12. PLEISTOCENE THROUGH MIOCENE CALCAREOUS ......I. RAFFI, J.-A. FLORES Table 1. Summary of Leg 138...

Pisias, N.G., Mayer, L.A., Janecek, T.R., Palmer-Julson, A., and van Andel, T.H. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 138

12. PLEISTOCENE THROUGH MIOCENE CALCAREOUS NANNOFOSSILS FROM EASTERNEQUATORIAL PACIFIC OCEAN (LEG 138)1

Isabella Raffi2 and José-Abel Flores3

ABSTRACT

The calcareous nannofossil biostratigraphy of the sediments retrieved during Leg 138 in the eastern equatorial Pacific Oceanis presented and discussed. The sedimentary sequences stidoed, at 10 of the 11 sites drilled, span the stratigraphic interval fromPleistocene to upper (Sites 847 and 848) and middle Miocene (Sites 851, 852, and 853), and three extend to the upper part of thelower Miocene (Sites 844, 845, and 846). Most of the zonal boundaries of the 1973 zonation of Bukry and standard 1971 zonationof Martini are recognized and used for the biostratigraphic classification of these low-latitude sediments. Additional biostrati-graphic events are discussed and in some intervals are used as secondary criteria for improving the biostratigraphic resolution. Afurther subdivision of upper Miocene Subzone CN9b of Okada and Bukry (1980) is proposed using the lowest and highestoccurrences of Amaurolithus amplificus.

Comments on the biochronology of calcareous nannofossils are given, with special reference to Miocene events, takingadvantage of the very good magnetostratigraphy and orbitally tuned time scale produced for the Leg 138 sites.

INTRODUCTION

During Ocean Drilling Program (ODP) Leg 138, the Neogenesediments of the eastern equatorial Pacific Ocean were sampled. Thisleg was the fifth in a series designed to recover high-quality sedimen-tary sections in the tropical oceans for detailed studies of globalclimate change during late Cenozoic time. During previous cruisessediments were drilled in the western equatorial Pacific (Leg 130), thewestern tropical Indian Ocean (Leg 117), the Peru Current region(Leg 112), and the equatorial Atlantic Ocean (Leg 108). The majorscientific objective of Leg 138 was to study Neogene paleoceano-graphic evolution in the highly productive waters of this region.Continuous sedimentary records, suitable for high-resolution pale-oceanographic studies, were constructed by splicing together strati-graphic sections recovered by drilling adjacent and offset holes withthe advanced hydraulic piston corer (APC) and the extended core bar-rel (XCB). Along two complementary north-south transects (95°Wand 110°W), 11 sites (42 holes) were drilled and more than 5500 mof cores was recovered (Fig. 1 and Table 1). Complete recovery ofthe stratigraphic section occurred at 8 of the 11 sites, and compositedepth sections were constructed by using multiple measurements ofsediment density (GRAPE) and other sedimentary parameters (seeHagelberg et al., in Mayer, Pisias, Janecek, et al., 1992). The sedi-ments recovered range in age from the Miocene to Pleistocene. In sixof the recovered sequences, high-quality magnetostratigraphic datawere obtained (Leg 138 Init. Repts. volume; Schneider, this volume),and these provide an excellent stratigraphic framework (e.g., Site845; Schneider, this volume). This allows us to calibrate several nan-nofossil events to the magnetic polarity time scale and provides newbiochronologic data for the Miocene (Table 2).

In this study, we comment on (1) the Pleistocene-to-Miocenecalcareous nannofossil biostratigraphy and biostratigraphic resolu-tion in the eastern equatorial Pacific; (2) the biostratigraphic classifi-

Pisias, N.G., Mayer, L.A., Janecek, T.R., Palmer-Julson, A., and van Andel, T.H.(Eds.), 1995. Proc. ODP, Sci. Results, 138: College Station, TX (Ocean Drilling Program).

2 Istituto di Geologia, Universita di Parma, c/o Villa Cybo, 00040 Castelgandolfo,Italy. (Current address: Universita degli Studi "G. D'Annunzio," Chieti - Facoltà di ScienzeMatematiche, Fisiche e Naturali, Italy.)

3 Universidad de Salamanca, Departamento de Geologia, Paleontologia, 37008Salamanca, Spain.

cation and age assignment of 10 of the 11 sites drilled during Leg 138;and (3) some aspects of the late Neogene chronostratigraphy andcalcareous nannofossil biochronology.

Details about the distribution patterns and biomagnetostratigra-phy of some middle and late Miocene calcareous nannofossil indexspecies are reported and discussed in Raffi et al. (this volume), whocollected quantitative data from some of the Leg 138 sections.

METHODS

Smear slides of each sample were prepared from unprocessedmaterial and were examined with a light microscope at ×1400 mag-nification under cross-polarized and transmitted light. Approximately2400 samples were examined. We examined approximately 200 fieldsof view in each slide, primarily those areas where the sample materialhad optimum density and where no appreciable piling of specimenshad occurred (with an average number of 50 specimens per field). Ineach slide, the nannofossil assemblage was characterized, and theabundance of nannofossils was estimated in a semiquantitative fash-ion. To check for the presence or absence of index species in criticalstratigraphic intervals, we used the quantitative counting techniquesof Thierstein et al. (1977), Backman and Shackleton (1983), Rio et al.(1990a, 1990b), and Fornaciari et al. (1990). To detect index speciesevents, we counted variable numbers of nannofossils, depending onthe abundance of all nannofossils in the assemblage and on theabundance of the specific index species. For example, the presenceor absence of index species belonging to discoasterids, helicoliths,sphenoliths, and ceratolithids was evaluated by counting a fixed num-ber of forms belonging to the group (100-200 discoasterids, 100 heli-coliths and sphenoliths, and 20-30 ceratolithids).

All the biostratigraphic events (Fig. 2) reported in Tables 3-12 andFigures 3-12 have been quantitatively defined.

In the range charts that we present of selected sites, only selectedsamples have been plotted. Abundance and preservation have beensemiquantitatively and qualitatively evaluated, respectively. The abun-dance code is as follows:

A (abundant) = usually >IO specimens observed per field;C (common) = 1-10 specimens per field;F (few) = 1 specimen per 1-10 fields; andR (rare) = <l specimen per 10 fields.

233

I. RAFFI, J.-A. FLORES

Table 1. Summary of Leg 138 sites.

Site

844845846847848849850851852853

Latitude

7°55.28N9°34.95N3°5.70NCH1.59S2°59.63SO°IO.98N1°17.83N2°46.22N5°17.57N7°12.66N

Longitude

9O°28.85W94°35.45W90°49.08W95°19.23W110°28.79W11O°31.18W110°31.29W110°34.31W110°4.58W109°45.08W

Waterdepth(m)

3425.03715.93295.63346.03867.33850.83797.83772.03871.63727.2

Table 2. Calcareous nannofossil events and age estimates.

Event

B acme Emiliania huxleyiB Emiliania huxleyiT Pseudoemiliania lacunosaReentrance medium Gephvrocapsa spp.T large Gephvrocapsa spp.B large Gephvrocapsa spp.T Calcidiscus macintxreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster pentaradiatusT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB Ceratolithus rugosusB Ceratolithus acutusT Triquetrorhabdulus rugosusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval R. pseudoumbilicusB Amaurolithus primusB Discoaster berggreniiB Discoaster loeblichiiB absence interval R. pseudoumbilicusB Minvlitha convallisT Discoaster hamatusB Discoaster neohamatusB Discoaster hamatusT Coccolithus miopelagicusB Catinaster coalitusT c Discoaster kugleriB c Discoaster kugleriB Discoaster kugleriT Coronocyclus nitescensT Calcidiscus macintvreiT Calcidiscus premacintyreiB Triquetrorhabdulus rugosusT Discoaster signusT Cydicargolithus loridanusT Sphenolithus heteromorphusT Helicosphaera ampliapertaB Discoaster signusEnd acme Discoaster deflandrei

Zone(base)

CNsl5CN14bCN14a(?)

CN13b(?)CN13aCN12dCN12cCN12bCN12aBCN12aACNlOcCNlOb

CNlOaCN9bCCN9bB

CN9bACN9aCN8b

CN8a

CN7

CN6

CN5b

CN5aCN4

Adoptedage

(Ma)

0.085-0.0730.260.4611.0281.241.4571.5971.67

1.96-0.112.44-0.062.61-0.092.76-0.013.65-0.05

3.804-0.0035.04-0.035.34-0.025.34- 0.025.56-0.045.88-0.026.50-0.066.80-0.27.24-0.128.35-0.118.43-0.088.85-0.39.43-0.049.36-0.129.56-0.11

10.39-0.1210.39-0.110.71-0.0111.34-0.0311.74-0.0412.20-0.212.12-0.0112.34-0.0112.65-0.0212.62-0.0212.73-0.0113.19-0.0113.57-0.05

15.8316.19-0.0116.21-0.01

Reference

1I122222333333333333333333333333333333333433

Notes: T = top occurrence, B = bottom occurrence, T c = top occurrence of commonand continuous species, and B c = bottom occurrence of common and continuousspecies. References are as follows: 1 = Thierstein et al. (1977), 2 = Raffi et al.(1993), 3 = this study, and Shackleton et al. (this volume), and 4 = reestimatedfrom Backman et al. (1990).

The qualitative evaluation of the state of preservation of the calcare-ous nannofossils found within each sample was made with the fol-lowing criteria:

G (good) = specimens exhibit little or no dissolution and/orovergrowth;

M-G (moderate to good) = specimens exhibit slight to moderatedissolution and/or overgrowth, and the identification of somespecies is hampered;

25°N

20 ;

15°-

10

—"m

• 854

" 853' 852

• 851• 850' 849

• 848

i i ^~

845 III;,* 844 ' : u ^fj ••:,::;,:

847 :;5J§|* A i lü i i i i

• 846

; : : : : : • : • : • : • •

-

m

ill5°S115°W 110° 105° 100° 95°

Figure 1. Location map for Leg 138 sites.90° 85° 80° 75°W

M (moderate) = specimens exhibit moderate dissolution and/orovergrowth, and identification is difficult at the specific level;and

P (poor) = specimens exhibit extreme dissolution and/orovergrowth.

These categories were determined on the basis of the "average" stateof preservation of the calcareous nannofossils examined on the smearslides. Considerable variation in the state of preservation of theindividual specimens can be observed in the same sample.

In the range charts, the degree of etching (E) and overgrowth(O) is also reported, following the criteria proposed by Roth andThierstein (1972) and modified by Roth (1983), as follows:

E=O and O=O: no sign of dissolution and overgrowth;E=l and O=l: slight dissolution and overgrowth;E=2 and 0=2: moderate dissolution and overgrowth; andE=3 and 0=3: severe effects of dissolution and overgrowth.

The taxa we consider here are reported in alphabetic order and bygeneric epithets in Appendix A. Bibliographic references for thesespecies are given in Loeblich and Tappan (1966, 1968, 1969, 1970a,1970b, 1971, 1973) and Aubry (1984, 1988, 1989, 1990). The taxo-nomic concepts we used in this study primarily followed those of Rioet al. (1990a) and are summarized below (see "Taxonomic Notes"section, this chapter).

BIOSTRATIGRAPHY

Here, we give a general discussion on the biostratigraphy obtainedin the sedimentary sequences recovered during Leg 138. We refer tothe zonal schemes of Martini (1971) and Bukry (1973, 1978), whichwere code numbered by Okada and Bukry (1980) and are regarded asthe "standard" for the biostratigraphic classification of Cenozoic ma-rine sediments. These zonations have been applied to all the strati-graphic intervals with relative ease. In addition to the known zonalboundaries in the Pleistocene and upper Miocene, we record otherbioevents that improve the biostratigraphic resolution of the two stan-dard zonations. In Figure 2, we report the recognized nannofossil events,the zones, and the adopted definitions, together with the adopted bio-chronology and chronostratigraphy. Details on the biostratigraphic re-sults of each section are reported in the individual site chapters below.

Pleistocene

The Pleistocene biostratigraphic classification is based on the bio-zonation of Gartner (1977) and on recent data from a biochronologicstudy of low- and mid-latitude, deep-sea records (Raffi et al., 1993).

234

PLEISTOCENE-MIOCENE NANNOFOSSILS

Table 3. Position of calcareous nannofossil events at Site 844.

EventInterval

(cm)Depth(mcd)

Interval(cm)

Depth(mcd)

T Pseudoemiliania tacunosa 844B-1H-CC/2H-1, 120 4.53-6.83Reentrance medium Gephyrocapsa spp. 844B-2H-4. 120/2H-5, 120 11.13-12.63T large Gephyrocapsa spp. 844B-2H-5. 120/2H-6, 120 12.63-14.13B large Gephyrocapsa spp. 844B-2H-7,47/2H-CC 14.90-15.61T Cakidiscus macintyrei 844B-2H-CC/3H-1, 120 15.61-17.45B medium Gephyrocapsa spp. 844B-3H-2, 60/3H-2, 120 18.35-18.95T Discoaster brouweri 844B-3H-3,85/3H-3, 120 20.10-20.45T Discoasterpentaradiatus 844C-3H-1.83/3H-2,83 21.78-23.28T Discoaster surculus 844C-3H-2,83/3H-3,83 23.28-24.78B Cemtolithus acutus 844C-4H-1, 30/4H-1, 50 32.55-32.75 844D-1H-3, 75/1H-3, 100 32.45-32.70T Discoaster quinqueramus 844C-4H-1.75/4H-1, 100 33.00-33.25 844D-1H-3, 125/1 H-3, 150 32.95-33.20T Amaurolithus amplificus 844C-4H-2, 125/4H-2, 145 35.00-35.20B Amaurolithus amplificus 844B-5H-2,60/5H-2,90 39.04-39.34T absence interval R. pseudoumbilicus 844B-5H-3, 90/5H-3, 120 40.84-41.14B Amaurolithus primus 844B-5H-4, 12O/5H-5,29 42.35^2.94T Minylitha convallis 844C-5H-3,2/5H-3,50 45.87^6.35B Discoaster berggrenii 844B-6H-4,60/6H-4, 150 51.70-52.60 844C-5H-7,30/6H-1,70 52.15-54.45B Discoaster pentaradiatus 844B-6H-4, 150/6H-5,60 52.60-53.20B Discoaster loeblichii 844B-6H-5,60/6H-6,60 53.20-54.70B absence interval R. pseudoumbilicus 844C-6H-2, 70/6H-2, 90 55.95-56.15B Minylitha convallis 844C-6H-5,5/6H-5,30 59.80-60.05T Discoaster hamatus 844B-7H-1, 100/7H-1, 120 59.90-60.10 844C-6H-5,5/6H-5,30 59.80-60.05B Discoaster hamatus 844B-8H-1,60/8H-2,60 69.26-70.76 844C-7H-5,2/7H-5,25 70.27-70.50T Coccolithus miopelagicus 844B-8H-2, 60/8H-3, 60 70.76-72.26 844C-7H-5, 125/7H-6, 2 71.50-71.77B Catinaster coalitus 844B-8H-4, 120/8H-5,60 74.36-75.26T c Discoaster kugleri 844B-10H-1,60/10H-2,60 91.73-93.23Be Discoaster kugleri 844B-11H-4, 60/11H-4, 120 108.60-109.20B Discoaster kugleri 844B-13H-4, 60/13H-4, 120 127.90-128.50T Coronocyclus nitescens 844B-I3H-4, 12O/13H-5,60 128.50-129.40T Cakidiscus premacintyrei 844B-14H-5, 120/14H-6. 120 140.53-142.03B Triquetrorhabdulus rugosus 844B-15H-4, 120/15H-5,60 148.70-149.60T Discoaster signus 844B-15H-7,40/16H-1, 120 152.40-154.88T Cyclicargolithusfloridanus 844B-17H-6, 120/17H-7,60 171.30-172.20T Sphenolithus heteromorphus 844B-19H-4,65/19H-4, 120 188.35-188.90B Reticulofenestra pseudoumbilicus 844B-19H-4, 65/19H-4, 120 188.35-188.90T Helicosphaera ampliaperta 844B-26X-3, 120/26X-4, 90 256.13-258.46B Discoaster signus 844B-28X-2, 60/28X-2, 122 273.33-273.95

Notes: T = top occurrence, B = bottom occurrence, T c = top occurrence of common and continuous species, and B c = bottom occurrence ofcommon and continuous species. Depths in meters composite depth (mcd).


EventInterval

(cm)Depth(mcd)

Interval(cm)

Depth(mcd)

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Cakidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster pentaradiatusB Cemtolithus acutusT Discoaster quinqueramusB Amaurolithus amplificusT absence interval R. pseudoumbilicusB Amaurolithus primusB Discoaster berggreniiB Discoaster hamatusT Coccolithus miopelagicusB Catinaster coalitusT c Discoaster kugleriB c Discoaster kugleriT Coronocyclus nitescensB Discoaster kugleriB Cakidiscus macintyrei (?)T Cakidiscus premacintyreiB Triquetrorhabdulus rugosusT Discoaster signusT CyclicargolithusfloridanusT Sphenolithus heteromorphusB Reticulofenestra pseudoumbilicusT Helicosphaera ampliapertaB Discoaster signusT acme Discoaster deflandrei

845 A-2H-CC/3H-1,42845A-3H-4, 42/3H-5, 42845A-3H-7, 42/3H-CC845A-4H-l,42/4H-2, 42845A-4H-3, 42/4H-4, 42845A-4H-6, 43/4H-7, 42845A-5H-3, 42/5H-3, 150845A-5H-7, 5O/5H-CC845A-8H-5, 94/8H-5, 120845A-8H-6, 120/8H-7, 12845A-10H-3, 120/10H-4, 13845A-10H-5, 90/10H-5, 145845A-11H-2, 32/11H-4, 32845A-13H-1, 145/13H-2,45845A-I5H-7, 50/16H-1,42845A-16H-1,42/16H-1, 120845A-16H-4, 32/16H-4. 78

845A-19H-l,78/19H-2, 42

845A-19H-l,78/19H-2,42

845A-20H-4.845A-20H-6,845A-22H-1,845A-23X-4,845A-23X-4,845A-27X-6,845A-29X-4,845A-29X-4,

78/20H-5, 4242/20H-6, 7878/22H-1, 14346/23X-4, 8646/23X-4, 8642/27X-6, 11840/29X-4, 120120/29X-5, 120

18.09-18.3822.88-24.3827.38-27.8529.15-30.6532.15-33.6536.66-38.1543.03-44.3849.11-49.6077.84-78.1079.60-80.0196.28-96.7198.98-99.53

104.68-107.68126.16-126.66156.46-158.85158.75-159.53163.15-163.61

190.54-191.68

190.54-191.68

206.21-207.35208.85-209.21223.61-224.26237.29-237.69237.29-237.69278.95-279.71294.73-295.53295.53-297.03

845B-12H-4, 150/12H-5, 150845B-15H-2, 120/15H-2, 145845B-15H-4, 20/15H-4.40

845B-16H-6, 120/16H-7, 20845B-17H-5, 10/17H-5, 42845B-18H-5, 10/18H-5, 78845B-18H-5, 78/18H-5, 120845B-18H-5, 78/18H-5, 1201845B-8H-7, 23/18H-7, 50

125.08-126.58157.10-157.35159.10-159.30

174.00-174.50181.00-181.32190.50-191.18191.18-191.60191.18-191.60194.00-194.99

Note: See note to Table 3.

235


CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY

Boundary species

Emiliania huxleyi

Pseudoemiliania lacunosa

reentrance medium Gephyro. spp.

medium Gephyrocapsa spp.

Discoaster brouweri

Discoaster pentaradiatusDiscoaster surculusDiscoaster tamalis

Sphenolithus spp.

R. pseudoumbilicuscommon D. asymmericus

Amaurolithus primusCeratolithus rugosusCeratolithus acutus

Discoaster quinqueramus

Amaurolithus amplificus

Amaurolithus amplificus

Amaurolithus primus

Discoaster berggrenii

Discoaster loeblichii

Discoaster hamatus

Discoaster hamatus

Catinaster coalitus

Discoaster kugleri |_

Sphenolithus heteromorphus I

1 Helicosphaera ampliaperta

Martini(1971)

NN21

NN 19

NN 16

NN15 + 14

NN 13

NN 12

NN 11b

NN 11a

NN 10

NN9

NN

NN7

NN6

NN5

NN4

Additional eventsin the eastern equatorial Pacific

large Gephyrocapsa spp

C. macintyrei

T. rugosus

J D. pentaradiatus

J M. convallis

J D. neohamatus

C. miopelagicus

common D. kuglericommon D. kugleriC. macintyreiC. nitescensC. premacintyreiT. rugosusD. signusC. floridanus

R. pseudoumbilicus

D. signusEnd acme D.deflandrei

Figure 2. Adopted chronostratigraphy and calcareous nannofossil biostratigraphy and biochronology. Geomagnetic polarity time scale after Candeand Kent (1992), with revised ages after Shackleton et al. (this volume).

236

Depth(mcd) OTHER EVENTS

reentrance medium Gephyrocapsa spp.

large Gephyrocapsa spp.

C. macintyrei


A. amplificus

A. amplificus

c ^ D. pentaradiatus

C=Ė= M. convallis

Absence interval of* R. pseudoumbilicus

C. miopelagicus^^_ D. calcaris

D. kugleri %

C. nitescens

10 20

5 10 15 C. premacintyrei

T. rugosus

D. signus %

Ft. pseudoumbilicus

D. signus %

5 10

D. deflandrei %

10 20 30 40 50


1 7 + 1 6

+12b

^77713a

12 | 10a

9bC

OTHER EVENTS

reentrance mediumGephyrocapsa spp

^ largeI I Gephyrocapsa spp.

I I C. macintyrei

mediumGephyrocapsa spp

A amplificus

A. amplificus

S <bI\ •s

J. 1I

D. pentaradiatus

/W. convallis

C. miopelagicus

7 ^ ^– D. calcaris

I I l10 20

\ D. kugleri

Figure 3. Chronostratigraphy and calcareous nannofossil biostratigraphy at Site 844. Magnetostratigraphy from site chapters in Mayer, Pisias,

Janecek, et al. (1992) and Schneider et al. (this volume). "Striped" areas at zonal boundaries represent intervals (sample spacing) within which

biostratigraphic events occur.

237


03 c— α>

- 1 0 - 1 - R

845A

845B

Magnetostratigraphy

OTHER EVENTS

reentrance medium Gephyrocapsa spp.

ss large Gephyrocapsa spp.

C. macintyrei


poorly preservednannofossils

^ Absence interval\ of R. pseudoumbilicus

\

\

D. kugleri %

5 10 15 20 25

D. signus °

D. deflandrei %

A. amplificus

C. miopelagicus

~ v C. nitescens

C. macintyrei= ^ = C. premacintyrei

• ^ T. rugosus

Ft. pseudoumbilicus

D. signus %

5 10 15 20

Figure 4. Chronostratigraphy and calcareous nannofossil biostratigraphy at Site 845. Notation as specified inFigure 3.



