Factors controlling the fluoride content of planktonic foraminifera: An evaluation of its paleoceanographic applicability

Geochimico

et Cosmochimica

0016-7037/93/$5.00

Acla Vol. 57, pp. 335-346

Copyright 0 1993 PergamonPressLtd. Printedin U.S.A.

+ .oo

Factors controlling the fluoride content of planktonic foraminifera: An evaluation of its paleoceanographic applicability YAIR ROSENTHALand EDWARD A. EWYLE Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02 139, USA (Received December 10, 1991; accepted in revisedform July 24, 1992)

Abstract-This work is an assessment of the effects of seawater salinity and temperature on the incorporation of fluoride into calcitic shells of planktonic foraminifera and of the subsequent modification of shell chemistry by postdepositional dissolution. A strong correlation between the fluoride content of most planktonic species and their calcification depth was observed, deeper-calcifying foraminifera have lower fluoride content and heavier ‘*O composition, perhaps suggesting a temperature control on the partition coefficient. However, species deviations from this general trend and the absence of correlation between fluoride and sea-surface temperature within an individual species suggest that the coprecipitation of fluoride in foraminiferal calcite is more strongly controlled by biological processes rather than by simple chemical relationships. Foraminiferal fluoride shows weak or no correlation with sea-surface salinity. Core-top planktonic foraminifera from the eastern equatorial Atlantic (Sierra Leone Rise) show a substantial decrease of fluoride and, to a lesser extent, Mg content with increasing water depth (293 l5 104 m). These decreasing trends with depth are attributed to selective dissolution, i.e., removal of the more dissolution-susceptible fraction with a different chemical composition than the residual, a process which previously has been suggested to account for variations in the isotopic composition of planktonic foraminifera. These findings indicate that foraminiferal fluoride cannot serve as a reliable tracer of oceanic paleo-temperature or salinity. It is possible, however, that fluoride may be useful for the study of dilution in deep-sea sediments. INTRODUCTION

1980; RUDE and ALLER, 199 1) . This study assesses the variations in F content of different planktonic foraminifera (i.e., low-Mg calcite) and attempts to unravel the causes for this variability and evaluate its paleoceanographic applications.

MUCH OFOUR UNDERSTANDING OF modem ocean dynamics has been inferred from measurements of the classical hydrographic properties: temperature, salinity, oxygen, and nutrients. Paleoceanographers would like to find tracers for these properties in the sedimentary record. Currently, the major tool for exploration of past temperature and salinity distributions in paleoceanographic studies has been the oxygen isotopic composition of foraminifera. However, because foraminiferal 8 ‘*O depends both on temperature and salinity of seawater (EPSTEIN et al., 1953; CRAIG and GORDON, 1965 ) , we are forced to make assumptions about one of these variables in order to extract the other. For surface waters, paleoecological temperature ( IMBRIE and KIPP, 197 1) and salinity ( CULLEN, 198 1) estimates can place some additional constraints ( DWPLESSY,1982; KEIGWIN and BOYLE, 1989; DUPLESSYet al., 1991), or perhaps in the future, by using the new II:, method (BRASSELL et al., 1986). No temperature-specific estimation technique for deep waters is available, and no direct salinity-specific method has been reported for surface or deep waters. A tracer specific for either temperature (in deep waters) or salinity (in surface and deep waters) would be of great utility in paleoceanographic studies. Marine carbonates contain relatively large and highly variable amounts of fluoride, with F/Ca ranging between 0.1 to 3.5 mmol/mol for calcite and 4.0 to 6.5 mmol/mol for aragonite (CARPENTER, 1969, who noted that calcium carbonate precipitation is the primary sink of dissolved fluoride in the ocean). Previous studies focused on the content of fluoride in aragonite and high-Mg calcite or in bulk assemblages of foraminifera (e.g., CARPENTER, 1969; OHDE and KITANO,

Geochemical Considerations CARPENTER( 1969 ) suggested that fluoride (F-) is primarily incorporated into the calcium carbonate crystal lattice structure during shell growth, although the nature of this coprecipitation process was not clear. KITANO and OKUMURA ( 1973) and OHDE and KJTANO ( 1980) suggested that the incorporation of F into high-Mg calcites might be coupled with the coprecipitation of Mg, whereas ICHIKUNI ( 1979) suggested that fluoride ions substitute for CO:- group in aragonites. If one assumes an ideal solid solution between CaCOr and CaF2 whereby two F anions substitute for one CO, group in the lattice:

2F- + [ CaC03 Isolid= [ CaF2]mlid+ CO:-

(1)

then the F content of the solid can be expressed as [ ~I,,,

= ~~)~,,,

(2)

where K is the partition coefficient of F between the solution and the precipitating solid. In this case, the F content of inorganic carbonates which precipitate in the ocean will depend largely on the changes in the CO:- activity of seawater (RUDE and ALLER, 199 1), since huoride is a conservative element in seawater. However, biogenic calcnication is regulated by biological processes and one cannot assume simple thermodynamic relations for biogenic carbonates. Knowing very little about the chemistry of the foraminifera’s calcifying medium we can only assume that this solution was either exactly saturated or oversaturated with respect to the precipitating mineral (i.e., Km 2 ac. hl; Ezp being the solubility product). In “ideal” thermodynamic situation, that is a constant temperature, pressure, and CO:- ion activity, the F content of a foraminifer will depend largely on the 335

336

Y. Rosenthal and E. A. Boyle

fluoride activity in the ambient water (assuming that the biological controls on Fare small) and reflect changes in water salinity (MILLER, 1974). In reality, one should anticipate kinetic effects on the partition coefficient (primarily temperature and calcification rate) and strong metabolic effects on the CO:- activity as suggested by the experiments of TER KUILE and EREZ ( 1987) and TER KUILE et al. (1989). A priori it is impossible to predict which effect (if any) will govern the coprecipitation process. This work provides a data base for assessing the influence of various factors and postdepositional processes on the fluoride content of planktonic foraminifera, in order to evaluate its potential for paleoceanographic uses. METHODS Sample Selection Core-tops chosen for this study underly surface waters representing a wide range of salinity (33.5-40.5k) and temperature ( 18-29’C; Table I ). Most samples are from the Atlantic Ocean, with additional samples from the Pacific and Indian oceans, and the Mediterranean and Red seas. The latter cores represent extreme oceanographic conditions with respect to sea-surface salinity and temperature. In order to limit uncertainties, cores with good stratigmphic documentation were chosen (Holocene core-tops, relatively high sedimentation rates, and known fauna1 abundances). All cores used for the fluoride-salinity and fluoride-temperature calibrations are shallower than the in situ hydrographic lysocline (i.e., ACO:- > 0 following the definition by BROECKERand PENG, 1982). Ten planktonic species were studied: four symbiotic shallow species (G. ruber, G. sacculife, 0. universa, G. aequilateralis) and six nonsymbiotic deep dwellers (P. obliquiloqulata. N. dutertrei. G. menardii. G. tumida, G. truncatulinoides, G. crassafirmis). Plankton-tows and b ‘*O studies (FAIRBANKSet al.,

