Award: OCE-1433989

Atmospheric deposition has been hypothesized to be the primary source of sea ice nitrate in the Arctic, and as such may represent an important external source of nitrogen (N) transported from the mid-continental latitudes that help support primary production in N-deplete waters of the Arctic Ocean. Isotopes of nitrate in sea ice and snow provide a unique opportunity to partition contributions of nitrate from the atmosphere to the ocean. The isotopic composition of nitrate (δ15N, δ18O, Δ17O) was measured in aerosols, snow, pack ice and seawater samples collected between 82N and 89N during the U.S. Arctic GEOTRACES 2015 expedition (Figure 1). Snow and aerosol samples were characteristic of atmospheric nitrate with generally low δ15N and high Δ17O and δ18O, while sea ice was higher in δ15N and lower in Δ17O and δ18O. Key outcomes of this project show that Δ17O> per mil occurs at various depths, indicating that atmospheric nitrate is an important component found in sea ice, but not as important as biologically produced nitrate from nitrification (or the oxidation of ammonium to nitrate) within sea ice. This is the first contribution that investigates open sea Arctic ice; uses triple isotopes of nitrate; and considers the influence of atmospheric deposition. Δ17O allows for the quantification of these two sources and indicates that biological nitrate represents >60% of the nitrate in sea ice, while atmospheric nitrate accounts for less than 40% (Figure 2). Overall, the findings of this study influence our understanding of how changes in sea ice distribution may influence nitrogen (and consequently carbon) biogeochemistry in ocean ecosystems at high latitudes through distinguishing the relative importance of biologically versus atmospherically sourced nitrate to the surface ocean. This project has contributed to the training of a graduate student, who has presented project findings at several major scientific conferences and is the lead author of a manuscript submitted to a peer-reviewed journal (S. Clark et al., Global Biogeochemical Cycles, in review). Specifically, the graduate student has been trained on several analytical methods, data analysis and computer coding; this project has also contributed to the professional development of technical staff. An undergraduate research project was also supported bia this project, and the undergraduate is employed in the scientific workforce in the U.S. Dissemination of project findings include: four conference presentations (2 oral talks and 2 poster presentations) at both international and national conferences, including the Association for the Sciences of Limnology and Oceanography, American Geophysical Union, International Symposium on Isotopomers and Polar2018. Finally, the data have been made publicly available on the BCO-DMO data repository. Last Modified: 05/29/2019 Submitted by: Meredith G Hastings