A 25-year record of atmospheric deposition of iron speciation in the East Mediterranean: The impact of pH

A 25-year record of wet deposition has been collected and analysed for Fe(II), soluble iron (DSRFe) and total Iron (Fe) at Finokalia station on Crete island in the East Mediterranean from 1997 to 2022. A significant temporal increase in rain pH values is observed, mainly due to the reduction in sulfur concentrations. Regardless of the pH value of the rain, the Fe(II)/DSRFe ratio appeared to remain always above 50%, indicating that a significant amount of Fe(II), hence bioavailable iron, enters the sea surface via rain. However, Fe(II)/DSRFe ratio gradually decreases from 0.70 to 0.52 with increasing pH until pH 7.0, while from pH 7.0 and above it increases again, reaching an average value of about 0.67 at very basic pH levels. This is related to the general decrease in Fe solubility with increasing pH and the respective association of the forms in which Fe(II) and Fe(III) exist. It is therefore evident that the observed increase in pH in wet deposition affects the amount of dissolved iron deposited in the oceans, particularly Fe(II), that is directly bioavailable to the marine ecosystem, with consequent impacts on marine productivity.