1982; HEMLEBENet al., 1989) suggest that the former four species calcify primarily in near-surface waters (0- 100 m) whereas the latter nonsymbiotic species continue to calcify in deeper, colder waters as they grow and sink downward. All of these species have calcitic shells. Foraminifera were handpicked from disaggregated sediments. To minimize the effects of natural variability between shells, specimens were analyzed from within a restricted size range (300-425 pm), and same morphology (i.e., G. saccu/r@r-without a final sac-like chamber: G. ruber-white variety; 0. universa-spherical shell; and encrusted shells of P. obliquiliqolata and G. truncatulinoides). Samples visibly contaminated with ferromanganese-oxides were excluded. A typical sample, weighing OS- I .O mg, contained between IO-30 shells. Samples were crushed mildly just sufficient to open the chambers, then ultrasonically cleaned (first in distilled water and later in methanol), and finally dried at room temperature before dissolution. Although it was impiausible that contamination by clay minerals or oxides would significantly affect the fluoride content of these shells (the fluoride content of these phases being too low to contribute much fluoride when present as minor contaminants), this possibility was evaluated by comparing analyses of split samples cleaned by the above short cleaning procedure with full oxidative/reductive cleaning following the protocol of BOYLE( 198 I ). There were no systematic differences between the two cleaning methods (Fig. I ), and small differences between individual samples are probably due to the natural variability between the fluoride content of individual shells. Analytical Procedure The spectrophotometric

technique of GREENHALGHand RILEY of fluoride in seawater was modified for the analysis of CaNOr solutions and scaled down for small samples. This method is based on the complexometric competition between F and alizarine for La, and the spectral shift between the complexed and uncomplexed forms of alizarine. The combined La-alizarine reagent (GREENHALGHand RILEY, 1961) was formulated to buffer the solutions at pH = 4.9, where the absorbance at 640 nm shows the least sensitivity to variations in pH. Samples were dissolved with weak HNOs. Dissolution was controlled to result in approximately constant Ca concentrations in order to minimize potential interfer-

( I96 1) for determination

Table 1. Location and water depth of cores used for the core-top salinity and temperature calibration. Isotope stratigraphy for these cores from (SAVIN and DOUGLAS, 1973; REISS et al., 1980; DURAZZI, 1981; DUPLESSY, 1982; BOYLE, 1984; MORLEY and SHACKELTON, 1984; MIX and FAIRBANKS, 1985; BOYLE and KEIGWIN, 1987; CURRYet al., 1988; MtX et al., 1990). COlS

Lat.

Long.

water

SmplP

cm Atlantic Ocean AI1107 PC65 EN66 GGCl6 EN66 GGC3a KNR64 PC5 RC13-228 V22-174 v25-59 V30-51K

31 05 04 16 22 10 01 19

Pacific Ocean 567 FFC26 v19-27

oow oaw 30W 4aw 12E 49w 29w 55w

2925 3152 2931 3047 3204 2630 3824 3409

2-3 o-2 o-2 7-12 6-7 4-5 6-V o-3

02 03N 00 47s

157 21E a2 07w

1611 1373

o-2 5-8

weditarrane8n KNRl34. 7 GGc14 KNR134.16 GGC 2

33 25N 38 OON

25 OOE 31 52E

2300 2740

7-a o-1

Red Sea AI193.19 AI193.19

29 11N 28 1aN

34 4aE 34 31E

550 884

o-2 o-3

72PG 74PC

00s 20N 55N 31N 20s 04s 2ZN 52N

38 21 20 74 10 12 39 19

ences from Ca (Ca = 0.017 M for shallow species and 0.04 M for the deeper ones, using 0.038 N and 0.096 N HNOr, respectively); the Ca concentration of the solutions was chosen to be equivalent to fluoride concentrations of 30-50 rmol/L. A test run of fluoride in solutions of increasing Ca concentration displayed minimal Ca interference ( 1 PM F for a 0. I M change in Ca). 50 pL of the combined La-alizarine reagent was added to 100 CCLsample and left for exactly I h before the analysis. Samples were analyzed in a 5 mm semimicro, self-masked cell, on a Shimadzu UV-120-02 spectrophotometer. Light absorption was determined at 640 nm rather than the commonly used 622 nm because the blank absorbance at this wavelength is three times lower than at 622 nm, whereas a standard-blank difference is essentially the same (a 30.5 pmol F/L standard was used for this calibration). Under these conditions light absorption changes linearly between I5 pmol F/L and 80 pmol/L. The reagent absorbance “blank” (the absorbance of the La-alizarine solution when no fluoride is present) was established by adding the reagent to a solution of CaCOr dissolved in dilute HNOa and purified of fluoride by ion exchange. The true fluoride blank certainly must be quite low, but it is difficult to establish by this analytical procedure. Samples were quantified by comparison to matrix-matched fluoride standards. Analytical reproducibility was monitored with three consistency standards (35.54,49.27, and 6 I .46 pmol F/L). Within-run precision of these consistency standards was 0.7% and inter-run reproducibility was better than 1%. Analysis of different splits from a finely crushed pooled sample suggests a procedural precision of 2%. Calcium and magnesium were analyzed on a Perkin-Elmer model 403 flame atomic absorption spectrophotometer. All samples were diluted with La solution (400 ppm La in a 0.05 N HCI and a 0.002 N HNOs solution) to eliminate suppression of the Ca signal due to phosphine in the acetylene. Long-term precision for Ca and Mg was about + I%. Thus, the overall analytical precision of the F/Ca in this study is about k2.296. However, reproducibility of replicate samples of foraminifera from the same core depth is seldom as good as the analytical precision. Reasons for the discrepancy between the analytical and sample reproducibility will be discussed in the following sections.

