Late pleistocene paleoclimates of the South Pacific based on statistical analysis of planktonic foraminifers

Palaeogeography, Palaeoclimatology, Palaeoecology , 22(1977): 61--78 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

LATE PLEISTOCENE PALEOCLIMATES OF THE SOUTtt PACIFIC BASED ON STATISTICAL ANALYSIS OF PLANKTONIC FORAMINIFERS

BOAZ LUZ

Department of Geology, The Hebrew University, Jerusalem (Israel) (Received June 2, 1976; revised version accepted October 22, 1976)

ABSTRACT Luz, B., 1977. Late Pleistocene paleoclimates of the South Pacific based on statistical analysis of planktonic foraminifers. Palaeogeogr., Palaeoclimatol., Palaeoecol., 22: 61--78. Statistical analysis applied to foraminiferal data from 78 South Pacific core tops enables the derivation of a transfer function that relates sea surface temperature to foraminiferal assemblages. Application of this transfer function to eight cores from the southern part of the East Pacific Rise yields estimates of the sea surface temperatures of the last glacial maximum, as well as the paleotemperature record of the past 150,000 years. Comparison of the last glacial temperature estimates with the recent sea surface temperature shows that the greatest change between glacial and present conditions (about 5°C) occurs in a climatically sensitive area near 50°S. Stratigraphic correlation of two cores from this area suggests that the last glacial started in this area with rapid cooling, and that the climate stayed generally cold until the end of the glacial. Similar general shape of the climatic record is found in the high latitudes of the North Atlantic as well as in the ice sheets of Greenland and Antarctica. In contrast to the similarity in the shape o f these high-latitude records, they differ distinctly from the foraminiferal oxygen isotope record of several deep-sea cores which indicates a general gradual increase of ice volume from the beginning of the last glacial to the maximum glaciation which occurred about 18,000 years B.P. In the study area the rate of sediment accumulation during the last glacial is about two to three times less than in the last interglacial. There is no indication of increased carbonate solution during the glacial, and it is suggested that the change in the accumulation rate results from a reduction in the supply of calcareous shells to the sediment. It seems that with cooling, the environment becomes less favorable to organisms producing calcium carbonate tests, and therefore carbonate production decreases during the glacial.

INTRODUCTION During the last decade, intensive studies of deep-sea cores have provided i n f o r m a t i o n a b o u t the stratigraphy a n d paleoclimatic history of the Pacific sector of the Sub-Antarctic. The research that was initiated by Hays (1965}, w h o u s e d radiolarians, was f o l l o w e d b y o t h e r w o r k e r s w h o used d i f f e r e n t g r o u p s o f m i c r o f o s s i l s : p l a n k t o n i c f o r a m i n i f e r s (Blair, 1 9 6 5 ; B l a c k m a n , 1 9 6 6 ; 61

62

Kennett, 1970); calcareous nannofossils (Geitzenauer, 1972); radiolarians (Hays, 1965; Hays and Opdyke, 1967; Huddlestun, 1971; Keany, 1973); diatoms (Hays and Donahue, 1972); and silicoflagellates (Jendrzejweski and Zarillo, 1972). Supplemented by paleomagnetic stratigraphy (Opdyke et al., 1966; Watkins and Godell, 1967), these studies contributed to the knowledge of changes in climatic conditions and their time of occurrence. However, in the above-mentioned papers, species abundances are used as qualitative climatic indicators, and no attempt is made to achieve quantitative estimates of climate. In the light of the work that is now being carried out by CLIMAP investigators, a work which involves drawing up global maps of the past climates, it seems highly important to obtain quantitative estimates of sea surface temperatures of the Pacific sector of the Sub-Antarctic. The transfer function method developed by Imbrie and Kipp (1971) has made it possible to derive objective and quantitative paleoclimatic estimates, based on fossil foraminiferal assemblages. A similar technique has been applied in the present paper in order to obtain paleotemperature estimates of the South Pacific. The transfer function has been derived from published data on the distribution of foraminiferal assemblages in South Pacific core tops (.Parker and Berger, 1971), Application of this transfer function to core samples from the southern part of the East Pacific Rise has yielded the record of ocean surface temperature during the past 150,000 years. MATERIAL AND DATA

Core selection for the present study has been guided by the need to work on cores which have relatively high rates of sediment accumulation, and by the need to use well-preserved foraminiferal fauna. These requirements are important in order to ensure maximum resolution of the climatic events, and TABLE I Core locations and temperature estimates 1 Core

2 Lat.

3 Long.

4 18K (cm)

5 TW18K (°C)

6 TWoK (°C)

Ell--3 E11--2 El1--1 RC12--225 E21--15 E25--10 DWBG 70 E20--18

56°54~S 56 ° 04'S 54°54'S 53°39'S 52 ° 01'S 50°06'S 48°29'S 44 ° 33rS

115°14'W 115 ° 05 ~W 114°42'W 123°08'W 120 ° 03'W 114°47'W 113°17'W 111 ° 20'W

60 140 40 30 40 40 30 30

0.56 0.56 0.67 1.12 1.08 1.13 2.64 9.24

2 3 3 4 5 6 6 9

Cols. 1--3: core designation and location; col. 4: depth to the level of 18,000 years B.P. (18K); col. 5: estimated 18K winter temperature; col. 6: present winter temperature (interpolated from Reid, 1969)

63

m i n i m u m distortion of the climatic information by selective destruction of the foraminifers. The East Pacific Rise is suitable for the purpose of the present study, because it accumulates sediments faster than most other areas of the South Pacific, and the sediments contain well-preserved foraminifers. Eight cores from the southern part of the East Pacific Rise have been used in the present study. The core material is from three institutions. All the Eltanin cores are from the Florida State University core collection, core RC12--225 is from Lamont-Doherty Geological Observatory, and core DWBG 70 is from Scripps Institution of Oceanography. The cores and their locations are listed in Table I. The foraminiferal data used in this paper are from two sources. Foraminiferal abundances in South Pacific core tops were taken from Parker and Berger (1971). Their data were used in the derivation of the transfer function. Counting of the down-core samples has been done by the present author. The t a x o n o m y of Parker and Berger was followed and the size fraction greater than 149 pm was counted. Calculation of species abundances is based on counts of randomly split samples which contain more than 300 specimens. Measurements of CaCO3 content were carried out by a gasometric technique modified after Hiilsemann (1966). THE TRANSFER FUNCTION

