Stable isotopic study of Oligocene-Miocene sediments from DSDP Site 354, Equatorial Atlantic

Marine Micropaleontology, 8 (1983/84 ) 121--139

121

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

STABLE ISOTOPIC STUDY OF OLIGOCENE-MIOCENE SEDIMENTS FROM DSDP SITE 354, EQUATORIAL ATLANTIC MILENA BIOLZI

Department of Geology, Swiss Federal Institute of Technology, Zurich (Switzerland) (Received July 16, 1982; accepted after revision April 18, 1983)

Abstract Biolzi, M., 1983. Stable isotopic study of Oligocene-Miocene sediments from DSDP Site 354, Equatorial Atlantic. Mar. Micropaleontol., 8 : 121--139. The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene--Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a 5180 increase of 1.45%0 in Globigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in 180 in both planktic and benthic forms (about 1.0%0 and 0.51/00, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallow Globigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediate Globigerina venezuelana; and deep Catapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in ~aO.

Introduction Foraminiferal tests and coccoliths are the predominant carbonate components of pelagic sediments. Oxygen- and carbonisotopic compositions of these marine calcareous organisms are influenced by environmental conditions, and isotope stratigraphy can be used to delineate paleoclimatic changes (Urey, 1947). Calcite precipitated in thermodynamic equilibrium with sea-water has an oxygen-isotopic composition depending on the isotopic composition of the water the organisms inhabited and the temperature.

0377-8398/83/$03.00

Continental ice build-up preferentially extracts '80 depleted water from the ocean, enriching it in ~80, whereas, on the contrary ice melting decreases the '80 content of the oceans. Since Emiliani's first curve of Pleistocene isotope temperatures (Emiliani, 1955} a great deal of new data have been collected, requiring significant changes in its interpretation. There is now evidence that much of the Pleistocene isotopic variations reflect continental ice accumulation during glacial times and interglacial ice melting, rather than changes in the water temperature (Shackleton, 1967; Duplessy et al., 1970).

© 1983 Elsevier Science Publishers B.V.

122 The factors controlling the carbon-isotope composition of the biogenic carbonate are more complex, b u t it has recently been demonstrated (Broecker and Broecker, 1974; Kroopnick et al., 1977; Williams et al., 1977) that 13C/12C ratios m a y also provide valuable paleoceanographic information. In this study, oxygen- and carbon-isotope analyses were made on Oligocene--Miocene planktic and benthic foraminifera, as well as on calcareous nannofossiis, from closely spaced samples from DSDP Site 354 in the Equatorial Atlantic. The purpose of this investigation was three-fold: (1) to reconstruct the major climatic-paleoceanographic changes across the Oligocene--Miocene boundary in the western Equatorial Atlantic; (2) to reconstruct the depth habitats in the water column o f different species of planktic foraminifera; and (3) to test the utility of calcareous nannofossils as surfacewater paleotemperature indicators by comparing the stable-isotope compositions of foraminifera and nannofossils from the same samples.

Site and samples descriptions DSDP Site 354 lies on the northwest flank of the Ceam Rise (Fig. 1) in the Equatorial Atlantic (0.5°53.95'N 44°11.78'W} at a water depth of 4052 m. The Middle Oligocene to Upper Miocene sediments consist of foraminifera and nannofossil chalks which extend from 282.5 m to 529.5 m below the sea floor. It is important to note that the section was discontinuously cored: 47.5 m of sediment lying between cores 7 and 8, 47 m between cores 8 and 9, 47.5 m between cores 9 and 10 and 47 m between 10 and 11 were not recovered. Part o f the section m a y also be missing in cores 9 and 7. Samples ages were determined from their planktic foraminifeml and nannofossil associations (Biolzi, 1982). The Oligocene/Miocene b o u n d a r y is placed at the top of nannofoasil Zone NP25 (Martini, 1971) and at the base of foraminiferal Zone Globorotalia kugleri (Bolli, 1966). However, because stratigraphic ages of most o f the

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Fig. 1. Location of Site 354 and some other DSDP Sites mentioned in the text (after Initial Reports of the DSDP, Leg 39).

samples for which the isotopic results are available in the literature are dated according to Blow's numerical zonation (1969), this zonal scheme is plotted in Fig. 2 and Table I, and not Bolli's {1966). The Oligocene/ Miocene boundary falls within Blow Zone P.22. Analytical procedures The stable-isotope analyses were performed on monospecific samples of planktic and benthic foraminifera. Each planktic sample analyzed consisted of about 20 to 30 individuals, each benthic sample a b o u t 5 to 12 individuals. At least one benthic and several planktic species were analyzed for most samples. When the scarcity of benthic fauna did not allow the measurement of a second monospecific sample, mixed calibrated benthic samples and in some cases randomly mixed samples were analyzed. One planktic species, Globigerina venezuelana, and one benthic species, Cibicidoides pseudoungerianus were analyzed throughout the examined

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TABLE I Stable-isotopic c o m p o s i t i o n s o f separated species and m i x e d samples

Samples

Biozones

Species

~ lsOpDB

7-1,95

N.8

G lobigerinoides trilobus Globoquadrina dehiscens Glo borotalia mayeri Globigerina venezuelana

--1.80 --1.56

Mixed benthics Stilostomella spp.

Cibicidoides pseudoungerianus Globoca~idulinasubglobosa 7-2,70

N.8

G loborotalia mayeri Globigerina venezuelana Mixed benthics

7-3,70

N.8

G lob ige rinoides trilobus Globoquadrina dehiscens Globorotalia mayeri Globigerina venezuelana Mixed henthics

Globocauidulina subglobosa Stilostomella spp. Cibicidoidespseudoungerianus 7-4,65

N.8

Globorotalia mayeri Globoquadrina dehiscens Globigerinoides ruber Globigerina venezuelana Mixed benthics

Oridorsalis umbonatus Stilostomeila spp. Cibicidoides pseudoungerianus 8-1,74

N.5

Globoquadrina dehi$cens Globorotalia mayeri Globigerina venezuelana Catapsydrax die~imili$ Mixed benthics Stilostomella spp.

