Collaborative research: US GEOTRACES PMT: Trace-metal concentrations and stable isotopes in the North Pacific

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The goal of the international GEOTRACES program is to understand the distributions of many chemical elements and their isotopes in the oceans. The National Science Foundation is supporting a U.S. GEOTRACES sampling expedition in the Pacific Ocean 2018, from Alaska to Tahiti. This award will focus on measurement of the stable isotopes of iron (Fe), nickel (Ni), zinc (Zn), copper (Cu), and cadmium (Cd) in seawater, filtered particles, and atmospheric aerosol particles on this expedition. The trace metals Fe, Ni, Zn, Cu, and Cd strongly influence marine biogeochemistry and carbon cycling. Measurements of isotope ratios -- the relative abundance of different forms of the same chemical element -- provide insights not possible from concentration measurements alone. The investigators will use isotope data to learn more about the inputs and outputs of these elements to the ocean as well as the biological processes that influence their distributions within the oceans, leading to greater understanding of the role these metals play in oceanic carbon cycling. The award will also host an international inter-lab comparison exercise for Cd isotope ratios at low concentrations in seawater. The project will support an early career investigator, a postdoctoral researcher, and undergraduate and graduate students. The investigators will measure delta-56Fe, delta-60Ni, delta-65Cu, delta-66Zn, and delta-114Cd at high spatial resolution along the U.S. GEOTRACES Pacific Meridional Transect (PMT). The resulting oceanic sections of all five isotope systems will enable them to distinguish between competing ideas about the controls on trace metal distributions. A wide variety of hypotheses about marine biogeochemical trace metal cycling, addressing topics of global importance will be explored and tested, including: 1) How do different sources, productivity and export regimes, coupled with overturning circulation, control the distribution of Fe, Ni, Zn, Cu and Cd and their isotopes in the Pacific? 2) Do oxygen minimum zones act as sinks for Cd, Zn, Cu and Ni, while acting as sources for Fe? and 3) What sources are most important for supplying Fe to the North Pacific Ocean? The PMT section crosses two high nutrient-low chlorophyll (HNLC) regions and two oligotrophic gyres, as well as transecting the oldest waters in the ocean, allowing the investigators to use high-resolution isotope datasets to investigate competing hypotheses about the effect of vertical, horizontal and in situ biogeochemical processes on the distribution of all five metals and their isotopes. Aerosol dust, volcanogenic and reducing margin sediments, and hydrothermal vents such as the East Pacific Rise have all been hypothesized as major contributors to the dissolved Fe cycle in the Pacific. Measurement of iron isotope signatures in aerosols and near to oceanic sources will enable the identification and quantification the importance of these different iron sources and processes in supplying iron to the iron-limited surface ocean, especially important for the two HNLC regions along the PMT section.