Permafrost thaw remobilizes mercury from northern Scandinavian peatlands

Abstract. With the ongoing warming of the Arctic, mercury (Hg) stored in permafrost peatlands is at risk of being remobilized with permafrost thaw due to ground subsidence, peat inundation and enhanced decomposition of organic matter. Once released, Hg may enter the pool available for microbial Hg methylation to the neurotoxic methylmercury (MeHg), which bioaccumulates in food webs and poses health concerns to northern communities and wildlife. Whether and to what extent Hg is remobilized from northern permafrost peatlands following thaw, however, remains poorly understood. Here, we collected 93 peat cores from five permafrost peatlands in Norway and Sweden from landforms representing intact and thawed conditions (peat plateaus/palsas and fens, respectively) to investigate Hg dynamics in sub-Arctic thawing permafrost peatlands. We found consistently lower Hg stocks in thawed features across all investigated sites, with median values of 5 mg Hg m⁻² in fens and 10 mg Hg m⁻² in peat plateaus. The lower stocks of Hg were a result of both lower concentrations of Hg in surface peat as well as overall lower peat mass of the fens, suggesting both organic matter degradation and Hg leaching from peat to play a role for Hg remobilization after thaw. Our study highlights the importance of accounting for landscape heterogeneity when studying Hg dynamics in permafrost peatlands. Hg concentrations and dry bulk density exhibited moderate variability, whereas carbon content remained relatively consistent throughout the peat profile. Given the landscape variabilities demonstrated with our data (e.g. peat depths), the large number of peat cores and within-core Hg measurements in this study supports our conclusion that Hg is lost from the peat matrix upon thaw. From Swedish and Norwegian permafrost peatlands alone (area of 130 km²), we estimate 5.6 kg of Hg to be remobilized annually. Should permafrost disappear entirely as projected from the region until the end of the century, up to 0.7 Mg of Hg could potentially be released into the environment. Our study demonstrates that permafrost thaw is driving substantial Hg remobilization and highlights that especially vulnerable regions such as the permafrost peatlands in Scandinavia could become an important Hg source to the Arctic Hg-cycle in the coming decades.