The role of organic complexation on ambient iron chemistry in the equatorial Pacific Ocean and the response of a mesoscale iron addition experiment

Abstract

We participated on the Iron‐Ex II study in the equatorial Pacific to investigate ambient iron chemistry and its response to a mesoscale iron addition experiment. As expected, dissolved iron values in the high‐nutrient, low‐chlorophyll surface waters of the South Equatorial Current (4–6°S, 105–109°W) were extremely low (∼20 pM). In studies of the chemical speciation of dissolved iron, we found two classes of Fe(III)‐binding organic ligands—a strong ligand class (L1) with a conditional stability constant and a mean concentration of 310 pM, and a weaker class (L2) with a conditional stability constant and a mean concentration of 190 pM. The total Fe(III)‐binding organic ligand concentrations were ∼25× higher than total dissolved iron concentrations (∼20 pM). Thermodynamic equilibrium calculations indicate that >99.9% of the ambient dissolved Fe(III) would be complexed with these organic ligands and exist as low‐molecular‐weight Fe(III) chelates, with <0.1% (15 fM or ∼10−14 M) existing as an inorganic Fe(III)′ fraction. In the primary Iron‐Ex II study, the total Fe(III)‐binding ligand concentrations increased by 400% to ∼2 nM within a day or two after the initial 2 nM mesoscale iron injection. Most of this increase was due to the stronger ligand class, L1, which increased from 0.3 nM to 1.3 nM. The weaker ligand class increased by a similar percentage, although the total magnitude was smaller. Similar increases in Fe(III)‐binding ligand concentrations were observed in a secondary Iron‐Ex II experiment within 17 h after an initial low‐iron 0.4 nM infusion. Results obtained from samples collected throughout the entire course of the iron‐enrichment experiments suggest that these Fe(III)‐binding organic ligands are produced rapidly in response to small iron additions to this low‐iron regime. The chelated iron appears to be directly or indirectly accessible for growth.