Mass balance for Nd in the Mediterranean Sea

Chemical Geology, 55 (1986) 45--50 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

45

[3]

MASS BALANCE FOR Nd IN THE MEDITERRANEAN SEA*

C.D. F R O S T 1' 2, R.K. O ' N I O N S 1 and S.L. G O L D S T E I N l 1Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ (Great Britain) 2Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 (U.S.A.)

(Received March 29, 1985; revised and accepted November 5, 1985)

Abstract Frost, C.D., O'Nions, R.K. and Goldstein, S.L., 1986. Mass balance for Nd in the Mediterranean Sea. Chem. Geol., 55: 45--50. The mass balance for Nd in the Mediterranean Sea has been assessed through the measurement of Nd isotope ratios in particulates from rivers entering the Mediterranean, sea-floor clays and ferromanganese crusts, and samples of atmospheric dusts, eNd-Valuesfor the Po and RhSne rivers respectively are --10.8 and --9.7, whereas the Nile, Seyhan and Tarsus have --3.3, --6.2 and --6.3. Sea-floor clays and ferromanganese crusts range from --2.0 to --10.1, and dust samples vary from --8.2 to --11.0. The relatively radiogenic Nd isotope composition of the Mediterranean seawater ( e N d = --6 to --7) compared to the Atlantic ( E N d = --12) reflects partial equilibration of the particulate input into the basin rather than merely the dissolved input.

1. Introduction The radiogenic tracer 143Nd has been exp l o i t e d effectively in specific studies o f R E E p r o v e n a n c e in the marine e n v i r o n m e n t a n d the m o v e m e n t o f o c e a n w a t e r masses (O'Nions et al., 1 9 7 8 ; Piepgras et al., 1 9 7 9 ; Piepgras a n d Wasserburg, 1 9 8 0 , 1 9 8 2 , 1 9 8 3 ; G o l d s t e i n a n d O ' N i o n s , 1 9 8 1 ) . It is n o w established t h a t t o a first a p p r o x i m a t i o n the s e a w a t e r a n d f e r r o m a n g a n e s e deposits f r o m a p a r t i c u l a r o c e a n e x h i b i t overall similarity in 143Nd/144Nd ratios, b u t t h a t the ratio differs significantly b e t w e e n one w a t e r mass a n d a n o t h e r . T h e residence t i m e o f Nd in the oceans is longer t h a n t h e m i x i n g *Department of Earth Sciences Contribution No. E.S. 595. 0009-2541/86/$03.50

times w i t h i n t h e m a j o r w a t e r masses b u t s h o r t e r t h a n the m i x i n g time o f the w h o l e o c e a n ( O ' N i o n s et al., 1 9 7 8 ; Piepgras et al., 1 9 7 9 ; Piepgras a n d Wasserburg, 1 9 8 0 ; G o l d s t e i n a n d O ' N i o n s , 1981). The relatively u n r a d i o g e n i c isotopic c o m p o s i t i o n o f Nd in seawater indicates t h a t the c o n t i n e n t s are the d o m i n a n t source o f N d in the oceans. We r e p o r t here S m - - N d i s o t o p i c d a t a f r o m the M e d i t e r r a n e a n d o c u m e n t i n g t h a t t h e Nd i s o t o p e c o m p o s i t i o n o f seawater is c o n t r o l l e d predominantly by particulate rather than dissolved i n p u t . T h e m o v e m e n t o f w a t e r masses w i t h i n a n d b e t w e e n o c e a n basins has been r e c o r d e d t h r o u g h m e a s u r e m e n t o f ~43Nd/144Nd a n d R E E a b u n d a n c e s , in p a r t i c u l a r o u t f l o w w a t e r f r o m the M e d i t e r r a n e a n has been identified

© 1986 Elsevier Science Publishers B.V.

