As a result of ongoing climate change, temperatures in the Arctic are rising at twice the rate of the global mean. Consequences from this rapid warming include the thawing of permafrost and subsequent release of substantial stocks of carbon and environmental pollutants, like the heavy metal mercury (Hg), from sub-arctic permafrost peatlands across the circumpolar north. The pan-Arctic permafrost region Hg pool has recently been suggested to represent a globally significant Hg reservoir. The remobilization of Hg within the environment poses a particular risk to Arctic wildlife and human residents. In the low-oxygen environments common to warming peat plateau complexes, where raised plateaus subside following permafrost thaw to form thermokarst collapse scars, inorganic Hg is readily converted to the organic form, methylmercury (MeHg), through microbial methylation. MeHg is a potent neurotoxin that bio-accumulates relatively easily in wildlife, especially higher trophic level species, and from there to humans through fish and marine mammal consumption.

In order to elucidate the impact of changing permafrost conditions on mercury mobilization in the environment, 14 cores representing three different landscape features common to Fennoscandian sub-arctic peatlands were analyzed from five sites in northern Sweden and Norway. The distribution with depth in the peat profile of total Hg (THg, meaning inorganic and organic forms of Hg) and MeHg concentrations, the fraction of THg converted to MeHg, and percent soil organic carbon (SOC) were examined. These parameters were compared between permafrost-elevated peat plateaus, post-thaw collapse fens, and intact fens unaffected by permafrost in recent history. As a result of this analysis, the ratio between THg and SOC that has been used to estimate THg storage in permafrost soils was determined to be lower in organic soils as compared to mineral soils, and lower generally, at 0.09 ± 0.08 µg THg/g C, than recent studies have suggested. The estimated THg pool of permafrost regions should thus likely be reduced. Continued warming may increase the likelihood of Hg poisoning in the Arctic, as MeHg production was found to be highest, across the landscape feature classes of this investigation, following permafrost thaw and collapse fen formation. Further study is warranted to quantify MeHg export from permafrost peatlands and the associated risk posed to Arctic inhabitants.