Oxygen isotopic analyses of planktonic foraminifera have provided a picture of many aspects of the evolution of the temperature structure of surface and near-surface oceans during the Miocene. In time slice studies oceanographic conditions have been interpreted from synoptic maps of isotopic data at between 22 and 27 locations in the Atlantic, Pacific and Indian Oceans. Three time slice intervals were examined: 22 Ma (foraminifera! zone N4B) and 16 Ma (N8) in early Miocene time; and 8 Ma (N17) in late Miocene time. In time series studies, the evolution of oceanographic conditions at single localities during an extended period of time were inferred from δ¹⁸O values of planktonic foraminifera.

Surface waters warmed throughout the early Miocene at almost all localities examined. At 22 Ma, the Pacific Ocean was characterized by relatively uniform temperatures in the equatorial region but a marked east-west asymmetry in the tropical South Pacific, with higher temperatures in the west. Between 22 Ma and 16 Ma, tropical Pacific surface waters warmed, but warmed more in the east than the west. At 16 Ma, the asymmetric distribution of temperatures in the South Pacific Ocean remained, and the latitudinal temperature gradient, inferred from the isotopic data, was gentler than that of either the late Miocene or Modern ocean.

Between the late early Miocene and late Miocene, surface waters at most low-latitude Pacific sites warmed while those at high latitudes cooled or remained unchanged. However, surface waters at high northern latitudes in the Atlantic Ocean as well as in the eastern equatorial Atlantic cooled, while water temperatures remained relatively unchanged at most South Atlantic sites. Surface waters warmed in the southernmost Atlantic, off the tip of South Africa. By 8 Ma, the east-to-west asymmetry of the temperature distribution in the tropical South Pacific Ocean had lessened. Surface water temperatures had become quite similar to those of the Modern ocean except that those in the equatorial Pacific Ocean were lower than today’s. This is reflected in the latitudinal gradient of surface temperatures at 8 Ma which is less steep than that of modern temperatures.

The pattern of surface temperatures and their evolution through the Miocene is consistent with the biogeographic distributions of planktonic foraminifera described by Kennett et al. (this volume). The isotopic data provide a more detailed picture of the evolution of Miocene surface temperatures than had been hitherto available, and serve as a framework against which hypotheses can be tested regarding the cause of the middle Miocene cooling of deep waters and the formation of the East Antarctic ice sheet.