Trace metal-Organic matter interactions in seawater: Exploring the impact of seawater chemistry on trace metal speciation in a warming and acidifying ocean.

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Trace metals (TMs), defined as less than 1 mg kg-1, are either important essential nutrients (Fe, Mn, Co, Cu, Ni, Zn) for microbial growth, or toxic (Cu, Pb, Cd) at elevated concentrations in seawater. The Ocean is currently experiencing deoxygenation, acidification, stratification and warming, resulting in changes in chemical speciation of TMs that are dependent on the physico-chemical conditions (e.g. pH, temperature and salinity). Whilst knowledge of dissolved and particulate metals provides information on total inventories and allows for identification of important sources of TMs to the marine environment, knowledge of chemical speciation is essential for understanding the biogeochemistry and bioavailability or toxicity of TMs. For example, previous work shows that inorganic Fe has a poor solubility in oxygenated seawater, but dissolved Fe concentrations are higher than expected because of complexation by organic matter. However current knowledge of TMs speciation is observed for a specific sample at laboratory conditions (e.g. pH=8.0, on the NBS scale), and therefore lack a mechanistic link to the intrinsic physico-chemical properties of seawater and their influence on metal binding to organic matter. Here I develop novel analytical and modelling tools, and utilize metal-resin/organic matter interactions to accurately determine TM speciation by ICP-MS over a wide range of pH values. I combine these measurements with an ion paring-organic matter (NICA-Donnan) model to develop a mechanistic description of the interactions and thereby improve our understanding of the roles of e.g. pH, temperature and ionic strength on marine TM cycling. Once this methodology is achieved, it will allow us to simultaneously determine TM speciation for multiple metals for the first time, including those frequently investigated before and TMs where recent evidence from isotope abundance points to an important role for binding to organic matter. The derived thermodynamic constants will also be incorporated into regional biogeochemical models, in order to obtain predictions of TM biogeochemical cycling at a mechanistic level under future ocean scenarios.