46 in the eastern Atlantic Ocean. Piepgras and Wasserburg (1983) have estimated the 143Nd/ 144Nd ratio of Mediterranean outflow as eNd ~ --6 and Elderfield and Greaves (1982) obtained eNd = --7 for a biogenic carbonate: both estimates indicate that Mediterranean outflow Nd is more radiogenic than eastern Atlantic seawater (eNd ~ --12). It is possible to characterise the different sources of Nd contributing to the Mediterranean Basin more directly than for a major ocean, and questions of the Nd mass balance may be addressed more readily. The purpose of the present contribution is to d o c u m e n t the characteristics of particulate inputs into the Mediterranean basin, and as part of an assessment of the mass balance producing relatively radiogenic seawater, to place limits on the degree of particulate interaction with seawater. 2. Results Sm--Nd isotopic analyses are presented on Table I for particulates from the Po, RhSne, Nile, Seyhan and Tarsus rivers, aeolian dusts, sea-floor clays and ferromanganese crusts from the Mediterranean basin. The relevant analytical methods have been described previously by Hamilton et al. (1983). Sr isotope ratios have been obtained for the leached residue of three sea-floor clay samples. Nd isotope compositions for particulates from the Seyhan and Tarsus rivers draining into the eastern Mediterranean (Fig. 1) have e~d-Values of - 6 . 2 and --6.3, respectively (Table I), somewhat lower than the value of --3.3 reported previously (Goldstein et al., 1984) for a sample from the Nile. The e~d-Values for the Seyhan and Tarsus rivers are much higher than those for the RhSne and Po rivers, which drain southern Europe, with end o f - - 9 . 7 and --10.8, respectively. This may be compared with the average eNd o f - - 1 1 . 5 reported by Goldstein et al. (1984) for the world's major rivers, and the higher values for the

Nile, Seyhan and Tarsus presumably reflect erosion of relatively juvenile Pan-African crust in NE Africa, and the southern Anatolian Fold Belt of southern Turkey. Three samples of atmospheric dusts collected in the eastern Mediterranean have eNd o f - - 8 . 2 , --11.0 and --12.1, within Goldstein et al.'s (1984) reported range for aeolian particulates. Five samples of sea-floor clay, widely distributed within the Mediterranean (Fig. 1) show a large range in eNd-Values from --2.0 to --10.1 (Table I). Although the available data are limited, the variations do appear to reflect their positions within the basin. Samples from the Seyhan delta and Cilicia basin have eNd similar or more radiogenic than values for the particulates from the Seyhan and Tarsus rivers. The relatively radiogenic Nd for a clay sample from the Tyrrhenian basin is likely to reflect an input from juvenile material in the Italian volcanic province, whereas the least radiogenic sample (eNd = --10.1) located off the edge of the RhSne fan is proximal to the RhSne river where particulates have end = --9.7. Nd isotope ratios of two ferromanganese crusts from the Tyrrhenian Sea (Fig. 1) have eNd of --5.0 and --6.8, similar to the estimated value of Mediterranean seawater of eNd = --6 o r - - 7 . The similarity in Nd

Fig. 1. Sketch map indicating location of analysed samples within the Mediterranean Basin. Solid squares are sea-floor clays or ferromanganese crusts (Tyrrhenian Sea). Cruise tracks shown in the eastern Mediterranean relate to atmospheric dust samples.

47

TABLE I Sm-Nd and 87Sr/86Sr isotopic data from Mediterranean sea-floor clay, ferromanganese crusts, aeolian dust and river particulate samples Sample

Sm (ppm)

Nd (ppm)

14'Sm/14*Nd

143Nd/~'4Nd(2o)

c~qd

TCR (*~)

4.836 4.952 7.551 1.995 2.173

26.85 25.77 34.49 10.01 10.81

0.1089 0.1162 0.1324 0.1205 0.1215

0.512084 0.512143 0.512469 0.512320 0.512313

(20) (24) (18) (10) (10)

--10.8 --9.7 --3.3 --6.2 --6.3

1.55 1.57 1.29 1.36 1.38

33.74 8.538

0.1254 0.1235

0.512289 (24) 0.512384 (34)

--6.8 --5.0

27.56 32.44 26.91 17.77 21.29

0.1098 0.1045 0.1015 0.1196 0.1231

0.512172 0.512233 0.512118 0.512369 0.512535

(22) (20) (20) (12) (14)

18.69 39.22 25.59

0.1163 0.1147 0.1187

0.512074 (20) 0.512019 (22) 0.512219 (20)

sTSr/S6Sr(2a)

Source .2

1.49 1.30

0.709161 (42)

5

--8.4 --7.6 --10.1 --5.8 --2.0

1.38 1.28 1.40 1.27 1.04

0.713428 (46) 0.712168 (44) 0.715363 (36)

--11.0 --12.1 --8.2

1.68 1.74 1.49

Rivers:

Po RhSne Nile *~ Seyhan Tarsus

F e r r o m a n g a n e s e crusts : T76-5A T76-15A

6.998 1.745

5

Sea-floor clays : RC9-179 RC9-194 RC9-197 SH74/1088 SH74/1060

5.009 5.611 4.518 3.515 4.355

6 6 6 4 4

A e o l i a n dusts:

