planktonic foraminiferal paleoceanography of the subantarctic Indian Ocean

QUATERNARY

RESEARCH

9, 71-86 (1978)

Comparison of RadiolarianlPlanktonic Foraminiferal Paleoceanography of the Subantarctic Indian Ocean DOUGLAS Graduate

School

F. WILLIAMS’ of Oceanography, Kingston, Rhode

AND JOHN KEANY~ University of Rhode Island 02881

Island,

Received July 6, 1976 A detailed paleoceanographic history of the Subantarctic region for the last million years was determined using paleomagnetic stratigraphy, radiolarian and planktonic foraminiferal ___I--mbiostratigraphy, and the oxygen isotope record from stages 1 to 13 (0.5 MY) in a deep-sea core (E45-74) from the southern Indian Ocean. Changes in the abundances of Antarctissa -_-&elEcjvi and-._. Neogloboquadrina pachjderma record 12 glacial/interglacial cycles. The paleoceanograph%--.&vents based on the combined results of these siliceous and calcareous indexes agree with changes in the global ice-volume record. Calcium carbonate dissolution selectively alters the planktonic foraminiferal fauna and causes test fragmentation and increased numbers of benthic foraminifera and radiolarians. Intense periods of calcium carbonate dissolution are associated principally with glacial episodes and are probably related to increased Antarctic bottom-water activity as well as changes in surface-water mass positions.

oceanographic history of the subantarctic Indian Ocean for the last million years by: (1) comparing several microfossil indicators of present-day oceanographic conditions to the ice-volume record of a deep-sea core; (2) assessing the effects of calcium carbonate dissolution and bottom current activity on the paleoclimatic indicators. The core selected for analysis (E45-74; 47”33’S, 114”26’E; 3744-m water depth) is located well above the carbonate compensation depth on the northern flank of the Southeast Indian Ridge and just north of the Polar Front. This region of the Indian Ocean. is of interest because of the important oceanographic processes which occur here today. The Polar Front is the active boundary between subantarctic and antarctic surface water masses that comprise the eastward-flowing Antarctic Circumpolar Current. Often described as the Antarctic Convergence, the Polar Front is a complex region where surface waters sometimes converge and other times diverge (Gordon, 1971, 1972). The boundary is also demarcated by pronounced microfossil and

INTRODUCTION

Few paleoceanographic studies have compared the records of both radiolaria and planktonic foraminifera to the oxygen isotopic record in a sedimentary sequence affected by calcium carbonate dissolution. Changes in the 1sO/160 ratio of planktonic foraminiferal calcite in deep-sea sediments primarily reflect changes in the oxygen isotopic composition of seawater caused by evaporation of water enriched in I60 and its incorporation in glaciers during glacial episodes (Shackleton, 1967; Dansgaard and Tauber, 1969). As a record of changing ice volumes, therefore, the WO curve of undisturbed marine sediments should provide an ideal stratigraphic framework by which to correlate events globally (Shackleton and Opdyke, 1973). In this study, we attempted to determine a detailed pale’ Present address: Department of Geology, University of South Carolina, Columbia, South Carolina 29208. * Present address: 116 TRW Building, Phillips Petroleum Co., Bartlesville, Oklahoma 74004. 71

0033-5894/78/0091-0071$02.00/O Copyright 0 1978 by the University of Washington. All rights of reproduction in any form reserved.

72

WILLIAMS

sedimentological changes in surface sediments (Kennett, 1969; Hays, 1965; Goodell, 1973). Calcareous oozes (foraminifera) predominate north of the Polar Front, while radiolaria and diatom tests are the major components of siliceous oozes south of the Polar Front. Biogeographic studies of radiolaria and planktonic foraminifera have established that the area1 distributions of certain species within both groups are related to physical and biological properties of water masses (Hays, 1965; Petrushevskaya, 1967; Be, 1969; Echols and Kennett, 1973). The sensitivity of these zooplankters to their environment makes it possible to reconstruct the paleoceanographic history of certain regions from the sedimentary records in deepsea cores. Radiolarian paleoclimatic methods are particularly valuable in the higher latitudes where calcareous sediments are susceptible to dissolution even in relatively shallow waters (Kennett, 1966; Fillon, 1972). The earliest application of radiolaria to paleoclimatic studies in the Southern Ocean (Hays, 1965) was based on oscillations of cold-water (antarctic) radiolarian assemblages to warmer-water radiolarian assemblages. More recently two techniques based on fluctuations of single species have been introduced. The antarctic radiolarian Antarctissa strelkovi (Keany, 1973) and the cosmopolitan radiolarian Cycladophora davisiana (Hays et al., 1976a) both increase in abundance during glacial periods. In well-preserved carbonate sequences found in Southern Ocean deep-sea cores, the percentage of Neogloboquadrina pachyderma, the composition of the total planktonic foraminiferal fauna, and species diversity appear to accurately reflect water mass movements (Kennett, 1970; Williams and Johnson, 1975; Vella et al., 1975; Williams, 1975). Below water depths of 4000 m in the southeast Indian Ocean, the tests of planktonic foraminifera become particularly susceptible to dissolution due to carbonate undersaturation (Williams, 1976a).

