Oxygen and carbon isotopes of Recent calcareous nannofossils as paleoceanographic indicators

Marine Micropaleontology, 5 (1980): 31--42

31

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

OXYGEN AND CARBON ISOTOPES OF RECENT CALCAREOUS NANNOFOSSILS AS PALEOCEANOGRAPHIC INDICATORS

DAVID E. GOODNEY 1 , STANLEY V. MARGOLIS, WALTER C. DUDLEY 2 , P. KROOPNICK and DOUGLAS F. WILLIAMS 3

Department of Oceanography, Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822 (U.S.A.) (Accepted August 16, 1979)

Abstract Goodney, D.E., Margolis, S.V., Dudley, W.C., Kroopnick, P. and Williams, D.F., 1980. Oxygen and carbon isotopes of Recent calcareous nannofossils as paleoceanographic indicators. Mar. Micropaleontol., 5: 31--42. 51sO and 513C values for several species of planktonic foraminifera and calcareous nannofossils from Recent deep-sea sediments have been studied in order to evaluate their paleoceanographic and paleotemperature potential. Nannofossils from Indian Ocean core-tops reflect isotopic temperatures as warm as, or warmer than, the temperatures reported by Williams et al. (1977) for shallow-dwelling planktonic foraminifera from the same samples. In general, deep-sea sediment samples from the world's major oceans indicate that nannofossil ~ 180 values are from 0.5 to 1 °/00 heavier than shallow-dwelling planktonic foraminifera. Although nannofossii 8180 values depart from thermodynamic equilibrium with oceanic surface water temperatures, the ~ 180 temperature trend parallels that of surface-dwelling planktonic foraminifera. Nannofossil 813C values also depart from equilibrium with surface water 613C-zCO: values. A comparison of nannofossil 6 ~3C data with that from planktonic foraminifera suggests that the rate of primary productivity in different water masses may be influencing the ~ 13C of carbonate-secreting phytoplankton and zooplankton.

Introduction

The oxygen isotopic composition of marine foraminifera has long been used as a paleotemperature and paleoceanographic tool.

1 Present address : Department of Chemistry, Willamette University, Salem, Oreg. 97301 (U.S.A.). 2Present address: Natural Science Division, Hilo College, University of Hawaii, P.O. Box 1357, Hilo, Hawaii 96720 (U.S.A.). Department of Geology, University of South Carolina, Columbia, S.C. 29208 (U.S.A.). Hawaii Institute of Geophysics Contribution No. 1021.

Recently, several investigators (Margolis et al., 1975; Savin, 1977) have shown that the oxygen isotopic composition of calcareous nannofossils is also a useful paleoceanographic indicator. In addition, it has been shown that the carbon isotopic composition of the biogenic carbonate may also yield valuable paleoceanographic information (Broecker and Broecker, 1974; Kroopnick et al., 1977; Williams et al., 1977}. The purpose of this investigation was to further test the utility of calcareous nannofossils as isotopic paleotemperature indicators and to evaluate the potential of the carbon isotopic-composition of the nannofossils.

32 Oxygen and carbon isotopic determinations were made on both nannofossils and several species of planktonic foraminifera in Recent core material from a variety of deep.sea locations. The aim of this comparison is to determine if calcareous nannofossils indeed reflect oceanic surface water oxygen and carbon isotopic values.

during computer processing (Goodney and Kroopnick, 1978). The relative enrichment or depletion is reported relative to the PDB isotope standard (Craig, 1957) using the notation:

Experimental procedures

where R = 13C/12C or 180/160. Duplicate preparations and measurements could be :performed with a precision of 0.1 °/00 or better.

