Controls on iron distributions in the deep water column of the North Pacific Ocean: Iron(III) hydroxide solubility and marine humic‐type dissolved organic matter

Abstract

Dissolved Fe in the western and central North Pacific Ocean was characterized by surface depletion, middepth maxima and, below that, a slight decrease with depth similar to the vertical distributions of nutrients, apparent oxygen utilization, Fe(III) hydroxide solubility, and humic‐type fluorescence (H‐flu) intensity. Dissolved Fe concentrations ([D‐Fe], <0.22‐ μ m fraction) in the deep water column were one‐half lower in the central region (0.3–0.6 nM) than the western region (0.5–1.2 nM) although the Fe(III) solubility ([Fe(III)sol], <0.025‐ μ m fraction) levels and distributions in deep waters were almost the same between both regions with middepth maxima (∼0.6 nM) at 500–1500‐m depth range and then a gradual decrease to ∼0.3 nM at 5000‐m depth. Higher [D‐Fe] than [Fe(III)sol] in the deep water column of the western region results from the higher production of dissolved Fe from the decomposition of sinking particulate organic matter in the western region than the central region because of the high atmospheric and/or lateral Fe inputs in the western region. Similarity between [D‐Fe] level and [Fe(III)sol] value at each deep water depth in the central region may be attributed to [D‐Fe] being nearly in the solubility equilibrium with Fe(III) hydroxide in seawater. Strong linear correlation between [D‐Fe] and H‐flu intensity in the central region and relatively similar linear relationships between [Fe(III)sol] and H‐flu intensity in the western and central regions are the first confirmation that humic‐type fluorescent dissolved organic matter may be responsible for [D‐Fe] in the deep water column as natural organic ligands complexing with Fe(III).