Speciation of Fe in the Eastern North Atlantic Ocean

Abstract

a b s t r a c tIn the Eastern North Atlantic Ocean iron (Fe) speciation was investigated in three size fractions: the dissolvable from unfiltered samples, the dissolved fraction (o 0.2 mm) and the fraction smaller than 1000 kDa ( o 1000 kDa). Fe concentrations were measured by flow injection analysis and the organic Fe complexation by voltammetry. In the research area the water column consisted of North Atlantic Central Water (NACW), below which Mediterranean Overflow Water (MOW) was found with the core between 800 and 1000 m depth. Below 2000 m depth the North Atlantic Deep Water (NADW) proper was recognised. Dissolved Fe and Fe in the o 1000 kDa fraction showed a nutrient like profile, depleted at the surface, increasing until 500-1000 m depth below which the concentration remained constant. Fe in unfiltered samples clearly showed the MOW with high concentrations (4 nM) compared to the overlying NACW and the underlying NADW, with 0.9 nM and 2 nM Fe, respectively. By using excess ligand (Excess L) concentrations as parameter we show a potential to bind Fe. The surface mixed layer had the highest excess ligand concentrations in all size fractions due to phytoplankton uptake and possible ligand production. The ratio of Excess L over Fe proved to be a complementary tool in revealing the relative saturation state of the ligands with Fe. In the whole water column, the organic ligands in the larger colloidal fraction (between 0.2 mm and 1000 kDa) were saturated with Fe, whereas those in the smallest fraction ( o1000 kDa) were not saturated with Fe, confirming that this fraction was the most reactive one and regulates dissolution and colloid aggregation and scavenging processes. This regulation was remarkably stable with depth since the alpha factor (product of Excess L and K 0 ), expressing the reactivity of the ligands, did not vary and was 10 13 . Whereas, in the NACW and the MOW, the ligands in the particulate (4 0.2 mm) fraction were unsaturated with Fe with respect to the dissolved fraction, thus these waters had a scavenging potential.Crown

Key role of organic complexation of iron in sustaining phytoplankton blooms in the Pine Island and Amundsen Polynyas (Southern Ocean)

“…At stations sampled a few days later (4-5 days) during the bloom (St. 106 and 107, Fig. 3 The Lt/DFe ratio highlights the saturation state of the organic ligands (Thuró czy et al, 2010b(Thuró czy et al, , 2011a(Thuró czy et al, , 2011b; this ratio is consistently 41 in open ocean water samples. A low ratio (close to 1) corresponds to ligands relatively saturated with Fe and indicates a low capacity of the ligands to bind and buffer additional Fe input.…”

“…The abundance of Fe in seawater is controlled by a balance between Fe input (via sediment resuspension, seaice and glacial melt, upwelling, atmospheric deposition, and hydrothermal inputs), stabilization processes via organic complexation that keep Fe in the dissolved phase, and by removal processes like (oxidative) precipitation and adsorptive scavenging (Baker and Croot, 2010;Boye et al, 2001;Croot and Johansson, 2000;Gledhill and Van Den Berg, 1994;Lannuzel et al, 2007Lannuzel et al, , 2008Lannuzel et al, , 2010Rue and Bruland, 1997;Klunder et al, 2011;Kuma et al, 1996;Nishioka and Takeda, 2000;Nishioka et al, 2001;Thuró czy et al, 2010bThuró czy et al, , 2011aThuró czy et al, , 2011b.…”

An Intercomparison of Dissolved Iron Speciation at the Bermuda Atlantic Time-series Study (BATS) Site: Results from GEOTRACES Crossover Station A

“…Thus, the difference in DFe between these profiles may represent an addition of inorganic Fe or weakly organically complexed Fe (i.e., FeL 2 ) to the November 2011 profile, analogous to the dissolution of particulate Fe by ambient strong Fe-binding ligands in the water column or samplers. If the difference in DFe concentrations between the two profiles is simply the result of an analytical offset, however, total ligand concentrations would be affected more than excess ligand concentrations since the change in DFe would not be "seen" by the added ligand (Thuróczy et al, 2010;Gledhill and Buck, 2012). Overall, because DFe is used in the calculation of ligand characteristics and samples were not exchanged for speciation analysis at the different analytical windows applied, it is challenging to delineate between the influences of analytical approach and DFe on the speciation results.…”

Distribution and Speciation of Dissolved Iron in Jiaozhou Bay (Yellow Sea, China)

“…The values of logK ′ were close to the values obtained by model ligands with organic matter such as siderophores produced by bacteria (Hider and Kong, 2010), porphyrins derived from pigments (Witter et al, 2000), and degradation products from marine and terrestrial organisms, among other humic substances (HS; Laglera and van den Berg, 2009). While interpreting results from CLE-aCSV one should further bear in mind that ligand-DFe coordination could be different from 1:1 ratio, and that not all DFe in a given sample is in an exchangeable form with respect to the added competitive ligand, having as a consequence a possible overestimation of L t and higher logK ′ (Cullen et al, 2006;Thuróczy et al, 2010;Gledhill and Buck, 2012).…”