Ventilation and Timescales of Oxygen and Nutrient Cycling in the Oxygen Deficient Waters Of The Eastern Tropical South Pacific

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Upward revision of global marine denitrification has led to speculation that there may be a large imbalance in the marine nitrogen cycle. In addition, the discovery of a new pathway (anaerobic ammonium oxidation, i.e., anammox) that leads to the removal of fixed nitrogen from marine sediments and anoxic regions of the water column has shaken long-held perceptions of the marine nitrogen cycle. As with denitrification, anammox converts fixed nitrogen to nitrogen gas, but does so without dissolved N2O as an intermediate stage. Large uncertainties remain in our knowledge of the rates and magnitudes of these N-loss pathways in oceanic oxygen minimum zones (OMZs) thus hampering efforts to constrain the marine nitrogen. In this project, a research team at the University of Washington will address this question: What is the in-situ rate and magnitude of oxygen utilization, denitrification, and anammox in the oxygen minimum zone of the Eastern Tropical South Pacific? They will approach this problem by making three significant oceanographic measurements that can be combined with an existing process study in the Eastern Tropical South Pacific aimed at distinguishing and identifying controls on anammox and denitrification in this region. First, they will measure water column distributions of dissolved nutrients along the hydrographic sections to help characterize nutrient sources and sinks in the oxygen minimum zone. Second, they will measure, to unprecedented coverage, dissolved N2O along the hydrographic sections as a diagnostic tracer for denitrification. Finally, they will measure transient anthropogenic tracers (CFCs and SF6) to quantify ventilation and circulation timescales within the OMZ. The tracer-derived timescales will be combined with measurements of dissolved oxygen and nutrients to quantify in situ rates of oxygen utilization and nutrient removal/regeneration. These rates will provide benchmarks to the rates determined in-vitro during shipboard incubation studies. Broader Impacts. This project will support an undergraduate participant through the Joint Institute for Study of the Atmosphere and Ocean (JISAO) summer internship program. JISAO typically supports 5-8 undergraduates to spend summer quarter in Seattle participating in atmospheric and oceanic research;a project such as this would be attractive to oceanography and chemistry majors in its technical aspects, and to journalism students in its outreach activities. One of the principal investigators will work with KUOW, the University of Washington public radio station, to design brief public interest stories on ocean science. The intern's final project will involve preparation of a brief radio segment for broadcast on KUOW on the background, significance and findings of the first year's research efforts, and participation in a scientific poster session with the other undergraduate interns at JISAO.