Dissolved Mercury Species:
Samples were collected according to Cutter and Bruland 2012, utilizing a trace metal clean rosette (GT-C rosette). Samples above 20 m were pumped from a hole drilled in the ice far away from the ship, using clean sampling techniques.

Sample bottles were cleaned according to Hammerschmidt et al. 2012.

Dissolved mercury analyses were performed at sea, with the exception of monomethylmercury. The full methodology may be found in the Hg Cook Book, and more detailed method descriptions can be found in Lamborg et al. 2012, Hammerschmidt and Bowman 2012, Bowman et al. 2015, Bowman et al. 2016.

Dissolved species of mercury were measured with a Tekran Model 2500 CVAFS Mercury Detector. Analysis of total mercury (HgT; Fitzgerald and Gill 1979, Bloom and Fitzgerald 1988, Bloom 1989) has a method detection limit of 0.10 pM, and procedural duplicates varied by 11 ± 5% (n = 17).

Elemental mercury (Hg0) was analyzed according to Fitzgerald and Gill (1979), Bloom and Fitzgerald (1988) and Bloom (1989). The method detection limit was 0.04 pM and procedural duplicates varied by 11 ± 14% (n = 8).

Monomethylmercury (MMHg) was analyzed according to Bowman and Hammerschmdit (2011), Tseng et al. (2004) and Munson et al. (2014). The method detection limit was 0.020 pM, and procedural duplicates averaged 11 ± 8% (n =5).

Dimethylmercury (DMHg) was analyzed according to Bowman and Hammerschmidt (2011), Tseng et al. (2004) and Baya et al. (2013). The method detection limit was 0.012 pM.

Particulate Mercury Species:
Sample collection with in-situ pumps is outlined in Bishop et al. 2012.

Particulate samples were frozen and transported to Wright State University (Dayton, OH) where they were analyzed within 6 months of collection. Filter punches were digested in acid-cleaned polypropylene containers with 8 mL of 2N HNO3 (Baker Instra-Analyzed). After analysis of MMHg, an aliquot of digestate was oxidized with BrCl solution for total Hg determination, according to Hammerschmidt & Fitzgerald (2006). All analyzes were performed on Tekran Model 2500 CVAFS Mercury Detector.

Particulate HgT analysis was done according to Fitzgerald & Gill (1979), Bloom & Fitzgerald (1988), and Bloom (1989). The method detection limit was 0.75 pmol/kg, and procedural duplicates averaged 7 ± 6 RPD (n = 16).

Particulate MMHg was analyzed according to Tseng et al. (2004). The detection limit for the method was 0.075 pmol/kg.

While at sea, variable gas pressure in the gas chromatograph caused irregular MMHg peak retention times, making it indecipherable from the inorganic Hg peak. Accordingly, seawater samples ranging from 0.25 to 2 L, were acidified to 1% with sulfuric acid and shipped frozen to Wright State University for analysis.