US GEOTRACES GP17-OCE and GP17-ANT: Mapping zinc speciation in the Southern Ocean overturning circulation to test the zinc scavenging hypothesis

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The Southern Ocean plays a significant role in the global carbon cycle because deep ocean waters rich in CO2 are brought into the surface layer and can exchange with the atmosphere. The growth of phytoplankton counteracts this carbon source by fixing CO2 into biomass, which can sink back to the deep sea. While the Southern Ocean is rich in macronutrients that support phytoplankton growth, key micronutrients such as iron, manganese and zinc are at very low abundance. Of these metals, only iron has been found to limit phytoplankton productivity on a large scale, but few observations of Zn and Mn have been made. This proposal seeks to map bioavailable Zn concentrations across an oceanographic transect from the South Pacific Ocean to the Amundsen Sea. At low abundance, Zn scarcity can limit phytoplankton, but Zn can also affect the uptake of other scarce metals, especially Mn. Measurements made as part of this project will determine the extent of limiting and toxic levels of Zn in this critical but undersampled region of the oceans. Such constraints are necessary for modelling changes to the oceanic Zn cycle – and its effect on phytoplankton activity – under past and future climate scenarios. As part of the GEOTRACES GP17 OCE and ANT expeditions, parameters governing the reactivity of Zn will be measured in high vertical and latitudinal resolution across a transect extending several thousand kilometers and crossing formation regions of globally important water masses. Voltammetry techniques will be used to determine the concentrations of free zinc (Zn’) and Zn-binding organic ligands dissolved in seawater for hundreds of samples. Together with other investigators in the GEOTRACES program, these measurements will constrain the reactivity of Zn’ in the deep ocean and define empirical relationships between Zn’ and phytoplankton metal uptake. This is essential for understanding the global distribution of Zn and other micronutrients in the oceans and their potential to affect primary production.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.