Water-column sample collection and in-situ measurements: Water-column samples for analysis of trace elements (dissolved, soluble and particulate), and continuous profiles of temperature, salinity, chlorophyll fluorescence and dissolved oxygen concentration were collected using a trace-metal clean conductivity-temperature-depth sensor (SBE 19 plus, SeaBird Electronics) mounted on a custom-built trace-metal clean carousel (SeaBird Electronics) fitted with custom-modified 5-L Teflon-lined external-closure Niskin-X samplers (General Oceanics), deployed using a non-metallic line with polymer jacket.

Shipboard processing of water-column samples: Upon recovery, the Niskin-X samplers were transferred into a shipboard Class-100 clean laboratory container, where seawater was filtered through pre-rinsed 0.2-micrometer (µm) pore AcroPak Supor filter capsules (Pall) into acid-cleaned 125 milliliter (mL) low-density polyethylene (LDPE) bottles (Nalgene) for shore-based dissolved trace element determinations, and, for selected samples, into acid-cleaned 60 mL LDPE bottles (Nalgene) for subsequent ultrafiltration through acid-rinsed 0.02 µm Anotop syringe filters (Whatman) following Ussher et al. (2010) for shore-based soluble trace element determinations. While drawing samples, the Niskin-X samplers were pressurized using 0.2 µm-filtered ultra-high purity compressed nitrogen gas. All samples used for dissolved- and soluble trace element determinations were acidified at sea to pH ~1.8 with ultrapure hydrochloric acid (Fisher Optima grade) then stored at room temperature until post-cruise analysis at Old Dominion University. In addition, from selected samples, a pre-cleaned perfluoroalkoxy alkane (PFA) cylindrical reservoir (Savillex) was filled with approximately 1.8 liters (L) of unfiltered seawater, which was passed through sequential 2 µm- and 0.2 µm-pore acid-cleaned 47-mm diameter polycarbonate membrane filters (Poretics) mounted in an in-line PFA filter holder (Savillex) using overpressure from 0.2 µm-filtered compressed air. The filters were then rinsed with 200 mL of ultrapure deionized water (Barnstead Nanopure) that had been adjusted to pH 8 with ultrapure ammonium hydroxide solution (Fisher Optima grade), placed in pre-cleaned polystyrene Petri dishes (Fisher), then air-dried under a Class-100 clean-air bench. The dried filters were stored in the sealed Petri dishes inside a desiccator at room temperature until post-cruise analysis at the NOAA Pacific Marine Environmental Laboratory.

Determination of DFe, sFe, DMn, sMn, DNi, DCu, DZn: Concentrations of these dissolved and soluble transition metals were measured in 0.2 µm-filtered (dissolved fraction) or 0.02 µm-filtered (soluble fraction) acidified seawater samples using a sector-field inductively-coupled plasma mass spectrometer (Thermo Fisher Scientific ElementXR) with an in-line separation-preconcentration system (Elemental Scientific SeaFAST SP3), following a modification of the method of Lagerström et al. (2013). Calibration standards were prepared in low-analyte concentration seawater, for which initial concentrations were determined using the method of standard additions, and were introduced using the same in-line separation-preconcentration procedure as the seawater samples. Scandium was used as an internal standard. Analyses were performed on a volumetric basis, so concentrations are reported in units of nanomole per liter (nM). Analytical blank concentrations were assessed by applying the in-line separation-preconcentration procedure including all reagents except loading air in place of the seawater sample ("air blank"), with the following mean blank concentrations: 0.002 nM DFe, <0.001 nM DMn, 0.003 nM DNi, 0.001 nM DCu, 0.001 nM DZn. Limits of detection were defined as the concentration equivalent to 3 times the standard deviation on the mean blank (n = 12), as follows: 0.042 nM DFe and sFe, 0.003 nM DMn and sMn, 0.021 nM DNi, 0.006 nM DCu, and 0.018 nM DZn. Estimated analytical precision, expressed as percent relative standard deviation on the mean (%RSD), is the average value of the %RSD obtained for replicate (separate-day) analyses of 58 different samples, as follows: 8.0% for DFe and sFe, 1.8% for DMn and sMn, 3.0% for DNi, 2.5% for DCu, and 5.8% for DZn. In terms of external consistency, we obtained the following mean concentrations for the GEOTRACES GSP seawater consensus material: 0.187 ± 0.037 nM DFe (n = 17; consensus value 0.155 ± 0.045 nM), 0.789 ± 0.016 nM DMn (n = 19; consensus value 0.778 ± 0.034 nM), 2.59 ±0.08 nM DNi (n = 19, consensus value 2.60 ± 0.10 nM), 0.581 ± 0.015 nM DCu (n = 19; consensus value 0.574 ± 0.053 nM), and 0.015 ± 0.003 nM DZn (n = 3; consensus value 0.030 ± 0.052 nM). Our reported GSP DZn concentration is for measurements in aliquots from a freshly-opened bottle of this consensus material; two other bottles yielded much higher concentrations and are therefore assumed to be contaminated.

