Zhu, Kechen, Achterberg, Eric Pieter , Bates, Nicholas R., Gerringa, Loes J. A., Middag, Rob, Hopwood, Mark J. and Gledhill, Martha (2023) Influence of changes in pH and temperature on the distribution of apparent iron solubility in the oceans. Global Biogeochemical Cycles, 37 (5). e2022GB007617. DOI 10.1029/2022GB007617.

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Abstract

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 ∼2 to 4-fold higher than observed dFe at depths less than 1000 m, ∼2-fold higher than the dFe between 1000-4000 m and ∼3-fold higher than dFe below 4000 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.

Key Points

Apparent iron solubility is driven by ambient pH, temperature (T) and dissolved organic carbon (DOC), and showed a 6-fold variation between surface (pH= 8.05 on the total scale, DOC= 71.8 µmol L-1, T= 20.4 °C) and deep oceanic waters (pH= 7.82, DOC= 38.6 µmol L-1, T= 1.1°C).

Higher values of apparent iron solubility were determined for deep Atlantic and Pacific waters, with lower values in subtropical gyres.

Calculated apparent iron solubility showed a similar trend in vertical distribution to dissolved iron, highlighting the importance of considering the impact of changes in ambient physico-chemical conditions on seawater iron chemistry.

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