18. A STUDY OF SEDIMENTATION AT DSDP HOLE 379A, BLACK SEA,BASED ON THE ISOTOPIC COMPOSITION OF STRONTIUM1

C D . Nardone and G. Faure, Department of Geology and Mineralogy,The Ohio State University, Columbus, Ohio

ABSTRACT

The β7Sr/β6Sr ratios and Sr concentrations of detritalnoncarbonate fractions of sediment deposited in the central basin ofthe Black Sea are controlled to a first approximation by varyingproportions of sialic and mafic components. The sialic componentconsists of weathering products of igneous, metamorphic, and sedi-mentary rocks of Precambrian to Tertiary age in the northernwatershed. The mafic component originates from ophiolitecomplexes and associated volcanic rocks and volcanogenic sedimentof Mesozoic to Cenozoic age in Turkey and Bulgaria. Quantitativeinterpretations of the data yield mixing equations from which the Srconcentrations of the sialic and mafic components are estimatedusing reasonable assumed values of their 87Sr/86Sr ratios. For themafic component we assumed 87Sr/86Sr = 0.705 ±0.001 andobtained a Sr content of 215 ±28 ppm. The concentration of Sr in thesialic component is 50.4±?J ppm, assuming that its 87Sr/86Sr ratio is0.735 ±0.005. The data suggest that slight variations of these valuesoccurred for limited intervals of time. The concentrations of thesialic component, calculated from the two-component mixingmodel, vary systematically as a function of depth in the hole andindicate a strong climatic influence on the input of sialic detritusfrom the northern watershed, as follows: Würm glacial: 59%;Würm/Riss interglacial: 51%; Riss glacial: 67%; Mindel/Rissinterglacial (?): 37%; Mindel glacial: 84%.

The 87Sr/β6Sr ratios of the calcium carbonate fractions in Cores 1through 22 range from 0.7070 to 0.7084 and are less than the valueof this ratio in modern seawater (0.709). The low values areattributable to the presence of reworked older coccoliths ofCretaceous and Eocene to Oligocene age which have lower 87Sr/86Srratios than indigenous species of Quaternary age. The average87Sr/86Sr ratio of carbonates deposited during the Würm/Rissinterglacial is greater than that of the Würm glacial period due tothe predominance of the indigenous species Emiliania huxleyi duringperiods of increased salinity (interglacials) in the Black Sea. Theisotopic composition of Sr in the carbonate phases therefore hassome value as an indicator of salinity in the Black Sea.

INTRODUCTION

This study is based on the hypothesis that the detritalnon-carbonate fraction of sediment deposited in thecentral basin of the Black Sea consists of twocomponents having different 87Sr/86Sr ratios and Srconcentrations. The sediment derived from thenorthern watershed originated by weathering ofigneous, metamorphic, and sedimentary rocks ofgenerally sialic composition and widely varying agefrom Precambrian to Tertiary. This component consistsprimarily of feldspar, quartz, garnet, illite, kaolinite(identified by Müller and Stoffers, 1974), and variousoxides which collectively have a low Sr concentration

'Laboratory of Isotope Geology and Geochemistry ContributionNo. 41.

(less than 100 ppm), but are enriched in 87Sr due todecay of 87Rb in the rocks from which they werederived. The second component originates primarily inTurkey and to a lesser extent from Bulgaria. It is char-acterized by the presence of mafic minerals (plagio-clase, pyroxenes, amphiboles, and montmorillonite)removed from ophiolite complexes and associatedvolcanic rocks and volcanogenic sediment of lateMesozoic to Cenozoic age (Müller and Stoffers, 1974;Brinkman, 1974; Vuchev, 1974; Kamen-Kaye, 1971).This component has a high Sr concentration (greaterthan 200 ppm) and low 87Sr/β6Sr ratio consistent withthe mafic composition and young geologic age of thesource rocks (Faure and Powell, 1972). The hypothesissuggests that the Sr concentrations and 87Sr/86Sr ratiosof detrital non-carbonate sediment in the central basinof the Black Sea should be controlled to a firstapproximation by the relative proportions of sialic

607

C. D. NARDONE, G. FAURE

(northern) and mafic (southern) silicate detritus presentin a given sample.

The hypothetical sialic component was transportedto the Black Sea by large meltwater streams and theirmodern successors that drain its northern watershed,including the Danube, Dnepr, Don, Kuban, Dnester,Bug, and others. The mafic component is contributedby several Turkish rivers such as the Kizil Irmak, YesilIrmak, Coruh, and many other lesser streams. Theinput of sediment from the northern watershed duringthe Pleistocene epoch probably varied between widelimits in response to the advances and retreats of thecontinental ice sheet. The amount of sedimenttransported by the rivers in Turkey (and Bulgaria)during the same period probably remained fairlyconstant because this region remained free of ice.Consequently, we propose that the ratio of sialic(northern) to mafic (southern) non-carbonate detritusdeposited at a given site in the Black Sea during thePleistocene epoch varied in response to the waxing andwaning of the ice sheets. The abundance of the sialic(northern) component increased during periods ofglaciation and decreased during interglacials after mostof the ice in northern Europe had melted. Theseclimatically controlled changes in the mineralogicalcomposition of the sediment should therefore bereflected by corresponding variations of its Srconcentration and 87Sr/86Sr ratio: high Sr concentra-tions and low 87Sr/86Sr ratios during interglacials andlow Sr concentrations and high 87Sr/86Sr during glacialperiods.

