Spatial variability of Fe(III) hydroxide solubility in the water column of the northern North Pacific Ocean

Solubilities (pH 8.0–8.2, 20°C) of amorphous hydrous ferric oxide (Fe(III) hydroxide) in seawater samples collected on two transects (38°30′–47°30′N along 170°00′E and 175°30′E longitudes) in the northern North Pacific Ocean were experimentally determined by a simple filtration (0.025 μm) involving γ-activity measurement of 59Fe. The vertical profiles of Fe(III) hydroxide solubility in the open-ocean waters have the following features in common: the solubility in the surface mixed layer (0–50 m) is high and variable (0.5–3.6 nM), sometimes corresponding with depth of high chlorophyll a concentrations, and is the highest (2.5–3.6 nM) in the boundary zone (42–44°N) between subtropical and subarctic water masses; the solubility minima (0.14–0.39 nM) occur at depth of 50–200 m, below the surface mixed layer, and there is a northward increase in the minimum value of solubility at the subsurface; the subsequent solubility levels appear to increase with depth in association with the increase in nutrient concentrations at lower latitude (0.3–0.7 nM) or to vary little in middepth waters with high nutrient through a water column at higher latitude (0.5–0.7 nM). The high Fe(III) hydroxide solubility observed in the surface mixed layer in the boundary zone is probably due to higher concentration or stronger affinity of natural organic Fe(III) chelators, which were possibly released by particular phytoplankton or cyanobacteria species through their metabolism. The fact that the solubility minima are present at narrow depth ranges in the subsurface suggests that the produced organic chelators are consumed or degraded in the surface layer. The subsequent increasing solubility in middepth waters would be due to the organic Fe(III) chelators produced through the decomposition and transformation of biogenic organic matter, resulting in a strong correlation between the Fe(III) hydroxide solubility and nutrient concentration in middepth waters (⩾50–100 m) below the depth of the solubility minima. The chemical composition of organic chelators in middepth waters may be different from those in the surface waters. These results support previous conclusions that natural organic Fe(III) chelators exist in significant concentration, controlling the dissolved iron concentration in oceanic waters, and have significant implications for the distribution of phytoplankton and cyanobacteria between oligotrophic and eutrophic systems.