Cibicidoides pseudoungerianus 8-2,75

N.5

Globigerinoides trilobus Globorotalia mayeri Gioboq uadrina dehiscens Mixed nannofossils

G lobigerina venezuelana Catap~ydrax diuimili8 Stilo~tomella spp. Cibicidoides pseudoungerianus

N.5

+0.38 +1.55 +1.80 (+1.57) +1.97 (+2.23) +2.13 --1.18 +0.17 +1.00

+2.46 +1.62

+1.59 +1.35 +0.94 +0.99 (+1.62) +0.86 +1.09 +1.29 +1.40 +0.74

--1.24 --1.17 --1.04 +0.41 +1.07 (+1.40) +1.30 + 1.37 (+1.17) +1.57

+2.22 +2.65 +1.33 +1.08 --0.81 +0.66 +0.87 (+1.29) +0.53

--1.21 --1.02 --0.95 +0.21 +1.29 +1.41 +1.49 (+1.13) +1.53

+1.26 +2.46 +1.94 +1.17 +0.63 +0.49 +0.66 (+1.23) +0.47

--1.55 --0.98 +0.30 +0.85 +0.99 +1.26 (+1.11) +1.51

+2.07 +1.30 +1.03 +0.85 +0.45 +0.15 (+0.86) +0.10

--1.33 -- 1.15 -- 1.00 --0.24

+2.02 +1.53 +2.20 +1.73

+0.10

+1.15

Oridorsalis umbonatus

+0.95 +1.15 (+1.02) +1.42 +1.46 +1.65

Globigerinoides trilobus Globoquadrina dehiscens Globorotalia mayeri

--1.33 --1.31 --0.48

Mixed benthics 8-3,70

-- 1.10

6 ~3CpDB

+1.06 +0.18 (+0.39)--0.37 +0.36 --0.30 +2.24 +2.27 +1.06

*On the left c o l u m n are r e p o r t e d the data measured, on the right c o l u m n are the data cor. rected for disequilibrium.

125

Samples

Bizones

Species

6

Mixed nannofossils

--0.09 +0.40 +0.90 +1.57 (+1.26) +1.66

+1.66 +0.95 +0.83 +0.48 (+0.95) +0.19

--1.10 --0.61 --0.10 +0.64 +1.33 +1.51 (+1.63) +1.53 (+1.15)+1.55

+ 1.28 +1.65 +1.33 +1.07 +0.38 --0.16 +1.48 (+1.10) +0.34

-1.39 --0.86 --0.86 +O.35 +1.12 (+0.92) +1.32

+1.19 +1.52 +1.06 +0.72 +0.42 (+1.03) +0.27

--1.31 --1.13 --0.63 +0.57 (+0.93) +1.33

+0.64 +1.03 +0.87 +0.58 (+0.52)--0.24

Globigerina venezuelana Ca tapsydrax dissimilis Stilostomella spp. Cibicidoides pseudoungerianus 9-1,70

N.5

Glo borotalia mayeri Mixed nannofossils

Globigerina venezuelana Catapsydrax dissimilis Mixed benthics

Oridorsalis umbonatus Globocassidulina subglobosa

Cibieidoidespseudoungerianus 9-2,73

N.5

Globorotalia mayeri Mixed nannofossils Globigerina venezuelana Ca tapsydrax dissimilis Mixed benthics Cibicidoidespseudoungerianus

9-3,70

N.4

G loborotalia mayeri Mixed nannofossils

Globigerina oenezuelana Catapsydrax dissimili~ Cibicidoides pseudoungerianus 9-3,96

N.4

Globorotalia mayeri Mixed nannofossils

Globigerina venezuelana Ca tapsydrax diuimilis Mixed benthics

Cibicidoides pseudoungerianus 9-4,76

P.22

Globorotalia mayeri Mixed nannofossils

Globigerina venezuelana Catapsydrax diuimilis Cibicidoides pseudoungerianus Mixed benthics 9-5,70

P.22

Globorotalia k ugleri Globorotalia may eri Mixed nannofossils

G lob igerina venezuelana Catapsydrax dissimilis Mixed benthics

Cibicidoides pseudoungerianus 9,cc

P.22

Globoro talia may eri Mixed nannofossils

Globigerina venezuelana Catapsydrax dissimilis Mixed benthics 10-1,70

P.22

Mixed nanno fossils

Globigerina venezuelana Catapsydrax di~imili8

~aOpDB

6

--1.43 --0.95 --0.69

+0.77 +1.16 +0.79

+0.52 +1.00 (+0.66)

~3CpDB

+0.61 1.66

--0.22 (+0.54)--0.22

--1.43 --1.07 --0.57 +0.67 (+0.85) +1.25 +1.26

+0.96 +1.35 +0.92 +0.86 (+0.92) +0.16 +0.13

--1.79 --1.37 --0.65 --0.04 +0.93 +0.16 (+1.11) +1.51

+1.76 +0.64 +0.95 +0.50 +0.39 +0.34 (+0.54)--0.22

--0.91 --0.37 --0.05 +0.56 +0.96

+0.68 +1.38 +0.92 +0.80 --0.69

--1.04 --0.64 +0.51

+1.40 +0.91 +0.62

126 TABLE I (continued) Sample

Bizones

Species

61 sOpDB

10-2,53

P.22

Mixed nannofossils

Mixed benthics

--0.95 --0.62 +0.43 +1.15

S tilosto mella Cibicidoidespseudoungerianus

(+0.95) +1.35

(+0.75)--0.01

-:0.8_3 --0.25 +0.68 +1.51 (+1.62) +2.02

+1.64 +0.76 +0.60 +0.26 (+0.60)--0.16

--0.59 --0.19 +0.94 + 1.42 +1.74 (+1.55) +1.95

+1.96 +1.20 +0.88 +0.71 +0.44 (+0.85) +0.09

--0.60 --0.08 +1.08 +1.70 +1.82 (+1.48) +1.88

+2.01 +0.96 +0.63 +0.05 +0.15 (+0.78) +0.02)

