Using Nitrogen Isotope Records to Constrain Changes to the Global Oceanic Fixed Nitrogen Budget during the Last Glacial Maximum in an Earth System Climate Model

Abstract:

Nitrogen is one of the major limiting nutrients in the ocean that prevents biological production and carbon export into the ocean interior, known as the biological carbon pump. The major source and sink processes for the oceanic fixed nitrogen budget, N fixation and denitrification, respectively, are sensitive to climate change. We force an Earth System Climate Model of intermediate complexity that includes three-dimensional modules ocean biogeochemistry and isotopes (MOBI) with boundary conditions from the Last Glacial Maximum (LGM: ~21,000 years ago) and show that the nitrogen isotope results are qualitatively consistent with a global sedimentary database. Water column denitrification, which occurs in oxygen minimum zones, decreases by a factor of ~2 due to increased oxygen solubility in the glacial surface ocean that is ~2°C cooler on the global average. Sedimentary denitrification also decreases by a factor of ~2 due more exposed continental shelves from reduced sea level. We conduct experiments showing how N fixation responds in a “Redfield” biogeochemical model with constant elemental stoichiometry (N:P=16) and another “non-Redfield” experiment that includes a higher N:P quota for nitrogen fixers (N:P=40) and preferential remineralization of dissolved organic phosphorus relative to nitrogen (2 times faster). This “non-Redfield” experiment produces a more realistic distribution of dissolved organic nitrogen and phosphorus in the modern ocean and stimulates additional N fixation due to less P limitation of N fixers. It predicts that the oceanic fixed nitrogen inventory during the LGM was ~9% larger than present, whereas the “Redfield” model predicts an increase of only ~5%. Our experiments suggest that the oceanic nitrogen inventory during the LGM was significantly larger than present-day and show the importance of including non-Redfield stoichiometry in marine biogeochemical models when estimating changes to N fixation.