history of antarctic seas. Biology of antarctic seas, II. Antarc-tic Research Series, 5: 125-183.

Hays, J. D., and N. D. Opdyke. 1967. Antarctic radiolaria, mag-netic reversals and climatic changes. Science, 158: 1001-1011.

Keany,J. 1973. New radiolarian paleoclimatic index in the PIio-Pleistocene of the southern ocean. Nature, 246(5429): 139-141.

Kennett, J. P., and P. Huddlestun. 1972. Late Pleistocene paleo-climatology, foraminiferal biostratigraphy, and tephrachron-ology, western Gulf of Mexico. Quaternary Research, 2(1): 38-69.

Kennett, J . P., and N. D. Watkins. 1976. Regional deep-seadynamic processes recorded by late Cenozoic sediments ofthe southeastern Indian Ocean. Geological Society of AmericaBulletin, 87: 321-339.

Petrushevskaya, M. G. 1973. Radiolarians in the bottom depositsof the Southern Hemisphere. Oceanology, 13(6): 860-869.

Watkins, N. D., and J . P. Kennett. 1972. Regional sedimentarydisconformities and upper Cenozoic changes in bottom watervelocities between Australia and Antarctica. Antarctic ResearchSeries, 19: 273-293.

Cenozoic biogeographic andbiostratigraphic development of

planktonic microfossils inthe Antarctic

JAMES P. KENNETTGraduate School of Oceanography

University of Rhode IslandKingston, Rhode island 02881

Knowledge of antarctic and subantarctic Ceno-zoic calcareous and siliceous planktonic microfossilshas greatly increased recently by the study of paleo-magnetically dated piston cores of Quaternary andPliocene age and of Deep Sea Drilling Project(DSDP) cores that have, for the first time, providedexcellent pre-Pliocene microfossil sequences. Gen-eral trends are summarized from results of num-erous investigations of several microfossil groups.

Present southern ocean biogeography is distinc-tive from other areas because of differences inspecies diversity, in species composition, in fre-quency variation, and in general faunal and floraldominance. Characteristic planktonic assemblagesare associated with antarctic, subantarctic, andsouthern subtropical (temperate) water masses thatare separated respectively by the Antarctic Conver-gence and the Subtropical Convergence. The Ant-arctic Convergence sharply separates assemblagesdominated by siliceous forms (diatoms, radiolaria,and silicoflagellates) to the south from calcareousassemblages (foraminifera, calcareous nannofos-

sils) to the north (Hays, 1965) and essentially marksthe southern distribution limit of calcareous nanno-fossils (Geitzenauer, 1972). This biogeographicprovinciality is circumpolar. Diversity is generallylow for all groups, with successive decrease asso-ciated with increasingly high-latitude water masses.Southward decrease in diversity even continueswithin the antarctic water mass. Decreased diversityis partly related to increased mixing in surficialwater masses and reduction in water mass stratifi-cation. The antarctic water mass near the AntarcticConvergence is the principal site of today's siliceousbiogenic productivity and sedimentation related tonutrient-rich upwelling of intermediate waters.These biogeographic features were not permanentthroughout the Cenozoic, but they have developedin conjunction with the evolution of southern oceanwater mass systems.

Cenozoic biogeographic patterns of the highsouthern latitudes are partly related to two trends.Nearly all evolution of calcareous planktonic micro-fossils takes place outside of the region, with sub-sequent migration into these water masses. Thus,there is virtually no endemism among calcareousmicrofossil groups at these latitudes. In contrast,evolution has been very conspicuous within thesiliceous groups especially during the Neogene(Chen, 1975; Petrushevskaya, 1975; McCollum,1975). Although Paleogene siliceous assemblagesare still not well known, endemism in the siliceousgroups seems much less apparent in the Paleogene,especially during the Eocene when assemblagesseem to have a much more cosmopolitan aspect.The low diversity of antarctic calcareous micro-fossils throughout the Cenozoic makes them onlybroadly useful for correlation. Higher diversity inthe Subantarctic makes them much more useful,although appearances and disappearances areoften clearly diachronous with warmer regions andare climatically controlled (Edwards and Perch-Nielsen, 1975).