Event

Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricusT Amauroüthus primusT Ceratolithus acutusB Ceratolithus rugosusB Ceratolithus acutusT Discoaster quinqueramusT circular reticulofenestridsT Amaurolithus amplificusB Amaurolithus amplificusB circular reticulofenestridsT absence interval R. pseudoumbilicusB Amaurolithus primusT Minylitha convallisB Discoaser berggreniiB Discoaster loeblichiiB absence interval R. pseudoumbilicusB Minylitha convallisT Discoaster hamatusB Discoaster hamatusT Coccolithus miopelagicusB Catinaster coalitusT c Discoaster kugleriB c Discoaster kugleriB Discoaster kugleriT C. floridanus + C. nitescensT Sphenolithus heteromorphusB Reticulofenestra pseudoumbilicusT Helicosphaera ampliapertaT acme Discoaster deflandreiDiscoaster signus

Interval(cm)

846B-2H-7, 20/2H-CC, 5846B-4H-6, 45/4H-6, 120846B-5H-3, 120/5H-4, 40846B-6H-3, 120/6H-4, 39846B-6H-5, 40/6H-5, 120846B-6H-6, 40/6H-CC, 5846B-8H-3, 43/8H-3, 120846B-1 OH-1,60/1 OH-1, 135846B-12H-2, 120/12H-3, 60846B-14H-4, 120/14H-5, 60846B-14H-CC, 13/15H-1.60846B-15H-CC/16H-l,60846B-17H-1, 120/17H-2, 50846B-19H-1.40/19H-1, 120846B-19H-3, 120/19H-4, 40846B-20H-3, 120/20H-4, 50846B-21H-4, 50/21H-4, 120846B-23X-2, 12O/23X-3, 50846B-23X-3, 5O/23X-3, 120846B-23X-CC, 10/24X-1.40846B-25X-2, 39/25X-3, 50846D-26X-4, 120/26X-5, 50849D-26X-6, 120/26X-7.23846B-29X-2, 60/29X-3, 60846B-29X-5, 60/29X-5, 120846B-30X-3, 60/30X-4, 60846B-30X-5, 60/30X-6, 60846B-32X-1, 60/32X-2, 60846B-32X-5, 60/32X-5, 120846B-33X-4, 120/33X-5, 60846B-33X-5, 120/33X-6, 60846B-33X-6, 120/33X-7, 34846B-35X-2, 60/35X-2, 120846B-36X-l,36/36X-2,60846B-38X-6, 50/38X-6, 120846B-38X-CC, 10/40X-1.40846B-38X-CC, 10/40X-1.40846B-38X-CC, 10/40X-1.40846B-41X-CC, 9/42X-1.79846B-42X-CC, 12/43X-CC, 3846B-42X-CC, 12/43X-CC, 3

Depth(mcd)

16.80-17.3937.10-37.8544.90-45.6055.70-56.3957.90-58.7059.40-60.0477.13-77.9096.15-96.90

118.00-120.40143.85-144.75147.71-150.80160.32-160.80171.95-172.75192.70-193.50196.50-197.20207.80-208.60219.45-220.15243.20-244.00244.00-244.70250.15-252.00262.69-264.30279.30-280.10282.30-282.83301.40-302.90305.90-306.50312.60-314.10315.60-317.10328.80-330.30334.80-335.40343.60-344.50345.10-346.00346.60-347.24359.20-359.80367.16-368.90394.10-394.80395.11-405.80395.11-405.80395.11-405.80421.87-425.49426.00-434.33426.00-434.33


Almost all the known Pleistocene nannofossil events have been ob-served. Because we only used the light microscope technique toexamine the nannofossils, we were not able to record the appearance(CN15/CN14b and NN21/NN20 boundary) and the increase in abun-dance of Emiliania huxleyi in the upper Pleistocene interval. Detec-tion of the other Pleistocene events follows the rationale in Rio et al.(1990b) and Raffi et al. (1993), which is primarily based on morpho-metric studies of gephyrocapsids. This group represents an importantcomponent of Pleistocene nannofossil assemblages (see discussionon taxonomy of gephyrocapsids in Raffi et al., 1993). AbiometricaHybased definition of the group provides a precise tool in the correlationof lower Pleistocene sequences (Raffi et al., 1993; Rio et al., in press).

In the interval from the Chron ln/lr.lr boundary (Brunhes/Matuyama boundary) to the top of Chron 2n (Olduvai), the nannofos-sil events recorded are (1) the reentrance of medium Gephyrocapsaspp. (mainly composed of G. omega-G. parallela morphotypes); (2)the disappearance of large and medium Gephyrocapsa spp.; (3) thefirst occurrence (F0) of large Gephyrocapsa spp.; (4) the last occur-rence (LO) of Calcidiscus macintyrei; and (5) the F0 of mediumGephyrocapsa spp.

The use of these events in the biostratigraphic classification of thelower-middle Pleistocene improves the stratigraphic resolution inthis time interval. Note that some events correspond to the boundarydefinitions of the "standard" zones, and most events are isochronousover wide areas (Raffi et al., 1993). The reentrance of medium Gephy-rocapsa spp. probably corresponds to boundary CN14a/CN13b, de-fined by the appearance of G. oceanica s.s. by Bukry (1973), whorecorded G. omega as occurring close to this boundary.

The appearance of medium-sized Gephyrocapsa spp. (>4 µm) hasbeen used to recognize the CN13b/CN13a boundary, originally de-

Table 6. Position of calcareous nannofossil events at Site 847B.

Event

B Emiliania huxleyiT Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricusT Amaurolithus primusT Ceratolithus acutusB Ceratolithus rugosusB Ceratolithus acutusT Discoaster quinqueramus

Interval(cm)

1H-3, 120/1H-4, 1202H-5, 120/2H-6, 514H-2, 60/4H-2, 1205H-1.60/5H-1, 1205H-7, 40/5H-CC6H-1.60/6H-1, 196H-3, 120/6H-4, 607H-6, 60/7H-6, 1209H-3, 60/9H-3, 1209H-6, 120/9H-7, 1512H-3, 60/12H-3, 12012H-5, 60/12H-5, 12014H-5.60/14H-5, 12414H-5, 124/14H-6, 6017X-3.60/17X-4, 6017X-5, 123/17X-6, 6018X-7, 3/18X-7, 1520X-3, 93/20X-4, 60

Depth(mcd)

3.20-4.7013.83-14.6430.43-31.0338.40-39.0047.20-47.7050.18-50.7755.28-56.1868.85-69.4586.08-86.6891.18-91.63

118.13-118.73121.13-121.73140.58-141.22141.22-142.08168.75-170.25172.38-173.25183.78-183.90197.18-198.35


fined (Bukry, 1973) by the F0 of Gephyrocapsa caribbeanica, aspecies difficult to recognize in the optical microscope. The use of theappearance of medium-sized Gephyrocapsa spp. allows one to definemore easily the CN13b/CN13a boundary. This event occurs in thesame stratigraphic interval as the FO of G. caribbeanica (after the LOof Discoaster brouweri and before the LO of Calcidiscus macin-tyrei). The appearance of medium Gephyrocapsa spp. represents thebest approximation of the Pliocene/Pleistocene boundary, as definedin the boundary stratotype section of Vrica (southern Italy) (AguirreandPasini, 1985).

239


Not zoned

20 14b

OTHER EVENTS

rθθntrance medium/× Gephyrocapsa spp.

- ^ – large Gephyrocapsa spp.

A, large Gephyrocapsa spp./\ ~rr- c. macintyreimedium Gephyrocapsa spp.

24X

25X

2βX

27X

28X

29X

30X

31X

32X

33X

34X

35X

36X

38X

39X

40X

41X j

Sphenolithus spp.

A. amplificus

A. amplificus circularreticulofenestrids

846D

25X

26X

Absence interval ofR.pseudoumbilicus

=fe= M. convallis

C. miopelagicus

D. kugleri %

C. nitescensC. floridanus

common D. signus

Acme of D. deflandreino D. signus

Figure 5. Chronostratigraphy and calcareous nannofossil biostratigraphy atSite 846. "Striped" areas at zonal boundaries represent intervals (samplespacing) within which biostratigraphic events occur.

OTHER EVENTS

reentrance mediumGephyrocapsa spp.



C. macintyrei


D. variabilis

Sphenolithus spp.

C acutus

Strong reworking of lowerMiocene nannofossils

Figure 6. Chronostratigraphy and calcareous nannofossil biostratigraphy at

Site 847. Notation as specified in Figure 5.

The extinction of Helicosphaera sellii is confirmed to be an unre-liable event in the Pacific, based on the diachroneity clearly demon-strated by Backman and Shackleton (1983) and Raffi et al. (1993),who compared Pacific Ocean records with those from the AtlanticOcean and the Mediterranean Sea (e.g., Takayama and Sato, 1987;Rio et al., 1990b). In the Leg 138 sites, this species is rare and is

240



Event

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster pentaradiatusT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricusT Amaurolithus primusT Ceratolithus acutusB Ceratolithus rugosusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval /?. pseudoumbilicusB Amaurolithus primusT Minylitha convallisB Discoaster berggrenii (?)B absence interval /?. pseudoumbilicusB Discoaster loeblichiiB Minylitha convallisT Discoaster hamatusT Discoaster neohamatusB Discoaster hamatusT Coccolithus miopelagicus

Interval(cm)

848A-1H-4, 60/1H-5, 60848B-3H-l,70/3H-l,80848B-3H-3, 60/3H-3, 120848B-3H-5, 120/3H-6, 40848C-3H-5, 20/3H-5, 40848C-3H-5, 40/3H-5, 50848C-3H-6, 120/3H-7, 10848B-4H-4, 40/4H-4, 80848B-4H-4, 40/4H-4, 80848B-4H-5, 23/4H-5, 85848B-4H-CC/5H-1,42848B-5H-1,95/5H-1, 120848B-5H-2, 50/5H-2, 120848B-5H-2, 127/5H-3, 62848B-6H-l,75/6H-2, 35848B-6H-4, 35/6H-4, 65848B-6H-5, 35/6H-5, 75848B-7H-l,50/7H-l,95848C-7H-2, 25/7H-2, 50848C-7H-6, 145/7H-7, 20848C-8H-6. 40/8H-6, 60848C-8H-6, 80/8H-6, 140848B-9H-5, 85/9H-5, 119848B-9H-5, 119/9H-6,40848B-9H-6, 119/9H-7, 10848B-9H-6, 119/9H-7, 10848B-10H-2, 120/10H-2, 150848C-10H-1, 100/lOH-l, 125848C-10H-2, 75/10H-2, 100848C-10H-5, 3O/1OH-5, 50848C-10H-5, 5O/1OH-5, 75

Depth(mcd)

5.3-6.816.35-16.4019.25-19.8522.85-23.5524.22-24.4224.42-24.5226.72-27.1230.45-30.8530.45-30.8531.78-32.4035.52-36.7737.30-37.5538.35-39.0539.12-39.9746.65-47.7550.75-51.0552.25-52.6556.90-57.3562.54-62.7969.74-69.9975.00-75.2079.94-80.5485.15-85.8985.89-86.6086.99-87.4086.99-87.4091.50-91.8093.32-93.5794.57-94.8298.62-98.8298.82-99.03



EventInterval

(cm)Depth(mcd)

Interval(cm)

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB Ceratolithus rugosusT Triquetrorhabdulus rugosusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval R. pseudoumbilicusB Amaurolithus primusB Discoaster berggreniiB absence interval R. pseudoumbilicusB Minylitha convallisT Discoaster hamatusB Discoaster neohamatusB Discoaster hamatusT Coccolithus miopelagicusT c Discoaster kugleri

849B-2H-3, 120/2H-4, 60849B-3H-6, 60/3H-CC849B-4H-2, 120/4H-3, 60849B-4H-6, 120/4H-7, 52849B-5H-2, 60/5H-2, 120849B-5H-4, 60/5H-4, 120849D-5H-6, 130/5H-7,0849B-7H-5, 60/7H-6, 60849C-7H-6, 60/7H-6, 90849B-10H-5, 60/1OH-6, 60849C-10H-3, 120/10H-3, 140849B-15X-5, 120/15X-6, 60849B-I7X-1, 120/17X-2, 60849B-17X-1, 120/17X-2, 60849B-18X-4, 60/18X-4, 120849B-21X-5, 120/21X-6, 60849B-22X-1, 120/22X-2, 60849B-23X-l,60/23X-l, 120849B-27X-3, 60/27X-4, 60849B-29X-4, 40/29X-4, 80849B-32X-5, 120/32X-6, 92849B-32X-5, 120/32X-6, 92849B-32X-CC/33X-1, 60849B-33X-4, 45/33X-4, 130849B-34X-3, 60/34X-4, 61849B-34X-5, 62/34X-6, 56849B-36X-CC/37X-1.60

12.85-13.7528.35-29.2633.45_34.3539.45-40.2743.75-44.3546.75-47.3559.25-59.4570.25-71.7577.05-77.35

101.10-102.60105.15-105.35156.80-157.70173.85-174.75173.85-174.75189.35-189.95224.40-225.30229.35-230.25239.30-239.90280.70-282.20304.00-304.40337.80-339.02337.80-339.02340.37-341.80346.75-347.60355.40-356.91358.42-359.86381.86-382.25

Depth(mcd)

849C-4H-6, 100/4H-6, 140849C-5H-1, 13/5 H-1,80

45.10-45.5047.28^7.95


scattered in the upper part of its range. Helicosphaera sellii shows apeak in abundance close to (just above and just below) the appearanceof medium Gephyrocapsa spp.

Pliocene

All the zones of Bukry (1973) and Martini (1971) can be differen-tiated in the Pliocene sections at all sites, except Sites 844 and 845,where nannofossil barren zones represent episodes of severe carbon-ate dissolution. Subzonal boundaries within Zones CN12 and CN11

are not easily recognized because of the scarcity of markers such asdiscoasterid and ceratolithid species.

Zone CN12 (Zones NN18-NN17-NN16)

The LO of Discoaster brouweri (CN13/CN12 and NN19/NN18boundary) was recorded in all Leg 138 sites. The boundary is charac-terized by the LO of Discoaster triradiatus, a species that occurs inabundance only at the end of the range of D. brouweri (Takayama,1970). This event was not clearly detected because D. triradiatus is

241


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14b

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13b

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Magneto848A stratigraphy OTHER EVENTS

1H

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reentrance medium=^= Gephyrocapsa spp.


= ^ = = ^ = C macintyrei

L̂ -̂ -i Sphenolithus spp.

i ^ p C. acutus

=^= ^. amplificus

=^= /\. amplificus

\

\ Absence interval of

^ R.pseudoumbilicusM. convallis ^=^= >

D. öe//us gr. % M• convallis5 10 1,5 = ^ 3

/×^×^ D. neohamatus

====-- C. miopelagicus

Figure 7. Chronostratigraphy and calcareous nannofossil biostratigraphy atSite 848. Notation as specified in Figure 3.

never present abundantly and consistently in the sections studied.This pattern of distribution also prevents the recognition of the in-crease in proportion (>20%) of D. trirαdiαtus relative to D. brouweri,a datum introduced by Backman and Shackleton (1983).

The successive and closely spaced extinctions of Discoαster pen-tαrαdiαtus and Discoαster surculus define the boundaries betweenSubzones CN12d-CN12c (Zones NN18-NN17) and SubzonesCN12c-CN12b (Zones NN17-NN16), respectively. The two eventsare often recorded together (Backman and Pestiaux, 1986; Rio et al.,1990b). In the Leg 138 sites, D. surculus and D. pentαrαdiαtus aregenerally rare or absent in the late Pliocene section. In all sites exceptSites 852 and 853, D. pentαrαdiαtus, when present, leaves the strati-graphic record together with D. surculus.

Note that the highly variable distribution and generally low abun-dance or absence of discoasterids, and thus their unreliability as bio-stratigraphic markers in the upper Pliocene, are particularly evident atsites strongly influenced by upwelling (such as Sites 849,850, and 851in the western transect and Sites 844, 845, and 846 in the eastern tran-sect). Similar low abundances and scattered occurrences of other markerspecies of discoasterids and other nannofossils have been observed atvarious levels in Leg 138 holes. These distribution patterns may havebeen influenced by varying productivity pressures, as suggested byChepstow-Lusty et al. (1989, 1992), who showed that regions of highproductivity are associated with lower discoasterid abundance.

Discoαster tαmαlis becomes extinct before D. surculus, thus defin-ing the boundary between Subzones CN12b and CN12a. In the mate-rial studied, D. tαmαlis, together with D. αsymmetricus and membersof the Discoαster vαriαbilis group, have an irregular distribution, beingmore common in sites located out of the equatorial zone.

In the lower part of Subzone CN12a (Zone NN16), we record theLO of the last representatives of the genus Sphenolithus (S. αbies and

S. neoαbies). This event occurs shortly above the LO of Reticulofenes-trα pseudoumbilicus and allows us to divide the CN12a Subzonefurther, defining Subzone "CN12aA" as proposed by Bukry (1991). Asimilar extinction of the last sphenoliths with respect to the final occur-rence of R. pseudoumbilicus has been recorded in oceanic sediments(Backman and Shackleton, 1983; Rio et al, 1990a; Bukry, unpubl. data,1990; Bukry, 1991) and by Rio et al. (1990b) in the Mediterranean.

Zone CN11

The LO of R. pseudoumbilicus, which defines the boundary ofZones CN12-CN11 (NN16-NN15), represents an easily recognizedevent at all the sites investigated except Site 845, where part of thePliocene interval contains no nannofossils. To recognize this bound-ary, we refer to the taxonomic concept of R. pseudoumbilicus asexpressed by Raffi and Rio (1979) and Backman and Shackleton(1983), and we consider the final exit of the large specimens of R.pseudoumbilicus (>7 µm). This event occurs in the upper part ofChron 2Ar (late Gilbert Chron) at Sites 848, 851, 852, and 853, as itdoes in many other areas (Rio, 1982; Backman and Shackleton, 1983;Rio et al, 1990a, 1990b).

Boundary CNllb/CNlla is defined by the beginning of the com-mon and continuous occurrence of D. αsymmetricus, a species thathas a generally scattered distribution in the upper Miocene of theeastern equatorial Pacific, as well as other oceanic regions (Bukry,1973), such as the western Indian Ocean (Roth, 1974; Rio et al.,1990b). Following Okada and Bukry's zonation, the D. αsymmetricusevent occurs above the LO of Amαurolithus primus and Amαurolithustricorniculαtus, which defines the boundary between Zones CN11and CN10. In the lower Pliocene section of the eastern equatorialPacific, the recognition of these two events has been hampered insome sequences because of (1) the low abundance of the two markers,(2) preservation problems, and (3) sediment reworking, which isparticularly evident at western transect Sites 852 and 853. The con-tinuous presence of D. αsymmetricus and the disappearance of A.primus have been recognized at Sites 846, 847, 848, 851, 852, and853; generally, the events are close each other.

Regarding the extinction datums of some ceratolithids species,such as representatives of the genus Amαurolithus (A. primus, A.delicαtus, and A. tricorniculαtus) and C. αcutus, note that data fromvarious authors (see Berggren et al., 1985) show some discrepancy inthe age estimates for these events. The discrepancy may be attributedto a poorly documented extinction pattern of these ceratolithids and/or from usage of different criteria when identifying the species. Mis-identification occurs in samples that contain nannofossils with calciteovergrowth and when specimens of different Amαurolithus and Cerα-tolithus species possess intergrade morphologic features. This is thecase in most of the lower Pliocene sequences recovered during Leg138. Ceratolithid species are irregularly distributed and are not easilydifferentiated because of the presence of overgrowth and intergrademorphotypes. Furthermore, at sites where ceratolithids are commonand well preserved in the lower Pliocene interval (as at Sites 851,852,and 853), it was not possible to obtain well-documented extinctionpatterns because of mixing in nannofossil assemblages caused bystrong reworking of upper Miocene sediments.

The presence of Discoαster tristellifer within Zone CN11 is note-worthy because it is particularly common in the interval correspond-ing to the lower range of D. αsymmetricus.

Zone CN10 (Zones NN13, pαrs-NNU)

Boundary CNlOc/CNIOb (NN12/NN13) is defined by the FO ofCerαtolithus rugosus. This boundary is recognized at the first appear-ance of rare but typical specimens of C. rugosus, although the presenceof forms intergrading between C. rugosus and its ancestor Cerαto-lithus αcutus sometimes makes the recognition of the event difficult.This difficulty may explain the different position of the C. rugosusevent with respect to Subchron 3n.4n (Thvera). In Site 852, the F0

242


Table 9. Position of calcareous nannofossil events at Hole 850B.

Event

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusT Amaurolithus primus (?)T Ceratolithus acutus (?)B Ceratolithus rugosusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusT circular reticulofenestridsB Amaurolithus amplificusT absence interval R. pseudoumbilicusB circular reticulofenestridsB Amaurolithus primusB Discoaster berggreniiB Discoaster loeblichiiB absence interval R. pseudoumbilicusT Discoaster hamatusB Discoaster neohamatusB Minylitha convallisB Discoaster hamatusT Coccolithus miopelagicusT c Discoaster kugleri

Interval(cm)

1H-4, 120/1H-5, 632H-4, 41/2H-4, 1203H-1, 120/3H-2, 543H-3, 120/3H-4, 543H-5, 120/3H-6, 623H-CC/4H-1.514H-5, 50/4H-5, 655H-6, 20/5H-7, 656H-l,20/6H-l,657H-CC/8H-l,608H-2, 80/8H-2, 10010H-l,59/10H-6, 15011X-CC/12X-1.7012X-l,70/12X-2, 12013X-4, 120/13X-5, 6014X-7, 30/15X-1,3517X-6, 58/18X-1,4219X-3, 60/19X-4, 6019X-6, 60/19X-7, 3420X-7, 43/2IX-1,6023X-6, 61/23X-7, 2023X-7, 20/24X-1.6126X-3, 58/26X-4, 6029X-6, 55/29X-7, 313OX-3, 60/30X-4, 6033X-2, 60/33X-3, 6033X-2, 60/33X-3, 6033X-3, 60/33X-4, 1436X-5, 47/36X-CC37X-1.60/37X-2, 6140X-7, 20/4IX-1,60

Depth(mcd)

8.70-9.6319.31-20.1124.85-25.6927.85-28.6930.80-31.7733.39-36.3642.45-42.6053.10-55.0556.20-56.6576.66-77.8579.55-79.7596.84-105.23

114.65-115.95115.95-116.45130.55-131.45144.10-144.50171.00-173.17185.55-187.05190.05-191.29201.28-201.85227.84-228.95228.95-230.86253.13-254.65286.10-287.36300.92-302.42318.75-320.25318.75-320.25320.25-321.29352.05-352.42355.85-357.36393.35-394.45


of Ceratolithus rugosus is recorded above the Thvera (in Subchron3n.3r); whereas in Sites 848 and 853, it apparently occurs within theThvera. In this same position, the event was detected by Backman andShackleton (1983) in the central equatorial Pacific, and by Rio et al.(1990a) in the western equatorial Indian Ocean. In some sections (Sites846, 848, 850, and 853), we detected an overlap of C. rugosus withspecimens of C. acutus. Although this observation agrees with thatmade by Rio et al. (1990a), most authors (see Berggren et al., 1985) donot report such an overlap.

