M. S. Srinivasan

Neogene planktonic foraminiferal biostratigraphy and evolution: Equatorial to subantarctic, South Pacific

Detailed planktonic foraminiferal zonations have been established for the Neogene (Latest Oligocene through present) in six DSDP sites in the South Pacific ranging from equatorial to subantarctic latitudes (48°S). Two basic zonal schemes are readily recognized: tropical and temperate. The tropical zonation is best developed in DSDP Site 289 and the temperate zonation in Sites 206, 207A and 284. Tropical and temperate zonations can be linked by a warm subtropical scheme in Site 208, because this sequence includes a mixture of tropical and temperate elements. A site located close to the Subantarctic Convergence (Site 281) contains a zonation largely of temperate character, but the present of cooler elements and some differences in biostratigraphic ranges have required a slightly different biostratigraphic scheme. Although two broad schemes are recognized, none of the biostratigraphic sequences are identical between any of the sites. This reflects differences in biogeography, evolution and diachronous extinction at various latitudes during the entire Neogene. Diachronism in biostratigraphic ranges continue to create difficulties in correlation across such wide latitudes. Our detailed work has required the establishment of new biostratigraphic zonations in certain parts of the Neogene sequence and modifications in some other parts. Otherwise, previously established schemes are followed as closely as possible. In the temperate region, a new zonation has been established for the Early Miocene to early Middle Miocene. For the remainder of the Neogene the zonation of Kennett (1973) has been largely used. The tropical zonation of Blow (1969) is employed in the equatorial Site 289, but with further subdivisions for Zones N4 and N17. For areas intermediate between tropical and temperate latitudes (Site 208), a modified Early Miocene zonation is established based on changes in tropical and temperate elements. The zonal schemes are established on taxa that exhibit both diachronous and isochronous ranges across the latitudes. Zones that are at least partly diachronous include the Globigerinoides trilobus and Globorotalia miozea Zones of Early Miocene age; perhaps the Globorotalia mayeri Zone (its base) of the Middle Miocene; the Globorotalia conomiozea Zone of the Late Miocene; and the Globorotalia crassaformis Zone of the Early Pliocene. A large number of datum levels are recognized based on first evolutionary appearances or extinctions. The most widely applicable datums are as follows: latest Oligocene — Globigerinoides F.A.; Early Miocene — Globoquadrina dehiscens, F.A., Globorotalia kugleri L.A., Catapsydrax dissimilis L.A. and Praeorbulina glomerosa F.A.; Middle Miocene — Orbulina suturalis F.A., Globorotalia peripheroacuta F.A., Fohsella lineage L.A., Globorotalia mayeri L.A.; Late Miocene — “Neogloboquadrina” continuosa L.A., Globoquadrina dehiscens L.A., Globorotalia cibaoensis F.A.; Early Pliocene — Globorotalia puncticulata F.A., Globorotalia margaritae F.A.; Early Pleistocene — Globorotalia truncatulinoides F.A. A number of other datums are identified which assist with correlation over more restricted latitudinal ranges. The evolution of most Neogene planktonic foraminifera is now well established for a wide range of water masses. Evolutionary lineages are primarily centered in the temperate and tropical regions. Tropical lineages have recently been reviewed by Srinivasan and Kennett (1981) and are not discussed in detail here. However, Sphaeroidinellopsis seminulina is now considered to have evolved directly into S. paenedehiscens during the Late Miocene and S. subdehiscens Blow is considered to be junior synonym of S. seminulina. A new evolutionary lineage is recognized in the warm subtropics (Site 208) whereby Globigerina woodi woodi gave rise to Globigerinoides subquadratus via Globigerina brazieri. The discovery of this lineage clearly demonstrates that Globigerinoides is a polyphyletic “genus”. Another major phylogenetic lineage is recognized within the temperate globorotaliids of Early Miocene age as follows: “N.” continuosa → Globorotalia zealandica incognita → G. zealandica zelandica → G. praescitula → G. miozea. Although parts of this lineage have been recognized earlier, the entire phylogeny has previously been underscribed. A new Early to Middle Miocene lineage is recognized in the subantarctic to temperate areas which involve a transition from Globorotalia praescitula to G. challengeri n. sp. via intermediate forms. Two major Neogene globorotaliid lineages — the Menardella of the tropics and Middle Miocene to Recent forms of Globoconella of the temperate areas — are both considered to have evolved from Globorotalia praescitula beginning in the Early Miocene. This evolution initially was restricted to temperate areas but has since separated into distinctly tropical and temperate phylogenetic elements.

