May 23, 2012
Circumpolar Rivers Most Responsible For High Levels Of Mercury In The Arctic
Brett Smith for RedOrbit.com
If global warming trends are to continue, thermometers across the globe aren´t the only place that mercury levels will be on the rise.
Both atmospheric forces and the flow of circumpolar rivers carry the toxic metal north into the Arctic Ocean, but it turns out that the amount of mercury streaming into Arctic ecosystems via rivers is double that being deposited through atmospheric means.
“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 Department of Earth and Planetary Sciences. “This is dangerous to both marine life and humans. The question from a scientific standpoint is, where does that mercury come from?”
Mercury is naturally occurring in the environment in extremely low quantities. It can be released into the atmosphere by volcanic eruptions, which are responsible for half of all atmospheric mercury emissions. Human activities have raised the environmental levels considerably, with coal combustion accounting for about 65 percent of artificial mercury emission.
Mercury can drift in the atmosphere for about a year before being deposited worldwide. Much of the mercury that ends up in Arctic soil, snow and ice reenters the atmosphere, which limits the metal´s impact on the Arctic Ocean.
Previous studies found that the level of mercury in the Arctic´s lower atmosphere changed with the seasons, increasing sharply from spring to summer. The research team used complex computer models based on factors like melting ice, microbe activity, and the air-ice-ocean interactions to investigate the source of the increased levels.
The team found that the only way to explain the variability was through the inclusion of a large source of mercury from circumpolar rivers. They estimated nearly twice as much mercury is entering the Arctic through rivers as from the atmosphere.
"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," said Daniel Jacob, a professor at the Harvard School of Engineering and Applied Sciences (SEAS).
"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."
Mercury is considered a biotoxin that can be poisonous if it reaches certain levels in an organism. Fish and shellfish have a natural tendency to concentrate mercury within their bodies. This mercury is then passed up the food chain are these animals are eaten.
"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," said principal investigator Elsie M. Sunderland.
"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."
The results of the study appeared in the journal Nature Geoscience on May 20.
Image 2 (below): While the atmospheric source of mercury in the Arctic was previously recognized, it now appears that twice as much mercury actually comes from the rivers. Credit: U.S. Fish & Wildlife Service