Models of iron speciation and concentration in the stratified epipelagic ocean

Abstract

Surface ocean iron speciation is simulated using a time‐dependent box‐model of light‐mediated redox cycling over a range of aeolian inputs of soluble iron in the stratified epipelagic ocean. At steady‐state, Dissolved iron (DFe) concentration increases with aeolian input of soluble iron up to 0.1 μmol m−2 d−1, and is limited by the solubility of ferric hydroxide at higher fluxes which causes the formation of colloidal iron. We demonstrate that even in the presence of ample excess ligand, rapid conversion of dissolved iron between oxidized and reduced forms in the tropical surface ocean exposes DFe to colloid formation and scavenging. This result provides an explanation for the much smaller range of interregional variability in DFe measurements (0.05–0.4 nM) than soluble Fe fluxes (0.01–1 μmol m−2 d−1) and dust fluxes (0.1–10 g m−2 yr−1) predicted by atmospheric models. We incorporate the critical behavior of the full chemical speciation model into a reduced, computationally efficient model suitable for large scale calculations.