Atmospheric input of manganese and iron to the ocean: Seawater dissolution experiments with Saharan and North American dusts

Dissolution of wind blown dust is a major source of iron, manganese and other trace nutrients in the ocean. Kinetic and thermodynamic values for the release of metals from dust are needed for computer models which incorporate dust as part of their ocean system. Here we investigate both the thermodynamic and kinetics parameters involved in the dissolution of metals from dust in seawater. We added dust from the Sahara and the Western United States in five different concentrations (0.01-5.0 mg/L) representative of those concentrations found in seawater after dust events, to open-ocean Pacific seawater. Sub-sampling of the reaction vessels took place on days 1, 2, 4, 7, 14, and 35 for the kinetic study. Results show different apparent thermodynamic constants for manganese (Mn) and iron (Fe). The final Mn concentrations are proportional to the added dust concentration. Fe concentrations reach a maximum of less than 2 nM, independent of the quantity and type of dust added. The Fe dissolution kinetics are faster than our sampling resolution. The first order rate constant for the dissolution of Mn from the Western US and Sahara dusts were 0.94 ± 0.04 frac(nmol Mn, day mg Dust), and 0.22 ± 0.01frac(nmol Mn, day mg Dust) respectively. We conclude that, Mn concentrations are limited by available Mn on the dust surface, while Fe concentrations are limited by the ligand concentrations in the seawater, which ultimately are determined by the biological community.