Date of Award

Spring 2022

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Julia G Bryce

Second Advisor

Patrick M Crill

Third Advisor

Charles T Driscoll

Abstract

Arctic ecosystems are a major global sink for both carbon (C) and mercury (Hg), both of which are influenced by anthropogenic activities. The accelerated climate-change-induced warming documented in the Arctic has led to permafrost thaw resulting in landscape evolution from hydrological, vegetation and microbial shifts. All of these resulting changes have the potential to influence how Hg, a toxic contaminant, is mobilized and re-cycled in this ecosystem. In this body of work, I investigate patterns of Hg speciation, elemental abundances, and stable isotopes across the thaw gradient in a permafrost peatland and interconnected freshwater lakes in Abisko, Sweden to assess changes in Hg storage and in Hg export pathways. In Chapter 2 (republished from Fahnestock et al., 2019), I report results suggest that during initial stages of permafrost thaw, when the active layer in the palsa deepens, export of gas-phase mercury may be an important pathway of Hg loss. As the thaw continues vegetation changes to a Sphagnum-dominated semi-thawed ecosystem, where Hg export into the atmosphere limits accumulation of Hg in peat and the peat bound HgT pool is depleted. In the final stage of thaw, characterized by fully thawed fens, more labile organic matter and a diverse microbial community result in greater Hg retention in fen peat and higher levels of porewater MeHg that may be exported to nearby lakes and streams. Hg stable isotopes employed in Chapter 3 provide evidence supporting the importance of vegetation as an important sink of Hg in Arctic peatlands, show that photoreduction processes are important across the landscape and that ombrotrophic bogs may retain more Hg from wet deposition. Taken together, these findings suggest that Hg cycling in thawing peatlands will be influenced by future climatological patterns that drive the hydrological conditions, particularly the hydrologic connectivity, of this ecosystem. Chapters 4 and 5 consider the role of Arctic lakes in an ecosystem undergoing permafrost thaw. In Chapter 4, findings show that lake sediments can store varying amounts of Hg and how abundance of total Hg in sediments are not necessarily linked with aqueous Hg concentrations in overlying waters or with total organic content of sediments. The high density of lakes in the Arctic region and likelihood of thermokarst pond formation and wetland expansion with continuing permafrost thaw may result in Hg accumulation in lake sediments. Stable Hg isotopes are document how lakes composed of different sediment composition, size and depth process Hg differently. Large variations in Hg pools, sources and methyl Hg concentrations across the three post-glacial lakes within one watershed highlight the need for increased study of both spatial and temporal Hg cycling in Arctic lakes.

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