miocene planktic foraminifera from the northern indian ...

SPECIAL PUBLICATION OF THE PALAEONTOLOGICAL SOCIETY OF INDIA No. 5; February, 2014; ISBN: 978-81-926033-2-2; pp. 111-119

MIOCENE PLANKTIC FORAMINIFERA FROM THE NORTHERN INDIAN OCEAN ARUN DEO SINGH and KOMAL VERMA CENTER OF ADVANCED STUDY IN GEOLOGY BANARAS HINDU UNIVERSITY, VARANASI -221005 Corresponding author’s email: [email protected] ABSTRACT The availability of Neogene deep-sea sections from the northern Indian Ocean provided by the Deep Sea Drilling Project (DSDP) enabled to examine Miocene planktic foraminifera in detail and to investigate their evolutionary lineages important in the tropical biozonations. The phyletic lineages were placed within chronologic framework accomplished by integration of planktic foraminiferal biostratigraphy based on datum levels (first/last appearance), magnetic stratigraphy and radiometric dates. These efforts have led to establish an improved planktic foraminiferal zonal scheme for the tropical Indian Ocean. An evaluation of the planktic foraminiferal datums reveals that not all of them are equally useful for global correlation. The first appearance of Praeorbulina glomerosa curva , Neogloboquadrina acostaensis and the last appearance of Catapsydrax dissimilis, Globigerinatella insueta are datum levels can be traced from tropical to temperate areas, thus useful in correlating sequences over a wide latitudinal range. Deep sea hiatuses in the Miocene stratigraphic records of the DSDP sites identified based on the absence of one or more planktic foraminiferal zones, are briefly discussed.

Keywords: Planktic foraminifera, Miocene, biostratigraphy, DSDP, northern Indian Ocean

INTRODUCTION Planktic for aminifer a represent a major calcareous microfossil group widely used for biostratigraphic subdivisions and correlation of deep sea cores. The Miocene Epoch (23.2 – 5.2 Ma) is a crucial period of the Cenozoic Era, as it witnessed major changes in ocean circulations and global climates affecting strongly biogeographic patterns and evolution of planktic foraminifera. The beginning of the Miocene was marked by the evolutionary radiations of planktic foraminifera leading to the developments of various lineages. A major faunal turnover triggered by a number of global oceanographic and climatic changes also characterizes this epoch. The paleoceanographic studies conducted in the world oceans reveal significant changes in planktic foraminiferal assemblages in different time slices of the Miocene period. These changes are recorded in terms of evolution, diversification, proliferation and extinctions of major foraminiferal groups. Detailed studies of Indian Ocean Miocene foraminifera were less attempted as compared to other oceans until the time of the International Indian ocean Expedition (IIOE) in 1970’s when continuous Neogene deep sea sediment sequences were recovered

from the JOIDES Deep Sea Drilling Project (DSDP). Neogene planktic foraminifera from the sites drilled through Leg 22 through 29 were initially examined at coarse sampling intervals (Fleisher, 1974; Vincent, 1977). Srinivasan and his coworkers at Banaras Hindu University carried out detailed investigations of Neogene foraminiferal assemblage records recovered Table 1: Location and water depth of the examined DSDP Sites. Sites 214 219 237 238

Latitude 0

11 20.21’S 9001.75’N 7009.99’S 11 009.21’S

Longitude 0

88 43.08’E 77 052.67’E 58 007.48’E 70 031.56’E

Water Depth (m) 1,165 1,764 1,640 2,832

at close stratigraphic intervals from the DSDP sites (214, 216, 217 of Leg 22; 219, 220 of Leg 23; and 237 and 238 of Leg 24) of the tropical Indian Ocean situated in its northern sector. As a result, a mass of new data on planktic foraminiferal evolution, diversification and faunal turnover were generated enabling to refine Indian Ocean Neogene biostratigraphy, initially developed for uplifted deep marine sequences in Andaman-Nicobar and to decipher oceanographic evolution of the Northern