Globigerina venezuelana Catapsydrax dissimilis

10-3,70

P.22

Mixed r r a ~ q

Globigerina venezuelana Catapsydrax dissimilis Mixed benthics

Cibicidoides pseudoungerianus 10-4,70

P.21

Mixed nannofossils

Globigerina venezuelana Catapsydrax diuimilis Mixed benthics

Stilostomella spp. Cibicidoidespseudoungerianus 10-5,64

P.21

Mixed nannofossils

Globigerina venezuelana Catapsydrax di~imili~ Mixed benthics

Stilostomella spp. Cibicidoides l~eudoungerianus 10-6,70

P.21

Mixed nannofo~sihs

Giobigerina venezuelana Catapsydrax di~imilis Globocassidulina subglobosa Cibicidoidespseucloungerianus Stilostomeila spp. Mixed benthics lO,cc

P.21

Globigerina venezuelana Mixed nannofossils

Globorotalia opima opima Catapsydrax diuimilis Mixed benthics

Cibicidoidespseudoungerianus 11-1,72

P.21

Glob igerina venezuelana

+1.35 +0.67

+0.49

(+1.22) +1.12 (+1.03) +1.43 +1.47 +2.12

+0.06 (+0.44)--0.32 +0.11 +0.14

--0.68 --0.49 +0.23 +0.81 +0.93 (+1.33) +1.73

+0.48 + 1.13 +1.55 +0.06 --0.69 (+1.04) +0.28 +0.77 +1.37

Gio bo talia opima opima Catapeydrax diuimilis

+0.20 +0.89

+0.70 +0.44

Glo bigerina venezuelana Glo bo rotalia opima opima Catapsydrax di~imilis

+1.58 +1.68 (+1.38) +1.78

--0.36 +0.10 (+0.49)--0.27

--1.45

+0.77

+0.12

+0.50

Cibicidoides p#eudoungerianu8 Stilostomella spp.

+0.62 +1.37 (+1.30) +1.70 +1.98

Globoq uadrina globularis Globigerina venezuelana Catapsydrax di~imili8

--1.33 --1.28 +0.59

Mixed benthics

P.21

+0.70

+0.15

--1.32 --0.88

Mixed benthics

11-2,46

--1.00 --0.29

+1.29 +0.70 +0.53 +0.15

Mixed nannofossils

Stilostomella spp. Cibicidoidespseudoungerianus 11-1,100 P.21

+1.32

6 IaCpDB

+0.43 +0.20 (+0.55)--0.21 --0.28 +0.67 +0.44 +0.08

127

Species

,5 ]sOpD B

Mixed benthics Stilostomella spp. Cibicidoides pseudoungerianus

+0.98 --0.31 +1.32 --0.82 (+1.04) +1.44 (+0.18)--0.58

11-2,98 P.21

Globigerina venezuelana Mixed nannofossils Catapsydrax diuimilis Mixed benthics Cibicidoides pseudoungerianus

--1.13 +0.77 --1.00 +1.40 +0.83 +0.40 +1.64 --0.27 (+1.43) +1.83 (+0.49)--0.27

11-3,98 P.21

Globigerina venezuelana Globoquadrina globularis Catapsydrax diuimilis Mixed benthics Cibicidoides pseudoungerianus

--1.35 +0.58 --1.25 +1.60 +0.46 +0.48 +0.81 --0.76 (+1.19) +1.59 (+0.47)--0.29

11-4,70 P.20

Gioboquadrina globularis Giobigerina venezuelana Mixed nannofossils Catapsydrax dissimilis Mixed benthics Cibicidoides pseudo ungerianus

--1.35 +0.52 --1.34 +0.56 --0.96 +1.06 +0.49 +0.22 +1.33 --0.07 (+1.24) +1.64 (+0.59)--0.17

11-5,70 P.20

Globigerina venezuelana Catapsydrax diuimilis Mixed benthics Stiloetomella spp. Cibicidoides pseudoungerianus

--1.51 +0.4~ +0.73 +0.20 +1.10 --0.27 +1.46 --0.54 (+1.27) +1.67 (+0.66) --0.10

11-6,70 P.20

Globoquadrina globularis Globigerina venezuelana Catapsydrax diuimilis Cibicidoides pseudoungerianus Mixed benthics

--1.48 +0.74 --1.44 +0.73 +0.49 +0.40 (+1.10) +1.50 (+0.64)--0.12 +1.78 +0.43

Sample

Bizones

section. Representative individuals of each species were picked from the fractions 180 p m to 450 p m (Plate I, 1--10). The analytical techniques used to determine the 1sO and 13C compositions have been described by Shackleton (1974). Each foraminifer was lightly crushed with a point of a needle. The samples were cleaned ultrasonically to remove adhering fine-grained material and chamber fillings. Isotopic analyses o f calcareous nannofossils were performed on fine-fraction samples obtained by washing the disaggregate bulk sediment through a 28 pm sieve. Scanning electron micrographs showed t h a t the < 2 8 ~m fraction consisted of relatively pure polyspecific calcareous nannofossiis (Plate I, 11). All samples were roasted in vacuo at 450 ° for 30

5 t3CpD B

min prior to isotopic analysis. To test the reproducibility of the data, multiple analyses were made on all nannofossil and foraminifera samples, wherever enough material was available. Results were obtained as per rail deviations from a laboratory standard and then related to the international PDB Standard by a calibrated conversion factor. The overall analytical precision averages 0 . 0 3 ° o . The data are listed in Table I. Stable-Isotope Results Oxygen-isotope composition Benthic foraminifera In order to estimate bottom-water conditions, Cibicidoides pseudoungerianus was