The m e t h o d used for deriving paleotemperature estimates is based on the technique developed by Imbrie and Kipp (1971). For details of the m e t h o d the reader is referred to t h a t paper and also to Kipp (1976). The latter deals with an improved version of the transfer function method. It should be noted that in the present work only well-preserved core tops were used. The reason for doing so is that the transfer function is intended for cores which were n o t subjected to calcium carbonate solution, or suffered only minor solution effects. In addition to that, in certain cases partly dissolved sea-bed sediments from the South Pacific are suspected of being pre-Recent (see Broecker and Broecker, 1974). Another difference is the application of a linear regression equation instead of a quadratic one. Experiments with different equations have shown that when using t h e m on down-core samples the linear equations give more reliable results, while some estimates derived from quadratic equations are not reasonable. The transfer function used in the present paper is based on 78 well-preserved samples (Table II) for which data were taken from Parker and Berger (1971). Only those core tops which are located above the lysocline as mapped by Parker and Berger were selected. Twenty-three species that are more abundant than one percent in at least one sample were used in the statistical analysis (Table III). The raw data matrix was subjected to Q-mode factor analysis which was run by the program CABFAC (Klovan and Imbrie, 1971). Four varimax factors were extracted and they account for about 96% of the original information.

64 T A B L E II T h e a b u n d a n c e o f f o u r v a r i m a x a s s e m b l a g e s in 78 S o u t h Pacific s e a - b e d s a m p l e s Sample

Communality

Varimax assemblage I

t

3 5 6 7 8 9

10 11 I~ 13

14 I~

12 17 18 I~ 20 21 ?~ 23 ?G 25 26 27 ?~ 29 ~0 3t 37 33 3~ 35 36 37 38 39 40 41 47 43 ~4 ~5 46 47 4~ 49 50 51 5~ 53 54 55 5h 57 5~

50 60

61 67 53 64 6~ 66 67 6~ 6~ 70 7~ 7~ 7~ 7~ 7'~ 77 79

A~P 30 AMP ~ AMP 35 AMP 36 CAP ~ C~O ~-2 C~P 9-3 CAP 15 CAP 24 CAP ~q CAP 4 0 - ? C~P 41 CaP 1~ CAP 37 Ca ~ 3R CAP ~q CAP 40 CAP 41 C ~P 47 CAP ~3 CaP 44 CAP 45 CAP 66 DWD P23 DWD 60 DWD 67 DWD ~3 9WO 6~ OWO 6n OWD 70 DW9 71 DW9 7~ DW9 75 9WO 120 DWO l ? l OWD I ~ 3 D~D 130 DWD I3G DWD 137 OWD 36 9WO 7~ GwD 79 DwD ~? LSD

LSD LS~ L~D LSD LSD LSD ISD LS9 MSN MSN MSN ~SN M~N ~SN P~O PR(1 PPO qlS ~IS PIS ~IS

~IR oIS

~IS

~IS ~ IS

5R

6? 64 65

6~

67 69 77 77 90 9~ ~ 103 lO& l~ 4R 57 67

65

0-4 0-3 0-3 9-4 0-4 0-3 0-3 0-4 0-3 0-~ O-x 0-~ 0-4 O-I O-I n-I 0-] O-I O-I !-4 0-] O-I 0--I ~-4 0-? 0-3 0-2 9 -~ 9-2 o-? 9-7 0-2 O-? 0-~ o-~ 9-3 '3-4 0-~ 0-3 n-3 :)-3 o-& n-~ 0-3 O-~ 0-3 3-4 0-3 O-? 0-3 0-3 O-? 1-5 n-~

.9300 .9792 .97Q6 .~8~7 ,9559 .9400 .9822 .9813 .95~8 .9844 .97]0 ,9755 .9737 .8615 ,97~7 .9621 ,9870 ,97U~ ,984~ ,9789 .9270 .9816

0--3 o~ ?-& 0-4 ~-4 9-3 0-3 0-5 0-~ o-~

.996~ .c711 ~q24

66 69 7~ ~ - 5 75 0 - 5 76 h - ~ 77 0 - 4 R? 9-? ~1 3 - ~

FLT 1 7 ~ 6 ~LT 1771 [{T II~9 ClT 1164 SOS ~ 95 SOSC 96 ~Dq c O~

O-~ CC 0-? )-~ I-? 1- ° ]-7

.9793

.g0Cl .9571 .9547 ,9833 ,9917 ,~777

,97~2 .947~ .9820 .~865 ,9442 .9444 .9710 .962] .986~ .97C~ .9786 .~442 g77g 9695 94b2 ~993 9305 93~6

9673 9623 95~ 9972 .9980

98L3 ~940

995C 9856 9733 9723 ~8(>6 ~e06 972~

97~i 9~4 q76t 9443

9955

rig( 1 9970 H3tq

963b

.8444 .7994 ,8050 • 8139 ,7071 .5941 ,6029 • 2913 ,5293 .9101 ,9418 .8484 .4710 .8129 .8993 ,8491 •9120 .8420 • ~137 ,894( .7821 .8679 ,9250 ,4558 -,0250 -.OOUO -,0069 ,1070 ,1674 .1430 .1563 .0730 ,1331 .3506 ,4518 ,4157 • 8128 .9666 • 9212 ,0115 .4784 • 486~ .9572 ,6693 .5914 .5629 ,6284 .6843 .76C0 • 7522 .7061 ,7661 -.0280 -.6279 -,0351 -..00~3 -.0597 .54~2 .5596 .5603 .4433 • 9423 .9~22 • q~OZ .9~29 ,87~9 • qlSO .~456 .7801 .5062 .5833 -.02~ -.OC3~ .C749 -.C352 ~8274 .8522

2 .0372 ,O&}O .040@ .0453 ,0352 .0089 ,0232 -,0111 .0206 .0430 ,0812 .0323 .0078 ,0188 ,042b ,0325 .0208 ,0~29 ,0992 ,0743 ,0438 .0551 ,0599 .0113 ,977b .9747 .7486 .9401 ,91~b .9139 ,942b .9840 ,9241 .0308 .0312 ,0214 ,0359 ,0529 .0584 .9325 .0404 ,Olb4 .0280 .0293 ,0199 .0255 ,0169 .0248 .0362 .02~8 ,0286 ,935~ .35~ .3524

.482b .9848 .9934 .0214 ,0283 .0127 .00~3 .0519 .0~30 ,0~0 .0539 ,0372 .04b~ ,0349 .03d3 .0580 .02~0 .3635 .6570 .9210 .493~ ,0430 .0466