Chain 10 Chain 19 Chain 23

5.517 7.439 5.024

143Nd/l*'Nd data normalized to 146Nd/l*4Nd = 0.7219. Reported errors are 20 on the mean. 87Sr/86Sr ratios are for residues from HOAc leaching. 4 . 6 _ 0 a n d end 0 =10. *~ Calculated for depleted mantle model where eNd* 2 S o u r c e o f samples: 1 = J. Simpson, Lamont Doherty Geological Observatory; 2 = J.-M. Martin, l~cole Normale Sup~rieure, Paris; 3 = B. Maynard, University of Cincinnatti; 4 = H.F. Shaw, Imperial College, London; 5 = A. Mottana, Universit~ degli Studi, Rome, Cattedra di Mineralogia, with permission of the Director for the Project Oceanografia e Fondi Marini of the Italian Research Council; 6 = Lamont Doherty Geological Observatory Core Library; 7 = R. Chester, University of Liverpool. .3 Nile data from Goldstein et al. (1984). i s o t o p e c o m p o s i t i o n o f f e r r o m a n g a n e s e deposits and seawater for a particular ocean has b e e n previously observed by Piepgras and Wasserburg (1980) for the Atlantic a n d Pacific Ocean basins. 3. D i s c u s s i o n T h e N d i s o t o p e c o m p o s i t i o n s f o r seafloor clays, a t m o s p h e r i c dusts, f e r r o m a n ganese crusts a n d river particulates in the

M e d i t e r r a n e a n a r e a , like t h o s e i n t h e m a j o r o c e a n s , all r e f l e c t t h e d o m i n a n t c o n t i n e n t a l Nd source of supply to the Mediterranean. T h e M e d i t e r r a n e a n v a l u e s {Fig. 2), h o w e v e r , are l a r g e l y i n t e r m e d i a t e b e t w e e n t h e m o r e radiogenic values for the Pacific Ocean a n d t h e less r a d i o g e n i c v a l u e s t y p i c a l o f the I n d i a n a n d A t l a n t i c Oceans (Piepgras e t al., 1 9 7 9 ; G o l d s t e i n a n d O ' N i o n s , 1 9 8 1 ; Piepgras and Wasserburg, 1980). Although direct measurements of the Nd isotope

48

(o)

MEDITERRANEAN ~

0

• ..

6

Riverparticulate e ~:~ Medit er~anean river particulate ] Aeoliandust [ ] Mediterranean aeoliandust Seyhan []

.-.-•..

114 2 o

(b)

3

I -16

-14

-12

-10

i -8

i -6

i -4

-2

SEAWATER

o

,

,

-16

-14

•

i -12

i n

i -10

Jn -8

n

i -6

-4

f -2

~Nd FERROMANGANESE 5 DEPOSITS

n3

2

SEAFLOOR CLAYS

o

~ m

-;e [77

~Nd

44

Medit........

-;o

-'e

-~

F"I~

-4 -'~ ENd [] Indian • Atlantic [] Pacific -12

Fig. 2. a. Comparison of eNd-Values for river and aeolian particulates from the Mediterranean region obtained as part of this study compared with published values for particulates from major rivers and atmospheric dusts from Goldstein et al. (1984). b. Comparison of eNd-Values reported for seawater, pelagic clays and ferromanganese deposits from the major oceans, with values for the Mediterranean basin (this study). The estimated eNd-values for Mediterranean seawater are from Elderfield and Greaves (1982) and Piepgras and Wasserburg (1983).

composition o f Mediterranean seawater are n o t available, an estimate of eNd = --7.1 b y Elderfield and Greaves (1982) and of eNd -~ --6 b y Piepgras and Wasserburg (1983} also are intermediate between eNd measured f r o m the Pacific and the Atlantic and Indian Oceans. Estimates o f th e residence time of Nd in seawater vary according t o the assumptions made a b o u t the degree of interaction

