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, Cruis e TN045, March 14-April 10, 1995. The Fe-binding capacity of organic seawat er ligands was measured in filtered seawater ( < 0.4 mu m) collected from s urface 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 nhl (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) wa s used to determine conditional stability constants and Fe-binding ligand c oncentrations in seawater. Conditional stability constants for FeL complexe s ranged from log K-FeL = 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 n M 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 produc ed during organic matter degradation. Ligand concentrations were consistent ly higher than total iron concentrations at every site measured, with an av erage "excess" ligand concentration of 2.15 +/- 1.50 (n = 10). "Excess" lig and concentrations in the OMZ were 2 to 20 times higher than surface waters (upper 100 m). Formation-rate constants (k(f)) and dissociation-rate const ants (k(d) ) between added Fe3+ and seawater ligands were measured using a kinetic approach at ambient seawater pH, allowing independent calculation o f the conditional stability constant, since K = k(f)/k(d). Using the kineti c approach, conditional stability constants ranged from log K-Fel = 30.5 +/ - 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 t otal ligand concentrations in the Arabian Sea are higher. This suggests tha t in areas that receive high Fe inputs through upwelling and/or atmospheric deposition, marine organisms may produce'excess' ligands to keep Fe solubl e in seawater for extended intervals. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.