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KEANY

This susceptibility to dissolution is species dependent. For example, a robust species such as Gioborotalia injlata is relatively resistant to dissolution compared to a thintested species like Globigerina bulloides (Berger, 1968). Temporal changes in the intensity of calcium carbonate dissolution can be determined by observing increases in test fragmentation (Thiede, 1973), in the number of benthic foraminifera (Gardner, 1975), and in the relative abundance of solution-resistant species (Berger, 1973). The effects of dissolution must be closely considered in paleoclimatic studies using planktonic foraminifera. METHODS

Samples with a volume of 8 cm3 were selected at IO-cm intervals to a depth of 850 cm in E45-74. Each sample was weighed dry and washed over a 63-pm-mesh Tyler screen. The washed residue was dried and weighed to determine the coarse fraction (weight percentage greater than 63 pm). The sample was then split into two equal aliquots using a modified Otto microsplitter. One aliquot was treated with 10% HCL to remove the carbonate fraction, and radiolarian slides were prepared using standard techniques. Radiolarian zones were established after Hays (1965) and total radiolarian populations (>500 specimens) were counted to determine the respective percentages of Antarctissa strelkovi and Cycladophora davisiana. AlthoughA. strelkovi intergrades morphologically with A. denticulata, consistent distinctions between the two species were made based on certain characteristics of the test (Petrushevskaya, 1967; Keany, 1973). The test of A. strefkovi is comparatively thinner and possesses more slender internal elements than that of A. denticulata. The test ofA. denticulata is more massive, with thicker walls and a thorax generally closed by a porous plate. Recently, Dow (1976) and Lozano and Hays (1976) were successful in utilizing these taxonomic distinctions for paleoceanographic studies

PACEOCEANOGRAPHY

OF SUBANTARCTIC

INDIAN

OCEAN

73

of radiolaria in the southern sectors of the tion provides a reliable record of global ice Indian and Atlantic Oceans. volume (Hays et al., 1976 b). CO2 gas was The second aliquot was sieved through a extracted from the foraminiferal calcite by 125pm-mesh screen and subsequently split reaction with 100% phosphoric acid at 50°C into a representative number of specimens in vacua. The isotopic analyses were per(>300) for foraminiferal counts. Determinaformed using a VG micromass 602-C mass tions were made of the frequencies of indispectrometer. The oxygen isotopic compovidual planktonic foraminiferal species, the sitions of G. bulloides are expressed as parts number of planktonic test fragments, the per thousand (%o) deviations from the PDB-1 number of benthic foraminifera, the number standard. Repeated analyses of a homogeof radiolarians, and the number of manganeous carbonate powder render a precision nese micronodules. Shannon- Wiener di- of 2 SD of the mean of 20.06 %o. Duplicate versity values were computed from the fo- analyses of G. bulloides populations were raminiferal assemblages for each interval. precise to better than one standard deviation The Shannon-Wiener index is a measure kO.15 %o. of diversity which considers the proportionment as well as the number of species in RESULTS each sample (Sanders, 1968). Indexes comPlanktonic Foraminiferal and Radiolarian monly utilized for determining calcium carBiostratigraphy bonate dissolution were also produced: percentage foraminiferal fragments to whole The sedimentary record of E45-74 can be subdivided into three planktonic foraminifforaminiferal tests; ratio of benthic to planktonic foraminifera; percent foraminifera era1 zones. The oldest horizon (0.65 my) is to radiolarians; the apparent accumulation defined by the last occurrence of G. puncticulata at 610 cm in the core, 20 cm above rate of manganese micronodules (milligrams the BrunheslMatuyama boundary (Fig. 1). per square centimeter per 1000 yr). A foramThe magnetic polarity log of E45-74 is from iniferal fragment is defined in this study as any portion of the foraminiferal test less Kennett and Watkins (1976). The youngest than 50% of the original test. The apparent zone (0.3 my) is marked at approximately accumulation rate of manganese micronod280 cm by the last occurrence of G. crassaule is derived by dividing the estimated mass formis and the first regular occurrence of G. truncatulinoides. The boundary of the of the nodules by the volume and sedimentation rate within the sample. The mass of G. crassaformislG. truncatulinoides zone is more difficult to define than that of the older micronodules was arbitrarily approximated G. puncticulata zone because specimens of using the weight percentage of the micro(as less than 1% of the nodules in the total number of particles in G. truncatulinoides fauna) were intermittently found in samples the sample. Thus, an apparent accumulation of micronodules is obtained due to errors in below the last occurrence of G. crassaforapproximating the sedimentation rate and mis (as approximately 5% of the fauna). The last occurrence of G. crassaformis is at 280 the mass of micronodules. Following these determinations, 20-25 cm, and G. truncatulinoides is no longer specimens of Globigerina bulloides were present below 310 cm. This zonation (first separated from each sample in the upper described by Kennett, 1970) cannot be ex500 cm for oxygen isotope analysis. G. bul- plained by temporal differences in dissoluloides occupies a deep habitat in this region tion as the index species have relatively high of the Subantarctic Indian Ocean (Williams, resistance to dissolution (Berger, 1968). The 1976a) and was chosen for analysis because ages for the zones in E45-74 agree closely of its abundance throughout most of the core with those proposed in a recent paleomagnetic/biostratigraphic study of four Subantand because its oxygen isotopic composi-