This hypothesis is supported by 87Sr/86Sr ratios andSr concentrations of alumino-silicate detritus frompiston core 1474 (42°23'N lat; 37°37'E long) publishedby Cooper et al. (1974). Their data show that the87Sr/86Sr ratios of the non-carbonate fraction of thiscore ranged from 0.7077 to 0.7101 (average 87Sr/86Sr =0.7088) in the upper 310 cm and then increased to0.7120 at 400 cm and 0.7150 at 600 cm. The increase ofthe 87Sr/86Sr ratio is accompanied by a decrease of theSr concentration from 173 ppm above 310 cm, to 122ppm at 400 cm and to 99.2 ppm at 600 cm. Usingcarbon-14 dates of Ross and Degens (1974), one caninfer that the change occurred around 13,800 years B.P.toward the end of the Würm glaciation. Cooper et al.(1974) also analyzed sediment from core tops andobserved that the 87Sr/86Sr ratios of the alumino-silicatefractions ranged from 0.7099 and 0.7130 near theTurkish coast up to 0.7179 along the eastern, northern,and western parts of the basin. These results aretherefore consistent with the hypothesis outlined above.

The calcium carbonate fraction of sediment of latePleistocene to Holocene age in the Black Sea iscomposed predominantly of coccoliths. Bukry (1974)observed that sediment of Holocene age (Units 1 and 2)contains the indigenous species Braarudosphaerabigelowi and Emiliania huxleyi, whereas Unit 3 ischaracterized by the exclusive presence of reworkedcoccoliths of Late Cretaceous to Eocene age. Accordingto measurements by Cox and Fauré (1974), the87Sr/86Sr ratios of the carbonate fractions of Holocenesediment (Unit 1) are identical to that of Sr in water ofthe Black Sea which has an 87Sr/86Sr ratio of 0.7093±0.0007. However, the carbonates of Unit 3, deposited

during the Würm glaciation, have a significantly lower87Sr/86Sr ratio of 0.7073 ±0.0003. This value iscompatible with the isotopic composition of Sr in theoceans during Late Cretaceous to Eocene time(Peterman et al., 1970) and is consistent with theidentification of the coccoliths. Therefore, Cox andFaure (1974) concluded that the 87Sr/86Sr ratios of thecarbonate phases are indirect indicators of the salinityin the Black Sea. During glacial periods, when thewater was brackish to fresh, redeposited coccoliths ofLate Cretaceous to Eocene age, having 87Sr/86Sr ratiosbetween 0.707 and 0.708, dominated the carbonatecontent of the sediment. During interglacials, the87Sr/86Sr ratios of the carbonates increased to 0.709 asindigenous Coccolith species re-established themselvesin more saline water resulting from the influx of waterfrom the Mediterranean Sea.

One of the objectives of the present study isconcerned with interpretations of 87Sr/86Sr ratios andSr concentrations of the detrital non-carbonatefractions of sediment from Hole 379A in terms of two-component mixing models in order to evaluate theinfluence of climatic variations on sedimentprovenance. A second objective is to confirm thehypothesis of Cox and Faure (1974) by analysis of thecarbonate fractions of sediment from Hole 379A. Theresults of these two investigations should be comple-mentary and should be compatible with the conclusionsderived by other methods described elsewhere in thisvolume. In addition, we hope that the results of thisstudy will demonstrate the usefulness of the isotopiccomposition of Sr in the interpretation of sedimentaryrocks of marine or nonmarine origin.

ANALYTICAL PROCEDURESThe sediment samples (5 to 10 g) were dispersed in

250 ml of double distilled and demineralized water,dried, weighed, and then treated with 250 ml of purified0AN HC1 in order to dissolve the carbonate fraction.The supernate was decanted and stored for laterisotopic analysis of Sr. Any remaining calciumcarbonate and most of the iron sulfides weresubsequently removed by additional leaching withpurified IN HC1. The carbonate (plus iron sulfide)contents of the samples were calculated from theobserved weight loss and were expressed as weightpercent relative to the washed and dried sediment.

Concentrations of Sr of detrital non-carbonatefractions were measured in duplicate by means of X-rayfluorescence using a Mo-target X-ray tube and a LiF(220) analyzer crystal (Diano Corp., Model XRD-6). Acalibration curve was established using the rockstandards of the U.S. Geological Survey (G-2, GSP-1,BCR-1, AGV-1, W-l) described by Flanagan (1973). Acorrection for mass absorption was made by means ofthe MoK-alpha Compton-scattered peak (Reynolds,1963). The standard deviation of the Sr determinationsis ± 1.7 ppm, based on duplicate analyses of 66 samples.A suite of 10 samples from Cores 36 to 68 was analyzed,both before and after removal of the acid-solublefraction, which enabled us to estimate the Sr contentsof their carbonate fractions.

