Mercury Accumulation in Permafrost Peatlands in Relation to Peat Accumulation and Permafrost History

Northern peatlands act as a globally important sink for atmospheric mercury (Hg). However, the environmental drivers that control long-term Hg accumulation rates (Hg-AR) in permafrost peatland systems remain understudied. With the aim to shed light on the influence of peat accumulation conditions and permafrost history on Hg accumulation, we have investigated Hg profiles in nine peat cores, which have previously been characterised using plant macrofossil analyses to reconstruct peat accumulation history. Across all cores, originating from sites in west-central Canada (n = 4) and northern Scandinavia (n = 5), highest Hg concentrations were found close to the peat surface with a maximum around 220 (µg Hg kg-1). Vertical profiles of Hg across successions of peat types in the cores revealed peaks of Hg concentrations in layers typical of drier surface conditions and slower peat accumulation. Calculated long-term Hg-ARs ranged from 0.3 to 2.6 µg Hg m-2 yr-1 between sites and were closely related to long-term carbon accumulation rates (C-AR). Our study shows that the net C-AR controls Hg-AR in Arctic permafrost peatlands. We further demonstrate that epigenetic permafrost twice as much Hg accumulated for the same amount of C, likely due to Hg enrichment during long-term peat decomposition in epigenetic as opposed to syngenetic permafrost peatlands.