WILLIAMS

AND KEANY

/

ID CORE

20

LO DEPTH

4a

5.0 IN

6D

7.0

8.0

METERS

90

,a

2.0 CORE

3.0

41)

5.0

DEPTH

IN

60

1.0

8.0

9.0

METERS

FIG. 1. Sedimentation rate diagrams for E45-74 based on the magnetic polarity and radiolariamplanktonic foraminiferal zonation (A); oxygen isotope stage boundaries continuous to the 12/13 boundary (B). The ages of the isotope stages are from Shackleton and Opdyke (1973) for V28-238. The numbers in the columns to the right of each diagram are sediment accumulation rates between each boundary (centimeters per 1000 yr).

arctic Indian Ocean cores (Vella and Watkins, 1975). Nearly coincident with the Brunhes/ Matuyama and G. puncticulata/G. crassaformis zone boundaries, Saturnulis circularis makes its last appearance at 620 cm in the core and defines the top of X zone (0.68 my) (Fig. 1). The extinction of Axoprunum anselenum defines the top of the younger zone (1Ir) at 410 cm (0.41 my). This radiolarian zonation is consistent with zonations of Hays (1965) and Hays and Opdyke (1967). Recently Hays and Shackleton (1976) found that the upper limit of A. anselenum (Stylatractus universus) and the w* boundary occur near the oxygen isotopic stage 12/stage 11 boundary (Termination V) approximately 0.41 my. Oxygen Isotope Stratigraphy

The al80 stages 1 to the oxygen rial Pacific

record of ‘E45-74 from isotope 13 is shown in Fig. 2 along with isotopic curves from an equatocore, V28-238 (Shackleton and

Opdyke, 1973), and a South Atlantic core, CH115-88 (Peters, 1976). The isotopic records of V28-238, E45-74, and CHl15-88 are based on analyses of G. sacculfir, G. bulloides, and mixed planktonic foraminiferal assemblages, respectively. All three curves are normalized to the Brunhes/Matuyama boundary and the numbering convention of Emiliani (1955, 1966) was used to describe the isotopic stages. Even-numbered intervals correspond to glacial episodes and oddnumbered intervals denote interglacial episodes. It is obvious from a comparison of the three isotopic records (Fig. 2) that sediment accumulation rates significantly varied in E45-74 and CH115-88 during the Brunhes Epoch, assuming near constant accumulation rates in V28-238. The isotopic record in CH115-88 is presented here to illustrate how varying sedimentation rates between the Brunhes/Matuyama (B/M) boundary and the core top will produce apparent differences in chronology between any given core with an excellent isotopic record (CH115-

PALEOCEANOGRAPHY

OF SUBANTARCTIC

V28-238 (Shackleton

and

INDIAN

OH

E45-74 OQdyka.19731

b"O %. wrt PDB

(this

75

OCEAN 115-88

lPcmrr.1976l

p~erl

b"O %, VA,,

b"O %. w,+ PDB *20

+I5

40

PDB

'05

0

FIG. 2. Comparison ofthe oxygen isotope stage boundaries in E45-74 (B) with the oxygen isotope records of V28238 (A) (Shackleton and Opdyke, 1973) and CH115-88 (C) (Peters, 1976). The isotopic curves are plotted relative to the Brunhes-Matuyama boundary to illustrate the effects of varying sedimentation rates in E45-74 and CH11588 on the positions of isotopic boundaries relative to V28-238. All 8r80 values are reported relative to the Chicago standard, PDB. The WO values of E45-74 are based on analyses of Globigerina bulloides to a depth of 460 cm.

88) and the accepted chronology of V28-238. The differences in the ages of the isotopic stage boundaries between cores E45-74 and V28238 are therefore interpreted as due to changing sediment accumulation rates in E45-74 and not due to substantial errors in chronology. The chronology provided by several biostratigraphic horizons in E45-74 also supports this observation. The ages ofthe l/2,9/10, lO/ll, 11/12, and 12/13 boundaries are in close agreement between the isotopic records of E45-74 and V28-238, but boundaries 314, 415, and 516 in E45-74 appear to be older than the corresponding boundaries in V28-238 by as much as 25,000-50,000 years. In fact, sedimentation rate variations between glaciayinterglacial episodes in E45-74 are substantial enough to make interpretation of the isotopic record difficult in many instances without the use of the biostratigraphic horizons. Isotopic stages 1 through 7 are reasonably

well defined and recognizable, but boundaries between stages 8, 9, and 10 are based on the G. truncatulinoideslG. crassaformis zonation. Similarly, by extrapolating from the B/M boundary in CH115-88, the apparent chronology of every isotopic stage boundary appears considerably younger in CH115-88 relative to V28-238 due to sedimentation rate differences. Using the chronology of the isotopic stage boundaries in V28-238 as a reference, the sedimentation rate diagram for E45-74 shows variations between 0.6 and 1.8 cm/1000 yr (Fig. 1B). From the 12/13 boundary to the Recent, the interglacial episodes have higher sedimentation rates than the preceding glacial episodes in four of six interglacial-glacial cycles. The average sedimentation rates in E45-74 from the Brunhes/Matuyama boundary to the core top are, however, in close agreement: 0.91 (paleomagnetic) and 0.90 cm/1000 yr (isotopic).

