Kinetics and thermodynamics of iron species in the Southern Ocean

Iron solubility and ligand measurements performed during this PhD work showed that strong organic ligands increase iron solubility and therefore the residence time of Fe in seawater. Measurements made during a mesoscale Fe fertilization experiment (EIFeX) in the Southern Ocean showed that both the soluble and colloidal fractions of dissolved organic ligands are thermodynamically stable in seawater. The composition of the ligand pool (in terms of the ratio of soluble to colloidal ligands) in the upper mixed layer (20 - 80 m) shifted towards a greater proportion of soluble ligands after the peak of biomass was reached in the Fe fertilization experiment. Prior to this, the soluble and colloidal ligand fractions were in some sort of equilibrium since dissolved ligand concentrations showed a linear correlation with iron solubility. After the peak of biomass was reached, the same ligand composition as observed below 80 m during the experiment was found in the whole upper mixed layer (20 - 300 m). Organic ligands appeared to have been directly released by bacteria. In contrast to the measurements from the Southern Ocean, measurements made in the Mauritanian upwelling zone showed a significant correlation between indicators of organic matter remineralization (pH, oxygen, and phosphate) and iron solubility in subsurface samples (40 - 80 m). The main increase in iron solubility could be attributed to the remineralization of organic matter. Lab based ultrafiltration experiments on ligand solutions made up in seawater showed that this technique is a powerful tool for separating the particulate and colloidal from the soluble iron fraction. Mass balance calculations showed that a significant portion of the added 55Fe had disappeared from the feed solution before the ultrafiltration was started and yet more disappeared during each cycle of ultrafiltration. Iron was adsorbing onto bottle walls and onto surfaces within the ultrafiltration unit. Future work using trace-metal ultrafiltration needs to recognize that these effects (wall sorption and filter loading) occur and must be taken into account in mass balance calculations