128 PLATE I

129

analyzed in most samples, Stilostomella spp. in a few, and Globocassidulina subglobosa and Oridorsalis umbonatus only sporadically. A mixed population of four, not constantly calibrated, benthic species (Cibicides spp., Anomalina spp., Globocassidulina subglobosa and Pullenia bulloides) was also analyzed in most samples. Their isotopic compositions are listed in Table I but are not plotted in Fig. 2 because of the lack of an adjusting factor for such a mixture. Uvigerina which may deposit its CaCO3 in isotopic equilibrium with water (Shackleton, 1974) was absent in the section. Since Cibicidoides pseudoungerianus is distributed throughout the whole sequence, its isotopic composition was plotted as representative of bottom-water changes. From isotopic investigations on living foraminifera, 6'SO of Cibicidoides( = Cibicides) deviates by about 0.400/00 from that of Uvigerina, and Globocassidulina deviates from Uvigerina by about +0.10%0 (Boersma and Shackleton, 1977a, b). Therefore, the values plotted in Fig. 2 ate adjusted by this amount. Woodruff et al. (1980) also mentioned a depletion of about 0.65°/00 in 180 o f Cibicidoides relative to that of Uvigerina. In the Zone P.20, the older portion of the studied section, the 1sO composition of Cibicidoides pseudoungerianus is more or less consistent at about +1.60°/0o. Two rather modest 6 ~80 fluctuations (about 0.40/00) in the Zone P.21 are followed by a positive shift which reaches a 'sO m a x i m u m at +2.00/0o, in the lower part of Zone P.22. In the two samples closest to the Oligocene/Miocene boundary, the benthic fauna was very poor and no Cibicidoides could be analyzed. Up to the end of the Early Miocene, the ~80 values of Cibicidoides ranges from +1.06°/0o to +1.66°/oo, with a maximum of about PLATE

+1.70%0 in sample 8-3,70. Above the hiatus where the biozone N.6 and N.7 are missing, the 5180 of Cibicidoides increases by about 0.50/00, reaching +2.00/00 .

Planktic foraminifera In order to estimate pelagic-water conditions, the following species have been analyzed: Globigerinoides spp. and Globoquadrina dehiscens in some Miocene samples, Globorotalia mayeri continuously in the Miocene sequence, Globigerina venezuelana throughout the section and Catapsydrax dissimilis up to its extinction, in Zone N.5. Globigerinoides is known to dwell in shallow water (Lidz et al., 1969; Berger, 1969; B~ and Tolderlund, 1971; Douglas and Savin, 1978}; in this study it indeed yields the lowest 8 ' sO values. Globigerina venezuelana, which ranges throughout the studied section, is taken as a reference for oxygen-isotopic changes in intermediate sea waters through the Oligocene--Miocene time. With a general trend toward higher 6 'sO values, its oxygen-isotope composition increases from about --1.5°/0o in the Middle Oligocene to about +0.50/00 in the Middle Miocene. The most striking event is the positive shift o f the 8 '80 of the calcite tests of Globigerina venezuelana from --1.45°/00 to about 0°/0o in Zone P.21. If interpreted as reflecting only temperature changes, such an isotopic difference would represent an extremely intensive cooling. The second main 180 enrichment took place in Zone N.5, with a shift of +1.2°/o0. After this episode, the 8180 of planktic foraminifera remained relatively stable, between +0.20/o0 and +0.5%0, up to the end of the section. It is possible that the two major 6180 shifts discussed are not as linear as they appear in

I

1--5. Analyzed planktic foraminifera (all 80× ): 1. Globigerina venezuelana; 2. Globorotalia raayeri; 3. Catapsydrax dissimilis; 4. Gioboquadrina dehiscens; 5. Glogiberinoides trilobus. 6--10. Analyzed benthic foraminifera (all 80× ): 6. Cibicidoides pseudoungerianus; 7. Globocassidulina subglobosa ; 8. Pullenia bulloides; 9. Oridorsalis umbonatus; 10. Stilostomella adolphina. 11. Scanning electron micrograph of the fine fraction sediment <28 # (500×).

130

Fig. 2 because between cores 8 and 9, and 10 and 11 two coring gaps of 47 m and 47.5 m, respectively, exist. The accumulation rate for the Oligocene and Early Miocene at this site was calculated to be in the order of 1.5 cm/ 1000 years (Perch-Nielsen et al., 1977). Therefore, the missing 47 m (47.5 m) of sediments correspond to a time span of about 3.13 m.y. (3.16 m.y.). During such a long period unrecorded changes and fluctuations possibly took place. Catapsydrax dissimilis is the other species analyzed over a long stratigraphic interval. It yields the most positive 5 ~sO among the planktic species, which confirms its presumed deep-water habitat. Its isotopic composition follows closely that of the other planktic foraminifera (highest ~ ~sO values in samples 8-3,70 and 10-5,64; a minimum in sample 9-2,73), but the isotopic variations are of lesser magnitude than those of Globigerina venezuelana. However, the closest similarity can be recognized between the shape of the oxygen isotope curve of Catapsydrax dissimilis and the profiles of the benthic species.

Nannofossils Nannofossils are the remains of unicellular, marine algae and the isotopic composition of the calcium carbonate they secrete strongly reflects the effect of respiration and photosynthesis. In Fig. 2, the ~80 values of calcareous nannofossils are plotted together with those of foraminifera. From sample 8-2,75 downwards to sample 10-6,70, the nannofossil 5~80 profile is closely parallel to the associated foraminifer profiles ff samples 9-2,73, 9-3,70 and 9-3,96 are excluded. Their 180 compositions fall between those of Globigerina venezuelana and Globorotalia mayeri, where the latter species has been analyzed. From sample 10,cc downwards, the behaviour of the profile of the few nannofossil samples available for analysis differs from the one above. The ~ ~sO of calcareous nannofossils are more positive than those of Globigerina venezuelana and, as far as the four analyzed samples are concerned, they do not reflect the positive shift recorded by