76 Calcium Carbonate Dissolution

WILLIAMS

Parameters

Seven parameters were utilized in an attempt to assess the effects of calcium carbonate dissolution on the sedimentation record in E45-74 (Fig. 3). Several of the parameters (percentage calcium carbonate, benthic to planktonic ratio (B/P), and percentage fragmentation of foraminiferal tests) are generally accepted as valuable indexes of dissolution in deep-sea sediments (Berger, 1973; Gardner, 1975; Thunell, 1976). When we use the term “correlation” in comparing the parameters, we are not implying a direct reciprocal relationship, but a mutual one in which changes of the two parameters may occur at approximately the same interval but differ in magnitude. We will attempt to explain exceptions which exist in the correspondence between the parameters and to supply correlation coefficients from linear regression analysis where appropriate. A more complete statistical treatment of the data from this core and several others is underway. Foraminiferal test fragmentation shows significant down core fluctuations with several minima (~5%) and maxima (~70%) (Fig. 3A). Lower fragmentation percentages usually signify a better preservation of the fauna but may also reflect winnowing (Kowsman, 1973; Yamashiro, 1975). The second parameter reflects the relative abundance of benthic to planktonic foraminifera (Fig. 3B). Benthic foraminifera are generally more resistant to dissolution than planktonics and therefore are valuable indicators of varying dissolution intensities, assuming of course that benthic productivity has not changed over a given time interval. A general increase in the B/P ratio occurs with increasing depth in the core. Particularly large increases in the B/P ratio take place between 400-430 and 680-720 cm, with minor increases at other intervals. Even these minor B/P increases are significant, however, as they represent increases in benthic foraminiferal abundance of at

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KEANY

least 50%. The relative abundance of benthic foraminifera among total foraminifera in a well-preserved sample is less than 2%. The relative abundance of benthics reaches maxima of 14-50% at 400-421 cm, 10% at 531 cm, and 13% at 721 cm. Minor increases at 100 and 280 cm reflect benthic percentages of 4-5%. These increases in the number of benthics coincide in most instances with increases in fragmentation of the planktonics, although the magnitude of the changes in each parameter is different. Despite intervals where large changes in fragmentation are not reciprocated in the B/P curve (3060, 180-230, and 550-600 cm), the two parameters are significantly correlated (r = +0.47). The abundance of planktonic foraminifera relative to radiolaria also undergoes significant fluctuations, with radiolarian abundance generally increasing with core depth (Fig. 3C). Individual periods of decreased foraminiferal abundance correlate closely with increases in foraminiferal fragmentation (r = -0.42) and in benthic foraminifera1 abundance (r = -0.51). The general trend of the foraminiferaYradiolarian curve is similar to the B/P variations except between 500 and 600 cm. The calcium carbonate record in E45-74 (Fig. 3D) shows twelve pronounced maxima (~60%) and four minima (~20%) in close similarity to the fluctuations in the other dissolution parameters. Decreases in CaC03 content are correlated with decreases in foraminiferal abundance (r = +0.53), increases in benthic foraminifera (r = -0.41), and increases in fragmentation (r = -0.45). Below 500 cm in the core the average percentage of CaCO, decreases from 60 to 30%. Minimum points in the coarse fraction curve (weight percentage >63 pm) also parallel the carbonate minima (r = +0.43) except at intervals of 320-380, 600-630, and 710-750 cm (Fig. 3E). Fluctuations in the coarse fraction (C.F.) are highly correlated with the foraminiferal/ radiolarian ratio (r = +0.75) although the C. F. is consistently lower below 200 cm in the core. C. F. changes are possibly the

w Gt 8

: E

z

H u

0*0304050607080

WHOLE

FRAGMENTS

FORAMINIFERAL

TESTS

00501

01SS0L”TION

RATIO

PLANKTONIC

BENTHIC

015

-

TO

41

80

TO

40

DlSSOL”TKJN

60

RAOIOLARIA

FORAMINIFERA

20 -

%

CARBONATE

80

60

40

(MIYAJIMA.1976,

20

lwt%=63pm)

COARSE

FRACTION

%

G

INFLATA

MN

MICRONODULE

hg/cm2/1000

ACCUMULATION

RATE YEARS)

FIG. 3. Seven parameters of calcium carbonate dissolution in E45-74. (A) Percentage of planktonic foraminiferal fragments to whole planktonics; (B) ratio of benthic to planktonic foraminifera; (C) percentage of foraminifera to radiolaria; (D) percentage of calcium carbonate (unpublished data from Miyajima, 1976); (E) weight percentage ofwashed sediment >63 pm;(F) percentage ofGIuborotaliu inflata oftotal planktonics, a solution-resistant species; (G) the apparent manganese micronodule accumulation rate in milligrams per square centimeters per 1000 yr.

