Iron speciation in precipitation in the North-Eastern Mediterranean and its relationship with Sahara dust

Precipitation samples collected at Erdemli, Turkey, during February 1996-June 1997 were analysed to determine iron content and speciation. The purpose of the measurements was to examine the atmospheric abundance of iron and to quantify its solubility in the region. Spectrophotometric analyses of Fe(II) and reducible Fe(III) in precipitation samples, along with measurements of pH, conductivity, filterable iron (Fe-filt), particulate aluminium (Al-par) and particulate iron (Fe-par) were performed to determine iron solubility, which principally affects its bioavailability. Backward trajectories corresponding to the sampling dates were analysed to determine the sources of atmospheric constituents arriving at the site. Among these, the mineral dust transported from the Great Sahara to the region is considered to be a rich source of iron. The concentration of Fe(II) varied from below detection limit (0.02 muM) up to 0.42 muM, while the maximum concentration of total reactive Fe (referred as Fe(II) + reducible Fe(III) = Fe-reac) was found to be 1.0 muM in precipitation. A strong correlation was found between particulate Fe and Al fractions, both of crustal origin. No correlation was observed between the soluble and insoluble fractions of iron. The soluble iron fraction, Fe(II) concentration varied independently from the concentrations of reducible Fe(III), Fe-filt, Fe-par, and from the pH of the precipitation. The Fe-filt fraction (size < 0.45 mum), measured by Atomic Absorption Spectrophotometer, and frequently interpreted to be the soluble iron fraction in the literature, was found to be significantly higher than the corresponding Fe-reac fraction in precipitation samples, most likely due to the colloidal iron content of the Fe-filt fraction passing through the 0.45 mum pore size filter. The volume weighted mean Fe-filt concentration of the precipitation samples collected during the episodic `red rain' events was found to be relatively higher. The geometric mean ratios of soluble Fe(II) and of Fe-reac to Total Fe (Fe-filt + Fe-par), were found to be 1.6% and 2.1%, respectively, while the mean ratio of Fe-filt to Total Fe was 9.6%. The flux of bioavailable iron (Fe-reac) fraction in most atmospheric wet deposition events was found to be sufficient for supporting the maximum primary production rates that are typical for the Eastern Mediterranean Sea.