Dataset: GN01 Dissolved Micronutrient Trace Metals

All trace metal-clean seawater samples in this dataset were collected following the prescribed US GEOTRACES protocol ("GEOTRACES cookbook": for more information see (Cutter et al., 2010; Cutter and Bruland, 2012)). Briefly, 24 Teflon-coated, acid-cleaned and preconditioned GO-FLO bottles were deployed on an epoxy-coated aluminum rosette and were tripped on ascent at 3 m/min. Once on deck, bottles were immediately capped with plastic shower caps on both ends to reduce contamination and subsequently kept in a clean sampling van under positive-pressure, HEPA-filtered air. All handling and subsampling was performed exclusively by designated, trace metal-clean personnel to preserve homogeneity and further reduce contamination.

During sampling for the GO-Flo samples, salinity and nutrient (unfiltered) samples were taken first, and then all Go-Flo bottles were pressurized to ~0.7 atm with HEPA-filtered air. Each bottle was fitted with an acid-cleaned and seawater-preconditioned 0.2 um AcroPak-200 capsule filter (Pall), and at least 500 mL was passed through each filter to rinse. Samples were collected in acid-cleaned (hydrochloric acid, 1 M (Fitzsimmons and Boyle, 2012)) LDPE bottles (Nalgene) after three 10% rinses of the bottle, cap, and threads and filled to the shoulder or as volume allowed.

In the absence of sea ice, surface samples were collected in trace metal clean fashion by taking a small boat upstream of the ship. The small boat was pointed into the oncoming current to sample "uncontaminated water" as it passed the boat, and a trained trace metal personnel wearing trace metal clean gloves and plastic sleeves submerged an acid-cleaned carboy under the water surface from the bow of the boat, always keeping the carboy pointed into the oncoming water. The carboy was immediately capped and bagged for transit back to the ship, where it was immediately filtered (0.2 um, Acropak-200) and sub-sampled into individual bottles, including our acid-clean 250 mL LDPE bottles.

A small subset of samples from designated "ice stations" (Stations 31, 33, 39, 43) were collected under the ice (approx. 1, 5, and 20 m) after the ice was drilled with a polypropylene/titanium trace metal coring system. Sampling was done using a polypropylene, battery-powered motor centrifugal pump with ½ inch FEP-lined Tygon tubing. Samples were collected and filtered (0.2 um, Acropak-200) into a 25 L acid-cleaned carboy and then sub-sampled back on the ship, including into our acid-clean 250 mL LDPE bottles, within 3 hours of collection.

At these same "ice stations", melt pond samples were collected (Marsay et al., 2018) by clearing surface snow with an acid-cleaned polyethylene shovel and then using a polyethylene/titanium trace metal coring system to drill through the upper ice. Melt pond water was pumped using a battery-powered polyethylene pump through pre-cleaned C-flex tubing into a pre-cleaned LDPE carboy. The melt pond sample in this carboy was filtered and sub-sampled into individual bottles, including our acid-clean 250 mL LDPE bottles, within 3 hours of collection.

Subsequently, all samples were acidified under clean air conditions to pH~2 with 0.012 M HCl (Optima, Fisher Scientific) within 1 week of sample collection and stored at room temperature until analyzed.

At least 9 months after acidification, samples were analyzed for trace metal (Fe, Ni, Cu, Zn, Pb, Cd, Mn) concentrations at Texas A&M University after pre-concentration on a SeaFAST-pico system (Elemental Scientific Inc.). This method follows a modified version of Lagerström et al. (2013) and is described completely for all elements in Jensen et al. (2020). Briefly, 10 mL of acidified seawater sample was weighed and then spiked with an isotope mixture containing 57Fe, 62Ni, 65Cu, 68Zn, 206Pb, and 111Cd. The mixture was then automatically loaded into the SeaFAST system. Monoisotopic metals such as Mn were measured via a 6-point standard curve using standards made up in low-metal seawater (maintained in-house) such that Mn concentrations spanned 0 to 10 nmol/kg; these standard curves were each run through the SeaFAST twice (once to start and once to end each run) as samples. The SeaFAST method buffers the acidified (pH~2) sample with a ~5.90 N ammonium acetate buffer (Optima, Fisher Scientific) to pH ~6.5 and loads the buffered mixture onto a pre-cleaned column containing Nobias PA1 resin (Sohrin et al., 2008). Each sample is then back-eluted with a solution of 10% (v/v) nitric acid and 1 ppb In (Optima, Fisher Scientific) to yield 400 uL, representing a 25-fold pre-concentration of each trace metal. Within days, each eluent is analyzed in low (204Pb, 206Pb, and 111Cd, 114Cd, 115In) and medium resolution (55Mn, 56Fe, 57Fe, 60Ni, 62Ni, 63Cu, 65Cu, 66Zn, 68Zn, 115In) on a Thermo Element XR high resolution ICP-MS in the Ken Williams Radiogenic Facility in the College of Geosciences at Texas A&M University.

Each sample run included ~80 aliquots from the SeaFAST. Every 10 SeaFAST samples on the XR, pure 10% (v/v) nitric with 1 ppb of 115In was run to extract a potential instrument blank coming from the instrument itself. Prior to running on the ICP-MS, the machine was “cleaned” with this mixture as a precaution against contamination from other samples, in addition to using a separate set of cones, nebulizer, spray chamber, and sample probe for SeaFAST samples only.