The deep-sea samples selected for this investigation came from t w o sources: (1) piston core tops from the Hawaii Institute of Geophysics core collection and ( 2 ) n a n n o fossils from trigger-weight cores from the Indian Ocean that were previously studied by Williams et al. (1977). The samples from the HIG piston cores were assigned a Recent age by identification of a modern foraminiferal fauna. The trigger-weight core samples have been verified as Recent by oxygen isotope stratigraphy (Williams, 1976). In addition all nannofossil samples were examined b y scanning electron microscope (SEM) to determine that only m o d e m species were present and to check for signs of dissolution and/or recrystaUization of the carbonate. Relatively pure but polyspecific nannofossil fractions consisting of isolated coccoliths and coccospheres were separated from the <:44 ~m fraction using a short centrifuge technique described b y Margolis et al. (1975). The purified nannofossil samples were roasted in v a c u o at a temperature of 450°C for 30 min and reacted in orthophosphoric acid at 25°C (+0.1 ° ). The foraminifera were analyzed according to the procedures o f Douglas and Savin (1971) and Williams et al. (1977). Monospecific samples were ultrasonically cleaned to separate nannofo~ils from the foraminiferal fragments. The foraminiferal samples were then roasted and reacted according to standard procedures. The carbon and oxygen isotope ratios of the derived CO2 were measured mass-spectrometrically. All necessary fractionation factors and corrections are applied to the raw data

5% 0 =

r Rsamp" - - 1 ] I-Rstd.

X 10 a

Results The results of the oxygen and carbon isotopic analyses of Recent nannofossils and foraminifera from the piston cores are presented in Table I. In Table I the results of the nannofossils and planktonic foraminifera at each core site are listed according to increasing ~ 1sO-CaCO3. Temperatures were calculated using Epstein's modified paleotemperature equation (Epstein et al., 1951; Friedman and O'Neil, 1977). For the temperature calculation the 51SO-H20 was estimated from Craig and Gordon (1965). The nannofossil isotope data for the Indian Ocean trigger-weight cores are given in Table II. Due to mixing and lack of biostratigraphic resolution, the assignment of a Recent age to the HIG piston core samples (Table i) can include sediments up to several thousand years old and perhaps as old as the last glacial period and therefore may yield isotopic temperatures as much as 5°C cooler than present-day oceanic surface temperatures. Assuming that the nannofossils were deposited synchronously with the planktonic foraminifera, valid isotopic comparisons can still be made. For most of the Indian Ocean core tops (Table II), oxygen isotope stratigraphy was used to determine a Recent age (Williams, 1976)~ and therefore the data from these samples can be better related with present-day surface water temperatures. The data in Tables I and II show, however,

33

0

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i

0 ¢q

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~0

0

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~ 0

o~

L~ 0

I

L~

v-~

O0

¢'1

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oo v-~

L'-t--4

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0 4#

34 TABLE II Nannofossil isotope data from Indian Ocean gravity core tops* Core

Latitude

Longitude

Depth (m)

8180 (°/00)

513C (°/00)

Size fraction**

V19-178

8°07'N

73°15'E

2188

--2.24

--0.55

B

V19-185

6°42'N

57°20'E

2867

--2.45

--0.75

B

V19-202

6°59'S

41°11'E

2589

--1.44 --1.58

--0.63 --1.05

A B

RCl1-147

19°04'S

112°45'E

1953

--0.84 --2.31

+0.94 -0.40

A B

V20-170

21°48'S

69 ° 14'E

2479

-0.24 --0.60

--0.11 --1.33

A B

RCl1-126

30°04'S

94°25'E

2336

+0.12

--0.16

A

E48-27A

38°32'S

79°54'E

3231

+0.40 +0.49

+0.23 --0.54

A B

E48-23A

39°31'S

83°43'E

3459

+0.74 +0.81

-0.47 --0.64

A B

E48-22A

39 ° 54'S

85 ° 25'E

3324

+1.00 +0.83

--0.46 --0.60

A B

E45-73A

47°33'N

114°26'E

3690

+0.61 +0.72

--0.77 --0.33

A B

*See Williams (1976) for detailed core descriptions and planktonic foraminiferal isotope data. **Size fraction of nannofossils are: A, < 15 um fraction; B, >15 to <44 um fraction.