800

700

600

500

400

300

200

100

g

:t:

TO

%

E45-74

78

WILLIAMS

result of winnowing as well as dissolution (Yamashiro, 1975). Globorotalia inflata is a solution-resistant species which prefers the transitional region of the southeast Indian Ocean and rarely reaches a percentage greater than 15% in surface sediments of the subantarctic region (Be and Tolderland, 1971; D. F. Williams, unpublished data). Abnormally high percentages of G. in&r in E45-74 do not reflect warm-water conditions at this site since the Subtropical Convergence is presently located at its southernmost extent (40”s) than at any time during the late Quaternary (Williams, 1976b). The G. infrata maxima of 20-42% of the total fauna at 210-230, 280-310,330-360, and 400 cm (Fig. 3F) are likely the result of dissolution of less resistant subantarctic species (Globigerina bulloides, G. quinqueloba, and Globigerinita glutinata). Below 400 cm, the overall percentage of G. inflata decreases because G. crassaformis and G. puncticulata are the dominant globorotalids. The oldest G. inputa maximum in E45-74 (a) corresponds with appropriate changes in each of the other five parameters in Fig. 3 (A-E): increase b is not accompanied by decreases in foraminiferal abundance or CaCO,; increases c and d are related to changes in the other parameters with these exceptions: The increases in benthics and radiolaria and decreases in CaCO, at maximum d are less than might be expected. The apparent accumulation rate of manganese micronodules in E45-74 shows several distinct peaks, particularly between 320-340 cm (0.34-0.36 my) and 400-430 cm (0.41-0.43 my). Micronodule accumulations attain maxima of 5-15 mg/cm2/1000 yr at these times (Fig. 3G). In the region south of Australia, manganese nodule deposits are closely associated with minimal sediment accumulation rates and strong bottom current activity (Kennett and Watkins, 1976). Evidence from bottom photographs suggests minimal bottom current activity at the present time in the immediate area of E45-74. The presence of manganese micro-

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nodules in the lower sections of E45-74 may reflect temporal changes in bottom currents. Coincident with the micronodule peak at 320-340 cm, increases in the percentages of G. in$ata and foraminiferal test fragmentation occur, suggesting winnowing as well as dissolution. Fish teeth, foraminiferal tests with manganese coatings, and quartz grains were also observed at both intervals. A large number of quartz grains associated with the earlier manganese micronodule peak were slightly rounded with frosted surfaces possibly indicative of subaqueous abrasion. Many of the benthic foraminifera showed evidence of dissolution with broken edges and pitted or dulled surfaces. Manganesecoated planktonic foraminifera, quartz grains, and rare fish teeth were also found associated with dissolution intervals earlier in the core. Particularly large numbers of quartz grains were found at 280,380-410,630-730, and 810 cm. Planktonic Foraminiferal Indicators

Paleoclimatic

From the total foraminiferal counts, three paleoclimatic indexes were obtained and are plotted in Fig. 4 along with the oxygen isotopic record from stages 1 through 13 (approximately 0.50 my). The ratio of G. bulloides to N. pachyderma (sinistral coiling) should be a useful fauna1 index of the watermass conditions in this region since G. bulloides is the dominant subantarctic species and N. pachyderma (sinistral) is the dominant antarctic species. Significant fluctuations in this ratio occur down to a depth of 400 cm in E45-74, but the magnitude of the fauna1 index during interglacial stage 5 does not equal the peak magnitudes during stages 1, 7, 9, and 11 for some unknown reason. Below interglacial stage 11, dissolution reduces the ratio to minimal values by selectively removing specimens of G. bulloides. Since N. pachyderma is more resistant to dissolution than G. bulfoides (Berger, 1968), fluctuations in the total percentage of N. pachyderma (sinistral plus dextral) should

PALEOCEANOGRAPHY

OF SUBANTARCTIC

INDIAN

OCEAN

79

800

900

FIG. 4. A plot of three planktonic foraminiferal paleoclimatic indexes in E45-74 compared with the oxygen isotope curve of Globigernia bulloides. The magnetic polarity record, the radiolarian/planktonic foraminiferal zonations, and the interglacial isotope stages (numbered after the convention of Emiliani, 1955, 1966) are also shown. The influence of calcium carbonate dissolution on the foraminiferal parameters is discussed in the text.

provide a more reliable indication of watermass changes in the lower sections of the core (Fig. 4 C). The percentage ofN. pachyderma varies between 10 and lOO%, with higher percentages reflecting cold antarctic water conditions. The equivalent of isotope stages l-27 can be identified in the N. pachyderma curve. Overall, the percentage of N. pachyderma increases with core depth, particularly below 400 cm (approximately 0.41 my). This trend may be due to dissolution effects on susceptible species or to generally cooler water temperatures in this region prior to 400,000 yr BP. Species diversity values were also computed from the foraminiferal fauna using the Shannon- Wiener index, and alternating intervals of high and low species diversity occur regularly down the core. When unaffected by dissolution, foraminiferal diversity values can be utilized to estimate mean surface-water temperatures in this region (Williams and Johnson, 1975). In the upper