The first appearance of C. acutus corresponds to the CNlOb/CNlOa boundary, as defined by Bukry (1973) together with the LOof Triquetrorhabdulus rugosus. Bukry noted that Subzone CNlOa isof very short duration and can easily go undetected in sections withcompressed sedimentation rates. In the Leg 138 sections, the FO ofC. acutus has been consistently recorded at all sites. It occurs simul-taneously with the extinctions of T. rugosus and T. rioensis at thosesites where the two triquetrorhabdulids are present. We note that, inthe Leg 138 sections, triquetrorhabdulids are generally rare in theirfinal range, and have a discontinuous distribution within their range,from the middle Miocene upward.

The Subzone CNlOa represents the best approximation for therecognition of the Miocene/Pliocene boundary by means of calcare-ous nannofossils.

Miocene

The zonal boundaries of the Miocene, as defined by Martini (1971)and Bukry (1973), have been recognized at all sites, with two excep-tions: (1) the base of Zone CN6 (NN8) at western transect Sites 849and 850; and (2) the top of CN7 (NN9) at eastern transect Site 845. Inthe upper Miocene, supplementary biostratigraphic events allow us todivide the rather long time interval (about 1.7 m.y.) corresponding toSubzone CN9b. Subzones CN7a-CN7b could not be differentiated.

Zone CN9 (NN11)

This zone is defined by the total range of the two related taxaDiscoaster quinqueramus and Discoaster berggrenii. The LO of D.

i o

OTHER EVENTS

reentrance medium^ Gephyrocapsa spp.

=^= large Gephyrocapsa spp.

^ ^ large Gephyrocapsa spp.=^F= C. macintyrei


Sphenolithus spp.

A. amplificus

circular reticulofenestrids

Absence interval ofR. pseudoumbilicus

D. bellus gr. %

5 10 15 20

D. neohamatus

C. miopelagicus

• * ~ D. calcaris

D. kugleri %


243


7///A

Not zoned

20 I 14b

12b

12aB

OTHER EVENTS

reentrance medium^ Gephyrocapsa spp.

" X " large Gephyrocapsa spp.

r^h v C. macintyreimedium Gephyrocapsa spp.

Sphenolithus spp.

C. acutus

A. amplificus

A. amplificus

r circular reticulofenestrids

Absence interval offt pseudoumbilicus

^ —^― D. neohamatus

M. convallis

C. miopelagicus

: 3 ^ D. calcaris

D. kugleri %

10 20 30


quinqueramus is used to mark the top of Zones CN9 and NN11, as D.berggrenii becomes extinct before D. quinqueramus (Bukry, 1973;Rio et al, 1990a). In our sections, the LO of D. quinqueramus waseasily detected; it follows the extinction of D. berggrenii, as it does inother regions.

Boundary CN9b/CN9a is defined by the entrance in the strati-graphic record of the genus Amaurolithus, namely, by the F 0 of A.primus. Although ceratoliths are generally rare, their appearance atthe base of Subzone CN9b was easily detected. The earlier forms,belonging to A. primus (see "Taxonomic Notes" section, this chapter),evolved rapidly, leading to the appearance of A. delicatus (whichoccurs shortly after the F0 of A. primus). Intergrading forms betweenthe two species are frequently found. Amaurolithus tricorniculatus,often recorded as appearing in Subzone CN9b (i.e., Berggren et al.,1985), is very rare and scattered in the material studied. Anotherspecies, A. amplificus, occurs after the FO of A. delicatus. The pres-ence of intergrade forms between Triquetrorhabdulus extensus-T.rugosus and A. amplificus just below the appearance level of A.amplificus possibly proves the Phylogenetic relationship betweenTriquetrorhabdulus and Amaurolithus previously suggested by otherauthors (Gartner, 1967; Perch-Nielsen, 1977, 1985).

The same observations on the A. amplificus distribution patternhave been made in the western equatorial Indian Ocean in Leg 115successions (Rio et al., 1990a). In the eastern equatorial Pacific, A.amplificus becomes extinct within the upper part of Subzone CN9b,in agreement with the previous findings of Bergen (1984) and Rio etal. (1990a). Compared to the Leg 138 paleomagnetic stratigraphy, theFOs and LOs of A. amplificus occur at the base and the top of Chron3An, respectively, and turn out to be isochronous events with recordsfrom the western equatorial Indian Ocean (Rio et al., 1990a). Thestratigraphic relationship of the lowest and highest occurrences of A.amplificus relative to the lowest occurrence of A. primus and thehighest occurrence of D. quinqueramus allows us to divide the Okadaand Bukry (1980) Subzone CN9b further, as suggested by Rio et al.(1990a). We can use these additional events to divide Subzone CN9binto three biostratigraphic units, defined as follows:

CN9bA: from the F0 of A. primus to the FO of A. amplificus;CN9bB: from the F 0 of A. amplificus to the LO of A. amplificus;

andCN9bC: from the LO of A. amplificus to the LO of D. quinqueramus.

The definitions and occurrences of these three new biostratigraphicunits are given in Appendix B in addition to any related remarks.

Within Subzone CN9b, the discoasterids are represented by sev-eral species, mainly D. quinqueramus, D. berggreni, D. surculus, andD. pentaradiatus. We note that in all CN9 zonal intervals, discoast-erids fluctuate in abundance and their preservation varies widely inthe different successions, sometimes preventing a precise identifica-tion of marker and secondary species. Following Rio et al. (1990a),we searched for a large form of Discoaster aff. brouweri (tabulated inLeg 115 range charts as Discoaster sp. 2), which seems to have arestricted stratigraphic distribution within the lower part of CN9b andthe upper part of CN9a in the western equatorial Indian Ocean. In ourcores, this discoasterid does not give the same clear biostratigraphicsignal as in the Indian Ocean.

Regarding the CN9a and NN1 la subzones, we use the appearanceof D. berggrenii to define their lower boundary, as indicated by Bukry(1973). In fact, the D. berggrenii morphotypes (the five-rayed dis-coasterids with a stellar knob in a distinct central area; Plate 2, Figs.1-2) appeared slightly earlier than D. quinqueramus morphotypessensu Bukry (1971) (Plate 2, Figs. 4-5). Although it is useful to distin-guish them for biostratigraphic purposes, D. berggrenii and D. quin-queramus have been considered as a single taxonomic unit in the rangecharts, as intergrades between them are common (Plate 2, Fig. 3). Inthe eastern equatorial Pacific, the appearance of D. berggrenii occurs


reentrance mediumc a Gephyrocapsa spp.

~^"~ large Gephyrocapsa spp._ ^ _ ~ ^ ~ C. macintyrei


Sphenolithus spp.

strong reworking of

nannofossils

A. amplificus

A. amplificus

Absence interval ofR. pseudoumbilicus

M. convallis

D. neohamatus

D. bellus g

calcaris =^= c. miopelagicus

D. kugleri %


in the lowermost part of Chron 4r (Subchron 4r.2r), and it is iso-chronous with the western equatorial Indian Ocean (Rio et al, 1990a).The morphological development of D. berggrenii and D, quinquerα-mus was gradual; these discoasterids probably evolved from otherfive-rayed forms that belong to the D. bellus group (Bukry, 1973), andintergrades between the two discoasterid groups are common.

Within Subzone CN9a, we record the occurrence, but in low abun-dance, of D. pentαrαdiαtus and D. surculus. Typical D. bellus andMinylithα convallis, present in the underlying intervals, disappear inthe upper part of this subzone, whereas the rare D. neohamatus, D.prepentaradiatus, and D. loeblichii have their LOs in the lower part.Similar co-occurrences of D. loeblichii and D. berggrenii have beennoted previously by Rio et al. (1990b) in the western equatorial IndianOcean, and by Proto-Decima et al. (1978), Mazzei et al. (1979), andParker et al. (1985) in the Atlantic Ocean.

AGE

JS

mid

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lat

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NN

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19

18

17

16

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13

12

11b

11a

10

9

8

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ned14b

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120

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12aB

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10c

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10a

9bC= "

9bB

9bA

9a

8b

8a

7

β

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1

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11

10 -

30 -

40 -

5 0 "

60 -

70 -

80 -

90 -

100 -

110 -

120 "

Mac

852B S t r a "

1H

2H

3H

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4H

5H

6H

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=

8H

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9H

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12H

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852D

852C

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netography OTHER EVENTS

I1

••

m—

m

•

z

m

4:>'/

i1—a1

z\ reentrance mediumL——' Gephyrocapsa spp.

^ large Gephyrocapsa spp.

I-Δ-, medium Gephyrocapsa spp.v C. macintyrei

' v i Sphenolithus spp

|

H strong reworking of1 upper Miocene

nannofossils

iv c. acufus

i ^ i 4. amplificus

r ^ ^ i A. amplificus

^ Absence intervalS^ of R. pseudoumbilicus

s^ ' ^ D. pentaradiatus

M. convallis

^——' c ^ = D. neohamatus

P• calcaris ç miopelagicus


Zone CN8 (NN10)

The upper part of this biostratigraphic interval is characterized bythe presence of Discoaster loeblichii, used by Bukry (1973), togetherwith D. neorectus, as a marker for dividing Zone CN8 into SubzonesCN8b and CN8a. D. loeblichii was recorded with great consistencyin almost all Leg 138 sections, except those where the scarcity and/orpoor preservation prevented recognition. At Sites 844,846, 849,850,851, 852, and 853, we differentiate the two subzones, although D.neorectus was found only sporadically.

Close to the F0 of D. loeblichii, we observed the lowest specimensof D. pentaradiatus. Other components of the discoasterid assemblageof CN8 are abundant or common (e.g., D. brouweri and D. bellus gr.)and scarce (Zλ neohamatus). In the lower part of the zone, where D.bellus gr. dominates, D. prepentaradiatus and D. calcaris are presentin low abundances.

The lower boundary of Zone CN8 is defined by the LO of Dis-coaster hamatus. This form is generally rare in the uppeπnost part ofthe range; however, the event can be consistently recognized in Leg138 material, with the exception of Site 845, where the correspondingstratigraphic interval is barren of nannofossils.

Just above the extinction of D. hamatus, the F 0 of M. convallis isrecorded. This event can be used to recognize the boundary betweenZones CN8/CN7 when D. hamatus is too rare (Rio et al, 1990a). Inthe Leg 138 material, the range of M. convallis, which encompassesZones CN8 and CN9a, shows variability. At the eastern transect sites,the species is generally rare and occurs sporadically, whereas it iscontinuously present at other sites. This irregular distribution of M.



Event

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricus (?)T Amaurolithus primusB Ceratolithus rugosusT Ceratolithus acutusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval R. pseudoumbilicusB Amaurolithus primusT Minylitha convallisB Discoaster berggreniiB Discoaster loeblichiiB absence interval R. pseudoumbilicusT Discoaster hamatusB Minrlitha convallisB Discoaster neohamatusB Discoaster hamatusT Coccolithus miopelagicusB Catinaster coalitusT c Discoaster kugleriB c Discoaster kugleri (?)

Interval(cm)

851B-IH-5, 95/1H-5, 140851E-2H-5, 15/2H-5, 45851C-2H-6, 25/2H-6, 70851B-3H-5, 30/3H-5, 60851B-3H-6, 100/3H-7, 46851C-3H-3, 70/3H-3, 140851B-4H-3, 100/4H-3, 150851B-5H-7, 65/5H-CC851B-5H-7, 65/5H-CC851B-7H-3, 58/7H-4, 60851B-7H-6, 25/7H-6, 60851B-7H-7, 45/8H-1.60851B-09H-2, 60/9H-3, 60851B-10H-2, 25/10H-2, 60851B-10H-2, 25/10H-2, 60851B-11H-2, 60/11H-2, 110851E-11H-6, 44/11H-6, 90851B-13H-3, 68/13H-4, 70851B-15H-6. 69/15H-7, 63851B-18X-2, 60/18X-3, 60851E-19X-4, 25/19X-4, 60851B-19X-1, 25/19X-1, 58851B-20X-CC/21X-l,25851B-22X-3, 60/22X-4, 60851B-22X-7, 31/23X-1.6O851E-25X-4, 25/25X-4, 56851B-25X-1.60/25X-2, 60851B-25X-4, 120/25X-5, 60851B-28X-2, 140/28X-3, 4085IB-28X-3, 40/28X-3, 80851B-29X-5, 71/29X-CC851B-32X-4, 55/32X-4, 120851B-34X-3, 120/34X-4, 65

Depth(mcd)

6.95-7.4019.15-19.4523.75-24.2027.45-27.7529.65-30.6131.25-31.9535.90-36.4051.95-52.1951.95-52.1967.03-68.5571.20-71.5572.90-75.0086.50-88.0097.20-97.5597.20-97.55

108.25-108.75116.24-116.70131.48-133.00157.54-158.98190.35-191.85199.25-199.60199.40-199.68224.88-227.90240.45-241.95246.16-252.65275.05-275.36276.10-277.60281.20-282.10314.30-314.8

314.80-315.20330.21-331.23358.80-359.45382.65-383.60



Event

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.T large Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster pentaradiatusT Discoaster surculusT Discoaster tamalisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricus (?)T Amaurolithus primusB Ceratolithus rugosusT Ceratolithus acutusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval R. pseudoumbilicusB Amaurolithus primusB Discoaster berggreniiB Discoaster pentaradiatusB Discoaster loeblichiiB absence interval R. pseudoumbilicusB Minylitha convallisT Discoaster hamatusB Discoaster neohamatusB Discoaster hamatusT Coccolithus miopelagicus

Interval(cm)

852B-1H-3, 110/1H-4, 60852B-2H-2, 65/2H-2, 140852B-2H-4, 65/2H-4, 140852B-2H-6, 140/2H-7, 40852B-3H-1.65/3H-1, 140852B-3H-1.65/3H-1, 140852B-3H-2, 65/3H-2, 140852B-3H-7, 35/3H-CC852B-4H-2, 65/4H-2, 140852B-4H-3, 65/4H-3, 140852B-5H-l,80/5H-l, 135852B-5H-2, 80/5H-2, 140852B-5H-3, 30/5H-3, 80852B-5H-7, 80/6H-1.80852B-6H-3, 135/6H-4, 35852B-6H-4. 35/6H-4, 80852B-7H-1,71/7H-I, 127852B-7H-3, 75/7H-3, 140852B-7H-CC/8H-1, 120852B-8H-CC/9H-l,70852B-9H-l,70/9H-2, 70852B-9H-6, 140/9H-7, 708 5 2 B - 1 1 H - 1 , 4 0 / 1 1 H - 1 , 8 0852B-11H-4, 145/11H-5, 40852B-11H-5, 40/11H-5, 80852B-11H-6, 140/11H-7, 83852B-12H-2. 70/12H-2, 110852B-12H-2, 70/12H-2, 110852B-12H-2, 120/12H-3, 143852C-13X-6, 40/13X-6, 50852C-13X-6, 50/13X-6, 90

Depth(mcd)

4.30-5.3011.30-12.0514.30-15.0518.05-18.5519.85-20.7019.85-20.7021.35-22.1028.55-29.2932.45-33.2033.95-34.7041.05-41.6042.55^3.1543.55-44.0550.05-52.6556.20-56.7056.70-57.1562.96-63.5266.00-66.6572.21-73.8582.77-84.6084.60-86.1092.80-93.60

105.80-106.20111.35-111.80111.80-112.20114.30-115.23118.30-118.70118.30-118.70118.80-119.03128.80-128.90128.90-129.30


convallis seems to be influenced by variability in either the preserva-tion of nannofossil assemblages or the productivity conditions, in thatthe form is rare and discontinuous at sites influenced by upwelling(i.e., 844 and 845).

A feature of the nannofossil assemblage of Zone CN8 is the turn-over detected within the placolith group, namely, an almost total dis-

appearance of large specimens (>7 µm) of Reticulofenestra pseudo-umbilicus. This placolith enters the stratigraphic record in the middleMiocene, close to the top of Zone CN4 (NN5), and exits in the middlepart of the Pliocene. The interval of absence of R. pseudoumbilicus,recorded in all the stratigraphic successions of Leg 138, starts fromthe lower upper Miocene (Zones CN8-NN10) and extends to the



Event

T Pseudoemiliania lacunosaReentrance medium Gephyrocapsa spp.B large Gephyrocapsa spp.T Calcidiscus macintyreiB medium Gephyrocapsa spp.T Discoaster brouweriT Discoaster pentaradiatusT Discoaster surculusT Discoaster tamatisT Sphenolithus spp.T Reticulofenestra pseudoumbilicusB c Discoaster asymmetricusT Amaurolithus primusT Ceratolithus acutusB Ceratolithus rugosusB Ceratolithus acutusT Discoaster quinqueramusT Amaurolithus amplificusB Amaurolithus amplificusT absence interval Å. pseudoumbilicusB Amaurolithus primusB Discoaster berggrenii

Interval(cm)

853B-1H-2, 120/1H-2, 130853B-1H-3, 80/1H-CC853B-2H-1, 120/2H-2, 65853B-2H-2, 65/2H-2, 120853B-2H-2, 140/2H-3, 25853B-2H-3, 25/2H-3, 65853B-2H-5, 65/2H-5, 120853B-2H-6, 120/2H-6, 140853B-2H-7, 30/3H-1.65853B-3H-4, 120/3H-4, 140853B-3H-4, 120/3H-4, 140853B-4H-1.65/4H-2, 65853B-4H-3, 140/4H-4, 25853B-4H-5, 140/4H-6, 80853B-4H-6, 80/4H-CC853B-5H-1.65/5H-1, 120853B-5H-1,65/5H-1, 120853B-5H-5, 65/5H-5, 120853B-6H-4, 27/6H-4, 65853B-6H-5, 65/6H-6, 65853B-7H-2, 147/7H-3, 30853D-7H-4, 120/7H-4, 140

Depth(mcd)

2.70-2.803.80-4.255.45-6.406.40-6.957.15-7.507.50-7.90

10.90-11.4512.95-13.1513.55-15.3520.40-20.6020.40-20.6021.95-23.4525.70-26.0528.70-29.6029.60-29.8932.40-32.9532.40-32.9538.40-38.9546.37-46.7548.25^9.7554.42-54.7569.47-69.67


lower part of Subzone CN9b. The bottom of this interval is charac-terized by the absence of all the representatives of genus Reticu-lofenestra, including the small (<7 µm) morphotypes; large forms ofR. pseudoumbilicus reenter, with low abundance, just below the ap-pearance level of A. amplificus. This temporary disappearance hasalso been observed in the western equatorial Indian Ocean (Young,1990; Rio et al., 1990a), and was interpreted as a regional strati-graphic feature, probably reflecting oceanographic-climatic instabil-ity (Rio et al., 1990a). Indications of the wider geographical extent ofthis "feature" in the R. pseudoumbilicus range are reported and dis-cussed in Raffi et al. (this volume).

Zone CN7 (NN9)

The total range of Discoaster hamatus defines the CN7 (NN9)zonal interval. Despite the low abundance of discoasterids at somesites (Sites 846, 849, and 850) and the presence of barren intervalsobserved at others (Sites 844 and 845), the consistent occurrence ofD. hamatus allows us to recognize this zonal interval in the materialwe investigated. Close to the F0 of D. hamatus, other five-rayeddiscoasterids appear. They have straight, tapering arms and belong tothe Discoaster bellus group. Their presence is useful in monitoringthe base of the zone where D. hamatus is rare.

We have not divided Zone CN7 into two subzones ("a" and "b"),as suggested by Bukry (1973) on the basis of the first evolutionaryappearance of C. calyculus from C. coalitus. In fact, in the Leg 138sections, the genus Catinaster is represented mainly by C. coalitus,whereas very rare and scattered specimens of C. calyculus have beenrecorded in intervals below the appearance of D. hamatus (i.e., atSites 844, 845, and 846). This finding agrees with the results from thewestern equatorial Indian Ocean (Rio et al., 1990a) and with theobservation of Perch-Nielsen (1985), who questioned the subdivisionof Zone CN7. Within this zone, the D. variabilis group and D. bolliiare common; in the upper part, the last, rare representatives of D.exilis disappear, and D. neohamatus appears.

Zone CN6 (NN8)

The short CN6 (NN8) zonal interval is defined by the F0 of D.hamatus (top) and the FO of C. coalitus plus the LO of Discoasterkugleri (bottom). In the material investigated, representatives of Cat-inaster are irregularly distributed. The zone is clearly recognized atSites 844, 845, and 851, where C. coalitus is well represented, even

AGE

middle-late Pleist

Pleist.

Φ

Φ Φ

I

C

ea

rlio

ce

0.

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OTHER EVENTS

i- -, Gephyrocapsa spp.^\ large Gephyrocapsa spp.


14strong reworking ' ^ y ' Sphenolithus spp

of upper Miocene

i 1 C. acufusV

r-—i A. amplificus

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Figure 12. Chronostratigraphy and calcareous nannofossil biostratigraphy at

Site 853. Notation as specified in Figure 3.

though C. calyculus is rare and scattered. At Site 846, the marker C.coalitus is very rare; and at Sites 849 and 850, it is found only in thebiozone above (CN7). At these sites, fluctuating but low abundancesalso characterize the distributions of the other discoasterids. We failedto find the rare and scattered last specimens of D. kugleri, recorded atSites 844 and 845, close to the C. coalitus appearance. These gener-ally low abundances of discoasterids have been observed all along theinterval encompassing Subzone CN5b (upper part)-CN6-CN7, cor-responding to the time interval from 10.8 to 9.5 Ma. As already notedat other levels in the upper Miocene (Zone CN9) and Pliocene, high-productivity conditions influence these equatorial successions, thenannofossil content, and discoasterid abundance, as suggested byChepstow-Lusty et al. (1989, 1992).

247


In the upper part of Zone CN6, and across the boundary betweenZones CN7/CN6, a turnover in the nannofossil assemblage occurs,mainly within discoasterids. Besides the appearance of D. bellus gr.,which becomes a dominant species in the overlying intervals, D.calcaris has a short and distinctive range. D. exilis declines graduallytoward the extinction, and D. brouweri has a more continuous distri-bution. According to Bukry (1973), we recorded the extinction ofCoccolithus miopelagicus within this zone, just below the upperboundary. In the eastern equatorial Pacific, this event seems to occurslightly later than at the mid-latitudes of the northern Atlantic(Olafsson, 1991) (see discussion in Raffi et al., this volume).

We detected a remarkable decline in the preservation state andcomposition of the nannofossil assemblages at Leg 138 sites in theinterval from the top of CN5b to CN7. This interval, which is charac-terized by reduced sedimentation rates and increased carbonate dis-solution, is comparable to other sites in the Pacific (Mayer et al.,1986) and may represent a dissolution event related to a general reor-ganization of Pacific circulation (Mayer, Pisias, Janecek, et al., 1992).