that shallow-dwelling foraminifera yield i s o t o p i c t e m p e r a t u r e s as w a r m as or w a r m e r than present summer surface water t e m p e r a t u r e s . This indicates t h a t t h e y m u s t have g r o w n u n d e r c o n d i t i o n s similar t o t h o s e p r e s e n t t o d a y . T h e i s o t o p i c d e p t h rankings f o r t h e f o r a m i n i f e r a species (Table I) are c o n sistent w i t h rankings o f o t h e r a u t h o r s (Lidz et al., 1 9 6 8 ; Berger, 1 9 6 9 ; Emiliani, 1 9 7 1 a ; Savin and Douglas, 1973). G. sacculifer and G. ruber c o n s i s t e n t l y give t h e w a r m e s t isot o p i c t e m p e r a t u r e s . G. sacculifer gives slightly w a r m e r t e m p e r a t u r e s in t h e e a s t e r n e q u a t o r i a l Pacific, a n d G. ruber gives slightly w a r m e r temperatures in t h e Caroline Basin. As expected, G. menardii gives isotopic

temperatures 6 ° - - 1 2 °C c o l d e r t h a n t h e s h a l l o w e r dwelling species since it secretes its calcite a t i n t e r m e d i a t e depths. F o r c o m p a r a t i v e p u r p o s e s Fig. 1 s h o w s t h e 180 o f t h e n a n n o f o s s i l s and t h e f o r a m i n i f e r a f r o m t h e s a m e l o c a t i o n p l o t t e d against t h e w a r m e s t i s o t o p i c t e m p e r a t u r e value c a l c u l a t e d f r o m t h e several species o f p l a n k t o n i c foraminifera. The temperature trend for the n a n n o f o s s i l s is parallel to t h a t o f t h e f o r a m i n ifera, a l t h o u g h a p p r o x i m a t e l y 1°/00 enriched in 180. A similar o f f s e t f o u n d in s a m p l e s f r o m T e r t i a r y c o r e s a m p l e s f r o m t h e S h a t s k y Rise were a t t r i b u t e d t o p o s t - d e p o s i t i o n a l overg r o w t h s o n t h e c o c c o l i t h s (Douglas a n d Savin, 1975). H o w e v e r , n o o v e r g r o w t h s w e r e

35

CORETOPS

\ t! - \......

-s0 I' . "."÷ x -2 0

•

÷ I~'x

xe

x

,.,. Q -I0 ec 0

r,o

x

a -a

a

x •

e

x\

Slope o f ~

~

a\

x

0.25%,/°C

~

0

10

20

a

30

x I ~lJ

I

I = = 30 °

= ~

I I 25"

~

= I

I I 20"

I

I

I

J I 15"

I

I

I

l I I0 °

I

TEMPERATURE

Fig. 1. ~ '~O vs. the calculated i s o t o p i c t e m p e r a t u r e for the w a r m e s t p l a n k t o n i c foraminifera. Data from the piston core t o p s (Table I). T e m p e r a t u r e for K K 7 1 - 2 5 and T R 1 2 6 - 2 3 is the m e a n yearly surface value. Solid line has s l o p e o f w a r m e s t p l a n k t o n i c foraminifera. Dashed lines indicate range o f n a n n o f o s s i l data. Legend: + = G . S a c c u l ~ f e r ; X = G . r u b e r ; A = G . bulloides; • = nannofossils; a = G . m e n a r d i i .

and size fractions used and the changes in depth habitat with latitude. The 513C data for the foraminifera from Recent sediments are difficult to directly correlate with environmental parameters (Tables I and II). For every sample except one, G. menardii has the highest 613C. The t w o species, G. tuber and G. sacculifer, which have similar isotopic temperatures, have 513C values which differ (except for KK71-78) by more than 0.6%o, with neither one consistently larger than the other. The 813C values for the nannofossils are lower (except for KK71-76) than all the planktonic foraminifera but larger than the values for the mixed benthic assemblage. This is in contrast with all other nannofossil 513C data obtained from Tertiary sediments, where nannofossil ~13C values are always larger than or equal to those

>

-3.0

I

I

I

I

~

I

INDIAN

I

OCEAN

I

I

CORE-TOP

I

I

SAMPLES

o

found on our samples during careful examination on the SEM. In Fig. 2 the 5180 results of the nannofossils from the Indian Ocean core tops are compared to the results of the foraminiferal analyses (Williams, 1976). The isotopic depth ranking o f the foraminiferal species is similar to that found with the HIG piston core tops from other locations, but the nannofossil depth ranking is significantly different; i.e., the nannofossil ~ 180 values are as warm as or warmer than foraminiferal values. The reason for this is unknown, although part of the difference may reflect a systematic offset between laboratories. An intercalibration between the isotope laboratories at Brown University, the University of Rhode Island, and the University of Hawaii is currently in progress. Notice, however, that the 8~80 temperature trend for the nannofossils and foraminifera is approximately the same although significant spread is introduced in the isotope data due to the different species

