Dataset: Fe(II) measured shipboard on the GP17-ANT RVIB Palmer cruise NBP24-01 in the Amundsen Sea from November 2023 to January 2024

All reagents were prepared in acid-washed opaque low-density polyethylene (LDPE) bottles, to prevent trace metal contamination or degradation of the reagents by light. The luminol reagent was initially prepared as a concentrate, by mixing 0.796 grams (g) sodium luminol (Sigma), 250 milliliters (mL) Optima grade ammonium hydroxide solution (Fisher), high-purity water (18.2 megohms per centimeter (MΩ cm⁻¹)), and approximately 45 mL Optima grade hydrochloric acid (Fisher) to reach a final volume of 1 liter (L) and a pH of 10.25. To produce the working reagent, the concentrated mixture was diluted in high-purity water by a factor of four and heated at 50 degrees Celsius (°C) for 9 to 12 hours in either an oven or a hot water bath. DTPA was prepared in high-purity water with final concentrations of 50 millimolar (mM) DTPA (Sigma-Aldrich) and 200 mM NaOH (Aldrich, trace metals basis). EDTA was prepared in high-purity water with a final concentration of 50 mM EDTA (Aldrich, trace metals basis) and 100 mM NaOH (Aldrich, trace metals basis). The MOPS (3-(N-Morpholino)propanesulfonic acid) buffer was prepared with a final concentration of 100 mM MOPS (Sigma) and 50 mM NaOH (Aldrich, trace metals basis) in high-purity water, so that the final pH was 7.2. All reagents were used within 1 month of preparation.

Because iron(II) oxidizes rapidly in seawater, standards were prepared individually and analyzed immediately. A primary stock of 10⁻² M Fe(II) was prepared by dissolving ammonium iron(II) sulfate in high-purity water acidified to pH 2 using Optima grade hydrochloric acid (Fisher). The low pH slows oxidation to negligible levels, but the standard was replaced monthly. A working stock was prepared by diluting the primary stock by a factor of 10⁴ in high purity, creating a 10⁻⁶ M solution at pH 6. This solution would oxidize to a significant extent within several hours, so it was always prepared fresh immediately before a calibration curve or an experiment using standards. This working stock was then added to iron(II) free seawater, acquired by filtering seawater through a 0.2 micrometer (μm) filter and storing in an opaque bottle at room temperature for at least 24 hours.

Dissolved iron(II) concentrations were measured using a luminol chemiluminescence-based FeLume flow-injection system (Waterville Analytical), with data collection and integration controlled by Waterville Analytical software. The hardware was similar to that used in a number of previous open-ocean studies. A standard quartz flow cell was used in conjunction with a Hamamatsu HC135 photon counter. The flow rate through the peristaltic pump was 2 milliliters per minute (mL min⁻¹). The photon counter integration time was 200 milliseconds (ms), with two measurements per second for up to 360 seconds. All samples were introduced in acid-washed Teflon bottles.