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Sediments recovered during Leg 57 provide a nearly complete lower Miocene to Holocene composite reference section for diatoms in the northwestern Pacific. High-and middle-latitude diatoms dominate the assemblages; however, because of the influence of the warm-water Kuroshio Current, low-latitude diatoms are consistently present in low abundance. Low-latitude diatom datums tied to paleomagnetic stratigraphy in the equatorial Pacific show little or no displacement in time at the 40°N latitude of the Leg 57 sites. Both highand low-latitude diatom zones are recognizable in the upper Miocene to Quaternary of Sites 438 and 440 and thus allow the first direct detailed correlation of these zonations for the upper Miocene. Eleven new subzones, which offer refinement of the existing high-latitude diatom zonation of the North Pacific, are proposed. In addition, the base of the Denticula lauta Zone is defined, and a new zone, the Actinocyclus ingens Zone, is proposed for the upper lower Miocene.A late Miocene hiatus spanning the interval from about 5.6 to 6.7 m.y.B.P. is identified at Sites 438 and 440. Correlative hiatuses occur throughout the middle-and high-latitude North Pacific both on the continental slopes and in the ocean basins. Onshore in California, this interval corresponds with an increase in the rate of sediment accumulation associated with the transition from laminated diatomites to overlying, more massive diatomaceous mudstones. These latest Miocene sedimentologic events apparently are related to a global cooling event.

Middle Miocene and upper Miocene diatoms recovered during Deep Sea Drilling Project Leg 63 allow refinement of eastern North Pacific diatom biostratigraphy and permit several ties with low-latitude biostratigraphy. Numerous datum levels provide excellent correlation between individual Leg 63 sites (23°N-34°N) and with Site 173 (40°N) off northern California. Three intervals of widespread hiatus development appear to be related to intensification of bottom-water activity (erosion and/or corrosion) associated with growth of the Antarctic ice sheet. These intervals include one near the end of the middle Miocene (ca. 12-11 Ma), one in the early late Miocene (ca. 10-9 Ma), and one in the late late Miocene (ca. 7.2-6.2 Ma). During the early to middle part of the Pliocene (ca. 4.5-2.5 Ma), diatom productivity declined off southern California, probably associated with an interval of relative climatic warming. Three new taxa are proposed, Annellus californicus var. hannai, n. var., Bogoroviapaleacea var. elongata, n. var., and Coscinodiscus temperei var. delicata, n. var.

Excellent reference sections for lower Oligocene through Holocene diatoms were recovered at DSDP Sites 572 to 575 in the central equatorial Pacific. Diatoms are generally common and well preserved at all sites, except in the upper Oligocene. These sections are zoned and correlated on the basis of diatoms, including the identification and correlation of 89 additional secondary datum levels in the Miocene and Pliocene. Paleomagnetic stratigraphy was obtained for the entire Denticulopsis nicobarica Zone (17.8 to 16.4 Ma) at Site 575, and upper lower Miocene diatom datum levels are correlated directly to paleomagnetic stratigraphy for the first time.Quantitative Five new diatom taxa are proposed: Actinocyclus ellipticus var. spiralis Barron, n. var., Coscinodiscus lewisianus van rhomboides Barron, n. var., C. loeblichii Barron, n. sp., C. yabei var. ellipticus Barron, n. var., and Thalassiosira tappanae Barron, n. sp.

