Award: OCE-1851430

The ocean covers most of the Earth’s surface, and its physical, biological, and chemical processes determine many aspects of Earth’s conditions. Biologically available nitrogen (or “fixed N”) is an essential nutrient for the microscopic plants (phytoplankton) in the ocean, and its supply can limit the growth of phytoplankton and the capacity of the ocean to fuel ocean ecosystems, including fisheries. Following the flow and transformation of fixed N is a powerful approach for understanding how the physics, biology, chemistry interact to determine the biological fertility of the ocean and its storage of carbon dioxide away from the atmosphere.

Nitrate (NO₃^-) is the primary form of fixed N in the sea. Both its nitrogen (N) and oxygen (O) atoms occur in more than one mass, or “isotope”: ¹⁴N and ¹⁵N for N and ¹⁶O and ¹⁸O for O. The ratios of these isotopes are affected by biological processes, and so isotopic measurements of nitrate reveal processes at work in the ocean that are otherwise hard to observe or disentangle. Moreover, the isotope ratios of nitrate in the ocean through time are recorded in the organic matter of the sediments that accumulate on the seafloor. These can be used to reconstruct important features of the past ocean, such as how the oxygenation of the ocean has changed through time.

This project produced analyses of the nitrate ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios in samples collected during the 2023 GO-SHIP IO5 zonal section across the South Indian Ocean from South Africa to Australia, the largest data set so far on the isotopic composition of nitrate in the Indian Ocean. The Indian Ocean is one of the three major basins of the global ocean, the one that has been studied the least, and it represents a gateway between the Atlantic and Pacific basins. The measurements of nitrate isotopes generated in this project are serving multiple purposes, all contributing to the project’s intellectual merit. One example of the discoveries that has arisen from the data involves the “oxygen-deficient zone” in the Arabian Sea, which removes N from the ocean through the process of “denitrification,” in which nitrate (rather than oxygen) is used by deep microbes to metabolize organic matter and thus survive. The measurements from GO-SHIP IO5 reveal that the impacts of denitrification, a shallow process, reach very deep into the ocean (down to roughly 3500 meters depth; see figure) and flow out into the rest of the global ocean, an unexpected finding. The next step will be to combine the data with other data sets from the other ocean basins to reconstruct the input-output “budget” of N in each ocean basin and the global ocean.

This project has had several broader impacts beyond the research itself. First, partly through this project, a website has been developed for visualizing data on ocean nitrate from throughout the global ocean (https://oceanicnitrateisotopes.geosciences.princeton.edu/). Second, in each year of the project, the principal investigator led a 1-day workshop for the Teachers as Scholars program of the Princeton University Program in Teacher Preparation, which brings middle and high school teachers to Princeton to learn about important scientific questions and research. Each workshop focused on different set of questions regarding how the ocean affects the Earth conditions that matter to humans. The workshop included hands-on demonstrations of how ocean data can be used for active learning in the classroom. Finally, undergraduates were involved in research through summer internships, allowing them to develop new skills and interests.

Last Modified: 02/12/2025
Modified by: Daniel M Sigman