Nitrogen cycling in oxygen deficient zones : insights from delta15N and delta18O of nitrite and nitrate

Abstract

The stable isotopes, δ15N and δ18O, of nitrite and nitrate can be powerful tools used to interpret nitrogen cycling in the ocean. In order to interpret isotope profiles, the isotope systematics of each process involved must be known. This thesis describes numerous experiments using both cultures of nitrifying organisms as well as natural seawater samples to determine the oxygen isotope systematics of nitrification. These experiments show that the accumulation of nitrite has a large effect on the resulting δ18ONO3. Also, the δ18ONO2 was developed as a unique tracer because it undergoes abiotic equilibration with water δ18O at a predictable rate based on pH, temperature and salinity. This rate, its dependencies, and how the δ18ONO2 values can be used as not only biological source indicators but also indicators of age are described. Finally, using the isotope systematics of nitrification as well as the properties of nitrite oxygen isotope exchange described in this thesis, the final chapter interprets multi-isotope nitrate and nitrite profiles in the Costa Rica Upwelling Dome using a simple 1D model. Overall, this thesis describes new nitrogen and oxygen isotopic tracers and uses them to elucidate the complicated nitrogen biogeochemistry in oxygen deficient zones.