Uvigerina proboscidea abundances and paleoceanography of the northern Indian Ocean DSDP Site 214 during the Late Neogene

Abstract This study attempts to understand the significance of Uvigerina proboscidea in paleoceanographic reconstructions at the northern (tropical) Indian Ocean DSDP Site 214 from the Late Miocene through the Pleistocene. In this interval at this site, U. proboscidea is the most abundant species of the benthic assemblage and shows abrupt frequency changes (about 1–74%). Based on relative percentages of U. proboscidea calibrated with oxygen and carbon isotope record and the sediment accumulation rates, the modern distribution of the species in the Indian Ocean, and other evidence, the peaks of abundance of U. proboscidea are inferred to represent times of high-surface productivity. This productivity is related to intensified trade winds during strong southwest (SW) Indian monsoons, causing widespread upwelling along equatorial divergence in the Indian Ocean. The sudden increase of U. proboscidea abundance at approximately 8.5–7.5 Ma reflects significant upwelling at the equatorial divergence. This event corresponds to the permanent build-up of West Antarctic ice sheets, and a major increase in SW Indian monsoons related upwelling in the northwestern Indian Ocean. The Chron-6 carbon shift at approximately 6.2 Ma is marked by another peak of abundance, reflecting widespread ocean fertility. The highest abundances of U. proboscidea and highest sediment accumulation rates occur between 5.8 and 5.1 Ma, which coincides with the greatest development of Antarctic ice sheets and strong southwest monsoons. The higher percentages at 3.2–3.1 Ma, approximately 2.4 Ma, and 1.6 Ma all represent phases of high productivity at the equatorial divergence.

Response of Northern Indian Ocean deep-sea benthic foraminifera to global climates during Pliocene-Pleistocene

Abstract The Northern Indian Ocean (DSDP Site 214) benthic foraminifera have been examined quantitatively to understand their response to the global climatic events and to determine the history of bottom-water circulation in the Northern Indian Ocean during Pliocene-Pleistocene. Uvigerina spp., Cibicides kullenbergi, C. wuellerstorfi, Globocassidulina pacifica, G. subglobosa, Oridorsalis umbonatus and Bulimina alazanensis are the dominant members of the benthic assemblage and reveal significant changes in distribution patterns. Based on benthic foraminiferal frequency changes, four intervals of major faunal and climatic turnover have been recognized at 5.2–5.1 Ma, 3.9–3.2 Ma, 3.2–3.1 Ma and 3.1–0.6 Ma. The interval between 5.2–5.1 Ma is marked by widespread Antarctic glaciation, deep-sea hiatuses, bottom-water cooling, increased upwelling, high sedimentation rates, highest Uvigerina abundances and intensified bottom-water circulation. During 3.9–3.2 Ma the bottom waters were warmer and bottom-water activity waned, with a sharp decline in Uvigerina abundance and increase in Cibicides . The interval between 3.2–3.1 Ma was a time of bottom-water cooling and coincides with a global benthic positive oxygen isotopic shift, Antarctic ice volume increase and possible initiation of Northern Hemisphere glaciation. The Late Pliocene and Pleistocene interval (3.1-Recent) is punctuated by major changes in benthic foraminiferal assemblages, warm/cold cycles, major glacial/interglacial intervals, increase in polar ice volume and intensification of upwelling. On the basis of cumulative percentages of Uvigerina and Cibicides , nine intervals of major bottom-water cooling have been inferred in the Pliocene and two in the Pleistocene at Site 214 reflecting the major changes in bottom-water circulation.

Secondary Calcification of the Planktonic Foraminifer Neogloboquadrina pachyderma as a Climatic Index.

Oscillations in the ratio of two principal types of secondary calcification of the test surface in Neogloboquadrina pachyderma populations closely parallel paleoclimatic oscillations over the last 6 million years in a deep-sea core drilled in the temperate South Pacific. The nature of secondary calcification in fossil planktonic foraminifera represents a useful index in interpreting Cenozoic climates.

Palaeotectonic implications of increased late Eocene-early Oligocene volcanism from South Pacific DSDP sites

Late Eocene-early Oligocene (42–35 Myr) sediments cored at two DSDP sites in the south-west Pacific contain evidence of a pronounced increase in local volcanic activity, particularly in close association with the Eocene–Oligocene boundary. This pulse of volcanism is coeval with that in New Zealand and resulted from the development of an Indo- Australian / Pacific Plate boundary through the region during the late Eocene. The late Eocene / earliest Oligocene was marked by widespread volcanism and tectonism throughout the Pacific and elsewhere, and by one of the most important episodes of Cenozoic climatic cooling.

