Sample Collection and Filtration
All seawater samples analyzed here for dissolved iron (dFe) concentration and isotope ratios (δ56Fe) were collected between 21 November and 4 December 2013 aboard the R/V Thomas G. Thompson as part of the U.S. GEOTRACES GP16 East Pacific Zonal Transect (Resing et al. 2015, Fitzsimmons et al. 2017, John et al. 2018). The EPZT encompassed 35 stations along an 8000 km transect that traversed from Peru to Tahiti (Peters et al. 2017, Moffett & German 2018). All seawater samples were collected as part of the standard U.S. GEOTRACES trace metal casts, using a trace metal clean carousel and CTD (Seabird) with a Kevlar conducting cable, fitted with 24 x 12-L Teflon-coated GO-Flo bottles (General Oceanics), following established GEOTRACES protocols (Cutter & Bruland 2012). Rosette deployments began with GO-Flo bottles open, allowing them to flush during deployment to depth, and the GO-Flo bottle was subsequently tripped on ascent at ~3 m/min, to minimize chances of contamination from rosette hardware. Upon recovery, GO-Flo bottles were transferred immediately into a clean van for subsampling and were pressurized using HEPA-filtered air. Each GO-Flo bottle was fitted with acid-cleaned Teflon tubing connected to a 0.2 um Acropak-200 capsule filter (Pall), allowing collection of filtered seawater for bulk dissolved Fe analysis into 4 L acid-clean low-density polyethylene (LDPE) bottles, after three 10% volume seawater rinses of the caps and threads (Cutter et al. 2017). Filtered seawater from Station 26 at 2650 m and 2700 m depths were combined to achieve sufficient volume for ultrafiltration processing.

Ultrafiltration and Sample Processing
Immediately after sample collection and 0.2 um filtration, 1 L of the 4 L homogenized sample was poured into a clean 1 L LDPE bottle for dissolved Fe analysis, and the remaining sample (~ 3 L) was ultrafiltered. Ultrafiltration took place using a cross flow filtration system (CFF) with a 10 kDa (~0.003 um for globular proteins; Erickson et al. 2009) regenerated cellulose membrane (Pellicon XL PLCGC) in single-pass mode (see Fitzsimmons and Boyle 2014 for detailed methods). Briefly, before collection, the CFF system was conditioned using ~500 mL of sample seawater, and the sample bottles, caps, and threads were rinsed with a minimal amount of ultrafiltered seawater. After conditioning, ~1 L of permeate (soluble fraction) and ~1 L of retentate were collected into separate 1 L LDPE bottles. All samples were then acidified to 0.012 M hydrochloric acid (HCl, Optima, Fisher) and stored for more than a year before analysis. Soluble Fe (sFe) is defined as the concentration in the permeate solution.

Dissolved Fe Concentration Analysis
Iron concentrations in the dissolved, permeate, and retentate phases were analyzed using a 15 mL aliquot of the 1 L samples. Samples were analyzed in the Sherrell lab at Rutgers University with a SeaFAST pico system (ESI, Omaha, NE, USA) and Thermo Element 1 HR-ICPMS following the SeaFAST methods of Lagerstrom et al. (2013). Samples were analyzed offline, as outlined by Jensen et al. (2020).

Dissolved Fe isotope ratio (δ56Fe) Analysis
Iron stable isotope ratios (δ56Fe) were determined using a double-spike multi-collector MC-ICPMS technique, based on Conway et al. (2013). All sample processing and Fe isotope analyses were carried out at the University of South Florida (USF). Chemical processing of samples took place in ISO-5 laminar flow hoods within the Marine Metal Isotope and Trace Element ISO-6 clean laboratory at the USF College of Marine Science. All ultrapure water used was from a Thermo Scientific Barnstead Genpure Ultrapure Water System; all plasticware was cleaned using standard procedures (Conway et al. 2013); and all reagents were Optima grade from Fisher Scientific. Briefly, the ~1 L of dissolved, permeate, and retentate seawater samples were spiked with a double spike containing 57Fe and 58Fe at a ratio of ~1:1, to provide a ~1:2 ratio of natural:spike Fe in samples. Following spiking, samples were amended to 10 mM H₂O₂ and left for 24 h after spiking. Samples were then brought to pH ~6, and Fe was extracted from seawater onto Nobias PA-1 chelating resin beads using a batch extraction technique. The Fe was then eluted from the Nobias resin using 3 M HNO₃, evaporated to dryness, refluxed with a conc. HNO₃ and conc. H₂O₂ mixture, evaporated to dryness, taken up in 1 M HCl, and purified using an AGMP-1 (BioRad) anion-exchange column chemistry technique following Sieber et al. (2019). Following column purification, samples were evaporated to dryness before being re-dissolved in 0.5 mL of 2% HNO₃ (v/v) for analysis by MC-ICPMS.

Samples were analyzed for Fe isotope ratios by Thermo Neptune Plus MC-ICPMS at USF’s Tampa Bay Plasma Facility at the USF College of Marine Science. Briefly, samples were introduced via a ~100 µL/min PFA nebulizer into an ESI Apex Ω desolvator (with Ar but no N₂ add gas), using a Ni Jet sampler cone and an Al X skimmer cone. Fe isotope ratios were measured using high resolution mode, and instrumental mass bias was corrected using the double spike technique. δ56Fe were calculated following an iterative procedure based on Siebert et al. (2001) and are expressed relative to the IRMM-014 international standard.

Problem report: Filtered seawater from Station 26 at 2650 m and 2700 m depths were combined to achieve sufficient volume for ultrafiltration processing.