This paper summarizes the biostratigraphy and magnetostratigraphy of the 11 sites drilled on the Kerguelen Plateau and in Prydz Bay, Antarctica, during ODP Leg 119. Excellent magnetobiochronologic reference sections were obtained at deep-water Sites 745 and 746 (0-10 Ma) and at intermediate depth Site 744 (0-39 Ma) on the southern Kerguelen Plateau. Site 738, an intermediate depth companion site for Site 744, contains a nearly complete lowermost Oligocene to Turonian carbonate section including a continuous sequence across the Cretaceous/Tertiary boundary. Northern Kerguelen Sites 736 and 737 (ca. 600 m water depth) constitute a composite middle Eocene to Quaternary reference section near the present-day Antarctic Polar Front. Biostratigraphic control is limited in Prydz Bay Sites 739-743. Glacial sequences cored on the continental shelf at Sites 739 and 742 appear to form a composite record, possibly from the uppermost middle Eocene to the Quaternary; the entire upper Oligocene and most of the Miocene, however, are removed at an unconformity. Preglacial sediments at Site 741 contain Early Cretaceous pollen and spores, but the red beds cored at Site 740 are unfossiliferous. Poorly-fossiliferous glacial sediments of probable Quaternary age were sampled on the upper slope at Site 743. A magnetobiochronologic time scale is presented for the Late Cretaceous and Cenozoic of the Southern Ocean based on previous studies and the results of Leg 119 studies.

Early Miocene through Pleistocene diatom datum levels are identified along with North Pacific diatom zones in the sections cored during Ocean Drilling Program Leg 145 at Sites 881 to 884 and 887. Recording of the first complete late early Miocene through Miocene magnetostratigraphies in the North Pacific at Sites 884 and 887 has allowed the first magnetostratigraphic calibration of over 40 diatom datum levels between Subchron C5En (18.817-18.317 Ma) and Subchron C3Bn (6.901-6.744 Ma). Absolute age estimates from these calibrations as well as from 20 younger (latest Miocene and Pliocene) magnetostratigraphic calibrations of diatom datum levels at Sites 881-884 and 887 are presented and compared with the published age estimates from the literature. Most of the late early Miocene through Pliocene diatom datum levels that have been widely used in the North Pacific for biostratigraphy appear to be roughly isochronous within the level of resolution constrained by sample spacing. Diachroneity across latitude, however, is revealed for a number of diatom events, including the first occurrences of Actinocyclus ingens var. nodus, Simonseniella barboi, and Neodenticula koizumii. Thalassiosira minutissima Oreshkina, nov. sp., is described. J.A. BARRON, A.Y. GLADENKOV 60°N 50 c 40 c 150°E 180° 150°F igure 1. Index map of North Pacific showing locations of Leg 145 drill sites. 120°M aruyama (1992), and Akiba et al. (1993) and is summarized in the Appendix. The reader should also consult Gladenkov and Barron (this volume) for taxonomic citations and illustrations of the Oligocene through early middle Miocene taxa discussed. Because we were not certain how to separate selected taxa of Crucidenticula and Denticulopsis that have been proposed by Yanagisawa and Akiba (1990), we have made the following groupings:

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Samples were examined for diatoms from 22 holes at 11 sites cored by ODP Leg 119 on the Kerguelen Plateau and in Prydz Bay, East Antarctica. Diatoms were observed in Oligocene through Holocene sediments recovered from the Kerguelen Plateau. The diatom flora from the Kerguelen Plateau is characterized by species such as Azpeitia oligocenica, Rocella gelida, Rocella vigilans, and Synedra jouseana in the Oligocene and Crucidenticula nicobarica, Denticulopsis hustedtii, Nitzschia miocenica, and Thalassiosira miocenica in the Miocene. This somewhat cosmopolitan assemblage gives way to a Pliocene and Holocene assemblage characterized by species such as Nitzschia kerguelensis, Thalassiosira inura, and Thalassiosira torokina, which are endemic to the Southern Ocean region. Samples examined from Prydz Bay are generally devoid of diatoms. The exception is Site 739, where diatoms occur sporadically in lower Oligocene and upper Miocene through Quaternary sediments.The Leg 119 diatom biostratigraphic results allow the development of a stratigraphic framework for the Indian sector of the Southern Ocean. This diatom zonation integrates diatom zonations developed previously for other sectors of the Southern Ocean. The zonation proposed here is based on biostratigraphic events of both geographically widespread and endemic species calibrated to the paleomagnetic stratigraphy. As such, this zonation has application throughout the Southern Ocean and allows correlation from the southern high latitudes to the low latitudes.