-2.0

÷ x~

~ \

x

~

-1.0 ~o c~ a

o

+

cl

o

-. o~

~o

1.0

2.0

3.0

I 30 °

I 25" AVERAGE

I 20 = SURFACE-

WATER

1 15"

I 10 °

TEMPERATURE

(C °)

Fig. 2. 8~80 vs. average surface water t e m p e r a t u r e for Indian Ocean trigger core tops. M o d i f i e d from Williams ( 1 9 7 6 ) ; foraminifera data. from Williams ( 1 9 7 6 ) ; n a n n o f o s s i l data given in Table II. Dashed line is drawn through the average value for the n a n n o f o s s i l data. Legend: × = G . r u b e r ; + = G . sacculifer;

A

=

G.

bulloides;

D =

G.

menardii;

v

=

• = calcareous nannofossils, < 1 5 u m fraction samples; o = calcareous nannofossils, > 15 to < 4 4 u m fraction samples. G.

quinqueloba;

36 of planktonic foraminifera (Margolis et al., 1975; Kroopnick et al., 1977). Discussion

The use of variations in the oxygen isotopic composition of certain foraminiferal tests as indicators of paleotemperatures is based on the assumption that the calcium carbonate is precipitated in isotopic (thermodynamic) equilibrium with seawater (Epstein et al., 1951, 1953; Emiliani 1955, 1966). There is evidence to both support and to refute the assumption of equilibrium carbonate precipitation by living and Recent fossil foraminifera. Smith and Emiliani (1968), using the 5'80 of Recent benthic foraminifera, calculated isotopic temperatures that were consistent with the temperature of the bottom water. Since monospecific and mixed samples agreed, it was concluded that benthic foraminifera give accurate isotopic temperatures. However, Duplessy et al. (1970) and Vinot-Bertouille and Duplessy (1973) using ancient and Recent benthic foraminifera, found wide variations in the 5180 and concluded that true temperatures cannot be calculated. The results of these studies may not be directly comparable since (1) the data of Smith and Emiliani (1968) were mostly from shallow-water regions where there was a large annual water temperature variation, (2) the data of VinotBertouille and Duplessy (1973) were on foraminifera that are taxonomically different from deep-sea benthic foraminifera, and (3) downslope transport of the samples was likely for the foraminifera studied by VinotBertouille and Duplessy (1973). Shackleton (1973) found that, although some species of benthic foraminifera do not deposit their tests in equilibrium, at least one genus, Uvigerina, closely approaches equilibrium. Planktonic foraminifera were also found by Shackleton et al. (1973) not to deposit their tests in isotopic equilibrium. Although the disequilibrium effects were small, the isotopic temperatures were warmer than surface

temperatures. Savin and Douglas (1973) found the isotopic temperature calculated from the tests of Recent shallow-dwelling planktonic foraminifera from the tropical South Pacific to be an average of 2.9 ° C colder than actual surface temperature. More recently Shackleton and Vincent (1978) have determined that the deviation of G. sacculifer from equilibrium is smaller than originally thought, but that two other species, G. rubescens and G. ruber, likely depart from isotopic equilibrium. The deviation is such that the species appear to yield temperatures higher than estimated summer values. Thus, the magnitude of the non-equilibrium (biological) isotopic fractionation for both planktonic and benthic foraminifera is apparently species dependent. Isotopic temperatures calculated from the 5'sO of planktonic foraminifera from the piston core samples measured in this work agree with the results of Shackleton et al. (1973) and Shackleton and Vincent (1978). The calculated temperatures for G. tuber and G. sacculifer are warmer than estimated yearly average temperatures and in most cases are warmer than estimated average summer temperatures. The actual water temperature estimates may be in error by -+1° C, but this uncertainty does not influence our conclusions. However, the discrepancy between the comprehensive study of Savin and Douglas (1973) using Recent planktonic foraminifera, and this work, cannot be completely resolved. Savin and Stehli (1973) have shown that selective solution after sedimentation biases the results to colder temperatures. However, since many of the samples reported here are from the same area (South Pacific) and approximately the same water depths as those reported by Savin and Douglas, there is no reason to believe that the differences are caused by differences in the extent of selective solution. Differences in laboratory procedures may again offer a partial explanation. Assuming equilibrium deposition of carbonate, temperatures calculated from the