112 Indian Ocean during the Neogene. This paper is an overview of the principal results of the Miocene planktic foraminiferal studies made on the those sites having fairly continuous records (Sites 214, 219, 237 and 238) with emphasis on the major evolutionary lineages providing important datum marker taxa applied in biostratigraphy of the tropical Indian Ocean. The DSDP sites 214, 237 and 238 are situated presently under the tropical-subtropical transition water

ARUN DEO SINGH and KOMAL VERMA

mass and site 219 in the northern part of the equatorial water mass (Fig. 1). Present day, these sites lie well above the carbonate lysocline as indicated by a general good foraminiferal preservation except for a shorter intervals in the Early Miocene. Core recovery was reasonably good at these sites except for a few stratigraphic intervals in the early Neogene interrupted with hiatuses and dissolution intervals. Details of the sites discussed are provided in Table 1.

Fig. 1. Location of the DSDP sites 214, 219, 237 and 238 in the Northern Indian Ocean.

Miocene Planktic Foraminiferal Assemblages Detailed examination of planktic foraminiferal assemblages from Miocene sections of the northern Indian Ocean DSDP sites enabled to record more than one hundred species. As the examined sites currently lie within the tropical belt and the Indian Ocean remained relatively stable during the last 25 M.y. , the tropical elements in the assemblages are therefore common and similar to those found in the equatorial Pacific throughout the Miocene. However, there have been occurrences of transitional to temperate or even sub-antarctic species during specific time slices in response to changes in surface ocean circulation linked to the global climate. The early Miocene fauna exhibit a tendency towards low diversity caused by carbonate dissolution, whereas, middle and late Miocene fauna

are diverse, numerically common to abundant and generally well preserved. In the early Miocene Catapsydrax dissimilis, Globorotalia peripheroronda, Globorotalia siakensis and various globigerinids and globoquadrinids dominate the assemblages. The middle Miocene assemblages are character ized by the Globorotalia (Fohsella and Menardella) lineages, Globigerinoides (bollii, obliquus, subquadratus), Sphaeroidinellopsis group, Globoquadrina and Globigerina (decoraperta, druryi, nepenthes). An interesting similar ity with the middle Miocene assemblages of the tropical Atlantic Ocean is observed in occurrences of Globigerinopsis aguasayensis and Globigerina bulloides. The late Miocene assemblages are characterized by globorotalids (Globorotalia merotumida-plesiotoumida and Globorotalia menardii

MIOCENE PLANKTIC FORAMINIFERA FROM THE NORTHERN INDIAN OCEAN

lineages), Neogloboquadrina acostaensis, Sphaeroidinellopsis, Globigerina (nepenthes and decoraperta), Candeina and in the latest Miocene by the initial appearance of Pulleniatina. Planktic Foraminiferal Lineages Several publications have appeared in the last four decades, presenting a synthesis on Neogene planktic foraminifera with special reference to their taxonomy and stratigraphic significance ( eg. Postuma, 1971; Stainforth et al., 1975; Blow, 1979; Saito et al., 1981;

113

Kennett and Srinivasan, 1983; Bolli and Saunders, 1985). Most of these studies have given emphasis on understanding the phylogenetic relationships among the taxa. This has led to a phylogenetic rather than a typological approach to planktic foraminiferal classification and enhanced the biostratigraphic refinements. In recent years, besides sur face ultr astructure studies using Scanning Electron Micr oscope, chemical composition of planktic foraminiferal tests, and genetic characterization based on the molecular evidences have enhanced our

