The organic complexation of dissolved iron along the U.S. GEOTRACES (GA03) North Atlantic Section

An Intercomparison of Dissolved Iron Speciation at the Bermuda Atlantic Time-series Study (BATS) Site: Results from GEOTRACES Crossover Station A

“…Dissolved Fe concentrations differed through the water column between the two occupations (Figure 2A; Rijkenberg et al, 2014;Sedwick et al, 2015). In both cases, however, organic Fe-binding ligands were reported in excess of DFe concentrations throughout the water column, and thermodynamically stable ("strong") organic DFe complexes were the dominant chemical form of DFe at all depths (Figures 2A-F; Buck et al, 2015;Gerringa et al, 2015).…”

“…In November 2011, Buck et al (2015) reported two ligand classes at BATS. The stronger L 1 -type ligand class was measured with very similar concentrations ([L 1 ] = 0.82-1.51 nM Eq Fe, average: 1.16 ± 0.21 nM Eq Fe, n = 37) and conditional stability constants (average log K cond FeL 1 ,Fe ′ = 12.42 ± 0.23, n = 37) to the single ligand class reported by Gerringa et al (2015) for this station in June 2010 ( Figure 2B).…”

“…DFe additions were allowed to equilibrate with natural ligands in the samples for at least 2 h, or overnight at 4 • C in the refrigerator (Buck et al, 2007). An addition of 25 µM SA (analytical window, α FeSA = 79; Buck et al, 2007) to each titration vial was then made and allowed to equilibrate for at least 15 min prior to commencing AdCSV analyses (Rue and Bruland, 1995;Buck et al, 2007Buck et al, , 2015. Each titration point was analyzed once, generating a total of 12 points in each sample titration.…”

“…Titration data generated by CLE-AdCSV with SA was interpreted using a combination of van den Berg/Ružić and Scatchard linearization techniques (Scatchard, 1949;Ružić, 1982;van den Berg, 1982) as described previously Bruland, 1995, 1997;Buck et al, 2007Buck et al, , 2012Buck et al, , 2015. In this approach, the sensitivity S for each sample titration was determined from the slope of the last few titration points.…”

“…Sample analyses for Fe-binding ligands using the SA method were conducted as described in Buck et al (2015) on a BioAnalytical Systems (BASi) controlled growth mercury electrode (static drop setting, drop size 14) coupled to a BASi Epsilon ε2 voltammetric analyzer. Frozen speciation samples were thawed at room temperature, shaken, and distributed in 10-mL aliquots into lidded Teflon vials (Savillex) that had been previously conditioned with the expected Fe and SA additions in buffered Milli-Q.…”