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Eltanin samples utilized in this study were obtained at the Florida State University Antarctic Research Facility. Some shipboard core-catcher slide preparations made by D. McCollum were utilized in this study. The shipboard processing procedure used on these core-catcher samples varied with the type of sediment and the degree of lithification. Sediments composed primarily of calcium carbonate were treated with 10% HC1 and the undissolved portion centrifuged and decanted several times to concentrate the diatom and silicoflagellate ZONAL CHARACTERISTICS Distephanus speculum Zone A Top: Not defined, but the zone includes all floras above the initial consistently common occurrence of long-spined Distephanus speculum morphotypes. Base: First consistently common occurrence of longspined Distephanus speculum in the absence of Distephanus boliviensis. Age: Early Pleistocene-Recent: Matuyama and Brunhes paleomagnetic epochs. Important characteristics of the zonal assemblage: This zone is characterized by the consistently common occurrence of long-and short-spined morphotypes of Distephanus speculum. Other species present sporadically through the zone are Distephanus speculum var. pentagonus, D. speculum var. septenarius, Dictyocha fibula, and Dictyocha aspera. Common to abundant Distephanus polyactis are found only in the lower portion of the zone which is within the Matuyama paleomagnetic epoch. Reference section: Site 269: 61°40.57'S, 140°04.21'E and E14-8: 59°.0'S, 160°17.4'W. Occurrence of zone: Continuously cored at Site 274 and discontinuously cored at Sites 269 and 266. Correlation: In paleomagnetically dated Eltanin piston cores the lower boundary of this zone is approximately correlative with the Gauss-Matuyama magnetic epoch boundary (2.43 m.y.B.P.). Distephanus speculum Zone B Top: First consistently common occurrence of longspined Distephanus speculum in the absence of Distephanus boliviensis. Base: Last consistently common occurrence of Distephanus boliviensis in the presence of a Distephanus speculum group dominated by short-spined morphotypes. Age: Late Pliocene: within Gauss paleomagnetic epoch. Important characteristics of the zonal assemblage: The zone is characterized by the consistently common to abundant occurrence of short-spined Distephanus speculum and rare to few long-spined Distephanus speculum. Short-spined D. speculum are consistently more abundant than long-spined D. speculum. Species present sporadically through the zone are Dictyocha pseudofibula, D. aspera, D. fibula, Distephanus boliviensis, D. sp. cf. boliviensis, and Distephanus speculum var. pentagonus. Abundant Distephanus speculum var. pentagonus forma armatus, observed at the base of this zone, at Site 266, may provide a useful datum within the lower portion of this zone. Reference section: Site 266: 56°24.13'S, 104°29.30'E and E14-8: 59°40.0'S, 160°17.4'W. Occurrence of zone: Continuously cored at Site 274 and intermittently cored at Sites 266, 267, and 269. Correlation: In paleomagnetically dated Eltanin piston cores this zone is p...

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Subantarctic sediments recovered in the Southwest Atlantic during Leg 71 provide well-preserved and often diverse silicoflagellate assemblages of middle Eocene, late Eocene-early Miocene, and late middle Miocene through Holocene age. Two holes, 511 and 513A, comprise a complete 370-meter upper Eocene to lower Miocene composite reference section which is divided into six new or revised silicoflagellate zones, in ascending order: the Mesocena occidentalis Zone, Naviculopsis trispinosa Oppel Zone, N. constricta/Dictyocha deflandrei Concurrent Range Zone, N. constricta-Corbisema archangelskiana Interval Zone, C. archangelskiana Range Zone, and Naviculopsis biapiculata Partial Range Zone. New species described from this upper Eocene to lower Miocene section include D. alta, Mesocena bispicata, and Distephanus crux paulii. Middle Eocene sediments hydraulically piston cored at Site 512 are assigned to the newly constructed Dictyocha grandis Range Zone and are subdivided into three subzones; the D. stelliformis Subzone, D. stelliformis-M. apiculata Interval Subzone, and M. apiculata Subzone. Paleomagnetic measurements and correlations of this middle Eocene section indicate deposition of the D. grandis Zone between-43.7 to 40.9 Ma. New species from this interval include D. anguinea, D. grandis, and D. stelliformis. Miocene to Holocene sediments present in hydraulically piston cored Sites 512 and 514 and the rotary cored Site 513 contain the M. circulus Zone, M. circulus/M. diodon Zone, M. diodon Zone, and Distephanus boliviensis Zone. The upper boundaries of these zones are correlated directly with magnetostratigraphy and lie in upper Chronozone 9, upper Chronozone 6, the lower Gilbert Chronozone, and the lower Gauss Chronozone, respectively. A detailed analysis was made of all silicoflagellate skeletal morphotypes present in the upper Eocene-lower Oligocene to determine the limits of species variability. Numerous sporadically occurring species were determined to be intraspecific variations of more abundant species and are placed in synonymy with these species.

The Neogene and Quaternary sedimentary record of Leg 71 and previously drilled sequences from the Southern Ocean reveal evidence of a major late Miocene change of oceanic and glacial conditions in the southern high latitudes during paleomagnetic Chron 9. The characteristics of late Miocene sedimentation and in particular the study of erosional patterns and ice-rafted debris suggest the following conclusions.