Zone CN5 (Zones NN7 and NN6)

The top of this zone is defined by the FO of C. coalitus and the LOof Discoaster kugleri. The bottom is defined by the LO of Spheno-lithus heteromorphus. The zone spans a rather long time interval, over2.5 m.y. In addition, Zone CN5 is divided into two subzones (CN5band CN5a) by the F0 of Discoaster kugleri (Bukry, 1973), which alsodefines the NN7/NN6 zonal boundary. The recognition of this bound-ary has been debated, as D. kugleri is considered a weak marker bymany authors (e.g., Gartner and Chow, 1985; Gartner, 1992; Forna-ciari et al., 1990). Substitute events suggested for dividing Zone CN5are (1) the LO of Cyclicargolithus floridanus, proposed by Bukry(1973) as a secondary marker for the base of Subzone CN5b; (2) theF 0 of Discoaster bollii, proposed by Ellis (1981); and (3) the LO ofCoronocyclus nitescens, proposed by Gartner and Chow (1985). De-tailed documentation of these and other events can help to establishmore reliable criteria for the definition of Subzones CN5b and CN5a(see discussion in Raffi et al., this volume). Among secondary bio-stratigraphic markers, Fornaciari et al. (1990) suggested that C. ni-tescens is more useful than D. kugleri, which is difficult to recognizein overgrown assemblages and which has a discontinuous distribu-tion. In our sediments, we recorded some of these alternative potentialmarkers, although the F0 of D. kugleri was recognized and used toplace the boundary between Subzones CN5b and CN5a. As expected,only through quantitative analysis on closely spaced samples has itbeen possible to determine the event precisely. D. kugleri is very rarein the lowermost range and is missing in many intervening samples.The distribution patterns at Sites 844, 845, and 846 (Figs. 3-5) pro-vide evidence for the "appearance" of D. kugleri: (1) a first rareoccurrence that is close to the extinction of C. nitescens (withinSubchron 5An at Site 845) in the Leg 138 successions, as it is in thewestern equatorial Indian Ocean (Fornaciari et al., 1990); and (2) thebeginning of common and continuous distribution, with a restrictedstratigraphic extension, occurring at the base of Chron 5r. This latterevent is probably the same event recorded as "Zλ kugleri lowestoccurrence" by Gartner (1992) in DSDP Site 608 (North Atlantic), assuggested by the calibration to the magnetostratigraphy. The highestoccurrence of D. kugleri is reported as occurring close to the appear-ance of C. coalitus (Bukry, 1973). In the material studied, we ob-served a clear event of "disappearance" for D. kugleri, which cor-responds to the end of its common and continuous distribution andhas been recorded in all the sequences that penetrated to SubzoneCN5b. At Site 845, this event occurs within Chron 5r (just aboveSubchron 5r.2n). The last specimens of D. kugleri, known to disap-pear close to the appearance of C. coalitus (Bukry, 1973), have beenobserved only at Sites 844 and 845 as rare and scattered occurrences(Figs. 3-4).

Within Subzone CN5b in the equatorial Indian Ocean, large-sizedCalcidiscus (>ll µm), labeled as C. macintyrei, enter the strati-

graphic record (Fornaciari et al., 1990). Detailed analyses performedon Leg 138 material to check this occurrence indicate that C. macin-tyrei is extremely rare and scattered. This species does not provide auseful biostratigraphic signal in this interval.

Within Subzone CN5a, in the middle part of Chron 5Ar at Site845, we recorded the FO of Triquetrorhabdulus rugosus. This wasrecorded at a similar stratigraphic position in the mid-latitude NorthAtlantic (Olafsson, 1991). The biostratigraphic usefulness of thisevent has also been shown in the equatorial Atlantic, Pacific, andIndian oceans (Olafsson, 1989; Fornaciari et al., 1990, 1993).

In the lower part of Subzone CN5a, we record the highest occur-rence of C. floridanus, within Chron 5AAr at Site 845. It is a distinctevent that occurs well below the F0 of D. kugleri. The final range ofC. floridanus has been shown to be biogeographically controlled(Olafsson, 1991; Fornaciari et al., 1993). Therefore, the use of theextinction event of C. floridanus for recognizing the base of SubzoneCN5b (base of Zone NN7) (Bukry, 1973) may cause confusion whencorrelating between different regions.

The nannofossil assemblage of Zone CN5 is dominated by heli-coliths, placoliths, and sphenoliths. Discoasterids fluctuate in abun-dance and are generally overgrown. Besides D. kugleri, other groupsthat are well represented in Subzone CN5b include D. exilis, D.musicus, and D. variabilis. D. bollii and D. brouweri are very rare inSubzone CN5b and are scattered in the upper part. D. signus becomesextinct within Subzone CN5a, just below the lowest occurrence of T.rugosus. Among the placoliths of CN5, R. pseudoumbilicus (>7 µm)is dominant and representatives of genus Calcidiscus occur discon-tinuously in low abundance. Close to the transition between SubzonesCN5b/CN5a (between the highest occurrence of C. nitescens and thelowest occurrence of T. rugosus), we observed the extinctions of Tri-quetrorhabdulus serratus and Calcidiscus premacintyrei. Additionalinvestigations on the distribution of these taxa, and of D. kugleri andC. floridanus, may clarify their stratigraphic relationship and allowfurther subdivision of Zone CN5, at least on a regional scale.

Zone CN4 (NN5)

This zone is defined by the LO of Sphenolithus heteromorphus(top) and the F 0 of Helicosphaera ampliaperta (bottom).The CN5/CN4 boundary is characterized by another turnover in the nannofossilassemblage, also documented in the equatorial Indian Ocean (Rio etal., 1990a). Large R. pseudoumbilicus, which are dominant in theoverlying interval, appears with rare specimens at the transition be-tween CN5 and CN4.

The extinction of S. heteromorphus is a neat event in the succes-sions studied. It marks the final occurrence of "spined" sphenoliths,which were dominant in the underlying intervals and have beenreplaced by small forms related to Sphenolithus abies. Helicoliths arecommon to abundant. In the discoasterid assemblage of Zone CN4,slender-armed forms like D. exilis, D. variabilis, and D. signus arepresent continuously, whereas the short-armed discoasterids of theDiscoaster deflandrei group gradually disappear. Within this strati-graphic interval, the discoasterid assemblage was often strongly over-grown, thus preventing the recognition of the different species ofdiscoasterids.

Zone CN3 (NN4)

Sediments belonging to this zone were recovered at eastern tran-sect Sites 844, 845, and 846. The lowermost parts of these sequencesfall within Zone CN3, that is, within the range of S. heteromorphus.This biostratigraphy provides the age control for the basement atthese sites. The biostratigraphic basement age of 16.5 to 17 m.y. isolder than the age previously estimated by tectonic subsidence orspreading models (Mayer, Pisias, Janecek, et al., 1992).

We used the highest occurrence of Helicosphaera ampliaperta todefine the upper boundary of this zone, although Bukry (1975, 1978)considered this species a secondary marker, owing to its irregular


distribution in many oceanic areas. He suggested the use of the appear-ance of C. macintyrei for the definition of the boundary between ZonesCN4 and CN3. Moreover, Bukry (1978) "declassed" the end of the D.deflandrei acme (not quantitatively defined) as a secondary marker inhis zonal scheme. This event was the primary criterion he proposed in1973 for defining the CN4/CN3 zonal boundary. In fact, this event canbe influenced by variable preservational and ecological factors andmay vary from place to place (Bukry, 1978; Parker et al., 1985). In Leg138 material, H. ampliaperta is rare in the upper part of the range, butits extinction is a rather clear event. Olafsson (1991) demonstrated thatthe disappearance of//, ampliaperta in the mid-latitude North Atlanticis also a distinct event. Interestingly, H. ampliaperta is absent in thewestern equatorial Pacific (Leg 130) (Fornaciari et al., 1993), thus con-firming the regional character of this biostratigraphic marker. Otherevents have been proposed for the definition of boundary CN4/CN3.We have not considered them, such as the lowest occurrence of Bukry'sC. macintyrei, as the application of different taxonomic concepts canlead to ambiguous biostratigraphic results (Fornaciari et al., 1990).Regarding the end of the D. deflandrei acme, we performed quantita-tive analyses on the discoasterid assemblages within the upper part ofZone CN3, following Rio et al. (1990a), to obtain the distributionpattern of D. deflandrei. Assuming the Rio et al. (1990a) definition forthe end of the acme (i.e. a drop below a value of 30% relative to thetotal discoasterids), the event occurs before the LO of//, ampliapertain the uppermost CN3. The D. deflandrei event coincides with thelowest occurrence of Discoaster signus (Figs. 3^-). Such concomitantevents were detected also in the western equatorial Indian Ocean (Rioet al., 1990a) and were used to distinguish the CN4/CN3 boundarywhere H. ampliaperta was missing.

The nannofossil assemblage of this part of Zone CN3 is dominatedby helicoliths, sphenoliths, and small placoliths as Reticulofenestraspp. and Dictyococcites spp. The distribution pattern of S. heteromor-phus is characterized by peaks in abundance at discrete levels, wherethe species is the dominant component of the nannofossil assemblage.

BIOCHRONOLOGY

The biochronology adopted in this work is derived from a recali-bration of the nannofossil events to the new time scale developed forLeg 138 sites (Shackleton et al., this volume). From 0 to 6 Ma, this newtime scale is orbitally tuned and provides new ages for the magneticreversals as well as the biostratigraphic events. Before 6 Ma, the Leg138 time scale is based on the new calibrations obtained in the tunedinterval, but it converges with the Cande and Kent (1992) paleomag-netic time scale by 14.8 Ma. In Table 2, we list the calcareous nanno-fossil events that define the zonal boundaries reported in Figure 2, aswell as additional events, together with their assigned age estimates.

The nannofossil biochronology of the Pleistocene and Pliocene iswell established, as demonstrated by a number of studies dealing withdirect correlations of the events to magnetostratigraphies and high-resolution isotope stratigraphies (e.g., Backman and Shackleton, 1983;Berggren et al, 1985; Raffi et al., 1993). Therefore, the biochronologicdata on Pleistocene and Pliocene nannofossil events reported in thiswork are derived from recent literature and from recalibration to anorbitally tuned time scale (Shackleton et al., this volume).

In the Miocene interval, the excellent magnetostratigraphies ob-tained at Sites 844, 845, 852, and 853 in the upper Miocene, andextending to the middle Miocene at Site 845 (see site reports inMayer, Pisias, Janecek, et al., 1992, and Schneider, this volume),allow us to refine the nannofossil biochronology of this time interval.The resulting biochronology substantially agrees with that obtainedin the equatorial Indian Ocean during ODP Leg 115 (Backman et al.,1990; Rio et al., 1990a). Differences between these low-latitude bio-chronologies include the B (bottom occurrence of) C. coalitus. In theLeg 138 material, this event was directly calibrated to magneto-stratigraphy and occurs in the lower part of Chron 5n.2n (age estimateof 10.7 Ma). It is considerably younger than the date of 11.6 Ma

(reestimated) obtained from Site 714 (Backman et al., 1990) by linearinterpolation between B D. hamatus and T (top occurrence of) S.heteromorphus. Data from Site 845 provide calibration for somemiddle Miocene events. Among them, T S. heteromorphus has anestimated age of ca. 13.58 Ma, obtained by linear extrapolation fromthe base of Chrons 5AAn and 5ABn. This age estimate results slightlyolder than the age (reestimated) of 13.49 Ma obtained from DSDPSite 608 (Backman et al., 1990). More detailed discussion of Leg 138Miocene nannofossil biomagnetostratigraphy and comparison withother low- and mid-latitude magnetobiochronologies is reported inRaffi et al. (this volume).

CHRONOSTRATIGRAPHY

The chronostratigraphic framework we adopt for this paper is thatused by Rio et al. (1990a), which divides the Miocene and Plioceneinto subseries-subsystems. This informal large-scale subdivision doesnot refer to European chronostratigraphic units of the Neogene. In fact,stage-age units designated in European sequences are often difficult torecognize outside the type regions. This is because they rarely haveupdated and detailed biostratigraphic and magnetostratigraphic agecontrols. On the other hand, reference to a correct chronostratigraphicframework is important, because the chronostratigraphy (precise defi-nition of chronostratigraphic boundaries) is "a common language"for international communication and correlation. The procedures fol-lowed in defining the subseries-subsystems reported in Figure 2 aresomewhat different from those adopted in the Initial Reports volume("Explanatory Notes" chapter in Mayer, Pisias, Janecek, et al., 1992),for they take into account recent revisions and difficulties encoun-tered in Neogene chronostratigraphy (Rio et al., 1990a, 1990c). Theadopted definitions of the chronostratigraphic boundaries and unitsare briefly explained below.

The Pleistocene

The Pleistocene is divided into middle-late and early Pleistocene,following the chronostratigraphy of Berggren et al. (1985) and Rio etal. (1991).

The middle-late/early Pleistocene boundary (top of Selinuntianstage, as proposed by Rio et al., 1991) approximates the time ofintensification of Pleistocene glaciations and the establishment of astrong 100,000-yr. eccentricity cycle in the ice-volume record. Withreference to calcareous nannofossil biostratigraphy, this boundaryapproximates (slightly postdates) the reentrance in the stratigraphicrecord of medium-sized Gephyrocapsa spp. (Rio et al., 1990b, 1991)between the base of Chron In (Brunhes) and the top of Subchronlr.ln(Jaramillo).

The definition of the Pleistocene/Pliocene boundary is based onthe stratigraphic principle of establishing a boundary stratotype. TheVrica section (Calabria, Southern Italy) has been defined and ac-cepted as the stratotype for the boundary (Aguirre and Pasini, 1985).Within this section, the top of the laminated level "e" in the Vrica sec-tion was selected as the marker horizon for the Pleistocene/Plioceneboundary (Aguirre and Pasini, 1985), and this level is just above thetop of Chron 2n (Olduvai) (Tauxe et al., 1983; Hilgen, 1991a) at anage of ca. 1.81 Ma (revised to the astronomically calibrated chronol-ogy of Hilgen, 1991a).

In the present work, we have recognized the boundary by the F0of medium Gephyrocapsa spp., which occurs in the equatorial Pacificjust above the top of the Olduvai Subchron, as it does at the boundarystratotype section in the Mediterranean (Rio et al., in press).

The Pliocene

We divide the Pliocene series into two intervals (early and latePliocene), following the generally used procedure for which theboundary is approximated by means of calcareous nannofossils,


namely, the highest occurrence of Reticulofenestra pseudoumbilicuswithin Chron 2Ar (close to the upper part of the Gilbert). It should benoted that Rio et al. (1991, 1994) have recently proposed a tripartitesubdivision into lower (Zanclean stage), middle (Piacenzian stage),and upper (Gelasian stage) Pliocene, as the time interval between thetop of the Piacenzian stage (ca. 2.6 Ma) and the Pliocene/Pleistoceneboundary (ca. 1.81 Ma) is not represented by a formally designatedchronostratigraphic unit (and stratotype section).

The Pliocene/Miocene Boundary

The recognition of the Pliocene/Miocene boundary was consid-ered controversial, owing to the difficulty in correlating this boundary(defined in the Mediterranean) to the open-ocean record. Sometimesdifferent procedures (magnetostratigraphy, planktonic foraminifersor calcareous nannofossils) are used for placing the boundary in thesame sequence, and this can lead to controversial identification of it(see discussion in Rio et al., 1990a). New results from land sectionsin Calabria (Zijderveld et al., 1986; Channell et al., 1988; Hilgen andLangereis, 1988) suggest that the boundary occurs slightly below theThvera Subchron (3n.4n) (in the upper part of Chron 3r), at anestimated age of about 5.32 Ma (Hilgen, 1991b). In this paper, weplaced the Pliocene/Miocene boundary within nannofossil SubzoneCNlOa, between the appearance of Ceratolithus acutus (top) andthe extinction of Discoaster quinqueramus (base), following Bukry(1973) who tentatively suggested the top of Subzone CNlOa as thebest approximation of this boundary, on the basis of correlations withMediterranean stratotypes.

The Late/Middle Miocene Boundary

Berggren et al. (1985) defined the late/middle Miocene boundary(Tortonian/Serravallian boundary) at the base of the Tortonian stageand placed it within Zone CN6 (NN8), corresponding to the F0 ofNeogloboquadrina acostaensis. We have drawn the boundary in theupper part of nannofossil Zone CN7 (NN9), in agreement with theprocedure followed by Rio et al. (1990a). Taking into account recentwork in Italian sections (Rio et al., 1990c, and unpubl. data), Rio andco-workers saw that the bio- and magnetostratigraphic position of thisboundary is unclear, owing to the poor quality of the data collected atthe Mediterranean stratotype section of the Tortonian. Contradictionsarise in the comparison and correlation of data from oceanic areaswith the Mediterranean stratotype (see discussion in Rio et al., 1990a,for further details).

The Early/Middle Miocene Boundary

We followed the general criterion adopted by calcareous nannofos-sil paleontologists for recognizing the middle/early Miocene boundaryand have drawn it at the top of Zone CN3 (NN4), just below the highestoccurrence of Helicosphaera ampliaperta. With regard to chrono-stratigraphic units, the middle/early Miocene boundary is equivalent tothe base of the Langhian stage (Berggren et al., 1985). This boundaryis approximated by the F0 of Preorbulina glomerosa curva (Cita andPremoli Silva, 1968), which is recorded above the F0 of Sphenolithusheteromorphus in the stratotype section, just below the LO of H.ampliaperta, within the upper part of Zone NN4 (Martini, 1971).

SITE SUMMARIES

The preliminary biostratigraphic results included in the site chap-ters of the Initial Reports volume (Mayer, Pisias, Janecek, et al., 1992)show that the shipboard nannofossil biostratigraphy was sufficientlydetailed to detect all major events. A general discussion of calcareousnannofossil biostratigraphy of the eastern equatorial Pacific, and asummary of the results at each site, are in the biostratigraphy chapterof this paper. We examined the middle and upper Miocene intervals bymeans of a new suite of more closely spaced samples. In this chapter,

we report refinements to the on-board nannofossil biostratigraphy anddiscuss biostratigraphic details and problems that arose during ourresearch. Biostratigraphic data were collected from composite depthsections constructed for each site (Hagelberg et al., 1992). These dataare reported in Tables 3-12 and are illustrated in Figures 3-12. Depthsof the data are reported in shipboard meters composite depth (mcd).

In the figures, we refer to the biostratigraphic scheme of Okadaand Bukry (1980), which provides more resolution in some strati-graphic intervals. Details on the distribution patterns of stratigraphi-cally indicative calcareous nannofossils in the middle and upperMiocene sections of some Leg 138 sites are reported in Raffi et al.(this volume). They collected quantitative data from many of thesamples used in this study. We have excluded Site 854 from this reportand refer the reader to the Initial Reports volume for Leg 138 for thenannofossil biostratigraphy of this sequence.

Eastern Transect:Sites 844, 845, 846, and 847

The eastern transect sites were drilled in the vicinity of 95°W (Fig.1), in the area where the equatorial current system interacts with thePeru Current. The primary objective at these sites was the reconstruc-tion of the paleoceanography in this area, namely, the history of thecurrent system and of oceanic productivity under the influence ofcoastal upwelling. Additional objectives included (1) calibrating low-latitude biostratigraphies to the paleomagnetic time scale, by meansof the anticipated magnetic stratigraphies at each site; (2) assisting inthe reconstruction of the local tectonic history by providing bio-stratigraphic ages for previously undated magnetic anomalies in thisregion; and (3) determining the backtracked history of these sites.

Site 844

Site 844 is located in the Guatemala Basin, in an Oceanographicregion known as the Costa Rica Dome, a surface upwelling area withassociated high open-ocean productivity. The site lies on the CocosPlate, on basement formed at the East Pacific Rise approximately 17.5m.y. ago (Mayer, Pisias, Janecek, et al., 1992).

The complete sediment sequence above the basalt basement wasretrieved at Site 844, and two lithologic units were recognized. In thePleistocene-upper Miocene interval, the recovered sediments areclay-rich and biogenic silica-rich oozes with minimal carbonate (UnitI). In the middle to lower Miocene interval, the sediment is microfos-sil (mainly nannofossil) ooze (Unit II). A good magnetostratigraphywas obtained in the late Neogene interval (Schneider, this volume).

The average sampling interval for nannofossil analysis was 60 cmin Holes 844B and 844D and in some cores from Hole 844C. Thenannofossil events we recognized are listed in Table 3. In the upperpart of the sequence (0-76 mcd), where siliceous microfossils (mainlydiatoms) dominate, the relative abundance and preservation of cal-careous nannofossils varies. Pleistocene nannofossil assemblages arediluted by siliceous microfossils and clay, but the major biostrati-graphic events were recorded, despite the scarcity of nannofossils andthe presence of barren intervals. A synthesis of the biostratigraphy atthis site is presented in Figure 3.

Stratigraphic sections devoid of nannofossils are present withinthe Pliocene and upper Miocene intervals, and these sections are oftencharacterized by reduced sedimentation rates. Consequently, in thelower Pliocene, we could not recognize Subzones CN12a, CNllb,CNlla, CNlOc, and CNlOb.

In the upper Miocene, we frequently encountered intervals contain-ing few and strongly etched nannofossils. Barren intervals were ob-served within the CN7 (NN9) Zone. Nevertheless, the primary bio-stratigraphic events have been recognized by means of detailed anal-ysis on short-spaced samples (about one sample every 0.048 m.y.).Nannofossils are abundant and well preserved in the middle and upperlower Miocene interval, except for discoasterids, which show strongovergrowths. The abundance of discoasterids varies throughout the

250


Miocene and is significantly low in some intervals (i.e., within CN7,CN6, and CN5). This discoasterid distribution pattern observed at Site844, and at other Leg 138 sites, is typical beneath areas of strongup welling and low abundances, and may be linked to enhanced up-welling conditions (Chepstow-Lusty et al., 1989, 1992). Within themiddle Miocene Zone CN5 (NN7 and NN6), the following eventswere observed (Fig. 3): T (top occurrence of) common and continuousD. kugleri; B (bottom occurrence of) common and continuous D.kugleri; T C. nitescens; T C. premacintyrei; B T. rugosus; TD. signus;and T C. floridanus. These events provide additional biostratigraphicinformation that may be useful for increasing the stratigraphic resolu-tion within the zonal subdivision of this time interval.

The lowermost part of Site 844 is placed within Zone CN3 (NN4),and is characterized by intervals of S. heteromorphus blooms.

Site 845

Three holes were drilled in the Guatemala Basin at Site 845, thenorthernmost site of the eastern transect. A continuous late Neogenerecord was constructed from these holes and a high-resolution magne-tostratigraphy was obtained from the Pliocene to the middle Miocene(Schneider, this volume). This high-quality polarity record allows usto calibrate the biostratigraphies and to obtain new biomagnetostrati-graphic data for the refinement of Miocene bioevents in a low-latitudeenvironment (see also Raffi et al., this volume).

In the Pleistocene-upper Miocene interval, the sediments are dia-tom and radiolarian clays (lithologic Unit I) with isolated occurrencesof pelagic carbonate. Below, in the middle-lower Miocene interval, thelithology is mostly nannofossil ooze (Unit II) with metalliferous sedi-ments at the bottom, just above the basalt basement. Nannofossil dataindicate a basement age of 17 m.y., which is older than the age estimatedfrom plate reconstructions (see Mayer, Pisias, Janecek, et al., 1992).

We sampled Hole 845A and five cores of Hole 845B at an aver-age interval of 70 cm. Detailed quantitative analyses were performedon closely spaced samples (about one sample every 0.025 m.y.) inthe lowermost Pliocene-middle Miocene interval. The nannofossilevents are summarized in Table 4 and Figure 4.

