Simulating marine nitrogen and iron biogeochemical feedbacks and drivers of ocean productivity and deoxygenation in the Anthropocene

Nitrogen and iron are the main limiting nutrients on phytoplankton growth in the ocean. Their nutrient budgets contain processes that are sensitive to environmental change including low oxygen thresholds. In this study, we use a global ocean biogeochemical model coupled within an Earth system model of intermediate complexity to quantify anthropogenic controls on marine nitrogen and iron cycling under warming and atmospheric nutrient pollutant scenarios. We performed model sensitivity simulations to isolate the individual and combined effects of marine nitrogen and iron cycle feedbacks on ocean productivity and deoxygenation. Our model simulations demonstrate strong stabilizing feedbacks when considering either the marine nitrogen and iron cycle individually. However, when the full set of marine nitrogen-iron feedbacks were included, enhanced nitrogen and iron source inputs outweighed sinks under anthropogenic scenarios. These marine nitrogen-iron biogeochemical feedbacks were responsible for driving a projected 4% increase in productivity and 27% expansion in the volume of oxygen deficient zones by year 2100 in the model, whereas a sensitivity simulation without these feedbacks resulted in a 9% decrease in productivity and 16% reduction in the volume of ODZs. Our model study suggests that positive amplifying feedbacks between the marine nitrogen and iron cycles may already be playing an important role increasing ocean productivity and deoxygenation in the Anthropocene.