Monomethylmercury (MMHg) is a toxic pollutant that bioaccumulates in aquatic food webs. A major mechanism that limits MMHg uptake by biota is photodemethylation in surface waters. Recently, the extent of mass-independent fractionation (MIF) of Hg isotopes preserved in fish is being used to quantify this MMHg sink. Here, the effects of different types and amounts of DOM on Hg MIF during MMHg photodemethylation were investigated to assess how variable MIF enrichment factors may be with respect to changing DOM binding sites. From experiments conducted with varying amounts of reduced organic sulfur (S(red)-DOM), the extent and signature of MIF is likely dependent on whether MMHg is dominantly bound to S(red)-DOM. Similar enrichment factors were observed for low MMHg:S(red)-DOM experiments, where S(red)-DOM was in far excess of MMHg. In contrast, significantly lower and variable enrichment factors were observed for experiments with higher MMHg:S(red)-DOM ratios. Additionally the relationship between the two odd Hg isotopes that display MIF (Δ(199)Hg/Δ(201)Hg) was consistent for the low MMHg:S(red)-DOM experiments, while lower Δ(199)Hg/Δ(201)Hg relationships were observed for the higher MMHg:S(red)-DOM experiments. These results suggest that both the extent and signature of MMHg MIF are sensitive to different ligands that bind MMHg in nature.

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Anthropogenic Pb is widespread in the environment including remote places. However, its presence in Canadian Arctic seawater is thought to be negligible based on low dissolved Pb (dPb) concentrations and proxy data. Here, we measured dPb isotopes in Arctic seawater with very low dPb concentrations (average ∼5 pmol ⋅ kg−1) and show that anthropogenic Pb is pervasive and often dominant in the western Arctic Ocean. Pb isotopes further reveal that historic aerosol Pb from Europe and Russia (Eurasia) deposited to the Arctic during the 20th century, and subsequently remobilized, is a significant source of dPb, particularly in water layers with relatively higher dPb concentrations (up to 16 pmol ⋅ kg−1). The 20th century Eurasian Pb is present predominantly in the upper 1,000 m near the shelf but is also detected in older deep water (2,000 to 2,500 m). These findings highlight the importance of the remobilization of anthropogenic Pb associated with previously deposited aerosols, especially those that were emitted during the peak of Pb emissions in the 20th century. This remobilization might be further enhanced because of accelerated melting of permafrost and ice along with increased coastal erosion in the Arctic. Additionally, the detection of 20th century Eurasian Pb in deep water helps constrain ventilation ages. Overall, this study shows that Pb isotopes in Arctic seawater are useful as a gauge of changing particulate and contaminant sources, such as those resulting from increased remobilization (e.g., coastal erosion) and potentially also those associated with increased human activities (e.g., mining and shipping).

Atmospheric deposition is an important source of trace elements (TEs) to the Arctic, including both anthropogenic (e.g., Pb) and micronutrient (e.g., Fe) metals. This study measured TE loadings, Pb isotopes, and dissolution in aerosols collected at Alert, Nunavut, Canada, from spring to summer of 2013 and 2014 and during the Canadian Arctic GEOTRACES Cruises (GN02 and GN03) in the summer of 2015. The aerosol loadings were 5–8 times higher and more enriched with anthropogenic metals (Pb and Cd) in the spring than in the summer consistent with the Arctic haze phenomenon. Pb isotopes reveal that Russia, Europe, and China were the likely source regions of this pollution in spring. Comparison of the Pb and Cd concentrations and Pb isotope data with previous studies suggests that atmospheric pollution in the Canadian Arctic has been relatively stable for the last couple of decades. Dissolution experiments were also performed to estimate the fraction of aerosol TEs that can potentially be dissolved and become bioavailable in seawater. The minimum and maximum dissolution estimates for Fe were 8 ± 5 and 65 ± 20%, respectively, which translate to a potential input of 5–40 kT year–1 of dissolvable Fe to the Canada Basin of the Arctic Ocean, comparable to the riverine Fe input (10–20 kT year–1) from Mackenzie River. Thus, aerosols could be an important Fe source to Arctic surface waters, especially with decreasing sea ice coverage, changes in stratification and in places far from shelf and river sources.

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