Quantitative planktonic foraminiferal analyses of lower Miocene sections reveal a hiatus encompassing part of foraminiferal zones N6 and N7 marking the extinction of Oligocene faunas that survived into the lower Miocene. The decline in the abundance of this Oligocene fauna can be dated at 22.2 Ma coincident with the evolution of Miocene species Globoquadrina dehiscens, G. altidpira and Globorotalia zealandica. This faunal event is marked by a cooling reflected in the oxygen isotope record and appears to mark the introduction of cool Antarctic bottom water into the Pacific Basin due to the opening of the Drake Passage that allowed deep circum-Antarctic circulation by about 22-25 Ma. Bottom circulation driven by Antarctic bottom water appears to have been established between 20 and 22 Ma in the Equatorial pacific and possibly also in the Atlantic and Indian Ocean. (mod. journ. abst.)