lated with surface salinity. The lack of F-S correlation for the shallowest dweller G. ruber (Fig. 2a) is certainly a significant discrepancy, however. The scatter in the data (Fig. 2) is significantly larger than the analytical precision (i.e., +2.2% or 0.8960in salinity space) and therefore should be considered as representative of real variability in foraminiferal fluoride. Sample reproducibility was about f7.5% for shallow species, G. sacculife, G. ruber, and 0. universa (F/Ca of 2.0-2.5 mmol/mol), and about f 13% for deep dwellers N. dutertrei and G. menardii (mean F/Ca of 0.8 mmol/mol). In salinity space these latter values translate into errors of +2.5?&1and &4.5?&1,respectively (calculated for mean oceanic salinity of 34.7%0). The large variability between replicate samples is greater than can be accounted for by spatial or short-term temporal changes in salinity. Spatial variability in foraminiferal salinity response arises from variations in the calcification depth between individual shells, whereas temporal variability results from the bottom-sediment mixing of individuals that lived at different times during a period of several thousand years (most of the cores in this study have an average sedimentation rate of 3 cm/ ky; hence, the bioturbation zone represents about 3 ky ). We have little knowledge of changes in salinity that may have occurred during the past 3,000 years, but we can say that temporal changes in salinity observed over the period of modern oceanographic observations (e.g., the Bermuda time series, STOMMELand WUNSCH, 1990) are not sufficient to produce the variability between replicate picks of individuals that we observed, and therefore other controls should be considered. To assess the variability within a single species we have analyzed G. sacculij2r shells of increasing size (all shells without a sac-like final chamber). It is often assumed that shells of different sizes represent different calcification depth. In a similar study BERGER et al. ( 1978) showed an enrichment of the oxygen isotopic composition of foraminifera from the west equatorial Pacific; the larger the shell size, the heavier

FIG. I. Comparison between F/Ca obtained in sample splits cleaned by two different procedures: I. Ultrasonic agitation in distilled H,O and methanol (short protocol). II. Full oxidative/reductive cleaning (full protocol; BOYLE, 198 1) .

RESULTS

AND

337

of fluorine in foram shells

Geochemistry

DISCUSSION

Salinity Effects

Core-top F/Ca (mean and standard deviation) in five planktonic species (G. ruber, G. sacculife, 0. universa, N. dutertrei, G. menardii) are given in Table 2. Also included are the annual sea-surface salinity and temperature for each core site ( LEVITUS,1982). Scatter plots of F/Ca against seasurface salinity show weak correlations for G. sacculife and 0. universa (Fig. 2b, c). A(F)/ A(salinity) estimates were derived from the slopes which were obtained from regressing the measured data against the surface salinity. The largest change of about 0.65 mmol/mol in F/Ca for a 6% change in salinity was observed in G. sacculife. This change is significantly lower than the change predicted by the theoretical model. The other three species show no discemable trend. Because N. dutertrei and G. menardii calcify primarily in deeper, subsurface water (HEMLEBEN et al., 1989), it is somewhat inappropriate to expect these values to be corre-

Table 2. Con-top F/Cn in planktonic foraminifera. The results ate repotted as mean values, standarddeviation of the means (i.e. SD/d@ and numberof replicate samples. (” II.&”stands for not-analyzed because samples were either absent or in very low abundance). F/Ca data are given in units of mmolhnol. Mean annual surface salinity and temperaturewere takenfrom LEWTUS(1982). P/Ca

core G.

WSAN Atlantic AI1107

t?Ka

P/CFt

ruhr

SD/dn n

F/Ca

F/C3

N. dutertrei

G. mnardii

MEAN

swhl

n

UMN

1

SD/&

n

0.04

2

Ogan 36.00

22.60

2.46

1

2.58

1

“.a.

0.91

0.04

2

0.91

EN66

GGC16

35.50

26.60

2.59

0.07

2

2.31

0.03

3

2.09

0.11

2

0.77

0.02

2

0.72

EN66

GGC38

35.50

26.60

2.62

0.02

3

2.26

0.06

1C

1.73

0.08

6

0.62

0.05

3

0.82

0.06

3

1

2.37

1.08

0.09

2

1.08

0.09

2

2

2.62

0.08 0.02

2 2

n.a.

0.01

1.00

0.09

3

1.08

0.05

3

1

2.56

0.03

2

0.04

4

0.08 0.79

0.07 0.04

2 2

0.96 0.86

0.09 0.05

2 2

1

0.95

1

0.95

1

0.65

1

0.65

1

KNR64

PC65

PCS

35.75

2.66

RC13-228

35.50

27.35 23.40

n.a.

V22-174

36.50

25.70

2.71

0.03

2

".a.

0.05

4

2.03 2.07

1

n.a.

2.28

0.03

7

1.83

0.01

4 Il.91

".a

n.a.

v25-

59

35.75

"30-

51K

36.00

27.15 21.40

2.52

FFC26

34.75

29.10

2.79

1

2.40

27

34.50

22.20

2.70

1

2.59

7 GGC14

39.00

20.90

2.63

0.16

2

2.73

0.05

6

1.97

0.06

5

*.a.

n.a.

GGC

38.00

18.70

2.50

0.05

2

2.71

0.07

4

2.05

0.10

2

n.a.

n.a.

2.74

0.06

4

".a.

n.a.

n.a.

0.03

2

2.67

0.06

5

2.31

0.06

4

n.a.

n.a.

Pacific S67 v19-

2.64

Oc%m 1

“.a

Mmditerranesn KNRl34. KNR134.16 Red

1

2

Sea

AI193.19

72PG

40.00

24.70

n.a.

AII93.19

74PC

40.00

24.70

2.79

Y. Rosenthal and E. A. Boyle

338

3 .o

h. G.sacculifer

a. G.ruber (white)

(wlo sac)

3.0

2.5

Z 2.5 (PE U‘ Z i;;E E 2.0

2.0

1.5 33

35

37

39 Surface Salinity ( 400 )

1.5 33

41

35

31

41

39

Surface salinity

(

o~oo )

c. 0. universa

35

e.

d. N. dutertrei

!

.

33

@I T

35

G. menardii

1.5 -I

1.5

0.5

41

39 ( doe)

Surface SaGity

.

I

31

.

1

1

39

Surface salinity ( d00 )

I 41

03

:

u

33

35

.

I

37

.

I

39

.