3

4

-.4642 -.5818 -°9?39 -,5041 -.b742 -,7600 -.7862 -.9467 -,8204 -.3927 -.27BI -.5045 -,8670 -.4473 -.4100 -.4909 -,3900 -.5178 -.9645 -.4166 -.5598 -°4746 -,3467 -,8673 -.0218 -.0330 -.0044 -,0082 .0243 .Olb4 .0198 -,0479 -,0708 -.90~6 -.8598 -,8932 -.547~ -.2080 --.3440 -,0090 -.844B -,8003 -,8i13 -°7045 -,7744 -.7375 -.7099 -.6794 -.5980 -,b329 -.6~03 -,OOd8 -.0050 -.0045 -.0083 -.0132 -.0261 -.~278 -.757~ -,7614 -,8936 -.3080 -.2805 -.2840 -.3076 -,309~ -.3570 -.~i16 -.6127 -.~467 -.7768 -.0048 -.00~7 .OOb7 -.0083 -,3878 -.4d~8

-.0092 -,0052 -,0041 -.0048 -,0036 -.0116 -.0068 -,0086 -,0037 -.0088 ,0037 -.0112 -.0083 -,O[b2 -,0093 -,0107 -,0026 -.0061 .0017 .0034 -.0028 -,0029 -,0030 -.0042 -.0157 .0594 -,6502 -.3104 -.3327 -.3497 -o1706 -.0775 -.0994 .0076 .0039 .0009 -.0072 --.0074 -.0040 -.3301 .0073 -.0043 -.0020 -,0075 --.0084 -,0059 -,0116 -.Q085 -,0060 -,0095 -,0068 -,0060 -.9328 -,9344 -,8732 -,0311 -,0375 -,0040 -.0048 -,0104 -.0074 -.0061 .0049 .0066 -.0051 -.0094 -.0076 -°0092 -.0057 .0117 -.0033 -.9303 -.7473 -,3648 --.8673 -,0071 -.0051

65 TABLE 111 Foraminiferal varimax assemblages (F t matrix); the figures indicate the relative importance of the species in the assemblages Species

O. universa G. conglobatus G. tuber G. ~enellus G. sacculifer G. siphonifera G. calida G. bulloides G. falconensis G. rubescens T. humilis G. quinquiloba G. pachyderma G. dutertrei G. conglomerata G. hexagona P. obliquiloculata G. inflata G. truncatulinoides G. crassaformis G. cultrata G. tumida G. glutinata

Varimax assemblages 1

2

3

4

0.011 --0.011 0.175 0.027 0.281 0.071 0.094 --0.051 0.000 0.046 0.088 --0.007 --0.028 0.034 0.193 0.032 0.098 --0.009 --0.020 0.014 0.050 0.082 0.896

0.050 --0.002 --0.011 --0.001 --0.027 --0.004 --0.007 0.846 0.014 --0.000 0.014 0.245 0.352 0.007 --0.012 0.003 --0.008 0.263 0.144 0.006 --0.004 --0.005 0.080

--0.025 --0.120 --0.949 --0.054 --0.057 --0.141 --0.011 --0.018 --0.010 --0.045 0.049 0.001 --0.005 --0.043 0.114 0.015 0.025 --0.010 --0.043 --0.022 0.002 0.035 0.182

0.030 --0.002 --0.004 0.001 --0.023 --0.005 --0.006 0.330 0.002 0.00:1 0.009 0.087 --0.934 --0.002 --0.009 0.002 --0.006 0.059 0.072 0.004 --0.003 --0.003 0.003

T h e s e f a c t o r s c a n be r e g a r d e d as f o r a m i n i f e r a l a s s e m b l a g e s (see T a b l e III). T h e a b u n d a n c e o f e a c h a s s e m b l a g e ( v a r i m a x f a c t o r loadings) in e a c h core t o p is given in T a b l e II and t h e m a p in Fig. I s h o w s t h e l o c a t i o n of t h e 78 core t o p s a n d t h e areas w h e r e t h e d i f f e r e n t a s s e m b l a g e s are d o m i n a n t . I t is clearly seen t h a t t h e a s s e m b l a g e s are a r r a n g e d in l a t i t u d i n a l b a n d s a n d t h u s t h e i r dist r i b u t i o n is p r o b a b l y c o n t r o l l e d b y climatic a n d h y d r o g r a p h i c c o n d i t i o n s . P a r k e r a n d Berger ( 1 9 7 1 ) , w h o s e d a t a are u s e d in t h e p r e s e n t p a p e r , p o s t u l a t e t h a t f o r a m i n i f e r a l d i s t r i b u t i o n in S o u t h Pacific sea-bed s e d i m e n t s is c o n t r o l l e d b y b o t h h y d r o g r a p h i c a n d s o l u t i o n c o n d i t i o n s . This is n o t in c o n f l i c t w i t h t h e c o n c l u s i o n s m a d e here, since t h e d a t a f r o m p a r t l y dissolved cores w e r e elimina t e d in t h e p r e s e n t s t u d y and t h e r e f o r e t h e f a u n a l p a t t e r n s reflect o n l y the hydrography. A stepwise regression was p e r f o r m e d in o r d e r t o find t h e r e l a t i o n s h i p s betw e e n t h e f a u n a l d i s t r i b u t i o n and t h e w i n t e r sea surface t e m p e r a t u r e ( t e m p e r a t u r e d a t a f o r t h e s a m p l i n g s t a t i o n s w e r e t a k e n f r o m Reid, 1 9 6 9 ) . T h e assemblage distribution shows good correlation with the ocean surface t e m p e r a t u r e . T h e m u l t i p l e c o r r e l a t i o n c o e f f i c i e n t is 0 . 9 7 8 , a n d t h e s t a n d a r d

66 140

140

180

i

~'*

~

G/UT/NATA

$

100

ASSEMBLAGE

RUBER--ASSEMBLAGE~ ~

BUL/OIDE$ ASSEMBLAGEi

4.5 ~ - -

60

~

25

Q

45

A',4M;,.OE 65 ' 140

•i

180

"

l

65

140

t00

60

Fig. 1. Sea-bed s a m p l e l o c a t i o n s a n d areas o f d o m i n a n c e o f t h e f o r a m i n i f e r a l v a r i m a x assemblages. E a c h assemblage is n a m e d a f t e r its m o s t a b u n d a n t species.

