Organic Iron Speciation in the Arctic Ocean

Objectives

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.

Methods

General

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).

Total and dissolved iron was measured directly on board by Flow Injection Analysis (FIA) after de Jong et al. 1998 in a cleanroom container. Reference M. Klunder (separate data set, see Dissolved_Fe_Arctic_Klunder_IPY35_NL)

Sampling statistics

4 deep stations have been sampled in order to characterize and show any difference between 3 basins in the Arctic Ocean: Nansen basins (2950 m and 3935 m), Amundsen basin (4500 m) and Makarov basin (3900 m). 3 shallow stations have also been sampled to show the influence of the river inputs on these basins: Barent Sea (175 m), Kara Sea (310 m) and Laptev Sea (56 m).