G. venezuelana. The total range of the ~sO values of the analyzed mixed nannofossil samples is about l°/00 . Carbon-isotope composition 5~3C values of foraminifera increase upward in the section. The three planktic foraminifera analyzed have ~3C compositions tightly clustered in a restricted range. Catapsydrax dissimilis and Globigerina venezuelana ~3C values plot parallel to each other with Globigerina enriched in 13C by about 0.250/0o relative to Catapsydrax. 513C of Globorotalia mayeri shows much more variation than the other planktic species. Somewhat similar also are the curves plotting the ~3C compositions of Catapsydrax dissimilis and Cibicidoides pseudoungerianus, if samples 8-2,75, 10,cc and 11-3,98 are excluded. The ~3C values of Cibicidoides pseudoungerianus in Table I and Fig. 2 are adjusted by --0.760/00 (N.J. Shackleton, pers. comm., 1982). The curve plotting the 5 ~3C of calcareous nannofossils closely follows the curves of the other planktic species, if samples 8-3,70 and 10-5,64 are excluded. Nannofossils are enriched in ~3C relative to the foraminifera; the difference between the 8 ~ac of nannofossils and that of the most positive foraminifer is greater in the lower than in the upper part of the section. Discussion

Oxygen isotopes The oxygen-isotope paleotemperature method is based on the assumption that calcium carbonate of marine organisms is precipitated in isotopic equilibrium with sea water. Investigations on living and fossil foraminifera have produced evidence both supporting and refuting this assumption. Evidence for isotopic disequilibrium in living planktic foraminifera was supported by Van Donk {1970), Shackleton et al. (1973), Vergnaud Grazzini (1973), Kahn (1977), Williams et al. (1977) and Berger et al.

131 (1978). Departures from isotopic equilibrium between benthic foraminifera and sea water have been shown by Duplessy et al. (1970), Buchardt and Hansen (1977), Erez (1978) and Graham et al. {1981). Shackleton (1974) has demonstrated that although some benthic species do secrete their tests out of equilibrium with sea water, the genus Uvigerina is at, or very close to, isotopic equilibrium. Douglas and Savin (1973), Saito and Van Donk (1974), Shackleton and Kennett (1975a), Savin et al. {1975), among others, have demonstrated that isotopic analyses provide a generally correct picture of temperature trends. The relationship between the temperature of precipitation and the isotopic composition of the organic carbonate is expressed in the empirically calibrated equation (O'Neil et al., 1969; Shackleton, 1974):

T= 1 6 . 9 - - 4 . 3 8 ( 5 c - - 5 w ) + 0.10 (5c-- 8w):

(1) where 5c is the oxygen isotopic composition of CO2 extracted from the carbonate and 5w is the oxygen isotopic composition of the water. In order to calculate paleotemperatures, the isotopic composition of the water which the foraminifera inhabited must be estimated. Shackleton (1967) estimated the mean oxygen-isotopic composition of the oceans (Gw) as --1.2%0 (PDB) prior to the Middle Miocene accumulation of the Antarctic ice-sheet. I have used this value to estimate isotopic temperatures. The parallel shapes of the curves shown in Fig. 2 and the agreement of the isotopic compositions of different benthic taxa from the same sample are good indicators that the data accurately depict oceanographic changes. The planktic data plotted in Fig. 2 clearly indicate a marked depth-temperature stratification in the pelagic zone. The high values and the small fluctuations exhibited by the 5180 of Cibicidoides pseudoungerianus testify to the cold, rather stable conditions of the Equatorial Atlantic bottom waters in the Middle Oligocene--Early Miocene. The in-

creasing ti 180 values in the Middle Miocene, reflected in the uppermost sample from Zone N.8, can be interpreted as the beginning of the Middle Miocene temperature decline associated with the ice-sheet accumulation in Antarctica. The parallelism of the 5180 curves for Catapsydrax dissimilis and Cibicidoides pseudoungerianus, particularly evident in the short-term fluctuations and the high 180 values of Catapsydrax dissimilis, suggest a deep habitat for Catapsydrax, presumably below the thermocline (Boersma and Shackleton, 1977b), in a regime of little environmental change. The small ranges of the isotopic variation of Catapsydrax (0.70/00) and Cibicidoides (0.96%0) more or les correspond to the rather homogeneous data reported from all oceans in the Oligocene-Early Miocene (Moore et al., 1981) and support the assumption that bottom and deepwater isotopic variations mostly reflected changes in the global ice volume and/or temperature changes at high latitudes already in the Oligocene--Miocene as in the Quaternary (Shackleton, 1967). The 51sO values of the intermediate and shallow-water species are more variable during the same time interval. A large increase in the ~ 1sO of Globigerina venezuelana (about 1.45%0 ) took place in the Oligocene (P.21, Globorotalia opima opima Zone). If interpreted only in terms of paleotemperature, a 1.45%0 increase would correspond to a cooling of 6°C, which is unlikely considering the stable climatic conditions at the present equatorial latitudes (Sverdrup et al., 1942; Kennett, 1982). Such a dramatic change could not reflect only accumulation of ice-sheets in the Antarctic because (1) an enormous ice-volume would be necessary to explain a difference of 1.45°/00 at equatorial latitudes (the difference between the isotopic composition of the present ocean and that of the ocean prior to the formation of the present ice-sheet is estimated at 1.2°/0o; Shackleton and Kennett, 1975b), and (2) such a large ice-accumulation on the Antarctic continent would have influenced the 1sO