error of estimate is 1.898°C {both adjusted for degrees of freedom). The regression equation and other regression statistics are given in Table I V . Temperature estimation was done similarly to the m e t h o d of Imbrie and Kipp (1971). According to their method the estimation proceeds in two steps: (1) An estimated varimax factor matrix (Table V) is calculated. This matrix shows the abundance of the core-top assemblages in the down-core samples. (2) The above matrix is multiplied by the vector of regression coefficients (from Table IV) and this yields another vector of temperature estimates. In this way the paleotemperatures were estimated in 8 cores, which are spread along the East Pacific Rise between the south latitudes 57 ° and 44 °. The results are given in Table I and shown in Figs. 3--7, and are n o w to be discussed. TABLE IV P a r a m e t e r s o f linear regression o f t h e w i n t e r t e m p e r a t u r e vs. t h e v a r i m a x assemblages

V a r i m a x assemblage 1 3 4 2 Constant term

a

b

c

d

12.353 --10.545 9.308 --1.301 9.975

0.740 0.931 0.957 0.957

0.860 0.964 0.978 0.977

4.536 2.375 1.898 1.922

a = regression c o e f f i c i e n t ; b = c u m u l a t i v e p r o p o r t i o n o f variance r e d u c e d ; c = m u l t i p l e c o r r e l a t i o n c o e f f i c i e n t ( a d j u s t e d for degrees o f f r e e d o m ) ; d --- s t a n d a r d e r r o r o f e s t i m a t e (°C, a d j u s t e d f o r degrees o f f r e e d o m ) .

67 TABLE V T h e a b u n d a n c e o f the foraminiferal e s t i m a t e d varimax assemblages in cores R C 1 2 - - 2 2 5 and E 2 1 - - 1 5

Core

Depth (cm)

Corn-

V a r i m a x assemblage

munality 1

RC ~C ~C ~C ~C ~C

I?225 l??V5 I~2V5 I?225 I~p~5 I72)s

~C ~C ~C ~C ~C RC oC RC ~C ~r ~ PC ~C oC [

I?Z~5 17775 12295 12225 17~5 12225

c

c f c r r F s

r

0 10 ZO 3C 40 sO 70 oO

O0

100 110 120 I~2~5 130 1 7 2 9 5 140 1 2 2 9 5 150 I??~5 16,0 I ~ 2 ~ 5 170 I?2~5 l ~ b 1~2~5 I~295 ~I 15 ?I 15 ? l 15 21 15 ~I ~ 5 ?I 15 ?? 15 ~1 15 91 15 ? l 15 71 15 ? l 15

71

r 91 F ~I ~1

21 f 91 E ~1 r 2! ~i

lS 15 15 ~5 15 15

15 t5 15

190 790 n I0 ~0

30

~q 5o 60 70 ~o oo I~0

llC 12n 1~0 140 150 160 170 lqn lO0 200

9~7 991 9q5 9q4 gqO 996 gGI 994 g84 9~6 992 983 989 974 g5o 9~8 9~6 994 q~4 g92 W94 q~4 974 976 97~ 9~3 955 ,9W2 994 9C7 ~(6 9C4 999 9q5 gS? g~3 98~ 980 9~I

GGO 9Q4 QQb

,005

2

3

4

.898

.042 .001 -.036 -.004 -)029 ,039 -.007 .011 -.002 -.001 .014 -.C14 -.030 .011 -.014

j749 .698 .542 .605 .539 .837 .661 .787 .190 .856 .807 .710 .289 .879 .891

-.006

-.400

,Q04 -.003 -.008 -.003 -.Q07 .002 -.004 -.002 ,.006 -.007 -.003 -.007 -.OIQ -.005 -.012

-.713 -.836 -.791 -.839 -.537 -.746 -.60~ -~G02 -.508 -.576 -.696 -.597 -.462 -.441

.626 -.020 -.028 -.633 ,024 -.Q33 .026 .010 --.015 -.03£ -.Olg -.031 -.031 -.027 ,014 -,011 -.027 ,C03 .CII -,GOg -,007 .007 -.017 -.004 -.032 -.028

.738 .554 .~66 .539 .622 .SZ9 .833 .820 .765 .534 .588 .515 .505 ,582 .716 ,771 .623 .753 ,764 .72C .723 .858 .740 .~43 .467 .469

,OOi -.0_05 -.006 -.008 .003 ,.012 -.001 -.009 --.008 ,.009 -.009 -,007 -.007 -.00~ -.000 -.007 -.007 -.604 -.002 -.005 -.00~ -,007 T.O07 -.002 -.006 -.006

-~655

-.663 -.829 -.~8~ -.837 -.779 -.5~4

-.52g -,551 -.627 -.8~0 -.799 -.852 -.859 -.81~ -,695 -,632 -.781 -,654 -.632 -.6~2 -.683 -.4g4 -.666 -.838 -.880 -.880

THE LAST GLACIAL IN THE STUDY AREA

The southern part of the East Pacific Rise was studied as part of a CLIMAP project of mapping the world-ocean surface temperature during the last glacial m a x i m u m (for compilation of CLIMAP results, refer to McIntyre and Moore et al., 1976). In order to map the last glacial m a x i m u m (which occurred 1 8 , 0 0 0 y e ~ s B.P. or 18K for the sake of brevity) we need, in addition to temperature estimates, a method of recognizing the time plane of 18K in the cores. Three different stratigraphic methods were utilized in the present work: (1) oxygen isotope variations; (2) the abundance of the radiolarian Cycladaphora davisiana; and (3) the abundance of the foraminifer

Glo bigerina pachyderma.