37

~80 o f planktonic foraminifera can be used to infer the depth habitats of foraminiferal species. Depth habitats inferred from the temperature calculated using the 61sO of foraminifera data presented here agree with the work of several investigators (Lidz et al., 1968; Shackleton, 1968; Emiliani, 1971a, b; Savin and Douglas, 1973; Saito and Van Donk, 1974; Williams, 1976). G . r u b e r a n d G. sacculifer have the shallowest depth habitats, although neither lived consistently in shallower water than the other. G . m e n a r d i i lives at an intermediate depth. No deepdwelling planktonic species were analyzed from the piston core samples. The a ~sO data for the Recent nannofossils indicates that there is non-equilibrium fractionation between the calcium carbonate and seawater. However, the CaCO3 of the nannofossils apparently responds to temperature or other parameters (such as a change in seawater 180) that controls the 5 ~sO of the foraminiferal calcite. Although there may be a systematic difference between laboratories, both the piston core and trigger-weight samples show the same trend. Investigations on the 6 ~3C of Recent planktonic foraminifera have shown t h a t foraminiferal CaCO3 is not precipitated in isotopic equilibrium with t h e ZCO2 of seawater [ZCO2 = CO2(aq) + HCO~ + CO~-] (Williams et al., 1977; Shackleton and Vincent, 1978). Williams et al. (1977) found that the fractionation was species dependent while Shackleton and Vincent (1978) observed that most surface-dwelling foraminifera deposit their CaCO3 out of isotopic equilibrium. More important, both of the above studies concluded that, although a relationship exists between the 613C of the foraminiferal calcite and the 513C of ~CO2 in the water column, the foraminifera axe n o t deposited in carbon isotopic equilibrium. The 6~3C values of the nannofossils analyzed in this study (Tables I and II) are lower than the expected range of 1.5.-4%0 f o u n d in previous investigations o f fossil material (Margolis et al., 1975). In Fig. 3A--C

the a 13C of the CaCO3 is plotted against the 6 lSO of each species for each station. Plots such as these would show a profile similar to a ~13C-Y,CO2 vs. depth profile (Kroopnick, 1974; Williams et al., 1977) if the CaCO3 of each species were deposited in equilibrium. There are obvious departures which further support the conclusion t h a t the CaCO3 is deposited out of equilibrium. However, the variations in 613C show systematic variations which are related to the 513C-Z CO2. The 613C of the nannofossils is obviously also not in isotopic equilibrium with the 6~3C-~CO2. This is contrary to the observa813C vs RD B -I

V 19- 202 0

I

I

I

I

RC 11- 147 0 I

2

2

AN rre

-2 glut~ N A rse

c~-1

err

e'"

ae~./ deh

aJ

mrs

Ro Ro. o( " " ° d u ' t

20

04sac ~

conglobJt

conglob ?e-C~sac

dut...e'~le h

N,~. ~e emen e,~"

....

"etum

. ~tum scite

2

A

I

crasj.

~" ,' etrunc d

I

i

B

I

I

I

I

I

813CvsPDB. -2 -3

-I ,

V 20-170 0 1 , ,

2 i

-2 ac~ c~.

0 _~ 0 ¢,o

A N

aeeerr err, e-SagO AN #conglob P O.e:. . . . . 6men " • • dut trunc s ~trunc d I

I

I

Fig. 3. a ~sO/a 13C plots for core top samples. Modified from Williams (1976); foraminifera data from Williams (1976). Legend: N = nannofossils; ae = G. a e q u i l a t e r a l i s ; bul = G. b u l l o i d e s ; conglob = G. conglobatus; cras = G. c r a s s a f o r m i s ; deh = S. d e h i s c e n s ; dut = N. d u t e r t r e i ; glut = G. g l u t i n a t a ; men = G. m e n a r d i i ; 0 = O. u n i v e r s a ; P.o. = P. obliq u i l a t a ; rr = G. t u b e r ; rs = G. r u b e s c e n s ; sac = G. s a c c u l i f e r ; scit = G. s c i t u l a ; tri = G. t r i l o b u s ; trunc d = G. t r u n c a t u l i n o i d e s dextral; trunc s = G. t r u n c a t u l i n o i d e s sinistral; turn = G. t u m i d a .

