Permafrost thaw could spike global mercury, study suggests

Northern hemisphere permafrost holds at least as much, if not more, mercury than all the rest of the world’s soil, a new study finds. If the frozen soil layer, typically found at higher latitudes, thaws as climate scientists predict, global mercury levels could rise dramatically by 2100 (Geophys. Rev. Lett. 2018, DOI: 10.1002/2017GL075571).

When elemental mercury emitted by volcanoes, smokestacks, and other natural and anthropogenic sources lands on the ground, it bonds to organic matter in the soil, according to Paul Schuster of the U.S. Geological Survey, who led the study. Microbes can degrade that organic matter and release mercury back into the atmosphere and waterways, but in frozen soil, that process stops. Over time, sedimentation traps mercury deeper and deeper in permafrost. Now climate scientists estimate the Northern Hemisphere could lose between 30% and 99% of its permafrost by 2100.

To understand what that might mean for global mercury levels, Schuster and his colleagues drilled and analyzed sediment cores from 13 sites in Alaska. Based on the concentrations of mercury they found in the permafrost and the soil above it, they calculated that areas with the frozen layer hold about 1.7 million metric tons of mercury, with half of that in the permafrost itself. On the basis of nine previous estimates, the average global soil mercury level was thought to be about 450,000 metric tons, according to the researchers. Mercury itself does not pose a large danger to humans. It becomes most toxic in water-soluble forms such as methylmercury.

Schuster predicts that mercury from thawing permafrost could eventually end up in oceans and waterways, where it would increase baseline mercury levels and could affect fisheries. Thomas Douglas, a chemist with the U.S. Army Corps of Engineers, says the important next step for researchers is to understand how—and how much—mercury will move through ecosystems as permafrost thaws.

The research could change the way scientists think about soil mercury, according to Elsie Sunderland, an environmental scientist at Harvard University. “In the past, when we thought about the amounts of mercury in soils, we were focused on the fractions that are most likely to be mobilized to the atmosphere or oceans. Permafrost wasn’t really on the horizon,” she says. “That’s really changing.”