US GEOTRACES GP17-OCE: Hydrogen sulfide as a strong ligand affecting the speciation and solubility of key trace metals

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Trace metals are present at very low parts per trillion concentrations in the ocean but in spite of their scarcity many, like iron and zinc, are essential to the growth of the microscopic plants called phytoplankton. Others, like mercury and arsenic, are toxic. In addition to very low concentrations, the essential or toxic nature of a trace metal also depends on the chemical form of the element – as a free dissolved ion all by itself or as charged ion interacting with other inorganic or organic ions. One of the inorganic ions in seawater that strongly interacts with trace metals is bisulfide, the ionic form of the foul-smelling gas hydrogen sulfide. Its reactivity with metals can be so strong that it forms metal-sulfide particles that sink out and are removed from the water. This research project will study how hydrogen sulfide interacts with trace metals like cadmium, copper, mercury, nickel, and zinc to form dissolved complexes that make these metals less reactive to other ions and phytoplankton, or insoluble particles that remove the metals. The researchers will conduct these studies on an oceanographic expedition from Tahiti to the ice edge around Antarctica to Chile as part of the U.S. GEOTRACES program with many other researchers measuring trace metals from the surface to the ocean bottom. All of this and more will be the subject of a graduate student’s doctoral research. In addition to learning how to work on a major oceanographic expedition and with other researchers, she will communicate her experiences and findings on a blog available to all who follow it. She will also help to create a presentation about her science that will be part of the “Sailing with GEOTRACES” virtual reality experience for schools, libraries/museums, festivals, and public presentations.The US GEOTRACES GP17-OCE cruise from Tahiti to the Antarctic ice edge to Chile will sample waters with large gradients in phytoplankton biomass and species, macronutrients, and trace metals. With anticipated changes in the concentrations and phases of hydrogen sulfide in the oxic water column, the transect offers an ideal opportunity to quantify the linked cycles of hydrogen sulfide and bioactive metals like cadmium, copper, and zinc, and the toxic element mercury. With an overall goal of quantifying the coupled cycles of trace metals and sulfide in the upper, oxic ocean, our specific research objectives for this research will be: (1) Quantify the sources and sinks of dissolved and particulate sulfide as it pertains to the cycling of key trace metals in order to accurately link their cycles; (2) Establish the conditions under which free sulfide reacts with and precipitates trace metals such as cadmium, copper, and zinc, and quantify this removal term; and (3) Evaluate whether nanoparticulate/colloidal pyrite exists in hydrothermal plumes and its contribution to iron transport relative to colloidal iron oxyhydroxides. A fully integrated set of shipboard measurements, laboratory studies, and data synthesis and speciation modeling are designed to meet these objectives. The approach includes close collaborations with investigations of dissolved and particulate trace metals and their speciation, as well as with those studying mixing processes using a variety of tracers.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.