PhD Defence - Charlotte Haugk

Abstract

Arctic warming is driving permafrost thaw with consequences for remobilization and transformation of contaminants stored in northern soils. One of particular concern is mercury (Hg), which has accumulated in large quantities in permafrost peatlands via atmospheric deposition over millennia. As these landscapes thaw, changes in peat structure, hydrological pathways and microbial communities may lead to the release of Hg and its transformation into the neurotoxic and bioaccumulative methylmercury (MeHg). This PhD thesis examines Hg accumulation and storage in permafrost peatlands and how thaw affects the cycling and transformation of Hg. It brings together evidence from historical peat records as well as experimental and natural permafrost thaw gradients.

In paper I, we investigated historical Hg accumulation in nine peat profiles of northern Scandinavia and Canada. This study is among the first to combine Hg data with macrofossil records and provides novel insights into how net Hg accumulation differs in syngenetic and epigenetic permafrost systems. Results reveal that the environmental conditions under which peat accumulated affect the Hg record within the peat profile. We could show that where peat formed before permafrost developed (epigenetic permafrost), more Hg accumulated relative to carbon compared to when peat and permafrost developed together (syngenetic permafrost). Permafrost thaw could also turn peatlands from long-term Hg stores into Hg sources to the surrounding environment. We examined changes in peat Hg storage across both natural and experimental thaw gradients in northern Scandinavian peatlands. In paper III, we compared intact peat plateaus and thawed fens across five permafrost peatlands and examined the landscape-scale distribution of Hg. Hg stocks in fens were approximately half those in peat plateaus, indicating substantial Hg loss following landscape transition after thaw. Furthermore, timing and thaw progression are discussed, as 17 years of experimentally accelerated thaw revealed no significant change in Hg or MeHg stocks (paper II). Beyond its effect on Hg storage, permafrost thaw can also influence MeHg production. In natural thaw gradients we found that MeHg concentrations and %MeHg were five times higher in fens than peat plateaus (paper IV). Fens also generally supported more diverse microbial communities and higher relative abundance of putative Hg methylators, further showing that fens that formed post-thaw resulted in conditions favorable for Hg methylation.

This thesis demonstrates that permafrost thaw can substantially alter Hg cycling in peatlands. It highlights the consequences of thaw, including changes in Hg mobilization, transformation and transport that could increase Hg exposure to humans and wildlife. These findings advance our understanding of the Arctic Hg cycle under continued climate warming.