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 K(FeL11,Fe(III)')/(cond) = 5 x 1012 M-1 and a mean concentration of 310 pM, and a weaker class (L2) with a conditional stability constant K(FeL22,Fe(III)')/(cond) = 6 x 1011 M-1 and a mean concentration of 190 pM. The total Fe(III)-binding organic ligand concentrations were ~25x 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.