Fig.2a. Important taxa of the Miocene planktic foraminiferal lineages and datum marker species. 1-7. Globigerinoides bisphericus-PrarorbulinaOrbulina lineage, 1. Globigerinoides bisphericus, X 75, 2-3. Praeorbulina sicana, X 75, X 150, 4. Pr. glomerosa curva, X 135, 5. Pr. glomerosa glomerosa, X 175, 6. Pr. glomerosa circularis, X 200, 7. Orbulina suturalis, X 150, 8-15. Globorotalia (Fohshella) lineage, 8. Gr. kugleri, X 304, 9. Gr. birnageae, X 170, 10. Gr. peripheroronda, X 90, 11. Gr. peripheroacuta, X 90, 12. Gr. praefohsi, X 65, 13. Gr. fohsi fohsi, X 72, 14. Gr. fohsi lobata, X 81, 15. Gr. robusta, X 70, 16-20. Globorotaia (Menardella) lineage, 16. Gr. archeomenardii, X 120, 17. Gr. praemenardii, X 120, 18. Gr. menardii, X 59, 19. Gr. limbata, X 40, 20. Gr. multicamerata, X 47, 21-23. Globorotalia (Globorotalia) lineage, 21. Gr. merotumida, X 120, 22. Gr. plesiotumida, X 120, 23. Gr. tumida tumida, X 44.

114 understanding on phylogenetic relationships among the fauna and their phenotypic variations (Huang, 1981; Kennett and Srinivasan, 1983; de Vargas et al., 2001; Kucera and Darling, 2002; Darling and Wade, 2008). The Neogene sequences of the DSDP sites 214, 219, 237 and 238 in the northern Indian Ocean have provided an unprecedented opportunity to recover continuous record of the planktic foraminifera and to investigate progressive evolutionary developments in various lineages in view of their importance in the Neogene biostratigraphy. Studies of the important Miocene planktic foraminiferal lineages enabled to document ancestor-descendant relationships through morphological gradations and the true evolutionary appearances and last appearances (extinctions) of the taxa vital for biostratigraphic correlation (Chaturvedi, 1987; Srinivasan and Singh, 1992a; Singh, 1995). It was observed that the evolution of a new species within these lineages occurred through a gradual and progressive morphological changes, thus supporting the theory of Phyletic Gr adualism (Singh, 1995). Further, oceanographic and climatic changes during the Miocene played major role in triggering morphological gradations among the taxa during the course of evolution. Three lineages within the globorotalids (Fohsella, Menardella and Globorotalia) and Neogloboquadrina, Pulleniatina, SphaeroidinellopsisSphaeroidina, Praeorbulina-Orbulina evolutionary bioseries provide bio-events significant for Miocene biochronology and intra- as well as inter-regional correlation of the tropical marine sequences . Important taxa of these lineages are illustrated through Scanning Electron Micrographs (Fig. 2a, b). Globorotalia (Fohshella) lineage [Early Miocene to Middle Miocene] includes Gr. kugleri, Gr. peripheroronda, Gr. birnageae, Gr. peripheroacuta, Gr. praefohsi, Gr. fohsi fohsi, Gr. fohsi lobata and Gr. fohsi robusta (in ascending order of their evolution). This is an important lineage in the tropical to the warm subtropical oceans and has been widely used in Early to Middle Miocene biostratigraphy. Globorotalia (Menardella) lineage [Middle Miocene to Recent] includes evolutionar y developments of Gr. archeomenardii, Gr. praemenardii, Gr. menardii, Gr. limbata and Gr. multicamerata within the Miocene. Other short ranging Pliocene-Pleistocene species of this lineage (such as Gr. miocenica, Gr. exilis and Gr. pertenuis) developed in tropical Atlantic province, occur


