Netherlands Polar Data Center

The distribution and biological availability of Fe is strongly controlled by its physical-chemical speciation within seawater, where colloids and Fe-organic complexes are dominant factors. In order to study the distribution and the biological availability of Fe, the natural Fe organic complexes over the whole water depth were determined in three different size fractions. Special attention was given in that distinct water masses present were sampled as well. Samples were collected by an ultra-clean sampling system using 24 Go Flo bottles fixed on an all-titanium frame and with a Kevlar cable. The concentration of iron binding ligands (organic compounds which strongly bind Fe) and their binding strength (conditional stability constant) are studied in 3 size classes here: unfiltered water, 0.2 µm filtered water and smaller than 1000 KDa ultra-filtrated water.

Under ultra clean conditions the 0.2 µm filtered seawater was ultra-filtrated using polyethylene hollow-fiber filters as to make an operational defined distinction between large colloidal and small colloidal Fe including the “truly dissolved” Fe (1000 KDa nominal weight, Stereapore, Mitsubishi-rayon Co. Ltd, Nishioka and al., 2000, 2005). The dissolved organic iron (0.2 µm filtered) as well as the truly dissolved iron (< 1000 KDa) were analysed by Maarten Klunder and Patrick Laan using a chemo luminescence method (FIA) with acidified samples (pH 1.8). Total iron will be measured 6-12 months after the acidification of the unfiltered sample. The natural ligand characteristics were determined by doing a complexing ligand titration with addition of iron (between 0 and 8 nM of Fe added) in buffered seawater (mixed NH3/NH4OH borate buffer, 5 mM). The competing ligand ‘TAC’ (2-(2-Thiazolylazo)-p-cresol) with a final concentration of 10 µM was used and the complex (TAC)2-Fe was measured after equilibration (> 15 h) by cathodic stripping voltammetry (CSV) (Croot and Johansson, 2000). The electrical signal recorded with this method (nA) was converted as a concentration (nM), then the ligand concentration and the binding strength were estimated using the non-linear regression of the Langmuir isotherm (Gerringa and al., 1995).

The voltammetric equipment consisted of a µAutolab potentiostat (Type I, II and III, Ecochemie, The Netherlands), a mercury drop electrode (model VA 663 from Metrohm). All equipment was protected against electrical noise by a current filter (Fortress 750, Best Power).

Seven stations were sampled on the zero meridian transect with a maximal depth of 4500 m. A total of 140 samples on 56 depths were sampled (28 of unfiltered, 56 of 0.2 µm filtered and 56 of 1000 KDa ultra-filtered). Among them, 11 depths characterizing the most important water-masses were sampled twice and kept frozen for later analyses while back at NIOZ (for the study of kinetic exchange between the different forms of iron).

Two profiles were sampled in the Weddell Sea for a total of 46 samples (8 of unfiltered, 19 of 0.2 µm filtered and 19 of 1000 KDa ultra-filtered). 8 depths were also sampled twice to characterize important water-masses.