During the Eocene (55 to 38 million years ago),sediments contain abundant calcareous microfos-sils even adjacent to the continent (Burns, 1975a).Antarctic planktonic microfossil assemblages arerelatively diverse compared with today, but theystill are lower than those of middle and high lati-tudes. Faunas may still be dominated by only oneor two species (Burns, 1975a, 1975b). Subantarc-tic Eocene planktonic foraminifera more closelyresemble temperate faunas (Jenkins, 1975). Bio-geographic differences exist between differentsectors of the southern ocean as a result of separa-tion by high-latitude land masses (Jenkins, 1974).Subantarctic planktonic foraminifera have slightlyhigher diversity in the Early Eocene. Calcareousnannofossils range southward to the continent andcontain low-latitude elements (Burns, 1975b). In

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the Subantarctic, Paleocene and Eocene calcareousnannofossils exhibit higher diversity than in theOligocene and Neogene. Antarctic radiolaria con-tain forms with relatively close affinities to tropicalforms (Chen, 1975). A major change occurs in ant-arctic biogeography near the Eocene-Oligoceneboundary. Since then, planktonic foraminiferahave had a characteristically polar aspect (Kaneps,1975). This event is considered to be linked to theinitiation of widespread glaciation (although noticecap formation) of Antarctica and to sea-ice forma-tion (Kennett and Shackleton, 1976). The antarc-tic faunal and floral provinces that begin to developat this time are related to the evolution of the south-ern ocean as Australia moved northward from Ant-arctica and perhaps to the opening of the DrakePassage.

During the Oligocene (38 to 22 million yearsago), diversity is relatively low in all groups through-out the world's oceans. In the Antarctic, the earlyOligocene planktonic foraminiferal fauna is mono-specific; in the later Oligocene, though, two speciesare recorded. In the Subantarctic, Oligocene plank-tonic foraminiferal faunas show much higher di-versity compared with the Antarctic, and the bio-stratigraphic succession is similar to temperateregions. Early and middle Oligocene faunas showthe lowest diversity for the entire Cenozoic, withincreasing diversity commencing again in the lateOligocene.

The Paleogene- Neogene transition is marked bya major change in the world's planktonic bio-geography. The development of modern patternstook place at this time, creating a consistently steepdiversity gradient between tropics and poles evenduring the most frigid climatic episodes (Kennettet al., 1972). This was not the case in the early andmiddle Oligocene, when assemblages were morecosmopolitan and had relatively low diversity evenin low-latitude areas. We suggest that the perma-nency of the steep diversity gradient and the dis-tinct latitudinal provinces during the Neogene andQuaternary resulted from the development of theAntarctic Circumpolar Current during the Oligo-cene, which permanently isolated the equatorialand southern polar planktonic assemblages.

During the Neogene, when the antarctic watermass remains cold, antarctic calcareous assem-blages are of consistently low diversity and faunasare often dominated by single species. In contrast,subantarctic assemblages are much more diverse.Antarctic siliceous faunas begin to become particu-larly important in the early Neogene, and siliceousbiogenic productivity increases as the AntarcticConvergence develops and intensifies (Kennett etal., 1975). Radiolaria begin to show distinct endem-ism (Chen, 1975; Petrushevskaya, 1975). The earlyand middle Miocene (22 to 11 million years ago)

show relatively higher diversity in both planktonicforaminifera and calcareous nannofossils relatedto a climax in Cenozoic warming (Jenkins, 1975).Discoasters are conspicuous in the early Mioceneof the northern Subantarctic (Edwards and Perch-Nielsen, 1975), while planktonic foraminiferalfaunas reach highest diversities in the middle Mio-cene of both the Antarctic (about four species) andthe Subantarctic (average of about 13 species). Sili-coflagellates also reach high diversities in the earlyto middle Miocene (Ciesielski, 1975).