sporadically in the tropical Indian ocean (Srinivasan and Singh, 1992a). This bioseries has been considered to be evolved in the cool subtropical oceans from Globorotalia (Globoconella) praescitula, and then migrated to the warm subtropical and tropical areas in the late Early Miocene (Kennett and Srinivasan, 1983). This has been interesting to record that this lineage became predominant constituent of the globorotaliid population only after the extinction of Fohsella lineage (Srinivasan and Singh, 1992a). The phylogeny of Globorotalia (Globorotalia) lineage [Middle Miocene to Recent] includes Gr. lenguaensis, Gr. paralenguaensis, Gr. merotumida, Gr. plesiotumida, Gr. tumida tumida and Gr. ungulata (Bandy, 1972). Except for the first two species due to their spor adic occurrences, the evolutionary sequence of this bioseries has been documented well in the examined sites. The evolutionary first appearance of Gr. tumida tumida occurs at the beginning of the Pliocene followed by the development of Gr. ungulata in the Pliocene. Neogloboquadrina lineage [Early Miocene to Recent] in the northern Indian Ocean includes N. continuosa, N. acostaensis, N. humerosa and N. dutertrei. The end member of this lineage (N. dutertrei) evolved in the late Pliocene. The earliest member of Pulleniatina lineage (Pu. primalis) evolved from its ancestor N. acostaensis in the late Miocene. Other species Pu. praecursor and Pu. obliquiloculata were gradually developed in the Pliocene. Sphaeroidinellopsis and Sphaeroidinella are the two groups restricted generally to tropical water mass and linked together phylogenetically. However, Sphaeroidinellopsis is known to be evolved in temperate to warm subtropical water mass from its ancestor Globigerina bulloides (Kennett and Srinivasan, 1983). Sphaeroidinellopsis lineage [Early Miocene to Pliocene] includes Ss. disjuncta, Ss. seminulina, and Ss. paenedehiscens. Ss. disjuncta also gives rise into Ss. kochi. The evolution of Sphaeroidinella from Sphaeroidinellopsis (Ss. paenedehiscens) took place in the Early Pliocene. The other planktic foraminiferal lineage important in the Miocene biostratigraphy at the examined sites is Praeorbulina-Orbulina bioseries. Pr. sicana de Stefani considered previously a synonomous with Globigerinoides bisphericus is now believed to be a descendant form of later in the evolutionary bioseries of Globigerinoides bisphericus-Prarorbulina-Orbulina. The DSDP sites of the northern Indian Ocean enabled to trace phylogenetic gradational developments within

115


Praeorbulina beginning with sicana to glomerosa curva, glomerosa glomerosa , glomerosa circularis and finally into Orbulina suturalis. Praeorbulina lineage includes short ranging taxa useful in distinguishing stratigraphic levels close to Early-Middle Miocene boundary. The planktic foraminiferal lineages discussed here are those important in the biostratigraphic correlation of the marine Miocene deep sequences. In order to evaluate the synchroneity of the levels of evolutionary appearances/disappearances of taxa within these lineages, used as datum markers, these lineages are placed within the chronological framework derived through an integrated approach of establishing the biomagnetochronology (Srinivasan and Singh, 1992a; Singh, 1995). Miocene Planktic Foraminiferal Datums and Biozones The deep sea cores from the northern Indian

Ocean DSDP sites provided an opportunity to document the Neogene planktic foraminiferal datums (first evolutionary appearance and disappearance) in order to enhance the biostratigraphic resolution. The datums representing conspicuous bio-events are widely applied in correlating deep sea sequences. A total of twenty planktic foramineral datums within the Miocene have been recorded. The ages of planktic foraminiferal events estimated by various workers are provided in Table 2. Following Hornibrook and Edwards (1971) these datums are grouped into three orders of reliability: First order- easily identifiable, common and persistent, and highly consistent range; Second order-easily identifiable, fair ly consistently present but not necessarily common; Third order-consistency of identification difficult or not common occurrence, geographic restrictions (Table 2).