41

Surface salinity ( d00 )

FIG. 2. Scatter plots of core-top F/Ca in planktonic foraminifera against sea-surface salinity. Each point represents a single analysis. Core data is given in Table I whereas mean values and the standard deviation of the means are given in Table 2 (analytical precision is f2.2%, i.e., about twice the symbol size). Note the change in F/Ca scale between shallow and deep species. its 6 ‘so. The heavier 6 ‘*O values were interpreted as reflecting calcification in deeper and colder water. Our study should be evaluated with two major caveats in mind. The first is that G. succufifer lives predominantly in relatively shallow water ( HE~~LEBENet al., 1989). The second caveat is that the assumption that big shells evolve from smaller ones and that larger shell means deeper calcification depth is not necessarily true ( HEMLEBENet al., 1989; OPPO and FAIRBANKS, 1989; LOHMANNand SCHWEITZER, 1990). Nevertheless, because analytical limitations prevent us from measuring fluoride in a single shell or a segment of it, this procedure is our best available approximation for ontogenic changes in the shell composition (e.g., OPPO and FAIRBANKS, 1989). As shown in Fig. 3a, shells of G. sacculifer from two shallow cores on the Sierra Leone Rise ( EN66 GGC38 and GGC 16 ) exhibit a decrease in their fluoride content with increasing

shell size. F/Ca drops from 2.49 f 0.04 mmol/mol in the 250-300 pm size fraction to 2.15 + 0.04 mmol/mol in the 700-800 pm fraction, a decrease of about 14% (values represent means plus 1CT/tin). The actual range of changes in the distribution coefficient of fluoride in these shells is probably larger than the observed range, because the two extreme ratios are integrated values and are not pure endmembers ( OPPO and FAIRBANKS, I989 ). In addition, we analyzed Mg for comparison; however, it shows no discemable change (Fig. 3b). A study of G. sacculifer shells from the Ontong-Java Plateau in the west equatorial Pacific shows no change with increasing shell size for both fluoride and Mg (Fig. 3c,d). As noted, all other measurements were done on the 300-425 pm fraction to minimize variability. From the preliminary work, it became apparent that coretops showing extensive signs of dissolution (e.g., EN66

339

Geochemistry of fluorine in foram shells a.

b.

Sierra LeolIe Kisc

Sierra Leone

Rise

6

2.8

I

200

400 Shell

600

200

8GU

600

.

’

.

800

Shell size (urn)

size (pm)

d. &tong-Java Plateau

c. Ontong-Java Plateau 2.8 -

z

400

6-

2.6 -

7

5-

uE 3 3E &E E

2.4 -

2.2 *

2.OI 200

400

600

800

Sheli size (urn) FIG. 3. Variations in the F/Ca and Mg/Ca ratios with increasing shell-size in G. sacculilprfrom the Sierra Leone Rise, east equatorial Atlantic (GGC38: closed squares; and GGC 16:open squares), and the Ontong-Java Plateau, west equatorial Pacific (567 FFC26).

GGC26,4745 m), yielded F/Ca that were substantially lower than values obtained in shallower core-tops from the same n&on (EN46 GGC38, 293 1 m). These latter observations suggested that dissolution on the seafloor might alter the fluoride content of planktonic shells during early stages of burial. In fact, on the coarsest global scale, sea-surface salinity is correlated with carbonate saturation (e.g., surface salinity and bottom water carbonate ion concentration increase in the same order: Pacific Ocean < Atlantic Ocean < M~te~n~n Sea < Red Sea). Hence, there is a chance that weak correlations between foraminiferal fluoride and sea-surface salinity could be due to postdepositional decreases of foraminiferal fluoride as a result of dissolution on the seafloor. Indeed, CARPENTER (1969) suggested that the F content of bulk assemblage of fomminifera seemed to depend on the state of preservation of the shells; lowest values were obtained in shells with the poorest preservation. His findings probably reflected changes in the bulk assemblage species composition, since high-F species such as G. ruber and G. sacculifer are dissolved preferentially to dissolution-resistant species such as low-F G. tumida. However, in this work we see di~lution~o~elated changes within a single species, and hence the effect of dissolution on foraminiferal fluoride must be investigated in more detail. Dissolution

Effects

The effects of dissolution were studied in a suite of cores from the southern slope of the Sierra Leone Rise in the eastern

basin of the equatorial Atlantic Ocean. The seven cores which were chosen (Table 3) have Holocene tops and all the studied species have abundance maxima at the core-tops. Stratigraphic and sedimentological data for these cores are published elsewhere (e.g., CURRY and LOHMANN, 1986). The cores are located between the latitudes 2 to 6’N and along 2O”W longitude and cover a depth range from 293 1 to 5 104 m. Mean annual sea-surface salinity along this transect is 35.3% with annual water temperature of 26’C; there are only minor variations both in mean surface temperature and salinity between the different core sites ( LEVITUS, 1982). Insitu water profiles exhibit constant salinities down to 250 m, whereas the thermocline shows strong gradient with temperature steeply decreasing from 26°C at the base of the mixed layer (about 40 m in this region) to about 12*C at 250 m. Thus, one would expect that if either salinity or temperature are the primary controls on the fluoride content of planktonic foraminifera, shells in all seven core-tops should exhibit the same F/Ca. Planktonic sedimentary foraminifera show substantial decreases in the fluoride content of these species with increasing water depth of the core (Fig. 4 and Table 4, which include the calcite saturation levels of the bottom water expressed as AC03; BAINBRIM;E,1981; BROECKER and PENG, 1982). Fluoride in N. dutertrei and G. menardii decreases by 45 to 55% between shallow and deep samples (Fig. 4d,e), by 25% in G. saccu~~~rshells (Fig. 4b), and by about lo- 15% in G. ruber and 0. universa (Fig. 4a,c). These variations am greater than the entire range observed in the core-top calibration

340

Y. Rosenthal and E. A. Boyle

Table 3. Location and water depth of cores from the Sierra Leone Rise. Also included are the mean surface salinity and temperature

(LEVITXJS,

1982).

EN66 GGC38

EN66 GGC26

06 04 04 03

39N 14N

19N 05N

21 20 20 20

54W 38W 13W 0l.W

shells and do not necessarily signify progressive changes in the fluoride content of individual shells or individual crystalhtes within a shell. Because the ovedying surface water properties are essentially the same for all of these cores, these F/Ca changes do not reflect variations in surface salinity or temperature. Al-

(Fig. 2). Moreover, F/Ca in all five species from the two deepest cores in the Sierra Leone Rise (GGC26 and GGC29 ) are su~~tialiy lower than values observed in any other site used for core-top fluoride-salinity calibration (Table 2, Fig, 2). It should be emphasized that these values represent variations in the mean F/Ca of an assemblage of single-species F/Cm mmollmol

F&I

2.0

25

00 a a g u s

0

00 00

4om-

E

-

DO

W

0

Grubcr

P.

0

0

00

0.

