Evaluating effect of ballast mineral on deep-ocean nutrient concentration by using an ocean general circulation model

The ballast parameterization separates sinking organic flux into two parts: the flux associated with ballast minerals and the flux independent of minerals. The ballast parameterization begins to be incorporated into global ocean biogeochemical models. However, parameters used in this parameterization such as those of calcium carbonate flux are not constrained enough and it has been difficult to evaluate quantitatively how much the ballast-induced flux affects nutrient concentration in deep ocean. In this study, we obtain optimized parameters of calcium carbonate flux by conducting 64 sensitivity simulations using export production estimated from satellite observation and an ocean circulation field simulated by a state-of-the-art ocean general circulation model. By comparing simulations with and without the ballast-induced flux with the optimized parameters, 6% of nutrient of the deep Pacific is estimated to be transported by the ballast-induced flux. Because this accounts for 30% of effects of the total biological pump, it indicates that the ability of the biological pump to carry nutrients to deep ocean significantly depends on the ballast-induced flux. Results of additional sensitivity simulations suggest that choice of model parameters associated with calcium carbonate flux strongly affects its estimation. Although the implementation of the ballast parameterization has potential ability to improve simulation of nutrient concentration, it is possible only when the model parameters of calcium carbonate flux are set appropriately.