Both atmospheric forces and the flow of circumpolar rivers carry mercury, a toxic element, north into the Arctic Ocean new research indicates. 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," study researcher Jenny Fisher, of Harvard University, said in a statement. "This is dangerous to both marine life and humans. The question from a scientific standpoint is: Where does that mercury come from?"

The study published May 20 in the journal Nature Geoscience.

Dirty mercury

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," study researcher Elsie Sunderland, also of Harvard, said in a statement. "It can cause long-term developmental delays in exposed children and impair cardiovascular health in adults."

Mercury remains in the environment without breaking down and 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."

"That's why these river sources are so important," Fisher said. "The mercury is going straight into the ocean."

River flows

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 percent of all freshwater discharge to the world's oceans.

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. The researchers modeled 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 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.

The rivers contribute about twice as much mercury to the Arctic Ocean than 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."