Award: OCE-1435002

Intellectual Merit: Our group measured the naturally occuring nitrogen (N) and oxygen (O) isotope ratios of nitrate (δ15N and δ18O) in samples from depth profiles collected along the U.S. GEOTRACES Arctic Ocean Section in August 2015. Nitrate isotope ratios were distinct directly below the ocean surface (in the Pacific Halocline), signaling modifications of Pacific-origin nutrients on the continental shelves upstream due to biological cycling (remineralization and benthic denitrification). The biological transformations evidenced from the nitrate isotope ratios provide important insights into the mechanisms of water mixing on the shelves upstream, and of water mass ventilation (i.e., how shelf waters enter the western Arctic Basin). In the Makarov basin, nitrate isotope ratios track the shoaling (i.e., shallowing) of the Pacific Halocline and the intrusion of waters from Eurasian Basin, resulting in a shallower subsurface nitrate reservoir. A shallower nitrate reservoir has implications for the delivery of nutrients to the sun-lit surface in an increasingly ice-free Arctic (Granger et al. in prep). Nitrate isotope ratios in sea ice were also measured in co-PI Hastings laboratory, indicating a proportionally important contribution of atmospheric nitrate to sea ice, comprising a source of new N to the oligotrophic (i.e., relatively infertile) surface of the Western Arctic Basins (Clark et al. in prep). Funding from this proposal also facilitate a collaboration with the Canada Arctic GEOTRACES effort, in which we measured nitrate isotope ratios in the Canadian Archipelago, Baffin Bay and the Labrador Sea. Nitrate isotope ratios revealed that primary production in Baffin Bay is fuelled dominantly by Pacific-derived nutrients, rather than the Atlantic nutrients (Lehmann et al. in review). The nitrate isotope ratios further diagnose that a large N deficit in deep waters of Baffin Bay (i.e., a loss of nutrient nitrate) results from benthic denitrification (i.e., the biological conversion to inert N2gas). Lehmann also analyzed and interpreted nitrate isotope ratios from the Western Pacific Boundary as part of this collaboration, detailed in Lehmann et al. (2018). Broader Impacts: Funding from this proposal provided support for a graduate student as well as two junior undergraduate technicians who have since secured positions in respective laboratories/graduate programs. Products: The research facilitated by this funding has resulted in 11+ conference presentations at national and international meetings, two published papers (Granger et al. 2018, Lehmann et al. 2018), a paper in review (Lehmann et al. in review), and three papers in preparation (Clark et al. in prep; Granger et al., in prep; Lehmann et al.in prep). Data from this project are archived with BCO-DMO (Granger 2018) and were submitted to the GEOTRACES data repository. Last Modified: 03/05/2019 Submitted by: Julie Granger