Effects of iron surface adsorption and sample handling on iron solubility measurements

Seawater samples from two separate cruises in the Southern Ocean (ANTXXI/3 (EIFeX) and ANTXXIII/9) were collected for measurements of iron solubility by 55Fe addition. For both sets of samples, a significant loss of the dissolved portion of the added Fe was observed during the 72 hour duration of each Fe solubility measurement incubation. The decrease in dissolved Fe was related to Fe precipitation and adsorption onto bottle walls. The dissolved Fe data can be successfully modeled assuming that two colloidal Fe species (organically complexed Fe and inorganic Fe) were quickly formed following the addition of dissolved Fe(III) to the seawater. Model results indicate that Fe dissociated from weak organic complexes was the main contributor to wall sorption during the first 6 h following Fe addition, and that most of the Fe deposited after the first 6 h arose from the dissociation of colloidal inorganic species. Effects of sample freezing on Fe solubility measurements are also discussed.

► During Fe solubility incubations at seawater pH 40–60% of the added Fe disappeared.
► Loss of dissolved Fe was modeled by a simple “4 + 2” species model of Fe cycling.
► Dissociation of weak FeLW complexes and FeC was responsible for the Fe kinetics.
► Both species, FeLW and FeC, are part of the colloidal iron fraction.