0 00

00 00

UJ

sooo-

,f

1

W

00

II---00

00

00

3ooo-

FICa

mmollmol

mmoltmol

-

G.sacculifcr

0

c. 0. univtrsa

AC03 F/CO

F/CC3

mmollmol

mmollmol 0.5

1.0

0.5

000 0

a3 00

00

0

CD

OQI

z OW

f. N. du&trlrti 6000

0

000

w

oal

-so00

1.0

003

3ooo 8 ;It a. a4ooo s

pm&kg

w , . G, mtnardii

i2---

FIG. 4.(a-e) Depth profiles of F/Ca in planktonic foraminifera from core-tops in the Sierra Leone Rise and (f) Changes in the water saturation for calcite with depth, expressed as AC03 (GEOSECS station 4 12, BAINBRIDGE, 198I )_ Note that the sharp decrease in fluoride for atI species starts at 4 km, about 700 m shallower than the estimated depth of the in situ hydrographic lysocline.

341

Geochemistry of fluorine in foram shells

Table 4. F/Ca and MgKa in planktonicforaminiferafrom core-topsin the Sierra Leone Rise. Two or three replicate samples (N) were analyzed in each core-top. Shells of G. ruber were absentfrom the deepest core due to dissolution. All ratios are given In units of mmol/mol. Also included are the bottom water saturation values represented as AC03 (GEOSECS, BAINBRIDGE, 1981) as defined by BROECKER and PENG(1982).

cafe

depth AC03 mol/l

EN66

EN66 GGCl

EN66 GGCl

EN66

EN66

EN66

EN66

35.0

GGC3

3152 3521

GGC2

GGC2

20.4

13.3

GGC2

GGC3

29.5

4270

4745

5104

12.0

2.1

-2.9

N

G. ruber

G.sacculifeer

O.universa

N.dutertrel

G.menardii F/Ca

F/Ca

F/Ca

Mg/ca

4.28

1.91

0.53

2.28

4.13

1.74

0.70

0.72 0.81 0.92

0.79

2.97

0.72

0.75

2.51

F/Ca

Mg/Ca

F/Ca

1 2 3

2.64 2.51 2.64

4.15

2.47

4.10

2.31

1 2

2.65 2.52

4.33 4.48

2.34

2.20

2.28

1.98

1 2

2.66 2.59

4.02 4.22

2.32

4.20

2.11

0.59

2.28

0.67

2.31

3.84

2.00

0.12

1.84

0.60

1 2

2.55 2.39

4.09 4.06

2.19

4.38

1.80

0.73

2.63

0.81

2.21

3.85

2.04

0.69

2.41

0.61

1 2 3

2.59 2.43

4.85 4.08

2.12

4.19

2.03

0.41

2.60

0.71

2.06

4.12

1.65

0.59

2.82

0.54

1 2 3

2.32 2.30

1 2 -3

absent

-

WCs

0.64

2.28

3.50 4.04

0.56

0.61

1.94

3.97

1.78

0.41

0.63

1.97

3.85

1.55

0.56

0.65 0.63

0.53 1.83

3.55

'1.4a

0.32

1.25

0.35

1.73

3.41

1.65

0.35

1.28

0.36

though it is possible that the meridional gradients in surface productivity may cause such variations, we suggest that these decreases are due to modification of the fluoride content of planktonic foraminifera on the seafloor during early stages of burial. Such changes could arise either from the preferential removal of F from calcite during carbonate dissolution, or by selective dissolution of individual shells (or parts of a shell) containing the highest fluoride concentrations. The former process would imply that a significant fraction of the fluoride is in interstitial rather than in lattice sites (and hence, preferentially leachable with respect to Ca; WALLS et al., 1977; RAGLAND etal,1979; ROSENTHAL and KATZ,~ 989).Inthe latter process, dissolution selectively removes a fraction of an assemblage of foraminifera shells leaving behind the more resistant residues. If the composition of the dissolution-susceptible fraction is different from the more resistant one, such a process would lead to a change in the mean chemical composition of this assemblage. This mechanism works both at the inter- and in&a-specific levels ( BERGER, 197 1). Selective dissolution in shells of single species was suggested in order to explain differences in the isotopic composition of planktonic foraminifera between relatively shallow (above the lysocline) and deep cores (e.g., SAVIN and DOUGLAS, 1973; BERGER and GARDNER, 1975; BERGER and KILLINGLEY 1977; BONNEAU et al., 1980). The current data supports the selective dissolution hypothesis. The decrease in F/Ca in all the species starts at about 4 km, approximately 700 m above the estimated depth of the hydrographic lysocline (Fig. 4f, BAINBRIDGE, 198 1). The sill depth of the Romanche and Vema Fracture Zones are both at 4 km, and these channels are the principal conduits

0.30

0.35

of deep water from the western to the eastern Atlantic basins via the Mid-Atlantic Ridge (WARREN, 198 1). Consequently, there are differences in the water properties and chemistry between the two water masses which fill the eastern basin above and below the sill depth (CURRY and LOHMANN, 1986). Core-tops show a slight increase in shell fragmentation below 4 km (CURRY and L~HMANN, 1986),similar to the dissolution trend indicated by the fluoride data. Analysis of Mg in three species, G. ruber, G. sacculifer, and N. dutertrei, show similar trends, although the Mg decreases are smaller than those obtained for fluoride (Fig. 5; Table 4). These results agree with earlier Mg data, both in core-top data ( SAVIN and DOUGLAS, 1973;Lo~~~setal.,1977)andinlaboratory studies of selective dissolution ( HECHT et al., 1975). The concomitant increase in shell fragmentation supports this conclusion. Additional evidence for this mechanism comes from down-core studies (in preparation), which show cyclic variations in the fluoride and Mg content of foraminifera, arguing against a unidirectional leaching of F From interstitial, non-lattice sites and in favor of a process linked to the wellknown carbonate dissolution cycles. These lines of evidence suggest that fluoride is primarily a lattice-bound element, although we are unable to prove at this stage whether the ICHIKUNI ( 1979) ion-exchange model is the primary mechanism for fluoride incorporation in biogenie shells. Similarly, we cannot rule out the possibility that F incorporation is coupled with other metals (e.g., Mg) as suggested previously for high-Mg calcites (KITANO and OKUMURA, 1973;O~~~ and KITANO, 1980).Asfarasfluoxide coprecipitation is concerned, it appears from these results that foraminiferal fluoride varies between shells (and

342

Y. Rosenthal and E. A. Boyle MglCe

2

3

MgKa mmollmcrl

Mg/Ca

mmol/mol

mmollmol

4

2

3

4 I

2 1 .

