Diatom preservation patterns in surface sediments of the Pacific sector of the Southern Ocean were examined. A belt of well-preserved diatoms is bounded on the north and south by sediments containing poorly preserved diatoms. The northern boundary correlates closely with the Subantarctic Front and appears to result from reduced phytoplankton productivity caused by temperature effects on metabolic processes. The southern boundary is caused by the damping effect that late winter/early spring sea-ice has on diatom productivity in the water column. Our evidence suggests that the southern boundary represents the spring sea-ice position rather than summer position. When these data are applied to late Quaternary sections in the Pacific sector it appears that there was only a modest northward expansion of winter sea-ice cover during glacial maxima.