Methylmercury (MeHg) is a neurotoxic compound that threatens wildlife and human health across the Arctic region. Though much is known about the source and dynamics of its inorganic mercury (Hg) precursor, the exact origin of the high MeHg concentrations in Arctic biota remains uncertain. Arctic coastal sediments, coastal marine waters and surface snow are known sites for MeHg production. Observations on marine Hg dynamics, however, have been restricted to the Canadian Archipelago and the Beaufort Sea (<79°N). Here we present the first central Arctic Ocean (79-90°N) profiles for total mercury (tHg) and MeHg. We find elevated tHg and MeHg concentrations in the marginal sea ice zone (81-85°N). Similar to other open ocean basins, Arctic MeHg concentration maxima also occur in the pycnocline waters, but at much shallower depths (150-200 m). The shallow MeHg maxima just below the productive surface layer possibly result in enhanced biological uptake at the base of the Arctic marine food web and may explain the elevated MeHg concentrations in Arctic biota. We suggest that Arctic warming, through thinning sea ice, extension of the seasonal sea ice zone, intensified surface ocean stratification and shifts in plankton ecodynamics, will likely lead to higher marine MeHg production.

The four profiles between the Siberian shelf/Laptev Sea and the North Pole (79 - 90°N) were sampled during the TransArc ARK XXVI/3 cruise in summer 2011 on the Research Vessel Polarstern and consist of 81 unfiltered acidified (HCl double-distilled, 0.4 %, v:v) samples. Acidification rapidly converts dimethylmercury (DMHg) into monomethylmercury (MMHg), and we thus measured methylmercury (MeHg) as the sum of MMHg and DMHg. For this study, we applied one of the best known reference methods, isotope dilution (ID), to a high sensitivity coupled gas chromatography - sector field ICP-MS (GC-SF-ICP-MS) method at the GET laboratory. MeHg and inorganic Hg species were extracted after derivatization, following previously published protocols, that we further improved for ultra-trace levels. Briefly, enriched spikes of 199iHg and 201MeHg (ISC Science, Spain) were added to a 115 mL aliquot of the sea water samples, targeting optimal ratios of 8.46 for 199iHgspike/202iHgsample and 4.25 for 201MeHgspike/202MeHgsample. The optimum spike to natural Hg isotope ratios was determined using the uncertainty magnification factor formula. After 24h of equilibration, pH was adjusted to 3.9 with NH3 (ULTREX® II Ultrapure Reagent, J.T. Baker, USA) and a buffer solution made up with acetic acid (glacial, ULTREX® II Ultrapure Reagent, J.T. Baker, USA) / sodium acetate (J.T. Baker, USA). A solution of 1 % (v:v) sodium tetra propyl borate (Merseburger Spezialchemikalien, Germany) was made up freshly, under cold conditions and avoiding contact with atmospheric oxygen. 1 mL of this solution was then added together with 200 µL hexane (Sigma Aldrich, USA). The glass bottles were hermetically sealed with Teflon-lined caps and vigorously shaken for 15 minutes. The organic phase was recovered and injected in the GC (Thermo Trace Ultra). The coupling to the high resolution ICP-MS (Thermo Element XR) and application of ultra-trace clean techniques allowed reaching detection limits as low as 0.001 pM. tHg was measured independently following the USEPA 1631 method at the GET laboratory. Potassium bromide (Sigma Aldrich, USA) and Potassium Bromate (Sigma Aldrich, USA) were heated for 4 h at 250°C to remove Hg traces before making up BrCl solution with freshly double-distilled HCl. We used a custom made semi-automatic single gold trap setup coupled to an cold vapor atomic fluorescence spectrometry (Brooks Rand Model III, USA), modified with mirrored quartz cuvette (Hellma Optics, Germany ) to achieve a detection limit of 0.025pM.

Supplement to: Heimbürger, Lars-Eric; Sonke, Jeroen E; Cossa, Daniel; Point, David; Lagane, Christelle; Laffont, Laure; Galfond, Benjamin T; Nicolaus, Marcel; Rabe, Benjamin; Rutgers van der Loeff, Michiel M (2015): Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean. Scientific Reports