Award: OCE-1233339

Intellectual Merit Nitrogen (N) is one of the elements required universally by plankton in the ocean. The dominant processes that control the amount of nitrogen in the ocean are poorly quantified, and we focus our attention here on denitrification, the bacterially-mediated process that dominates nitrogen removal from the ocean. The eastern tropical South Pacific Ocean is home to one of the ocean?s naturally occurring low-oxygen regions, where the process of denitrification can thrive. While representing only 0.1% of the ocean?s volume, these low oxygen regions have wide-ranging effects on the chemical composition of the ocean. In this project, we measured the N isotope ratio (15N/14N) of nitrate and nitrogen gas in seawater, and nitrate in marine aerosols, collected as part of the GEOTRACES GP16 section in the south Pacific Ocean. Nitrogen isotope ratios record the activities of processes that produce and consume nitrate, including denitrification. Our goal was to better understand the contribution of denitrification in the south pacific ocean to the nitrogen isotope ratios in the broader pacific basin. The GEOTRACES GP16 section allowed us to track the 15N/14N signal produced by denitrification half way across the pacific ocean. Using 15N/14N and other tracers, including the oxygen isotope ratios (18O/16O) in nitrate, as well as the concentration and 15N/14N ratio of nitrogen gas, we were able to identify the role denitrification plays in setting the distribution of nitrate in the south pacific. This fundamental understanding of how the distribution of nitrate is controlled will enable us to better interpret past and future perturbations to the marine nitrogen cycle, which may have seen periods of increased or decreased denitrification activity. Broader impacts Nitrogen cycling in low oxygen regions, such as those studied in this project, provides essential services to human society by limiting the impact of anthropogenic nitrogen inputs on ocean ecosystems. In low oxygen regions this excess nitrogen is converted to forms that are unavailable for most life, thus effectively removing it from the burden of anthropogenic nutrient inputs. Better understanding nitrogen cycling in natural oceanic oxygen deficient zones will allow us to better predict and manage regions experiencing anthropogenically-driven hypoxia, as many of the same processes are present and active in these regions. This project supported a PhD student at Stanford University, who used the data from this project for the bulk of their PhD thesis. During this project, the student received essential training and had the opportunity to participate in the GP16 cruise, attend planning, project, and data meetings, present the data at national and international meetings, and publish the work as first author. Through this project, the PI and graduate student also undertook mentoring of undergraduate and high school students in the laboratory. These students received training in oceanography, as well as laboratory and data analysis protocols, including the powerful tool of isotope ratio mass spectrometry. The undergraduates have applied to graduate school to pursue scientific research, and the high school students have gone on to study STEM fields in college. The results have been freely shared through data workshops, national and international meetings, publications, and submission of the data to public databases. As well, Casciotti has hosted laboratory visitors interested in acquiring skills for nitrate isotope analysis. We have trained about half a dozen new laboratories in the United States, China, India, and Japan to make nitrate isotope analyses, during the course of this project. Thus, these efforts have resulted in capacity building in the field of chemical oceanography, and particularly in nitrogen isotope biogeochemistry. Last Modified: 12/19/2017 Submitted by: Karen L Casciotti