How does ocean biology affect atmospheric p CO 2 ? Theory and 1 Models

3 This paper examines the sensitivity of atmospheric pCO2 to changes in ocean biol4 ogy that result in drawdown of nutrients at the ocean surface. We show that the global 5 inventory of preformed nutrients is the key determinant of atmospheric pCO2 and the 6 oceanic carbon storage due to the soft tissue pump (OCSsoft). We develop a new the7 ory showing that under conditions of perfect equilibrium between atmosphere and ocean 8 atmospheric pCO2 can be written as a sum of exponential functions of OCSsoft. The 9 theory also demonstrates how the sensitivity of atmospheric pCO2 to changes in the soft 10 tissue pump depends on the preformed nutrient inventory and on surface buffer chem11 istry. We validate our theory against simulations of nutrient depletion in a suite of re12 alistic general circulation models (GCMs). The decrease in atmospheric pCO2 following 13 surface nutrient depletion depends on the oceanic circulation in the models. Increasing 14 deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases 15 the atmospheric pCO2 sensitivity to surface nutrient forcing. Conversely, stratifying the 16 Southern Ocean decreases the atmospheric CO2 sensitivity to surface nutrient depletion. 17 Surface CO2 disequilibrium due to the slow gas exchange with the atmosphere acts to 18 make atmospheric pCO2 more sensitive to nutrient depletion in high ventilation mod19 els and less sensitive to nutrient depletion in low ventilation models. Our findings have 20 potentially important implications for both past and future climates. 21