Aliquots of about 0.5 g of acid-leached sedimentwere dissolved in a mixture of HF + H2SO4 and Sr was

608

STUDY OF SEDIMENTATION BASED ON ISOTOPIC COMPOSITION OF STRONTIUM

extracted from the resulting solutions by cationexchange chromatography. The isotopic compositionof Sr was measured on a solid-source mass spec-trometer (Nuclide Corp., Model 6-60-S). The measured87Sr/86Sr ratios were corrected for isotope fractiona-tion to 86Sr/88Sr = 0.1194. Four samples were analyzedin duplicate from which we calculate a standarddeviation of ±0.00036 for the fractionation-corrected87Sr/86Sr ratios. The Eimer and Amend SrCCh standardwas analyzed six times in the course of this study andgave an average 87Sr/86Sr ratio of 0.70806 ±0.0002 (lσ).This value is in excellent agreement' with resultsobtained by other investigators.

NON-CARBONATE FRACTIONSThe sediment from Hole 379A has been subdivided

into nine units on the basis of megascopic composi-tional criteria. Detailed descriptions of each unit aregiven in the Site Report for 379A and will not berepeated here. Table 1 contains a brief summary of thelithologic character of each unit and indicates theirtentative correlations with the glacial chronology ofnorthern Europe.

The interpretation of the 87Sr/86Sr ratios and Srconcentrations of the detrital non-carbonate fractions(Table 2) is based on the hypothesis presented earlierthat the sediment contains two components derivedfrom the northern and southern watersheds of theBlack Sea. The Sr concentration ( Srm) of a two-component mixture is given by:

TABLE 2Analytical Results for Non-Carbonate Fractions of

Sediment From DSDP Hole 379A, Black Sea

= Sr/+Srv(l-/)(1)

where Srs and Sry are the concentrations of thiselement in the sialic (northern) and mafic (southern)components, respectively, and where/is the fraction ofthe sialic component in the mixture by weight. The87Sr/86Sr ratio of such mixtures is:

ü -£.86Sr Sr

(2)m

where a and b are constants whose values aredetermined by the Sr concentrations and 87Sr/86Srratios of the end members (Boger and Faure, 1974,1976; Shaffer and Faure, 1976). Equation 2 is themixing equation which is a straight line in coordinatesof 87Sr/86Sr and 1/Sr and is a hyperbola when 87Sr/86Srratios are plotted versus the Sr concentrations.

TABLE 1Lithologic Units and Tentative Glacial Chronology of Sediment

From DSDP Hole 379A, Black Sea

LithologyThickness

(m) Cores Tentative Age

1 Nanno ooze 0.302 Sapropel 0.403 Terrigenous mud 654 Diatomaceous nanno mud 355 Sapropel 0.306 Terrigenous mud 1307 Nanno diatomaceous marl 458 Terrigenous mud 1809 Calcite-rich mud 170

1?1?

1-78-1111

12-2425-2930-49

HoloceneHoloceneWurm G.Riss/Wúrm I.G.Riss/Würm I.G.Riss G.Mindel/Riss I.G.Mindel G.

Sample(Interval in cm)

Depth(m) Unit

Carbonate(%)

Non-Carbonate

87Sr

Sr ppm 86Sr

1-1.1-4,4-3,4-4,

70201050

4-5, 666-1,426-2, 426-4,46-5,6-5,6-6,7-2,7-4,7-5,8-1,

321263811612614

8-5, 269-2, 209-3, 629-4,39-5, 13610-1, 11010-3, 1510-4, 7010-5, 2310-6, 4511-1, 11011-2,7111-3,3011-4,5012-1,5312-2, 2412-3,14013-1,2513-3, 3013-4, 2413-5,2113-6,514-1,20144, 1614-6, 3015-2, 1415-4, 1315-5, 716-1,4016-2,16-3,

1532

16-4,5618-2,4819-1, 13019-1, 14019-2, 17

10184271311

50-68 -700,000 yr

Note: G = glacial. I.G. = interglacial. The tentative chronology is that of Stofferset al., this volume.

19-5,20-3,21-2,21-6,22-6,36-4, 7338-4, 7638-6, 1539-5, 2543-5,92454,11151-2,2654-2,1758-4, 89604, 6468-5, 100

0.704.70

30.1032.0033.6645.4246.9249.5451.3252.2652.8855.5158.6661.2663.1469.2673.7075.6276.5379.3682.1084.1586.2087.2389.9591.1092.2193.3095.0099.53

100.74103.40108.25111.30112.74114.21115.55117.20121.66124.80127.64130.63132.07135.40136.65138.32140.06154.98163.30163.40163.67169.01174.84181.77187.63196.61320.23338.26340.65348.25384.92401.61451.76478.67518.39536.14610.00

3?3333333333333444444444444444666666666666666666666666666

24.225.217.123.525.621.419.420.615.920.017.223.921.819.116.619.622.315.015.220.720.317.115.317.119.427.816.213.118.818.217.626.423.920.521.518.123.820.820.323.915.519.819.913.612.413.613.020.420.011.718.524.726.025.724.926.323.614.513.911.723.325.532.933.442.637.432.8

121.0105.890.894.0

126.2130.8126.4136.4250.6147.2109.9110.297.893.4

116.8110.2100.9166.4150.2126.1168.8178.9110.9104.8180.2127.6218.8138.4118.8114.6101.0115.7125.4115.6130.9105.2122.0105.7114.4105.2213.6115.6111.187.676.067.880.495.2