38

tions of Kroopnick et al. (1977) who showed that in Tertiary cores, coccoliths were closer to isotopic equilibrium with respect to 613C-~COs than planktonic foraminifera. Since the nannofossils are the skeletal remains o f coccolithophorids which are capable of photosynthesis as well as respiration, there is no reason to believe they precipitate calcium carbonate by the same mechanism as foraminifera. There is, moreover, every reason to believe that rates o f photosynthesis and respiration in surface waters would have an effect on the 5 ~3C of the coccotiths (Dudley, 1976; Goodney, 1977). In areas o f upwelling or during the warm season, when rates o f productivity are high, the ~ 13C of the coccolithophorid CaCO3 may reflect the lower 613C values of the ~CO2 in surface waters. Using just the 6~3C-CaCO3 of the coccoliths, it is not possible to infer if variations are caused b y changes in the source 513C or by changes in the magnitude of biological fractionation. Note that the systematic difference between laboratories shows up again in Fig. 3, but the difference in no w a y affects the conclusions. Nannofossil 6 ~sO and 513C values plot consistently in the left-hand side in Fig. 3A--C (the Indian Ocean cores) indicating more negative 5 ~3C and warmer 51~O values than for planktonic foraminifera. In some cases such as R C l 1 - 1 4 7 (Fig. 3B), the nannofossil data plot distinctly separate from the planktonic foraminifera trend, while in others some planktonic foraminifera data plot close to the nannofossil value and are separated from the bulk of the other planktonic foraminifera (V19-202, Fig. 3A). V19-202 also shows little difference in the 5180 o f several species of planktonic foraminifera and nannofossils, b u t a 3%o difference in 613C. The same is true for V20-170 (Fig. 3C), the nannofossil ~ 13C being the most negative. Since some surfacedwelling planktonic foraminifera contain symbiotic photosynthetic zooxanthellae, the possibility exists that at some stage in their growth these species m a y utilize COs that has been depleted in ~:C b y the symbiote to produce test CaCO3, and thereby the 6~3C

values become more similar to coccolith values. Alternatively both foraminifera and coccoliths can utilize different carbon reservoirs during calcification and thus the 5~3C value for the CaCO3 may represent various mixtures of carbon from these two reservoirs. This may explain some of the overlap and variability in ~3C values for coccoliths and different species of planktonic foraminifera. A plot of all the isotopic data for G. tuber shows that the fractionation of ~3C is relatively constant and independent of 5180 (temperature) (Fig. 4). Since the fractionation is so constant, G. ruber may be suitable for inferring long-term variations in the ~3C-~CO2 of the oceans. More data o f this kind, and intercalibration of laboratories are necessary to further test this hypothesis. The 5 ~3C-6180 field of G. menardii is smaller than G. ruber (Fig. 5) since G. menardii tends to inhabit deeper waters where the hydrographic conditions are more stable and constant from location to location. The 613C of G. menardii

3

-2

-1

813C vS P D 0

1

3

2

-1¸ -3

-2

~A

/

l1,

c~ c~

© Aj 1

2

3

4

I

I

I

.

.

.

.

.

.

.

.

Fig. 4. Plot o f 5180 vs. 613C f o r Globigerinoides tuber. L e g e n d ( s o u r c e o f d a t a ) : • = this w o r k ; • = Williams ( 1 9 7 6 ) .

