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This thesis deals with the marine nitrogen (N) cycle. Two questions are addressed: “Do pre-industrial estimates of rates of N-fixation and denitrification yield a balanced marine Ncycle?” and: “How did the marine N-cycle, and particularly the N inventory, evolve across the last glacial termination?”. Observations and models are combined to address these questions. Tracers reflecting the N anomaly (N∗) and stable N isotopes (δN) are used to constrain a newly developed geochemical multi-basin box model and the Biogeochemical Elemental Cycling (BEC) model. Two probabilistic box model approaches were developed to simultaneously estimate the preindustrial rates of N-fixation and denitrification, as well as to compute the marine N budget. The approaches yield rates of N-fixation of 118 TgN yr−1 and 134 TgN yr−1 and rates of denitrification in the water column and sediment of 51 TgN yr−1 and 81 TgN yr−1, respectively. These rate estimates plus updated estimates of atmospheric deposition and riverine input of N yield a marine N-cycle that is balanced to within 15 TgN yr−1. A N residence time of 4,500 yr is computed. This residence time being shorter than the time-scale of the last glacial termination, the question is raised as to whether and how much the marine N-cycle has contributed to the variations of atmospheric level of carbon dioxide (CO2) on this time-scale, through changes in the global inventory of N. The change in the marine N inventory across the last glacial termination is evaluated using the box model developed for pre-industrial investigations. This is done by determining the temporal evolution of N-fixation and denitrification, which are affected by climate forcings. Deglaciation simulations are constrained by sediment δN records from several sites located in different ocean basins. A decrease of 26 to 48% in the global N inventory is simulated across the last deglaciation. This decrease is primarily a consequence of a 35% decrease in the global rate of N-fixation due to a strong iron forcing during the early deglaciation. A subsequent increase in N-fixation due to a strong internal N-feedback permits the global rate of N-fixation to remain nearly unchanged between the last glacial period and the Holocene. The decrease in the global inventory of N across the last deglaciation implies that global marine productivity possibly decreased, and hence the carbon sequestered in the deep ocean as well. This makes the marine N-cycle a potential candidate for explaining a part of the

Constraining the pre-industrial and deglacial marine nitrogen cycle by combining models and observations