Table 2: Miocene planktic foraminiferal events recorded in the northern Indian Ocean DSDP sites and their estimated ages. Neogene planktic foraminiferal events

Order of reliability Hornibrook & Edwards (1971)

Ryan et. al. (1974)

Saito (1977)

Saito (1984)

Hornibrook (1981)

Miller et. al. (1985)

Berggren et. al. (1983)

FAD Gr. tumida tumida

I

LAD Gq. dehiscens

II

6.2

5.0

FAD Pr. primalis

I

5.8

6.2

5.8

5.8

FAD Gr. plesio tumida

I

7.7

6.8

7.7

FAD N. acostaensis

I

10.8

10.0

9.1

LAD Gr. siakensis

I

12.0

11.2

10.4

LAD Gr. fohsella group

I

FAD Gr. fohsi lobata

I

FAD Gr. menardii

I

13.4

FAD Gr. fohsi fohsi

I

15.5

13.9

FAD Gr. praefohsi

I

14.7

14.7

FAD Gr. peripheroacuta

I

14.9

15.3

14.6

FAD O. suturalis

I

16.0

15.5

FAD Pr. glomerosa curva

I

LAD Cs. dissimilis

I

18.5

18.0

17.6

FAD Gt. insueta

III

19.4

18.6

18.0

LAD Gr. kugleri

I

20.5

20.0

FAD Gq. dehiscens

I

5.0

Berggren et. al. (1985)

5.2 5.3

10.2

10.2

12.0

12.4

5.3

10.4 11.2

11.5

13.1

13.1

13.9

13.9

12.0

14.9

14.9

19.5

15.2

15.2

16.5

16.5

19.5

16.5 17.6

17.6 21.8

24.0

23.2

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Fig. 2b. Important taxa of the Miocene planktic foraminiferal lineages and datum marker species. 1-4. Neogloboquadrina lineage, 1. N. continuosa, X 190, 2. N. acostaensis, X 110, 3. N. humerosa, X 130, 4. N. dutertrei, X 75, 5-8. Pulleniatina lineage, 5-6. Pr. primalis, X 60 X 60, 7. Pr. praecursor, X 70, 8. Pu. obliquiloculata, X 60, 9-13. Sphaeroidinellopsis – Sphaeroidinella lineage, 9. Ss. disjuncta, X 65, 10. Ss. seminulina, X 70, 11. Ss. kochi, X 90, 12. Ss. paenedehiscens, X 200, 13. Sphaeroidinella dehiscens, X 63, 14. Gt. insueta, X 110, 15. Catapsydrax dissimilis, X 80, 16. Globorotalia siakensis, X 80, 17. Globoquadrina dehiscence, X 70

The planktic foraminiferal biostratigraphy based on the datum levels at each site was integrated with the available radiometric dates, paleomagnetostratigraphy to accomplish a refined chronological framework (Srinivasan, 1989; Srinivasan and Chaturvedi, 1992; Srinivasan and Singh, 1992b; Singh and Srinivasan, 1995). Furthermore, these efforts have enabled to testify the synchroneity of the planktic foraminiferal datums and their usefulness in biostratigraphic correlation. An evaluation of these datums reveals that not all those

recorded planktonic foraminiferal datums are useful for global correlation. The first appearance of Pr. glomerosa curva , N. acostaensis and the last appearance of Cs. dissimilis, Gt. insueta are datum levels can be traced from tropical to temperate areas, thus useful in correlating sequences over a wide latitudinal range. The extent of others is limited to tropical to warm subtropical areas. Neogene planktic foraminiferal biostratigraphy for the Indian Ocean DSDP sites established previously

117


by the shipboard scientists was based on Blow’s (1969) zonal concept. But, it is rather difficult to apply Blow,s zonal scheme to low-latitude Indian Ocean sites due to apparent rarity or absence of zonal taxa eg. Catapsydrax stainforthi, because of a strong dissolution noted in the lower Miocene sections. Studies enabled to establish an improved planktic foraminiferal zonal scheme for the tropical Indian Ocean and sixteen zones were recognized based on the stratigraphic ranges of datum marker species (Srinivasan, 1989; Srinivasan and Chaturvedi, 1992; Srinivasan and Singh, 1992; Singh and Srinivasan, 1995) [Fig. 3]. Srinivasan (1988, 1989) and Sharma and Srinivasan (2007) observed that biostratigraphic zonations established for the northern S it e D SD P 28 9 ( Tr opi cal P ac if ic ) Sr ini vasa n & K e nn et t 1981 a, b

