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Factors controlling the bioavailability of Atmospheric Iron in the maRine ecosYstems

Final Activity Report Summary - FAIRY (Factors controlling the bioavailability of Atmospheric Iron in the maRine ecosYstems)

Input of bioavailable iron by dust deposition at the surface of the ocean was postulated to explain an increase in the biological pump in the Southern Ocean during the glacial period (Martin, 1990). During the past decade, an increasing interest of the international community towards the 'iron hypothesis' has brought the scientists to investigate the actual role of iron in contrasted environments. As we learn more about the biogeochemistry of this element, more questions are coming and more than ten years after John Martin had postulated the 'iron hypothesis', there is still a huge international and interdisciplinary effort to understand the role of iron in the functioning of the world's oceans. By increasing the pool of bioavailable iron in surface seawater, atmospheric aerosol dissolution not only affects primary productivity but, through the uptake of atmospheric CO2 (Cooper et al., 1996) and the stimulation of biological DMS production (Turner et al., 1996), is also linked to climate regulation. The major source of iron to the open ocean is through the deposition of atmospheric dust particles, and a very important question remains concerning the bioavailability of iron introduced to the water column after a dust input. In FAIRY, we have been able to parameterise some key processes that control the pool of dissolved iron in ocean surface.

We have shown that kinetic aspects for both iron dissolution and scavenging processes are very important as we have identified very fast adsorption and desorption processes. In both cases, we have shown the significance of the dissolved organic matter related to biological activity:

(1) Fe dissolution is dependent on the water's ligand field rather than the type or quantity of dust deposited on the surface. Dissolved inorganic Fe is saturated at 0.1 nM in natural sea water. Because of these thermodynamics, an increase in dust deposition will not increase Fe in the ocean, unless initial Fe inputs fertilise the ocean in such a way that the biologically community within the region are accustomed to dust deposition events and can produce ligands which will hold Fe in the dissolved form. Ultimately, the Fe concentrations are determined by the concentrations of Fe binding ligands - and thus depends on biological activity.

(2) dissolved organic matter present in seawater can aggregate and form particulate material (particulate organic matter) that will compete with mineral particles such as dust for iron adsorption. Those new parameterisation are very important for our knowledge of biogeochemical cycles: they can be used in models of natural ecosystems to understand the role of particles, including atmospheric particles on the export of DFe to deep waters and the competition that may exist between different types of particles.

The complex view of atmospheric iron pathway in surface waters of the ocean can be better understood thanks to the results found in FAIRY.