By Roberta Attanasio, IEAM Blog Editor
Greenland. Credit: Rene Schwietzke (CC BY 2.0).
Greenland, the largest island in the world not considered its own continent, lies above the Arctic Circle with the exception of its Southern tip. Ice—the Greenland Ice Sheet—is everywhere but is rapidly becoming a major contributor to sea level rise as it melts because of climate change. Now, a new study shows that the resulting freshwater (or meltwater) runs off to the ocean taking along an unexpected and toxic companion—mercury, a chemical that when transformed into methylmercury bioaccumulates and biomagnifies in fish, shellfish, and animals that eat fish, causing nervous system damage and other deleterious effects in humans and wildlife.
Although it is not clear where the meltwater mercury comes from, the study—which focuses on the Southwest Greenland meltwaters—indicates that environmentally high mercury concentrations persist in downstream fjords, the long, narrow, deep bodies of water created by glaciers. Thus, meltwater mercury has a potential impact on coastal food webs. Co-author Jemma Wadham said: “We’ve learned from many years of fieldwork at these sites in Western Greenland that glaciers export nutrients to the ocean, but the discovery that they may also carry potential toxins unveils a concerning dimension to how glaciers influence water quality and downstream communities, which may alter in a warming world and highlights the need for further investigation.”
Mercury is a chemical element that occurs naturally in the earth’s crust. However, human activities that include mining, fossil fuel combustion, and waste incineration have caused mercury pollution to become widespread at the global level. Mercury emissions generally disperse widely in the atmosphere and can travel long distances before being eventually deposited into water or onto land, where it can be washed into water. Once deposited, certain microorganisms can transform it into the highly toxic methylmercury form that builds up in fish, shellfish, and other animals that eat fish, causing nervous system damage. Because of bioaccumulation (higher concentrations in tissues of aquatic plants and animals than in water) and biomagnification (higher concentrations at increasingly higher levels in the food chain), across the world, most human exposure to mercury derives from eating fish and shellfish contaminated with methylmercury.
Fjords funnel glacial ice to the Atlantic Ocean along Greenland’s southeastern coast. Credit: NASA Goddard Space Flight Center (CC BY 2.0).
For the study, researchers sampled waters from three different rivers and two fjords next to the ice sheet. Mercury was one of the elements they measured, although they did not expect to find high concentrations of it. Instead, they found that dissolved mercury levels were much higher than the typical dissolved mercury content found in rivers.
Jon Hawkings, lead author of the study, said that the export of dissolved mercury from the region they studied—up to 42 tons per year, which is about ten percent of the estimated global riverine export of mercury to the oceans—must be considered globally significant. “This is among the highest concentrations of dissolved mercury ever measured in natural waters. And environmentally high levels persist in downstream fjords, posing a potential risk of accumulation in coastal food webs.”
These findings take on particular importance because Greenland’s waters support a rich marine ecosystem. Fishing is Greenland’s primary industry, and the country is a major exporter of cold-water shrimp, halibut, and cod.
Rob Spencer, senior author of the study, said: “For decades, scientists perceived glaciers as frozen blocks of water that had limited relevance to the Earth’s geochemical and biological processes, but we’ve shown over the past several years that line of thinking isn’t true. This study continues to highlight that these ice sheets are rich with elements of relevance to life.”
The researchers believe that the source of meltwater mercury is very likely Earth itself, and not anthropogenic activity such as fossil fuel combustion. It is likely that mercury ended up in the meltwater due to erosion of rocks by the moving glaciers. If further studies confirm their hypothesis, scientists and policymakers will need to adjust the management of global mercury pollution accordingly in the future, taking into account that mercury from climatically sensitive environments like glaciers could be much more difficult to manage.