400 cm of E45-74, the species diversity values provide temperature estimates which vary from approximately 7-8°C during interglacial episodes to 0°C during glacial episodes. These temperature estimates correspond closely to modern temperatures of subantarctic and antarctic surface waters located north and south of the Polar Front (Gordon, 1972). Below 400 cm in E45-74, the diversity values show the same trend as do the N. pachyderma and G. bulloidesl N. pachyderma (s) curves and appear to be biased by dissolution toward unusually low values. However, the possibility still exists that temperatures were cooler prior to 400,000 yr BP. A comparison of the iY80 record and the three paleoclimatic indexes shows a general agreement, although departures are evident in the levels of the maxima and minima of each parameter. For example, isotope stage 2 occurs slightly deeper and stage 3 is less well developed in the fauna1 records than in

80

WILLIAMS

AND

the #*O curve. Sharp decreases in diversity during stages 7 and 11 are not reciprocated by changes in the curves, and stages S-10, 14, and 20 are indistinguishable in the diversity curve. During the stage S-10 interval, significant changes in micro-nodule accumulation, and the dissolution parameters may have led to the discrepancy. Of the three foraminiferal indexes, changes in total N. pachyderma appear to have the best correlation with the isotope curve, although not a perfect one. Despite these problems, it is still possible to determine a history of the water-mass movements in the subantarctic using the planktonic foraminiferal assemblages.

KEANY

Radiolarian

Paleoclimatic

Indexes

The percentages of the radiolarians, Antarctissa strelkovi and Cycladophora davisiana, are shown with the PO record of E4574 (Fig. 5). High percentages of A. strelkovi are indicative of cooler water temperatures (Keany, 1973; Lozano and Hays, 1976; Watkins et al., 1976; Dow, 1976), while C. davisiana may be related to the structure of antarctic surface waters (Hays et al., 1976 b). A. strelkovi has an average abundance of 30-40% throughout the core but with noticeably diminished numbers during glacial stages 2 and 4 compared with values found at the other glacial

E45 - 74 OXYGEN COMF! :w”

( 6’*0 +3.5

% C. DAVISIANA

% A. STRELKOVI

ISOTOPIC

OF G. BULLOIDES %. w.r.t. PDB) +3.0

+2.5

50

40

30

20

IO

25

20

WARM -

I5

IO

5

C

FIG. 5. A plot of two radiolarian paleoclimatic indexes (percentage ofAnrarctissa strelkovi and Cycladophora davisiana) compared with the oxygen isotope record of E45-74. The positions of the isotope stage boundaries are shown on each curve, and the stages are numbered after the convention of Emiliani (1955, FM).

PALEOCEANOGRAPHY

OF.SUBANTARCTIC

stages. Fluctuations in the A. strelkovi record are significantly correlated with the oxygen isotope curve (r = +0.634), and, except for stages 3 and 4, the positions of the isotope stage boundaries in the A. strelkovi record are in good agreement (Fig. 5). Using the similarity of the two records, therefore, it is possible to infer isotope stages 13-27 in the A. strelkovi record. The positions of these stages are in good agreement with their equivalents in the N. pachyderma curve (Fig. 4) and lend support to our interpretations, although the shapes of the curves are different. C. davisiana has its greatest abundance (27%) during isotope stage 2 (t = 18,000 yr BP), consistent with values for this interval reported by Hays et al. (1976 a) (Fig. 5). For the remainder of the core, the magnitude of variations in C. davisiana decrease to an average value of 6-10%. Except for stages 6, 8, 9, 14, and 17, minima and maxima in C. davisiana abundance have approximately the same positions as interglacial and glacial stages in the A. strelkovi and al80 records. However, the overall correlation of C. davisiana to the other paleoclimatic variables is not particularly significant and supports the contention of Hays et al. (1976 b) that the abundance of C. davisiana is not controlled by sea-surface temperature: WO, r = +0.24; A. strelkovi, r = +0.29; N. pachyderma, r = +0.28; G. bulloideslN. pachyderma, r = -0.23.

