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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 δ 18 O 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.

Oxygen and carbon isotopic ratios of planktonic and benthonic foraminifera have provided information about the evolution of the oceans at low- and mid-latitude sites in the Miocene eastern North Pacific Ocean. DSDP Site 495 (12° N; 91° W) provides a record of early and middle Miocene oceanographic conditions in the eastern equatorial Pacific. Oxygen isotopic evidence indicates that G. sacculifer, D. altispira and G. siakensis were shallow-dwelling, tropical planktonic species. G. venezuelana deposited its test at greater depths, probably below the thermocline. Carbon isotopic evidence conflicts with that of the oxygen isotopes in that it suggests that G. siakensis calcified under conditions similar to those of G. venezuelana. Temperature variability at Site 495 during early and middle Miocene time was relatively small. However, while middle Miocene deep waters at this site cooled, simultaneously with a major phase of growth of the Antarctic ice sheet, surface and near-surface waters warmed. The oxygen isotopic record at Site 470 in the eastern North Pacific (29° N; 117° W) indicates that middle and late Miocene surface temperatures at this site were relatively stable, but were probably lower than modern surface temperatures. At Site 173 (40° N; 125° W) middle and late Miocene surface temperatures were consistently lower than those at the more southerly Site 470, and were also significantly more variable. There is no indication that surface temperatures have changed significantly at Site 173 since late Miocene time. The inferred greater variability of surface temperatures at Site 173 may reflect greater variability of the intensity of upwelling at that site than at Site 470 during Miocene time. At Site 495 both the planktonic and benthonic foraminiferal carbon isotopic records vary sympathetically with published benthonic foraminiferal isotopic records from the Atlantic and Pacific Oceans, indicating that the carbon isotopic ratios at that site largely reflect global fluctuations in the isotopic composition of dissolved inorganic carbon. At Site 470 the planktonic carbon isotopic record fluctuates sympathetically with published benthonic records, indicating that the middle and late Miocene 13 C/ 12 C ratios of dissolved inorganic carbon in surface waters at this site reflected global fluctuations in 13 C/ 12 C. The planktonic carbon isotopic record at Site 173 could not be correlated with global carbon isotopic fluctuations, indicating that, in part, local effects controlled the 13 C/ 12 C ratio of dissolved inorganic carbon in surface waters at that site.

Abstract The maximum temperature to which a shale has been heated as a result of burial can, in some instances, be estimated using oxygen isotope geothermometry. The isotopic fractionation, or difference between O<sup>18</sup>/O<sup>16</sup> ratios of two coexisting minerals which have reached isotopic equilibrium with one another, is temperature dependent. Hence, if two coexisting minerals, which have isotopically equilibrated with one another, can be separated from a shale, and if the variation of the equilibrium isotopic fractionation between these minerals is known, the temperature of equilibration can-be estimated. Quartz and coexisting illite or mixed layer illite/smectite is a promising pair for isotope geothermometry of shales. A preliminary equilibrium fractionation curve for this pair is given by: where exp refers to the fraction of layers in the mixed-layer clay which are expandable. The results of three isotope geothermometry studies are summarized. Mineralogic and O 18 /O 16 data for coexisting quartz and illite from the altered volcanic rocks of the active hydrothermal region at Broadlands, New Zealand were used to investigate isotopic equilibration and to serve as a basis for calibration of the quartz-clay isotope geothermometer. Mineralogic and O l8 /O l6 data for coexisting clay-sized quartz and illite /smectite from one of three deep wells in the Gulf Coast indicate that isotopic equilibrium is approached between these two minerals at a well temperature above about 100° C. The illite/smectite apparently exchanges oxygen with pore waters in an approach toward isotopic equilibrium during the reaction: smectite + Al + K 𠆒 illite + Si. The released Si forms quartz which dominates the finest quartz fractions and forms overgrowths on detrital grains. This quartz apparently forms in isotopic equilibrium with pore waters. Isotopic temperatures derived from coexisting quartz and illite from the Precambrian Belt argillites range from 225° C to 310° C and generally increase down-section. This temperature range is compatible with the bulk mineralogy and probable depth to which the rocks have been buried. Oxygen isotope geothermometry can not yet be used routinely. However, it may be more possible to do so after additional information is obtained on factors such as chemical alteration and mineralogic reactions that control isotopic exchange.