68

The best method for recognizing the 18K level in deep-sea cores is probably oxygen isotope stratigraphy. It has been suggested that the isotopic ratio is controlled primarily by global changes in ice volume (Broecker and Van Donk, 1970; Shackleton and Opdyke, 1973). In this case the changes in isotopic composition in cores should be nearly synchronous. N. J. Shackleton (personal communication) has analyzed oxygen i~otopes in core E 2 0 - - 1 8 (the northernmost core in this study), and found that 180 reaches a m a x i m u m 30 cm below the core top. This depth was taken as the 18K level. Unfortunately, oxygen isotope data were not available for the rest of the cores, and therefore it was necessary to look for other stratigraphic clues. Hays et al. (1976) have proven the usefulness of C. davisiana as an indicator of the last glacial maximum. They have shown that the abundance of this sPecies reaches a pronounced m a x i m u m at the same level where m a x i m u m

0

Ell-I

E11-3

Ell-2

% C. dovisiana 5 I0

% C. davisiana 5 I0

15

0

15

% C. davisiana 0 5

r

TO

% G. pachyderma 30 60 90

cm

0 20

40

ISKI

i

~

,

I

~

I

,

40

60

6O

80

8O

I00 120

IO0 120

140

140

160

160

I

180

180 I

200 220

/ I I

2OO 2ZO

24O

24O

260

260

280

28O

Fig. 2. A b u n d a n c e of C. davisiana in three cores (supplied by J. D. Hays). The first peak b e l o w the core t o p is taken to indicate the level of the m a x i m u m glaciation w h i c h occurred a b o u t 18,000 years B.P. (18 K). Note the correspondence between the abundance of C. davisiana and G. pachyderma in core E l l - - 1 .

glacial conditions are indicated by oxygen isotopes. C. davisiana abundance was used to indicate the 18K level in the three southernmost cores in the study area (Fig. 2). North of the location of these cores the species is not abundant enough and cannot be used as an indicator of the 18K level. In core E l l - - 1 the foraminifer G. pachyderma has an abundance peak which coincides with the C. davisiana m a x i m u m (Fig. 2). Assuming that the G. pachy-

69

derma peak is not t o o diachronous, this peak is used to approximate the 18K level in the remaining cores. Once the depth of the 18K was determined and the temperature estimated, it was possible to proceed to the next step, and to compare the conditions during the last glacial with those of today. The estimates of the 18K winter temperature and the recent observed winter temperatures are plotted in Fig. 2. Today the winter temperature in the study area increases gradually from about 2°C at latitude 57°S to about 9°C at 44°S. The 18K temperature distribution is different. All the cores except the northernmost one register a temperature drop, with a m a x i m u m temperature change occurring around 50°S (see Fig. 3c). Between 57 and 50°S the temperature ~radient shown is °C 12

°C

°C 12

I0

I0 PRESENT

8

6 4 Ol 2

2 510

40 Q

? Ooo

i 60

° S let.

° S let

b

il i

18K

8

AT

•

" e !

•

• Slat.

c

Fig. 3. Sea surface temperature over the sites of the eight cores listed in Table I. a. The present winter temperature (interpolated from Reid, 1 9 6 9 ) . b. Glacial m a x i m u m winter temperature estimates, c. The difference between the present temperature and the glacial m a x i m u m temperature.

very shallow. However, it is quite possible that over the location of the southernmost cores the temperature dropped below 0.5°C. This would make the gradient steeper. The reason for the uncertainty is that the foraminiferal assemblage in cores E l l - - 2 and E l l - - 3 became monospecific during the glacial. The glacial levels in these two cores contain only one species, G. pachy derma. But this species is found in Antarctic waters as cold as --l°C (B6, 1969), and thus it is only the limitation of the transfer function technique which precludes the estimation of temperatures lower than 0.5°C. Between 50 and 44°S the 18K temperature gradient is steeper than today. This suggests that the lateral pressure gradient was greater during the last glacial and consequently the velocity of the West Wind Drift was faster in this area. It should be noted however, that the control points between 50 and 44°S are sparse and any further conclusions concerning this steeper temperature gradient should await the temperature estimation in additional cores. In any case, the trend of the glacial-to-present temperature change shown in Fig. 3 is probably real, and it can be concluded that the South Pacific became less and less sensitive to climatic fluctuations, north of 50°S. To summarize, during the time of the last glacial maximum, Antarctic cold

70

water transgressed against the relatively stable Pacific central waters. The largest temperature change is recorded in cores located around 50°S. This results from this area being climatically sensitive and from the fauna and the transfer function being sensitive to changes in hydrographic conditions. Based on these findings, tl~e important conclusion from the present study is that the most suitable area for studying past climatic variations in the South Pacific is around 50°S. THE L A S T 1 5 0 , 0 0 0 Y E A R S

Two cores ( E 2 1 - - 1 5 and RC12--225) which are located in a climatically sensitive area were selected. The upper 200 cm of these cores were analyzed for both CaCO3 content and foraminifer species abundance at 10-cm intervals. The transfer function described earlier in this paper was applied to the faunal data and the paleotemperatures were estimated.

Results The results of the CaCO3 analyses, abundance of G. pachyderma and winter sea surface temperature estimates are shown in Figs. 4 and 5. Both E21--15 E~1-15 CARBONATE BO

80

(Z)

G"

PACHY~RMA

TW

('C;

I~

5

0 10

O~ I0

I

eO 3O

4O 5O 60 70 BO 90

2O 30 4O 5O 643 70 SO

[3 m 73

5o?

ZOO 110

110

/2O

leo

IBO

13o ~

140

140

150

150

160

160

170

170

180

±5O

190

190

L=O0

2O0

210

-

50

BO

ICO

3O

5O

7O

SO

0

±

2

3

4

5

rq

210

Fig. 4. Carbonate content, percent G. pachyderma and v4nter temperature estimates (TW) in core E 2 1 - - 1 5 .

7:1 RC12-~2~ G-

CARBONATE 6O

PAEHYOEI~

8O

TW

~b~, i

[ "C) 2

3

4

5 0

0

lO

~.0 2O

~0

50 6O

80

I

gO

4O ~0

/

~o

7o

~

t3o

~

IC~D R

~0

~.~0

14o

~

15o

170

17o

IE30 190

l~t:?

2OO 2~.0

Fig 5. Carbonate content, percent G. pachyderma and winter temperature estimates (TW) in core RC12--225.

and RC12--225 record two warm and two cold periods, with three minor fluctuations during the lower warm period. This pattern is clearly indicated by the TW (winter sea surface temperature) curves, and by the abundance of the cold species G. pachyderma. The close correspondence between these two curves indicates that the temperature variations are controlled by transgressions and regressions of cold Antarctic waters (represented by G. pachyderma) against the warmer waters north of the polar front. The calcium carbonate content is high throughout the cores, and averages about 85% with small fluctuations which do not exceed 10%. Inspection of the carbonate and the temperature trends reveals a tendency of decreasing carbonate content with lowering of the temperature. This pattern is especially clear in core E21--15. The implications of the percent carbonate changes will be discussed later. Before this can be done, it will be necessary to fix certain time levels in the cores.

Stratigraphic correlations Time correlation of core E21--15 with core RC12--225 is possible by the use of the TW curves, and the correlation of the two cores is shown in Fig. 6.

