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.

Controls on methylmercury concentrations in lakes and streams of peatland‐rich catchments along a 1700 km permafrost gradient

“…Although the relationship between the abundance of the hgc A gene and in situ biological methylation is unclear, these results suggest that its potential activity may have been decreased at higher MeHg and SO 4 ‐S concentrations. As groundwater‐rich fens are abundant in the Interior Plains (Ecosystem Classification Group 2009; Quinton et al 2009), they are potentially important external MeHg sources for these lakes, as fens readily convey water in their channels (Quinton et al 2009) and have been previously identified as Hg II methylation hotspots in boreal sites (Gordon et al 2016; Fahnestock et al 2019; Poulin et al 2019; Tarbier et al 2021). Lakes that receive high DOM inputs from their surrounding catchment, as is typical for peatland lakes in the Interior Plains (Olefeldt et al 2013), have also previously been shown to have high MeHg concentrations but low rates of in situ methylation (Bravo et al 2017; Branfireun et al 2020).…”

“…Thus, certain regions in wetlands and lakes are often favorable locations for methylation (Mitchell et al 2008 a ; Branfireun et al 2020). In addition, permafrost thaw in peatlands often leads to land surface collapse, inundation, and increased groundwater interactions, creating Hg II methylation hotspots (Gordon et al 2016; Fahnestock et al 2019; Poulin et al 2019; Tarbier et al 2021).…”

“…Climate warming and permafrost thaw (Gibson et al 2021) will likely affect concentrations of neurotoxic methylmercury (MeHg) in northern peatland‐rich catchments (Gordon et al 2016; Tarbier et al 2021). Many northern boreal catchments have widespread peatlands with large stocks of atmospherically deposited mercury (Hg) bound to soil organic matter (Schuster et al 2018).…”

“…Many northern boreal catchments have widespread peatlands with large stocks of atmospherically deposited mercury (Hg) bound to soil organic matter (Schuster et al 2018). Favorable environmental conditions for MeHg production render peatlands as important sources of MeHg in boreal catchments (St. Louis et al 1994), but the presence of permafrost in peatlands may influence MeHg production, degradation, and transport through its influence on groundwater connectivity and soil environmental conditions (Gordon et al 2016; Poulin et al 2019; Tarbier et al 2021). Still, linkages between permafrost conditions and downstream MeHg delivery in northern catchments are poorly understood (Zolkos et al 2020).…”

Arctic methylmercury cycling

“…Remaining reservoirs (soil, snow, ice and air) were estimated from the stocks of Hg (Dastoor et al, 2022;Table 3.8.1 in AMAP (2021), key data for this review are also shown in Table S1) and the best estimates of %MeHg for each environmental compartment (Tables 1 and S1). The MeHg pool in surface soils was estimated by multiplying the total Hg pool by the median %MeHg in surface soils (St. Pierre et al, 2015: 0.82 %) and wetlands (Tarbier et al, 2021: 1.7 %, not impacted by recent permafrost thaw) and assuming a 25 % coverage of wetlands for the Arctic Region (Kåresdotter et al, 2021). Methylmercury reservoirs in snowpack and glaciers were estimated using %MeHg measurements for snow (6.1 %, range: 0.11-11 %; St.…”