156.7229.2

95.3106.5100.9113.699.1

103.987.771.064.282.568.4

199.4104.975.099.2

117.2

0.71170.71180.71420.71380.71150.71070.71350.71080.70770.71000.71530.7134

0.7152

0.7169

0.70930.71450.71410.71790.7239

0.71020.71160.71080.71540.71060.71490.7135

0.7079

0.71450.71760.7136

0.7199

0.70800.70730.7125

0.7110

0.71150.7103

0.7306

609

C. D. NARDONE, G. FAURE

These relationships tell us that the mixing hypothesiscan be tested by plotting the 87Sr/86Sr ratios of detritalnon-carbonate fractions versus the reciprocal of theirSr concentrations. The validity of the hypothesis isindicated by the goodness of fit of the data points to astraight line which can be fitted by least-squaresregression. If the fit is satisfactory, the equation canthen be used to estimate Srs and Srv by substitutingreasonable assumed values of the 87Sr/86Sr ratios. Theresults can be used in Equation 1 to calculate theconcentrations of sialic detritus (/) of any given samplein the suite from its observed Sr concentration. Theeffect of changing climatic conditions of sedimentationin the Black Sea should then be reflected by systematicvariations in the concentration of sialic detritus derivedfrom the northern watershed.

The analytical results compiled in Table 2 indicatethat the calcium carbonate (plus iron sulfide) contentsof the samples of Units 1 through 8 are fairly constant(range 11.7% to 27.8%) and average 19.2%. Unit 9 is acalcitic mud with a higher carbonate content of 35.8%.The Sr concentrations range from 64.2 to 250.6 ppmand average 120.2 ppm. Five samples from Units 3, 4,6, and 8 (6-5, 32 cm, 11-2,71 cm, 15-2, 14 cm, 19-1, 140cm, 45-4, 111 cm) have anomalously high Sr concentra-tions of 200 ppm or more. These samples consist ofsand-sized particles and represent turbiditic sands withgraded bedding that are intercalated with theterrigenous mud. Their 87Sr/86Sr ratios are lower thanthose of all other samples we have analyzed (0.7073 to0.7093) and average 0.7080. The high Sr content andlow 87Sr/86Sr ratios suggest that the turbidite layers areenriched in mafic minerals and presumably originatedfrom the continental shelf and slope of the Turkishcoast. The sandy layers were deposited by turbiditycurrents and therefore do not satisfy our sedimentationmodel.

All of the samples for which both 87Sr/86Sr ratios andSr concentrations are available have been plotted inFigure 1A, using different symbols to identify thelithologic units. Most of the samples are from Units 3and 6 representing sediment of Würm and Riss age,respectively. The Würm/Riss interglacial interval (Unit4) is represented by only four samples, one of which is aturbidite. Additional analyses for Units 4, 8, and 9 arein progress and will be reported elsewhere. One samplefrom Unit 8 (38-4-76, Mindel glaciation) has a Srcontent of 71.0 ppm and an 87Sr/86Sr ratio of 0.7306.This is the highest isotopic ratio of the samples we haveanalyzed and suggests a very high content of sialicdetritus in Unit 8.

When the samples from Units 3 and 6 are examinedseparately (Figures IB and D), the positive correlationof the 87Sr/86Sr ratios with reciprocals of the Srconcentrations is quite apparent. However, certainsamples deviate markedly from the linear patterns.These anomalous samples include the turbidites as wellas small sets of samples which are stratigraphicallyrelated and appear to form distinctive linear arrays.Nevertheless, eight samples from Unit 3 fit a mixingline whose equation is:

87

86

Sr n×n,r 1.930= 0.6966+

SrSr(3)

Eleven of the samples of Unit 6 fit a very similar linerepresented by:

87,

86(4)

Sr Sr

The similarity of the mixing equations for these twounits indicates that the 87Sr/86Sr ratios and Srconcentrations of the sialic and mafic components ofsediment that entered the Black Sea during the Würmand Riss glaciations remained essentially the same. Forthis reason we combined Equations 3 and 4 to obtain asingle mixing equation that is satisfied by most, but notall, of the samples we have analyzed:

87(

86-= 0.6958

Sr Sr(5)

This equation was plotted in Figure 1C in order todetermine to what extent the samples of Unit 4(Würm/Riss interglacial) fit the general mixingequation. We conclude only that the fit is poor, but thatit permits the assumption that the sediment in this unitconsists of mixtures of two components similar to thoseof Units 3 and 6.

Samples 1-4, 20 cm, 4-3, 10 cm, and 4-4, 50 cm, forma subset (labeled 3-1) within the upper 32 meters ofUnit 3 to which we have also assigned 7-5, 126 cm.These four samples lie on a mixing line whose equationis:

87

86

Sr n 1.826= 0.695+

SrSr(6)

In addition, the samples from Cores 19, 20, 21, and 22in Unit 6 can be distinguished in Figure ID by aseparate line (subset 6-1):

87,

86= 0.686

Sr Sr(7)

A second subset (6-2) is suggested by the samples fromCore 12 (12-1, 53 cm, 12-2, 24 cm) which form adistinctly different line whose equation is:

87

86

Sr n , „ 5.110= 0.673 + ——

SrSr(8)

These three subsets deviate from the main groupperhaps because the mineral compositions of either thesialic or the mafic component, or both, changed duringcertain intervals of time. Such changes in the mineralcomposition may have been caused by subtle changes inthe processes that transported terrigenous sediment toHole 379A. In any case, the data indicate that the Srconcentrations and 87Sr/86Sr ratios of the sedimentcomponents assumed different values for relatively