DISCUSSION Dissolution Intensities in the Subantarctic Indian Ocean

Changes in dissolution intensity may occur diachronously in the major ocean basins, and some disagreement exists conceming the mechanisms controlling temporal variations of calcium carbonate dissolution in Quatemary deep-sea sediments. Increased carbonate dissolution appears as an interglacial phenomenon in Pacific sediments (Arrhenius, 1952; Hays et al., 1969; Berger, 1973: Thompson and Saito, 1975) and as a

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glacial phenomenon in Atlantic and Gulf of Mexico sediments (Gardner, 1975; Damuth, 1975; Thunell, 1976). In the subantarctic Pacific, Kennett (1970) noted intensified dissolution during some glacial episodes, and other evidence suggests that dissolution events may actually lag climatic interglacials (Luz, 1973; Luz and Shackleton, 1975; Ninkovitch and Shackleton, 1975). Our detailed analysis of several dissolution parameters in E45-74 suggests that the intensity of calcium carbonate dissolution has significantly varied in the vicinity of E45-74 during the last million years. However, due to E45-74’s location close to the Polar Front and the control that the Polar Front exerts on the composition of the underlying sediments (Goodell, 1973), it is difficult to separate dissolution effects from changes exerted by migrations of the Polar Front. The effectiveness and reliability of each dissolution parameter will depend on the degree to which it is independent of the Polar Front, and, in order to determine an accurate paleoceanographic history of the subantarctic region, an evaluation of the effects of dissolution on the paleoceanographic indicators is necessary. Dissolution weakens the tests of planktonic foraminifera and leads to fragmentation. Other workers have demonstrated that winnowing by currents will affect the number of fragments and the coarse fraction of a sediment sample (Kowsman, 1973; Yamashiro, 1975). Winnowing should decrease the number of fragments by selectively removing the smaller particles, thereby increasing the coarse fraction. Figure 3 clearly shows, however, that the coarse fraction decreases as the fragmentation increases, suggesting dissolution as the control of these variables in E45-74 and not winnowing. Fluctuations of the Polar Front may influence the benthic/planktonic ratio, foraminiferal/radiolarian ratio, and percentage of calcium carbonate by decreasing the total input of foraminifera from the surface waters. In surface sediments south of the Polar Front,

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radiolaria are the dommant components, with very few benthic or planktonic foraminifera (D. F. Williams, unpublished data). Because the number of benthics does not significantly increase south of the Polar Front, we believe down-core increases in benthics reflect dissolution variations more than Polar Front migrations. We are not presently able to determine what proportion of the variations in the foraminifera/ radiolaria and percentage CaCO, are due to dissolution or to surface water-controlled changes in the supply of carbonate. Finally, variations in the coarse fraction curve and the percentage of solution-resistant G. inflata are most likely due to fluctuations of dissolution intensity and not the Polar Front. From a comparison of the dissolution parameters in Fig. 3, at least 10 periods of intensified dissolution are recorded since the Jaramillo Event in E45-74, and these dissolution intervals appear to be associated primarily with glacial episodes (Fig. 6). Stages 11 and 19 are exceptions, however, during which accumulations of manganese

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micronodules and quartz grains occur. From the distribution of the dissolution events in the core, an overall picture emerges that the sediments in this region were poorer in CaCO, between 1.0 and 0.3 my as compared with 0.3 my and the present. This change occurred approximately at isotope stage 7. The coincidence of manganese micronodules and frosted quartz grains with evidence of dissolution during stages 11, 12, and 19 suggest that antarctic bottomwater activity may have been the primary cause of the dissolution and sediment changes at these times. The other dissolution intervals may be due to changes in the depth of the foraminiferal lysocline during the last million years and not bottom current activity. Presently, dissolution-altered foraminiferal assemblages are found in surface sediments of this region below 4000 m (Williams, 1976a), implying that the foraminiferal lysocline shallowed repeatedly by 200-300 m during the last million years. Grain size analyses are needed to determine the extent of sediment redeposition and transport by bottom currents along the ridge

FIG. 6. A plot showing the calcium carbonate dissolution record and planktonic foraminiferaVradiolarian paleoclimatic records in E45-74. The variation in A. strelkovi and the paleomagnetic/biostratigraphic zonations of E45-77 are shown for comparison. The hatched dissolution intervals are based on a comparison of the dissolution indexes in Fig. 3. The interglacial episodes through the equivalent of isotope stage 27 are inferred from the correlation of the planktonic foraminiferal/radiolarian paleoclimatic indexes to the isotope record.

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flask (Yamashiro, 1975), and work is needed on cores with higher sedimentation rates to define more closely the number and timing of the dissolution events in this region of the Subantarctic. Paleoceanographic Changes in Subantarctic Surface Waters

The detailed analysis of the planktonic faunas and dissolution record in E45-74 enables us to evaluate the usefulness of certain parameters for paleoclimatic reconstructions in high-latitude deep-sea sediments. Changes in sediment accumulation rates resulting from dissolution intervals often make interpretation of the fauna1 records difficult because individual events become blurred. Even the glacial/interglacial boundaries within the oxygen isotope record are not clearly defined in all instances, particularly in stages 8 and 12. Despite these problems, the S180 record of E45-74 for the last 500,000 yr (stages 1- 13) may be used to determine short-term changes in sedimentation rates which are not discernible by using solely the biostratigraphic and paleomagnetic zonations. Of the foraminiferal and radiolarian paleoclimatic indexes examined in the context of isotopic stages l- 13, changes in the total percentage of N. pachyderma and A. strelkovi appear to compare best with the oxygen isotope record (Fig. 6). Increased abundances ofA. strelkovi and N. pachyderma are significantly correlated with each other (r = +0.30) and with glacial episodes inferred from the isotope record (r = +0.63), pachyderma).

