Planktonic foraminiferal shells buried in deep ocean sediments record lower &dgr;13C values of surface water dissolved inorganic carbon during glacial times than during Holocene. In the Southern Ocean and at high northern latitudes, a drop of between 0.3? and 0.9? is observed, whereas the mean change in low and middle latitudes is only ¿0.15?. However, a stronger biological carbon pump sufficient to explain the 80 ppmv lower atmospheric pCO2 values during glacial times would raise the surface ocean &dgr;13C values of dissolved inorganic carbon by about 1.0?. Here the results of a three-dimensional ocean circulation model study are presented which demonstrate that the increase of &dgr;13C values in the sea surface due to a strengthening of the biological carbon pump is counteracted by processes which drive the &dgr;13C values in the opposite direction. This was found by performing simulations employing the three-dimensional Hamburg Model of the Oceanic Carbon Cycle (HAMOCC) combined with a [CO2(aq)] dependent parameterization of the biological carbon isotope fractionation. The difference in the biological carbon isotope fractionation between Glacial and Holocene is responsible for a lowering of &dgr;13C values in surface water dissolved inorganic carbon by about 0.3?. The additional effects of the glacially elevated CO32- concentration (0.25--0.50?) combined with the 0.35? lowering of &dgr;13C values for the whole ocean due to a transfer of terrestrial organic carbon from the biosphere to the ocean-atmosphere reservoir also contribute to a further &dgr;13C drop of 0.6--0.85?. Hence a small glacial decrease of the planktonic foraminifera &dgr;13C of the order of 0.25? instead of an increase is predicted. ¿ 1999 American Geophysical Union