Environmental scientists at Harvard have discovered that the Arctic accumulation of mercury, a toxic element, is caused by both atmospheric forces and the flow of circumpolar（极地附近的） rivers that carry the element north into the Arctic Ocean. 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（水文学的） 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," 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?"

The results of the study, which was led jointly by Harvard School of Engineering and Applied Sciences (SEAS) and Harvard School of Public Health (HSPH), appeared in the journal Nature Geoscience on May 20.

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（甲基水银） 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," 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 is considered a persistent bioaccumulative toxin because it remains in the environment without breaking down; as it travels up the food chain, from plankton（浮游生物） 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."