132

b o t t o m - w a t e r composition as well as the surface--intermediate waters. The '80/'60 ratios of the deep planktic and benthic foraminifera do not show a change comparable to that of Globigerina venezuelana. Therefore, a further explanation is required. Positive oxygen-isotope shifts in the biozone P.21 were identified in different localities: Site 167, in the Equatorial Pacific (Douglas and Savin, 1973); Site 357, in the low latitude south Atlantic (Boersma and Shackleton, 1967); Site 366, in the eastern Equatorial Atlantic (Boersma and Shackleton, 1977b and Vergnaud Grazzini and Rabussier Lointier, 1980). Dorman (1966) and Devereux (1967) found also temperature minima in the Oligocene of Australia and New Zealand, respectively. The amplitude of most of these generally synchronous shifts is about 0.5% 0; only at Site 366, located at the same latitude but 25 ° further westward than Site 354, it is higher (about 1.5%0 ) with larger lsO fluctuations in planktics than in benthics. This event is explained by the different authors as a global change in the isotopic composition of the ocean water due to ice-accumulation in the Antarctic region. But the larger isotopic shifts recorded at Sites 366 and 354 (about 1.00°/00 and 1.45°/00, respectively) could also reflect local phenomena, the onset of cold surface currents, the migration of the site due to plate tectonic motions, or a change in the oceanic circulation pattern might have amplified the minimal glacial effect at equatorial latitudes. The different nature of the samples analyzed here and those used by Vergnaud Grazzini and Rabussier Liontier (1980) could be responsible for the different amplitude of the shifts found in the two sites (monospecific in the first case, multispecific in the latter). Formation of some shelf ice was quite probable in the Antarctic region by Oligocene time, even if the Antarctic icesheet was not fully developed (Margolis and Kennett, 1971; Leclaire, 1974; Frakes and Kemp, 1972; Shackleton and Kennett, 1975b; Savin et al., 1975; Kennett, 1982); cold surface water could consequently have charac-

terized the latitudes lower than 60 ° S. The incursion of a cold, South Atlantic water mass into the equatorial region could have influenced the oxygen-isotopic composition of the biogenic calcite precipitated in that water. The modern Benguela Current is a cold surface current which forms in the South Atlantic (about 60°S) and flows northwestward in a counter-clockwise gyre. After following the west coast of Africa, it is deflected westward, flowing into the equatorial area between 0 ° and 20°S and is known as the South Equatorial Current. Approaching the coast of South America, the South Equatorial Current diverges into two branches: the Brazil Current which flows southward, and the Guiana Current which crosses the Equator and enters the North Atlantic Ocean, influencing the character of the water along the northeastern coast of South America, the Caribbean Sea and the Gulf of Mexico. Between depths of 100--200 and 600--700 m, the waters of the Equatorial Atlantic south of about 10°N are mainly of South Atlantic origin (Sverdrup et al., 1942}. After the development of the circum-Antarctic current during the Oligocene causing major reorganization of circulation patterns in the South Atlantic (Kennett, 1982), only minor changes occurred in the South Atlantic circulation system {Fig. 3). In the Middle Oligocene, cold South Atlantic water was probably transported into the equatorial region, much as it is today. Reconstructions of the position of the continents (Sclater et al., 1977} suggest that during the Oligocene Site 354 might have been located further south than the present latitude. Therefore, I conclude that South Atlantic water transported by the South Atlantic circulation system reached low latitudes in the Late Oligocene and influenced the temperature of the equatorial waters between about 100 and 600 m causing the '80 enrichment recorded by Gl ob ige rina venezue lana. Another 8 ~sO increase, in the Early Miocene (Zone N.5), is recorded in the shallowintermediate water species (Globorotalia mayeri, 0.91°/00 ; calcareous nannofossils,

133

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Paleotemperat

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Fig. 3. I n f e r r e d Middle Oligocene o c e a n c i r c u l a t i o n ( a f t e r F r a k e s a n d K e m p , 1972).

0.95O/o0 ; Globigerina venezuelana, 1.26%o ) as well as in the deep and b o t t o m ones, but with smaller amplitude (Catapsydrax dissimilis, 0.65O/o0; Cibicidoides pseudoungerianus, 0.34°/0o ). Ice-volume effect and temperature variation could have been responsible for this fluctuation. As known for the Quaternary (Shackleton, 1967), the ~ 8 0 fluctuations in b o t t o m and deep waters formed in polar regions essentially reflect changes in the global ice-volume, whereas the fluctuations in the surface-intermediate waters result from a combination o f both ice-volume and temperature changes. In other words, an increase in ~ 'SO in benthics would mean an increase in global ice-volume while a corresponding 51sO increase in planktics is a result of an increase in ice-volume plus a decrease in surface-water temperature. In core 9 a major increase in 51SO of both planktic and benthic foraminifera is observed. The planktic increase is on average 0.60/00 greater than in the benthic. Therefore this isotopic change is interpreted as a combination of temperature decrease and corresponding increase in the global ice-volume.

The '80 compositions of calcareous nannofossils show a rather complex interrelationship with the 180 compositions of foraminifera. From sample 8-2,75 downwards to sample 10-6,70 the profiles plotting the 5 '80 o f nannofossils and those of planktic foraminifera run approximately parallel. This would suggest that the isotopic composition of the nannofossils was generally controlled by the same parameters which controlled the isotopic composition of the shallow and intermediate dwelling foraminifera, but the former are systematically enriched in both 1sO and 13C relative to the latter. This is rather puzzling because oxygen values for the nannofossils indicate cooler temperature and deeper habitat than are indicated for Globorotalia mayeri, whereas the carbon data, if interpreted in a straightforward way, would indicate that the nannofossils should have occupied a shallower depth than

G. mayeri. From sample 10-6,70 downward the ~sO compositions of the few available nannofossil samples have a different relationship to the value for planktic foraminifera. From

