The Physical Chemistry of Atmospheric Iron and Its Geochemical Behavior in the Surface Waters of the Open Ocean

Abstract

The physical chemistry of atmospheric iron present in remote marine aerosols collected from over the North Pacific has been systematically studied. 90- 100% of iron present in remote marine aerosols is in the non-aluminosilicate form. The Fe(II) concentrations in remote marine aerosols has been determined for the first time, and more than 50% of the iron is present as Fe(II). Photochemical reduction, including sulfur or organic reductants, and the subsequent dissolution of iron are probably the dominant transformation mechanisms affecting atmospheric iron during its long range transport from Asia to the North Pacific. It is postulated that the key reaction to produce Fe(II) and to increase the solubility of iron in marine aerosols is [Fe(III)(OH)(H2O)5]2+ +H2O + hv ----> [Fe(II)(H2O)6]2+ + OH or Fe(III) + S(IV) + hv -----> Fe(II)aq + The "saturated concentration" of operationally defined (using a 0.4-μm filter) dissolved atmospheric iron in seawater was found to be ~10-17 nmol/kg (~5-8 nmol/kg if 0.05-μm Millipore filters were used). If the total iron concentration was less than 2 nmol/kg (the open Pacific Ocean is in this category), ~50% of the atmospheric iron dissolved in seawater. More than 99% of the dissolved iron in the surface waters of the central North Pacific is provided by atmospheric input. The annual input of atmospheric iron to the central North Pacific is ~8 x 1011 g/yr and to the entire North Pacific is ~15 x 1012 g/yr. The dissolved iron and particulate iron from atmospheric input to the world ocean is estimated to be ~16 x 1012 and ~16 x 1012 g/yr respectively compared with the dissolved iron and particulate iron from the global riverine input to the world ocean of ~l x 1012 g/yr and ~110 x 1012 g/yr respectively [GESAMP, 1989]. Adsorption and biomass uptake are responsible for controlling the iron concentration in seawater. Although adsorption is one of the predominant mechanisms responsible for controlling the dissolved iron in the open ocean, the addition of the mineral aerosols to the open ocean still results in the dissolution of atmospheric iron in seawater. Atmospheric iron present in remote marine aerosols may be involved in two important atmospheric processes: 1) its long range transport to the open ocean provides a critical micronutrient for organisms in the surface waters of the open ocean, and iron may be the limiting nutrient in some oceanic areas, and 2) reduction of atmospheric Fe(III) may be a major