39

-3

-2 I

B13C vs P D B 0 I

-I [

-3

1 I

2 ]

3 i|

-1

G menardii

-2

-1 c~

o 6O

4

I

[

I

I

I

where 5180 varies between 0 to +3%0. The 513C values in Fig. 6 show similar high values for high-latitude cores. The relationship is by no means perfect, and more core top data are necessary for quantification. The departure of the 513C o f carbonates from equilibrium may be due to the direct inclusion of metabolic CO2 ("vital effects") or to changes in the local rate of productivity which can control the 813C of the HCO~ reservoir. The 513C data from certain foraminifera and the nannofossils are plotted as functions of latitude in order to evaluate the effects on the 5~3C of coccolithophores of variables such as species composition, 5'3C~,CO2, and rate of productivity (Figs. 7 and 8). The nannofossil and foraminifera curves are roughly mirror images. This relationship is expected if the 813C-ZCO~ of the ocean reservoir is to some extent affected b y the

Fig. 5. Plot o f 5~sO vs. 61aC for Globorotalia menardii. L e g e n d (source o f data): * = this w o r k ; • = Williams (1976).

-3

-2

BI3C vs P.DB. 0

-1

1

2

3

-3

was consistently among the highest and therefore approached the expected value for incorporation of surface water HCO~ (Kroopnick et al., 1977). Thus G. menardii is also a likely candidate for inferring paleo-~13C~C02. In Fig. 6 the 5 ~3C vs. 81sO data for all the nannofossils are presented. The range of values is large due to the large variation in age of the samples. For core samples of Recent age, the 5 ~3C variation is only 1%0. The general (upper left to lower right) trend could be caused b y differences in surface water ~13C o f ~CO2 or species diversity changes as a function of temperature, latitude, and productivity. Since the temperature dependence of the ~3C fractionation is small (Kroopnick et al., 1977), the latter possibilities are more reasonable. The reason for the observation in earlier works on nannofossil carbon isotopes that their 8 ~3C was more positive than planktonic foraminifera is that the earlier work was on temperate to Subantarctic core material

\

-2

,

-1 c~ ~2

~o

\...

0

1

:o

•

•

oO o~

~ o

2

3

4

t

I

I

I

I

I

t

Fig. 6. Plot o f ~ 180 vs. 5 lsC for nannofossils. L e g e n d (source o f data): • = this w o r k ; • = this w o r k , I n d i a n Ocean gravity c o r e t o p s ; o = Site 284 DSDP, K r o o p nick et al. (1977); + = Site 279 DSDP, Margolis et al. ( 1 9 7 5 ) ; ~ = Site 281 DSDP, Margolis et al. (1975); [] = Site 277 DSDP, Margolis et al. (1975); v = Core E55-26, Margolis et al. (1975).

40

I

I

I

INDIANOCEAN

I CORE

I

TOPS

Planktonic E o r a m i n i f e r ° ~

A men

oE

%o

-k-

/o

"-

~J

-

I

/

. . . . . . . . . . . . .

Nonnofossils -1

I 40 °

I 20"

I 0° LATITUDE

l 200

I 40 °

Fig. 7. Plot of 5 ~C vs. latitude for Indian Ocean core tops. See caption, Fig. 3 for abbreviations legend. Foraminifera isotope data from Williams (1976). Nannofossil isotope data, this work.

coccolithophorids or other phytoplankton. For example, in areas of low productivity the coccolithophorids would not appreciably alter the ~ ~sC of the environment so the foraminifera CaCO3 would only be slightly enriched in '3C. On the other hand, during periods o f high productivity the p h y t o p l a n k t o n would r

i

~ . . . . . CORE

Planktonic

v

Foraminifera

A.....

i~

-- 7 - - - - - - - - -

~OP~<

A"

A

~'

C~

2

l

~0

_

, - , ,

_ _

u ~o -1

Nannofossils

T/ t u b e r ~. met ~ •soc • nonno$ • 6ul

-2

I

I

40"

20"

I O°

I 20"

I ~ 4.O"

1 I

s

LATITUDE

Fig. 8. Plot of ~ lsC vs. latitude for piston core tops from Table II. See caption, Fig. 3. for abbreviations legend.

