Atmospheric dissolved iron deposition to the global oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and dust mineralogy

Mineral dust deposition is suggested to be a sig- nificant atmospheric supply pathway of bioavailable iron (Fe) to Fe-depleted surface oceans. In this study, mineral dust and dissolved Fe (Fed) deposition rates are predicted for March 2009 to February 2010 using the 3-D chemical transport model GEOS-Chem implemented with a compre- hensive dust-Fe dissolution scheme. The model simulates Fed production during the atmospheric transport of mineral dust, taking into account inorganic and organic (oxalate)- promoted Fe dissolution processes, photochemical redox cy- cling between ferric (Fe(III)) and ferrous (Fe(II)) forms of Fe, dissolution of three different Fe-containing minerals (hematite, goethite, and aluminosilicates), and detailed min- eralogy of wind-blown dust from the major desert regions. Our calculations suggest that during the year-long simula- tion ∼ 0.26 Tg (1 Tg = 1012 g) of Fed was deposited to global oceanic regions. Compared to simulations only taking into account proton-promoted Fe dissolution, the addition of ox- alate and Fe(II)/Fe(III) redox cycling to the dust-Fe mobi- lization scheme increased total annual model-predicted Fed deposition to global oceanic regions by ∼ 75 %. The imple- mentation of Fe(II)/Fe(III) photochemical redox cycling in the model also allows for the distinction between different oxidation states of deposited Fed. Our calculations suggest that during the daytime, large fractions of Fed deposited to the global oceans is likely to be in Fe(II) form, while noc- turnal fluxes of Fed are largely in Fe(III) form. Model sensi- tivity simulations suggest Fed fluxes to the oceans can range from ∼ 50 % reduction to ∼ 150 % increase associated with the uncertainty in Fe-containing minerals commonly found in dust particles. This study indicates that Fed deposition to the oceans is controlled by total dust-Fe mass concentrations, mineralogy, the surface area of dust particles, atmospheric chemical composition, cloud processing, and meteorological parameters and exhibits complex and spatiotemporally vari- able patterns. Our study suggests that the explicit model rep- resentation of individual processes leading to Fed production within mineral dust are needed to improve the understand- ing of the atmospheric Fe cycle, and quantify the effect of dust-Fe on ocean biological productivity, carbon cycle, and climate.