Changes in ocean circulation are often credited as the primary control on large-scale climate change during the Miocene. This study investigates the latest Oligocene to middle Miocene evolution of Southern Ocean circulation by evaluating stable isotopic trends of shallow- and deep-dwelling planktonic foraminifera, as well as ϵp records reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altanic Ocean (Deep Sea Drilling Project site 516). Changes in ϵp at site 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in ϵp at ~20.3 Ma is interpreted to reflect an increase in upper water column nutrient concentrations, caused by the onset or strengthening of the Antarctic Circumpolar Current (ACC). Measurable alkenone concentrations disappear by ~17 Ma, prior to a collapse in surface-to-thermocline Δ18O and Δ13C gradients. This is interpreted as reflecting a severe decrease in mixed layer nutrient concentrations and reduced proto-Antarctic Intermediate Water influence. The Δ18O gradient was reestablished by 14.5 Ma, coincident with the hypothesized East Antarctic ice sheet expansion, suggesting a direct relationship between increased strength of the ACC and the largest climate shift of the middle Miocene. ¿ 2000 American Geophysical Union