Variation in Fe-organic complexation with depth in the Northwestern Atlantic Ocean as determined using a kinetic approach

1-nitroso-2-napthol (1N2N) was used as a complexing ligand to study Fe speciation and the kinetic interaction of Fe3+ with excess organic ligands in seawater. Two independent experimental approaches measured Fe speciation in seawater collected from the Northwestern Atlantic Ocean during an October 1996 cruise. Competitive ligand equilibration–cathodic stripping voltammetry (CLE–CSV), at pH 6.9 with complete equilibrium established for the samples, determined an average 1.8±0.8 nM ‘excessʹ of Fe-binding ligands. The conditional stability constants of these FeL complexes were invariant with depth, and ranged from log =22.2±0.5 at 500 m, to log =22.9±0.3 at 200 m. Kinetic experiments in which Fe3+ was added to pH 8.0 seawater from six depths revealed log values ranging between 20.1–22.7. Formation rate constants, kf, for inorganic Fe′ complexation by excess organic ligands ranged from 1.13×106 M−1 s−1 at 200 m to 4.21×104 at 2874 m. Dissociation rate constants, kd, for the recovery of inorganic Fe′ as Fe(1N2N)3 from FeL complexes ranged from 3.92×10−5 s−1 in surface water to 1.0×10−7 s−1 at 2000 m. The kinetic results show significant differences in the formation and dissociation rates for Fe-organic complexes, indicating different ligands are responsible for Fe complexation with depth. Results obtained using these two methods suggest that ‘excessʹ organic ligands in seawater complex Fe with conditional stability constants log K22, and slow the hydrolysis and/or polymerization of added Fe3+, thereby keeping Fe soluble, and perhaps more biologically available in seawater.