Distinct trends in dissolved Fe speciation between shallow and deep waters in the Pacific Ocean

The chemical speciation of dissolved Fe was investigated in the Pacific Ocean to elucidate the regional variability of organic ligands. The concentration and conditional stability constants (K′Fe′L, with respect to inorganic Fe) of organic ligands were analyzed by a Competitive Ligand Equilibration-Adsorptive Cathodic Stripping Voltammetric (CLE-ACSV) method using 2-(2-thiazolylazo)-p-cresol (TAC). In the surface waters of the Pacific Ocean, a low concentration of dissolved Fe was observed (0.20 ± 0.14 nM, n = 8) with an excess of organic ligands (0.27 ± 0.14 Eq of nM Fe, n = 7); thus, most of the dissolved Fe was estimated to be complexed with these ligands. A 2D cross-section diagram along 160°W and 170°W showed a roughly increasing trend in the concentration of organic ligands toward the north of the deep waters, suggesting the accumulation of ligands in the biogeochemical cycle of deep water. In the equatorial Pacific and central subtropical North Pacific, the concentration of organic ligands was observed to exceed that of dissolved Fe throughout the water column. On the contrary, the concentration of dissolved Fe at depths ranging from 1000 to 3000 m was higher than that of organic ligands in the western North Pacific, suggesting that the excess dissolved Fe can exist as colloidal Fe and/or organic/inorganic complexes with the ligands that were not detectable by our method. A greater concentration of excess dissolved Fe relative to the concentration of organic ligands was also observed in the deep (below 1000 m) waters of the subtropical South Pacific. These differences regarding the saturation state of organic ligands ([L]/[D–Fe]) between the surface and deep waters probably reflect different sink and source mechanisms of organic ligands within the water column. The extent of the organic complexation of Fe varies with the biogeochemical processes in the ocean interior and plays a key role in oceanic Fe cycling.

► We investigate the speciation of dissolved Fe in the Pacific Ocean.
► Most of the dissolved Fe was complexed with organic ligands in the surface waters.
► Excess dissolved Fe with respect to the organic ligand concentration was found in the deep waters.
► The organic ligands tended to be more saturated with Fe in the deep waters.
► Biogeochemical processes affect the extent of organic Fe complexation in seawater.