NISO-MARBL · Leveraging nitrogen isotopes to constrain nitrogen budgets and internal cycling using a global ocean biogeochemistry model under present and future climates

Nitrogen (N) is essential for all life on Earth, including phytoplankton living in the surface ocean. As such, the availability of N limits primary productivity across much of the global ocean, exerting a control on the biological carbon pump and climate. Yet, uncertainties in estimates of global N sinks and sources affect our understanding of ocean fertility. Since the N cycle is uniquely mediated by biological processes, an incomplete understanding of ecosystem dynamics limit our ability to quantify nutrient budgets and cycling. Studying N isotopes can fill this knowledge gap, as they provide novel information about ocean circulation, N cycling, and ecosystem change. By combining N isotopes and biogeochemical ocean modeling, this project aims to improve our quantitative understanding of the physical and biological processes controlling the N budget and N recycling in the present and future ocean. I will incorporate N isotopes into MARBL, the ocean biogeochemistry component of the Community Earth System Model. To place additional constraints on the N cycle, I will also add two novel tracers to the model, heterotrophic bacteria and the oxygen isotope ratios of nitrate. Using this framework, I will address the N budget including inter-basin balancing of N loss, and N recycling including the bacterial production of ammonium fueling phytoplankton. I will investigate these themes in the context of both the present day and future ocean to explore the sensitivity of the N cycle to warming and deoxygenation. The recent and rapid expansion of N isotope measurements and measurement facilities over the past five years provide an opportune window in which to develop and validate the state-of-the-art N isotope model. This work will provide coherent interpretations of existing modern and paleoceanographic data and will advance our understanding of the role that the ocean N cycle plays in driving fertility, carbon sequestration, and climate.