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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.
Highlights
For thousands of years, carbon (C) and mercury (Hg) have been sequestered and “locked” in Arctic and subarctic permafrost soils
Warming of permafrost soils across the circumpolar north is expected to continue at an accelerated rate.[7]
Further deepening of the active layer and the degradation and collapse of palsas and peat plateaus is likely, given the already relatively warm soil temperatures of the area and the ongoing warming of the Arctic.[11,23−25] Here, we have studied the formation of Hg-methylation hotspots using existing thaw gradients at five Fennoscandian sites
Summary
Carbon (C) and mercury (Hg) have been sequestered and “locked” in Arctic and subarctic permafrost soils. Our data suggest net methylation in collapse fens (top 50 cm), representing relatively recently thawed conditions, to be an order of magnitude higher in comparison to net methylation in the active layer (top 50 cm) of intact peat plateaus These observations reveal a risk of increased MeHg exposure to human and wildlife upon permafrost thaw and support earlier work demonstrating the creation of Hg methylation hotspots.[19−21,43] Lehnherr et al.,[19] for instance, observed potential methylation rates of HgII in two thaw ponds which resemble the rates observed in more productive systems south of the permafrost region, such as temperate wetlands and lakes. Photos from study sites, geochemical analysis, analysis of HgT and MeHg, ground temperatures in Tavvavuoma, summary statistics (HgT, MeHg, % MeHg, RHgTC, and ancillary parameters in peat and mineral layer; peat soil classes and with peat depth), one-way ANOVA and Tukey’s post-hoc (comparing HgT, MeHg, % MeHg, and RHgTC between sites, comparing HgT, MeHg, % MeHg, and RHgTC and ancillary parameters between core classes), two-way ANOVA and Tukey’s post-hoc (comparing HgT, MeHg, % MeHg, and RHgTC and ancillary parameters between core classes and with depth), depth-distribution of HgT conc. by site, depthdistribution of % SOC by core class and by site, depthdistribution of all parameters for each site, depthdistribution of HgT concentration normalized to C by core class and by site, HgT as a function of % SOC (RHgTC), depth distribution of % MeHg across all sites and core classes, and discussion on the potential effect of sampling transportation (PDF)
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