.

m

0

4

00

DO

0

cl

0

0

,

0

00 011

m

0

m

G. ruher

9.

I

0

m

0

4

q

m

0

3 I ,

1

b. G.sacculifer

0

N. durerrrei

FIG. 5. Depth profiles of Mg/Ca ratios in planktonic foraminifera from core-tops in the Sierra Leone Rise (see Tables 3 and 4 for the data). possibly between different portions of individual shells), and it is evident that higher F levels are related to the dissolution susceptibility. This heterogeneity must be a response to changes in environmental or biological conditions (e.g., temperature, salinity, metabolic, or calcification rates). In both cases, the changes in these conditions (environmental or biological) during the life history of foraminifera lead to concomitant changes in the uptake of fluoride in the calcifying shells. As mentioned earlier, salinity is essentially constant in the upper water column of the Sierra Leone Rise region, and therefore salinity variations cannot influence variations in fluoride coprecipitation in planktonic foraminiferal calcite in this region. In contrast, the sharp local thermocline allows for the possibility that temperature or temperature-related factors (e.g., metabolic rate) may play an important role in determining the fluoride content of planktonic foraminifera. Interestingly, those species that live over the widest depth range, and therefore experience the largest temperature change (N. dutertrei and G. menardii), also exhibit the largest decrease in fluoride with increasing water depth, whereas the

shallowest species (G. ruber) shows the smallest decrease in F/Ca (Fig. 4). Temperature Effects I: Inter-Specific Variability Figure 6 presents mean values of 6 I80 as well as F/Ca and Mg/Ca ratios in different planktonic foraminifera from the shallowest core in the Sierra Leone Rise (EN66 GGC38-293 1 m). The foraminifera species are ordered by increasing calcification depth, as inferred from plankton-tows and 6 “0 data (FAIRBANKSet al., 1982; HEMLEBENet al., 1989). The in situ thermocline is also depicted (Fig. 6). In all the analyses we used only the 300-425 pm size fraction, in an effort to minimize variability in the calcification depth. Examination of these plots indicates that the inter-specific variations in F/ Ca ratios are generally in accord with the changes in 6 “0; deeper calcifying species show heavier 6 I80 values and lower fluoride. The enrichment in 6 I80 reflects the decrease in water temperature with increasing calcification depth ( BERGER et al., 1978; FAIRBANKSet al., 1979, 1982). However, F/Ca exhibits anomalously high values for the two deepest calcifying species, G. truncatulinoides and G. crassaformis. These

FlCa 6 3 G. ruher

’ xo (do0 rwn, 2 I 0 -I -2 I

MglCa mmollmol

mmollmol

0

I

1

.

2 I

.

3 ,

0

2

4

6

R

10

0

G. sacculifer 0.

universa

G.

aequilaleralis

100

I’. obliquiloqulala G. menardii

200

N. duferrwi (; rumtda G. rruncurulinoides G crassaformis

/ _ ,_ r Temperature “C 20

/”

FIG. 6. Chemical (F/C& Mg/Ca mmol/ mol ratios) and isotopic ( 6 I80 in PDB) composition of planktonic foraminifera in EN66 GGC38 (293 1 m water depth), from the Sierra Leone Rise. The species are ordered by increasing calcification depth. Also included is the in situ thermochne (solid line represents mean annual temperature; dashed lines represent seasonal temperatures).

30

343

Geochemistry of fluorine in foram shells two species add a calcitic crust in cold, deep waters (HEMLEBEN etal., 1989; LOHMANN and SCHWEITZER, 199O),as reflected by the oxygen isotopic composition of these samples, which is about 2%0 heavier than the other species. For these two species we analyzed only encrusted specimens. Mg/Ca show a similar trend of decreasing ratios with increasing calcification depth. This observation agrees with earlier studies (SAWN and DOUGLAS, 1973; BENDER et al., 1975 ) . One species (0. u~jve~~u), however, deviates from this general trend showing anomalously high Mg values (Fig. 6). The latter observations were confirmed by four additional core-tops from the Atlantic and Pacific oceans (Table 5 ) . F/ Ca and Mg/Ca are compared to the oxygen isotopic composition of different species from nearby cores where isotopic data was available (Fig. 7 ) . Measured S “0 was corrected for differences in surface salinity between the various sites using the global ocean average 0.5% change in 6 “0 for Ik change of salinity (CRAIG and GORDON, 1965; BROECKER, 1986). There is a good correlation between F/Ca and 6 “0, suggesting that temperature might have a substantial influence on the fluoride content of foraminifera, at least between species (inter-specific level). Similarly, Mg shows a relatively good, albeit less smooth, correlation with b’*O. Deviating from these correlations, as shown earlier, are the deepest dwelling species, G. truncatulinoides and G. crassaformis, both for fluoride and Mg. These anomalies suggest that other processes are important, most probably biologically mediated. From field and laboratory studies it has been proposed that G. t~n~atulinoides lives and grows over a wide depth range in the Sargasso Sea, from the sea-surface and down to about 1000 m ( HEMLEBENet al., 1985). It appears to build its calcitic shell throughout the entire depth range by adding chambers while sinking through the thermocline, and by adding a massive calcitic crust upon encounte~ng cold, deep water at the base of the main thermocline. A similar picture was revealed from the studies of LOHMANNand SCHWEITZER ( 1990) in other oceanographic regimes. G. crassaformis is believed to have a similar life history (HEMLEBEN et al., 1989). MELIERES (1977) showed that G. t~ncatulinoides crust has distinctly different c~s~ll~aphy (large crystals) than the chambers (small crystals), and suggested that the

calcification rate of the crust is substantially slower than the chambers. Chemical analysis demonstrates compositional differences between the crust and the chambers (DUCKWORTH, 1977), Recently, chemical and isotopic studies confirmed this disparity between the crust and the rest of the shell (G. P. Lohmann and IL C. Lohmann, unpubl. data). It appears, therefore, that biological processes such as calcification rate might exert a control on the chemical composition of the shell. The extent of this effect varies in response to changes in metaboiic rates, which might be related to the ambient temperature ( HEMLEBENet ai., 1989). Despite the anomalies (i.e., excluding G. truncatulinoides and G. crassafirmis), the correlation between the fluoride content of the planktonic species and their oxygen isotopic composition is impressive enough to raise hopes for utilization of fomminifem1 fluoride as paleothe~ometer, as suggested recently by OPDYKE et al. ( 199 1) . However, in order to assess the applicability of fluoride as a proxy of seawater temperature, one must demonstrate a response of the fluoride content in shells of a single species to changes in the ambient temperature. Temperature

