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Circumpolar rivers responsible for Arctic mercury-- says study

Environmental scientists at Harvard have observed that the Arctic accumulation of mercury, a toxic element, is caused by both atmospheric forces and the flow of rivers-- the Lena, Yenisei and Ob that carry the element north into the Arctic Ocean.

"The Arctic is a unique environment because it's so remote from most human instigated 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?"

Lena, Yenisei and Ob are among ten 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. 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 natives are more vulnerable to the effects of methyl-mercury exposure because they consume large amounts of fish and marine mammals as part of their traditional diet.

Mercury being a naturally occurring element that has been enriched in the environment by human activities such as coal combustion and mining. When converted to methyl-mercury 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 and become more dangerous.

"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."

"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."

The study was published in the journal Nature Geoscience.