610

STUDY OF SEDIMENTATION BASED ON ISOTOPIC COMPOSITION OF STRONTIUM

87Sr

86

A: All samples

Sr

0.720

0.710

0.700 L 1 1 L

B: Unit 3 (Würm Glacial)

. < - • •

3-1

j i I

10 12 14x10" 8 10 12 14x10-3

87C: Unit 4 (Würm/Riss Interglacial)

0.710 -

0.700

D: Unit 6 (Riss Glacial)

14X10 -36 8 10 12 14 x TO'3 4 6 8 10

1/Sr, ppm' 1 1/Sr, pprrf1

Figure 1. Plots of ^Sr/^Sr ratios of detrital non-carbonate fractions of DSDP Hole 379A versus reciprocals of their Srconcentrations. The lithologic units are defined in Table 1. Colinear arrays of data points 3-1, 6-1, and 6-2 are subsets ofthese respective units which presumably reflect changes in the compositions of the sialic (northern) and mafic (southern)components. Turbidite sands are identified by the letter T.

short intervals of time. Finally we note that anothersample from Unit 6 (16-2, 15 cm) deviates significantlyfrom the "main sequence." We attribute the anomalouscomposition of this sample to the short-term hetero-geneity of the sialic component, but are unable todetermine whether it belongs to a third subset withinUnit 6.

In general, the foregoing examination of the dataindicates that most of the samples in Units 3 and 6 fit asingle mixing equation (5) and therefore can be treatedas mixtures of two components. The Sr concentrationsof the sialic and mafic components can be estimated bysubstituting their assumed 87Sr/86Sr ratios intoEquation 5. No measurements of the isotopic composi-tion of Sr in the Mesozoic volcanics of Turkey havebeen reported. However, the 87Sr/86Sr ratio of the maficcomponent should be less than 0.7073 (the lowest valuein turbidite Sample 19-1, 140 cm) and greater than0.7030, which is representative of young oceanictholeiites (Peterman and Hedge, 1971). The only datathat may be relevant here are the 87Sr/86Sr ratios ofmafic rocks from the Troodos Massif on Cyprusreported by Peterman et al. (1971). They obtainedratios ranging from 0.7040 to 0.7057 with an average of0.7049 for 10 rock samples. On this basis we select avalue of 0.705 ±0.001 for the 87Sr/86Sr ratio of themafic component and estimate its- Sr content fromEquation 5 as 215 ±?6 ppm.

The 87Sr/86Sr ratio of the sialic componentis very probably greater than 0.730 because Sample38-4, 76 cm has a ratio of 0.7306. On the other hand,it seems very unlikely to us that its 87Sr/86Sr ratiois higher than 0.740 because this value implies anaverage age of about 880 million years for the rocks inthe northern drainage basin. This date is based on aRb/Sr ratio of 1.02 reported by Cooper et al. (1974) foracid-leached core-top sediment from the Black Sea. Therocks exposed in the present watershed north of theBlack Sea are predominantly of Phanerozoic age(Müller and Stoffers, 1974). However, it is probablethat detritus of Precambrian age may have reached theBlack Sea during glacial periods. Nevertheless, weregard 0.740 as an upper limit and assume a value of0.735 ±0.005 for the 87Sr/86Sr ratio of the sialiccomponent. This value leads to an estimated Sr contentof this component equal to 50.4 ±U ppm. The low Srcontent indicates that the sialic component delivered toHole 379A consists primarily of clay minerals and otherweathering products of sialic rocks. The higher Srcontent of the mafic component may be due to thepresence of plagioclase and unweathered particles ofmafic volcanic rock. The Sr concentrations of the sialicand mafic components implied by the mixing equations(3 to 8) are listed in Table 3. The estimated Srconcentrations of the sialic component range from 45.7to 82.4 ppm, those of the mafic component from 134 to

611

C. D. NARDONE, G. FAURE

TABLE 3Strontium Concentrations of the Sialic and Mafic

Components Derived From the Mixing Equations (3 to 8)fori 8 7Sr/ 8 6Sr) v = 0.705 ±0.001,(87Sr/86Sr‰ = 0.735±0.005

TABLE 4Summary of Analytical Results and Estimated Abundancies of

the Sialic (Northern) Component for Cores From DSDP Hole 379A

Unit orSubset

36

3+63-16-16-2

Cores

1-712-21

1-211-4

19-2212

Equation

345678

Sr Concentration (ppm)Sialic (Sr$)

50.350.650.445.751.882.4

Mafic (Srp)

230204215183134160

230 ppm. The variation is not excessive and may beaccountable by slight changes in the mineral composi-tion of the sediment components delivered to the site ofdeposition.

The concentrations of the sialic component in all ofthe samples were estimated from Equation 1 bysubstituting their Sr concentrations (Table 2) and thevalues of Srs and Srv listed in Table 3. Except for thepreviously identified subsets, we assumed Sr5 == 50.4ppm and Srv =215 ppm for all samples, even thoughwe are presently unable to demonstrate that they satisfythe combined mixing Equation 5 because their 87Sr/86Srratios have not yet been measured. Samples belongingto subsets identified in Figure 1 were evaluated by useof the appropriate values of Sr5 and Srv listed in Table3. The results were averaged for each core and are listedin Table 4 together with average values for thecarbonate contents, Sr concentrations and 87Sr/86Srratios. These data were plotted versus their respectivecore numbers in Figure 2 in order to facilitate theirevaluation.

