NSF Award
2318938
The marine nitrogen cycle plays a large role in the ocean’s ability to take up and store carbon dioxide from the atmosphere. This is because N is a key nutrient for photosynthesis in the surface ocean. However, the ways in which fixed N (which can be used as a nutrient) is converted to N2 gas (which cannot) in the ocean interior are not fully understood. In particular, there are many open questions about the production and consumption of N2O (an intermediate gas in the N cycle), a potent greenhouse gas. This project aims to better understand the dynamics of fixed N loss in the interior of the Atlantic Ocean through the measurement of ~950 well preserved dissolved gas samples, modern water column measurements, and shipboard incubation experiments. The major goals are to map the distribution of N2O and its isotopic composition, quantify excess N2 produced by fixed N loss, and explore biogeochemical pathways. The project will include lab and field opportunities for students (one PhD student and two undergraduate students). The project will also broaden the reach of this large set of dissolved gas samples by consuming only a small quantity of gas and making the samples widely available for members of the scientific community. Finally, the large data sets generated from this work will be made publicly available, supporting other projects beyond the scope of this study. <br/><br/>This study will fill large data gaps in the Atlantic interior for the abundance and isotopic composition of N2O. With such a large set of samples that were collected, stored, and will be analyzed in the same way, there is an unprecedented opportunity to constrain the spatial variability of N2O and its isotopes without the challenges of inter-laboratory calibration, and without requiring dozens of cruises and years of preparation. Similarly, this work will build upon new analytical advances that permit the measurement of N2/Ar ratios and N2 isotopes as top-down tracers of denitrification. Comparison of stored gas tanks from the 1980s to modern measurements in North Atlantic regions of deep-water formation (where water mass ages are younger than 40 years) provides a unique opportunity to explore temporal changes in the N cycle over recent decades. Shipboard incubation experiments will provide new mechanistic insight into N2O cycling, and the large data set of N2O abundance and isotopic composition will directly aid in ongoing work by colleagues to employ machine learning techniques to map N2O variability in the global ocean.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
