Can nitrogen cycle feedbacks lead to runaway denitrification and deoxygenation of the ocean?
Friday, 19 December 2014: 9:30 AM
Over millennial timescales, the oceanic reservoir of fixed nitrogen (N) is regulated by a balance between N loss in sediments and low oxygen (O2) waters, and N2-fixation by diazotrophic phytoplankton. The two processes are coupled by selection for diazotrophs under N-limited conditions that arise from denitrification, although the spatial scale of this coupling is debated. A strong local coupling has been argued against because the export of newly fixed N would boost nearby denitrification rates, stimulating further N2-fixation in overlying water, resulting in runaway N loss and deoxygenation of the ocean. Using a global ocean circulation model with a prognostic ecosystem and biogeochemical cycles, we show that no such runaway feedback arises. Even when the majority of N2-fixation occurs directly above suboxic zones, N sources and sinks quickly reach a stable equilibrium without widespread N depletion. Taking a systems dynamics approach, we derive a “feedback factor” (f) for the interaction of N inputs and losses, which depends on the stoichiometry of diazotroph biomass, ventilation of the tropical thermocline, and upwelling rate of denitrified waters. Under modern conditions of ocean circulation and O2 solubility, f is much lower than 1 – the value that separates stable and runaway amplification of a system response – and the oceanic N reservoir is remarkably insensitive to spatial proximity of N sources and sinks, as might accompany the alleviation of diazotroph Fe-limitation. Based on these parameters, we identify climatic conditions that might push the system into a runaway feedback regime, allowing for periods of catastrophic N loss and deoxygenation during Earth’s history.