The concentration of the sialic component, indicatedby the foregoing treatment of the data, increases fromabout 57% in Core 1 to 67% in Core 7. It decreases to40% in Core 10 and then rises again to about 64% inCore 12. Thus we place the upper boundary of theWürm/Riss interglacial at the bottom of Core 7 and itslower boundary at the top of Core 12. These results arecompatible with the age assignment in Table 1. Theabundance of sialic detritus remains high from Core 12to Core 18 which represent the Riss glacial period.Stoffers et al. (this volume; Table 1) placed the lowerboundary of the Riss sediment at the base of Core 24.However, we infer low concentrations of sialic detritusin Cores 19 through 22 (subset 6-1) and prefer to placethe lower boundary of Riss age sediment at the base ofCore 18. Therefore, Mindel/Riss interglacial sedimentextends from the top of Core 19 to the base of Core 29.We have no samples from Cores 23 through 35 and areunable to define the boundary between the Min-del/Riss and the underlying Mindel sediment. Accord-ing to Stoffers et al. (this volume; Table 1), the Mindelextends through Cores 30 to 49. We have data for onlyfour cores from this interval which suggest a highaverage content of sialic detritus of about 84%. Thecalcite-rich muds which underlie the Mindel section(Cores 50 to 68, Unit 9) appear to have somewhat lowerconcentrations of the sialic component with an averageof about 70%.

Core

146789

101112131415161819202122363839435154586068

No. ofSamples

236324543533414121121111111

Carbonate(%)

24.7(2)22.1(3)19.7(2)21.6(3)18.1(2)18.3(4)17.8(5)19.9(3)20.7(3)21.6(5)21.7(3)19.9(2)13.2(4)20.4(1)21.1(3)26.0(1)25.3(2)26.3(1)23.6(1)14.2(2)11.7(1)23.3(1)32.9(1)33.3(1)42.6(1)37.4(1)32.8(1)

Non-Carbonate FractionSr

(ppm)

113.4(2)103.7(3)130.1(5)100.4(30113.5(2)135.9(4)148.7(5)128.3(3)110.4(3)119.8(5)108.4(3)113.4(2)80.0(4)95.2(1)

119.5(3)100.9(1)106.4(2)103.9(1)87.7(1)67.6(2)82.5(1)68.4(1)

104.9(1)75.0(1)99.2(1)

-117.2(1)

87Sr86Sr

0.7118(2)0.7132(3)0.7121(5)0.7143(2)

--

0.7169(1)0.7143(2)0.7209(2)0.7117(5)0.7142(2)0.7145(1)0.7170(3)

-0.7102(2)0.7110(1)0.7115(1)0.7103(1)

-0.7306(1)

-------

Sialic Comp.(%)

56.761.951.566.761.648.040.252.663.857.864.761.783.272.738.640.233.436.677.389.580.489.066.885.070.3

-59.4

Note: The numbers in parentheses indicate the number of samplesincluded in each average. The estimates of the abundance of thesialic component were obtained from Equation 1 using the data inTable 3 and the measured Sr contents.

We conclude that the interpretation of Sr concentra-tions and 87Sr/86Sr ratios of the detrital non-carbonatefractions, by means of the two-component mixinghypothesis, is valid as a first approximation andprovides useful information about the stratigraphicvariations of the abundance of the sialic componentderived from the northern watershed of the Black Sea.The effect of climatic variations during the Pleistoceneepoch can be recognized and is compatible with ageassignments based on sedimentological and min-eralogical criteria. Additional measurements of87Sr/86Sr ratios may permit us to define the mixingequations more precisely and to improve the accuracyof the estimated concentrations of the sialiccomponent.

CARBONATE FRACTIONSThe carbonate fraction of sediment in Cores 1

through 49 of Hole 379A consists primarily of Coccolithooze. Chemically precipitated carbonate mineralspredominate only in Unit 9 (Cores 50 to 68). Thecalcareous nannoplankton of Cores 1 through 12consists of occasional floods of the indigenous speciesEmiliania huxleyi (Lohman) and Gephyrocapsacaribbeanica Boudreaux and Hay, together withreworked species of Cretaceous and Eocene toOligocene age (Percival, this report). The indigenousspecies disappear in Section 5 of Core 11 and are notfound again until Core 25 in which Braarudosphaerabigelowi (Gran and Braarud) makes a brief appearance.

612

STUDY OF SEDIMENTATION BASED ON ISOTOPIC COMPOSITION OF STRONTIUM

60

Sr, ppm 87Sr /86Sr

80 100 120 0.710 0.720

Sialic Comp.

20 40 60 80%

10

12

14

16

18

20

22

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38

40

42

44

46

48

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Holocene

Würm Glacial

Würm/RissInterglacial

Riss Glacial

HolsteinianInterglacial

0m

0m

Mindel Glacial

7

Figure 2. Stratigraphic variation of average Sr concentrations, ^Sr/^Sr ratios and estimated abundancesof the sialic components in cores of DSDP Hole 379A. The sediment deposited during glacial periods isenriched in sialic detritus from the northern watershed, as expected. The placement of the boundaries of theWürm/Riss interglacial is consistent with that in Table 1. The top of Mindel/Riss interglacial sediment istentatively placed at the top of Core 19 due to the low content of sialic detritus in Cores 19 to 22 ofsubset 6-2.

