Showing papers in "Geological Society of America Memoirs in 1985"

Miocene planktonic foraminiferal biogeography and paleoceanographic development of the Indo-Pacific region

Abstract: •Present address: Department of Geological and Geophysical Science, Princeton University, Princeton, New Jersey 08544.

The evolution of Miocene surface and near-surface marine temperatures: Oxygen isotopic evidence

Abstract: 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 <5O 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 wanned 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 lowlatitude 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 •Present address: Department of Geological and Geophysical Science, Princeton University, Princeton, New Jersey 08540.

Depth stratification of planktonic foraminifers in the Miocene ocean

Abstract: A depth stratification of planktonic foraminifers based on oxygen isotopic ranking is proposed for the Miocene. Species are grouped into surface, intermediate, and deep dwellers based upon oxygen isotopic composition of individual species. The depth stratification is applied to planktonic foraminiferal populations in three Miocene time-slices (21 Ma, 16 Ma, and 8 Ma) in the equatorial, north, west, and east Pacific. The late Miocene time-slice is compared with modern Pacific GEOSECS transect water-mass profiles of temperature and salinity in order to illustrate the similarities between the depth ranking of planktonic foraminifers and temperature and salinity conditions. The geographic distribution of inferred surface, intermediate, and deep water dwellers was found to be very similar to modern temperature profiles: surface dwellers appear to be associated with warmest temperatures (>20°C), upper intermediate water dwellers with temperatures between 10 and 20°C, and lower intermediate and deep water dwellers with temperatures below 10°C. Tropical high-salinity water appears to be associated with the upper intermediate Globorotalia menardii group in the modern ocean. Depth stratification applied to two Miocene time-series analyses in the equatorial Pacific (Sites 77B and 289) indicates increased vertical and latitudinal provincialism between early, middle, and late Miocene time. The early and middle Miocene equatorial Pacific was dominated by the warm surface water group, which shows distinct eastwest provincialism. This provincialism is interpreted as the periodic strengthening of the equatorial surface circulation during polar cooling phases. During the late Miocene the upper intermediate group increased and the surface group declines. At the same time the east-west provincialism disappeared. This faunal change may have been associated with the major Antarctic glaciation and resultant strengthening of the general gyral circulation and the strengthening of the Equatorial Countercurrent due to the closing of the Indonesian Seaway at that time. \"Present address: Department of Geological and Geophysical Science, Princeton University, Princeton, New Jersey 08544

A multiple microfossil biochronology for the Miocene

Abstract: A multiple microfossil biochronology is presented for the Miocene which allows resolution of time approaching 100,000 years. Carbonate stratigraphy is integrated to greatly enhance this resolution. Graphical correlation techniques were applied to over 20 DSDP (Deep Sea Drilling Project) sections to identify 175 planktonic foraminiferal, calcareous nannofossil, radiolarian, and diatom datum levels between 24.0 and 4.3 Ma which show the most consistent (isochronous) correlations. Ages are estimated for these datum levels through 72 direct correlations to paleomagnetic stratigraphy and extrapolation between the correlation points. The resulting Miocene time scale resembles previously published time scales except for the early Miocene, where recent paleomagnetic correlations result in changes. The three CENOP (Cenozoic Paleoceanography Project) time slices (-21,16, and 8 Ma) are characterized biostratigraphically (planktonic foraminifers, calcareous nannofossils, radiolarians, and diatoms) and in terms of carbonate stratigraphy. The ages of the time slices are estimated as follows: the early Miocene time slice (21.2-20.1 Ma; given as 22 Ma in this volume), the late early Miocene time slice (16.4-15.2 Ma), and the late Miocene time slice (8.9-8.2 Ma). An alternate time scale utilizing a paleomagnetic Anomaly 5-paleomagnetic Chron 11 correlation is also presented. Estimated ages for microfossil zones and datum levels in the late middle and early late Miocene (14-7 Ma) utilizing the alternate time scale are generally younger than those for the more traditional time scale. The late Miocene time slice has an estimated age of 8.0-7.0 Ma.

Evolution of the Miocene ocean in the eastern North Pacific as inferred from oxygen and carbon isotopic ratios of foraminifera

Abstract: •Present address: Department of Geological and Geophysical Science, Princeton University, Princeton, New Jersey 08540.