Collaborative Research: US GEOTRACES GP17-OCE: Dissolved concentrations, isotopes, and colloids of the bioactive trace metals

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The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. Many trace metals, which are by definition present in very low amounts, are essential for life and thus considered nutrients for phytoplankton growth. Other elements can be useful for tracing other ocean processes, and some (such as lead) are important because they are pollutants. This project will address three main objectives: 1) to measure the dissolved concentrations and size partitioning of micronutrients iron, manganese, zinc, copper, cadmium, nickel, and pollutant lead in seawater from Tahiti towards Antarctica and then back northeast to South America; 2) to analyze the isotope ratios - the relative abundance of different forms of the same chemical element - of iron, zinc, cadmium, nickel, and copper in seawater, suspended particles, aerosols, and porewaters from this section, as isotopes provide diagnostic insights into sources, sinks, and metal transformations beyond what is provided by concentration measurements alone; and 3) to use ocean models to constrain the rates of ocean processes and effects of ocean circulation on trace metal and isotope distributions. This project spans three US labs, and four graduate students and several undergraduate students will participate. It will also provide ultrafiltered samples and data to several other collaborating groups as a service.The U.S. GEOTRACES GP17-OCE expedition, planned for late 2021 or 2022, aims to determine the distribution of trace elements and isotopes along a transect spanning regions of global importance to nutrient and carbon cycling, crossing the South Pacific Gyre, the iron-limited waters of the Antarctic Circumpolar Current, Pacific Deep Waters with hydrothermal inputs, and waters near the Chilean margin. The South Pacific Gyre and Pacific sector of the Southern Ocean are climate-critical regions for the transfer of heat, carbon, and nutrients within the global ocean, and they are a region where phytoplankton growth is typically limited by low concentrations of iron (Fe) in the surface ocean. The investigators will use a variety of analytical approaches, along with analyses made by other project collaborators, to address four major objectives related to dissolved micronutrient biogeochemistry in this region: 1) What are the relative fluxes of dissolved metals from dust, sediments, and Antarctic continental inputs to surface waters of the South Pacific gyre and ACC waters? 2) What fluxes of trace metals, especially Fe, are delivered by Pacific Deep Water to the Southern Ocean for upwelling to the surface waters? Are these fluxes hydrothermal or benthic in origin? Are dissolved metals supplied by the Pacific-Antarctic Ridge to the abyssal Southern Ocean? 3) How do micronutrient metal uptake stoichiometries change across gradients of primary production, community composition, and nutrient limitation from the gyre to ACC waters? How are these affected by metal speciation? 4) How do pre-formed biological signatures in surface waters, combined with regeneration and scavenging along the transport path, influence the regional and global distribution of TEIs?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.