The middle Miocene delta (super 18) O increase represents a fundamental change in the ocean-atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from approximately 20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high delta (super 13) C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep-waters warmed in the early Miocene by approximately 3 degrees C ( approximately 20 to 16 Ma) and cooled by a similar amount during the middle Miocene delta (super 18) O increase (14.8 to 12.6 Ma), corresponding to the increase ( approximately 20 Ma) and subsequent decrease ( approximately 16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 per mil), relatively rapid ( approximately 0.5 m.y.) delta (super 18) O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long-term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2 degrees to 3 degrees C decrease in deepwater temperatures to produce the middle Miocene delta (super 18) O shift. Copyright 1992 by the American Geophysical Union.