Iron(III) hydroxide solubility and humic-type fluorescent organic matter in the deep water column of the Okhotsk Sea and the northwestern North Pacific Ocean

Vertical distributions of Fe(III) hydroxide solubility were studied in the Okhotsk Sea and the northwestern North Pacific Ocean during May and June 2000. Fe(III) solubility minima (0.35-0.45 nM) were present in a narrow depth range (80-100 m) below the surface mixed layer at all stations. In general, the Fe(III) solubility levels in intermediate and deep waters are characterized by mid-depth maxima (0.76-0.86 nM) at 800-1250 m depth and, below that, a slight decrease to 0.4-0.6 nM with depth in association with increase in nutrient, apparent oxygen utilization (AOU) and humic-type fluorescence intensity. The most significant correlation between the Fe(III) solubility and humic-type fluorescence in intermediate and deep waters suggests that the distribution of humic-type fluorescent organic matter may control the distribution of Fe(111) solubility in deep ocean waters. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters may be controlled primarily by Fe(III) complexation with natural organic ligands, such as marine dissolved humic substances released through the oxidative decomposition and transformation of biogenic organic matter in intermediate and deep waters. In addition, high Fe(III) hydroxide solubility values (1.0-1.6 nM) were observed in the surface mixed layer at a station in the northwestern North Pacific Ocean where a phytoplankton bloom was observed. The higher Fe(III) solubility in the surface waters was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators, which may be released by dominant phytoplankton and/or bacteria during the spring bloom and probably have a different chemical composition from those found in intermediate and deep waters. (C) 2003 Elsevier Ltd. All rights reserved.