Coupled Iron-Phosphorus Cycling in Surface Seawater Mediated by Photoreduction of Fe-rich Dust

Nutrient concentrations are exceedingly low in the oligotrophic waters of the open ocean. One source of the micronutrient iron (Fe) to these regions is deposition of continentally-derived dust. Photolytic reduction of Fe is believed to be a key process in rendering bioavailable the Fe present in dust particles delivered to surface seawater. Fe(III)-oxyhydroxides, in general, have a strong affinity for sorption of phosphate, and Fe-oxyhydroxides present in dust derived from continental soils can be highly enriched in phosphate. Therefore, solubilization of Fe(III) phases via photoreduction may liberate phosphate to surface seawater. Because biological productivity in oligotrophic regions of the open ocean can be limited by phosphate as well as by iron, this process could be an important avenue for providing two essential, limiting nutrients to support primary productivity in these regions. Alternatively, if Fe-rich dust delivered to surface waters has unsaturated surface sorption sites, dust may actively scavenge phosphate from surface waters, pushing the system further toward phosphate limitation. Depending upon the balance between these two processes, continentally derived dust may either partially alleviate or enhance phosphate limitation in oligotrophic waters. We will present results of experiments designed to determine whether phosphate-containing Fe(III)-oxyhydroxides, subjected to photoreduction, release phosphate to seawater. In a series of incubation experiments, synthetic iron minerals (ferrihydrite, goethite) and natural dust samples (Saharan dust, Chinese loess) are suspended in artificial seawater and exposed to sunlight in the presence/absence of citrate. We observe an increase in both dissolved Fe(II) and Fe(Total) during daylight hours, which we attribute to the photoreduction process. Fe concentrations are at least 5 times higher in the presence of citrate, consistent with other studies exploring the effect of organic ligands on the Fe-photoreduction process. Phosphate sorbed onto synthetic ferrihydrite appears to enhance the release of Fe during photoreduction. The net effect of this process on dissolved phosphate is still being evaluated.