The late Miocene (11 to 5 million years ago)shows a marked northward movement of the Ant-arctic Convergence, resulting from antarctic cool-ing, and shows a corresponding expansion of theantarctic planktonic province (Hayes and Frakes,1975). This resulted in lower diversity of calcareousfaunas and floras.

Pliocene (5 to 1.8 million years ago) and Qua-ternary biostratigraphic events have been placed ina chronological framework using paleomagneticstratigraphy and thorium dating. In subantarcticsequences, the Pliocene and Quaternary have beendivided into zones based on upward sequential dis-appearance of planktonic foraminifera (Kennett,1970), upward sequential disappearance of radio-larians (Hays and Opdyke, 1967), and stratigraphicsuccession of other groups. Pliocene planktonicforaminifera are essentially monospecific in bothantarctic and subantarctic sequences.

During the Quaternary (1.8 million years ago tothe present), the antarctic water mass shows itsgreatest northward expansion, and siliceous bio-genic productivity, associated with the AntarcticConvergence, reached its maximum (Hayes andFrakes, 1975; Kennett et al., 1975). In the Subant-arctic, planktonic foraminiferal diversity is muchhigher than in the Pliocene. Although calcareousnannofossil diversity may be high, only three tofive species are very abundant (Geitzenauer, 1972),while sometimes the floras are almost monospecific.Large northward shifts of antarctic and subantarc-tic water masses occurred, but there were no south-ward penetrations much beyond those of today.Several radiolaria and foraminiferal species dis-appear or appear close to paleomagnetic reversalssuch as the Brunhes-Matuyama boundary (690,000years ago) and do not seem closely related to majorclimatic events (Hays and Opdyke, 1967; Hays andDonahue, 1972; Kennett, 1970).

This research was supported by National ScienceFoundation grant DPP 75-15511.

References

Burns, D. A. 1975a. Distribution, abundance, and preservationof nannofossils in Eocene to Recent Antarctic sediments. New

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Zealand Journal of Geology and Geophysics, 18(4): 583-595.Burns, D. A. 1975b. Nannofossil biostratigraphy for antarctic

sediments, leg 28, Deep Sea Drilling Project. In: Initial Re-ports of the Deep Sea Drilling Project, 28 (Hayes et al., editors):589-598. Washington, D.C., U.S. Government Printing Office.

Chen, P.-H. 1975. Antarctic radiolaria. In: Initial Reports of theDeep Sea Drilling Project, 28 (Hayes et al., editors): 437-513.Washington, D.C., U.S. Government Printing Office.

Ciesielski, P. F. 1975. Biostratigraphy and paleoecology ofNeogene and Oligocene silicoflagellates from cores recoveredduring antarctic leg 28, Deep Sea Drilling Project. In: InitialReports of the Deep Sea Drilling Project, 28 (Hayes et al., editors):625-691. Washington, D.C., U.S. Government Printing Office.

Edwards, A. R., and K. Perch-Nielsen. 1974. Calcareous nanno-fossils from the southern Southwest Pacific, Deep Sea DrillingProject leg 29. In: Initial Reports of the Deep Sea Drilling Proj-ect, 29 (Kennett et al., editors): 469-539. Washington, D.C.,U.S. Government Printing Office.

Geitzenauer, K. R. 1971. The Pleistocene calcareous nanno-plankton of the subantarctic Pacific Ocean. Deep-Sea Re-search, 19: 45-60.

Hayes, D. E., and L. A. Frakes. 1975. General synthesis, DeepSea Drilling Project leg 28. In: Initial Reports of the Deep SeaDrilling Project, 28 (Hayes et al., editors): 919-942. Washing-ton, D.C., U.S. Government Printing Office.

