Intellectual merit Nitrogen is one of the major nutrients required universally by plankton in the ocean, and its availability can affect the oceanÆs ecology, productivity, and carbon cycle. While the cycling of nitrogen by marine plankton is connected to that of other nutrient cycles, it is also unique in that its largest input and output terms are biologically controlled. These terms in the oceanic nitrogen budget are poorly characterized, and the focus of this project was on quantifying nitrogen inputs through nitrogen fixation in the North Atlantic Ocean. Developing robust estimates of the global rate and distribution of nitrogen fixation from direct shipboard measurements of nitrogen fixing activity is complicated by the inherent spatial and temporal variability of this biologically mediated flux. Thus, this project focused on developing geochemical approaches for estimating nitrogen fixation inputs from natural abundance isotope measurements. Before the initiation of this project, nitrate stable isotope measurements were sparse in the Atlantic, being focused primarily in the Sargasso Sea. This project has mapped the distribution of nitrate isotopes across the North Atlantic Ocean with unprecedented spatial coverage and measurement precision, and it will enable the assessment of nitrogen fixation in the North Atlantic Ocean. The major goals of this project were to develop an nitrogen isotope budget for nitrogen fixation in the tropical Atlantic based on the GEOTRACES North Atlantic section, together with two additional Atlantic sections, one running across the South Atlantic, and one running north/south between the two sections. By combining these data with an oceanographic model we aimed to calculate how much nitrogen fixation was needed to explain the data. All sample analyses, as well as the initial description and interpretation have been completed, but more can be done with the data as we combine it with other nitrate isotope data from the basin, and with other parameters measured in GEOTRACES. Another goal of this project was to evaluate the importance of atmospheric deposition of nitrogen to the north Atlantic, relative to the magnitude of nitrogen fixation. With atmospheric nitrogen loading and deposition to the ocean increasing, human perturbations of the nitrogen cycle are beginning to reach remote areas of the open ocean. These areas, which are frequently limited by nitrogen supply may see an increase in primary productivity and production of the greenhouse gas nitrous oxide. It is important to understand the links between anthropogenic atmospheric deposition and the natural processes of nitrogen delivery to the open ocean in order to assess the future impact of human activities on marine ecology and greenhouse gas production. Broader impacts The broader impacts of the study include the mentoring of a postdoctoral investigator and the inclusion of undergraduates in state-of-the-art research. Two of the principal investigators on this project are early career women scientists, and this project has helped expand their recognition and retention in the oceanographic community. As mentioned above, the project provided a high-quality nitrate isotope data set for the North Atlantic, which can be used by the broader community. Our data has been submitted to the Biological and Chemical Oceanography Data Management Office, along with full procedural documentation. The researchers involved in this project have worked to build teaching and research connections with Bermuda Institute of Ocean Sciences, with the goal of producing an inseparable mix of educational, undergraduate research, and graduate research opportunities involving ocean science. The project included training for a variety of undergraduate, graduate, and postdoctoral researchers. The project has also furthered national and international capacity for nitrate isotope measurements. We have reached out to laboratories in 12 different countries, i...
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