134 more negative 5 ~sO than those of Globigerina venezuelana, they change to a more positive one and apparently do not follow the positive shift recorded in the 5 'sO of G. venezuelana. The isotopic literature reports a general parallel trend between nannofossil and shallow-dwelling foraminiferal profiles but also departure from the isotopic equilibrium with surface waters (Margolis et al., 1975; Goodney et al., 1980). Goodney et al. (1980) report 5~sO of recent nannofossfls from Pacific and Mediterranean by about 0.5 to 1.0°/00 heavier than those of shallow-dwelling planktic foraminifera. If an adjusting factor for disequilibrium fractionation by approximately these values is applied to the nannofossil ~sO compositions obtained here, these would be as light as or even lighter than the 6~sO of the shallow
plex and less well understood than those influencing their '80/'60 ratios. The '3C content of bicarbonate in sea water is controlled by inorganic and biological processes: (a) the bulk ~3C content depending on the ~3C content of the incoming carbon in rivers; (b) selective removal of '2C by phytoplankton during photosynthesis in surface waters, which leaves the remaining inorganic carbon near the surface enriched in '3C; (c) oxidation of the organic matter that falls into deep water; and (d) atmosphere/ocean exchange, which adds '3C-depleted CO2. Any effect leading to non-equilibrium fractionation or "vital effect" during the shellbuilding processes, e.g. metabolic processes within the cell (Berger et al., 1978; Erez, 1978), ~2C increasing with increasing nutrient availability (Berger et al., 1978), changes in the source of carbon incorporated in the calcite tests (Douglas and Savin, 1978) can affect the 5~3C value of the biogenic carbonate. The carbon-isotope data plotted in Fig. 2 suggest that carbon-isotope compositions of foraminiferal tests reflect depth stratification. Nannofossils and surface dwelling foraminifera are enriched in '3C, while deep dwelling and bottom waters species are depleted in ~3C. In the m o d e m Atlantic, the ~ac of the surface water CO: ranges between 1%0 and 2°/o0 due to the equilibration with atmospheric CO2 (Kroopnick, 1974, 1980) with small variations in upwelling areas and high latitudes. 5 ~3C values approach 0°/00 in underlying deeper waters due to the oxidation of organic matter. A minimum in 5 ~3C often occurs in the region of the dissolved O2 minimum (Deuser and Hunt, 1969; Kroopnick et al., 1972; Kroopnick, 1974, 1980; Shackleton and Vincent, 1978; Bender and Keigwin, 1979). The high 8 ~3C measured in the Oligocene--Miocene surface water is consistent with the carbon isotope composition of the m o d e m Atlantic surface-waters.

Carbon isotopes

5 ~aO versus 523C -- Depth habitat

The factors influencing the carbon-isotope compositions of foraminifera are more corn-

Oxygen- and carbon-isotope compositions of calcite shells have been the basis for several

135

studies of foraminifera temperature and depth stratification (Emiliani, 1954; Lidz et al., 1968; Oba, 1969; Hecht and Savin, 1972; Savin and Douglas, 1973; Saito and van Donk, 1974; Boersma and Shackleton, 1977b; Douglas and Savin, 1978). Investigations on living foraminifera from plankton tows have actually demonstrated that the faunal composition of foraminifera assemblages in the water column usually changes with depth in the upper few hundred metres (Schott, 1935; Bradshaw, 1957; B~ and Lott, 1964; Berger, 1969). This vertical stratification is closely related to water density which in turn is controlled by temperature and salinity. If the 5180 and 5'3C values recorded by the planktic foraminifera (Table I) are interpreted as reflecting the environmental conditions in which the calcite tests were secreted, the following relative dwelling depth-habitats are inferred: Globigerinoides and Globoquadrina shallowest (lightest ' 8 0 , heaviest 13C compositions}; followed by intermediate -- dwelling

Globorotalia mayeri and Globigerina venezuelana, with the deepest habitat suggested for Catapsydrax dissimilis. In Fig. 4 the 5'80 versus the 5'3C of the benthic and the most representative planktic foraminifera analysed are plotted. The central lower position of Cibicidoides pseudoungerianus values indicates heavier (colder) '80 composition and more negative 5'3C, very matching its sediment-surface habitat. The deep dwelling Catapsydrax dissimilis 6 '8C and 513C values fall in a slightly higher and more left-hand side field, attesting to warmer, '3C-enriched waters. The partial overlapping of Globorotalia mayeri and Globigerina venezuelana fields denotes close habitat conditions between the two groups. The elongated field in which Globigerina venezuelana values are distributed indicates more intensive changes in the 1sO and '3C contents of the near-surface calcite tests than those from bottom and deeper waters and reflects the input of different water masses into the intermediate and surface ocean strata. The relatively small 5180/513C fields of Catapsydrax

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dissirnilis and Cibicidoides pseudoungerianus reflect the more stable hydrographic conditions in the deep and bottom waters and the lack of influence of the above mentioned circulation system on the deep sea layers. Conclusions Significant changes were found in the oxygen and carbon isotope compositions of Oligocene--Miocene planktic foraminifera

Globorotalia mayeri and Globigerina venezuelana from Site 354. Only minor fluctuations were recorded for the isotopic compositions of the deep planktic species Catapsydrax dissimilis and the benthic species Cibicidoides pseudoungerianus. The fluctua tions reflect not only temperature and icevolume changes, but changes in circulation patterns. The large 'SO enrichment recorded by Globigerina venezuelana in the biozone P.21 can not be attributed to temperature decrease and ice accumulation on the Antarctic continent only, as it has no equivalent in the deep pelagic and bottom water signals.

136

The incursion of cold South Atlantic water into the Equatorial region, flowing northward via the South Atlantic current system offers the best explanation for the isotopic pattern observed. Ice-volume effects and temperature decreases are, on the other hand, considered responsible for the second significant stage of 180 enrichment, this one registered from surface-dwelling to bottom dwelling organisms, in Zone N.5. The results obtained for the Sierra Leone Rise sediments, at Site 366, confirmed the strong influence of the South Atlantic water circulation on the isotopic composition of the surface calcareous organisms inhabiting the Equatorial Atlantic region, as suggested by Vergnaud Grazzini and Rabussier Lointier, 1980. Both oxygen and carbon isotopes provide significant information on the stratification of planktic organisms in the water column; three groups characterized by different depthhabitats have been identified: Globigerinoides, Globoquadrina, Globorotalia mayeri and nannofossils have low ~sO values and high ~3C values, typically associated with relatively warm surface water; the intermediate isotopic composition of Globigerina venezuelana indicates a depth habitat between 100 and 600 m; Catapsydrax dissimilis has high ~sO values and low 13C values, typical of a very deep pelagic habitat, probably below the thermocline. An isotopic paleotemperature of about 18°C is inferred for the intermediate water in Zone P.20, then, through a general ~80/~60 ratio increase, the temperature would have fallen to about 9°--11°C in the Middle Miocene (Zone N.8). Average temperatures in the range of 10°--7°C and 6°--3°C, throughout the sequence, are obtained from deepand bottom-water foraminifera, respectively. In the Miocene section, Globorotalia mayeri gives the highest isotopic temperature (about 17°--15°C). A comparative investigation of the isotopic composition of planktic foraminifera and calcareous nannofossils indicates for the latter a 8'SO enrichment relative to the

shallow-dwelling planktic recorded in recent fauna.

