenced) sources of mercury, yet we know that the concentrations of mercury in Arctic marine mammals are among the highest in the world," commented lead author Jenny A. Fisher, from Harvard's Atmospheric Chemistry Modeling Group and the Department of Earth and Planetary Sciences (EPS). "This is dangerous to both marine life and humans. The question from a scientific standpoint is, where does that mercury come from?"
Using an advanced modeling system known as GEOS-Chem, the team found that the only adjustment that could explain the dramatic increase in lower atmosphere mercury levels during the summer months was the addition of a considerable source from circumpolar rivers.
The largest rivers flowing into the Arctic are the Lena, the Ob and the Yenisei in Siberia. Together they contribute almost ten per cent of the total volume of water discharged into the world’s oceans.
While airborne mercury can remain so for up to year before falling in rain or snow, this precipitation is distributed worldwide and often when the mercury falls in snow on the Arctic it is re-released back into the atmosphere. When river water flows into the ocean, however, the surface becomes super-saturated and mercury is transferred to the lower atmosphere.
"That's why these river sources are so important," added Fisher. "The mercury is going straight into the ocean."
"At this point we can only speculate as to how the mercury enters the river systems, but it appears that climate change may play a large role," remarked Daniel Jacob, Professor of Atmospheric Chemistry and Environmental Engineering at SEAS.
Another potential factor is run-off from gold, silver and mercury mines. The findings indicate that the problem could get much worse as increasing global temperatures see areas of permafrost thawing and levels of precipitation increasing. There are particular concerns for inhabitants of the Far North.
"Indigenous people in the Arctic are particularly susceptible to the effects of methylmercury exposure because they consume large amounts of fish and marine mammals as part of their traditional diet," Sunderland said. "Understanding the sources of mercury to the Arctic Ocean and how these levels are expected to change in the future is therefore key to protecting the health of northern populations."