Impact of pH, temperature and dissolved organic matter on iron speciation and dissolved iron inventories in seawater

Phytoplankton growth has been shown to be limited by a low supply of iron (Fe) in large parts of the world’s surface ocean. In oxic seawater, the thermodynamically favored Fe form is Fe(III), which is rapidly precipitated and scavenged out of solution. Iron bound to organic matter has been shown to dominate Fe speciation and to buffer dissolved Fe (DFe) concentrations over the solubility of inorganic Fe (Fe´). Our current knowledge of Fe speciation suggests that an excess binding capacity of organic matter relative to Fe typically exists in seawater, but the sources, nature and residence time of the Fe-binding ligand pool are still largely unclear. Organic speciation of Fe is usually determined via competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), but its data interpretation has some limitations, e.g. the absence of pH and temperature dimensions. The oceans are currently experiencing acidification, warming, deoxygenation and stratification, and therefore it is important to understand the impact of changing seawater chemistry conditions (e.g. decreasing pH) on Fe speciation. Therefore, I applied an ion paring-organic matter model (NICA-Donnan), to thermodynamically calculate ambient Fe speciation and derive the apparent Fe(III) solubility (SFe(III)app). Iron speciation is calculated by the competition between inorganic complexes and organic complexation with the NICA-Donnan model, using DFe concentrations from various seawater samples. The SFe(III)app is calculated in a oversaturated system by setting an input of DFe(III) to 10 nmol L-1, at ambient ocean pH, temperature and dissolved organic carbon (DOC) concentrations. This will result in the precipitation of Fe hydroxide, as ferrihydrite assumed in our system. The SFe(III)app is defined as the sum of all aqueous inorganic species and Fe bound to organic matter at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). I combined these predictions with observational DFe as well as Fe(II) data, to build a comprehensive picture on Fe speciation and DFe inventory in the ambient oceanic water column, with further feedbacks on primary productivity. In Chapter 3, I first calibrated predictions of Fe speciation with four different NICA parameter sets representing a range of binding sites strengths and heterogeneities, by comparing those predictions to determinations of Fe speciation via CLE-AdCSV in samples collected from the Celtic Sea. The results showed a constant low DOC concentration resulted in a slight improvement in the fit of titration data to the simulated titrations, suggesting that the changes in dissolved organic matter composition that occur alongside changes in DOC concentration dilute the Fe binding site pool. Using the optimized parameters, the calculated SFe(III)app was within the range of DFe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at the furthest offshore stations. This supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding to organic matter. In Chapter 4, I further derived Fe(III) NICA constants for marine DOM from samples collected across the Peruvian shelf and slope, via the approach PEST-ORCHESTRA. Using the constants, the modelled SFe(III)app showed a ca. 2 fold increase in the oxygen minimum zone compared to surface waters. The increase results from a one order of magnitude decrease in H+ concentrations which impacts both Fe(III) hydroxide solubility and organic complexation. Using the Fe(II) measurements, I calculated the dissolved Fe(III) concentrations (DFe-FeII). The results highlight that the underlying distribution of ambient DFe(III) largely reflected the modelled SFe(III)app and an important role of ambient pH and temperature on the speciation and solubility of Fe. Finally, I investigated correlations of predicted SFe(III)app and measured DFe at ocean basin scales, using data obtained during a series of GEOTRACES cruises (Chapter 5). A similar trend was observed in the vertical distributions of horizontally averaged SFe(III)app and DFe. Combining the regression analysis and proportions of DFe relative to predicted SFe(III)app at the basin scale, the results suggest the distribution of DFe is not solely a function of sinking organic matter remineralization processes, but also regulated by relative changes in ambient pH and temperature. pH has a larger impact on SFe(III)app than DOM at basin scales, based on a solubility gradient of Fe hydroxide that is driven by ambient temperature. Therefore, consideration of the impact of pH and temperature on organic Fe complexation is as important for the speciation and solubility of Fe as the characterization of Fe-binding ligands, since the global distributions Fe-binding ligand (and DOC concentrations) are relatively invariant at the basin scale.