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This dataset includes the concentrations of iron (Fe) bound to humic-like substances and total iron-binding capacity of humic-like substances from discrete depth profile and towfish samples collected on the GP17-ANT cruise NBP24-01 on the R/V IB Nathaniel B. Palmer in the Amundsen Sea from 29 November 2023 to 28 January 2024. This project investigates the impact of Fe-binding humic-like substances on Fe biogeochemistry in the Amundsen Sea. The project used a combination of depth profile and surface towfish samples to characterize the contributions of humic-like substances to Fe biogeochemistry across gradients in primary production, water masses, and bathymetric features in the Amundsen Sea. Understanding the distributions of Fe-binding humic-like substances provides insight into the impact of compositional changes of the Fe-binding ligand pool on Fe biogeochemistry and may help elucidate specific sources of these ligands. 

The method of competitive ligand exchange followed by adsorptive cathodic stripping voltammetry (CLEAdCSV) allows for the determination of dissolved iron (DFe) organic speciation parameters, i.e., ligand concentration (LFe) and conditional stability constant (log Kcond Fe′L ). Investigation of DFe organic speciation by CLEAdCSV has been conducted in a wide range of marine systems, but aspects of its application pose challenges that have yet to be explicitly addressed. Here, we present a set of observations and recommendations to work toward establishing best practice for DFe organic speciation measurements using the added ligand salicylaldoxime (SA). We detail conditioning procedures to ensure a stable AdCSV signal and discuss the processes at play during conditioning. We also present step-by-step guidelines to simplify CLE-AdCSV data treatment and interpretation using the softwares ECDSoft and ProMCC and a custom spreadsheet. We validate our application and interpretation methodology with the model siderophore deferoxamine B (DFO-B) in a natural seawater sample. The reproducibility of our application and interpretation methodology was evaluated by running duplicate titrations on 19 samples, many of which had been refrozen prior to the duplicate analysis. Nevertheless, 50% of the duplicate analyses agreed within 10% of their relative standard deviation (RSD), and up to 80% within 25% RSD, for both LFe and log Kcond Fe′L . Finally, we compared the sequential addition and equilibration of DFe and SA with overnight equilibration after simultaneous addition of DFe and SA on 24 samples. We found a rather good agreement between both procedures, with 60% of samples within 25% RSD for LFe (and 43% of samples for log KcondFe′L ), and it was not possible to predict differences in LFe or log KcondFe′L based on the method applied, suggesting specific association/dissociation kinetics for different ligand assemblages. Further investigation of the equilibration kinetics against SA may be helpful as a potential way to distinguish natural ligand assemblages.