In the Pleistocene and upper Pliocene, the conventional biostrati-graphic events were recognized. Nannofossil abundances are generallylow throughout the interval because of clays and biosiliceous compo-nents. Most of the Pliocene section is barren of nannofossils, as thebasal part of the upper Miocene corresponding to Zones CN8 and CN7(NN10 and NN9). In the rest of the upper Miocene section (CN9-NN11), nannofossil assemblages are poorly preserved and show strongdissolution at some levels. In this interval, we could not detect T A.amplificus. Because discoasterids are considered a solution-resistantform of nannofossil (Ramsay, 1972, 1977; Ramsay et al., 1973), weexpected their abundance to increase in the dissolved assemblagesencountered in the upper Miocene (i.e., the upper part of Core 138-845A-13H, just above a barren interval; Fig. 4). On the contrary, weobserved generally low abundances of discoasterids in this interval. Infact, abundances are low in some samples that contained stronglyetched nannofossils. If this part of the Miocene was characterized byenhanced productivity, then our observation supports the contention(Chepstow-Lusty et al., 1989, 1992) that discoasterid abundances arereduced when productivity increases. Detailed quantitative analysis isneeded to clarify this pattern and to understand the influence of envi-ronmental factors on nannofossils in the eastern equatorial Pacific (seeFlores et al., this volume, for upper Pliocene interval).

In the middle and lower Miocene interval, the assemblages arerich with well-preserved nannofossils. Only discoasterids, whoseabundance fluctuates even in this interval, are strongly overgrown insome samples. The excellent paleomagnetic record obtained at thissite allowed us to calibrate directly the B of C. coalitus and theadditional events within Zone CN5 (Zones NN7 and NN6) that arerecorded in the same biostratigraphic interval at Site 844 (Figs. 3-4).Despite overgrowths that hamper the recognition of most of the

discoasterids, we were able to identify the acme end of D. deflandreiin the lower Miocene section. By quantitative analysis, we were ableto determine the appearance of D. signus in the upper part of CN3(NN4). In the lowermost samples, we were able to find an acme in theabundance of 5. heteromorphus (Plate 1, Fig. 7).

Site 846

Site 846 is located approximately 300 km south of the GalapagosIslands, where the South Equatorial Current interacts with the PeruCurrent. The four holes drilled at this site recovered a sedimentarysequence of more than 400 m, deposited since 16.5 Ma. The upper200 m of the composite sedimentary section constructed at Site 846is based on APC coring and is considered stratigraphically continu-ous. In the lower portion of the hole, recovered by XCB coring, smallgaps occur in the composite section. The entire sedimentary sectioncan be divided into two lithologic units: one consisting of alternatingcarbonate ooze (mostly nannofossil) and siliceous ooze (diatom andradiolarian) from 0 to about 350 mcd (Unit I); the other consisting ofnannofossil ooze from about 350 to 455.3 mcd (Unit II).

We sampled Hole 846B and two cores of Hole 846D at an averageinterval of 70 cm. In these samples, we recognized almost all theconventional biostratigraphic events from the Pleistocene back to theupper part of the lower Miocene. These results are summarized in Table5 and Figure 5. Range charts are reported in Tables 13 and 14. Rework-ing of upper Miocene nannofossils was observed in several intervalswithin the upper Pliocene. Nannofossils are generally abundant andwell preserved throughout the section, with the exception of the lowerpart of the upper Miocene (CN8-CN7-CN6), where we observedpoorly preserved assemblages. This interval corresponds to the disso-lution event seen at other Leg 138 sites. This event was related to amajor paleoceanographic event observed in the Pacific (Mayer et al.,1986; Mayer, Pisias, Janecek, et al., 1992). In the same interval, theabundances observed in discoasterid assemblage are remarkably low.These low abundances were also observed upward in the section, inintervals close to the first appearance of Amaurolithus spp. The occur-rence of these discoasterid-depleted intervals appears linked to particu-larly high abundances of diatoms. The distribution of ceratolithids isalso irregular, generally characterized as rare and scattered. After theirentrance in the stratigraphic record, A. primus and A. delicatus areoften missing from the samples. The scarcity of ceratolithids alsoeffects the distribution of A. amplificus, which seems to have a shorterrange at Site 846 than at Sites 844, 851, 852, and 853.

Although core recovery was poor in the lowermost part of thesedimentary sequence, the nannofossils from the few available sam-ples of Cores 138-846B-42X and -43X allowed us to place thesesediments in the upper part of Zone CN3. The assemblage in the darkreddish brown metalliferous sediments atop the basalt basement, ischaracterized by very high abundances of S. heteromorphus.

Site 847

This site, located 280 km west of the Galapagos Islands, is theequatorial divergence site of the eastern transect. The sedimentarysequence was deposited in a complex tectonic area, at the transitionbetween crust formed at the Galapagos Spreading Center and the EastPacific Rise (Mayer, Pisias, Janecek, et al., 1992). The four holesdrilled at Site 847 provide a continuous sequence spanning the last 7m.y. that primarily consists of nannofossil ooze with abundant dia-toms in some intervals. Estimated sedimentation rates are quite con-stant (around 30 m/m.y.), except in the early Pliocene time interval,when sedimentation rates increase (Mayer, Pisias, Janecek, et al.,1992). The average nannofossil sampling interval of 60 cm from Hole847B provides a temporal resolution approximately 20 k.y.

In the Pleistocene and upper Pliocene, calcareous nannofossils areabundant and preservation is generally good. Most of the conven-tional biostratigraphic events were identified (Table 6 and Fig. 6;range chart reported in Table 15), although in some intervals nanno-

251


Table 13. Distrubution of calcareous nannofossil taxa in the Pleistocene-Pliocene interval at Site 846.

Zone(CN)

15 + 14B

14a

13b

13a

12d + c

12b

12ab

12aA

11

10c

10b

10a

Core, section,interval (cm)

2H-5, 602H-6, 542H-6, 1202H-7, 202H-CC, 53H-I.424H-1.404H-5, 424H-5, 1204H-6, 454H-6, 1204H-7, 204H-7, 545H-I,495H-3, 405H-3, 1205H-4, 405H-6, 426H-1, 396H-3, 396H-3, 1206H-4, 396H-5, 406H-6, 206H-6, 406H-CC, 57H-5, 408H-2, 1208H-3, 438H-3, 1208H-3, 1408H-4, 699H-4, 601 OH-1,6010H-I, 13510H-2, 60IOH-3,6010H-5, 60I1H-5, 60I2H-2, 120I2H-3. 60I2H-5, 6013H-I.6013H-7. 4014H-3, 60I4H-3, 120I4H-4, 60I4H-4, 12014H-5, 6014H-CC, 315H-1.60I5H-3, 6015H-7, 4015H-7, 6315H-CC, 1316H-1.6016H-4,6017H-1.5OI7H-1, 120I7H-2, 50I7H-2. 120I7H-5. 5018H-I.5018H-3. 5018H-6, 50I8H-7, 13I8H-CC, 13I9H-I.4019H-I, 12019H-3, 4019H-3, 120I9H-4, 4020H-3, 502OH-3, 12020H-4, 502OH-5, 5021H-3, 5021H-4, 50

Depth(mbsf)

13.615.0415.716.216.7916.9226.432.4233.233.9534.735.235.5435.9938.939.740.443.4245.3948.3949.249.8952.452.752.953.7455.566.767.4368.268.469.1978.683.684.3585.186.689.699.1

103.2105.6108.6112.1120.9124.6125.2126.1126.7127.6130.56131.1134.1139.9140.13140.45140.6145.1150150.7151.5152.2156159.5162.5167168.13168.97168.9169.7171.9172.7173.4181.5182.2183184.5191192.5

Depth(mcd)

14.215.6416.316.817.3918.9729.5535.5736.3537.137.8538.3538.6941.1944.144.945.648.6251.8954.8955.756.3958.959.259.460.2463.876.477.1377.978.178.8988.7596.1596.997.6599.15

102.15112.6118120.4123.4128.55137.35141.75142.35143.25143.85144.75147.71150.8153.8159.6158.83160.15160.8165.3171.25171.95172.75173.45177.25181.35184.35188.85189.98190.82192.7193.5195.7196.5197.2207.1207.8208.6210.1217.95219.45

Abundance

AAAAAAAAAAAAAAAAAAAAAA

cAAAAAAACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Preservation

GGGGGGGGGGGGGGGGGGCiGGGG(iGGGGMGMGGGGGGCiGMMGGGMGGVIMGGGGVIGGGGMVI\1

MMVI

MVI

MGVIGGMGVIVIMMG

Etching

11111000111010111I01011I110121->11010010]1000]101011111111I11I11I01101001011111

Overgrowth

00000000000000000000000000000000000100011001200000000I000012002000000000022120

SmallGephyrocapsa

AAAAC

ccAAFCFFC

cccFcAGFF

P. lacunosa G. omega

R

FFA FA FA RA RAAAAAAAAAACACAACACAAAACAAACC

ccccAC

cAACcc

G• oceanica

cf.

FFFF

FFR

cf.

FCCcccFFcf.

LargeGeph vrocap.sa

Rcf.FCCFR

H. sellii

FFFFFFC

CFR

F

Rcf.F

RF

Notes: For an explanation of the abundance and preservation codes, see text. For genus names, see Appendix A.


Table 13 (continued).

C. macintyrei D. brouweri D. triradiatus D. pentaradiatus D. surculus D. tamulis D. asvmmetricus Sphenolithus spp. R. pseudoumbilicus C. rugosus A. delicatus

ccF

c

FC

FCCFFCFCFl•

FFlFFFFCRFFllC11K

FFFRllll

Ccl

K

RCFFCFFC1FlFFFFFFVFCFFFCC

cFl••

FCRFRl•

FFFFFFFFFFFCCFF

FF

R

F

R

:f.

R

R

l

RRR

RFR

R

F

RFlFFlRFCFFFFFR

RRR

FFFFFVFllFllllFFFFFFCl •

FF

FFR

1-FFR

R

FFFFFCFCF

FRF

RR

cf.

cf.

cf.

R

F

RCCcccccccFFccccccFFFFl•

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ccccccccccc1

cRFF

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cc( •

cA

cf.

cf.cf.

cf.

cf.cf.

cf.

FRFFlCRFRRRRFcf.F

FFll

RR

RR

FRR

FRFRRRR



Zone(CN)

15+ 14b

14a

13b

73a

I2d + c

12b

12aB

l2aA

11

10c

10b

10a

Core, section.

interval (cm)

2H-5, 602H-6, 542H-6, 1202H-7, 202H-CC, 53H-1.424H-1.404H-5. 424H-5, 1204H-6, 454H-6, 1204H-7, 204H-7, 545H-I.495H-3, 405H-3, 1205H-4, 405H-6, 426H-1.396H-3, 396H-3, 1206H-4, 396H-5, 406H-6, 206H-6, 406H-CC, 57H-5, 408H-2, 1208H-3, 438H-3, 1208H-3, 1408H-4, 699H-4. 601 OH-1.6010H-1, 13510H-2. 6010H-3, 60IOH-5,601IH-5, 6012H-2, 12012H-3, 6012H-5, 60I3H-I.60I3H-7.40I4H-3, 60I4H-3, 120I4H-4, 60I4H-4, 120I4H-5, 60I4H-CC, 315H-1,6O15H-3. 6015H-7, 4015H-7, 6315H-CC, 1316H-l,6016H-4, 6017H-1.5017H-1, 12017H-2, 5017H-2, 12017H-5, 50I 8 H - 1 , 5 0

18H-3, 5018H-6, 50I8H-7, 13I8H-CC, 1319H-l,40I9H-1, 120I9H-3,4O19H-3, 12019H-4, 4020H-3, 5020H-3, 12020H-4, 502OH-5, 502IH-3, 5021H-4, 50

A. primus

R

R

RRRR

R

RRRR

A. tricorniculatus C. acutus

R

R

RIRlF

R

R

C. armalus

C

cf.

cf.

cf.

cf.

I-lI-Fl•

F

F1-R

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RCcf.

cf.

cf.

C. leptoporus C.

C

cccccccccccF

ccccccccccccl

cccccccccccccccccccccccccccccccccccccccf.

cccccccccccc

cristatus

F

C

R

FR

FRFR

FR

RFR

R

R

FFFF

FRFF

FFF

F

R

C. telesmus

l

RR

I

cf.

cf.F

R

R

1

C. pelugicus

RFCRC1C

cFcc('( •

( •

( -

( •

ccci•

crci•

ci

c1l•lFIl('Cl

cCCC

r

("FR1

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cci ;

c( •

lFCC

D. intercalaris

R

FI-

R

Rl•

l

l•

l•

llFFl•

F

R

C

l•

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D. variahilis

cf.

R

F

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I-F

RCRFICl•

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fossil assemblages are difficult to interpret because of dilution bybiosiliceous material. Among Pliocene discoasterids, D. pentara-diatus is very rare in the upper Pliocene, thus preventing the identifi-cation of Subzone CN12b (Zone NN17).

In the lower Pliocene and upper Miocene intervals (below 145mcd; range chart in Table 16) preservation deteriorates, overgrowthsaffect discoasterids, and sometimes it was not possible to differentiatethe species. Variations in the relative abundance of discoasterids,

ceratolithids, and triquetrorhabdulids were observed, mostly in theupper Miocene interval.

The lower part of the Site 847 section was placed in SubzoneCN9b, because of the presence of representatives of genus Amauro-lithus, together with generally rare and overgrown D. berggrenii andD. quinqueramus.

In the lowermost part of the sequence, we observed strong rework-ing of lower Miocene nannofossils. These older forms {Sphenolithus

254



G. rotula H. carteri gr. H. hyalina Pontosphaera spp. Reticuhfenestra spp. R. roturia R. cluviger Scyphosphaera spp. Syracosphaera Thoracosphaera Umbilicosphaera spp.

ccf.

cf.

F

F

FFFFFCFCCFCFFFRFCFFRCCF

FFFFFFFFFFFCccF

FCC

ccccFCcccccFFCcc

cFFCcFCFCCu.

C

cccccFcFFFCCCCcFFCFCCCCCCFtL

,

F

CCcFCCCCCFC

ccFF

LL,

R

A

F

FFRFFF

FFFFFFFFFR

F

FRFF

CFF

F

FFF

FF

F

F

F

A

FR

F

AAAAACCAC

ccccFFFACCcccFF¥CCcFFCcFFCAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFAAAAAAAAAAAA

cc

FC

cccRFCFCCccFC

conicus, S. dissimilis, S. belemnos-dissimilis, and Helicosphaera in-termedia) are particularly abundant in the calcareous veneer scrapedfrom the cherts that are the deepest samples retrieved at Site 847. Thefinding of lower Miocene reworked material in the sediments at Site847 is particularly intriguing as it raises new questions regarding thecomplex tectonic history of an area with crust thought to be about 9m.y. old (Hey, 1977).

Western Transect:Sites 848,849,850,851,852, and 853

The western transect sites were drilled at 110°W (Fig. 1), in an areawhere the equatorial current system is removed from the direct influ-ence of the eastern boundary currents. Studies of the sedimentarysequences retrieved along this transect are designed to reconstruct the

255


Table 14. Distribution of calcareous nannofossil taxa in the Miocene interval at Site 846.

Zone(CN)

10a

9bC

9bB

9bA

9a

8 b

8a

7

6

5b

5a

4

3


21H-4, 5021H-4, 12021H-5, 5021H-7, 5022H-3, 5023X-1.5023X-3, 5023X-3, 12023X-4, 5023X-5, 5023X-CC, 1024X-1.4024X-3, 4024X-5, 4024X-7, 2525X-1.5025X-2, 3925X-3, 4025X-5, 16

I38-846D-26X-1, 12026X-2, 12026X-3, 12026X-5, 12026X-6, 12026X-7, 2328X-3, 6029X-l,6029X-2, 6029X-3, 6029X-4, 6029X-4, 12029X-5, 6029X-5, 12030X-1.603OX-2, 6030X-3, 6030X-4, 6030X-7, 4031X-4, 6031X-7.4032X-1.6032X-2, 6032X-3, 6032X-4, 6032X-5, 6032X-5, 12032X-6, 6032X-6, 12032X-7, 1033X-l,6033X-4, 6033X-4, 12033X-5, 6033X-5, 12033X-6, 6033X-6, 12033X-7, 3434X-2, 6034X-4, 6034X-7, 6035X-1, 12035X-2, 12035X-4, 12035X-6, 12036X-1,3636X-4, 4036X-6, 6837X-1,5837X-3, 2638X-2, 5038X-3, 5038X-5, 12038X-6, 5038X-6, 12038X-CC, 1040X-1.4040X-2, 10040X-3, 12040X-CC, 1041X-3.6041X-CC, 942X-1.7942X-I, 11342X-CC, 1243X-CC, 344X-3. 60

Depth(mbsf)

192.5193.2194197200.5207.7210210.7211.5213216.15216.6219.622.6

252.45226.2227.59229.1231.86

241242.5244247248.5249.03258.5264.8266.3267.8269.3269.9270.8271.4274.5276277.5279283.3288.6292.9293.7295.2296.7298.2299.7300.3301.2301.8302.2303.4307.9308.5309.4310310.9311.5312.14

358.2359359.7360.01370.7372.8374.5377.9383.5386.77390.39390.73390.9399.23412.5

Depth(mcd)

219.45220.15220.95223.95230.95241244244.7245.5247250.15252255258260.85261.3262.69264.2266.96

278.8276.3277.8280282.3282.83293.6299.9301.4302.9304.4305305.9306.5309.6311.1312.6314.1318.4323.7328328.8330.3331.8333.3334.8335.4336.3336.9337.3338.5343343.6344.5345.1346346.6347.24

393.3394.1394.8395.11405.8407.9409.6413418.6421.87425.47425.83426434.33447.6

Abundance

AAAAAAAAAAAAAAAAAAA

AAAAAAAAAAA

cAAAAAAAAACAACCACACACC

cccccccAAAAAAAAAAAAAAAAAAAAAAAAAAA

Preservation

MMGGMMMMMMVIGMMVIGGMG

GMGMGGMMGGGGGVIVIVIVIMGMGVIVIVIVIVIVIMMMMMMMVIVIV1VIGGGGGGGGGGGGGGVIMMMVIVIVIMVIVIMVIMVIM

Etching

1

11222121212

222222121222222211100000000000221222

Overgrowth

11001122221011]0010

020100110000012222020

((

;

))

D. bro

F

R

FFFRFFCFRCFCFCF

FFFFRCFFFFFRRFRRRR

FRet

F

R

F

C. macintyrei Sphenoiithus spp.

FFFFFFFRFFRFRF

F

R

F

RRF

:f.

:f.

RR

R

R

F

F

F

CF

F

R

F

FR

C

R

R

FRRRRRR

R

CCA

ccACcccA

cACcc:A

cc

cccc:ccccAAAAAAAAAACAAAAACC

ccccFccccccccccccccccccccccccccccA

ccAA

cAA

256



R. pseudoumbilicus A. delicatus A. primus A. tricσrniculatus T. rugosus D. quinqueramus D. berggrenii A. amplificus D. loeblichii M. convallis

ccF

cccACCCFFC

cc

c

c

F

F

RF

FFFFFRRRcf.

R

FF

R

RFRRR

cf.F

RFCCR

F

RA

F

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R

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cf.

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.

257



Zone Core, section,

(CN) interval (cm) D. neohamatus D. bollii D. hamatus C. miopelagicus C. coalitus D. kugleri C. nitescens C.floridanus S. heteromorphus H. timpliaperta

10a 21H-4,5021H-4, 12021H-5, 50

9bc 21H-7,5022H-3,50 R23X-1.5023X-3,5023X-3, 120 F

9bB 23X-4,50 cf.23X-5, 5023X-CC, 10 cf.24X-l,40 R24X-3,40 cf.24X-5, 4024X-7, 2525X-l,50 cf.25X-2,39 R25X-3, 40

9bA 25X-5, 16

138-846D-26X-1, 12026X-2, 12026X-3, 12026X-5, 12026X-6, 12026X-7, 2328X-3, 6029X-l,60

9a 29X-2,6029X-3, 6029X-4. 6029X-4, 12029X-5,6029X-5, 120

8b 30X-l,60 F3OX-2,60 F3OX-3,60 F30X-4,60 F3OX-7, 4031X-4,60 C31X-7,40 R

8a 32X-1.6O32X-2, 6032X-3, 6032X-4, 6032X-5,6032X-5, 12032X-6, 6032X-6, 120

7 32X-7, 1033X-1.6033X-4, 6033X-4, 12033X-5, 60

6 33X-5, 12033X-6, 6033X-6, 12033X-7, 3434X-2, 6034X-4, 6034X-7. 6035X-1, 120 F C R35X-2, 120 F C F35X-4, 120 F C C35X-6, 120 F C C

5a 38X-6, 12038X-CC, 1040X-1,4040X-2, 100

4 40X-3, 12040X-CC, 1041X-3, 6041X-CC, 942X-1,7942X-1, 113

3 42X-CC, 1243X-CC. 344X-3, 60

R

cf.

cf.RRRcf.RRRFFFFFRRFR

R

cf.

c\\

cf.cf.cf.CCcRCRR

C

cACCCCCCCAAAVC

cACACC

ccccccAA

cAC

RRR

5b 36X-1,36 F A F36X-4, 4036X-6,68 R A R37X-1,58 F C R37X-3, 2638X-2, 5038X-3, 5038X-5, 12038X-6,50 R

FR

FCRCCCFFC

cf.cf.

FFFCclFRRRll

FACCFCACAAA

FlRFF



D. deflandrei D. signus C. leptoporus C. pelagicus D. adamanteus D. bellus gr. D. calcaris D. exilis D. fonnosus D. moorei D. musicus

ccccccccccccccccccc

ccccccccccccccFFFFCFF

FF

FFCFFFFFCCcccccccccccccRRRFC

F

F

FCFCFFR

CCFFR

CCF

C

ccFRRFCFC

cFCCFFF

CFFFFCC

F

FCRRFFCFFFFFFFCCCFFFF

R

Rcf.

FRFF

tL

RF

RR

RFu_

FFR

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F

F

R

R

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R

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CCR

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cf.

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cf.

