20. Indian Ocean Neogene Planktonic Foraminiferal Biostratigraphy and its Paleoceanographic Implications

  1. J.R. Heirtzler,
  2. H.M. Bolli,
  3. T.A. Davies,
  4. J.B. Saunders and
  5. J.G. Sclater
  1. Edith Vincent

Published Online: 23 MAR 2013

DOI: 10.1029/SP009p0469

Indian Ocean Geology and Biostratigraphy

Indian Ocean Geology and Biostratigraphy

How to Cite

Vincent, E. (1977) Indian Ocean Neogene Planktonic Foraminiferal Biostratigraphy and its Paleoceanographic Implications, in Indian Ocean Geology and Biostratigraphy (eds J.R. Heirtzler, H.M. Bolli, T.A. Davies, J.B. Saunders and J.G. Sclater), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP009p0469

Author Information

  1. Scripps Institution of Oceanography, La Jolla, California, 92093

Publication History

  1. Published Online: 23 MAR 2013
  2. Published Print: 1 JAN 1977

ISBN Information

Print ISBN: 9780875902081

Online ISBN: 9781118664919

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Summary

The Neogene planktonic foraminiferal biostratigraphy of the Indian Ocean is discussed mainly from material of 16 DSDP sites between 12°N and 48°S, which were above the CCD throughout Neogene times, and where 4074 m of calcareous sediments were continuously cored. Foraminiferal faunas are common to abundant and well preserved in Upper Miocene through Pleistocene sediments at all these sites, except the deeper Somali Basin Site (236) and the shallow Gulf of Aden sites (231, 232 and 233), where the faunas are affected by significant dissolution. Comparison of Upper Neogene assemblages from the last four sites with well preserved faunas from the other sites provides data on the solution susceptibility of extinct species. Middle and Lower Miocene faunas are moderately to poorly preserved at all sites where they occur. Foraminiferal events are evaluated in relation to planktonic zonations of other fossil groups with special attention to paleontological events calibrated to the paleomagnetic-radiometric time scale. The tropical Neogene biostratigraphy is similar to that of the tropical Pacific. Further taxonomic reevaluation of the Globorotalia connoidea-G. inflata bioseries in sections of mid-latitude sites is needed for a more detailed biostratigraphic interpretation of these sites. Latitudinal control of appearances or disappearances of many planktonic foraminiferal species is well documented through the Neogene.

It was not possible to use the first appearance of Globorotalia truncatulinoides to delineate the Pliocene/Pleistocene boundary in the Indian Ocean because of the discrepancy between its level and the extinction level of Discoaster brouweri. The first G. truncatulinoides occurs well above (up to 70 m) the extinction level of D. brouweri in expanded sections whereas in slowly accumulating sequences it is coincident or slightly above this nannofloral event. There is, in general, good agreement among various fossil groups for the recognition of the Miocene/Pliocene boundary at low-latitude sites, but at none of the mid-latitude sites is this boundary clearly identified. The Oligocene/Miocene boundary is best estimated on a foraminiferal basis by the initial appearance level of Turborotalia kugleri.

Changes in faunal content and in rates of accumulation of calcareous sediments at these 16 sites, together with changes in the isotopic composition of foraminiferal tests at subantarctic Site 281 are tentatively related to global climatic changes associated with Antarctic glaciation. The two distinct regimes which occur today in the eastern and western Indian Ocean appear to have been in existence since the late Middle Miocene. Sedimentation was reduced in both Oligocene and Early Miocene throughout most of the Indian Ocean and resumed significantly only at the end of the Early Miocene. During the Early and Middle Miocene relatively warm surface-water temperatures (optimum for the development of the globoquadrinid group) prevailed in mid as well as low latitudes. Intensified bottom water circulation associated with the buildup of the East Antarctica ice sheet resulted in a significant decrease in sediment accumulation rates in the temperate and subtropical southeast Indian Ocean during the Late Miocene, but sedimentation was not reduced during that time either on the crest of the central and northern Ninetyeast Ridge, or in the western Indian Ocean, where sediments accumulated in the early Late Miocene (prior to 6.3 m.y.) at rates comparable to those of the Middle Miocene. Bottom currents, however, were apparently intense throughout the ocean in late Middle Miocene and earliest Late Miocene, as evidenced by the general telescoping of fossil zones at that time and by an interval of mixed, reworked foraminiferal faunas spanning the Middle/Late Miocene boundary throughout the northwestern Indian Ocean. The widespread presence of this interval with reworked sediments, which have been apparently winnowed during transport, may possibly be related to increased tectonic activity and circulation readjustments in the northwest Indian Ocean after the opening of the Gulf of Aden and the closure of the Indo-Tethyan seaway. A marked increase in sediment accumulation rates and in the abundance of siliceous fossils at all low-latitude sites in Late Miocene, at approximately 6.3 m.y., corresponded to the time of maximum Antarctic ice accumulation and of the onset of the modern equatorial circulation pattern.

Cooling during the Late Miocene moved the area of optimum conditions for globoquadrinids north of 25°S. The extinction of Globoquadrina dehiscens was paleoceanographically controlled, decreasing in age from south to north with progressive cooling. The uppermost Miocene (Kapitean) severe cooling recorded throughout the Pacific is not recognizable in the Indian Ocean from present data. There is faunal evidence in the temperate, subtropical and tropical areas for water temperatures cooler in Late than Early Pliocene. A substantial increase in velocity of bottom water associated with the Late Pliocene cooling, which removed Upper Pliocene to Quaternary Pediments from the South Indian and South Australian Basins was felt throughout the Indian Ocean. Telescoping of Upper Pliocene sequences (especially pronounced in the uppermost Pliocene) occurs throughout the eastern ocean as far north as the Andaman-Nicobar region. In the western part of the ocean, disturbances of Upper Pliocene and lowermost Pleistocene sedimentary sequences are evidenced at many sites. Sites 214 and 216 on the crest of the central and northern Ninety-east Ridge appear to remain in an area better protected from bottom currents than any other sites during the entire Neogene and, thus, provide the best undisturbed sequences for biostratigraphic investigations.

In the southern subtropical area, cool conditions prevailed in the earliest Pleistocene; a marked warming occurred between −1.42 and −1.18 m.y., followed by a relatively cool interval with small-scale temperature fluctuations up to −0.52 m.y. and from that time on a warming trend occurred. A warming trend is recorded as well in cores from the subantarctic southeast area during the last 0.8 m.y., with superimposed temperature fluctuations which were essentially synchronous with north hemisphere climatic changes. The 1O°C surface isotherm did not shift north of 36°S during the Pleistocene.