Cracks in sea ice are funneling additional mercury to the Arctic surface, raising concerns about the toxic element seeping into the food chain of the delicate ecosystem, according to a new study.
The research, published yesterday in Nature, finds that channels of open water in Arctic ice, known as leads, are stirring up air so that mercury is pumped from higher in the atmosphere to air close to the surface. Warming temperatures are increasing the amount of seasonal sea ice that melts every summer, which in turn helps create the leads, said study lead author Christopher Moore, an assistant research professor at the Desert Research Institute.
"As more and more of that seasonal sea ice is around as the Arctic changes, then there is the potential that this mechanism can occur over a larger and larger area," said Moore. Environment Canada, the Desert Research Institute and NASA jointly funded the research.
He emphasized that the new study does not definitively make conclusions that additional mercury is getting deposited on snow or ice or entering the food chain. Much more research is needed to outline what is ultimately happening in the region, the scientists said.
Yet the findings suggest that could happen as more mercury is hovering at the surface. When converted to a toxic form, mercury can enter the food chain, threatening the food supplies of native Arctic peoples dependent on marine animals and wildlife.
"There's a lot of people who rely on hunting and fishing, and of course, the main exposure of humans and wildlife to mercury is through consumption of fish," said Daniel Obrist, a research professor at the Desert Research Institute and co-author of the paper. "We know the Arctic is affected by mercury pollution."
An accidental discovery
The research team discovered the increased mercury levels in the air serendipitously, as they originally were studying other atmospheric chemistry dynamics off the coast of Barrow, Alaska. After documenting increased levels of both mercury and ozone over the Chukchi Sea, they compared the measurements to satellite data of sea ice and models of air mass movements.
Because the sea ice leads are not permanent, and can open and close quickly in the ice over day intervals, the scientists were able to compare the mercury ups and downs with the formation of the cracks. Mercury levels would jump only when air masses moved over the ice leads, they found.
Because ozone witnessed similar fluctuations, the mercury had to have come to the surface from higher in the atmosphere, the scientists said. "Ozone isn't emitted from the seawater," noted Moore.
Because open water is much warmer than the air above it, heat from ice cracks stirs up the atmosphere much in the same way boiling water mixes up air above a stove, explained Obrist. "It's basically convection" that is helping pull the mercury down to the surface, he said.
For mercury to become toxic to the Arctic ecosystem, it typically undergoes a "depletion" event, where it is oxidized, removed from the atmosphere and deposited on snow or ice. A next step of research is trying to figure out the degree that is occurring, and where, said Obrist. There's a lot that's not known about the chemistry with the depletion process and how it interacts with climate change, added Moore.
Another study in Nature Geoscience this week, for example, found high levels of chlorine near the same site in Barrow, Alaska, but it's not known at this point whether climate change is driving that process, said study co-author Greg Huey, an atmospheric chemist at the Georgia Institute of Technology. Chlorine could help convert mercury so it is deposited from the air to snow or ice, he said.
Many unanswered questions
Scientists are trying to determine how much deposited mercury makes it into the Arctic Ocean, and how much of that is in turn changed to its toxic and bioavailable form, said Jenny Fisher, a postdoctoral fellow at Australia's University of Wollongong who did not participate in the study.
Mercury in the atmosphere comes from natural sources like volcanoes, as well as human activities like coal burning. According to a report from the Arctic Monitoring and Assessment Program, air currents can transport mercury to the Arctic from mid-latitudes in a matter of days.
The research comes at a "turning point," as 94 countries recently signed the Minamata Convention, a global treaty curbing mercury pollution, said Son Nghiem, a NASA scientist in a statement.
Joel Blum, a professor at the University of Michigan who did not participate in the new paper, praised the quality of the data but emphasized that people should not yet be alarmed by the findings. There is so much that is not understood about Arctic mercury, and there are offsetting forces that can reduce levels, he said. Recent studies on the issue have produced mixed results, he said.
Several years ago, for instance, scientists documented elevated levels of mercury in snow, causing worrisome headlines, noted Blum. Since then, studies have reported that much of the mercury that gets deposited on snow and ice is re-emitted into the atmosphere, he said.
Another recent study indicated that declining sea ice can lower toxic levels of methylmercury, which is sensitive to sunlight, he noted. One theory is that sea ice is preventing the breakdown of mercury by blocking sunlight.
"It is an ongoing quest to figure out what's going on," Blum said.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500