Mercury in permafrost

Arctic permafrost is thawing. By the end of this century, the amount of year-round frozen soil could be reduced by 30–99%. That’s important because microbial activity, which steadily breaks down dead organic matter in other soils, is effectively halted in permafrost. But when the permafrost thaws, it could resume. Motivated by the need to understand how a sudden thaw could affect global greenhouse gas levels, researchers have been compiling maps and databases of just how much carbon is stored in permafrost soils.

Now the US Geological Survey’s Paul Schuster and colleagues have piggybacked on that work to look at the related question of how permafrost regions store mercury. Mercury enters the soil through plants—by binding to cellular receptors in place of nutrients such as iron or magnesium—so where there’s plant matter, there’s also mercury. The researchers drilled soil cores from 13 geologically diverse permafrost sites in northern Alaska—including the one in the photo—and measured their mercury and carbon contents. The mercury–carbon ratio naturally varied from sample to sample, but it was consistent enough, at 1.6 ± 0.9 µg of mercury per gram of carbon, that the researchers could use published soil carbon maps to derive an estimate of how much mercury is contained in Northern Hemisphere permafrost regions.

That estimate turned out to be huge: 1656 ± 962 kilotons, split roughly equally between the permafrost itself and the seasonally thawing active layer that overlies it. In comparison, all other soils on Earth store a total of some 450 kilotons of mercury, and the ocean holds some 350 kilotons.

But merely knowing how much mercury is stored and vulnerable to release doesn’t answer the question of where it goes when it’s liberated. Mercury circulates slowly through the environment, mostly in its relatively benign elemental and inorganic forms rather than as methylmercury, the organic neurotoxin that accumulates in fish. How harmful the newly released mercury is to humans and wildlife depends on the subtle interplay of biological, geological, and chemical processes. (P. F. Schuster, Geophys. Res. Lett., in press, doi:10.1002/2017GL075571 ; photo by Seth Spawn.)