Nitrate isotopic signatures (δ15N-NO3- and δ18O-NO3-) in East Antarctic sea-ice and the underlying water column (SIPEX II results)

Nitrate is an important nitrogen source for organisms as it is the largest pool of fixed nitrogen available in ocean water and also in sea ice. During biological processing small isotope fractionations occur which affect the isotopic signature of nitrate (δ15N-NO3- and δ18O-NO3-) and contain information on past and ongoing processes influencing nitrate. Although isotopic signature analysis for nitrate has become a common tool for investigating the N-cycling in freshwater and marine systems, measurements in sea ice are still extremely scarce. During the SIPEX II expedition (Sept.-Oct. 2012; R/V: Aurora Australis), samples were taken in both sea ice and the underlying water column.
Nitrate isotopic signals in the water column show little variation and were close to deep ocean values (δ15N = 4.8‰, δ18O = 2.4‰). Close to the surface of the water column a small elevation was observed for both nitrogen and oxygen suggesting some biological activity was ongoing. In the sea-ice, slightly higher values of δ15N = 5.5‰ were observed at the water-ice interface and which increased to 14‰ in the center and top layers of the ice cores. Increasing δ15N values coincide with an increase of δ18O and decrease of nitrate concentration suggesting assimilation or denitrification of nitrate being one of the main processes. Solely based on nitrate isotopy differentiating between these two processes is difficult, however anoxic conditions and denitrification have been observed before. At some stations the δ15N and δ18O results deviate from the 1:1 slope generally linked with assimilation and denitrification, indicating possibly other processes are also taking place. Further analysis of the data is required to better constrain the processes taking place.