Vertical distributions of iron(III) hydroxide solubility and dissolved iron in the northwestern North Pacific Ocean

Abstract

Detailed vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable (0.3‐2.4 nM), corresponding with the depth of high chlorophyll a concentrations, and the solubility minima (0.2‐0.4 nM) occurred at 75‐125 m depth. The vertical profiles of Fe(III) hydroxide solubility in mid‐depth and deep waters are characterized by mid‐depth maxima (0.6‐0.7 nM) and, subsequently, a slight decrease to 0.4‐0.5 nM with depth, which is markedly similar to nutrient and dissolved Fe depth profiles. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands, which were released through the oxidative decomposition and transformation of biogenic organic matter in mid‐depth and deep waters.