Profiles of dissolved iron-binding ligands and their conditional stability constants from R/V Knorr KN199-04, KN204-01, subtropical North Atlantic Ocean, 2010-2011 (U.S. GEOTRACES NAT project) (NCEI Accession 0278050)

Acquisition Description:
Sample collection and filtering: Water column samples were collected by the U.S. GEOTRACES sampling team from 24 modified 12 L Teflon-lined GoFlo samplers (General Oceanics) deployed on the GEOTRACES clean CTD rosette system. The U.S. GEOTRACES sampling team filtered the samples through 0.2 µm Supor Acropak filter cartridges (Pall) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface samples (~2 m depth) were collected underway either on approach or leaving each station using the Ken Bruland lab towfish (“GeoFish”) deployed to collect surface samples from outside the wake of the ship. Seawater from the towfish was pumped directly into the Bruland lab clean air laboratory van and filtered through 0.2 µm Supor Acropak filter cartridges. Filtered seawater samples for Fe speciation (organic complexation) analyses were collected in 500 mL acid-cleaned Nalgene narrow-mouth fluorinated high-density polyethylene bottles that were conditioned with a Milli-Q fill for at least two weeks, and rinsed three times with filtered sample prior to sample collection. All filtered seawater samples for dissolved iron speciation from leg 1 were provided directly to the PI (Buck), who either analyzed the sample shipboard or froze the sample at -20 ºC for laboratory-based analyses post cruise.

Dissolved iron speciation (organic complexation) analyses: Dissolved Fe speciation (organic complexation) was analyzed using a competitive ligand exchange- adsorptive cathodic stripping voltammetry (CLE-ACSV) method with salicylaldoxime as the added competing ligand (Buck et al. 2007, 2010, 2012), modified from the original Rue and Bruland (1995) method. Dissolved Fe additions of 0 to 7.5 nM were used in the titrations, for a total of at least 10 points in each titration. Deposition times of 90 to 120 s were applied to the analyses. All analyses were performed on Bioanalytical Systems (BAS i ) Controlled Growth Mercury Electrodes (static mercury drop setting, size 14) with Epsilon e2 (BAS i ) electrochemical analyzers. There are no reference samples available for iron speciation/ organic complexation measurements of iron in seawater, though this method was shown to compare well with other labs and techniques in the GEOTRACES intercalibration exercises (Buck et al. 2012, 2016).