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R

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F

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FFF

R

FFFCF

CFCRA

FFFF

R FRCAACF

R FF

R FFCFFCFF

C C CC C R F F R CC F cf. F F C CA R F R R CA F R A F

259



Zone(CN)

Core, section,interval (cm) D. neorectus D, pseudovariabilis D. variabilis G. ratula H. carteri gr. H. intermedia Pontosphaera spp. Reticulofenestra spp. R. rotaria

9bB

9bA

VI

21H-4, 5021H-4, 12021H-5, 5021H-7, 5022H-3, 5023X-l,5023X-3, 5023X-3, 12023X-4, 5023X-5, 5023X-CC, 1024X-1.4024X-3, 4024X-5, 4024X-7, 2525X-l,5025X-2, 3925X-3, 4025X-5, 16

138-846D-26X-1, 12026X-2, 12026X-3, 12026X-5, 12026X-6, 12026X-7, 2328X-3, 6029X-l,6029X-2, 6029X-3, 6029X-4, 6029X-4, 12029X-5, 6029X-5, 12030X-1.6030X-2, 6030X-3, 6030X-4, 6030X-7, 4031X-4, 6031X-7,4032X-1.6032X-2, 6032X-3, 6032X-4, 6032X-5, 6032X-5, 12032X-6, 6032X-6, 12032X-7, 1033X-1.6033X-4, 6033X-4, 12033X-5, 6033X-5, 12033X-6, 6033X-6, 12033X-7, 3434X-2, 6034X-4, 6034X-7, 6035X-1, 12035X-2, 12035X-4, 12035X-6, 12036X-1.3636X-4, 4036X-6, 6837X-1.5837X-3, 2638X-2, 5038X-3, 5038X-5, 12038X-6, 5038X-6, 12038X-CC, 1040X-1.4040X-2, 10040X-3, 12040X-CC, 1041X-3, 6041X-CC, 942X-1.7942X-1, 11342X-CC, 1243X-CC, 344X-3, 60

cf.cf.cf.cf.cf.cf.

cf.

cf.cf.

cf.

cf.

cf.

260



Thoracosphaera T. milowi T. serratus Umbilicosphaera spp

Note: For an explanantion of the abundance and preservation codes, see thetext. For genus names, see Appendix A.

development of the equatorial current system during late Neogeneunder the influence of global climatic change. All the western transectsites are located on crust generated at about 10 to 12 Ma. The sites lieabout 900 km west of the East Pacific Rise, with the exception of thesouthernmost Site 848, which is located about 650 km away. Becausesufficient time was saved during this part of the leg, because offavorable weather and drilling conditions, we were able to drill anextra site (Site 850) along the transect.

We report on six of the seven western transect sites. Site 848 liessouth of the equator. Sites 849 and 850 lie within the equatorial diver-gence zone. Sites 851, 852, and 853 were drilled progressively north ofthe equator. We excluded from this report the northernmost Site 854 (forbiostratigraphic results on nannofossils see Initial Reports volume).

Site 848

The sedimentary sequence drilled at this southernmost site of thewestern transect is almost 100 m thick, and extends to the uppermostpart of middle Miocene. Excellent magnetostratigraphy was obtainedin the Pleistocene-Pliocene interval (upper 46 m of the section) andin the lower part (the 20 m above the basement), which is middle-upper Miocene in age.

Our biostratigraphic work is based on closely spaced samples (10-40 cm) from Hole 848B and from five cores of Hole 848C. Nannofossilbiostratigraphic results are summarized in Figure 7, and the events arelisted in Table 7. Range charts are reported in Tables 17 and 18. Nanno-fossils are generally abundant and have variable preservation through-out the sequence. Poor nannofossil assemblages (in terms of abun-dance and preservation) were observed in the lower part of the section,and correspond to an interval with low sedimentation rates (4-6 m/m.y.) and enhanced dissolution (Mayer, Pisias, Janecek, et al., 1992).

Precise biostratigraphic assignment was difficult in some intervalsbecause of the absence of marker species and to dissolution, whichprimarily effected discoasterids. Discoasterid identification was par-ticularly vexing in the oldest sediments retrieved at Site 848, whereovergrowths made the nannofossils almost unrecognizable. Further-more, representatives of genus Catinaster are missing, thereby fur-ther complicating the biostratigraphy. In the lower part of the section,biostratigraphic control was provided by the highest occurrence of C.miopelagicus and lowest occurrence of D. hamatus. Poor preserva-tion of discoasterids was observed within the entire Miocene interval.Notwithstanding the strong overgrowths, we detected the lowest oc-currence of D. neohamatus and tentatively placed the boundary CN8a/CN8b at the lowest observed specimens of D. loeblichii. Even theCN8/CN9 boundary (B D. berggrenii) was placed with some diffi-culty, as the marker D. berggrenii was strongly overgrown, very rare,and scattered in the lower part of its range.

The consistent occurrence of ceratolithids in the uppermost Mio-cene and lower Pliocene is noteworthy. Specimens of genera Amauro-lithus and Ceratolithus are unusually abundant in some levels andallowed us to define their biostratigraphic events. In the upper part oflower Pliocene (mainly within Zone CN11), we observed a strongreworking of middle-upper Miocene nannofossils such as C. coali-tus, M. convallis, D. berggrenii, and D. quinqueramus. The samereworking was observed at Sites 850, 851, 852, and 853.

Site 849

The sedimentary sequence at Site 849 accumulated above a crustwith an age of 11-12 Ma (Mayer, Pisias, Janecek, et al., 1992). As thesite is located within the high productive equatorial divergence zone,it can provide important information about the development of oceanicconditions in this region during late Neogene. Four holes were drilledat this site, and a 320-m composite section was constructed from therecovered cores. This interval spans the time from the Pleistocene tothe middle Miocene. The dominant lithology is diatom-nannofossilooze with biosiliceous interbeds of variable thickness. The sedimen-tation rates at Site 849 are variable and follow a general pattern ob-served throughout the eastern equatorial Pacific (see Mayer, Pisias,

261


Table 15. Distribution of calcareous nannofossil taxa in the Pleistocene-upper Plicene interval at Site 847.

Zone(CN)


Depth(mbsf)

Depth(mcd)

SmallAbundance Preservation Etching Overgrowth Gephyrocapsa E. huxleyi P. lacimosu G. oceanica Gephyrocapsa sp. 3

15

14b

14a

13b

13a

I2d+12c

12b

12aB

1H-2, 1201H-3, 120IH-4, 1201H-5, 152H-1, 1202H-3, 1202H-4. 1202H-5, 502H-5, 1202H-6, 512H-6, 1202H-7, 243 H-1,503H-3, 503H-7, 604H-1.604H-1. 1204H-2, 604H-2, 1204H-3, 604H-3, 1204H-4, 604H-5, 604H-7, 104H-7, 605H-1.605H-1, 1205H-2. 605H-2, 1205H-3, 605H-5, 605H-7, 405H-CC, 106H-l,606H-1. 1206H-2, 606H-3, 606H-3, 1206H-4, 606H-5, 607H-1.607H-4, 607H-5, 607H-5, 1207H-6. 607H-6, 1207H-7, 107H-7, 408H-1.608H-1, 1208H-2, 608H-2, 1208H-3, 608H-3, 1208H-4, 608H-4, 1208H-5. 1208H-6, 608H-6, 1209H-l,609H-1, 1209H-2, 609H-2, 1209H-3, 609H-3, 1209H-4, 609H-5, 609H-6, 609H-6, 1209H-7, 159H-7, 4010H-2, 6010H-4, 60

2.74.25.77.27.79.210.711.512.212.9913.6814.2216.519.526.0226.126.727.628.229.129.730.632.134.635.135.636.237.137.73S.641.644.445.0944.535.636.53838.639.54154.659.160.661.262.162.763.163.464.164.765.666.267.167.768.669.270.771.672.273.674.275.175.776.677.278.179.681.181.782.1582.484.687.6

2.74.25.77.27.839.3310.8311.6312.3313.1413.8314.3716.8519.8525.8528.9329.5330.4331.0331.9332.5333.4334.9337.4337.9338.43939.940.541.444.447.247.8949.5850.1851.0852.5853.1854.0855.5861.3565.8567.3567.9568.8569.4569.8570.1572.2872.8873.7874.3875.2875.8876.7877.3878.8879.7880.3883.0883.6884.5885.1886.0886.6887.5889.0890.5891.1891.6391.8893.8396.83

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Janecek, et al., 1992). Sediment accumulation is particularly high inthe latest Miocene/early Pliocene (as high as 100 m/m.y.). These highrates suggest that the site was within the highly productive equatorialdivergence zone by that time.

The average sample spacing for nannofossil analysis at Site 849was 60 cm (Hole 849B, five cores from Hole 849C, and from one coreof Hole 849D). Because sedimentation rates varied, our 60-cm sam-ple spacing translates in a range of temporal resolutions varying from

1 sample/0.04 m.y. to 1 sample/0.012 m.y. The nannofossil events werecognized are listed in Table 8, and the biostratigraphy is summa-rized in Figure 8.

Nannofossil preservation is generally moderate to good. Placo-liths, mainly reticulofenestrids, are the most important component ofthe nannofossil assemblages. Within this group, we identified mor-photypes with round outlines and central opening, labeled as "circularreticulofenestrids" in the figures (see "Taxonomic Notes" section,



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this chapter). At Site 849, these reticulofenestrids appear when thelarge morphotypes R. pseudoumbilicus reenter the stratigraphic rec-ord in Subzone CN9bA. The distribution of "circular reticulofenes-trids" has also been checked at Sites 846,847, and 850. At these sites,the distribution is also restricted to Subzone CN9b, up to levelscorresponding to the lower range of A. primus and close to the lowestoccurrence of A. amplificus.

Abundances of discoasterids at Site 849 fluctuate throughout thesequence and are generally low, as expected in an area of high pro-ductivity. Even ceratolithids are very rare and discontinuously distrib-uted. Some biostratigraphic events were detected with difficulty, suchas the lowest and highest occurrences of A. amplificus and the lowestoccurrence of D. hamatus. Other markers are missing, such as C.coalitus and C. calyculus.

263



Zone(CN)

Core, section,interval (cm) C. leptoporus C. cristatus C. telesmus C. pelagicus D. intercalaris D. variabilis G. rotula H. carted H. wallichii Pontosphaera

15

14b

14a

13b

13a

I2d+12c

12b

12aB

IH-2, 1201H-3, 1201H-4, 1201H-5, 152H-1, 1202H-3, 1202H-4, 1202H-5, 502H-5, 1202H-6, 512H-6, 1202H-7, 243H-l,503H-3, 503H-7, 604H-l,604H-1, 1204H-2, 604H-2, 1204H-3, 604H-3, 1204H-4, 604H-5, 604H-7, 104H-7, 605H-l,605H-1, 1205H-2, 605H-2, 1205H-3, 605H-5, 605H-7, 405H-CC, 106H-l,606H-1, 1206H-2, 606H-3, 606H-3, 1206H-4, 606H-5, 607H-l,607H-4, 607H-5, 607H-5, 1207H-6, 607H-6, 1207H-7, 107H-7, 408H-l,608H-I, 1208H-2, 608H-2, 1208H-3, 608H-3, 1208H-4, 608H-4, 1208H-5, 1208H-6, 608H-6, 1209H-l,609H-1, 1209H-2, 609H-2, 1209H-3, 609H-3, 1209H-4, 609H-5, 609H-6, 609H-6, 1209H-7, 159H-7, 4010H-2, 6010H-4, 60

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Within the oldest sediments retrieved at Site 849, we recorded thefinal range of common D. kugleri. This finding allows us to assign thelowermost part of the sequence to the middle part of Subzone CN5b.

Site 850

Site 850 was drilled just north of the equator, halfway betweenSites 849 and 851. This location is suitable for investigating the

narrow equatorial divergence zone. Unfortunately, the site was onlydrilled once, because of time pressures. The hole was cored with theAPC to 74 mbsf and then cored with the XCB from there to thebasement. The 400-m section recovered from this site spans the timeinterval from the Pleistocene to the middle Miocene. A brief intervalwith a strong magnetic signal was identified between 60 and 75 mcd,spanning the Subchron 2An.lr (Kaena) to the 2An-2Ar (Gauss-Gilbert) boundary. The sedimentary section at Site 850 represents a

264



Rhabdosphaera Reticulofenestra spp. Scyphosphaera Svracosphaera Thoracosphaera Umbilicosphaera Reworked

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Notes: For an explanation of the abundance and preservation codes, see text. For genus names, see Appendix A. Single asterisk (*)= one specimen observed.

single lithologic unit of radiolarian and diatom-nannofossil ooze, nannofossil events are listed in Table 9. Nannofossils are abundantwith several layers of laminated diatom ooze and thinly bedded chert throughout the sequence and their preservation varies. Placolithsin the lower part of the section. The sedimentation pattern is similar generally dominate the assemblages. Discoasterids and ceratolithidsto that reconstructed at Site 849 (Mayer, Pisias, Janecek, et al., 1992). fluctuate in abundance and are very rare or absent in some intervals,

We took 6 to 12 samples per core from Hole 850B for nannofossil as was seen at Site 849. A detailed nannofossil biostratigraphy in theanalysis. The resulting biostratigraphy is shown in Figure 9, and the upper Miocene and Pliocene intervals was difficult to generate on

265

3

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!... i ! 1 1 1 1 1 S 1 1 i ! § ICore, section, Depth Depth = S g 3 8 ~ 1 ~ I £• = | §> ^ §j •§

Z o n e ( C N ) i n t e r v a l ( c m ) ( m b s f ) ( m c d ) A b u n d a n c e P r e s e r v a t i o n E t c h i n g O v e r g r o w t h | ^ ^ S - o i = i . « 2 3 ^ 8 . 5 - ^ ^ iù o a r a . < J Q Q Q Q Q Q t o • a j U < ç ^ t ;

12H-1, 120 102.7 115.73 A G 0 0 * C C C C R R * F12aB 12H-2,60 103.6 116.63 A G 0 0 C C C F F * R R

12H-2. 120 104.2 117.23 A MG I 0 C R C * * * *12H-3,60 105.1 118.13 A MG 1 1 A C C F F * * F12H-3, 120 105.7 118.73 A G 0 0 F R C A R F *CF

12aA 12H-4,60 106.6 119.63 A G 0 0 R R C C F R R CR12H-4, 120 107.2 120.23 A G 0 O F R F F F * R R R12H-5.60 108.1 121.13 A G 0 0 R C F R R R CF12H-5, 120 108.7 121.73 A G 0 0 F F A C * * R * * C C FI2H-6,63 109.63 122.66 A G 0 0 R F F R R * C A RI2H-6, 120 110.23 123.26 A G 0 0 R R R A R * A A FI2H-7, 14 110.67 123.7 A MG I O R F C R R R F A FI2H-7.60 111.13 124.16 A G 0 0 R F F F R * C C RI3H-1.7O 111.7 125.7 A G 0 0 F F R R F C C RI3H-2,60 113.1 127.1 A G 0 0 F C R F C C

I IB 13H-4. 150 117 131 A G 0 0 F F * A A F13H-5,60 117.6 131.6 A G 0 1 F F F R F C C R *14H-1.60 121.1 136.58 A MG 1 1 F C R F C C F14H-2, 124 123.24 138.72 A G 0 0 R F F R F R A A R14H-3.60 124.1 139.58 A G 0 0 F F F F F A A R14H-3, 124 124.74 140.22 A G 0 0 C C R R F A A R14H-4, 124 126.24 141.72 A G 0 0 C F F AA14H-5.60 127.1 142.58 A G 0 0 F A F F F C C * *

LJaJL _ 14H-5, 124 127.74 143.22 A MG 1 1 A F C R A A F14H-6,60 128.6 144.08 A MG 1 1 F C F F * A C R *14H-6, 124 129.24 144.72 A M 1 2 C F * A A R14H-7,50 130 145.48 A MG 1 1 F F F F F F R *15H-2,60 132.1 148.05 A MG 1 1 F C R F F F R R15H-4.60 135.1 151.05 A MG 1 1 F F F C F R R15H-6.62 138.12 154.07 A MG 1 1 C F R F F C R *15H-7,49 139.49 155.44 A MG 0 2 R C C F A R R *16H-1.140 140.9 157.35 A MG 0 2 F A C C C F C R

10c 16H-5,60 144.6 161.05 A MG 1 1 F F F F C C F F R16H-6,60 146.1 162.55 A MG 0 1 F F R R C F R R F17H-1.60 146.4 165.75 A MG 1 2 F F F F C F R17H-2.60 147.9 167.25 A MG 1 1 F F F F A F17H-3.60 149.4 168.75 A G 0 0 F F F F C R R17H-4,60 150.9 170.25 A G 0 1 F F F F A F *17H-5.60 152.4 171.75 A G 0 1 F F R F * F R *17H-5, 123 153.03 172.38 A M I 2 C C R C R R R R

~ 17H-6,60 153.9 173.25 A M G I 1 F F R F C R17H-6. 123 154.53 173.88 A MG I 2 C C C A17H-7. 154.85 174.2 A MG 1 1 F F R F * C R R18H-2,60 157.6 176.95 A MG 1 I F F F F * C

10b 18H-3.60 159.1 178.45 A MG I 1 C F F F * C18H-4.60 160.6 179.95 A MG 0 1 F C F A18H-5,60 162.1 181.45 A MG I 1 C F F C C *18H-6,60 163.6 182.95 A MG 1 1 F A F C C18H-7,3 164.53 183.88 A G 0 1 F C F R R A A

~ 18H-7, 15 164.65 184 A MG 0 2 C C F C R19H-1.60 165.3 184.65 A M G 1 2 FF F C C19H-1, 123 165.93 185.28 A G 0 0 R F F R * C F19H-2. 123 167.43 186.78 A MG 1 1 C F C R C C *

10a 19H-5. 123 168.93 188.28 A MG 0 2 R C C F A A *19H-7,5 169.25 188.6 A MG 0 2 C F F R A F *19H-7.40 169.6 188.95 A MG 1 1 F F R F C C *20X-I.60 174.5 193.85 A MG 1 2 F F A C20X-1.122 175.12 194.47 A MG 0 2 R C F C A A

~~20X-2,60 176 195.35 A MG 1 I R R F A C20X-2, 122 176.62 195.97 A G 0 0 C * * A C R20X-3.60 177.5 196.85 A MG 1 1 F F R R C C20X-3.93 177.83 197.18 A G 0 1 F C R C C C

~20X-4 ,60 179.01 198.36 A MG 1 1 R R C C20X-4. 123 179.64 198.99 A MG 1 1 R C C C A A *20X-5.60 180.51 199.86 A MG I 2 R F C C20X-6.60 182.01 201.36 A M G I 2 F F R C C21X-1,60 184.1 203.45 A MG 1 1 R F A C2IX-2,60 185.6 204.95 A G I 0 F R F A C *21X-3,60 187.1 206.45 A M 0 2 R cf AA C21X-4,60 188.6 207.95 A G 0 1 F R F A A2IX-5,60 190.1 209.45 A MG 0 2 F R cf A A

9b 21X-6.60 191.6 210.95 A M G 0 2 F R F A C2IX-7.40 192.9 212.25 A M G 0 2 F R F A A22X-1.60 193.8 213.15 A M 0 2 R R c f A A22X-2, 123 195.93 215.28 A M G 0 2 FC AA22X-3,60 196.8 216.15 A G 0 1 R R AA C22X-4, 123 198.93 218.28 A M 1 2 CR R AC22X-5.60 199.8 219.15 A MG 0 2 F F F AA C *22X-6, 123 201.93 221.28 A MG 0 2 R cf * AA A23X-1.60 203.4 222.75 A MG 0 2 F cf F A A *

Table 16. Distribution of calcareous nannofossil taxa in the lower Pliocene-upper Miocene interval at Site 847.

Si a

•a •? I , IQ & cm **i ^> ""** ^3 &»- > J ^ j r* *•> * * J

Core.section, g | | | | § | | 5 | | j { f "g | | } | | | | | | | | | 1 8Z o n e ( C N ) i n t e r v a l ( c m ) * ® 5 * « ^ « ^ e > . . ; 3 • * ^ ; • ~ * « < • ; § • β g S S S g δ ^ ^ ^ . S

1 2 H - 1 . 1 2 0 A R F R F F R A12aB 12H-2,60 C * R R C C F A F

12H-2, 120 F R F R F R A F12H-3,60 * C R R R F C A F12H-3, 120 A C * F C F A C X

12aA 12H-4,60 A R R F F C A R R12H-4. 120 A F * F A R A RI2H-5.60 * C F C C A X12H-5, 120 A R * F A R A R X12H-6,63 C F F F C R A12H-6, 120 R A R R C C A X12H-7, 14 A R C R R A X X12H-7,60 A F R R F C A R13H-1.70 CR R R F C A R13H-2.60 C R R F R F A

1 IB 13H-4, 150 AR * * * R C A13H-5,60 C * F R C C A14H-1.60 C F * F F C R A C R R14H-2, 124 F A * * F F C A F X14H-3,60 A F R F C F A F14H-3, 124 A R F F F A A F14H-4, 124 C A * F R C F A A F X14H-5,60 C R F F F C A

±la:Z 14H-5, 124 C A R F * F C A R X14H-6,60 C C F R F F C R A C R14H-6, 124 A F R C C A R14H-7,50 C F R * F F R A C R15H-2,60 C R F R C C R A C15H-4,60 C R R F F C F A C15H-6,62 C R F C R A C F

10c 15H-7,49 A F R C C R A C F X16H-1, 140 C F F F F C R A C R R16H-5,60 C C F R R C A C R16H-6.60 C R R C A C17H-l,60 F R F F * R C R A F F17H-2,60 C F C C C A F17H-3,60 C C F R F F F A CI7H-4,60 R C F R F F A F17H-5,60 * * C F F R F F A F17H-5, 123 A A F F R F F A F R17H-6,60 R A R F R R R F A C17H-6, 123 F A F A R F C A F R17H-7, 154.85 C C A R F C R A F R18H-2,60 R C F R F C A C R

10b 18H-3,60 CC F F F C A C F18H-4,60 F A R A R F C A C F F18H-5.60 F C F R C F A C18H-6,60 F C C F R C F A C R R18H-7,3 F A R R F R A C18H-7, 15 C R F R F R A C19H-1.60 C R F R * R F A F19H-1, 123 c f C R F * F A F19H-2, 123 * C C A C C F AI9H-5, 123 A F A R R C A

10a 19H-7,5 A * A R C C A19H-7,40 C C A R F F A20X-1.60 C F A R F F A20X-1, 122 C A A F R C F A20X-2,60 * C F A R * A X20X-2, 122 * A F A R F R A20X-3,60 * C F C F C C A2OX-3.93 * C C C R R F R A20X-4,60 F C C C F C A20X-4, 123 C C R R F R A2OX-5,60 F C F R F R A20X-6,60 C F F F F F F A21X-l,60 R C C F F F F F A21X-2,60 F F F C F F A21X-3,60 F C F C F F F A X21X-4,60 C C F C F F F A

9b 21X-5,60 F C R C c f C F F A21X-6,60 F C F C R F F F A21X-7,40 c f F C R C R F F F A22X1,60 R F C F C F F R A22X-2, 123 R C C R C F F A22X-3.60 F R F C C F C A22X-4, 123 R cF A C C C R A X X22X-5,60 F C C A F F A22X-6, 123 R R C R R F F C R A X X23X-1.60 R C C F F F R A




Core, section,Zone (CN) interval (cm)

23X-4, 12323X-7, 4024X-1, 12224X-4, 122

9b 24X-7,4025X-3, 6025X-4, 6025X-5, 6025X-6, 6025X-7, 4026X-CC27X-CC

Depth(mbsf)

207.03210.7213.72218.22221.9225.8227.3228.8230.3231.6232.04241.54

Depth(mcd)

226.38230.05233.07237.57241.25245.15246.65248.15249.65250.95251.39260.89

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board because of the scarcity of some marker species. We could notlocate some Pliocene events precisely (i.e., T A. primus and B com-mon D. asymmetricus) or place the relative zonal boundaries. Withthe careful shore-based work of this study, however, we were able tosubstantially improve the upper Miocene biostratigraphy and recog-nized all the conventional and some secondary events (Fig. 9).