N18

M i oce ne P lank ti c F or ami nif e ra l Z one s in N or t he rn Ind ian O c e an D SD P Si te s

Pu . p rim a lis ( 2 14 , 2 19 , 2 37 , 2 38 )

N1 7 A

Gr. ple sio tu m ida ( 21 4, 21 9 , 23 7 , 23 8)

5.8 M a

Gr . p lesiot um ida

FAD

7.7 M a

N. ac ost aen sis

FA D

10 .2 M a

Gr . siak e nsis

LA D

10.4 M a

F ohse lla lineag e

LA D

11 .5 M a

Gr . fo hsi r obust a

FAD

12.6 M a

Gr. fo h si lo b ata ( 21 4, 23 7)

G r . foh si lo ba t a

F A D 13.1 M a

Gr. fo h si fo hs i ( 21 4, 23 7)

Gr . fo hsi fo h si

F AD

Gr . p r aef ohsi

F A D 1 4.7 M a

Gr . p er ip her oac ut a

F A D 1 4.9 M a

G t . insuet a O . sut ur alis G s. sica nus Pr . g lom er o sa cu r v a

LAD FA D 15 .2 M a FA D FA D 16.5 M a

Cs. diss imilis

LA D 17.6 M a

G t . insuet a

FA D 1 8.8 M a

Gr . k ugler i

LA D 2 1.8 M a

Gq. deh iscens

FA D 2 3.2 M a

MI DDLE EA R LY

N E ILL IAN

N. ac o sta en sis ( 21 4, 23 7, 23 8)

Gr. m en ard ii ( 2 14 , 2 38 )

HA V E LO CK IA N

Gr. sia ken si s ( 21 4, 23 7 )

N11

Gr. pra efo hs i ( 23 7)

N10

Gr. pe riph er oa c uta ( 23 7)

N9

Gr. pe riph er oro nd a ( 23 7)

N8

Pr. glo m er os a ( 2 37 )

N7

Gt. ins ue ta (2 14 , 2 37 )

N6

Cs . s tain forth i ( 2 14 , 2 37 )

N5

Cs . d iss im ilis (2 1 4)

N 4B

Gq . d eh isc e ns (2 1 4)

N 4A

Gs . p rim o rdiu s (2 14 )

O N GEIA N

13.9 M a

ING LIS IA N

J A R A W A IAN

SERIES

LA TE O LIG OC E NE

5.3 M a

FA D

Gr. fo h si ro b us ta (2 14 , 2 19 , 2 37 , 2 38 )

N12

LA D

Pu. p r im alis

N14 N13

Gq. de hisce ns

F A D 5 .2 M a

ARCHIPELAGO

N16

An dam an N ic ob ar M ari ne S ta ge s Sh arm a & Sr ini vasa n, 200 7

Gr. tum id a tu m ida ( 21 4, 21 9, 23 7, 23 8)

N1 7 B

N15

M IOCE NE

Im por ta nt Pl ank tic F or amin if er al D atu ms

Gr . t u mid a tum ida

LA T E

EARLY

PLIOCENE

EPOC H

Indian Ocean DSDP sites are correlatable with those in the uplifted marine sequences of the Andaman-Nicobar (Fig. 3) and in the equatorial Pacific DSDP sites (Srinivasan and Kennett, 1981a, b). The planktic foraminiferal datums used in demarcating the Miocene epoch boundaries are: First appearance of Globoquadrina dehiscens , Late Oligocene/Early Miocene; First appearance of Praeorbulina glomerosa curva, Early Miocene/Middle Miocene; First appearance of Neogloboquadrina acostaensis, Middle Miocene/ Late Miocene; First appearance of Globorotalia tumida tumida, Late Miocene/ Early Pliocene.