foraminifera, as

Acknowledgements I am grateful to J. McKenzie for her critical reading of the manuscript and suggestions for improvement, to N. Shackleton who introduced me to the study of the stable isotopes and reviewed the manuscript, to K. Hsii and H. Oberhaensli for the helpful discussions and to U. Gerber for the photographic work. This work is contribution N.207 of the Laboratory of Experimental Geology, Zurich. References Anderson, T.F. and Cole, S.A., 1975. The stable isotope geochemistry of marinecoccoliths: a preliminary comparison with planktonic foraminifera. J. Foraminiferal Res., 5, 3: 188--192. B~, A.W.H. and Lott, L., 1964. Shell growth and structure of planktonic foraminifera. Science, 145(3634) : 823--824. B~, A.W.H. and Tolderlund, D.S., 1971. Distribution and ecology of living planktonic Foraminifera in surface waters in the Atlantic and Indian Oceans. In: B.M. Funnel and W.R. Riedel (Editors), Micropaleontoiogy of Oceans. Cambridge University Press, London, pp. 105--149. Bender, M.L. and Keigwin, L.D., 1979. Speculations about the upper Miocene change in abyssal Pacific dissolved bicarbonate C-13. Earth Planet. Sci. Lett., 45: 383--393. Berger, W.H., 1969. Ecologic patterns of living planktonic Foraminifera. Deep-Sea Res., 16: 1-24. Berger, W.H., Killingley, J.S. and Vincent, E., 1978. Stable isotopes in deep-sea carbonates: Bob Core ERDC-92, West Equatorial Pacific. Oceanol. Acta, I, 2: 203--216. Biolzi, M., 1982. The Oligocene/Miocene boundary in the Equatorial Atlantic D S D P Site 354. Results of studies on planktic foraminifera and calcareous nannofossils. Riv. Ital. Paleontol. Stratigr.~88(1): 113--131. Blow, W.H., 1969. Late middle Eocene to Recent planktonic foraminiferal biostratigraphy. Proc. 1st Int. Conf. Plank. Micr., 1: 199--442. Boersma, A. and Shackleton, N.J., 1977a. Tertiary oxygen and carbon isotope stratigraphy, Site 357 (mid latitude south Atlantic). In: P.R. Supko, K. Perch-Nielsen et al.,InitialReports of the Deep

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Sea Drilling Project, 39. U.S. Government Printing Office, Washington, D.C., pp. 911--924. Boersma, A. and Shackleton, N.J., 1977b. Oxygen and carbon isotope record through the Oligocene, DSDP Site 366, Equatorial Atlantic. In: Y. L a n c e lot, E. Seibold et al., Initial Reports of the Deep Sea Drilling Project, 41. U.S. Government Printing Office, Washington, D.C., pp. 957--962. Bolli, H.M., 1966. Zonation of Cretaceous to Pliocene marine sediments based on planktonic Foraminifera. Boll. Inf. Assoc. Venez. Min. Petrol., 9/1: 3--32. Bradshaw, J.S., 1957. Ecology of Living Planktonic Foraminifera in the North and Euqatorial Pacific Ocean. Thesis, Univ. California, Los Angeles, 256 pp. Broecker, W.S. and Broecker, S., 1974. Carbonate dissolution on the western flank of the east Pacific Rise. In: Hay (Editor), Studies in Paleo-Oceanography. Soc. Econ. Paleontol. Mineral., Spec. Publ., 20. Buchardt, B. and Hansen, H.J., 1977. Oxygen isotope fractionation and algal symbiosis in benthic foraminifera from the Gulf of Elat, Israel. Bull. Geol. Soc. Den., 26: 185--194. Cati, F. (Editor), 1981. In search of the Paleogene/ Neogene Boundary Stratotype, Part 1: Potential Boundary stratotype sections in Italy and in Greece and a comparison with results from the Deep-Sea. G. Geol., 44: 1--210. Deuser, W.G. and Hunt, J.M., 1969. Stable isotope ratios of dissolved inorganic carbon in the Atlantic. Deep Sea Res., 16: 221--225. Devereux, L, 1967. Oxygen isotope paleotemperature measurements on New Zealand Tertiary fossils. N.Z.J. Si., 10: 988--1011. Dorman, F.H., 1966. Australian Tertiary paleotemperatures. J. Geol., 7 4 : 4 3 ~ 1 . Douglas, R.G. and Savin, S.M., 1973. Oxygen and carbon isotope analyses of Cretaceous and Tertiary foraminifera from the central North Pacific. In: E.L. Winterer et al., Initial Report o f the Deep Sea Drilling Project, 17. U.S. Government Printing Office, Washington, D.C., pp. 591---606. Douglas, R.G. and Savin, S.M., 1978. Oxygen isotopic evidence for the depth stratification o f Tertiary and Cretaceous planktic foraminifera. Mar. Micropaleontol., 3 : 175--196. Duplessy, J.C., Lalou, C. and Vinot, A.C., 1970. Differential isotopic fractionation in benthic foraminifera and paleotemperatures reassessed. Science, 168: 250--251. Emiliani, C., 1954. Depth habitats of some species o f pelagic foraminifera as indicated by oxygen isotope ratios. Am. J. Sci., 252: 149--158. Emiliani, C., 1955. Pleistocene temperatures. J. Geol., 63 : 538--578. Emiliani, C., 1971. Depth habitats of growth stages of pelagic foraminifera. Science, 173: 1122--1124.

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