72

RC12-~25 TW 0

÷

e

E21-i5

("g)

?

TW

4,

s

o

÷

e

(°[)

a

,4 ~

0

0

i0

&O

2O

2O

3O

3O

40

40

5O

5O

60

60

70

70

8O

8O

9O i{30

I(30 llO

' 1 1 0

120 130

140

140

'

150

[xso

150

I ±F~O

170

170

180 190

.1_90

210

210

Fig.

6.

Correlation o f winter temperature estimates (TW) in cores E 2 1 - - 1 5 and R C 1 2 - - 2 2 5 .

It is possible to correlate not only the major climatic trends, but also the minor temperature fluctuations and thus it seems that these fluctuations are real features of the climatic record in the study area. It is important to place the climatic events in the absolute time scale, and Fig. 7 shows an attempt to do so. The time stratigraphic model in Fig. 7 is based on the assumption that major climatic changes in the study area took place at the same time as major changes in the extent of glaciations and in global climates, as indicated by oxygen isotope curves. That is, the bases of lower and upper warm periods in E21--15 and R C 1 2 - - 2 2 5 occurred at the same time as terminations II and I (Broecker and Van Donk, 1970) respectively, and that the cooling at the end of the lower warm period occurred at the end of the last interglacial as indicated by oxygen isotopes. Fig. 7 is drawn according to the time scale of Broecker and Van Donk, which is adapted in this paper. Two lines of evidence lend some support to the above stratigraphic model: (1) estimates of the average accumulation rates, and (2) the time of a rapid change in the abundance of the coccolith Emiliania huxleyi. Core E 2 1 - - 1 5 has been dated by an excess ~°Th method (Geitzenauer, 1969). It was found that the age at 300 cm down the core is 3 0 0 , 0 0 0 -+ 7 5 , 0 0 0 years. Based on

73

this date the accumulation rate of the core can be estimated as 0.88--1.4'7 c m / 1 0 0 0 years. Alternatively, the accumulation rate can be determined by foraminiferal biostratigraphy. Kennett (1970) has found that the age of the base of the Globorotalia truncatulinoides zone, in several cores including E21--15, is about 2 0 0 , 0 0 0 years. The base of this zone in E21--15 is 200 cm, and thus the average rate of accumulation is estimated as 1 c m / 1 0 0 0 years. If the level of 170 cm in this core is correlated with termination II (127,000 years B.P.), the average accumulation rate is calculated as 1.34 c m / 1 0 0 0 years. This rate of accumulation is consistent with the rates calculated above by other means. Geitzenauer (1972) has found that E. huxleyi increases sharply in abundance at the depth of 50 cm in E21--15, near the base of the upper cold period found in the present paper. In many North Atlantic cores an increase in the abundance of E. huxleyi occurs near the base of the last glacial (75,000 years B.P.) (A. McIntyre, personal communication). Thus, it seems justified to correlate the cooling at 80 cm in E21--15 with the base of the last glacial.

TW

C °E)

TW

(°C)

o

o

lo

lO

~o

2o

3O

3o

4O

40 5o

60

6o

7O

7o

BO

8o

9O

9°8©

lO0

loo

ii0

xxo

U]

I~01

120

[D

130

130

140 '

140

-< ;0

1%0 160 I 170

160 170 ±E~D

2oo ]

200

21o

210

Fig. 7. Time correlation o f the winter temperature estimates in cores E 2 1 - - 1 5 and R C 1 2 - 225. See the t e x t for e x p l a n a t i o n .

74

Paleoclimatic implications Assuming that the time stratigraphic model of Fig. 7 is correct, several interesting features of the climatic record in the study area emerge. Worth noting is the shape of the climatic record during the last glacial. After a rapid cooling at 75,000 years B.P., the climate remains steadily cold, until the rapid warming at the end of the last glacial. It is quite possible that the fine structure of the climatic record is lost due to the low accumulation rate during the glacial. However, the curves would still indicate the general climatic trend in the area. This climatic record is not unique to the study area, similar records were reported from several high-latitude locations. The foraminiferal record of the high-latitude North Atlantic has been heavily studied by several workers including Phleger et al. (1953) and McIntyre et al. (1972). Recently, a transfer function has been applied to core V23--82 b y Sancetta et al. (1973). The paleotemperature curve of this core indicates rapid cooling at 75,000 years B.P. and generally low temperature throughout the last glacial with relatively small fluctuations. Rather a similar climatic trend is recorded by the 5180 curves of two ice cores, one from Greenland and the other from Antarctica (Dansgaard et al., 1971; Langway et al., 1973; G o w et al., 1973). In these t w o cores, variations in the atmospheric temperature are recorded and thus it is possible that we have an indication of a general pattern in the mode of climatic response of both the atmosphere and the ocean in the high latitudes. In this regard it is interesting to point o u t the distinct difference between the climatic records discussed thus far and the foraminiferal 5'sO record. It has been demonstrated by Broecker and Van Donk (1970) that the general trend of the Atlantic 51sO record of the last 130,000 years follows a sawtooth pattern. Similar trends are found in Pacific cores (Shackleton and Opdyke, 1973; Shackleton in Luz, 1973; Luz and Shackleton, 1975). Shackleton (1967) and Shackleton and O p d y k e (1973) argue that the ~'sO record indicates primarily variations in the volume of glacier ice. In this case the isotopic record registers a gradual increase of the ice volume during the last glacial. This is quite different from the record of the high-latitude temperature discussed above, which shows rapid cooling 75,000 years B.P. and relatively steady low temperature during the entire glacial. Besides the general climatic trend in the study area, there are several relatively short climatic events worth mentioning. In the time span between 127,000 and 75,000 years B.P. there are three temperature peaks and two temperature lows (Fig. 7). It is tempting to correlate the temperature highs with the three warm episodes of the last interglacial which are well represented in the Barbados high sea-level terraces as well as in the European pollen record (Mesolella e t al., 1969; v a n der Hammen et al., 1971; Matthews, 1973, Fig. 1). It should be remembered however, that the climatic details of the last glacial are not very well represented in many deep-sea cores. In some cases it is probably the low accumulation rate and burrowing organisms of the deep-

