Cracked sea ice & Arctic mercury

Cloud plumes rise from cracks of open water in the Arctic Sea ice cover.

VIGOROUS mixing in the air above large cracks in the Arctic Sea ice, which exposes seawater to cold polar air, pumps atmospheric mercury down to the surface, a field campaign by the National Aeronautics and Space Administration (NASA) of the United States has found. This process can lead to more of the toxic pollutant entering the food chain, which can negatively affect the health of fish and the animals who eat them, including humans.

Following the cracking of sea ice off the coast of Barrow, Alaska, which created open seawater channels called leads, scientists found that concentrations of mercury near the ground level had increased. The researchers were in the Arctic for the NASA-led Bromine, Ozone, and Mercury Experiment (BROMEX) in 2012.

“None of us had suspected that we would find this kind of process associated with leads,” said Son Nghiem of NASA’s Jet Propulsion Laboratory. Nghiem is the BROMEX principal investigator. He and other co-authors published the findings in the journal Nature on January 15.

The mercury-pumping reaction takes place because open water in a lead is much warmer than the air above it. Because of that temperature difference, the air above the lead churns like the air above a boiling pot. The mixing is so strong that it actually pulls down mercury from a higher layer of the atmosphere to near the surface. The mixing, marked by dense clouds spewing out of the leads, extends up into the atmosphere for about 400 metres. The scientists think that this may be the height where the mercury pumping occurs.

Almost all of the mercury in the Arctic atmosphere is transported there in gaseous form from sources in areas farther south, from sources such as wildfires, coal burning and gold mining. Scientists have long known that mercury in the air near ground level undergoes complex chemical reactions that deposit the element on the surface. Once the mercury is completely removed from the air, these reactions stop. However, this newly discovered mixing ice forces down additional mercury to restart and sustain the reactions.

Leads have become more widespread across the Arctic Ocean as climate change has reduced the Arctic Sea ice cover. “Over the past decade, we’ve been seeing more new sea ice rather than perennial ice that has survived for several years. New ice is thinner and saltier and cracks more easily. More new ice means more leads as well,” said Nghiem.

To understand the effects of the leads, the team took ground-based measurements of mercury and other chemical species over the frozen Chukchi Sea and over snow-covered land. They used images from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite to observe sea ice and a National Oceanic and Atmospheric Administration (NOAA) model of air transport to gain insights into what was upwind of their mercury measurements.

Nghiem points out that this new finding has come at a turning point for action on Arctic mercury pollution. The Minamata Convention, a global treaty to curb mercury pollution in which Arctic vulnerability is particularly noted, has been signed by 94 nations since it was opened for signatures in October 2013.