Title:

Mercury isotopic constraints on mercury cycling across a permafrost thaw gradient in Stordalen Mire, Sweden

Warming global temperatures, particularly in the Arctic, are accelerating permafrost thaw and releasing previously sequestered mercury (Hg). Limited constraints on the size of Arctic terrestrial Hg pools, coupled with uncertainties in our knowledge of the responses of ecosystem Hg dynamics to climate change, complicate long-term forecasts of Hg impacts on ecosystem health and adaptability. Isotopes of Hg afford a means of deciphering the major pathways controlling elemental biogeochemical cycling and may provide insight into key mechanisms controlling Hg cycling across the permafrost thaw gradient. Stordalen Mire in Abisko, Sweden (68°21′N, 19°02′E) a thawing peatland, contains a permafrost thaw sequence represented by three habitats across the mire including: (i) permafrost-dominated, well-drained palsas occupied by Betula Nana, Eriophorum Vaginatum, Rubus Chamaemorus and Empetrum hermaphroditum, (ii) Sphagnum spp.- dominated semi-thawed sites with variable water table depths, and (iii) fully thawed fen sites containing vegetation dominated by Eriophorum angustifolium. Three lake sites, chosen for their varying total Hg contents and proximity to the thawing mire sites, include: Villasjön < 1.5 m water depth), Mellarsta Harrsjön a stream-fed lake (max depth < 7 m), and Inre Harrsjön, connected to Mellarsta Harrsjön, (max depth <5 m). Mercury isotope results from peat and lake sediment cores showed both variations in mass-dependent fractionation given by δ202Hg and in mass independent fractionation given by Δ201Hg and Δ199Hg across all sites and varying with depth. The δ202Hg values for all samples ranged from -2.22 ‰ to -0.65 ‰ and Δ199Hg varied from -0.58‰ to 0.09‰. Several sites showed a marked decrease in Δ199Hg with depth. The Δ199Hg/ Δ201Hg of 1.34 for lake sediments from the shallow lake Villasjön, together with low surface water methyl Hg contents, suggest that the paired chemical signatures may be explained by photochemical demethylation of MeHg being an important control on Hg cycling in this lake but less so for the other two lakes in this study.

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