Biological pump and vertical mixing in the southern ocean: Their impact on atmospheric CO2

Abstract

A global box model simulating nitrogen and carbon cycling in the ocean has been developed. The distinctive feature of the model is the detailed description of the seasonal cycles of the oceanic upper mixed layer (UML) ecosystem. Unlike other ocean regions, phytoplankton productivity in the Southern Ocean is assumed to be limited by low iron availability, leading to twofold decrease in the phytoplankton growth rate. Calculated ecosystem and carbon cycle characteristics are in a good agreement with available observational data and conceptual models of generalized phytoplankton seasonal cycles in the world ocean. The model estimates of the global ocean new production outside of shelf regions and the preindustrial atmospheric pCO2 are 9.9 Gt C/yr and 282 ppm, respectively. Results of numerical experiments with the model showed that the potential new production which might be reached by allowing phytoplankton maximum growth rate to increase is 29 Gt C/yr (76% of this increase is contributed by the Southern Ocean) and corresponds to an atmospheric pCO2 of 205 ppm; however, this would require an unrealistic tenfold increase in growth rate. The large contribution of the Southern Ocean is accounted for by the high‐nutrient, low‐chlorophyll (HNLC) conditions existing in this region caused by the high dissolved inorganic nitrogen concentrations below the UML, deep mixing during the austral summer, and iron limitation of phytoplankton productivity. A realistic (twofold) increase in the phytoplankton growth rate in the Southern Ocean which can be considered as a maximal effect of iron fertilization results in the lowering of atmospheric pCO2 by only 10 ppm. Changes in the UML depth in the Southern Ocean (a wintertime shallowing and summertime deepening of the UML in comparison with preindustrial conditions) could lead to a decrease of atmospheric pCO2 by 15 ppm at the most. The combined effect of iron fertilization and these changes in vertical mixing might constitute about 30–35 ppm, that is, less than one half of the lowering of 80 ppm during the last glaciation.