Collaborative Research: US GEOTRACES GP-17-ANT: Molecular speciation of trace element-ligand complexes in the Southern Ocean and Antarctic shelf

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The supply of iron and other biologically essential trace elements to surface waters is a critical regulator of biological growth in the Southern Ocean. Sea ice volume and the physical dynamics of the Southern Ocean are currently experiencing rapid changes. An understanding of the corresponding changes in metal cycling is needed as these elements are essential for the growth of phytoplankton which plays an important role in modulating atmospheric CO2. One of the major knowledge gaps is understanding how soluble forms of these metals are generated and transported to the surface ocean. Many trace elements are insoluble in seawater and precipitate close to their sources unless they are bound to a dissolved organic molecule, or ligand, that keeps the metal in solution. Little is currently known about what these ligands are, where they come from, or how they affect the reactivity and fate of metals in the Southern Ocean. The study proposed here is designed to identify the organic ligands that bind to metals and determine the ligand sources and reactivity along the Antarctic continental margin. The proposed project will survey the molecular speciation of six biologically important trace elements (Fe, Cu, Zn. Ni, Co, Mn) along the US GEOTRACES cruise track along the Antarctic shelf in the Amundsen Sea in order to better understand trace element cycling across the region. The goal of the GEOTRACES expedition is to identify processes and quantify fluxes that control the distributions of trace elements and their isotopes to the Southern Ocean. As part of this effort, the investigators will assess biological ligand production as the ecosystem shifts from low productivity iron-starved conditions in the Antarctic Circumpolar Current to higher productivity along the upwelling continental margin, with a particular focus on assessing production within the Amundsen Sea polynya blooms and sinking organic matter remineralization zones. Second, changes in the accumulation, saturation, and composition of ligands in Circumpolar Deep Water will be measured as the water passes across the Antarctic shelf and receives inputs from sediments and glacial meltwaters. Finally, the role of sea ice on metal-ligand dynamics will be investigated. Metal ligands are a central parameter of numerical ocean models that predict and estimate metal distributions, and results from this project will provide those models with knowledge of the processes that supply ligands and affect ligand concentrations. Two graduate students and undergraduate interns will be supported and trained as part of this project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.