Nitrogen cycle inferred from N and O isotopes in a changing Arctic Ocean

The rapid decline of sea ice extent in the Arctic Ocean will likely impact the overall Arctic ecosystem and associated biogeochemical cycles. As a first indicator, Arctic primary production has significantly increased during the last decades. All evidences highlights the necessity to better constrain the mod- ern, yet overlooked, Arctic nitrogen (N) cycle to make future primary production projections more reliable. We analysed nitrate δ15N and δ18O and water δ18O on full water column profiles during two summer cruises (July-August 2014) in the Arctic Ocean: SWERUS (R. V. Oden) consisting of across-slope sections over the East-Siberian and Laptev shelves; and ARK-XXVII (R. V. Polarstern) in the vicinity of Fram Strait. The coupling of the N and O isotopes is especially useful to constrain nitrification, the mi- crobially-mediated oxidation of ammonium into nitrate. Preliminary interpretations indicate a strong decoupling between nitrate δ15N and δ18O in the upper ocean, increasing and decreasing upward, respectively. This feature is unique in comparison to the other oceanic basins, and indicates that most of the nitrate in the upper ocean has been produced by nitrification within the Arctic Ocean, either over the shelves or in the deep basins.