Iron speciation in the Arabian Sea

Fe(III) speciation was measured in seawater collected as part of the United States Joint Global Ocean Flux (US JGOFS) Arabian Sea Process Study, Cruise TN045, March 14–April 10, 1995. The Fe-binding capacity of organic seawater ligands was measured in filtered seawater (<0.4 μm) collected from surface depths and throughout the oxygen minimum zone (OMZ). Seawaters from three stations on the southern line (S2, S9, and S11) were examined. Total Fe concentrations measured at the three sites ranged from: 1.25±0.21 nM to 1.30±0.01 nM (S2); 1.67±0.50 nM to 2.63±0.54 nM (S9); and 1.40±0.11 nM to 1.70±0.29 nM (S11). Cathodic stripping voltammetry (CSV) with 1-nitroso-2-napthol (1N2N) as the competitive ligand (pH 6.9) was used to determine conditional stability constants and Fe-binding ligand concentrations in seawater. Conditional stability constants for FeL complexes ranged from log KFeL=21.6±0.1 to 22.5±0.9 at the three sites. Total ligand concentrations ranged from 1.47±0.06 nM to 6.33±1.16 nM over all sites, but increased by a factor of 2–3 from the surface to the oxygen minimum zone (OMZ), suggesting that Fe-binding ligands may be produced during organic matter degradation. Ligand concentrations were consistently higher than total iron concentrations at every site measured, with an average “excess” ligand concentration of 2.15±1.50 (n=10). “Excess” ligand concentrations in the OMZ were 2 to 20 times higher than surface waters (upper 100 m). Formation-rate constants (kf) and dissociation-rate constants (kd) between added Fe3+ and seawater ligands were measured using a kinetic approach at ambient seawater pH, allowing independent calculation of the conditional stability constant, since K=kf/kd. Using the kinetic approach, conditional stability constants ranged from log KFeL=20.5±0.1 to 22.9±0.1. Although log K values are comparable in magnitude to those reported in the Pacific and Northwestern Atlantic Oceans, measured total ligand concentrations in the Arabian Sea are higher. This suggests that in areas that receive high Fe inputs through upwelling and/or atmospheric deposition, marine organisms may produce `excess’ ligands to keep Fe soluble in seawater for extended intervals.