Understanding of Earth's transition from a warm, ice-free Cretaceous to today's bipolar glaciation is hotly debated. The Oligocene-Miocene boundary is marked by a brief glacial event followed by an interval of colder temperatures. Changes are small compared to the major Antarctic ice build-up at the Eocene-Oligocene boundary and establishment of a permanent Antarctic ice-sheet in the mid-Miocene. However, fossil evidence from low latitudes, including the faunal turnover which originally defined the Oligocene-Miocene boundary, indicates a reversal in trans-Atlantic flow, i.e. from westward to eastward, at this time. Modelling results suggest that a combined narrowing of the Tethys Seaway and deep opening of Drake Passage, and hence inception of Antarctic circumpolar circulation, drove reorganization of the patterns of ocean circulation. Despite evidence for a shallow Drake Passage opening in earliest Eocene time and subsequent deepening, a comparison of Southern Ocean isotopic records suggests that circumpolar circulation did not exist prior to c. 26 Ma. In fact, sedimentary records of a grain-size current-speed indicator from the Tasman Gateway reveal a singular, marked increase immediately preceding the initial Miocene event. The likely driver of this increase is inception of the full Antarctic Circumpolar Current. Among possible causes of early Cenozoic climate deterioration, the opening of seaways appears to play the major role. Extreme orbital configurations and pCO (sub 2) -drawdown may act as reinforcing factors.