Effects II: f&a-Specific

Variability

Although the above observations support the concept of temperature effect on the fluoride content of foraminifera, the results from single-species core-top calibrations are far less encoura~ng (Table 2, Fii. 8). ~monstmtion of a response of the fluoride content of a single species to temperature is essential to validating the applicability of fluoride as a tracer of seawater temperature. Plots of F/Ca against mean annual sea-surface temperatures show poor correlations for all five species studied here. In fact, for G. saccul@r we obtain an inverse correlation (Fig. 8b). Plotting F/Ca against seasonal rather than annual temperatures did not improve the correlations. The correlation between F/Ca in different planktonic species (i.e., inter-specific variations) and their 6 ‘*O composition indicates a 2 mmol/mol change in FfCa for a 2% change in 6 “0 ( Fig. 7a). Provided that tem~mture is the principal cause for the 6180 change, this relation can be converted to

Table 5. Chemical (F/Ca, Mg./Q ratios in mmol./mo1)composition of planktonic forsminifera in core-topsfmm different oceanic rqimes. The species are ordered by increasing calcifiiation depth. The 8180 data (in o/00 PDB) was obtained from nearby cores ( SHACKELTON,1977;BERGER et al., 1978; DURAlj?, 1981) and this in study (for core EN66 GGC38). Sierra

Leone

Ontong-Java

Rise

Plateau

EN66.GGC38

-1.50 -1.55 O.universa -0.57 G. a0quilaterali.s -0.60 P.obliquiloqulata -0.27 -0.70 G.menardii N.dutertrei -0.30 G. sacculifer

0.27 G. truncatulinoides

1.84 2.04

ERDC92

S67FFC26

6180

F/Ca

W/Q

2.60 2.37 1.91 1.63 1.29 0.79

4.13

-2.15

2.69

5.67

4.20 7.40 3.40 2.48 2.71 2.59 1.69 2.00 1.82

2.39 1.91 1.56 1.13 0.95 0.99 0.64

4.48

0.68 0.47 1.31 1.12

-2.10 -1.90 -1.60 -1.25 -0.55 -1.00 0.70

4.50 3.80 3.59 3.28 2.60

Caribbean sea

Equatorial Atlantic V25-60

v25-59

6180

P/C%

bsd&

-1.54 -1.09 -1.34

2.64 2.56 2.05

4.29 3.91 6.72

1.59

1.61

North Atlantic

V26-164

KNR64.PCS

r30-49

&SO

P/CZ+ ng/ca

8180

P/C~

-1.56 -0.99

2.52 2.31 1.63

-2.01 -1.74 -1.16

2.66 2.35

7.60 5.05

1.80

V30-51k

1.90

-0.11

0.96 0.88 0.60 1.36 1.40

-0.54

2.31 2.37 2.71

1.08 0.79

2.85

1.21

3.20

8.19

-0.15

1.19

1.45 0.68 0.79 0.62 1.39 1.39

-&?a

4.16 7.55 2.04 2.07

2.08 3.56 3.59

344

Y. Rosenthal and E. A. Boyle

SierraLeoneRise Omng-JavaPi&au E!q.Atlantic Caribbean Sea N-E Allantic

3

2 Corrected

1

0 6

-I

-2

-3

’ * 0 k,/OO PDB)

lo ] b.

0'

3

2

1

0

Corrected 6 “0

-1

-2

-3

(o/00 PDB)

FIG. 7. Correlation between the F/Ca (a) and Mg/Ca (b) ratios and 6 “0 of planktonic foraminifera from core-tops in different oceanic regimes (data showed in Table 5). 6”O data was corrected

for differences in the water salinity between the various sites. The points encircled by the solid line represent samples of G. truncutulinoides and G. crassaformis.

a 0.25 mmol/mol change in F/Ca per 1“C change in temperature. It follows that if temperature is the primary control on the fluoride content of foraminiferal calcite (and if biological processes play no role) then this dependence predicts a 2.5 mmol/mol change for the entire temperature range studied in this work ( x 10°C). This apparent temperature dependence is represented by the dashed lines overlain on the intra-specific F-T plots (Fig. 8). The large discrepancy between the predicted slope and the measurements is obvious. One could argue that planktonic foraminifera have “special considerations” (e.g., seasonality, inter-annual variability, etc.) which mask a true temperature response, and therefore that the lack of correlation does not discredit entirely temperature control of fluoride uptake. However, the studies of CURRY and MATTHEWS ( 198 1) indicate unequivocally that the oxygen isotopic composition of planktonic foraminifera closely traces changes in sea-surface temperature; it would be surprising if those “special considerations” did not also mask the response of 6 “0 at the same time. Evidently, the absence of a significant response of the fluoride content within a single species to the ambient water temperature casts doubt on the utility of fluoride as paleothermometer. CONCLUSIONS We have investigated the effects of salinity and temperature on the incorporation of fluoride into calcitic shells of planktonic foraminifera, as well as the subsequent modification of the shell chemistry by dissolution on the seafloor. The data

suggests that the F/Ca is species dependent and that the F content in planktonic foraminifera follows depth habitat. There is a good correlation between the fluoride content of most planktonic foraminifera and their corresponding oxygen isotopic composition, where deeper-calcifying species have lower fluoride content and heavier 6 “0. This result might be taken as an indication that temperature is the principal control on the fluoride content of foraminifera; however, specific deviations from this general trend (e.g., G. truncutulinoides and G. crassaformis), and the fact that there is no correlation between fluoride and sea-surface temperature within a single species, lead us to believe that biological processes dominate the coprecipitation of fluoride in foraminifera1 calcite. Such processes (e.g., calcification rate) are likely to be temperature dependent to some extent. The similarities between the behavior of F and Mg in foraminiferal shells provide support for this hypothesis. Indeed, earlier studies suggested that although the Mg content of foraminifera shows a weak correlation with the ambient water temperature, it is dominated by biological processes (e.g., SAVINand DOUGLAS, 1973; BENDER et al., 1975). It is even less convincing to assert that foraminiferal fluoride responds to changes in sea water salinity; core-top planktonic foraminifera show weak or no correlation with sea-surface salinity. A study of dissolution effects on the chemical composition of planktonic foraminifera suggests that even the weak correlations that were obtained (e.g., G. sacculifer) may be spurious, driven by a dissolution artifact. Core-top planktonic foraminifera from the eastern equatorial Atlantic (Sierra Leone Rise) show a substantial decrease of the fluoride content with increasing water depth (29315 104 m) A similar trend, although less significant, is seen for Mg. These depth trends are attributed to selective removal of the more dissolution-susceptible fraction with a different chemical composition from the residual shells. A similar process was suggested previously to account for variations in the oxygen isotopic composition of planktonic foraminifera. It appears, therefore, that fluoride is not distributed homogeneously in a population of shells; this heterogeneity may occur either within individual shells or between shells, thus reflecting variations in the partition coefficient during shell ontogeny. The decrease in fluoride content with increasing shell size (in G. sacculijh shells from the Sierra Leone Rise) provides an example for such ontogenic changes. The data here support the notion that fluoride occupies lattice sites in the calcite shells, although the nature ofthis substitution and its fundamental controls are not yet understood. In summary, the data presented here show that foraminiferal fluoride does not reflect directly either temperature or salinity of seawater and therefore could not be used as a tracer of these properties. It is possible that fluoride can be used for the study of dissolution in deep-sea sediments, both in modern and ancient oceans. The marked decrease in planktonic foraminiferal fluoride with increasing core-depth suggests that it may be possible to infer changes in the depth of the lysocline from foraminiferal fluoride. It would be useful also to examine the changes in the F content of benthic foraminifera in shallow sediments where sharp temperature gradients exist. This should provide a better control on the calcification depth and might help in understanding the correlation between foraminiferal fluoride and depth habitat.

