Permafrost Thaw Increases Methylmercury Formation in Subarctic Fennoscandia

ABSTRACT: 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.