613

C. D. NARDONE, G. FAURE

The isotopic composition of Sr in the indigenouscoccoliths is very probably identical to that of the waterin the Black Sea at the time of deposition. The present87Sr/86Sr ratio of the Black Sea is 0.7093 ±0.0007 (Coxand Faure, 1974) which is typical of modern oceanwater. However, the reworked older coccoliths havesignificantly lower 87Sr/86Sr ratios, depending on theirage. Measurements by Peterman et al. (1970) indicatethat the 87Sr/86Sr ratio of the oceans increased from0.70713 (Hauterivian) to 0.70730 (Albian) in EarlyCretaceous time and rose to 0.70746 during the LateCretaceous. The corresponding values of this ratio inthe oceans during the middle Eocene and Oligoceneepoch were 0.70780 and 0.70804, respectively. Since thecarbonate fractions of sediment above Unit 9 at Hole379A are mixtures of indigenous and reworked oldercoccoliths, their 87Sr/86Sr ratios should presumablyvary between 0.707 and 0.709.

The 87Sr/86Sr ratios of carbonate fractions from 15samples representing Cores 1 through 22 (Table 5)actually range from 0.7070 to 0.7084. Four samplesfrom Unit 3, deposited during the Würm glaciation,have an average 87Sr/86Sr ratio of 0.7075 ±0.00014 (1σ). The highest value occurs in Core 4 (Sample 4-4, 50cm, 87Sr/86Sr = 0.7078) which contains an abundanceof Emüiania huxleyi (Percival, this volume) as well asabundant older species. This core also contains radio-larians indicative of "marine" conditions (Schrader,personal communication). The slight increase of the87Sr/86Sr ratio in Core 4 is therefore compatible withthe paleontological data and signals a temporaryincrease in salinity of the Black Sea.

The 87Sr/86Sr ratios of carbonate fractions in Cores 8to 11 (Unit 4, Würm/Riss interglacial) are significantlyhigher than those of Unit 3 and average 0.7080±0.00014 (1 σ). These cores all contain floods ofindigenous coccoliths mixed with reworked olderspecies which are generally "rare" to "common." Thehigh 87Sr/86Sr ratio of the carbonate phase during theWürm/Riss interglacial period is consistent with theabundance of Emüiania huxleyi and suggests highersalinity in the Black Sea at this time. These resultsconfirm the hypothesis of Cox and Faure (1974) andstrengthen their suggestion that the 87Sr/86Sr ratio ofthe carbonate fraction is controlled by the mixture ofindigenous and reworked older coccoliths.

The carbonate fractions of Unit 6 (including Cores12 through 18), deposited during the Riss glaciation,generally have high 87Sr/86Sr ratios averaging 0.7079±0.00022 (1 σ). The only low value (0.7070) wasobserved in Sample 12-1, 53 cm. These cores contain noindigenous coccoliths and only occasional mollusk shellfragment in Cores 16 and 17 (Percival, this volume)which indicates fresh-water conditions in the Black Seaduring the Riss glaciation. The higher 87Sr/86Sr ratiosof the carbonate fractions in this interval may be due todominance of reworked coccoliths of Eocene toOligocene age which have higher 87Sr/86Sr ratios thanspecies of Cretaceous age.

One sample from Core 22 (22-6, 11 cm) has an87Sr/86Sr ratio of 0.7084 which is similar to that of Core18. This result taken by itself does not resolve thequestion whether the top of the Mindel/Riss-age

TABLE 58 7Sr/ 8 6Sr Ratios of Carbonate Fractions,

DSDP Hole 379A, Black Sea

Sample(Interval in cm)

1-1,704-4,506-5, 327-4, 16

Average for Würπ8-5, 269-5,136

10-4, 7011-4,50

8 7 Sr/ 8 6 Sr

0.70730.70780.70720.7076

Age

Wurm GlacialWurm GlacialWurm GlacialWurm Glacial

I Glacial: 0.7075±0.00014 (lσ)0.70780.70790.70840.7079

Wurm/Riss InterglacialWurm/Riss InterglacialWurm/Riss InterglacialWurm/Riss Interglacial

Average for Würm/Riss Interglacial: 0.7080+0.00014 (lσ)12-1,5312-2, 2413-4,2415-4, 1316-4, 5618-2,48

0.70700.70820.70840.70770.70830.7080

Riss GlacialRiss GlacialRiss GlacialRiss GlacialRiss GlacialRiss Glacial

Average for Riss Glacial: 0.7079±0.00022 (lσ)22-6, 11 0.7084 Mindel/Riss Interglacial?

sediment occurs at the top of Core 25 (Stoffers et al.,this volume) or at the top of Core 19 as suggested inFigure 2.

The Sr concentrations of calcite in Cores 36 to 45 ofUnit 8 range from 628 to 859 ppm and average 719 ±37ppm (1 σ). Those of Cores 51 to 68 (Unit 9) are verysimilar and have a mean Sr content of 658 ±20 ppm (1σ). These results are quite normal for calcite whichgenerally contains less Sr than aragonite but more thandolomite.

CONCLUSIONSThe results of this study can be summarized as

follows:1) The Sr concentrations and 87Sr/86Sr ratios of

detrital, noncarbonate sediment deposited at DSDPHole 379A are controlled to a first approximation bythe relative abundances of a sialic component derivedfrom the watershed north of the Black Sea and a moremafic component that originated in Turkey andBulgaria.

