Harvard study finds circumpolar rivers most responsible for high levels of mercury in the Arctic.
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| The Lena River delta. The Lena is one of several major rivers that flows northward into the Arctic Ocean. |
Cambridge, Mass. - May 21, 2012 - Environmental
scientists at Harvard have discovered that the Arctic accumulation of
mercury, a toxic element, is caused by both atmospheric forces and the
flow of circumpolar rivers that carry the element north into the Arctic
Ocean.
While the atmospheric source was previously recognized, it now
appears that twice as much mercury actually comes from the rivers.
The revelation implies that concentrations of the toxin may further
increase as climate change continues to modify the region's hydrological
cycle and release mercury from warming Arctic soils.
"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," says lead author Jenny A. Fisher, a postdoctoral
fellow in 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?"
The results of the study, which was led jointly by Harvard School of
Engineering and Applied Sciences (SEAS) and Harvard School of Public
Health (HSPH), appeared in the journal Nature Geoscience on May 20.
Mercury is a naturally occurring element that has been enriched in the
environment by human activities such as coal combustion and mining. When
converted to methylmercury by microbial processes in the
ocean, it can accumulate in fish and wildlife at concentrations up to a
million times higher than the levels found in the environment.
"In humans, mercury is a potent neurotoxin," explains co-principal
investigator Elsie M. Sunderland, Mark and
Catherine Winkler 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 considered a persistent bioaccumulative toxin because it
remains in the environment without breaking down; as it travels up the
food chain, from plankton to fish, to marine mammals and humans, it
becomes more concentrated and more dangerous.
"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
says. "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."
Sunderland supervised the study with Daniel Jacob, Vasco McCoy Family Professor
of Atmospheric Chemistry
and Environmental Engineering
at SEAS, where Sunderland is also an affiliate.
Mercury enters the Earth's atmosphere through emissions from coal
combustion, waste incineration, and mining. Once airborne, it can drift
in the atmosphere for up to a year, until chemical processes make it
soluble and it falls back to the ground in rain or snow. This deposition
is spread worldwide, and much of the mercury deposited 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," says Fisher. "The mercury is going straight into the ocean."
The most important rivers flowing to the Arctic Ocean are in Siberia:
the Lena, the Ob, and the Yenisei. These are three of the 10 largest
rivers in the world, and together they account for 10% of all freshwater
discharge to the world's oceans. The Arctic Ocean is shallow and
stratified, which increases its sensitivity to input from rivers.
Previous measurements had shown that the levels of mercury in the
Arctic lower atmosphere fluctuate over the course of a year, increasing
sharply from spring to summer. Jacob, Sunderland, and their team used a
sophisticated model (GEOS-Chem)
of the conditions in the Arctic Ocean and atmosphere to investigate
whether variables like melting ice, interactions with microbes, or the
amount of sunlight (which affects chemical reactions) could account for
the difference.
Incorporating those variables, however, was not enough.
The GEOS-Chem model, which is backed by rigorous environmental
observations and more than a decade of scientific review, quantifies the
complex nuances of the ocean-ice-atmosphere environment. It takes into
account, for example, ocean mixing at various depths, the chemistry of
mercury in the ocean and the atmosphere, and the mechanisms of
atmospheric deposition and re-emission.
When the Harvard team adapted it for their Arctic mercury
simulations, the only adjustment that could explain the spike in
summertime concentrations was the incorporation of a large source to the
Arctic Ocean from circumpolar rivers. This source had not been
recognized previously.
As it turns out, approximately twice as much mercury in the Arctic Ocean originates from the rivers as from the atmosphere.
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| The researchers' new model describes the known inputs and outputs of mercury to the Arctic Ocean. (Image courtesy of Jenny Fisher.) |
"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," says Jacob. "As global temperatures rise, we begin to see areas
of permafrost thawing and releasing mercury that was locked in the soil;
we also see the hydrological cycle changing, increasing the amount of
runoff from precipitation that enters the rivers."
"Another contributing factor," he adds, "could be runoff from gold,
silver, and mercury mines in Siberia, which may be polluting the water
nearby. We know next to nothing about these pollution sources."
As the contaminated river water flows into the Arctic Ocean, Jacob
says, the surface layer of the ocean becomes supersaturated, leading to
what scientists call an "evasion" of mercury from the ocean into the
lower atmosphere.
"Observing that telltale supersaturation, and wanting to explain it,
is what initially motivated this study," says Fisher. "Relating it to
Arctic rivers was detective work. The environmental implications of this
finding are huge. It means, for example, that climate change could have
a very large impact on Arctic mercury, larger than the impact of
controlling emissions to the atmosphere. More work is needed now to
measure the mercury discharged by rivers and to determine its origin."
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