A N DA M AN IA N

Fig. 3. Miocene planktic foraminiferal zones, datums and Andaman-Nicobar Stages (after Sharma and Srinivasan, 2007). [Estimated ages of planktic foraminiferal datums are after Saito, 1977; Berggren et al., 1985].

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Incompleteness of Planktic Foraminiferal Record: Paleoceanographic/climate Linkage Although core recovery at the DSDP sites 214, 219, 237 and 238 was reasonably good, the planktic foraminiferal records were interrupted with the hiatuses particularly within the Miocene at specific stratigraphic levels. Highly impr oved high resolution biostratigraphies at these sites have enabled to identify

breaks in stratigraphic records ( deep sea hiatuses) based on the absence of one or more planktic foraminiferal zones and efforts have been made to understand their paleoceanographic significance ( Singh and Mohan, 2007 and references therein). The temporal and spatial extent of the Miocene hiatuses at different sites are provided in Table 3.

Table 3: Miocene deep sea hiatuses at the northern Indian Ocean DSDP sites (after Singh and Mohan, 2007). Hiatus Event (Keller and Barron, 1987)

Site

Biostratigraphy

Chronology

NH 4

237

Gr. siakensis Zone / N. acostaensis Zone

10 – 10.5 Ma

NH 3

238

Gr. fohsi fohsi Zone / Gr. siakensis Zone

11 – 13 Ma

NH 2

214, 219

Gr. glomerosa Zone / Gr. praefohsi Zone

14 – 16 Ma

A condition of non-deposition in pelagic regime can occur due to dissolution and/or physical removal (erosion) of sediments. Generally, missing sediment record occurs when rate of sediment removal exceeds to the rate of sediment supply to ocean floor. The rate of sediment removal depends on intensity of bottom water activity and dissolution within the water-column and sediment-water interface. There have been a number studies on the spatial and temporal distributions of the deep sea hiatuses in the world oceans and their implications in the Cenozoic paleoceanography (Kennett and Watkins, 1975; Keller and Barron, 1987). Previous studies indicate that most of the hiatuses were either related to changes in bottom circulation pattern or major tectonic activity (Kennett, 1977; Miller et al., 1987). The Neogene deep sea hiatuses are generally suggested to be caused by intensified deep water circulation linked with the polar cooling. The chronology of the Miocene hiatuses recognized in the northern Indian Ocean DSDP sites indicates that these hiatuses are correlatable with the hiatuses identified from the other oceans ( Table 3; Keller and Barron, 1987). Singh and Mohan (2007) suggested that these hiatuses (corresponding to NH2, NH3 and NH4) were attributed to the physical erosion of sediments most likely by the intensified AABW (Antarctic Bottom Water ) current during global climate cooling. Based on the foraminiferal records (abundances of solution resistive vs solution susceptible taxa), Srinivasan and Singh (1991) identified two intervals of strong carbonate dissolution; the first one at Site 214 during 17-20 Ma, and the other during 16.5-17 Ma at

Site 237, when abundances of solution susceptible forms decreased drastically corresponding to increased abundances of dissolution resistant taxa. Also, these were the intervals of abrupt reduction in accumulation rates. The dissolution interval at Site 214 is broadly correlatable with the earliest Miocene hiatus NH1 of Keller and Barron (1987). It is plausible that large influx of corrosive Antarctic Bottom Waters to the two sites would have caused enhancement in carbonate dissolution during these intervals. Sometimes, a strong dissolution might cause removal of solution susceptible zonal marker taxa like Gt. insueta , Praeorbulina and Orbulina species. In such cases, absence of planktic foraminiferal biozones defined by solution susceptible taxa may not essentially be suggestive of a hiatus, instead it may represent an interval of strong dissolution. Ther efore, a caution should be taken while differentiating dissolution interval from a deep sea hiatus in pelagic sequences. AKNOWLEGEMENTS Authors are grateful to Prof. M. S. Srinivasan for reviewing the manuscript and useful suggestions. Deep Sea Drilling Project is thankfully acknowledged for providing samples. REFERENCES Bandy, O. L. 1972. Origin and development of Globorotalia (Turborotalia)pachyderma (Ehrenberg). Micropaleontology, 18 (3): 294-318. Blow, W. H. 1979. The Cainozoic Foraminiferida. Vols I and II, Leiden, E. J. Brill., 1413 pp.


