Dataset: Concentration of labile dissolved zinc (dZn) from trace metal clean samples collected on the GEOTRACES GP17 expedition across the South Pacific and Southern Oceans in December 2022 to January 2023 on the R/V Revelle (RR2214)

Sampling on GP17-OCE:
Clean seawater samples were collected using a GEOTRACES CTD carousel outfitted with 24, 12-liter (L) GoFlo bottles (General Oceanics). Additional samples were collected at 3 meters (m) depth using a surface tow-fish that was operated while the ship was underway. For more information about sampling regimes, see the BCO-DMO project page for the GP17 expedition (https://www.bco-dmo.org/project/905972) or the GP17-OCE cruise report (linked therein).

Prior to the cruise, 250 milliliters (mL) fluorinated polyethylene (FLPE) bottles were rinsed in citranox detergent for 24 hours and then allowed to soak for 1 week in ca. 10% hydrochloric acid. After each step, bottles were rinsed several times in ultrapure (18.2 mega-ohm cm-1) water and cleaned bottles were stored, double-bagged when not in use. Sample bottles were triple-rinsed and then filled with seawater filtered through a 0.8 / 0.2 micrometer (um) capsule filter (Acropak) in a trace-metal-clean sampling van. Samples were stored at 4 degrees Celsius prior to analysis at sea.

Analytical methods:
A high-throughput electrochemical system was utilized at sea to measure labile dissolved zinc This system consisted of a Metrohm 858 sample processor, Metrohm 800 Dosino burettes, a mercury working electrode and 663 electrode stand, and an Autolab PG101 Potentiostat. The working electrode was set within a PFA analysis cup with a Ag/AgCl reference electrode and glassy carbon counter-electrode. Standard additions of Zn were added using a 2 mL Metohrm 800 Dosino. Sample was loaded into the analysis cup via a 5 mL Metrohm Dosino 800 buret. The cup was rinsed via a set of peristaltic pumps (Metrohm 843) with non-contaminated, filtered seawater collected from the underway tow-fish system. All operations were conducted in NOVA 12.X software.

To prepare for analysis, 12 mL of samples were transferred to conditioned perfluoroalkoxy alkane (PFA) vials and amended with a 7.5 mm borate buffer (pH = 8.1) and 108 uM ammonium pyrrolidine dithiocarbamate (APDC) and allowed to equilibrate for 8-16 hours. At a Zn binding strength of 10^8.4 for a the Zn(PDC)₂ complex, the equivalent 'alpha' value for this competitive ligand exchange experiments is 2.95 relative to Zn' (in other words, 75% of unchelated or weakly chelated Zn would be bound to PDC; a strong ligand at 1 nanomolar (nM) concentration with a binding constant of 10^9.5 would compete equally with PDC). After this incubation period, the vials were moved to an autosampler and 10 mL was loaded into the analysis cup and the concentration of the electroactive Zn(PDC)₂ complex was determined by cathodic stripping voltammetry using a hanging drop mercury electrode.

The voltammetric protocol consisted of an initial, 150 second purge with high purity N2 gas, followed by 7 measurement cycles consisting of 1) an additional 30 seconds purge, 2) a 120 seconds adsorption period at -0.6 volts (V), 3), a brief, 10 second quiescent period, and 4) a linear sweep between -0.6 V to -1.3 V at a speed of 10 volts per second (V/s). The Zn reduction peak was identified at -1.1 V and quantified by measuring peak area over a linearly-interpolated baseline. For the first 3 cycles, the background Zn was measured. The final 4 cycles consisted of successive and automated standard additions (2 nM for nearly all samples) to determine sample-specific Zn sensitivity. The concentration of labile dissolved Zn was determined by the mean of the peak area of the background Zn, divided by the sensitivity.

Analytical Performance:
For all samples measured, median R2 for linear regression of standard additions was 0.996 and the median relative standard deviation of the triplicate baseline scans was 4.5%. An analysis blank of 0.2 nM was subtracted, reflecting the input of Zn from reagents and electrochemical noise.

Performance of the electrochemical system was assessed by in-house standards. For high concentrations, a batch of seawater collected at 1000 m was analyzed throughout the cruise (5.32 +/- 0.51 nM; n = 52). For low concentrations, a sample from the upper water column of Station ALOHA was measured to be 1.12 +/- 0.36 (n = 61).

To assess accuracy, we measured the Zn concentration with the SAFe D2 standard after neutralizing the pH with ammonium hydroxide and a larger addition of borate buffer. We found a mean value of 6.98 +/- 0.62 nM, which overlaps with the reported "consensus" value for this sample (7.43 +/- 0.25 nM).