Sampling at the sediment-water interface was carried out using a benthic lander called Susane built at IFREMER by Joel Knoery (Knoery et al., 2019). It was deployed using a 0.322 cable from the port side squirt boom on the Oceanus. The lander was deployed at the seafloor and syringes sampled water at 10-centimeter (cm) intervals while it sat on the seafloor. Transfer of sampled water to the analytical system is described below.

Water column samples were collected using a Seabird Rosette equipped with an SBE50 pressure sensor and an autofire module. It was pre-programmed and deployed on amsteel cable and fired at select depths). Bottles were 5 liter (L) Niskin-style acquired from Ocean Test Equipment. They were teflon-coated and had external teflon-coated springs.

All iron (Fe) samples were filtered using Pall Gelman Supor 0.45 micrometer (μm) polyethersulfone filters with either Millipore Swinnex polypropylene 25-millimeter (mm) filter holders or Advantec-MFS type PP47 47 mm polypropylene inline filter holders, as recommended by the GEOTRACES Cookbook version 3.0 (Cutter et al., 2017). Nitrogen (N2) gas was used to pressurize the Go-Flo bottles during filtration. Filtered fractions for Fe(II) measurement were transferred to a syringe using a three-way Luer-lock adaptor to prevent the introduction of air, then measured immediately. Fe(II) was measured via chemiluminescence with luminol, consisting of a continuous 2 milliliters per minute (mL min-1) flow of sample and luminol solution at a 1:1 ratio into a standard quartz flow cell with a Hamamatsu HC135 photon counter via a peristaltic pump. The signal value for each sample was determined as the mean of the signal for 30 seconds (n=50), once the signal reached a plateau. Because luminol chemiluminescence is not selective for Fe(II), each sample was also treated with diethylenetriamine pentaacetate (DTPA), a selective Fe(II) chelator used as a masking ligand. The DTPA-treated samples, with a final concentration of 0.5 millimolar (mM) DTPA, serve to correct for chemiluminescence caused by compounds besides Fe(II). DTPA-treated samples were processed with the exact same analytical train as untreated Fe(II) samples. The order of sample measurement was randomized to prevent systematic error due to sample oxidation. Fe(II) calibration was performed each time the system was powered on and calibration curves consisted of six or more concentrations within the appropriate working range of the measured Fe(II). These concentrations were created by spiking a seawater solution that aged for at least 24 hours in an amber bottle with an appropriate volume of 1 micromole (μM) Fe(II) working stock in milliQ water. This working stock was created fresh before calibration from a 0.01 mole (M) Fe(II) standard solution at pH 2, prepared monthly by dissolving Optima grade hydrochloric acid (Fisher) and ammonium ferrous sulfate (Fluka) in milliQ water. Luminol solutions were prepared first using a stock solution with 0.796 grams (g) of sodium luminol (Sigma), 250 mL of Optima grade ammonium hydroxide (Fisher), approximately 45 mL of Optima grade hydrochloric acid (Fisher), and milliQ water. The working luminol reagent was produced by diluting this stock to one-fourth of its starting concentration, then heating it at 50º Celsius for 9 to 12 hours. The DTPA solution was created by diluting 2.4584 g of DTPA (Millipore-Sigma) with sodium hydroxide (Aldrich, trace metal basis) in milliQ water for a final concentration of 50 mM DTPA and 200 mM NaOH. Additional details of this method including reagent preparation, instrumental settings, and procedures have been previously reported (Bolster et al., 2018).

Total dissolved Fe (dFe) samples were collected into 500 mL LDPE bottles. Within one month of collection, these samples were acidified to a pH of approximately 1.7 by adding 1 mL of hydrochloric acid (Optima, Fisher Scientific) per 500 mL of seawater. After one year of storage, samples were prepared in triplicate using the seaFAST-pico (Elemental Scientific) offline method, where 10 mL of acidified sample was added to a Nobias resin chelation column and then eluted with ultra-pure, distilled 5% nitric acid (Optima, Fisher Scientific) for a final volume of 0.5 mL. This method is similar to the method described in Jackson et al. (2018) and Rapp et al. (2017). Fe concentrations were quantified with a Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma-Mass Spectrometer and Apex desolvation system in medium resolution. A 1 part per billion (ppb) indium eluent is used during sample preparation as an internal standard to correct for instrument drift. Instrument optimization and tuning are performed daily with a 1 ppb indium and uranium tuning solution. Procedural blanks were prepared in triplicate from pH 1.7 hydrochloric acid to correct for contamination during acidification of samples. Sample concentration of Fe was determined via the isotopic dilution method in which acidified samples were spiked with a multi-element standard, including 57Fe, enriching it over the natural isotopic abundances.

Oxygen (O2) concentrations from the benthic boundary gradient sampler were measured less than an hour from recovery using a Presens TX-3 microoptode in a flow cell fitted to the sampling syringe outlets. Between each O2 measurement, brief flushes of nitrogen gas were also used to check for calibration shifts and remove contamination between samples. The order of sample measurement was randomized to reduce the influence of any contamination post-recovery.