Oxygen isotopic analyses of planktonic foraminifera have provided a picture ofmany aspects of the evolution of the temperature structure of surface and near-surfaceoceans during the Miocene. In time slice studies oceanographic conditions have beeninterpreted from synoptic maps of isotopic data at between 22 and 27 locations in theAtlantic, 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 lateMiocene time. In time series studies, the evolution of oceanographic conditions at singlelocalities during an extended period of time were inferred from 18O values of planktonicforaminifera.Surface waters warmed throughout the early Miocene at almost all localities examined.At 22 Ma, the Pacific Ocean was characterized by relatively uniform temperaturesin 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 Pacificsurface waters warmed, but wanned more in the east than the west. At 16 Ma, theasymmetric distribution of temperatures in the South Pacific Ocean remained, and thelatitudinal temperature gradient, inferred from the isotopic data, was gentler than that ofeither the late Miocene or Modern ocean.Between the late early Miocene and late Miocene, surface waters at most lowlatitudePacific sites warmed while those at high latitudes cooled or remained unchanged.However, surface waters at high northern latitudes in the Atlantic Ocean aswell as in the eastern equatorial Atlantic cooled, while water temperatures remainedrelatively unchanged at most South Atlantic sites. Surface waters warmed in the southernmostAtlantic, off the tip of South Africa. By 8 Ma, the east-to-west asymmetry of thetemperature distribution in the tropical South Pacific Ocean had lessened. Surface watertemperatures had become quite similar to those of the Modern ocean except that those inthe equatorial Pacific Ocean were lower than today's. This is reflected in the latitudinalgradient of surface temperatures at 8 Ma which is less steep than that of moderntemperatures.The pattern of surface temperatures and their evolution through the Miocene isconsistent with the biogeographic distributions of planktonic foraminifera described byKennett et al. (this volume). The isotopic data provide a more detailed picture of theevolution of Miocene surface temperatures than had been hitherto available, and serveas a framework against which hypotheses can be tested regarding the cause of themiddle Miocene cooling of deep waters and the formation of the East Antarctic ice sheet.
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