Biogeochemical constraints on the modern and glacial oceanic nitrogen cycle

This dissertation contributes to the understanding of the global oceanic budget of biologically available (or “fixed”) nitrogen. I present three studies that help constrain the budget of fixed N in the modern ocean. Estimates for the primary terms in the N budget are obtained by analyzing large-scale biogeochemical tracers through the use of simple mixing models, a geochemical box model, and a general circulation model. In the third study, I also present a model for changes in the N budget on glacial/interglacial time scales.; These studies support the emerging view that N₂ fixation is the dominant source of oceanic fixed N, with global rates nearly an order of magnitude higher than earlier estimates. The estimates of N₂ fixation presented here are at the upper end of the range of reported estimates. However, a simple isotopic balance also supports an upwardly revised rate of global sediment denitrification, so that my estimated N₂ fixation rates do not balance the modern N budget.; The results of the time-dependent model offer strong support for the presence of feedbacks between the major fixed N sources and sinks, and the N inventory. By comparing time-dependent model simulations with isotopic records from sediment cores, I conclude that feedbacks in the N cycle are strong enough to regulate the N inventory against large perturbations due to climatic forcing on glacial/interglacial time scales. This finding therefore allows only a limited role for oceanic nutrient changes as a cause for glacial/interglacial changes in atmospheric CO₂.