A. strelkovi;

r = +0.325,

N.

The glacial/interglacial fluctuations indicated by these two paleoclimatic indexes closely complement each other despite differences in magnitude and instances where events are not distinctly seen. For example, stages 7, 14, 18,20, and 22 are slightly better defined in the N. pachyderma curve than in the A. strelkovi curve, and stages 10 and 17 are better defined in the A. strelkovi curve. Changes in C. davisiana, foraminiferal species diversity, and the ratio of G. bulloides to N. pachyderma corroborate the N. pachy-

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derrna and A. strelkovi

records in most instances. However, calcium carbonate dissolution reduced the effectiveness of using species diversity or the G. bulloideslN. pachyderma ratio for paleoclimatic reconstructions, particularly in the lower sections of the core. The decreased fluctuations in C. davisiana abundance prior to 400,000 yr ago are possibly a reflection of smaller temperature variations during that time in its deep-water habitat. A synthesis of the foraminiferal, radiolarian, and isotopic records in E45-74 (Fig. 6) suggests that 12 major changes occurred in the water-mass conditions of the subantarctic region since the beginning of the Jaramillo Event (0.95 my). These water-mass changes are represented by the repeated invasion of fauna characteristic of modern antarctic waters into the region presently under subantarctic conditions. Such paleoceanographic fluctuations of the surface waters are related to glacial/interglacial episodes in the polar regions. The fauna1 changes recorded in E45-74 are substantiated by the A. strelkovi record of another subantarctic core, E45-77 (46”27’S; 114”25’ E’ 3740 m) which records paleoceanographic fluctuations of the antarctic and subantarctic water masses into the late Matuyama (to interglacial episode 3 1, approximately 1.2 my). The fauna1 results of E45-74 are further substantiated by paleoceanographic studies involving the Subtropical Convergence (STC) in the southeast Indian Ocean. During the last 500,000 yr, the STC is known to have been repeatedly displaced by waters characteristic of the Polar Front (Williams, 1976b; Be and Duplessy, 1976). Using the detailed stratigraphy in E45-74, the timing of these subantarctic water-mass changes appears to be synchronous with that of similar water-mass changes in other oceans (Hays et al., 1969; Kennett, 1970; Shackleton and Opdyke, 1973). CONCLUSIONS

The utilization of more than one paleoclimatic index makes possible a more de-

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tailed reconstruction of the paleoceanographic history of the Subantarctic than the use of either the radiolarian or foraminiferal fauna alone. Changes in the percentages of Neogloboquadrina pachyderma and Antarctissa strelkovi correlate well with the oxygen isotope record from isotopic stages 1 through 13 and apparent discrepancies between the fauna1 and isotopic records are the result of dissolution or bottom current changes. Below stage 13, the N. pachyderma and A. strelkovi curves were utilized to infer glacial-interglacial changes through the equivalent of isotope stage 27 in the late Matuyama. The record of A. strelkovi is particularly useful in intervals where the foraminiferal fauna has been affected by calcium carbonate dissolution. The results of this study suggest that expansions of the Antarctic water mass have occurred repeatedly during the last million years in response to polar glaciations. These watermass changes are similar in magnitude to those documented in the upper Brunhes (Williams, 1976; Kennett, 1970; Vella et al., 1975). Distinct periods of intense calcium carbonate dissolution are marked by increases in fragmentation of the foraminiferal assemblages, coarse fraction percentage, benthic foraminiferal numbers, and the abundance of the transitional species, Globorotalia inflata, which is highly resistant to dissolution. CaCO, content and radiolarian abundance in subantarctic sediments are influenced by the Polar Front as well as dissolution, and it is not possible at this time to determine what proportion of the temporal variations in these parameters is due to Polar Front migrations or dissolution. At least 10 periods of intensified dissolution are evident in sediment deposited since the Jaramillo Event in E45-74, and these dissolution periods occur primarily during glacial periods. Generally, dissolution was more intense in the vicinity of E45-74 prior to 300,000 yr ago, suggesting a general change in the depth of the foraminiferal lysocline in the subantarctic region during the upper

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Brunhes. The existence of manganese micronodules and fish teeth with other dissolution parameters in some intervals of the core suggests that bottom current activity strongly influenced sediment accumulation rates in E45-74 during glacial episodes 12 (0.41-0.43 my) and 10 (0.34-0.36 my). ACKNOWLEDGMENTS This research was performed as partial fulfillment of the authors’ dissertation research. We gratefully wish to thank N. D. Watkins, J. P. Kennett, and W. L. Prell for their helpful comments on the manuscript; M. Miyajima for unpublished calcium carbonate data on E45-74; M. A. Sommer and R. K. Matthews for the use of the VG micromass 602-C mass spectrometer in the Benedum Stable Isotope Laboratory at Brown University; D. Cassidy for providing samples from the Eltanin core collection; N. Healy-Williams for laboratory assistance; A. Doherty for drafting; D. Scales for photographing the plates; and N. Meader for conscientiously typing the manuscript. This research was supported through NSF Grant OPP75-15511 to J. P. Kennett.

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