between particulates and seawater. If the Nd in particulates were isolated from seawater t hen the residence time could be as high as ~ 3-103 a (Goldstein and O'Nions, 1981), alternatively complete equilibration of the particulates and seawater yields (2-3)-102 a (Goldberg et al., 1963; Wildeman and Haskin, 1965). The actual residence time of Nd in the oceans is intermediate between these values because sea-floor clays do n o t have eNd identical t o seawater, as would be expect ed under circumstances of complete equilibration, and because the residence time of Nd must be shorter than t he ~103-a estimate for the mixing time for the entire ocean (Broecker and Peng, 1982). In the case o f the Mediterranean, if particulate Nd is isolated from dissolved Nd, t hen the eNd of Mediterranean seawater will only reflect the dissolved river inputs and the inflow of Atlantic seawater. The Atlantic seawater input into the Mediterranean is by far the greatest source of dissolved Nd, contributing an estimated Nd mass flux of 2.2 g s -1 (Table II). Unless the dissolved Nd load of European rivers is e x t r e m e compared with published values (Martin and Meybeck, 1979; Martin et al., 1976; Keasler and Loveland, 1982), then the rivers are unlikely to c o n t r i b u t e m uch more than ~ 10% to the dissolved Nd input budget. In this case the eNd-Value of Mediterranean seawater should be very close to that of the Atlantic Ocean, around eNd = - - 1 2 . If, at the ot her ext rem e, particulate Nd entering the Mediterranean equilibrated with the dissolved Nd (Table II), Mediterranean seawater would yield an eNd = - - 6 , equal to the estimated present-day seawater value. Because the Nd flux into the Mediterranean associated with particulates is - 50 times larger than the dissolved flux, only ~ 10% is required to cont ri but e to the Nd dissolved in Mediterranean water. This conclusion is consistent with Elderfield and Greaves's (1982) observation t h a t surface layers of Atlantic seawater are enriched in REE m ost

49 TABLE II Mass balance of Nd in the Mediterranean area (A) Mass balance of dissolved Nd in the Mediterranean Sea Water flux (103m3s -') Atlantic Ocean Black Sea Rivers

1,110 6.35

Nd concentration Nd flux (lO-"gg-') (gs-')

eNd

0.2 0.2

2.2 0.013

--12

4 4

0.39 0.036

--10 --3

:

Southern European Nile

10.1 0.9

Average eNd of dissolved Nd

--12

(B) Mass balance of Nd in river particulates and aeolian dusts in the Mediterranean basin Sediment flux (106gs -~) Southern European rivers Nile river Aeolian dusts

1.9 3.2 0.54

Nd concentration Nd flux (10 6gg-~) (gs-L)

eNd

25 35 30

--10 --3 --10

48 112 16

Average end of particulate and dust Nd contents

--6

Note the Turkish rivers (Tarsus and Seyhan) have higher eNd than Southern European rivers. Data from this study and Martin et al. (1976), Martin and Meybeck (1979), Piepgras and Wasserburg (1980), Keasler and Loveland (1982). Aeolian dust flux assumed to be 10% of river sediment flux (Garrels and MacKenzie, 1971; Prospero, 1981). end assumed equal to eNd of dissolved Nd in seawater (Piepgras and Wasserburg, 1980). end of Black Sea assumed equal to eNd of Southern European river water. p r o b a b l y d u e t o dissolution o f p a r t i c u l a t e i n p u t . A n alternative s o u r c e o f relatively r a d i o g e n i c Nd t o o f f s e t t h e u n r a d i o g e n i c A t l a n t i c i n p u t is s u b m a r i n e h y d r o t h e r m a l fluids. H o w e v e r , on t h e basis o f available d a t a f o r vent fluids r e p o r t e d b y Michard et al. ( 1 9 8 3 ) o f Nd = 6 0 0 pg g-l, it seems unlikely t h a t such a s o u r c e w o u l d be signific a n t in t h e Mediterranean. In s u m m a r y , it is c o n c l u d e d t h a t t h e relatively radiogenic Nd i s o t o p e c o m p o s i t i o n o f M e d i t e r r a n e a n seawater reflects the p a r t i c u l a t e i n p u t into the basin r a t h e r t h a n t h e dissolved input. In o r d e r t o c o u n t e r balance t h e signature o f A t l a n t i c seawater

Nd (eNd = - - 1 2 ) , w h i c h is the d o m i n a n t dissolved c o m p o n e n t , ~ 10% o f the Nd associated with p a r t i c u l a t e i n p u t m u s t equilibrate w i t h dissolved Nd. Acknowledgements R. Chester, J.-M. Martin, B. M a y n a r d , A. M o t t a n a , H. Shaw a n d J. S i m p s o n are t h a n k e d f o r d o n a t i n g samples f o r this s t u d y . The research was s u p p o r t e d b y N E R C , the R o y a l S o c i e t y , t h e University o f W y o m i n g College o f Arts a n d Sciences, and a N S F g r a d u a t e fellowship held b y C.D.F. at Cambridge.

50

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