The central Scotia Sea, located between the South American and Antarctic plates, is an integral part of the marine conduit that permits eastward deep-water flow from the Pacific Ocean to the Atlantic Ocean. The geologic history of the central Scotia Sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete, deep Antarctic Circumpolar Current, widely believed to have been a key factor in the history of Antarctic glaciation. Here, we present new evidence on the nature and age of the central Scotia Sea floor. Multibeam surveys and the first dredged samples indicate that a now-submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid-Miocene climatic optimum. Inception and development of a full deep Antarctic Circumpolar Current may therefore have been important, not in the drop in global temperatures at the Eocene-Oligocene boundary as long surmised, but in the subsequent late Miocene global cooling and intensification of Antarctic glaciation.