345

Geochemistry of fluorine in foram shells b. Gsacculifrr (w/o sac)

a. C. ruber (while) I

3.0 m

Z

Gl

2.5 -

q

,/ I

.

II

I/

q

,I

2.0 -

80

I

2.5 -

,I

wE v> i;;o ;

I

. m e

a

0i

q

I

3.0 -

,I I/

1’ I/

,I

.

1.5 18

I

‘a

m

,I

2.0 -

I/

I

.

I

.

I

.

20 22 24 26 28 Mean annual SST (“C)

.

1.5 I8

30

1 .:. 20

* 22

24

I. 26

1. 28

30

Mean annual SST (“C)

-I 18

20

22

24

26

28

30

Mean annual SST (“C) e. G. menardii

d. N. durertrei

i 18

20

22

24

26

28

Mean annual SST (“C)

30

18

20

22

24

26

28

30

Mean annual SST (“C)

FIG. 8. Calibration of core-top F/Ca in planktonic foraminifera against sea-surface temperature. Each point represents a single analysis. Core data is given in Table 1 whereas mean values and standard deviations are given in Table 2. The analytical precision of these measurements is rt2.2% (i.e., about twice the size of a symbol). Note the difference in F/ Ca range between species. The dashed lines represent an apparent temperature line, as predicted from the inter-specific variations in F/Ca (see text). Acknowledgments-We thank Michael Bender for suggesting the spectrophotometric fluoride technique, and Bill Curry and Pat Lohmann for discussions about and samples from Sierra Leone Rise cores. Core curation at the Woods Hole Oceanographic Institution, Lamont-Doherty Geological Observatory, and Hawaii Institute of Geology is supported by the National Science Foundation. Y.R. thanks Delia Oppo, Rob Sherrell, and Debra Colodner for discussions and comments on this paper. Critical reviews of this paper were appreciated. This research was supported by NSF grant OCE 7483 1. Editorial handling: F. J. Miller0 Jr. REFERENCES BAINBRIDGEA. E. ( 1981) GEOSECS Atlantic Expedition, Vol. 1, Hydrographic Data. National Science Foundation. BENDERM. L., LORENSR. B., and WILLIAMSF. D. ( 1975) Sodium, magnesium and strontium in the tests of planktonic foraminifera. Micropaleontology 21, 448-459.

BERGER W. H. ( 1971) Sedimentation of planktonic foraminifera. Mar. Geol. 11,325-358. BERGERW. H. and GARDNER J. V. (1975) On the determination of Pleistocene temperatures from planktonic foraminifera. J. Foram. Res. 5, 102-l 13. BERGERW. H. and KILLINGLEYJ. S. ( 1977) Glacial-Holocene transition in deep-sea carbonates: Selective dissolution and the stable isotope signal. Science 269, 563-566. BERGER W. H., KILLINGLEYJ. S., and VINCENTE. (1978) Stable isotopes in deep-sea carbonates: Box core ERDC-92, West Equatorial Pacific. Ocean. Acta 1, 203-216. E~ONNEAU M. C., VERGNALJD-GRAZZINI C., and BERGER W. H. ( 1980) Stable isotope fractionation and differential dissolution in recent planktonic foraminifera from Pacific box-cores. Ocean. Acta 3, 377-382. BOYLE E. A. ( 198 1) Cadmium, zinc, copper, and barium in foraminifera tests. Earth Planet. Sci. Lett. 53, 1 l-35. BOYLEE. A. ( 1984) Sampling statistic limitations on benthic foraminifera chemical and isotopic data. Mar. Geol. 58, 2 13-224.

346

Y. Rosenthal and E. A. Boyle

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N. J. (1988) Changes in the distribution of 613C of deep water ZCOz between the last glaciation and the Holocene. Puleoceunography 3,3 17-342.

DUCKWORTHD. L. ( 1977) Magnesium concentration in the tests of the planktonic foraminifer Globorotalia truncatulinoides. .I. Foram. Res. 4, 304-3 12. DUPLESSYJ.-C. ( 1982) Glacial to interglacial contrasts in the northern Indian Ocean. Nature 295,494-498. DUPLESSYJ.-C., LABEYRIEL., JUILLET-LECLERCA., MAITRE F.. DUPRAT J., and SARNTHEINM. ( 199 1) Surface salinity reconstruction of the North Atlantic Ocean during the last glacial maximum. Ocean. Acta 14, 3 1 l-324. DURAZZI J. T. ( 198 1) Stable-isotope studies of planktonic foraminifera in north Atlantic core tops. Palaeogeogr. Palaeoclimatol. Palaeoecol. 33, 157- 172.

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KITANO Y. and OKUMURA M. (1973) Coprecipitation of fluoride with calcium carbonate. Geochem. J. 7, 37-49. LEVITUSS. ( 1982 ) Climatological Atlas of the World Ocean. NOAA Professional Paper 13. LOHMANNG. P. and SCHWEITZERP. N. ( 1990) Globorotalia truncatulinoides’ growth and chemistry as probes of the past thermocline: I. Shell size. Paleoceanography 5, 55-75. LORENSR. B., WILLIAMSD. F., and BENDERM. L. ( 1977) The early nonstructural chemical diagenesis of foraminiferal calcite. J. Sediment. Petrol. 47, 1602-1609.

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