Nitrate isotope anomaly as a tracer of biogeochemical processes

The general aim of the thesis is testing the potential of the nitrate MIF (massindependent fractionation) signature as a tracer of biogeochemical processes. Nitrate δ15N, δ17O and δ18O of water samples collected at the Marano lagoon (Italy) have been analysed, by combining the denitrifier method with the N2O thermal decomposition in a gold furnace. No clear correlation between the magnitude of the capital delta (Δ17O=δ17O+0.5*δ18O) and the local atmospheric deposition has been found. Moreover, sewage treatments with ozone might be responsible for some high Δ17O values associated to sampling points close to populated areas. Overall, the signal is low (0.6‰ on average, which would correspond to roughly 2% of atmospheric nitrate in sample water) and the error associated to the measure is no less than 75%. To test the assumption that the atmosphere is the only source of a MIF signature, a set of nitrate minerals of different origin was analysed. The capital delta in a specimen of buttgenbachite (with formula Cu36(NO3)2Cl8(OH)62*4−10H2O) from Likasi mine (Congo, 4‰±1) and in a sample of nitromagnesite (Mg(NO3)2*6H2O) collected at Pozalagua Cave (Spain, 10.3‰±0.4) would indicate the possibility that a significant isotope anomaly could be generated due to geochemical processes. So far the MIF in atmospheric nitrate has been interpreted as the result of chemical reactions only, but it could be demonstrated that a series of mass dependent processes might generate an apparent Δ17O. The study of the MIF is a relatively new field and the present work underlined some limits of this novel tracer. Further work should be focused on identifying the systems and the conditions for what the nitrate MIF can be considered as a conservative tracer. Particularly, attention should be paid to the effects of biological processes involved in the nitrogen cycle, such as nitrification and denitrification, and transport processes