With the rise in global temperatures, northern peatlands could release increasing amounts of mercury into the atmosphere. Such was the conclusion made by researchers from Macalester College, the University of Minnesota and the United States Forest Service, who, for the first time ever, measured gaseous mercury emissions in a full-scale experiment with an artificially warmed ecosystem conducted over a year. The experiment showed that elevated carbon dioxide concentrations, for instance, have virtually no effect on mercury emissions.
Mercury is one of the most dangerous environmental pollutants. Once released into the atmosphere, it can be transported over long distances and deposited back on land and in water bodies. Methylmercury, a compound that accumulates in fish and other organisms, gradually moving up the food chain and affecting the nervous systems of animals and humans, is especially dangerous. Northern peatlands, which are widespread in Canada, Scandinavia and Russia, serve as one of the largest natural repositories of mercury accumulated over the past centuries. However, it remained unclear until now whether these mercury reserves would be released as the climate warms.
In order to find an answer to this question, the scientists used the SPRUCE experimental site in the U.S. state of Minnesota. The site contains ten large open chambers where air and soil temperatures are artificially maintained at levels between 2.25°C and 9°C above ambient. In half of the chambers, CO₂ concentrations were further increased to about 900 ppm to simulate conditions anticipated by the end of the 21st century. Gaseous mercury was measured via passive monitors installed approximately one meter above the peatland surface. Using data on air exchange in the chambers, the researchers then calculated how much mercury was emitted or absorbed per square meter of surface area. Observations continued from December 2020 through November 2021.
In the course of the experiment, the air temperature in the chambers varied from –4.6°C to 30.4°C, and mercury fluxes ranged from absorbing some 4 nanograms per square meter per hour to emitting 21 nanograms per square meter per hour. Analysis revealed a clear relationship between temperatures and emission rates. On average, a 1°C increase in air temperature led to a rise in mercury flux of about 0.6 nanograms per square meter per hour during the warm season and 0.9 nanograms in winter. For soils at a depth of 10 cm, the impact proved even stronger: in winter, every additional degree increased emissions by some 2.3 nanograms per square meter per hour.
However, elevated carbon dioxide levels did not have a statistically significant effect on emissions. The scientists believe that the main cause might be the increased activity of soil microorganisms, which convert mercury compounds into a volatile form more quickly at higher temperatures.
Another unexpected result was that the peatlands emitted mercury almost as actively in winter as they did in summer. The researchers attribute this to the fact that plants essentially stop absorbing gaseous mercury during the cold season, whereas microorganisms continue to work even at near-zero temperatures. This means that winter warming, which occurs faster than summer warming in northern latitudes, could have an especially strong impact on future mercury emissions.
During the next stage, the researchers plan to look into the role of microorganisms, vegetation and the water regime of the peatlands in these processes. Ultimately, they hope to find ways to conserve the peatlands in order to keep the mercury bound in the soil instead of releasing it back into the atmosphere.