75 sea sediment which prevents high resolution of the paleoclimatic records. One would expect the same to be the case in the study area, where the accumulation rates are low during the last interglacial (about 1.8 cm/1000 years) but in spite of the low rates, there are three well-separated warm events. Thisprobably indicates t h a t the climatic fluctuations during the last interglacial were pronouncea, and that tl~e area in general is sensitive to climatic variations. IMPLICATIONS FOR THE CARBONATE BUDGET The stratigraphic model of Fig. 7 requires that the glacial accumulation rates be 2--3 times less than the rates during the interglacial. Assuming that the model is correct, there are three mechanisms which could account for the decrease in the glacial rates of accumulation: (1) increased carbonate solution; (2) increased ocean-bottom erosion; and (3) decrease in calcium carbonate production. Of these three possibilities the first two should be eliminated for the following reasons. Examination of the calcareous fossils reveals that they are in excellent state of preservation and thus carbonate solution was probably minor, and at any rate there is no indication of increased solution during the glacial. Therefore, the first possibility is not likely to explain the change in the accumulation rate. The second possibility seems n o t probable because there is no indication of sediment winnowing. Had winnowing taken place, it would have concentrated coarser grains in the glacial section, but this is not the case. In the course of sample preparation, the percent of sediment greater than 63 pm has been recorded, and it was found that the percentage is not greater during the colds. On the contrary, the non-carbonate fraction which, at least in part, is finer (clay-size minerals) than the carbonates, is more abundant in the cold intervals (Figs. 4 and 5). It seems then that production o~ calcium carbonate was less during the last glacial. This would account for both lower accumulation rate and lower carbonate content during the glacial. In the absence of large variations in the non-carbonates accumulation, a reduction of 50% in carbonate accumulation would result in less than 10% change of CaCO3 if the initial c o n t e n t is 90%. These figures are in good accord with the data of core E21--15 (Fig. 4). In core RC12--225, a significant change in the accumulation rate of the noncarbonates probably t o o k place as well. This would explain why the carbonate content in this core (Fig. 5) is relatively steady through time. Large variations in calcium carbonate production t o o k place in the high latitudes of the North Atlantic (McIntyre et al., 1972). Cooling and warming caused by movements of the Atlantic polar front, resulted in fluctuations in the rate of coccolith-carbonate production, with total disappearance of coccoliths in polar-water conditions. Extreme climatic conditions with complete elimination of coccolithophorids were never the case in the area of the present research. However, the mechanism of climatic control over the production of biogenic calcium carbonate seems to have been effective in the high latitudes of the South Pacific.

76

It has been postulated that the production changes in the high latitudes play an important role in the global carbonate budget (Olausson, 1971; Berger, 1973; Luz and Shackleton, 1975). In the light of the findings of the present research, it is necessary to reemphasize the importance of the fluctuations in the high latitudes. In the past, attention was drawn to cores which show large fluctuations in their carbonate content. In many such cores the carbonate c o n t e n t variations result from an interplay between carbonate production and dilution by terrigenous material -- decreased production accompanied by increased dilution and vice versa. However, it is possible that many other cores containing information a b o u t changes in carbonate production were neglected because they fail to show percent carbonate fluctuations. CONCLUSIONS

The area near 50°S over the East Pacific Rise is sensitive to climatic variations, and cores from this area register a b o u t 5°C temperature difference between the peak of the last glacial and the present. The climatic record of this area shows a sharp cooling at the beginning of the last glacial (75,000 years B.P.) with relatively stable temperatures until the end of the glacial. This is similar to the climatic record of the high-latitude North Atlantic, as well as the climatic records of the Greenland and the Antarctic ice cores. In contrast, these high-latitude records differ from the ~ ~sO record of several deep-sea cores which suggests a general gradual increase of ice volume during the last glacial. It will be important for the understanding of climatic changes to find o u t why the temperatures of the high latitudes behave in different ways from the glacial ice volume. The finding of the present study suggests that the rate of CaCO3 production in the study area decreases during the last glacial. It seems that the cooling during the glacial made the area less favorable to organisms producing calcareous tests. This is similar to the pattern of calcium carbonate production in other high-latitude locations, and adds more significance to the changes in carbonate production in these areas as an important c o m p o n e n t of the global CaCO3 budget.

ACKNOWLEDGEMENTS

I thank Z. Reiss for careful reading of the manuscript and for his useful suggestions. I also thank J. Hays, A. McIntyre and N. Shackleton for the examination of several samples used in this paper and for helpful discussions. Part of the work in this study was carried out while the author was a postdoctoral fellow at Lamont-Doherty Geological Observatory, and was supported by an IDOE/NSF grant to the CLIMAP project (grant No. GX28671). Thanks are due to D. Cassidy, curator of the Florida State University core collection, for the samples from the Eltanin cores. Washed samples from core DWBG 70

77

were supplied by F. Phleger of Scripps Institution of Oceanography, who is greatly acknowledged here. Core RC12--225 is from the Lamont-Doherty Geological Observatory core collection, whose curatorial services are supported by the NSF grant No. GA29460 and by the Office of Naval Research, grant No. N00014-67-A-0108-0004.

