Methylmercury (MeHg) forms in anoxic environments and can bioaccumulate and biomagnify in aquatic food webs to concentrations of concern for human and wildlife health. Mercury (Hg) pollution in the Arctic environment may worsen as these areas warm and Hg, currently locked in permafrost soils, is remobilized. One of the main concerns is the development of Hg methylation hotspots in the terrestrial environment due to thermokarst formation. The extent to which net methylation of Hg is enhanced upon thaw is, however, largely unknown. Here, we have studied the formation of Hg methylation hotspots using existing thaw gradients at five Fennoscandian permafrost peatland sites. Total Hg (HgT) and MeHg concentrations were analyzed in 178 soil samples from 14 peat cores. We observed 10 times higher concentrations of MeHg and 13 times higher %MeHg in the collapse fen (representing thawed conditions) as compared to the peat plateau (representing frozen conditions). This suggests significantly greater net methylation of Hg when thermokarst wetlands are formed. In addition, we report HgT to soil organic carbon ratios representative of Fennoscandian permafrost peatlands (median and interquartile range of 0.09 ± 0.07 μg HgT g –1 C) that are of value for future estimates of circumpolar HgT stocks.

<p>With ongoing climate change, temperatures in the northern latitudes are increasing more than twice as fast as the global mean. This causes thawing of permafrost and the release of carbon and contaminants, including mercury (Hg), which have thus far been immobilized in the frozen soil. The potential release of Hg, and microbial transformation of mobilized inorganic Hg to monomethylmercury (MeHg), presents a risk to ecosystems and human health. MeHg is a neurotoxic substance that is readily taken up and biomagnified in aquatic food webs to dangerous concentrations. Arctic communities are particularly vulnerable to Hg pollution as a result of a diet that often includes high trophic level fish and marine mammals. Despite the ecological and societal consequences of elevated Hg levels and the potential for increased Hg conversion to MeHg in post-thaw wetland environments, much of the Hg cycle in the high North is poorly understood.</p><p>While global and northern latitude Hg budgets have been estimated, the effect of permafrost thaw on MeHg formation has not yet been fully investigated. Here, we compared concentrations of total Hg (HgT) and MeHg in intact permafrost samples from palsas and peat plateaus with samples from recently thawed collapse fens and from peatlands unaffected by permafrost dynamics in order to investigate whether permafrost thaw impacts net MeHg formation in peatlands. Our study includes five subarctic permafrost peatland sites located in northern Sweden and Norway. Concentrations of HgT and MeHg in the soil cores ranges from 1.1 to 210 and 0.005 to 28 ng g-1 dry weight, respectively, with higher concentrations in the upper soil horizons. No differences were observed in average HgT and MeHg concentrations between the five sites, including both coastal and inland locations. Interestingly, we observe higher concentrations of MeHg and MeHg:HgT ratios in the collapse fens as compared to the permafrost cores, showing increased net methylation of Hg upon permafrost thaw.</p>