In the lower part of Site 850, we did not detect the lowest occur-rence of C. coalitus; therefore, we were not able to discriminate ZoneCN6 as we had at Site 849. In the assemblages of the lowermost Cores138-850B-41X and -42X, we recognized the interval with commonD. kugleri, which terminates at the top of Core 138-850B-41X. Thisfinding indicates that the oldest sediments retrieved at Site 850 can beassigned to the middle part of Subzone CN5b.

Site 851

Site 851 is located beneath the highly productive surface watersalong the northern edge of the South Equatorial Current, on top of crustgenerated at about 11-12 Ma. The upper 150 m of the site were tripleAPC cored, and the remainder was double XCB cored to basement.

The 390-mcd section spans the time interval from the Pleistoceneto the middle Miocene. The sedimentary section is considered a singlelithologic unit composed of foraminifer-nannofossil ooze and diatom-nannofossil ooze. A clear paleomagnetic record for the Pleistocene andpart of the Pliocene (top 80 mcd) was obtained at this site. This record,together with the biostratigraphy, provided high-quality age controlpoints for reconstructing sedimentation rates. The sedimentation ratepattern is typical of pelagic sedimentation in this region of the equato-rial Pacific (Mayer, Pisias, Janecek, et al., 1992). There is a drop in thesedimentation at around 10 to 8 Ma. This drop is associated with anincrease in dissolution. This event was observed in all the other Leg138 sites that penetrated to this point.

We sampled Hole 85 IB, two cores of Hole 851C, and five coresof Hole 85IE, at a spacing that averages 40 cm. Nannofossils areabundant and generally well preserved throughout the sequence. Theyprovide a detailed and well-constrained biostratigraphy, as summa-rized in Figure 10. The nannofossil events recognized are listed inTable 10.

The abundance of discoasterids in the upper Pliocene interval isparticularly low. D. tamalis and D. surculus become extinct con-comitantly, and D. pentaradiatus is virtually absent in Zone CN12,whereas it is common to abundant in underlying intervals (particu-larly in Cores 138-851B-8H to -12H).

In the upper Miocene, discoasterids are abundant and well pre-served, as is the entire nannofossil assemblage. Common reworked

forms from Zones CN9 and CN6 and Subzone CN5b are present insome levels of the upper Pliocene, within Subzone CN12a and ZoneCN11. Poorly preserved assemblages were observed in the uppermiddle Miocene interval (within Zones CN6 and CN7), correspond-ing to the interval of increased dissolution and low sedimentationrates outlined above.

Close to the appearance level of C. coalitus (bottom of CN6),discoasterids are very rare and fluctuate in abundance throughout themiddle Miocene interval. The presence of common to abundant D.kugleri in the lowermost part of the Site 851 sequence (Fig. 10) allowsus to assign it to Subzone CN5b. This feature in the distribution of D.kugleri is correlatable between eastern transect Sites 844, 845, and846 and western transect Sites 851 and 850. The bottom and top ofcommon D. kugleri, calibrated at Site 845 as occurring in the lowerpart of Chron 5r, provide an age constraint for the basal sediments ofSite 850 and 851, which accumulated after 11.8-11.9 Ma.

Site 852

Site 852 was drilled to provide material to study the interactionbetween the south Equatorial Current and the North Equatorial Coun-tercurrent during the late Neogene. The site is located near the bound-ary between the two currents, north of the equator. Based on platereconstructions, the site ever passed beneath the equatorial divergence.A continuous sedimentary record for the upper Pleistocene through theuppermost middle Miocene was recovered from four holes (852A-852D) drilled at Site 852. A single lithologic unit was described,composed of a mixture of foraminifer-nannofossil ooze, with someoxide-rich beds, and radiolarian-nannofissil ooze. Because this site islocated north of the high-productivity zone, the microfossil constitu-ents differ from the more equatorial sites and the sedimentation ratesare lower at Site 852. A good magnetic stratigraphy was obtainedfrom the Pliocene to the lower upper Miocene in Holes 852B and852C.

In this study, we sampled Hole 852B and the lowermost cores ofHoles 852C and 952D at an average spacing of 40 cm. Calcareousmannofossils are abundant and generally well or moderately wellpreserved throughout the entire sequence. They provide a detailedand well-constrained biostratigraphy. The nannofossil events recog-nized at Site 852 are reported in Table 11 and illustrated in Figure 11.

In the Pleistocene and Pliocene, nannofossils are well preservedand abundant. In this interval, we observed reworked nannofossilsfrom the upper Miocene Zone CN9 to CN6 (NN11 to NN9). In thePliocene and uppermost Miocene, discoasterids and ceratolithids arecommon or abundant. The upper Miocene discoasterid assemblage is

268



S .2 g

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particularly rich and diverse; Zλ quinqueramus and Zλ berggreni arecommon throughout their stratigraphic range. Discoasterids in theupper part of middle Miocene (in Zones CN6 and CN7) show mod-erate overgrowth. Between 10.5 and 8.5 Ma, we observed strongdissolution of the nannofossil assemblage, and some samples werebarren. This interval coincides with a time of very low sedimentationrates (as low as 3 m/m.y.) and is well correlated as a dissolution eventamong the Leg 138 sites that penetrated to this point in the strati-graphic record.

In samples from the lowermost part of Site 852, we recordedcommon specimens of Catinaster coalitus and D. calcaris and thehighest occurrence of C. miopelagicus. These events allowed us toplace this interval in Subzone CN6.

Site 853

Site 853 is presently located beneath the region dominated by theNorthern Equatorial Countercurrent (NECC). The site was drilled torecover a sedimentary record suitable for studying fluctuations of theNECC and the influence of the Northern Hemisphere trade winds. Acontinuous section from the upper Pleistocene through the upper partof the upper Miocene was recovered from the five holes cored at Site853. The sedimentary section is about 73 m thick. It is composed ofclayey nannofossil oozes with levels containing abundant foramin-ifers and iron and manganese oxides in the upper part, and some levelsof almost pure nannofossil ooze. Siliceous microfossils are virtuallyabsent, except in the upper Pliocene and Pleistocene intervals, sug-gesting low-productivity conditions. Site 853 was never located be-neath the equatorial divergence during its sedimentary history(Mayer, Pisias, Janecek, et al., 1992). The excellent polarity recordtogether with the calcareous nannofossil biostratigraphy provides awell-constrained stratigraphy.

For this study, we sampled Hole 853B and the lowermost core ofHole 853D at an average spacing of 50 cm. Nannofossils are generallyabundant or common throughout the section, and show good to mod-erately good preservation. The nannofossil events recognized at Site853 are reported in Table 12 and are summarized in Figure 12.

Within the Pliocene interval, and mainly in lower Zone CN12 andin Zone CN11, we observed assemblages with rare and poorly pre-served nannofossils, together with common reworked forms from theupper Miocene (Zones CN9, CN7, and CN6). This observation wasalso made at Sites 851 and 852. The mixture of reworked and in situnannofossils in these Pliocene assemblages made difficult the recog-nition of some extinction events of long-ranging forms (like A. pri-mus, R. pseudoumbilicus, S. abies, and S. neoabies), which are com-monly present in upper Miocene assemblages.

Ceratolithids and discoasterids are generally common and abun-dant throughout the Pliocene and upper Miocene section, reflectingthe relatively low-productivity conditions at this site, far from theequator. The lowermost part of the Site 852 sequence is placed inZone CN8. In this interval, the nannofossil assemblage is charac-terized by abundant and slightly overgrown discoasterids, amongwhich D. neohamatus and the D. bellus group dominate. Common M.convallis and abundant S. abies and S. neoabies are also observed.

SUMMARY AND CONCLUSIONS

We presented a chronostratigraphic classification of the calcar-eous nannofossil biostratigraphy of Leg 138 sediments from thePleistocene to the late early Miocene. Most of the zonal boundariesof the standard zonations of Okada and Bukry (1980) and Martini(1971) are recognized in these eastern equatorial Pacific sediments.

The biostratigraphic classification of the Pleistocene was aug-mented with the biozonation proposed by Gartner (1977) and by arecent biochronologic study of low- and mid-latitude, deep-sea se-quences (Raffi et al., 1993). As we used only standard light micro-scope techniques, we did not recognize the appearance of Emilianiahuxleyi (CN15/CN14b and NN21/NN20 boundaries) or the begin-ning of its dominance in the upper Pleistocene interval.

Detailed biostratigraphic classification in Pliocene Zones CN12,CN11, and CN10 (Zones NN18 to NN12) was difficult at some sitesbecause of the scattered occurrence or absence of the primary andsecondary species of discoasterids and ceratolithids. At these sites, wefound that discoasterids and ceratolithids were unreliable as bio-stratigraphic markers in much of the Pliocene and in some intervalsof the middle and late Miocene because of their highly variabledistributions and generally low abundances or absences, in somecases. This is particularly evident in the sedimentary sequences ofsites located beneath the equatorial divergence zone and/or influ-enced by upwelling (in the western transect Sites 849, 850, and 851and in the eastern transect Sites 844, 845, and 846). The rare anddiscontinuous occurrences of discoasterids are associated with highsurface-water productivity, thus supporting the results of Chepstow-Lusty et al. (1989, 1992), who showed that discoasterid abundancesare influenced by varying productivity pressure.

In the Miocene interval, we recognized the zonal boundaries asdefined by Martini (1971) and Bukry (1973) at all sites, with someexceptions: (1) the boundary CN7a/CN7b; (2) the base of Zone CN6(NN8) at western transect Sites 849 and 850; and (3) the top of CN7(NN9) at eastern transect Site 845. This was because of the sporadicoccurrence or absence, and the different stratigraphic ranges, of thedefining markers (C. coalitus, C. calyculus, and D. hamatus). The

269


Table 17. Distribution of calcareous nannofossil taxa in the Pleistocene-Pliocene interval at Site 848.

Zone(CN)


Depth Depth small large(mbsf) (mcd) Abundance Preservation Etching Overgrowth P. lacunosa Gephyrocapsa G. oceanica G. omega Gephyrocapsa H. sellii

15 + 14b

14a

13b

13a

12 d+12c

12b

12aB

12aA

lib7

lla

l()c

10b

10a

138-848A-1H-1,6O1H-2, 601H-3, 601H-4, 601H-5, 601H-6, 60

138-848B-2H-3, 602H-4, 602H-5. 602H-6, 602H-7. 703H-l,603H-1.703H-1,8O3H-1, 1203H-2, 603H-3, 603H-3, 1203H-4, 403H-4, 803H-4, 1203H-5, 403H-5, 803H-5, 1203H-6, 403H-6, 80

144-848C-3H-5, 203H-5, 403H-5, 503H-5, 603H-5, 903H-5, 1003H-5, 1203H-5, 1403H-6, 203H-6, 503H-6, 803H-6, 1203H-7, I03H-7, 203H-7, 303H-7, 403H-7, 503H-7, 604H-3, 404H-3, 1204H-4, 404H-4, 804H-4, 1204H-5, 234H-5, 854H-5, 1204H-6, 204H-6, 1204H-6, 1454H-7, 204H-7, 505H-1.425H-1.955H-1, 1205H-2, 505H-2, 1205H-2, 1275H-3, 625H-3, 1205H-4, 405H-4, 1205H-5. 405H-5, 1205H-6, 405H-6, 1205H-7, 456H-1.756H-2, 356H-2, 756H-3, 356H-3, 756H-4, 356H-4, 756H-5. 356H-5, 75

0.62.13.65.16.68.1

5.87.38.810.111.912.312.412.512.913.815.315.916.61717.418.118.518.919.620

21.221.421.521.621.92222.222.422.72323.323.724.124.224.324.424.524.624.625.426.126.526.927.4328.0528.428.929.930.1530.430.731.1231.6531.932.733.433.4734.3234.935.636.437.137.938.639.440.1540.9542.0542.4543.5543.9545.0545.4546.5546.95

0.82.33.85.36.88.3

8.610.111.612.914.716.2516.3516.4516.8517.7519.2519.8520.5520.9521.3522.0522.4522.8523.5523.95

24.224.424.524.624.92525.225.425.72626.326.727.127.227.327.427.527.628.9529.7530.4530.8531.2531.7832.432.7533.2534.2534.534.7535.0536.7737.337.5538.3539.0539.1239.9740.5541.2542.0542.7543.5544.2545.0545.846.6547.7548.1549.2549.6550.7551.1552.2552.65

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270



C. macintyrei D. brouweri D. triradiatus D. pentaradiatus D. surculus D. tamalis D. asymmetricus Sphenolithus R. pseudoumbilicus C. rugosus A. primus A. tricorniculatus

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Zone Core, section,(CN) interval (cm) C. acutus T. rugosus/rioensis D. quinqueramus C. leptoporus C. cristatus/telesmus C. pelagicus C. radiatus D. decorus D. intercalaris D. pansus

138-848 A-1H-1,6O C F C

15+14b lH-2,60 C R FlH-3,60 C R F1H-4,60 C F R1H-5,60 C R RlH-6,60 C F C

138-848B-2H-3,60 C R C

14a 2H-4,60 C R C2H-5,60 C R R2H-6,60 A F F2H-7,70 C R3H-I.60 A R3H-1.70 A R F3H-1.80 A R F3H-1, 120 A R3H-2,60 A F3H-3,60 A F3H-3, 120 A R F3H-4,40 A F C

13b 3H-4,80 A C F3H-4, 120 * A R R3H-5,4O A F C3H-5,80 A R C3H-5, 120 A R F3H-6,40 C F C3H-6,80 A F F

144-848C-3H-5,20 C F F *3H-5,40 C F C3H-5,50 C F F3H-5,60 C R C3H-5,90 A A F3H-5, 100 C F F

13a 3H-5, 120 A C R C3H-5, 140 A C C3H-6,20 A C C3H-6,50 C C F3H-6,80 C A C3H-6, 120 A C C3H-7, 10 A F C3H-7,20 A C C3H-7,30 A C F3H-7.40 A A R F

12d 3H-7,50 A C F C+ 3H-7,60 A C F C

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12aB 4H-6, 120 A F A F4H-6, 145 A C R F R4H-7,20 A C R R R4H-7,50 A C C * F

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273


Table 18. Distribution of calcareous nannofossil taxa in the Miocene interval at Site 848.

Zone(CN)


Depth(mbsf)

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7H-6, 257H-6, 1007H-6, 1257H-6, 1457H-7, 20

7H-7, 508H-1.8O8H-1, 1408H-2, 808H-2, 140

8H-3, 408H-3, 1008H-4, 208H-4, 100

8H-5, 208H-5, 1408H-6, 808H-6, 1408H-7, 409H-2, 1199H-3, 50

9H-3, 1009H-3, 1199H-4, 409H-4, 100

9H-5, 369H-5, 1199H-6, 409H-6, 119lOH-1,40

10H-1, 12010H-2,47

10H-2, 120

IOH-3,4610H-3, 10810H-3, 12610H-4, 1010H-4, 4010H-4, 8510H-4, 15010H-5, 40

10H-5, 12010H-6, 40

10H-6, 120

10H-7, 1010H-7, 2010H-7, 6011H-1,2OHH-1,50

11H-1, 11511H-2, 3511H-2, 12011H-3,4O

11H-3, 12011H-4, 4011H-5, 2011H-5, 50

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50.65

51.5552.4

53.0553.9

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60.7561.561.7561.9562.2

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64.865.465.9

66.567.268

68.769.9

70.871.4

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72.772.8973.6

74.275.0675.8976.677.3978.679.4

80.1780.981.6682.2882.4682.8

83.183.5584.284.685.486.1

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87.887.988.2

88.85

89.5590.4

91.1

91.992.693.994.2

53.7554.1555.356.9

57.3558.2559.1

59.7560.6

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64.2564.7565.566.2567

67.7568.569.2569.569.7

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77.87979.980.5

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82.4983.2

83.884.66

85.4986.2

86.9989.2

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92.8893.0693.4

93.794.1594.895.2

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138-848C-7H-I.257H-1. 1007H-2, 257H-2, 507H-3, 507H-3, 1007H-4, 257H-4, 1007H-5, 257H-5, 1007H-6, 257H-6, 1007H-6, 1257H-6, 1457H-7, 207H-7. 508H-1.808H-1, 1408H-2, 808H-2, 1408H-3, 408H-3. 1008H-4, 208H-4, 1008H-5. 208H-5, 1408H-6, 808H-6, 1408H-7, 409H-2. 1199H-3, 509H-3, 1009H-3. 1199H-4, 409H-4, 1009H-5, 369H-5, 1199H-6, 409H-6, 11910H-l,4010H-1, 12010H-2, 4710H-2. 12010H-3, 46I0H-3, 10810H-3. 12610H-4, 1010H-4, 4010H-4, 8510H-4, 15010H-5, 4010H-5. 12010H-6, 4010H-6, 12010H-7. 1010H-7, 2010H-7, 6011H-1,2OHH-1,5011H-1. 11511H-2, 3511H-2, 12011H-3, 4011H-3, 12011H-4, 4011H-5, 2011H-5, 50

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276



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lowest occurrence of C. coalitus was directly calibrated to the mag-netostratigraphy at Site 845 as occurring in the lower part of Chron5n.2n (age estimate of 10.7 Ma).

Moreover, in the Miocene interval, we discussed supplementaryevents that improve upon the biostratigraphic resolution provided bythe standard zonations.

In the upper Miocene interval, the stratigraphic relationship of thelowest and highest occurrences of A. amplificus relative to the eventsB A. primus and T D. quinqueramus allowed us to divide further arather long time interval (about 1.7 m.y.), corresponding to SubzoneCN9b, into three distinguishable biostratigraphic subunits. The low-est and highest occurrences of A. amplificus were calibrated to themagnetostratigraphy at Sites 844, 845, 852, and 853 as occurring atthe bottom and top of Chron 3An, respectively, and are isochronouswith the occurrence of A. amplificus in the western equatorial IndianOcean (Rio et al., 1990a). In the interval between lower SubzoneCN9b and Zone CN8, we observed a significant turnover within theplacoliths, corresponding to the temporary and almost complete dis-appearance of large specimens (>7 µm) of R. pseudoumbilicus. Thisinterval seems to have a wide geographical extent, as it has also beenobserved in the western equatorial Indian Ocean (Young, 1990; Rioet al., 1990a) at a similar stratigraphic level. This turnover is thoughtto reflect oceanographic-climatic instability (Rio et al., 1990a).

In the middle Miocene interval, the following additional eventswere recorded within Zone CN5 (from the top): T C. nitescens, T C.premacintyrei, B T. rugosus, T D. signus, and T C. floridanus. The TC. floridanus event is easily detected in Leg 138 material and isrecorded above the highest occurrence of S. heteromorphus and wellbelow the lowest occurrence of D. kugleri. This finding supports thecontention that the final range of C. floridanus is biogeographicallycontrolled, as shown by Olafsson (1991) and Fornaciari et al. (1993).The lowest occurrence of D. kugleri was recognized and used to placethe boundary between Subzones CN5b and CN5a by means of aquantitative analysis on closely spaced samples. This marker speciesis generally rare and scattered in most of its range, except for a briefinterval in which it is common and continuously distributed. Thisinterval with common and typical D. kugleri is correctable betweenall different sequences and was calibrated as occurring within Chron5r (estimated age interval: 11.3-11.9 Ma) at Site 845. At this samesite, the other nannofossil events of Zone CN5 have also been cali-brated to the magnetostratigraphic record.

In the lower Miocene interval, corresponding to the uppermostpart of Zone CN3, we detected the acme end of D. deflandrei and thelowest occurrence of D. signus, which have similar stratigraphicpositions in the western equatorial Indian Ocean (Rio et al., 1990a).

TAXONOMIC NOTES

A complete list of taxa considered in the present study is reported inAppendix A. We followed the taxonomic concepts explained in Rio et al.(1990a); concepts concerning the most important taxa are summarized below.Species representatives of genera Dictyococcites, Pontosphaera, Rhabdo-sphaera, Scyphosphaera, Syracosphaera, Toracosphaera, and Umbilico-sphaera are grouped under the single genus epithet in the range charts.

Gephyrocapsids

Rio (1982) and Rio et al. (1990b) have shown that gephyrocapsids can beconsistently used for biostratigraphic purposes, and Raffi et al. (1993) and Rioet al. (in press) have shown that they provide an accurate and precise tool forcorrelation in early Pleistocene sequences. In the material studied, the taxo-nomic concepts followed for recognition in light microscope of gephyrocap-sids are those summarized in Raffi et al. (1993). They discriminate within thegroup the following biometrically based taxonomic entities:

1. Gephyrocapsids <4 µm in size, "small Gephyrocapsa spp.";2. Gephyrocapsids >4 µm and <5.5 µm in size, with an open central area,

"medium Gephxrocapsa spp.";3. Gephyrocapsids >5.5 µm in size, labeled "large Gephyrocapsa spp."

Within the medium-sized gephyrocapsids, there are morphotypes with anopen central area and a bridge nearly aligned with the short axis of the placolith,similar to G. oceanica as intended by many authors. These gephyrocapsids arecomparable to G. omega Bukry (= syn. G. parallela Hay and Beaudry) andmake up a large proportion of the medium-sized gephyrocapsid stock thatreappeared in the lower Pleistocene after the interval of temporary disappear-ance of medium-sized forms.

Reticulofenestrids

Within reticulofenestrids, we distinguished the following species andgroups: "circular reticulofenestrids," Reticulofenestra spp., and R. pseudo-umbilicus.

To "circular reticulofenestrids," we ascribed forms that are 5-8.5 µm insize, with a circular outline and relatively large central opening, known as R.rotaria Theodoridis or R. pseudoumbilicus var. rotaria Young. These reticulo-fenestrids seem restricted to late Miocene.

Reticulofenestra spp. includes the small-sized reticulofenestrids R. haqii,R. minuta, and R. minutula.

For distinguishing R. pseudoumbilicus, we followed taxonomic conceptsof Raffi and Rio (1979) and Backman and Shackleton (1983), and consideredadding to this species reticulofenestrids larger than 7 µm. In the easternequatorial Pacific, R. pseudoumbilicus appears at the boundary between ZonesCN4 and CN5, and has low abundance in its lower range. It becomes adominant element of nannofossil assemblages in Zone CN5. These reticulo-fenestrids virtually disappears from the stratigraphic record for a long intervalin the late Miocene. The Pliocene extinction of large R. pseudoumbilicus is aclear event in the Leg 138 material, which occurs in the upper part of Chron2Ar (late Gilbert), as it does in other areas (Backman and Shackleton, 1983;Rio et al., 1990a, 1990b).

Calcidiscus

We ascribed to C. macintyrei species only circular Calcidiscus specimensequal to or larger than 11 µm, using the "10-µm size" as a break point to makethe distinction between C. leptoporus and C. macintyrei (see discussion inFornaciari et al., 1990). Following this taxonomic concept, in the equatorialIndian Ocean (Fornaciari et al., 1990), the first appearance of C. macintyreioccurs within Zone CN5, at a higher stratigraphic level than that indicated byBukry (1978). In the middle Miocene of the eastern equatorial Pacific, C. mac-intyrei is very rare and discontinuously present in the lower part of its range.

