Dataset: Concentrations of dissolved iron and manganese determined in water-column and near-surface seawater samples collected on the US GEOTRACES GP17-OCE cruise on R/V Roger Revelle (RR2214) in the South Pacific and Southern Oceans from Dec 2022 to Jan 2023

Water-column samples for trace metal analysis were collected in modified 12-liter (L) Teflon-lined GoFlo samplers (General Oceanics Inc.) deployed on the U.S. GEOTRACES clean CTD rosette system with a total of 24 samplers. All samples were filtered as soon as possible after recovery by GEOTRACES sampling personnel using a pre-cleaned, sample-rinsed 0.2-micrometer (µm) Supor Acropak filter cartridge (Pall Corp.) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface seawater samples (~2 meters depth) were collected while underway near each station using a towed fish sampling system towed outside of the ship's wake; using a pneumatic Teflon bellows pump, seawater from the towed fish was pumped into a clean-air 'bubble' in the ship's laboratory, where it was filtered through a pre-cleaned, sample-rinsed 0.2 µm Supor Acropak filter cartridge. The sample collection, filtration, and subsampling followed established GEOTRACES protocols. The filtered seawater samples for post-cruise analysis of dissolved iron and maganese were collected in 125-milliliter (mL) acid-cleaned Nalgene wide-mouth low-density polyethylene (LDPE) bottles, and acidified at-sea to pH ~1.7 using Fisher Optima grade ultrapure hydrochloric acid diluted to ~6N using ultrapure deionized water (Barnstead Nanopure). These acidified samples were stored at room temperature for several months prior to analysis at Old Dominion University. Samples that were used for shipboard dissolved iron analysis were similarly collected in 125 mL acid-cleaned LDPE bottles and acidified to pH ~1.7 for at least 12 hours prior to analysis. For the samples used for shipboard analysis, several sets of LDPE bottles were repeatedly re-used; after use, these bottles were emptied, rinsed with ultrapure deionized water, filled with dilute ultrapure hydrochloric acid and stored for at least 12 hours, and then rinsed with ultrapure deionized water before rinsing and filling with new seawater samples.

Shipboard dissolved iron (DFe) determinations used flow-injection analysis (FIA) with colorimetric detection after in-line preconcentration on resin-immobilized 8-hydroxyquinoline (Sedwick et al., 2005; 2015), modified from the method of Measures et al. (1995). Analyses were performed on a volumetric basis, so the DFe concentrations are reported in units of nanomole per liter (nM). Analytical precision and accuracy were monitored via the analysis of in-house reference seawater samples during each day of shipboard measurements, whereas seawater consensus reference materials were analyzed only on selected days, due to limited available volumes. Repeat, separate-day analyses of an in-house reference seawater sample yielded a mean DFe concentration of 0.294 ± 0.038 nM (one sigma, n = 9). These measurements yield an estimated overall analytical uncertainty of ±12.9%, expressed as one relative standard deviation on the mean, and a detection limit of 0.11 nM DFe, estimated from three times the standard deviation on the mean. All samples were analyzed at least twice (at two separate times of the day) during each analysis. Analyses of seawater consensus reference material SAFe D2 #294 yielded a mean concentration of 1.03 ± 0.05 nM (one-sigma, n = 2), which compares favorably to the consensus value of 0.91 ± 0.022 nM DFe. In addition, analysis of GEOTRACES seawater consensus reference material GSP #113 yielded a mean concentration of 1.42 nM, which is much greater than the consensus value of 0.155 nM DFe; because this sample bottle had been opened multiple times over several years and had only a small volume remaining, it was likely contaminated with iron prior to shipboard analysis.

Post-cruise determinations of dissolved iron (DFe) and dissolved manganese (DMn) were made using inductively-coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific ElementXR) with in-line separation-preconcentration (Elemental Scientific SeaFAST SP3), modified after the method of Lagerström et al. (2013). Calibration standards were prepared in low-analyte concentration filtered seawater for which initial concentrations were determined using the method of standard additions (Sedwick et al., 2022). Calibration standards were introduced using the same in-line separation-preconcentration procedure as the seawater filtrate samples, with indium used as an internal standard. Analytical blank concentrations were assessed by applying the in-line separation-preconcentration procedure, including all reagents and loading air in place of the seawater filtrate sample ("air blank"), yielding mean blank concentrations of 0.000 ± 0.005 nM for DFe (n = 8), and 0.004 ± 0.001 nM for DMn (n = 14). Analytical limits of detection, defined as the concentrations equivalent to three times the standard deviation on the mean blank, were 0.016 nM DFe and 0.003 nM for DMn. Mean concentrations of multiple, separate-day determinations of GEOTRACES sample number 15704 seawater collected during the GP15 cruise were 0.11 ± 0.03 nM DFe (n = 21) and 0.83 ± 0.03 nM DMn (n = 23). Previous analyses of this in-house reference seawater using ICP-MS had yielded mean concentrations of 0.16 ± 0.02 nM DFe (n = 7) and 0.82 ± 0.03 nM DMn (n = 9) while analyzed alongside seawater consensus reference materials GSP and SAFe D2 that yielded mean concentrations of 0.0115 ± 0.01 nM and 0.968 ± 0.007 nM DFe, and 0.786 ± 0.011 nM and 0.468 ± 0.004 nM DMn, respectively (consensus concentrations for the GSP and SAFe D2 seawater are 0.155 ± 0.045 nM and 0.956 ± 0.024 nM DFe, and 0.778 ± 0.034 nM and 0.36 ± 0.05 nM DMn, respectively). Analytical precision at the concentration levels of the in-house reference seawater, expressed as ± one relative standard deviation on the mean, are ± 32% for DFe (noting the relatively low concentration of the in-house reference water) and ± 3.1% for DMn. In addition, the typical reproducibility of repeat sample analyses during a single analytical run were ± 8.0% for DFe and ± 3.6% for DMn.