The flux of biogenic silica out of the oceanic mixed layer (the export flux) must balance the import of high-silicate deep waters associated with mass exchange between the surface and deep ocean. Recent regional estimates of the export flux limit it to 50--80 Tmol yr-1. In order to reproduce such low export fluxes, coarse-resolution general circulation models must have low pycnocline diffusivity and a lateral exchange scheme which involves advection of tracers by unresolved mesoscale eddies. Failure to capture low rates of vertical exchange will result in climate models which overpredict the uptake of anthropogenic carbon dioxide and fail to capture the proper locations and rates of density transformation. ¿ 1999 American Geophysical Union