Micropaleontological analyses have been completed on six piston and drill cores of early Pliocene age (Gilbert magnetic epoch; 4.30 to 3.32 m.y. B.P.) recovered from the Southern Ocean between 110° E. and 160° W. long. All cores are dated by radiolarians, diatoms, and in part by paleomagnetic methods. Stratigraphic correlations are based on paleomagnetic data and recently developed Southern Ocean diatom and silicoflagellate zonations. Silicoflagellate temperature curves constructed from the generic ratios of Dictyocha/Distephanus define seven temperature oscillations and suggest that a dramatic change in the thermal structure of the Southern Ocean occurred during the Gilbert magnetic epoch, with a fluctuating drop in surface-water temperatures from >10°C (present-day northernmost subantarctic) to <1.5°C (present-day Antarctic). Inferred surface-water temperatures between 4.30 and 3.95 m.y. B.P. suggest that conditions were then too warm for the existence of the west Antarctic ice sheet.