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Saito, T., Thompson, P.R. and Breger, D. 1981. Systematic index of Recent and Pleistocene Planktonic Foraminifera.190pp. Univ. of Tokyo Press, Tokyo. Sharma V. and Srinivasan M.S. 2007. Geology of Andaman – Nicobar: The Neogene. Capital Publishing Company, New Delhi, pp. 163. Singh, A.D. 1995. Neogloquadrina, Pulleniatina and Sphaeroidinella-Sphaeroidinellopsis lineages in the northern Indian Ocean: Their Paleoceanograhic relations and biostratigraphic significance. J. Geological Society of India, 46: 163-177. Singh , A.D. and Mohan, R. 2007. Planktic foraminiferal evidence for Neogene deep sea hiatuses in the Northern Indian Ocean. Micropaleontology : Application in Stratigraphy & Paleocenography, pp. 315-330. Srinivasan, M.S. 1988. Late Cenozoic sequences of Andaman – Nicobar Islands; Their regional significance and correlation. Indian Jour. Geology, 60 (1): 11-34. Srinivasan, M.S. 1989. Recent advances in Neogene planktonic foraminiferal biostratigraphy, chemostratigraphy and paleocenography: Northern Indian Ocean. Jour. Paleont. Soc. of India, 34: 1-18. Srinivasan, M.S. and Chaturvedi, S.N. 1992. Neogene planktonic foraminiferal biochronology of the DSDP sites along the Ninetyeast Ridge, northern Indian Ocean. In: Ishizaki, K., Saito, T. (Eds.), Centenary of Japanese Micropaleontology, pp. 175-188. Srinivasan M.S. and Kennett, J.P. 1981a. A review of Neogene planktonic foraminiferal biostartigraphy: Applications in the Equatorial and South Pacific, SEPM Special publication, no. 31, p. 395-432. Srinivasan M.S. and Kennett, J.P. 1981b. Neogene planktonic foraminiferal biostartigraphy and evolution: Equatorial to Subantarctic, South Pacific. Marine Micropaleontology, 6: 499-533. Srinivasan M.S. and Singh A.D. 1992a. Neogene Globorotalia lineages in the tropical Indian Ocean: Their paleoceanographic implications. Indian Journal of Geology, no. 64(4), p. 323-328. Srinivasan M.S. and Singh, A.D. 1992b. Neogene planktic foraminifera biochronology of DSDP sites219 (ChagosLaccadive Ridge), Arabian Sea. Proceedings of Indian National Science Academy, 58A (4): 335-354. Srinivasan M.S. and Singh, A.D. 1995. Neogene planktic foraminifera biochronology of the Central Indian Ocean DSDP sites 237 and 238. Journal Geological Society of India, 45: 445-462. Stainforth, R.M., Lamb, J.L., Luterbacher, H., Beards, J.H. and Jeffords, R.M. 1975. Cenozoic planktonic foraminiferal zonation and characteristics of index forms. Univ. of Kansas Paleont. Contr., Art., 62: 1-425. Vincent, E. 1977. Indian Ocean Neogene planktonic foraminiferal biostratigraphy and its paleocenographic implications. In: J. Heirtzler et. al. (eds.), Indian Ocean Geology and Biostratigraphy. Amer. Geophys. Union, pp. 469-484.