Determination of pFe-large, pFe-small, pMn-large, pMn-small, pAl-large, pAl-small: The concentrations of Fe, Mn, and Al in suspended particles separated on 2 µm- and 0.2 µm-pore membrane filters (defined as large- and small-particulate fractions, respectively) were measured using energy dispersive X-ray fluorescence (ED-XRF), following the method of Buck et al. (2021). ED-XRF analysis was conducted under a vacuum atmosphere using thin-film principles on a Thermo Fisher Scientific Quant’X equipped with a rhodium target X-ray tube and an electronically-cooled, lithium-drifted solid-state detector. X-rays for primary sample excitation were passed through graphite and metal filters for optimum control of peak-to-background ratios, using the excitation conditions described by Buck et al. (2021). Four separate quality assurance/quality control procedures were conducted: (1) daily energy adjustment was performed for an energy channel alignment of the Quant’X; (2) weekly calibration verification using a series of multi-element samples; (3) weekly analysis of the NIST 2783 standard reference material; and (4) monthly analysis of 10 acid-washed filter blanks. Calibrations were performed using commercially available thin-film standards (MicroMatter Inc.), as well as low-concentration standards for pFe and pMn (<1000 nanograms per square centimeter (ng cm-2)) that were prepared as described by Buck et al. (2021). Field blanks, for which acid-cleaned 0.2 µm- and 2 µm-pore polycarbonate membranes were mounted in the filtration assembly, rinsed with 200 mL of pH 8 ammonium hydroxide solution, then air-dried and stored as for the samples, yielded the following measured blank concentrations that were subtracted from corresponding sample values: 4.5/5.1 ng cm-2 for Fe (0.2 µm/2 µm), below the minimum detection limit for Mn (0.2 µm/2 µm), and 11.78/11.77 ng cm-2 for Al (0.2 µm/2 µm). The minimum detection limit (MDL) for individual elements using ED-XRF is defined as 3 times the square root of the background intensity measured from a standard of known concentration (Bertin, 2012; Buck et al., 2021), which yields the following MLD values: 0.95 ng cm-2 Fe (equivalent to ~0.13 nM pFe), 1.27 ng cm-2 Mn (equivalent to ~0.18 nM pMn), and 9.4 ng cm-2 Al (equivalent to ~2.8 nM pAl). Analytical precision and accuracy of the ED-XRF method were assessed from analyses of the NIST-2783 standard reference material (air particulate on filter media). Mean recoveries ± one standard deviation for individual elements in NIST-2783 by ED-XRF were 108% ± 2.6% for Fe, 104% ± 2.5% for Mn, and 92% ± 3.5% for Al (n = 320).

Temperature: In-situ temperature was measured using a conductivity-temperature-depth sensor (SBE 19 plus, SeaBird Scientific), with data processed using the SBE Data Processing software.

Salinity: Salinity was calculated from in-situ conductivity, as measured using a conductivity-temperature-depth (CTD) sensor (SBE 19 plus, SeaBird Electronics), with data processed using the SBE Data Processing software.

Fluorescence: In-situ chlorophyll fluorescence was measured using a WET Labs ECO-FL(RT)D deep chlorophyll fluorometer with 125 micrograms per liter (μg L-1) range mounted on the CTD rosette, with data processed using the SBE Data Processing software.

Dissolved Oxygen: In-situ dissolved oxygen concentration was measured using an SBE 43 Dissolved Oxygen Sensor mounted on the CTD rosette, with data processed using the SBE Data Processing software.