Seawater dissolved iron concentrations

In the late 1980s, improvements in the measurement methods for trace metals in seawater lead to the observation that dissolved iron concentrations in the open ocean are... 

Although the details of the equilibrium model are still uncertain, the general trends are likely reliable. As shown in Figme 5.16, most of the Fe(III) in seawater is predicted to be in the form of the FeL complex. The equilibrium model also predicts that this degree of complexation should enhance iron solubility such that 10 to 50% of the iron delivered to the ocean as dust will eventually become dissolved if equilibrimn is attained. If this model is a reasonable representation for iron speciation in seawater, uptake of [Fe(III)]jQjgj by phytoplankton should induce a spontaneous dissolution of additional particulate iron so as to drive the dissolved iron concentrations back toward their equilibrium values. 

The results of our laboratory experiment show that sub-nanomolar iron additions to our low-iron (0.22 nM dissolved Fe) growth medium mediated an increase in the Fuco Chl a ratio and a decrease in the Hex Chl a ratio of colonial P. antarctica at an irradiance of 20 pE m 2 s 1. Not surprisingly, our data reveal a strong correlation (r2 = 0.82) between the ratio of Fuco to Hex (Fuco Hex ratio) after 25 and 31 days incubation and the initial dissolved Fe concentration of our experimental treatments (Fig. 3a). This observed increase in the Fuco Hex ratio with dissolved iron availability appears to extend well above the range of dissolved iron concentrations used in our laboratory experiment. For instance, when grown under iron-replete conditions (-500 nM dissolved Fe, in a 20-fold seawater dilution of LI medium) at an irradiance of 20 pE m 2 s-1, the same strain of colonial P. antarctica exhibited a Fuco Hex ratio of -0.45 (Fig. 3b), which was around 40 times higher than the Fuco Hex ratio (ca. 

Profiles of dissolved iron in seawater show the influence of both biotic and abiotic processes. At stations in the Northeast Pacific, dissolved iron concentrations are low in surface waters, reflecting biological uptake. Iron concentrations also show peak values at depth, corresponding to the oxygen minimum zone (Martin and Gordon, 1988), suggesting the abiotic reduction of Fe(III) back to the soluble Fe(II). 

Fig. 6.25 Dissolved iron concentrations in Gulf of Alaska (NE Pacific) seawater showing nutrient-like profile (compare with Fig. 6.20). After Martin et al. (1989), with permission from Elsevier Science.

A significant proportion of the needs for reference materials for seawater trace metal studies would be addressed by the preparation of these materials. Although the total iron concentration of these reference materials should be provided, these materials clearly will be useful for studies of other important metals such as zinc, manganese, copper, molybdenum, cobalt, vanadium, lead, aluminum, cadmium, and the rare earth elements. With careful planning, such water samples should be useful for analysis of dissolved organic substances as well. The collection sites should be chosen carefully to provide both a high and a low concentration reference material for as many metals as possible. 

The committee recommends, but assigns a lower priority to, the preparation of reference materials from other locations. For example a standard for dissolved iron in a coastal seawater matrix containing high concentrations of dissolved organic material would be particularly useful in addressing matrix effects associated with such materials. 

A full imderstanding of the speciation of dissolved iron requires consideration of ligands other than water and hydroxide. The most important ones are listed in Table 5.6 along with their concentration ranges in seawater and freshwater. For Fe(III) in seawater at pH > 4, the formation of complexes with hydroxide is most important, but at pH <4, sulfete, chloride, and fluoride pairing predominates (Figure 5.15b). To predict the equilibrimn speciation at low pH, these anions need to be added to the mass balance equation fiar Fe(III) (Eq. 5.20). Seawater with low pH tends to have low O2 concentrations. Under these conditions, most of the dissolved iron is present as Fe( II), which undergoes complexation with sulfide and carbonate. 

Primary production in the ocean is controlled by major nutrients, such as nitrate and phosphate, but also by certain trace metals. Dissolved iron was hypothesized (over 50 years ago) to be a key nutrient limiting primary production rates in the sea. However, credible data for the concentration of dissolved iron in seawater have only become available in the last 8 years. Iron is present in surface seawater at concentrations less than 0.5 nanomole per kilogram. These low concentrations of dissolved iron suggest that it is, in fact, a nutrient that can limit primary production in the ocean (Martin et al., 1989). The role of iron in limiting productivity of the ocean can be resolved only when measurements of dissolved iron at concentrations below 1 nanomole per kilogram become routine. There is evidence that other trace metals could also control phytoplankton growth. 

