High Concentrations of Toxic Mercury in the Arctic from Circumpolar Rivers
Environmental scientists have known that high levels of the toxic element, mercury, have been accumulating in the Arctic Ocean for some time. It was believed to be mostly caused by atmospheric sources stemming from the combustion of coal. However, a new study from the Harvard School of Engineering and Applied Sciences and the Harvard School of Public Health has found that the great majority of Arctic mercury arrives via circumpolar rivers. Some of the largest rivers in the world flow north into the Arctic in Eurasia and North America.
The implications of this finding are significant for predicting future levels of the toxic heavy metal. The levels in the Artic will likely be increasing from the thawing of Arctic soils, releasing mercury into the hydrological cycle.
"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 at Harvard. "This is dangerous to both marine life and humans."
Mercury is a naturally occurring element that has been enriched in the environment by human activities. It is considered a persistent bioaccumulative toxin because it does not break down in the environment. It travels through the food chain from the lowly plankton all the way up to humans. As it travels up, it becomes more concentrated and lethal. The greatest concern is for the indigenous people of the Arctic who consume large amounts of locally-caught fish and marine mammals.
Through combustion, mercury is emitted into the atmosphere. Once chemical processes make it soluble, it falls back to Earth as rain or show. Some fall directly into the Arctic Ocean, but most fall on the large watersheds of the far north.
The largest rivers flowing into the Arctic are in Siberia: the Lena, the Ob, and Yenisei. They are three of the ten largest rivers in the world and account for 10% of all freshwater discharge into the world's oceans. Other major rivers include the Kolyma in eastern Siberia, the Mackenzie in Canada, and the Yukon in Canada and Alaska.
The Harvard researchers found that rivers contributed more than double the mercury than the atmosphere by noticing a spike in mercury concentrations during the summer. This could only be explained by the increased river flow from circumpolar melting.
"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 co-author, Daniel 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 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."
This study has been published in the journal Nature Geoscience
Image credit: Greg Fiske, WHRC