greatly deplete the inorganic reservoir in 12C relative to lsC and the foraminifera CaCO3 would become enriched in 13C. The core top data (Fig. 8) show a large negative excursion in the coccolith 613C values at about 10--I8°S which roughly corresponds to a positive excursion in the planktonic foraminifera values. This excursion is also seen in. the Indian Ocean data (Fig. 7). The 613C-~CO2 data for Atlantic surface waters similarly shows positive excursions at 10°N and S with a large depletion at the equator (Kroopnick, 1980). We can only speculate with these limited data as to whether these curves are influenced by productivity, variations in 6 ~3C of Z CO2 due to local upwelling, selective solution, or some unknown fractionation processes. The complex interrelationships of these diverse processes in the natural system must be more fully investigated before the 5~3C of fossil carbonate can be used for paleoceanographic studies. Conclusions A comparative investigation of the isotopic composition of calcareous nannofossits and planktonic foraminifera from Recent sediments has revealed the following about their use as pateotemperature and paleoceanographic indicators: The ~ 'sO composition of calcareous nannofossils does not in all cases indicate deposition in isotopic equilibrium with oceanic surface waters. However, a definite temperature-dependent trend exists and the slope of this trend parallels those for some species of near-surface
41

t o p s , and for i n t e r l a b o r a t o r y calibration. T h e ~ 13C values f o r f o r a m i n i f e r a are n o t e x p e c t e d f o r t h e u p t a k e o f ~CO2 f r o m o c e a n surface waters. In m o s t cases t h e 513C o f t h e nannofossils are m o r e negative t h a n p l a n k t o n i c f o r a m i n i f e r a f r o m t h e same core. S o m e species o f p l a n k t o n i c f o r a m i n i f e r a m a y be useful in tracing t h e p o s i t i o n o f t h e shallow o x y g e n m i n i m u m as suggested b y Williams et al. ( 1 9 7 7 ) and S h a c k l e t o n a n d V i n c e n t ( 1 9 7 8 ) . T h e a n t i t h e t i c relationship b e t w e e n the 513C o f p l a n k t o n i c f o r a m i n i f e r a a n d nannofossils w h e n p l o t t e d against latit u d e a n d t h e 513C o f dissolved b i c a r b o n a t e suggests t h a t t h e rate o f p r i m a r y p r o d u c tivity m a y i n f l u e n c e t h e ~13C o f b o t h o f t h o s e fossil groups.

Acknowledgments This research was s u p p o r t e d b y N S F grants D E S 7 5 - 1 9 3 8 6 , O C E 7 7 - 2 1 0 9 9 , and O C E 7 8 0 8 6 2 8 . We t h a n k Ms. C. W e n k a m f o r age dating o f p i s t o n c o r e samples and Dr. J.P. K e n n e t t f o r t h e A t l a n t i c and C a r i b b e a n p i s t o n core t o p samples. T h e core samples f r o m t h e L a m o n t - D o h e r t y Geological O b s e r v a t o r y c o r e c o l l e c t i o n are c u r a t e d u n d e r N S F grants D E S 7 5 - 2 1 3 6 6 and D E S 7 6 - 0 2 2 0 2 .

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Kroopnick, P.M., Margolis, S.V. and Wong, C.S., 1977.13C variations in marine carbonate sediments as indicators of the CO~ balance between the atmosphere and oceans. In: N.R. Anderson and A. Malahoff (Editors), The Fate of Fossil Fuel CO~ in the Oceans. Plenum, O.N.R. Seri.,pp. 295--321. Lidz, B., Kahn, A. and Miller, H., 1968. Depth habitats of pelagic foraminifera during the Pleistocene. Nature, 217: 245--247. Margolis, S.V., Kroopnick, P.M., Goodney, D.E., Dudley, W.C., and Mahoney, M.E., 1975. Oxygen and carbon isotopes from calcareous nannofossils as paleoceanographic indicators. Science, 189: 555--557. Saito, T. and Van Donk, J., 1974. Oxygen and carbon isotope measurements of Late Cretaceous and Early Tertiary foraminifera. Micropaleontology, 20: 152--177. Savin, S.M., 1977. The history of the earth's surface temperature during the past 100 million years. Annu. Rev. Earth Planet. Sci., 5: 319--355. Savin, S.M. and Douglas, R.G., 1973. Stable isotopes and magnesium geochemistry of Recent planktonic foraminifera from the South Pacific. Geol. Soc. Am. Bull., 84: 2327--2342. Savin, S.M. and Stehli, F.G., 1973. Interpretation of oxygen isotope paleotemperature measurements: effect of the O~8/O 16 ratio of sea water, depth stratification of foraminifera, and selective solution. Coll. Int. C.N.R.S., 219: 183--191.

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