O. 2-pm CritiCap Supor capsule filter (Pall Corporation) into the 1-1 polycarbonate bottle under class-100 filtered air. Subsamples of this 0.2-pm filtered station B seawater were collected for subsequent iron measurements, which confirmed that dissolved Fe concentrations were relatively low (0.17 0.1 nM, n = 4). Based on the concentrations of FeCl3 (11.65 pM) and EDTA (11.71 pM) in the LI medium, the resultant 1-1 inoculum of colonial P. antarctica had initial dissolved Fe and EDTA concentrations of approximately 1.42 and 1.25 nM, respectively. 

In seawater (pH = 8.1) the reported iron concentration ranges from to 10 - M with a mean value of perhaps 10 " M (cf. Holland 1978 Hem 1985). It is interesting to question whether this amount represents dissolved or suspended ferric iron. Examination of Fig. 12.4 shows that the dominant aqueous species in seawater at pH = 8.1 is Fe(OH) . (Fe(IIl) chloride complexing can be ignored.) Assuming that Fe(III) is controlled by the solubility of amorphous HFO (p p = 37.1) as an upper limit, and with free energies from Table A 12.1, we find for the reaction Fe(OH)3(am) = Fe(OH)3, that = [Fe(OH)y] = 10" M. This value is close to the mean iron concentration reported... 

Above a pH of 3 and in the absence of chelators dissolved iron is only present as ferrous iron under natural conditions. Therefore, the colored complex that results from the reaction between ferrous iron and a reagent can be analysed colorimetrically and correlates with the concentration of total dissolved iron. Most conveniently, one can mix a drop of Ferrozine solution (Stookey 1970), one drop of H SO (diluted 1 4) and 1 ml of pore water in the glove box, wait until complex formation is completed (20-30 minutes) and quantify the iron concentration by the intensity of the color at a wavelength of 562 nm. To avoid matrix effects standards should be prepared with artificial seawater. 

Mullins [37] has described a procedure for determining the concentrations of dissolved chromium species in seawater. Chromium (III) and chromium (VI) separated by co-precipitation with hydrated iron (III) oxide and total chromium are determined separately by conversion to chromium (VI), extraction with ammonium pyrrolidine diethyl dithiocarbamate into methyl isobutyl ketone, and determination by AAS. The detection limit is 40 ng/1 chromium. The dissolved chromium not amenable to separation and direct extraction is calculated by difference. In waters investigated, total concentrations were relatively high (1-5 xg/l), with chromium (VI) the predominant species in all areas sampled with one exception, where organically bound chromium was the major species. 

Howard [27] determined dissolved aluminium in seawater by the micelle-enhanced fluorescence of its lumogallion complex. Several surfactants (to enhance fluorescence and minimise interferences), used for the determination of aluminium at very low concentrations (below 0.5 pg/1) in seawaters, were compared. The surfactants tested in preliminary studies were anionic (sodium lauryl sulfate), non-ionic (Triton X-100, Nonidet P42, NOPCO, and Tergital XD), and cationic (cetyltrimethylammonium bromide). Based on the degree of fluorescence enhancement and ease of use, Triton X-100 was selected for further study. Sample solutions (25 ml) in polyethylene bottles were mixed with acetate buffer (pH 4.7, 2 ml) lumogallion solution (0.02%, 0.3 ml) and 1,10-phenanthroline (1.0 ml to mask interferences from iron). Samples were heated to 80 °C for 1.5 h, cooled, and shaken with neat surfactant (0.15 ml) before fluorescence measurements were made. This procedure had a detection limit at the 0.02 pg/1 level. The method was independent of salinity and could therefore be used for both freshwater and seawater samples. 

Spencer and Sachs [29] determined particulate aluminium in seawater by atomic absorption spectrometry. The suspended matter was collected from seawater (at least 2 litres) on a 0.45 tm membrane filter, the filter was ashed, and the residue was heated to fumes with 2 ml concentrated hydrofluoric acid and one drop of concentrated sulfuric acid. This residue was dissolved in 2 ml 2 M hydrochloric acid and the solution was diluted to give an aluminium concentration in the range 5-50 pg/1. Atomic absorption determination was carried out with a nitrous oxide acetylene flame. The effects of calcium, iron, sodium, and sulfate alone and in combination on the aluminium absorption were studied. 

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