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Abstract: 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 72hour 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 6h following Fe addition, and that most of the Fe deposited after the first 6h arose from the dissociation of colloidal inorganic species. Effects of sample freezing on Fe solubility measurements are also discussed. [Copyright &y& Elsevier]

Abstract: Electrochemically derived iron speciation data from four vertical profiles to 1000m depth were obtained during the SAZ-Sense voyage to offshore waters south of Australia in summer (January/February, 2007). The dual aims of this study were firstly to devise a new operational definition to represent the ‘complexing capacity’, or total concentration of iron-complexing ligands, and subsequently derive vertical profiles of these ligand classes. Secondly, to compare the vertical trends for each ligand class with vertical distributions in oceanic properties thought to control ligand production (i.e. siderophores produced by bacteria and particle remineralisation). Based on simulated ligand titrations, we operationally defined ΣL as the overall class of ligands, which represents all iron-complexing ligands detectable under the analytical conditions chosen. The stability constant of ΣL is a weighted average for these ligands. The ligand titration data suggests the presence of an excess of iron-complexing ligands throughout the water column with an average concentration of [ΣL]=0.75±0.20nM (n=47), and an average stability constant of =21.50±0.24 (n=47). Here, based on the range of observed stability constants we define a distinctly different class of extremely strong ligands (L 1) to be the ligand class with a stability constant of ≥22 , whereas ΣL ranged from 21.00 to 21.95 for . L 1 had an average concentration and stability constant of 0.42±0.10nM (n=14) and 22.97±0.48 (n=14), respectively. L 1 was only found in three of the four depth profiles, and was restricted to the upper ocean (i.e. <200m depth), whereas ΣL was observed at all sampling depths down to 1000m. Heterotrophic bacterial abundances (a proxy for siderophore production) were always the highest in the surface mixed layer (50–72m depth for the 4 stations) then decreased sharply, whereas POC downward flux (a proxy for remineralisation) was greatest below the surface mixed layer then decreased exponentially. It has been suggested that siderophores control L 1 production whereas the remainder of ΣL may be set by particle breakdown (Hunter and Boyd, 2007). Hence we should expect some vertical partitioning of L 1 (present<70m depth) and ΣL (present over the water column). However, profiles at all stations in subtropical, subantarctic, and polar waters exhibited distinguishable concentrations of L 1 to 200m depth (i.e. straddling both regions of putative L 1 and ΣL production). There remain issues with the separation of different ligand classes, such that since [L 1]≤[Fe], deeper in the water column, the concentration of L 1 cannot be resolved, and hence the provenance of both L 1 and ΣL cannot be clearly assigned. [Copyright &y& Elsevier]

Labile Fe(II) distributions were investigated in the Sub-Tropical South Atlantic and the Southern Ocean during the BONUS-GoodHope cruise from 34 to 57 ° S (February-March 2008). Concentrations ranged from below the detection limit (0.009 nM) to values as high as 0.125nM. In the surface mixed layer, labile Fe(II) concentrations were always higher than the detection limit, with values higher than 0.060 nM south of 47 ° S, representing between 39% and 63 % of dissolved Fe (DFe). Apparent biological production of Fe(II) was evidenced. At intermediate depth, local maxima were observed, with the highest values in the Sub-Tropical domain at around 200 m, and represented more than 70 % of DFe. Remineralization processes were likely responsible for those sub-surface maxima. Below 1500 m, concentrations were close to or below the detection limit, except at two stations (at the vicinity of the Agulhas ridge and in the north of the Weddell Sea Gyre) where values remained as high as ~0.030-0.050 nM. Hydrothermal or sediment inputs may provide Fe(II) to these deep waters. Fe(II) half life times (t ½) at 4°C were measured in the upper and deep waters and ranged from 2.9 to 11.3 min, and from 10.0 to 72.3 min, respectively. Measured values compared quite well in the upper waters with theoretical values from two published models, but not in the deep waters. This may be due to the lack of knowledge for some parameters in the models and/or to organic complexation of Fe(II) that impact its oxidation rates. This study helped to considerably increase the Fe(II) data set in the Ocean and to better understand the Fe redox cycle. [ABSTRACT FROM AUTHOR]

Abstract: Vertical distributions of iron and iron binding ligands were determined in 2 size classes (dissolved <0.2μm, soluble <200kDa, e.g., ~0.03μm) in the Southern Ocean. Colloidal iron and complexing capacity (>200kDa–<0.2μm) were inferred as the difference between the dissolved and soluble fractions. Dissolved iron and ligands exist primarily in the soluble size range in the surface waters, although iron-complexing colloids still represent a significant portion of the dissolved pool and this fraction increases markedly with depth. This work presents evidence for the colloidal nature of a significant portion (37–51% on average) of the ‘dissolved’ organic Fe pool in these oceanic waters. From the data it was not possible to discern whether iron colloids exist as discrete organic complexes and/or inorganic amorphous colloids. Iron-complexing colloids are the most saturated with iron at the thermodynamic equilibrium, whereas soluble organic ligands occur in larger excess compared to soluble iron. It suggests that the exchangeable fraction for iron uptake through dissociation of Fe complexes likely occurs in the soluble fraction, and that soluble ligands have the potential to buffer iron inputs to surface waters whereas iron colloids may aggregate and settle. Expectations based on Fe diffusion rates, distributions and the stability of the soluble iron complexes and iron colloids also suggest that the weaker soluble Fe complexes may be more bio-available, while the strongest colloids may be a major route for iron removal from oceanic waters. Investigations of the size classes of the dissolved organic iron thus can significantly increase our understanding of the oceanic iron cycle. [ABSTRACT FROM AUTHOR]