Development of a consistent thermodynamic model of trace element - organic matter interactions in the Ocean.

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Trace elements can limit productivity in the ocean, and thus influence the amount of carbon dioxide that marine phytoplankton remove from the atmosphere. The most critical trace element for phytoplankton growth is iron, which limits productivity in approximately 30 % of the worlds' ocean. Progress with respect to incorporation of iron into biogeochemical models and climate models is required, in order to account for its influence on the impact of carbon dioxide on the earths' climate. Incorporation of iron into biogeochemical and climate models needs to be underpinned by a fundamental understanding of the chemistry of iron in seawater, since iron chemistry is affected by ocean pH, temperature and the concentration of dissolved organic carbon. Organic matter is known to have a major influence on iron chemistry, but interactions between iron and organic matter are currently described in a simplistic way that do not relate to fundamental principles. In this project we propose to parameterise the chemistry of iron in seawater using state of the art approaches that realistically represent the complexity of organic matter and the way organic matter interacts with iron and other trace elements in the marine environment. Importantly our approach adopts the NICA model - a model that is grounded in thermodynamic theory and has been tried and tested in fresh water aquatic environments. We hypothesise that use of the NICA model for interpretation of iron chemistry in seawater will improve our ability to link iron chemistry to the behaviour of dissolved organic carbon and ocean acidity. Our objectives are to determine the parameters required to apply the NICA model in seawater. We will apply the model to conditions observed in the plume of the River Amazon which constitutes the largest riverine input to the ocean. Our approach will allow us to examine the role that organic matter and pH play in the offshore transportation of iron, thereby improving our understanding of the factors that control riverine fluxes of iron to the ocean.