Dataset: Dissolved iron speciation results from 2019 Bermuda Atlantic Iron Time-series (BAIT) cruises EN631, AE1909, AE1921, AE1930 in the Western Subtropical North Atlantic Gyre from March to November 2019 (BAIT Project)

Sampling and analytical methodology: 

Sample collection and filtering: Water column samples were collected by the BAIT sampling team from 12 trace metal clean modified 6-L Niskin-X samplers (General Oceanics Inc.) deployed on a clean CTD rosette system (Sedwick et al. 2005). The BAIT sampling team filtered the samples through 0.2 µm Supor Acropak filter cartridges (Pall) inside a clean-air laboratory van.

Filtered seawater samples for iron speciation (organic complexation) were collected in 500 mL acid-cleaned Nalgene narrow-mouth fluorinated high-density polyethylene (FPE) bottles that had been filled with Milli-Q for at least two weeks after acid cleaning and rinsed three times with filtered seawater prior to sample collection (Buck et al. 2012). All filtered seawater samples for dissolved iron speciation were frozen at -20 ºC shipboard and shipped to the University of South Florida for laboratory-based analyses post cruise.

Dissolved iron speciation (organic complexation) analyses:

Dissolved iron speciation (organic complexation) was analyzed using competitive ligand exchange- adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime as the added competing ligand (Buck et al. 2015), modified from the original Rue and Bruland (1995) method. A borate-ammonium buffer was added to all vials, for a final pH of 8.2 (NBS scale; Buck et al. 2007, 2012). Dissolved iron additions of 0 to 10 nM were used in the titrations, for a total of 15 points in each titration. CLE-AdCSV analyses were performed on Bioanalytical Systems (BASi) Controlled Growth Mercury Electrodes (static mercury drop setting, size 14) with Epsilon e2 (BASi) electrochemical analyzers. Deposition time of 120 s was applied to the analyses. There are no reference samples available for iron speciation measurements of iron in seawater, though this method has been shown to compare well with other labs and techniques in GEOTRACES intercalibration exercises (Buck et al. 2012, 2016).