Potential effect of atmospheric dissolved organic carbon on the iron solubility in seawater

• Published in: Marine chemistry Vol. 194; pp. 124 - 132
• Main Authors: Meskhidze, N., Hurley, D., Royalty, T.M., Johnson, M.S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.08.2017
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/j.marchem.2017.05.011

Atmospheric aerosols are an important source of soluble iron (sol-Fe) to the global oceans. After deposition to seawater, sol-Fe will either complex with Fe-binding marine organic ligands and enter the ocean dissolved Fe (DFe) pool, or form oxyhydroxide particles (PFe) and precipitate out. Since oceanic DFe is commonly assumed to be bioavailable, the importance of atmospheric sources of sol-Fe for ocean biogeochemistry is determined by both: total fluxes of sol-Fe and the fraction of sol-Fe that is converted to DFe in the ocean. The results from these laboratory studies show that in 20min from the time of mixing with seawater, nearly all sol-Fe gets oxidized and converted to PFe. The addition of dicarboxylic acids (oxalic and malic) had minor influence on the conversion rate of sol-Fe to PFe. However, the addition of α-hydroxy-carboxylic acids (citric and tartaric) to Fe solutions prior to mixing with seawater was found to considerably increase the concentration of seawater DFe. After 10days of the experiment, 15% and 50% of the sol-Fe in citric and tartaric acid solutions, respectively, remained in a DFe form. Numerical simulations for sol-Fe laden dust deposition events to the ocean show that if α-hydroxy acids are present in aerosol solution upon deposition to the surface ocean, over 95% of sol-Fe can potentially bind with marine organic ligands; this fraction reduces to <20% in the absence of atmospheric organics. A possible mechanism is provided to explain the differences in binding strength between aerosol sol-Fe and atmospheric dissolved organic carbon species commonly found in maritime aerosols. [Display omitted] •We explore the processes and elucidates the mechanisms that may affect atmospherically-delivered iron in the surface ocean•In 20min from mixing with seawater, nearly all aerosol soluble iron gets oxidized and converted to particulate forms•The presence of α-hydroxy-carboxylic acids in aerosol solution can extended soluble Fe lifetime after mixing with seawater•Simulations show that in the absence of organic acids, < 20% of aerosol soluble iron gets complexed by marine organic ligands•Simulations show that in the presence of organics, up to 95% of aerosol soluble iron can enter the ocean dissolved iron pool