Dissolution of cobalt and zinc from natural and anthropogenic dusts in seawater
Abstract. Atmospheric dust inputs to the surface ocean are a major source of trace metals likely to be bio-available for phytoplankton after their dissolution in seawater. Among them, cobalt (Co) and zinc (Zn) are essential for phytoplankton growth and for the distribution of the major groups such as coccolithophorids, cyanobacteria and diatoms. The solubility in seawater of Co and Zn present in natural and anthropogenic dusts was studied using an open-flow reactor with and without light irradiation. Those dusts can be transported in the atmosphere by the wind before being deposited to the surface ocean. The analyses of cobalt and zinc were conducted using voltammetric methods and the global elemental composition of dust was determined by ICP-AES. This study highlighted the role of the dust origin in revealing the solubility characteristics. Much higher dust solubility was found for zinc as compared to cobalt; cobalt in anthropogenic particles was much more soluble (0.78%) in seawater after 2 h of dissolution than Co in natural particles (0.14%). Zinc showed opposite solubility, higher in natural particles (16%) than in anthropogenic particles (5.2%). A natural dust event to the surface ocean could account for up to 5% of the cobalt inventory and up to 50% of the Zn inventory in the mixed layer in the Pacific Ocean whereas the cobalt and zinc inventories in the mixed layer of the Atlantic Ocean might already include the effects of natural dust inputs and the subsequent metal dissolution. Anthropogenic sources to the surface ocean could be as important as the natural sources, but a better estimate of the flux of anthropogenic aerosol to the surface ocean is needed to further estimate the anthropogenic inputs. Variations in natural and anthropogenic inputs may induce large shifts in the Co/Zn ratio in the surface ocean; hence it could impact the phytoplankton community structure.