2) The estimated values of the abundance of thesialic component, derived from a two-componentmixing model, vary systematically as a function ofdepth and confirm that the input of this componentincreased during periods of glaciation and decreasedduring interglacials in northern Europe.

3) Short-term variations in the Sr contents and87Sr/86Sr ratios of the two sediment componentsdelivered to this site can be recognized (providedsamples are available) and can be accommodated bythe derivation of discrete mixing equations relevant tospecific stratigraphic intervals.

4) The 87Sr/86Sr ratios of carbonate fractions arecontrolled by the relative abundances of indigenousand older reworked coccoliths, provided thatchemically precipitated carbonate minerals are notdominant.

5) Saline conditions in the Black Sea during theWürm/Riss interglacial are reflected by high 87Sr/86Sr

614

STUDY OF SEDIMENTATION BASED ON ISOTOPIC COMPOSITION OF STRONTIUM

ratios (0.7080 ±0.00014), whereas fresh-waterconditions during the Würm glacial period areindicated by a lower average value of this ratio (0.7075±0.00014.)

6) The 87Sr/86Sr ratios of carbonate phases of Rissage sediment are high (0.7079 ±0.00022) even thoughindigenous coccoliths are absent, perhaps because ofthe dominance of reworked Tertiary coccoliths whichhave higher 87Sr/86Sr ratios than those of Cretaceousage.

ACKNOWLEDGMENTSWe thank David A. Ross of the Woods Hole Oceano-

graphic Institution for the opportunity to participate in thestudy of core samples recovered during Leg 42B of the DeepSea Drilling Project. The samples were collected byE.T.Degens and Peter Stoffers during the cruise and weretransmitted to us by W.G.Deuser. This study was supportedby the Oceanography Section of the National ScienceFoundation through Grant DES75-03203.

REFERENCESBoger, P.D. and Faure, G., 1974. Strontium-isotope

stratigraphy of a Red Sea core: Geology, v. 2, p. 181-183., 1976. Systematic variations of sialic and volcanic

detritus in piston cores from the Red Sea: Geochim.Cosmochim. Acta, v. 40, p. 731-742;

Brinkman, R., 1974. Geologic relations between Black Seaand Anatolia. In Degens, E.T., and Ross, D. A. (Eds.), TheBlack Sea—geology, chemistry and biology: Am. Assoc.Petrol. Geol. Mem. 20, p. 63-76.

Bukry, D., 1974. Coccoliths as paleosalinity indicators. InDegens, E.T. and Ross, D.A. (Eds.), Black Sea—geology,chemistry, biology: Am. Assoc. Petrol. Geol. Mem. 20,p. 353-363.

Cooper, J.A., Dasch, E.J., and Kaye, M., 1974. Isotopic andelemental geochemistry of Black Sea sediments. In Degens,E.T. and Ross, D.A. (Eds.), Black Sea—geology,chemistry and biology: Am. Assoc. Petrol. Geol.Mem. 20, p. 554-565.

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Flanagan, F.J., 1973. 1972 values for internationalgeochemical reference samples: Geochim. Cosmochim.Acta, v. 37, p. 1189-1200.

Kamen-Kaye, M., 1971. A review of depositional history andgeological structure in Turkey. In Campbell, A.S. (Ed.),Geology and history of Turkey: Libya (Petrol. Explor.Soc), p. 111-138.

Müller, G. and Stoffers, P., 1974. Mineralogy and petrologyof Black Sea basin sediments. In Degens, E.T. and Ross,D.A. (Eds.), The Black Sea—geology, chemistry andbiology: Am. Assoc. Petrol. Geol. Mem. 20, p. 200-248.

Peterman, Z.E. and Hedge, C.E., 1971. Related strontiumisotopic and chemical variations in oceanic basalts: Geol.Soc. Am. Bull., v. 82, p. 493-500. '

Peterman, Z.E., Coleman, R.G., and Hildreth, R.A., 1971.Sr87/Sr86 in mafic rocks of the Troodos Massif, Cyprus:U.S. Geol. Surv. Prof. Paper, 750-D, p. 157-161.

Peterman, Z.E., Hedge, C.E., and Tourtelot, H.A., 1970.Isotopic composition of strontium in sea water throughoutPhanerozoic time: Geochim. Cosmochim. Acta, v. 34,p. 105-120.

Reynolds, R.C., 1963. Matrix correction in trace elementanalysis by X-ray fluorescence: Estimation of the massabsorption coefficient by Compton scattering: Am.Mineralogist, v. 48, p. 1133-1143.

Ross, D.A. and Degens, E.T., 1974. Recent sediments in theBlack Sea. In Degens, E..T. and Ross, D.A. (Eds.), BlackSea—geology, chemistry and biology: Am. Assoc. Petrol.Geol. Mem. 20, p. 183-199.

Shaffer, N.R. and Faure, G., 1976. Regional variation of87Sr/86Sr ratios and mineral compositions of sedimentfrom the Ross Sea, Antarctica: Geol. Soc. Am. Bull.,v. 87, p. 1491-1500.

Vuchev, V.T., 1974. Black Sea studies in Bulgaria—A briefsurvey. In Degens, E.T. and Ross, D.A. (Eds.), The BlackSea—geology, chemistry and biology: Am. Assoc. Petrol.Geol. Mem. 20, p. 90-96.

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