Researchers at Harvard have found that the accumulation of toxic mercury in the Arctic is being caused both by atmospheric processes and by circumpolar rivers depositing it there.
The atmospheric source has been known for some time, but it turns out twice as much mercury is being brought into Arctic ecosystems by the flow of rivers as from atmospheric deposition.
This finding suggests that, as climate change continues, the concentration of mercury in the Arctic will rise dramatically, primarily from changes in the hydrological cycle and warming of the Arctic soil.
“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?”
Mercury is naturally occurring in the environment in extremely low quantities, but human activities, such as coal combustion and mining, have raised the environmental levels considerably. When it’s converted to methyl mercury by microbial processes, it can then accumulate in fish and wildlife in levels up to a million times higher than those 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, also, simply doesn’t break down. It will remain in the environment continuing to increase in quantity and accumulating in the food chain.
“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.”
Mercury enters the atmosphere through emissions from coal combustion, waste incineration, and mining. Once there, it drifts around for a year or so, until chemical processes make it soluble. It then falls to the earth in rainfall or snow. This happens worldwide, regardless of where the emissions are originally from.
A lot of the mercury deposited in the Arctic snow and ice though, is later re-emitted into the atmosphere, limiting its 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 main rivers flowing into the Arctic Ocean are all in Siberia: the Lena, the Ob, and the Yenesei. They are three of the ten largest rivers in the world, together accounting for 10% of the freshwater discharges into the world’s oceans. And the Arctic Ocean is shallow and heavily stratified, furthering the impact of the rivers.
Previously it had been found that the level of mercury in the Arctic lower atmosphere varied seasonally, increasing sharply from the spring to summer. The research team used different computer models to investigate whether that variability was from melting ice, increased microbial activity, or sunlight.
None of those could account for the difference though. The only way to explain the variability was through the inclusion of a large source from circumpolar rivers. As it currently appears, 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,” 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 river water flows into the Arctic Ocean it supersaturates the top layer of water, leaving a clearly recognized signature.
“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.”
Source: Harvard University
Image Credits: NASA, U.S. Fish and Wildlife Services