Novel approaches to the evaluation of iron and phosphorus availability in dust deposited to the oceans

Ocean is a major sink for atmospheric CO2. The carbon uptake capacity of a large part of the global ocean is however limited by the amount of nutrients iron and/or phosphorus in surface waters. Therefore, understanding the origins and fate of iron and phosphorus in surface oceans is important in modeling the climate system and therefore predicting climate change. One of the primary external sources of iron and phosphorus found in the surface waters in the open ocean is through atmospheric dust. Such iron and phosphorus from the dust can increase ocean carbon uptake and alter ocean biogeochemistry, thus affecting climate. The magnitude of the impact of dust input on oceanic carbon uptake and climate is dependent on total dust deposition fluxes as well as the bioavailability of iron and phosphorus in the dust. Global models seem able to simulate the former reasonably well but not the latter. One important reason is that most iron and phosphorus in desert dust are unreactive and thus not bioavailable but they can become much more reactive after being transported in the atmosphere. For example, the fraction of dissolved (<200nm) to total iron and phosphorus (defined as solubility) are orders of magnitude higher in dust over more remote oceans than over desert regions. It is now understood that solubility of iron, and probably phosphorus, in dust is controlled to a large extent by processes in the source area and in the atmosphere. Mechanistic understanding of some of the processes have been developed and/or parameterized into global models. However, major gaps remain on the solubility, lability and bioavailability of iron and phosphorus in dust deposited to the ocean, particularly from the wet deposition. These gaps significantly affects the ability of global models to represent the current climate system and therefore to predict climate change. The aim of this work is to evaluate the solubility and lability (availability) of iron and phosphorus in dust, particularly in rainwater deposited to the oceans. The objectives are: (1) To elucidate the fundamental parameters and processes controlling the concentration and partition of labile, soluble (<1nm), dissolved, and total iron and phosphorus in dust from rainwater; (2) To clarify how iron interact with phosphorus and trace metals in rainwater and affect their solubilities; and (3) To quantify the labile and/or soluble, dissolved and total iron and phosphorus deposition fluxes at five sites downwind of dust source regions for model constraining. These objectives will be met by field measurements and laboratory simulations employing state-of-the-art separation techniques (e.g., flow field flow fractionation, FFF) and in-situ labile trace metal speciation techniques (i.e., diffusive gradients in thin Films, DGT) coupled with high resolution ICP-MS and Electron and Atomic Microscopy. The scientific results will be fed to global models to improve the estimation of atmospheric nutrient deposition fluxes, and can be readily fed to Met Office and NERC Earth system models to predict the impact of atmospheric nutrient input on ocean productivity and climate in the present and the future.