Hays, J . D. 1965. Radiolaria and Late Tertiary and Quaternaryhistory of antarctic seas. Antarctic Research Series, 5: 125-184.

Hays, J . D., and J . G. Donahue. 1972. Antarctic Quaternaryclimatic record and radiolarian and diatom extinctions. Ant-arctic Geology and Geophysics: 733-738.

Hays, J. D., and N. D. Opdyke. 1967. Antarctic radiolaria, mag-netic reversals and climatic change. Science, 158: 1001-1011.

Jenkins, D. G. 1974. Initiation of the proto Circum- AntarcticCurrent. Nature, 252: 371-373.

Jenkins, D. G. 1974. Cenozoic planktonic foraminiferal bio-stratigraphy of the southwestern Pacific and Tasman Sea,Deep Sea Drilling Project, leg 29. In: Initial Reports of theDeep Sea Drilling Project, 29 (Hayes et al., editors): 449-467.Washington, D.C., U.S. Government Printing Office.

Kaneps, A. G. 1975. Cenozoic planktonic foraminifera fromantarctic deep-sea sediments, Deep Sea Drilling Project, leg28. In: Initial Reports of the Deep Sea Drilling Project, 28 (Hayeset al., editors): 573-587. Washington, D.C., U.S. GovernmentPrinting Office.

Kennett, J. P. 1970. Pleistocene paleoclimates and foraminiferalbiostratigraphy in subantarctic deep-sea cores. Deep Sea Re-search, 17: 125-140.

Kennett, J . P., R. E. Burns, et al. 1972. Australian-antarctic con-tinental drift, paleocirculation changes and Oligocene deep-sea erosion. Nature Physical Science, 239: 51-55.

Kennett, J . P., R. E. Houtz, et al. 1974. Cenozoic paleoceanog-raphy in the southwest Pacific Ocean, antarctic glaciation andthe development of the Circurn-Antarctic Current. In: InitialReports of the Deep Sea Drilling Project, 29 (Kennett et al., edi-tors): 1155-1169. Washington, D.C., U.S. Government Print-ing Office.

Kennett, J . P., and N. J. Shackleton. 1976. Oxygen isotopic evi-dence for the development of the psychrosphere 38 Myr ago.Nature, 260: 513-515.

Diatom investigations in theSouth Atlantic

ANDREW M. COMBOS, JR.Antarctic Research Facility

Department of GeologyFlorida State University

Tallahassee, Florida 32306

Table 1. Locations of leg 36 drill sites.

Latitude LongitudeHole (°S.) (°W.)

326 56035.00' 65018.20'327 50052.28' 46047.02'328 49048.67' 36039.53'329 50°39.31' 46005.73'330 50055.19' 46053.00'331 37053.00' 38006.92'

Diatoms from six Deep Sea Drilling Project(DSDP) sites (table 1) and six USNS Eltanin, Vema,and Robert Conrad piston cores from the southwestAtlantic (table 2) have been investigated in detail.Neogene diatoms were encountered at nine of theAtlantic sites, commonly occurring as thin veneersof diatomaceous ooze overlying older sediments.Thick accumulations of Miocene to Recent diato-maceous sediments were penetrated at two sites:DSDP site 329 on the Maurice Ewing Bank, andDSDP site 328 in the Malvinas (Falkland) OuterBasin. The section at site 329 consists of nearly 400meters of middle to late Miocene nannodiatomooze that exhibits much reworking. This accumula-tion is thought to have resulted from the eddyingof sediments carried by bottom currents flowing

Table 2. Piston cores used in this study.

LatitudeLongitudeCore (°S.) (°W.)

RC 15-84 50028.6' 44043

V 18-112 51040' 48029'V 17-107 51008' 54022'RC 12-237 4704571 57038.5'E 6-9 53059 55058'V 31-60 51007' 53004'

over the Falkland (Malvinas) Plateau. A relativelyundisturbed Miocene to Recent section of clayeyand silty biosiliceous ooze was recovered at site 328.

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