Predicting Iron(III) Speciation in the Peruvian Upwelling Region

Kechen Zhu1, Martha Gledhill1, Mark James Hopwood1, Jan E. Groenenberg2, Anja Engel1 and Eric P Achterberg1, (1)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (2)Wageningen University, section soil quality, Wageningen, Netherlands
Iron is an essential macronutrient for phytoplankton growth, and limits productivity in high nutrient - low chlorophyll regions of the surface ocean. The solubility and bioavailability of Fe is a function of chemical speciation. And the complexation of Fe(III) with organic matter plays a key role in buffering dissolved Fe in the ocean. Here we combined pH and metal titrations in a novel approach with non-ideal competitive adsorption in Donnan-like gels (NICA-Donnan model), to investigate Fe(III) speciation in the Peruvian upwelling region. A speciation modeling program ORCHESTRA in combination with model independent parameter estimation and uncertainty analysis (PEST) (Janot et al. 2017), was used to derive non-ideal competitive adsorption (NICA) constants for Fe(III) binding to organic matter. We found NICA Fe(III) constants were different in the oxygen minimum zone (OMZ) compared to those out of OMZ. Thus, we combined our derived NICA Fe(III) constants with observed Fe(II) concentrations to obtain a comprehensive assessment of Fe speciation in the Peruvian upwelling system at ambient pH and DOC concentrations.