Methodology: Details of the methods for the cruise are given in DiMento et al. (2019). Details of the overall method and approach for dissolved gaseous mercury and atmospheric mercury methods are given in Andersson et al. (2008), Mason et al. (2017), Soerensen et al. (2014), and Soerensen et al. (2013). Analytical methods are detailed in DiMento et al. (2019) with additional information in the papers listed above and in Munson et al. (2014), Morton et al. (2013), and Gichuki & Mason (2014). See "Related Publications" below for complete citations.

Sampling Procedures: Melt pond water samples were collected using a battery powered peristaltic pump (Pegasus Alexis) to fill acid-washed 20 L carboys (low density polyethylene – LDPE) through C-Flex tubing (Masterflex) that had been acid-cleaned with 0.1 M ultrapure HCl and thoroughly rinsed with ultrapure water before use (Marsay et al., 2018). At each melt pond, the tubing was used as a probe to ascertain that the pond did indeed have a bottom layer of ice, although it was not possible to determine any connectivity with seawater through flaws in the ice, or the presence of a "false bottom" formed from refreezing of fresh meltwater. Samples for TE analysis were also collected from snow, sea ice, and from seawater beneath the sea ice at each station. Snow samples for TE analysis were collected using an acid-cleaned polyethylene shovel to transfer the snow column into an acid-cleaned LDPE drum insert (CDF Corporation), which was held inside a plastic bucket, and which was subsequently sealed before transfer to the ship. The sample was allowed to melt at room temperature on the ship (8–10 h), and then homogenized by gentle shaking (Marsay et al., 2018).

Methylmercury concentrations were determined following the ascorbic acid-assisted direct ethylation method (Munson et al., 2014) using a Tekran 2700 instrument and autosampler to automate the purging, trapping, and detection via cold vapor atomic fluorescence spectroscopy (CVAFS). Samples were thawed then acidified to 1% (v/v) H2SO4 and left to digest overnight before neutralizing with 8N potassium hydroxide (KOH), buffering with 4M acetate, adding 2.5% (w/v) ascorbic acid and finally 1% (w/v) sodium tetraethyl borate (NaTEB) to ethylate the methylmercury. Total mercury concentrations were determined by dual gold-amalgamation CVAFS utilizing a Tekran 2600 instrument in accordance with U.S. EPA Method 1631. Briefly, waters were digested with bromine monochloride (BrCl) followed by a pre-reduction step with hydroxylamine hydrochloride (NH₂OH·HCl). Inorganic Hg(II) was then reduced to Hg⁰ using stannous chloride (SnCl₂) prior to automated analysis on the Tekran.