The role of ocean biology in setting atmospheric pCO2: the preformed nutrient theory

The sensitivity of atmospheric pCO2 to changes in (1) ocean circulation and (2) high latitude ocean biology has great implications for understanding the lower atmospheric CO2 content of the glacial atmosphere. Here we present a simple theory for how atmospheric pCO2 varies with preformed PO4 inventory and the ocean carbon storage due to the soft tissue pump. Under conditions of perfect equilibrium between atmosphere and ocean and assuming that total ocean nutrient is conserved, atmospheric pCO2 can be written as a sum of exponential functions of the preformed nutrient inventory. We validate our theory against simulations in a suite of realistic ocean general circulation models (GCMs). We then proceed to discuss two important points as follows: (1) Changes in physical forcing can drive large changes in atmospheric pCO2, even with minimal changes in surface nutrient concentration. If Southern Ocean deepwater formation strengthens, the preformed nutrient inventory and thus atmospheric pCO2 increase. (2) As shown by previous studies, an increase in high latitude nutrient utilization can decrease atmospheric pCO2 significantly. Here we show that the response of atmospheric pCO2 to changes in surface nutrients depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO2 sensitivity to surface nutrient forcing. In conclusion, atmospheric pCO2 is more sensitive to changes in surface biology changes in climates characterized by higher deep ocean ventilation.