Nitrogen isotopes in a global ocean biogeochemical model : constraints on the coupling between denitrification and nitrogen fixation

• Main Author: Somes, Christopher J.
• Format: Thesis
• Language: English
• Published: 2009
• Subjects: Pacific; Southern Ocean
• Online Access: https://oceanrep.geomar.de/id/eprint/14334/; https://oceanrep.geomar.de/id/eprint/14334/1/somes_thesis.pdf

We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth System Climate Model designed for millennial timescale simulations. The model includes prognostic tracers for the stable nitrogen isotopes, ¹⁴N and ¹⁵N, in the nitrate (NO₃ˉ), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO₃ˉ assimilation, water column denitrification, and zooplankton excretion are considered as well as the input of newly fixed nitrogen by diazotrophs. A global database of δ¹⁵NO₃ˉ observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce the observed patterns such as high subsurface values in denitrification zones, the meridional and vertical gradients in the Southern Ocean, and the meridional gradient in the Central Equatorial Pacific. The observed subsurface minimum in the Atlantic is underestimated presumably owing to too little nitrogen fixation there. Sensitivity experiments show that algal NO₃ˉ assimilation, nitrogen fixation and water column denitrification have strong effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotopes distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for diazotrophs and stimulates additional nitrogen fixation. Water column denitrification has a strong but rather localized effect on the nitrogen isotope distribution in model versions without iron limitation of diazotrophy, in which a tight coupling of nitrogen fixation exists. However, including iron limitation of diazotrophy inhibits a tight coupling between water column denitrification and nitrogen fixation in the Eastern Pacific and shifts the main location of nitrogen fixation from the ...