Influence of Changes in pH and Temperature on the Distribution of Apparent Iron Solubility in the Oceans

An insufficient supply of the micronutrient iron (Fe) limits phytoplankton growth across large parts of the ocean. Ambient Fe speciation and solubility are largely dependent on seawater physico-chemical properties. We calculated the apparent Fe solubility (SFe(III)(app)) at equilibrium for ambient conditions, where SFe(III)(app) is defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to organic matter formed at a free Fe3+ concentration equal to the solubility of Fe hydroxide. We compared the SFe(III)(app) to measured dissolved Fe (dFe) in the Atlantic and Pacific Oceans. The SFe(III)(app) was overall similar to 2-4-fold higher than observed dFe at depths less than 1,000 m, similar to 2-fold higher than the dFe between 1,000 and 4,000 m and similar to 3-fold higher than dFe below 4,000 m. Within the range of used parameters, our results showed that there was a similar trend in the vertical distributions of horizontally averaged SFe(III)(app) and dFe. Our results suggest that vertical dFe distributions are underpinned by changes in SFe(III)(app), which are driven by relative changes in ambient pH and temperature. Since both pH and temperature are essential parameters controlling ambient Fe speciation, these should be accounted for in investigations of changing Fe dynamics, particularly in the context of ocean acidification and warming.