The demise of older, thicker sea ice in the Arctic could have repercussions far beyond the steady decline in habitat essential for ringed seals, walruses and polar bears. A new NASA-led study has found that it could set in train a chemical process that results in more toxic mercury being deposited in the northern polar region.
Writing in a paper that has been accepted for publication in the Journal of Geophysical Research-Atmospheres, Son Nghiem of NASA's Jet Propulsion Laboratory and colleagues note that as thick, multiyear ice declines it is replaced by newer, seasonal and perennial ice, which is saltier than its more weathered kin. This is significant, notes a press release from the American Geophysical Union, which publishes the journal, because of the interaction between the salt in sea ice, frigid temperatures and sunlight:
When these mix, the salty ice releases bromine into the air and starts a cascade of chemical reactions called a "bromine explosion." These reactions rapidly create more molecules of bromine monoxide in the atmosphere. Bromine then reacts with a gaseous form of mercury, turning it into a pollutant that falls to Earth's surface.
The study began as a way to learn more about bromine explosions, which were first observed in the Canadian Arctic in the late 1980s, which have been observed during springtime in both polar regions, and which also deplete tropospheric ozone. The authors wanted to establish whether these explosions took place in the troposphere near the planet's surface, or higher up, in the stratosphere.
When satellite observations in 2008 detected increased bromine, Nghiem and colleagues used a combination of field measurements and atmospheric models to track whether it was transported by air currents south into the interior of Alaska and Canada. When it became clear that mountain ranges were blocking that transport, they had their answer about the explosion's location. "If the bromine explosion had been in the stratosphere, 5 miles (8 km) or higher above the ground, the mountains would not have been able to stop it and the bromine would have been transported inland," Nghiem says. They then calculated that the likely trigger was changes in sea ice salinity.
Air currents carry mercury into the Arctic atmosphere primarily from Asia, and particularly China, where emissions appear to be increasing as a result of industrial expansion and small-scale coal burning. Microorganisms can uptake the mercury and convert it into methylmercury, which is toxic and fat-soluble and accumulates in animals as it passes up the food chain. As a result, some human populations, such as the Inuit of Greenland and northeastern Canada, which eat high levels of marine mammals and fish, are exposed to levels of methylmercury in excess of World Health Organization guidelines.
Nghiem is leading an Arctic field campaign this month for NASA's Bromine, Ozone, and Mercury Experiment (BROMEX), which it is hoped will provide further insights into bromine explosions and their consequences, including for tropospheric ozone levels and mercury deposition.
IMAGE: Icebergs, and small bits of floating ice from icebergs that have disintegrated, near the Kangilerngata Sermia glacier, Disko Bugt (Disko Bay), West Greenland. (Jenny E. Ross, Corbis)