Control on dissolved iron concentrations in deep waters in the western North Pacific: Iron(III) hydroxide solubility

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Abstract
The vertical profiles of nutrient, AOU (apparent oxygen utilization), and Fe(III) hydroxide solubility (Fe(III) solubility) in the western North Pacific commonly show that their concentrations increased with depth below the surface mixed layer and have strong gradients around the North Pacific Intermediate Water (NPIW) in a potential density (σθ) range of 26.7–27.0. They had the maximum values at a σθ range of 27.0–27.5, and then rapidly decreased with depth at higher σθ than 27.5. The similarity of their profiles versus σθ suggests that they are controlled by similar processes with an intrinsic timescale such as deep‐ocean circulation. The vertical profiles of dissolved Fe were also characterized by mid‐depth maxima and, subsequently, a slight decrease with depth, which is markedly similar to nutrient and Fe(III) solubility depth profiles. This implies that the major source of dissolved Fe in the deep ocean is release during the remineralization of biogenic organic matter. In the present study, we attempted to confirm that the dissolved Fe depth profiles are controlled by the sinking particulate organic matter (POM), the production of dissolved Fe from POM during carbon remineralization, the scavenging of dissolved Fe and the temporal fixed Fe(III) solubility depth profile. Using the production rate (α) of dissolved Fe (1/α = 5–10 years), the calculated depth profile of dissolved Fe is in remarkable agreement with the observed profile of dissolved Fe with mid‐depth maxima. Therefore we concluded that dissolved Fe concentrations in deep ocean waters are controlled primary by the Fe(III) solubility.