Mercury storage in sub-arctic lake sediments in Stordalen Mire, Abisko Sweden

Abstract:

Permafrost thaw driven by climate change in northern high latitudes is thought to play a significant role in enhancing the mobilization of previously sequestered peatland mercury (Hg) to the atmosphere and hydrosphere. Though studies have shown sub-arctic lake Hg dynamics are impacted by adjacent permafrost thaw, the magnitude and long-term effects of Hg mobilization in sub-arctic lakes remain poorly constrained. Three lakes from the well-characterized Stordalen Mire in Abisko, Sweden were sampled in order to study intra- and inter- lake variability in total lake sediment Hg. These three lakes were chosen due to their proximity to a thawing permafrost peatland and were characterized for their carbon and mercury dynamics. Coring sites included: (1) 6 cores taken at Villasjön, a shallow lake less than 1.5 meters water depth with a previously established methane (CH₄) ebullition gradient; (2) 2 cores from Mellarsta Harrsjön, a stream-fed lake with a maximum water depth < 7 m; and (3) 2 cores from Inre Harrsjön, connected to Mellarsta Harrsjön, with a maximum water depth of <5 m. Radiocarbon ages constrain the formation of the three lakes to ~ 3400 years ago. We found both significant inter- and intra- lake variations in total sediment Hg. Within Villasjön, the cores associated with the lowest CH₄ ebullition have markedly lower total Hg relative to the cores located in areas with the highest observed CH₄ ebullition. The depth of maximum sediment Hg content also varies across the ebullition gradient such that the cores from areas with high ebullition rates had high total Hg as deep as ~30 cm whereas maximum total sediment Hg in the low ebullition cores was located in the top 5 cm. From the inter-lake perspective, Mellarsta Harrsjön and Inre Harrsjön, which contain overall lower methane ebullition fluxes relative to Villasjön, were found to contain significantly less Hg. If sediment Hg is mobilized during CH₄ ebullition then this pathway of Hg mobilization needs further understanding.