The vertical distributions of dissolvable (unfiltered) Fe concentrations in semi-closed oceanic regions, such as the Japan Sea and the Bering Sea, are characterized by a gradual increase with depth in the intermediate and deep waters. However, there is a rapid decrease in the dissolvable Fe concentration over the narrow depth range between deep and bottom waters with constantly lower concentrations observed in the bottom waters of the Japan Sea (Japan Basin), probably from the injection of newly formed bottom water. In addition, the rapid increase in dissolvable Fe concentrations in bottom waters in the western North Pacific Ocean may be due to the resuspension of sediments from the seafloor or the slope. However, there are no differences of labile dissolved (filtered) Fe concentrations and Fe(III) hydroxide solubility between deep and bottom waters, revealing that dissolved Fe concentrations in deep waters may be controlled primarily by Fe(III) complexation with natural organic ligands. Dissolved Fe concentrations, humic-type fluorescence intensity, and Fe(III) solubility correlate well with PO4 with different slopes between intermediate and deep waters in the North Pacific Ocean. However, there is no different depth regime between them in semi-closed oceanic regions. The most statistically significant correlation between the Fe(III) solubility and fluorescence intensity was found with a nearly linear relation in intermediate and deep waters at all stations of the semi-closed and open oceanic regions. This result suggests that the distributions of humic-type fluorescent organic matter may be responsible for Fe(III) solubility and dissolved Fe concentrations in deep oceanic waters.
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