Dataset: Dissolved trace metal concentrations in water column samples and incubation experiments from STING II cruise EN704 on R/V Endeavor over the West Florida Shelf in July 2023

Sample collection

Water column samples were collected in July 2023 aboard the R/V Endeavor using a trace metal clean rosette (SeaBird) outfitted with 12-L Niskin-X samplers (Ocean Test Equipment), a trace metal clean pump system, or a custom surface pump “towfish” system (Mellett and Buck, 2020). Seawater samples for dissolved (D) trace metal analysis were filtered through 0.2 µm Pall Acropak Super membrane filter capsules and collected in acid-cleaned and triple-rinsed 125-mL low density polyethylene (LDPE) bottles.

Samples from seawater incubation experiments were filtered through consecutive acid-cleaned 5 µm and 0.4 µm polycarbonate track-etched (PCTE, Whatman) filters in a dual stage Teflon filter rig (Savillex) on a custom acrylic tower. Filtrate from the 0.4 µm filters was collected in acid-cleaned and triple-rinsed 125-mL LDPE bottles for subsequent dissolved trace metal measurements. The 5 µm and 0.4 µm filters were folded in eighths, placed in acid-cleaned snap-cap polypropylene centrifuge tubes, and frozen at -20 ºC for labile particulate metals.

All dissolved trace metal samples were acidified shipboard with ultrapure hydrochloric acid (HCl, Optima) to 0.024 N HCl, double bagged in buckets and stored at room temperature until analyzed at Oregon State University (OSU).

Analysis of dissolved trace metal concentrations

Analysis of seawater samples for the concentrations of dissolved (<0.2 or <0.4 µm) aluminum (Al), scandium (Sc), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), gadolinium (Gd), lutetium (Lu) and lead (Pb) were performed using an automated SeaFAST pico (Elemental Scientific, Inc.) in line with a Thermo Scientific iCAP-RQ Quadrupole CCT ICP-MS. This instrument is maintained at OSU in the Keck Collaboratory for Plasma Spectrometry.

Trace metals were eluted from the Nobias-1A resin in seaFAST using 10% nitric acid (Optima, Fisher) containing 1 ppb indium as an internal standard and quantified using nine-point calibration curves in filtered (<0.2 µm) surface seawater collected offshore in the Gulf of Mexico in 2018 (Hollister et al. 2020) and stored unacidified in an acid-cleaned and double-bagged polypropylene in the dark; prior to use for the seawater matrix, a 2-L aliquot was pulled from the carboy and acidified to 0.024 N hydrochloric acid (HCl, Optima) to match the sample matrix. Subsamples of two in-house seawater quality control samples (collected previously from offshore surface waters, and from the EPZ GEOTRACES GP16 transect) were measured in each run and were also analyzed in tandem with community-established reference materials (SAFe-D1, SAFe-D2, GSC and NASS-7). All reference material results are provided in the supplemental file. Analytical blanks for the seaFAST method were obtained using air blanks, and limits of detection were calculated as three times the standard deviation of the averaged air blanks in each analytical run (Hollister et al. 2020).

Samples were analyzed a second time following the same procedure outlined above but after 90 minutes of UV-oxidation to ensure complete recovery of any organic-bound Cu and Co (Biller and Bruland 2012). The UV-oxidation was carried out on aliquots of samples in 15-mL Teflon vials (Savillex) with custom quartz lids (Hollister et al. 2020). For all elements except Cu and Co, results from analyses after UV-oxidation were combined with no UV results to achieve an additional analytical replicate.

Sample analyses for trace metals were performed by senior researcher Dr. Salvatore Caprara in the College of Earth, Ocean and Atmospheric Sciences at Oregon State University.