Organic Ligands And The Dissolution Of Iron- Laden Dust In Seawater

Atmospherically supplied dust deposition has proven to be a critical source of iron (Fe) to high nitrate - low chlorophyll (HNLC) oceanic regions. The low solubility (Ksp = 2 x 10-39) of the hydrolysis species of Fe (Fe (III) oxyhydroxide solids) renders Fe to become less bioavailable. Fe is kept in the dissolved form by organic ligands above this solubility limit. The dissolution of Fe (III) bearing minerals in the presence of siderophores has been the subject of numerous recent studies. For our study, different dust dissolution experiments were carried out on Australian dust and iron minerals (goethite and lepidocrocite) using surface ocean water from different latitudes collected during the GEOTRACES Pacific ocean cruise in June 2011 and Iron Cycle III - Spring Bloom voyage in September 2012 to determine the effect of the presence or absence of siderophore (desferrioxamine B (DFB)), oxalate and light on the dissolution of Fe from dust. Short- and long- term dissolution experiments were performed at ambient seawater pH. Iron species were measured by electrochemical methods or chemiluminescence, and HPLC-ESI-MS was used to study structural changes of the organic ligands. In all cases iron dissolution was observed, with the largest increase being observed in the presence of DFB and light. Addition of the weak ligand and electron donor oxalate had no significant effect on the dissolution. The results indicate that light and the complexing capacity of the ligands critically affects the dissolution process. Preliminary results show that a maximum dissolved iron concentration was reached three hours after the addition of the dust, which subsequently decreased again. This observation might be due to the onsetting precipitation of iron. We will also present results from an artificial iron-mineral dissolution experiment conducted under comparable conditions.