Elliptical Calcidiscus referred to Calcidiscus premacintyrei are consis-tently present in Zones CN3 and CN4, and become extinct in the upper part ofSubzone CN5a.

Discoasterids

Discoasterids recorded in the sediments of Leg 138 show variable distri-bution and preservation patterns between the different sites and at differentstratigraphic intervals. Rich and well-preserved discoasterid assemblages havebeen observed mainly at sites located out of the equatorial divergence zoneand out of the influence of upwelling. Overgrown discoasterids were recordedin the Miocene interval, making the identification of the different speciesdifficult and sometimes impossible, particularly within the lower and middleMiocene nannofossil assemblages.

Pliocene Discoasterids

Pliocene discoasterid species were identified following concepts outlinedin Backman and Shackleton (1983). In this time interval, the assemblages aredominated by Discoaster brouweri. D. pentaradiatus is common or abundantin the early Pliocene; Discoaster surculus, D. variabilis, D. decorus, D.tristellifer, and, overall, D. tamalis and D. asymmetricus are subordinate.

Late-early Miocene through late Miocene Discoasterids

The massive six-rayed forms, which characterize discoasterid assemblagesin the early and middle Miocene, were replaced by slender forms withbifurcated and pointed tips in the late Miocene. This turnover in the discoast-erid assemblages, which took place gradually within the late-middle Miocene,is particularly evidenced by the appearance in the stratigraphic record offive-rayed discoasterids. The species of five-rayed discoasterids observed inthe eastern equatorial Pacific are the D. bellus group, D. hamatus, D. berg-grenii, D. quinqueramus and Discoaster sp. 1 (sensu Rio et al., 1990a).

Specimens belonging to Discoaster bellus gr. are characterized by smallsize (6-8 µm) and poorly developed central area. Morphotypes with intergrade

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morphologic features between D. bellus and D. hamatus and between D. bellusand D. berggrenii are included in the group.

Discoaster hamatus is characterized by five long rays with a spine extend-ing and bending sharply near the tips (Plate 1, Fig. 1). Specimens relativelythin and smaller (8-10 µm in size) than standard have been observed in thematerial studied.

To D. berggrenii, we ascribed forms even with a poorly developed centralknob, with a distinct central area, following Rio et al. (1990a). In the materialstudied, D. berggrenii appears and becomes extinct earlier than D. quinquera-mus, according to Bukry (1971); in the range charts, the two species werelumped together because intergrades between typical D. berggrenii and D.quinqueramus are commonly recorded. Large specimens of D. quinqueramus(about 16 µm) were observed in the final range of the species. Small five-rayeddiscoasterids, which are common and continuously distributed up to thehighest occurrence of D. quinqueramus, are referred to as Discoaster sp. 1(Plate 2, Fig. 5). This form is smaller in size than D. berggrenii and D.quinqueramus (6-8 µm vs. 8-15 µm) and has a poorly developed central areawith a very small knob (see Rio et al., 1990a, pi. 2, fig. 9).

The six-rayed species of slender discoasterids with pointed tips consideredhere are D. braarudii, D. brouweri, D. intercalaris, D. neohamatus, D.neorectus, and Discoaster sp. 2 (sensu Rio et al., 1990a). D. braarudii and D.neorectus are found with rare and scattered specimens; the medium-sized D.intercalaris is rather common, but it occurs discontinuously. The first and rareD. brouweri appears in the upper part of Subzone CN5, and becomes commonand continuously distributed in Zone CN7. In the eastern equatorial Pacific,within Subzones CN9a and CN9b (lower part), we observed a large discoast-erid (20-30 µm in size) morphologically similar to D. brouweri and referredto as Discoaster sp. 2 (Plate 2, Figs. 6-7), which was observed by Rio et al.(1990a) in the equatorial Indian Ocean. In the material studied, it is rare anddiscontinuous, and does not provide a meaningful biostratigraphic signal.

D. neohamatus was easily recognized, even in overgrown assemblages. Thefeature of its long pointed rays is a "spine" at tips, which sharply bends in onedirection; it can be recognized even in strongly overgrown specimens. It is lessprotruding in specimens found in the terminal range of the species, whereintergrade forms between D. brouweri and D. neohamatus were observed.

Among slender discoasterids with bifurcating rays, we considered thegroups of five-rayed (D. moorei, D. pentaradiatus, and D. prepentaradiatus)and six-rayed {D. bollii, D. calcaris, D. aff. calcaris, D. challenged, D. exilis,D. icarus, D. loeblichii, D. pansus, D. perclarus, D. pseudovariabilis, D.surculus, D. subsurculus, D. signus, D. tuberi, and D. variabilis) discoasterids.The four-rayed species D. blackstockae is found only in spot samples withsingle specimens.

D. moorei is easily recognized by its asymmetrical rays, even in overgrownassemblages, and it is distributed scattered and in low abundances from ZonesCN3 to CN6. D. prepentaradiatus was distinguished from D. pentaradiatusfor the poorly developed central area and for the shape of its arms, which areshorter and bend downward and lack birefringence. D. pentaradiatus showsa certain degree of morphologic variability, consisting in arms more or lessslender and bifurcations more or less developed.

Among six-rayed bifurcating discoasterids, some species are not easilydifferentiated because many intergrading forms exist. The classifications of D.variabilis and D. surculus take into account their high degree of variability inmorphologic features. D. perclarus and D. icarus are found to be very rare andscattered. Typical D. calcaris (Plate 1, Fig. 2) is common and has restrictedrange in intervals close (just below) the lowest occurrence of D. hamatus. D.aff. calcaris is a discoasterid observed and described by Rio et al. (1990a; seepi. 7, figs. 4-6) in the equatorial Indian Ocean. We found this form as rare andscattered within Subzone CN5b. We could not distinguish D. signus and D.tuberi, the observed specimens having often the tips of bifurcated rays broken.Therefore, we designated with a single species epithet, D. signus, thosediscoasterids with characteristic central prominent knob, recognizable even instrongly overgrown assemblages.

The middle Miocene discoasterid assemblage is characterized by six-rayedforms belonging to D. musicus (= D. sanmiguelensis) and D. kugleri group,having a broad central area of variable size, larger than the length of the rays;species circumscription within this group is sometimes difficult when over-growth is present. In the material studied, D. kugleri (Plate 1, Figs. 3-5) ispresent with typical specimens having a flat central area that lacks the largestar-shaped knob of D. musicus specimens.

Another middle Miocene form observed is the small discoasterid D.adamanteus, recognized even in strongly overgrown assemblages, where, onthe contrary, it was difficult to distinguish species like D. aulakos, D. vari-abilis, and D. exilis. The same problems of identification arose in the earlyMiocene interval, in which discoasterids are generally poorly preserved.

Distinction within D. dilatus-D. extensus group proved impossible, whereasthe D. deflandrei group was discriminated, sometimes with difficulty. Thisgroup dominates the assemblages of the upper part of the early Miocene.

Ceratolithids

The horseshoe-shaped nannofossils belonging to the Ceratolithaceae fam-ily are a minor but distinctive component of the assemblages during the latestMiocene and early Pliocene. In the Leg 138 material, ceratolithids are presentwith variable abundances and are found overall at sites located north of theequatorial zone. In these sequences, qualitative observations on rich andwell-preserved assemblages helped to clarify the Phylogenetic relationship ofthe group. Appearance and extinction events of members of the ceratolithidgroup provide meaningful biostratigraphic signals in the late Neogene (sum-marized below, listed in stratigraphic succession).

Late Miocene ceratolithid events are as follows:

1. B (bottom occurrence of) A. primus,2. B A. delicatus,3. B A. amplificus, and4. T (top occurrence of) A. amplificus.

Early Pliocene ceratolithid events are as follows:

5. BC. acutus,6. B C. armatus,1'. B C. rugosus,8. T C. acutus,9. T C. armatus, and10. T A. primus, A. delicatus, and A. tricorniculatus.

The first representative of the group is the species A. primus, within whichtwo morphotypes can be distinguished. The primitive specimens are compa-rable with the holotype (see Bukry and Percival, 1971, pi. 1, fig. 12), have athick arch, and evolve rapidly to more delicate crescent-shaped forms, whichoccur together with another delicate species, A. delicatus. In the materialstudied, the distinction between these two morphotypes is clear but does notseem to be stratigraphically useful, as they occur closely one after the other.

An interesting finding in some Leg 138 successions was the observationof transitional forms between Triquetrorhabdulus rugosus-T. extensus and A.primus, observed close to the appearance level of ceratolithids. The presenceof such intergrading forms (Plate 3, Figs. 3, 4, and 6) confirms previoussuggestions of a Phylogenetic relationship between Triquetrorhabdulus andAmaurolithus (Gartner, 1967; Perch-Nielsen, 1977, 1985). A similar Phyloge-netic relationship was documented also at higher levels, where we observednannofossils with transitional morphological features between T. extensus andA. amplificus (Plate 3, Figs. 7-8) just below the appearance of A. amplificus.Gartner and Bukry (1975) pointed out that the robust and asymmetrical A.amplificus could be related to A. primus and A. delicatus, although such arelationship is unclear. The presence of the intergrading forms indicates thatA. amplificus probably evolved directly from Triquetrorhabdulus. These inter-grading forms are also present within the range of typical A. amplificus and inthe equatorial Indian Ocean (Rio et al., 1990a). The stratigraphic range of A.amplificus in the eastern equatorial Pacific is restricted to Chron 3An, as in theequatorial Indian Ocean (Rio et al., 1990a; Raffi et al., this volume).

A. tricorniculatus, an asymmetrical delicate ceratolithid with a pronouncedapical spine, occurs discontinuously with rare specimens in sediments of theinvestigated sections; intergrade forms between A. primus, A. delicatus, andA. tricorniculatus are rather common.

Similar intergrading between different species was observed among thebirefringent ceratolithids C. acutus, C. armatus, C. rugosus, C. cristatus, andC. telesmus. Therefore, species assignments and recognition of some eventswere sometimes difficult, and were also hindered by overgrowth problems.For this reason, we could not confidently recognize the events B C. armatusand T C. armatus; the event T C. acutus was also difficult to detect.

We point out the finding of forms that seem related to ceratolithids (shownin Plate 4, Figs. 5-7). These are birefringent rod-shaped forms, similar tospecies of the lower Cretaceous genus Ceratolithina (see Perch-Nielsen,1988), and were found as rare and scattered in the lower part of Zone CN7(NN9). Similar forms were also observed by one of us (LR.) at Sites 710 and714 in the western equatorial Indian Ocean in the same stratigraphic interval.

Triquetrorhabdulids

Species ascribed to the genus Triquetrorhabdulus observed in Leg 138Miocene sediments are T. extensus, T. milowii, T. rugosus, and T. serratus.

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T. rioensis and T. farnsworthii were included in the taxonomic unit T.rugosus, as T. rioensis has the same stratigraphic distribution as T. rugosus,and T. farnsworthii was considered a well-preserved morphotype of T. rugo-sus, common in the upper part of its range. T. extensus was found in the upperMiocene interval together with intergrade forms between Amaurolithus spe-cies (see above).

T. milowii was found to be rare and sporadic. T. serratus is common in thelower and middle Miocene interval, and disappears within Subzone CN5a, justabove the appearance of T. rugosus.

Helicoliths

Helicolith nannofossils, which are not solution resistant, occur continu-ously in most of the Leg 138 sequences. Helicoliths are missing or representedby strongly etched specimens in the intervals where dissolution affects nanno-fossil assemblages, as in the upper Miocene and lower Pliocene intervals atSites 844 and 845.

The species recognized are listed in Appendix A. Among them, we includedinto the group of H. carted specimens with slightly variable morphology. Inthe late early Miocene, H. ampliaperta is generally rare but clearly distinguish-able within the helicolith assemblage dominated by H. intermedia, recordedup to Zone CN5, and H. granulata.

Sphenoliths

Sphenoliths are a major constituent of the Miocene and early Pliocenenannofossil assemblages of the eastern equatorial Pacific. We observed char-acteristic high-abundance intervals (blooms) of these nannoliths in the Leg 138sequences. The simultaneous extinction of S. abies and S. neoabies in thePliocene (Backman and Shackleton, 1983) provides a useful event; the twospecies have been grouped together with S. moriformis and S. compactus (asSphenolithus spp.) in the range charts, as differentiations among these speciesare not always clear. The typical spined sphenolith S. heteromorphus isabundant in the material studied, and its extinction event provides a neatbiostratigraphic signal in the middle Miocene.

ACKNOWLEDGMENTS

We would like to thank J. Backman and D. Rio for their criticalreviews, comments, and suggestions, which improved the quality ofthis paper. Special thanks go to J.W. Farrell and A. Sposato for thehelp given in some phases of this work. Funding for this study wasprovided by CNR Grant No. AI91.00913.05 to I. Raffi.

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Date of initial receipt: 12 April 1993Date of acceptance: 31 December 1993Ms 138SR-112


APPENDIX A

Calcareous Nannofossils Considered in This Chapter(in Alphabetic Order of Generic Epithets)

Amaurolithus amplificus (Bukry and Percival, 1971) Gartner and Bukry, 1975Amaurolithus delicatus Gartner and Bukry, 1975Amaurolithus primus (Bukry and Percival, 1971) Gartner and Bukry, 1975Amaurolithus tricorniculatus (Gartner, 1967) Gartner and Bukry, 1975Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan,

1978Calcidiscus macintyrei (Bukry and Bramlette, 1969) Loeblich and Tappan,

1978Calcidiscus premacintyrei Theodoridis, 1984Catinaster calyculus Martini and Bramlette, 1963Catinaster coalitus Martini and Bramlette, 1965Ceratolithus acutus Gartner and Bukry, 1974Ceratolithus armatus Muller, 1974Ceratolithus cristatus Kamptner, 1950Ceratolithus rugosus Bukry and Bramlette, 1968Ceratolithus telesmus Norris, 1965Coccolithus miopelagicus Bukry, 1971Coccolithus pelagicus (Wallich, 1877) Schiller, 1930Coccolithus radiatus Kamptner, 1955Coronocyclus nitescens (Kamptner, 1963) Bramlette and Wilcoxon, 1967Cyclicargolithus floridanus (Roth and Hay in Hay et al., 1967) Bukry, 1971Discoaster adamanteus Bramlette and Wilcoxon, 1967Discoaster asymmetricus Gartner, 1969Discoaster aulakos Gartner, 1967Discoaster bellus Bukry and Percival, 1971Discoaster berggrenii Bukry, 1971Discoaster blackstockae Bukry, 1973Discoaster bollii Martini and Bramlette, 1963Discoaster braarudii Bukry, 1971Discoaster brouweri Tan (1927) emend. Bramlette and Riedel, 1954Discoaster calcaris Gartner, 1967Discoaster challenged Bramlette and Riedel, 1954Discoaster decorus (Bukry, 1971) Bukry, 1973Discoaster deflandrei Bramlette and Riedel, 1954Discoaster exilis Martini and Bramlette, 1963Discoaster hamatus Martini and Bramlette, 1963Discoaster icarus Stradner, 1973Discoaster intercalaris Bukry, 1971Discoaster kugleri Martini and Bramlette, 1963Discoaster loeblichii Bukry, 1971Discoaster misconceptus Theodoridis, 1984 = Discoaster pentaradiatusDiscoaster moorei Bukry, 1971Discoaster musicus Stradner, 1959Discoaster neohamatus Bukry and Bramlette, 1969Discoaster neorectus Bukry, 1971Discoaster pansus (Bukry and Percival, 1971) Bukry, 1973Discoaster pentaradiatus Tan (1927) emend. Bramlette and Riedel, 1954Discoaster perclarus Haq in Haq et al., 1967Discoaster prepentaradiatus Bukry and Percival, 1971Discoaster pseudovariabilis Martini and Worsley, 1971Discoaster quinqueramus Gartner, 1969Discoaster sanmiguelensis Bukry, 1981 = Discoaster musicusDiscoaster signus Bukry, 1971Discoaster subsurculus Gartner, 1967Discoaster surculus Martini and Bramlette, 1963Discoaster tamalis Kamptner, 1967Discoaster triradiatus Tan, 1927Discoaster tristellifer Bukry, 1976Discoaster tuberi Filewicz, 1984Discoaster variabilis Martini and Bramlette, 1963Geminilithella rotula (Kamptner, 1956) Backman, 1980Gephyrocapsa oceanica Kamptner, 1943

Gephyrocapsa omega Bukry, 1973Gephyrocapsa parallela Hay and Beaudry, 1973Helicosphaera ampliaperta Bramlette and Wilcoxon, 1967Helicosphaera carteri (Wallich, 1877) Kamptner, 1954Helicosphaera carteri var. wallichii (Lohmann) Theodoridis, 1984Helicosphaera hyalina Gaarder, 1970Helicosphaera intermedia Martini, 1965Helicosphaera sellii Bukry and Bramlette, 1969Minylitha convallis Bukry, 1973Orthorhabdus serratus Bramlette and Wilcoxon, 1967 = Triquetrorabdulus

serratusPseudoemiliania lacunosa (Kamptner, 1963) Gartner, 1969Reticulofenestra haqii Backman, 1978Reticulofenestra minuta Roth, 1970Reticulofenestra minutula (Gartner, 1967) Haq and Berggren, 1978Reticulofenestra pseudoumbilicus (Gartner, 1967) Gartner, 1969Sphenolithus abies Deflandre in Deflandre and Feit, 1954Sphenolithus heteromorphus Deflandre, 1953Sphenolithus moriformis (Brönnimann and Stradner, 1960) Bramlette and

Wilcoxon, 1967Sphenolithus neoabies Bukry and Bramlette, 1969Triquetrorhabdulus extensus Theodoridis, 1984Triquetrorhabdulus farnsworthii (Gartner, 1967) Perch-Nielsen, 1985 = Tri-

quetrorhabdulus rugosusTriquetrorhabdulus milowii Bukry, 1971Triquetrorhabdulus rioensis Olafsson, 1989Triquetrorhabdulus rugosus Bramlette and Wilcoxon, 1967Triquetrorhabdulus serratus (Bramlette and Wilcoxon, 1967) Olafsson, 1989

APPENDIX B

Definition, Occurrence, and Remarks for the Three BiostratigraphicUnits That Divide Okada and Bukry (1980) Subzone CN9b

CN9bA. Amaurolithus primus Subzone

Definition: Top - lowest occurrence of Amaurolithus amplificus; bottom- lowest occurrence of A. primus.

Occurrence: The type locality is Hole 853B from Sample 138-853B-7H-2,147 cm, to -6H-4,65 cm. Other occurrences are recorded at Leg 138 Sites 844,845, 846, 848,849, 850, 851, and 852, and at Leg 115 Sites 707,709,710,711,and 713.

CN9bB. Amaurolithus amplificus Subzone

Definition: Top - highest occurrence of A. amplificus; bottom - lowestoccurrence of A. amplificus.

Remarks: Bergen (1984) indicated that A. amplificus became extinctwithin the upper part of CN9b in sediments from the tropical Atlantic Ocean.Similar distribution range for A. amplificus has been observed by Rio et al.(1990a) in the equatorial Indian Ocean, where it was calibrated to the polaritytime scale and corresponded to Chron 3An. This same stratigraphic positionfor A amplificus is recorded in the eastern equatorial Pacific.

Occurrence: The type locality is Hole 853B from Sample 138-853B-6H-4,27 cm, to -5H-5, 120 cm. Other occurrences are recorded at Leg 138 Sites 844,846, 848, 849, 850, 851, and 852, and at Leg 115 Sites 707,709,710,711, and713.

CN9bC. Amaurolithus delicatus Subzone

Definition: Top - highest occurrence of Discoaster quinqueramus; bottom- highest occurrence of A. amplificus.

Occurrence: The type locality is Hole 852B from Sample 138-852B-7H-CC to -7H-3, 140 cm. Other occurrences are recorded at Leg 138 Sites 844,846, 848, 849, 850, 851, and 853, and at Leg 115 Sites 707, 709, 710, 711,and 713.


• » # .

6b 7Plate 1. 1. Discoaster hamatus Martini and Bramlette; ×2000. Sample 138-845A-15H-5, 149 cm; parallel light. 2. Discoaster calcarisGartner; ×2000. Sample 138-845A-16H-1, 120 cm; parallel light. 3-5. Discoaster kugleri Martini and Bramlette; ×2000. Sample 138-846B-35X-3, 120 cm; parallel light. 6. Coccolithus miopelagicus Bukry; ×2000. Sample 138-846B-35X-3, 120 cm; (a) parallel light; (b) crossed

7. Acme in abundance of Sphenolithus heteromorphus Deflandre; X1400. Sample 138-845A-31X-CC; crossed nicols.nicols.

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Plate 2. All specimens at parallel light. 1. Discoaster berggrenii Bukry (primitive form); ×2000. Sample 138-853D-7H-4, 60 cm. 2. Discoasterberggrenii Bukry; ×2400. Sample 138-844B-5H-2, 29 cm. 3. Discoaster berggrenii-Discoaster quinqueramus intergrade; ×2000. Sample 138-853D-7H-l,30cm. 4. Discoaster quinqueramus Gartner; ×2000. Sample 138-853D-7H-1,30 cm. 5. Discoaster quinqueramus Gartner (left) and Discoastersp. 1 (right); ×2000. Sample 138-853B-6H-3, 65 cm. 6, 7. Discoaster sp. 2; ×2000; (6) Sample 138-853B-7H-2, 147 cm; (7) Sample 138-853D-7H-1,30 cm. 8. Discoaster neorectus Bukry; ×2000. Sample 138-853D-7H-1, 30 cm. 9, 10. Discoaster loeblichii Bukry; ×2000; (9) Sample 138-846B-30X-2, 60 cm; (10) Sample 138-853B-7H-4, 30 cm.


7 8 9Plate 3. All specimens ×4000, parallel light. 1,2,5. Amaurolithus primus (Bukry and Percival); (1, 2) Sample 138-853B-7H-3, 30 cm;(5) Sample 138-853B-7H-2, 147 cm. 3, 4, 6. Triquetmrhabdulus-Amaumlithus intergrade. Sample 138-853B-7H-2, 147 cm. 7, 8.Triquetrorhabdulus extensus-Amaurolithus amplificus intergrade; (7) Sample 138-853B-6H-4, 27 cm; (8) Sample 138-844B-5H-2, 129cm. 9. Amaurolithus amplificus (Bukry and Percival). Sample 138-844B-5H-2, 129 cm.

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6a 6b 7Plate 4. 1. Amaurolithus primus (Bukry and Percival); ×4000. Sample 138-853B-7H-2,147 cm; parallel light. 2. Triquetrorhabdulus extensus-Amaurolithus amplificus intergrade; ×4000. Sample 138-853B-6H-4, 65 cm; parallel light. 3. Amaurolithus amplificus (Bukry and Percival).Sample 138-853B-5H-6, 25 cm; parallel light. 4. Triquetrorhabdulus extensus Theodoridis; ×4000, lateral view. Sample 138-844C-4H-1, 50 cm.5-l.Ceratolithus{l)\ (5) Sample 138-845B-15H-2, 145 cm; (6,7) Sample 138-845B-15H-2, 100 cm; (5a, 6a, 7) crossed nicols; (5b, 6b)parallel light.

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