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Tree-rings are a promising high-resolution archive for gaseous atmospheric mercury (composed primarily of Hg0) reconstruction, but the influence of cambial age (ring number from pith) and tree-specific differences are uncertainties with potential implications for interpreting tree-ring Hg signals. We address these uncertainties and reconstruct the last 400 years of Hg0 change using a tree-ring Hg data set from 20 white spruce ( Picea glauca) trees from a pristine site in central Yukon. Cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are prevalent and must be normalized to a common mean to accurately constrain long-term trends in the mean tree-ring Hg record. Our record shows stable, low Hg0 concentrations prior to ∼1750 CE, a persistent rise from ∼1750-1950 (increasing more rapidly post-1850), a pause from ∼1951-1975, and then a resumed increase to record-high levels at present. This general pattern is reflected in other proxy-based Hg reconstructions worldwide. This study provides a novel long-term Hg0 reconstruction in the Western subarctic from one of the most widely distributed boreal tree species in North America and, therefore this proxy may also hold potential for investigating broader spatial patterns in Hg0 cycling across the subarctic and northern boreal forest.

Caribou, which rely on lichens as forage, are a dietary source of monomethylmercury (MMHg) to many of Canada's Arctic Aboriginal people. However, little is understood about the sources of MMHg to lichens in the High Arctic. We quantified MMHg, total mercury (THg) and other chemical parameters (e.g., marine and crustal elements, δ(13)C, δ(15)N, organic carbon, calcium carbonate) in lichen and soil samples collected along transects extending from the coast on Bathurst and Devon islands, Nunavut, to determine factors driving lichen MMHg and THg concentrations in the High Arctic. Lichen MMHg and THg concentrations ranged from 1.41 to 17.1 ng g(-1) and from 36.0 to 361 ng g(-1), respectively. Both were highly enriched over concentrations in underlying soils, indicating a predominately atmospheric source of Hg in lichens. However, MMHg and THg enrichment at coastal sites on Bathurst Island was far greater than on Devon Island. We suggest that this variability can be explained by the proximity of the Bathurst Island transect to several polynyas, which promote enhanced Hg deposition to adjacent landscapes through various biogeochemical processes. This study is the first to clearly show a strong marine influence on MMHg inputs to coastal terrestrial food webs with implications for MMHg accumulation in caribou and the health of the people who depend on them as part of a traditional diet.

Atmospheric deposition of metals originating from a variety of sources, including bitumen upgrading facilities and blowing dusts from landscape disturbances, is of concern in the Athabasca oil sands region of northern Alberta, Canada. Mercury (Hg) is of particular interest as methylmercury (MeHg), a neurotoxin which bioaccumulates through foodwebs, can reach levels in fish and wildlife that may pose health risks to human consumers. We used spring-time sampling of the accumulated snowpack at sites located varying distances from the major developments to estimate winter 2012 Hg loadings to a ∼20 000 km(2) area of the Athabasca oil sands region. Total Hg (THg; all forms of Hg in a sample) loads were predominantly particulate-bound (79 ± 12%) and increased with proximity to major developments, reaching up to 1000 ng m(-2). MeHg loads increased in a similar fashion, reaching up to 19 ng m(-2) and suggesting that oil sands developments are a direct source of MeHg to local landscapes and water bodies. Deposition maps, created by interpolation of measured Hg loads using geostatistical software, demonstrated that deposition resembled a bullseye pattern on the landscape, with areas of maximum THg and MeHg loadings located primarily between the Muskeg and Steepbank rivers. Snowpack concentrations of THg and MeHg were significantly correlated (r = 0.45-0.88, p0.01) with numerous parameters, including total suspended solids (TSS), metals known to be emitted in high quantities from the upgraders (vanadium, nickel, and zinc), and crustal elements (aluminum, iron, and lanthanum), which were also elevated in this region. Our results suggest that at snowmelt, a complex mixture of chemicals enters aquatic ecosystems that could impact biological communities of the oil sands region.

