Methylated mercury species in Canadian high arctic marine surface waters and snowpacks

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 ∼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 result in 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 relatively little to marine pools of both Hg(II) and MMHg at snowmelt.