Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO₂

We use both theory and ocean biogeochemistry models to examine the role of the soft-tissue biological pump in controlling atmospheric CO₂. We demonstrate that atmospheric CO₂ can be simply related to the amount of inorganic carbon stored in the ocean by the soft-tissue pump, which we term (OCS_soft). OCS_soft is linearly related to the inventory of remineralized nutrient, which in turn is just the total nutrient inventory minus the preformed nutrient inventory. In a system where total nutrient is conserved, atmospheric CO₂ can thus be simply related to the global inventory of preformed nutrient. Previous model simulations have explored how changes in the surface ' concentration of nutrients in deepwater formation regions change the global preformed nutrient inventory. We show that changes in physical forcing such as winds, vertical mixing, and lateral mixing can shift the balance of deepwater formation between the North Atlantic (where preformed nutrients are low) and the Southern Ocean (where they are high). Such changes in physical forcing can thus drive large changes in atmospheric CO₂, even with minimal changes in surface nutrient concentration. If Southern Ocean deepwater formation strengthens, the preformed nutrient inventory and thus atmospheric CO₂ increase. An important consequence of these new insights is that the relationship between surface nutrient concentrations, biological export production, and atmospheric CO₂ is more complex than previously predicted. Contrary to conventional wisdom, we show that OCS_soft can increase and atmospheric CO₂ decrease, while surface nutrients show minimal change and export production decreases.