Thawing of Arctic permafrost, the largest long-term reservoir of mercury (Hg), is accelerating the release of this toxic element to the environment. Remobilization as a result of global warming trends can result in physicochemical transformations of Hg with adverse impacts for aquatic and terrestrial ecosystems. To assess possible consequences, researchers studied Hg reallocation from previously frozen soils into surrounding environments Schematic of biogeochemical Hg cycling across permafrost thaw. Red arrows denote gaseous flux for each stage of the thaw sequence. Source: M.F. Fahnestock et al.north of the Arctic Circle in Abisko, Sweden.

Three different landscapes were examined: palsa, or frozen permafrost, semi-thawed bog areas and fens characterized by flowing water and thawed peat. Analysis of air, water and soil samples revealed the highest methylmercury concentrations in fens, leading to a revised picture of Hg cycling in these regions.

Export of total gaseous Hg appears to be a primary pathway of Hg loss during initial stages of permafrost thaw. Vegetation evolves to form Sphagnum-dominated ecosystems in which Hg export into the atmosphere limits its accumulation in peat. The appearance of fully thawed fens in the last stage of thaw results in a more biologically mobile Hg pool spurred by favorable redox conditions and a diverse microbial community capable of methylation. Under these conditions, fen peat retains more Hg and methylmercury export to lakes and streams increases, posing biomagnification and food chain hazards.

Researchers from the University of New Hampshire, Rochester Institute of Technology, Syracuse University, The Ohio State University and Stockholm University contributed to this study, which is published in Geochemical Perspectives Letters.