The Oligocene-Miocene boundary in the South Pacific

Qualitative and quantitative planktonic foraminiferal trends have been examined across the Oligocene-Miocene transition in three Deep Sea Drilling Project sites (DSDP 289, 208, 206) in the South Pacific ranging from the equator to temperate regions. Primary objectives were to determine main changes in planktonic foraminiferal assemblages over the Oligocene-Miocene transition. Of available planktonic foraminiferal datums in the South Pacific sequences, Globoquadrina dehiscens is the most useful for designation and correlation of the Oligocene-Miocene boundary. The stratotype Aquitanian stage (earliest Miocene) in France contains Gq. dehiscens . Although the base of the Aquitanian stratotype is marked by an unconformity, biostratigraphic data elsewhere within the basin suggest that the evolutionary appearance of Gq. dehiscens occurred close to the time of the oldest stratotypic sediments. Therefore, the evolutionary appearance of Gq. dehiscens seems to represent one of the most reliable datums for interregional correlation of the Oligocene-Miocene boundary, one that occurs within the biostratigraphic range of Globorotalia ( Fohsella ) kugleri and marks the boundary between zones N4A and N4B. Globigerinoides first evolved during the late Oligocene and is not coincident with the Oligocene-Miocene boundary. Unlike the Eocene-Oligocene boundary, the Oligocene-Miocene boundary is not marked by a crisis in the Oligocene planktonic foraminiferal assemblages. Most Oligocene forms continue their range upward into the early Miocene, where most are replaced by typical Neogene forms. The only important Oligocene phylogenetic lineage to be essentially eliminated during the Oligocene-Miocene transition is Catapsydrax , the last important forms of which disappeared during earliest Miocene. Evolution of Gq. dehiscens effectively heralds the beginning of major evolutionary radiations in planktonic foraminifera that led to the distinctive Neogene phylogenies. Appearance of this form was followed by evolutionary radiation of Globigerinoides into a number of species; initial evolution of Globorotalia ( Globoconella ) incognita , which forms the earliest ancestral form of Globoconella ; evolution of the Globigerina woodi group; Sphaeroidinellopsis disjuncta (the ancestral stock of the Sphaeroidinellopsis - Sphaeroidinella lineage); Globorotalia ( Fohsella ) peripheroronda from Gr. (F.) kugleri (representing the continued development toward the classic Fohsella forms); and typical forms of the Dentoglobigerina stock represented by Dentoglobigerina altispira . These evolutionary radiations are reflected by a general increase in simple species diversity through early Miocene. At all three sites, species diversity is lowest in the interval near the Oligocene-Miocene boundary. Among the three sites examined, diversity is highest in warm-subtropical Site 208, because at this latitude faunas include both tropical and temperate forms. The earliest Miocene is marked by high frequencies of Gr. (F) kugleri in tropical areas and Gq. dehiscens in warm-subtropical to temperate areas.

Pleistocene oceanographie changes indicated by deep sea benthic foraminifera in the northern Indian Ocean

AbstractAn attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence. The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW). Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by low abundance ofU. proboscidea and more diverse and equitable benthic fauna.The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary.

Globorotalia menardii neoflexuosa new subspecies from the northern Indian Ocean

Abstract Globorotalia menardii neoflexuosa new subspecies is proposed for Globorotalia menardii flexuosa figured by Be and McIntyre (1970), (non Pulvinulina tumida var. flexuosa Koch , 1923) from the northern Indian Ocean. It is suggested that the name G. tumida flexuosa (Koch) be restricted to the flexuose forms of G. tumida and that the name G. menardii neoflexuosa be limited to the flexuose forms of G. menardii (s.s.). G. tumida flexuosa (Koch) is an extinct species, making its last appearance during the last interglacial (Zone X), while G. menardii neoflexuosa new subspecies, which became extinct in the Atlantic near the end of the X climatic episode (approximately 80,000 years ago), is still a member of the living planktonic fauna in the northern Indian Ocean.

Foraminifera and age of the type section of the Tahuian Stage

Abstract Eight foraminiferal samples were collected from the type section of the Tahuian Stage, South Canterbury, New Zealand. The position of samples is shown in a lithological column and the distribution of Foraminifera is shown on a range chart. The Tahuian Stage is found to be equivalent partly to Bortonian and partly to Kaiatan (middle and upper Eocene respectively).

Neogene deep sea benthic foraminiferal diversity in the Indian Ocean: Paleoceanographic implications

The species diversity indices, as defined by the number of species,S; Shannon-Wiener index,H(S) and Buzas-Gibson index,É, of DSDP sites 219, 220, 237 and 238 were measured to determine the benthic foraminiferal diversity patterns in the Indian Ocean deep sea sequences during the Neogene. The Time-Stability hypothesis could satisfactorily explain the observed diversity patterns. The general patterns of diversity suggest environmental stability during the Neogene. However, few small fluctuations in diversity during the Middle Miocene (c.14·8 Ma), Late Miocene (c.6·0 Ma) and Late Pliocene (c.2·0 Ma) may possibly be the effects of Antarctic Bottom Water (AABW) activity in this region. The benthic foraminiferal diversity in the tropical Indian Ocean is more than the high latitudinal areas with comparable depths.