Distribution and speciation of dissolved iron in the Sulu Sea and its adjacent waters

The Sulu Sea is a deep ocean basin that is isolated from the surrounding ocean by chains of islands and shallow sills less than 420 m in depth. Vertical distributions of dissolved (<0.22 μm) iron and natural organic iron-complexing ligands were determined in November–December 2002 for the Sulu Sea and its adjacent waters (the Philippine Sea, Celebes Sea and South China Sea) to elucidate effects of hydrography on the chemical speciation of dissolved iron. In the Sulu Sea, vertical profiles of dissolved iron concentrations ([D-Fe]) had peaks at less than 500 m depth. High [D-Fe] observed in the low-salinity surface waters suggest that river run-off from surrounding islands might be an important iron source for the Sulu Sea. The [D-Fe] values below 500 m were 0.43–1.68 nM; they decreased with increasing depth. The maxima were detected at 125–500 m depths. Unusually elevated temperatures of deep seawater in the Sulu Sea seem to be responsible for the shallow regeneration of dissolved iron from sinking organic matter. The maximum [D-Fe] that were found around 125–500 m depth were also partly attributable to the iron released from large amounts of particulate Fe from the continental slope that were resuspended in the deep seawater. Concentrations of natural organic iron-complexing ligands ([LT]) in the Sulu Sea were 0.4–1.3 nM. In the Sulu Sea, although water temperature, salinity and AOU were constant in the deep water, distributions of [D-Fe] and [LT] differed among stations. These heterogenous distributions were also observed in adjacent seas. The conditional stability constants of these ligands for Fe3+ ( K Fe 3 + L cond ) varied by two orders of magnitude, with values of 1022.3–1024.1; no significant difference was found between the Sulu Sea and its adjacent seas. This study found that measured [D-Fe] was higher than [LT]; excess [D-Fe] can exist as colloidal Fe and/or organic/inorganic complexes with weak ligands that were not detectable in this study. The presence of excess [D-Fe] for [LT] also was observed in adjacent waters, indicating that high temperatures in the deep water did not characterize excess [D-Fe] in the Sulu Sea. The chemical speciation of iron based on the thermodynamic equilibrium shows that iron exists as organically bound (>99%) in the surface water. The extent of organic complexation of dissolved iron at 500 m is lower (43–92%), suggesting the importance of iron dissolution process from particles in deep waters and the possible existence of colloidal iron or weak ligands. These results suggest that the vertical distributions of [D-Fe] and [LT] in the Sulu Sea and Celebes Sea differ from those obtained for other open and coastal seas. Geological features such as those near islands and deep basins in the Sulu Sea and Celebes Sea seem to influence the dynamics of iron strongly.