REFERENCES Bd, A. W. H., 1969. Planktonic Foraminifera. Am. Geogr. Soc. Antarct. Map Folio Ser., 11: 9--12. Berger, W. H., 1973. Deep-sea carbonates: Pleistocene dissolution cycles. J. Foram. Res., 3: 187--195. Blackman, A., 1966, Pleistocene Stratigraphy of Cores from the Southeast Pacific Ocean. Dissertation, University of California, San Diego, Calif., 200 pp. Blair, D. G., 1965. The distribution of Planktonic Foraminifera in deep-sea cores from the Southern Ocean, Antarctica. Thesis, Florida State University, Tallahassee, Fla., 141 pp. Broecker, W. S. and Broecker, S., 1974. Carbonate Dissolution on the western flank of the East Pacific Rise. In: W. W. Hay (Editor), Studies in Paleo-Oceanography. SEPM Spec. Publ., 20: 44--57. Broecker, W. S. and Van Donk, J., 1970. Insolation changes, ice volumes and the 018 record in deep-sea cores. Rev. Geoph. Space Phys., 8: 169--198. Dansgaard, W., Johnsen, S. J., Clausen, H. B. and Langway, C. C., 1971. Climatic record revealed by the Camp Century ice core. In: K. K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale University Press, Newhaven, Conn., pp.37--56. Geitzenauer, K. R., 1969. Coccoliths as late Quaternary paleoclimatic indicators in the Southern Ocean. Nature, 223: 170--172. Geitzenauer, K. R., 1972. The Pleistocene calcareous n a n n o p l a n k t o n of the subantarctic Pacific Ocean. Deep-Sea Res., 19: 45--60. Gow, A. J., Epstein, S. and Sharp, R. P., 1973. Climatological implications of stable isotope variations in deep ice cores from Byrd Station, Antarctica. In: R. F. Black, R. P. Goldthwait and H. B. Willman (Editors), the Wisconsinan Stage. Geol. Soc. Am. Mem., 136: 323--326. Hays, J. D., 1965. Radiolaria and late Tertiary and Quaternary history of Antarctic seas. Antarct. Res. Ser. Am. Geophys. Union, 5: 125--183. Hays, J. D. and Donahue, J. G., 1972. Antarctic Quaternary climatic record and radiolarian and diatom extinctions. In: J. A. Raymond (Editor), Antarctic Geology and Geophysics. International Union of Geological Sciences Series, B, 1 : 7 3 3 - - 7 3 8 . Hays, J. D. and Opdyke, N. D., 1967. Antarctic Radiolaria, magnetic reversals, and climatic changes. Science, 158: 1001--1011. Hays, J. D., Lozano, J., Shackleton, N. J. and Irving, G., 1976. An 18,000 years B.P. reconstruction of the Atlantic and Western Indian Sectors of the Antarctic Ocean. In: R. M. Cline and J. D. Hays (Editors), Investigations of Late Quaternary Paleo-Oceanography and Paleo-Climatology. Geol. Soc. Am. Mem., 145: 337--374. Huddlestun, P., 1971. Pleistocene paleoclimates based on Radiolaria from subantarctic deep-sea cores. Deep-Sea Res., 18: 1141--1143. Hl'ilsemann, J., 1966. On the routine analysis of carbonates in unconsolidated sediments. J. Sediment. Petrol., 36: 622--625. Imbrie, J. and Kipp, N. G., 1971. A new micropaleontological method for paleoclimatology: Application to a late Pleistocene Caribbean core. In: K. K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale University Press, Newhaven, Conn., pp.71--181.

78 Jendrzejewski, J. P. and Zarillo, G. A., 1972. Late Pleistocene paleotemperature oscillations defined by silicoflagellate changes in a subantarctic deep-sea core. Deep-Sea Res., 19: 327--329. Keany, J., 1973. New radiolarian paleoclimatic index in the Plio-Pleistocene of the Southern Ocean. Nature, 246: 139--141. Kennett, J. P., 1970. Pleistocene paleoclimates and foraminiferal biostratigraphy in subantarctic deep-sea cores. Deep-Sea Res., 17: 125--140. Kipp, N. G., 1976. A new transfer function for estimating past sea-surface conditions from the sea-bed distribution of planktonic foraminiferal assemblages in the North Atlantic. In: R. M. Cline and J. D. Hays (Editors), Investigations of Late Quaternary PaleoOceanography and Paleo-Climatology. Geol. Soc. Am. Mere., 145: 3--42. Klovan, J. E. and Imbrie, J., 1971. An algorithm and Fortran IV program for large scale Q-mode factor analysis. J. Int. Assoc. Math. Geol., 3: 61--77. Langway, C. C., Dansgaard, W., Johnsen, S. J. and Clausen, H., 1973. Climatic Fluctuations During the Late Pleistocene. In: R. F. Black, R. P. Goldthwait and H. B. Willman (Editors), The Wisconsinan Stage. Geol. Soc. Am. Mere., 136: 317--322. Luz, B., 1973. Stratigraphic and paleoclimatic analysis of Late Pleistocene tropical Southeast Pacific cores. Quaternary Res., 3 : 5 6 - - 7 2 . Luz, B. and Shackleton, N. J., 1975. CaCO3 solution in the tropical east Pacific during the past 130,000 years. In: W. V. Sliter, A. W. H. B~ and W. H. Berger (Editors), Dissolution of Deep-Sea Carbonates. Cushman Found. Foram. Res., Spec. Publ., 13: 142--150. Matthews, R. K., 1973. Relative elevation of Late Pleistocene high sea level stands: Barbados uplift rates and their implications. Quaternary Res., 3: 147--153. McIntyre, A. and Moore Jr., T. C. and members of the CLIMAP project, 1976. The surface of the ice-age Earth. Science, 191: 1131--1137. McIntyre, A., Ruddiman, W. F. and Jantzen, R., 1972. Southward penetration of the North Atlantic Polar Front: faunal and floral evidence of large-scale surface water mass movements over the last 225,000 years. Deep-Sea Res., 19: 61--77. Mesolella, K. J., Matthews, R., Broecker, W. S. and Thurber, D. L., 1969. The astronomical theory of climatic change: Barbados data. J. Geol., 77: 250--274. Olausson, E., 1971. Quaternary correlations and the geochemistry of oozes. In: B. M. Funnel and W. R. Riedel (Editors), Micropaleontology of the Oceans. Cambridge University Press, Cambridge, pp. 375--398. Opdyke, N. D., Glass, B., Hays, J. D. and Foster, J., 1966. Paleomagnetic study of antarctic deep-sea cores. Science, 154: 349---357. Parker, F. L. and Berger, W. H., 1971. Faunal and solution patterns of planktonic Foraminifera in surface sediments of the South Pacific. Deep-Sea Res., 18: 73--107. Phleger, F. B., Parker, F. L. and Pierson, J. F., 1953. North Atlantic Foraminifera. Rep. Swed. Deep-Sea Exped., 7 : 1--122. Reid Jr., J. L., 1969. Sea surface temperature, salinity, and density of the Pacific Ocean in summer and winter. Deep-Sea Res., Suppl., 16: 215--224. Sancetta, C., Imbrie, J. and Kipp, N. G., 1973. Climatic record of the past 130,000 years in North Atlantic deep-sea core V23--82 with the terrestrial record. Quaternary Res., 3: 110--116. Shackleton, N. J., 1967. Oxygen isotopic analyses and Pleistocene temperature reassessed. Nature, 215" 1 5 - 1 7 . Shackleton, N. J. and Opdyke, N. D°, 1973. Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28--238: oxygen isotope temperatures and ice volumes on a l 0 s and 106 year scale. Quaternary Res., 3: 39--55. Van der Hammen, T., Wijmstra, T. A. and Zagwijn, W. H., 1971. The floral record of the late Cenozoic of Europe. In: K. K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale University Press, Newhaven, Conn., pp.391--424. Watkins, N. D. and Godell, H. G., 1967. Geomagnetic polarity change and faunal extinction in the Southern Ocean. Science, 156: 1083--1087.