Role of regional bioproductivity in atmospheric CO2 changes

One process that has been proposed to explain the glacial-interglacial chan ge in atmospheric CO2 is a change in the strength of the ocean's biological pump, that is, changes in ocean export bioproductivity. Using a coupled oc ean-atmosphere box model that incorporates carbon chemistry and a nutrient budget, we investigate the effect on atmospheric CO2 levels of changes to o cean export productivity in different regions. The primary discovery is the role of water column stratification in controlling the extent to which atm ospheric CO2 is affected by regional changes in the biological pump. Strati fication manifests itself in two competing ways. On the one hand, increasin g stratification causes an increase in atmospheric CO2 sensitivity to expor t bioproductivity. This results in modeled CO2 being almost an order of mag nitude more sensitive to low-latitude bioproductivity changes than it is to high-latitude changes. On the other hand, increasing stratification decrea ses the rate at which nutrients mix up into the surface layer, thereby sett ing a limit on the overall magnitude of bioproductivity increase and atmosp here CO2 decrease that is possible in stratified ocean regions. One of the implications of this work is to point to the potential importance of change s in the strength of low-latitude ocean bioproductivity as a cause of the g lacial-interglacial atmospheric CO2 change.