Methylmercury (MeHg) is a potent neurotoxin that has been demonstrated to biomagnify in Arctic freshwater foodwebs to levels that may be of concern to Inuit peoples subsisting on freshwater fish, for example. The key process initiating the bioaccumulation and biomagnification of MeHg in foodwebs is the methylation of inorganic Hg(II) to form MeHg, and ultimately how much MeHg enters foodwebs is controlled by the production and availability of MeHg in a particular water body. We used isotopically enriched Hg stable isotope tracers in sediment core incubations to measure potential rates of Hg(II) methylation and investigate the controls on MeHg production in High Arctic wetland ponds in the Lake Hazen region of northern Ellesmere Island (Nunavut, Canada). We show here that MeHg concentrations in sediments are primarily controlled by the sediment methylation potential and the quantity of Hg(II) available for methylation, but not by sediment demethylation potential. Furthermore, MeHg concentrations in pond waters are controlled by MeHg production in sediments, overall anaerobic microbial activity, and photodemethylation in the water column.

Distribution of total mercury (THg), gaseous elemental Hg(0) (GEM), monomethyl Hg (MMHg), and dimethyl Hg (DMHg) was examined in marine waters of the Canadian Arctic Archipelago (CAA), Hudson Strait, and Hudson Bay. Concentrations of THg were low throughout the water column in all regions sampled (mean +/- standard deviation; 0.40 +/- 0.47 ng L(-1)). Concentrations of MMHg were also generally low atthe surface (23.8 +/- 9.9 pg L(-1)); however at mid- and bottom depths, MMHg was present at concentrations sufficient to initiate bioaccumulation of MMHg through Arctic marine foodwebs (maximum 178 pg L(-1); 70.3 +/- 37.3 pg L(-1)). In addition, at mid- and bottom depths, the % of THg that was MMHg was high (maximum 66%; 28 +/- 16%), suggesting that active methylation of inorganic Hg(II) occurs in deep Arctic marine waters. Interestingly, there was a constant, near 1:1, ratio between concentrations of MMHg and DMHg at all sites and depths, suggesting that methylated Hg species are in equilibrium with each other and/or are produced by similar processes throughout the water column. Our results also demonstrate that oceanographic processes, such as water regeneration and vertical mixing, affect Hg distribution in marine waters. Vertical mixing, for example, likely transported MMHg and DMHg upward from production zones at some sites, resulting in elevated concentrations of these species in surface waters (up to 68.0 pg L(-1)) where primary production and thus uptake of MMHg by biota is potentially highest. Finally, calculated instantaneous ocean-atmosphere fluxes of gaseous Hg species demonstrated that Arctic marine waters are a substantial source of DMHg and GEM to the atmosphere (27.3 +/- 47.8 and 130 +/- 138 ng m(-2) day(-1), respectively) during the ice-free season.

We sampled seawater and snowpacks in the Canadian high Arctic for methylated species of mercury (Hg). We discovered that, although seawater sampled under the sea ice had very low concentrations of total Hg (THg, all forms of Hg in a sample; on average 0.14-0.24 ng L(-1)), 30-45% of the THg was in the monomethyl Hg (MMHg) form (on average 0.057-0.095 ng L(-1)), making seawater itself a direct source of MMHg for biomagnification through marine food webs. Seawater under the ice also contained high concentrations of gaseous elemental Hg (GEM; 129 +/- 36 pg L(-1)), suggesting that open water regions such as polynyas and ice leads were a net source of approximately 130 +/- 30 ng Hg m(-2) day(-1) to the atmosphere. We also found 11.1 +/- 4.1 pg L(-1) of dimethyl Hg (DMHg) in seawater and calculated that there could be a significant flux of DMHg to the atmosphere from open water regions. This flux could then resultin MMHg deposition into nearby snowpacks via oxidation of DMHg to MMHg in the atmosphere. In fact, we found high concentrations of MMHg in a few snowpacks near regions of open water. Interestingly, we discovered a significant log-log relationship between Cl- concentrations in snowpacks and concentrations of THg. We hypothesize that as Cl- concentrations in snowpacks increase, inorganic Hg(II) occurs principally as less reducible chloro complexes and, hence, remains in an oxidized state. As a result, snowpacks that receive both marine aerosol deposition of Cl- and deposition of Hg(II) via springtime atmospheric Hg depletion events, for example, may contain significant loads of Hg(II). Overall, though, the median wet/dry loads of Hg in the snowpacks we sampled in the high Arctic (5.2 mg THg ha(-1) and 0.03 mg MMHg ha(-1)) were far below wet-only annual THg loadings throughout southern Canada and most of the U.S. (22-200 mg ha(-1)). Therefore, most Arctic snowpacks contribute