Deep-Sea Sedimentation: Processes and Chronology - ACS

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Deep-Sea Sedimentation: Processes and Chronology S. KRISHNASWAMI and D. L A L Physical Research Laboratory, Navrangpura Ahmedabad 380 009, India

The application of naturally produced and artificially injected radionuclides to determine the chronology of marine sedimentary processes is discussed in this review, with particular reference to models describing their transport to the sea floor and distribution within the sediment pile. The particlereactive radionuclides belonging to the U-Th series, cosmic-ray produced and artificially injected nuclides have proven useful in this pursuit. Studies on the radiogenic descendants of U and Th ( Th, Th, Pb and Th) in sea water reveal that the particle reactive radionuclides are removed to sediments from overlying water in short time scales, 10-100 years; however, their scavenging mechanisms have not yet been clearly identified. The removal most likely occurs through a combination of two processes, adsorption onto sinking particles and scavenging at the sediment-water interface. Evaluation of the relative significance of these two processes has been one of the intriguing problems in marine geochemistry in recent years. Radionuclide profiles in well preserved sediment cores indicate that their distribution in the sediment pile, particularly near the sediment-water interface, is dominated by particle reworking processes, much unlike the commonly assumed undisturbed grain-by-grain deposition of open ocean sediments. The 'proper' interpretation of tracer profiles in sediments, therefore, rests on a knowledge of the processes affecting the tracer distribution and their relationship with time. The availability of several radionuclides with different halflives and source functions and mathematical models to describe the sedimentary processes considerably helps in resolving at least some of the problems. The state-of-the-art in these studies is reviewed with a case study of the application of deep-sea sediments to resolve changes in cosmic ray production of radioisotopes in the past. 238

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0097-6156/82/0176-0363$06.50/0 © 1982 American Chemical Society

Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


364

N U C L E A R AND

C H E M I C A L DATING T E C H N I Q U E S

Deep-sea sediments have long been considered as an i d e a l r e p o s i t o r y f o r the r e t r i e v a l of records p e r t a i n i n g t o earth's past h i s t o r y . Major events such as g l o b a l c l i m a t i c changes, temporal v a r i a t i o n s i n the meteorite and cosmic dust i n f l u x and v o l c a n i c a c t i v i t y are some of the key problems i n e a r t h s c i e n c e s , which may be i l l u m i n a t e d through the study of records preserved i n deep-sea sediments. The i n t r o d u c t i o n of 'time' dimension i n these s t u d i e s i s c e n t r a l f o r t h e i r understanding i n a c h r o n o l o g i c a l frame. As i n several important d i s c o v e r i e s i n a s t r o p h y s i c s and earth s c i e n c e s , the development of a method f o r determining the chron ology of ocean sediments was i n c i d e n t a l and was a consequence of the s t u d i e s on the o r i g i n of p e n e t r a t i n g r a d i a t i o n s observed by i o n i z a t i o n chambers on board ships. These s t u d i e s i n d i c a t e d the presence of s i g n i f i c a n t concentrations of radium i n deep-sea sediments [ l ] . an observation which paved the way f o r development of the 'radium method' f o r d a t i n g deep-sea sediments [2,3]. In subsequent years several major advances were made both i n the understanding of the marine geochemistry of r a d i o n u c l i d e s and i n the techniques f o r t h e i r measurement. These advances helped i n e s t a b l i s h i n g by about 1960, r a d i o a c t i v e geochronometry of marine deposits as a r o u t i n e e x e r c i s e i n p u r s u i t of an 'age' [4,5,6,7]. However, during the l a s t few years' p r e c i s e measurements of r a d i o n u c l i d e s i n c a r e f u l l y c o l l e c t e d and w e l l preserved sediment cores have revealed t h a t t h e i r d i s t r i b u t i o n i n the sediment p i l e i s a r e s u l t a n t of several complex processes [8,9,10], much u n l i k e the simple p a r t i c l e by p a r t i c l e accumulation model envisaged e a r l i e r . These f i n d i n g s have emphasized the n e c e s s i t y to decouple the e f f e c t s of various processes on the r a d i o n u c l i d e d i s t r i b u t i o n i n the sediment p i l e before an age-depth r e l a t i o n s h i p can be derived. In t h i s a r t i c l e we plan t o focus on two aspects ( i ) the t r a n s p o r t of r a d i o n u c l i d e s to the ocean f l o o r and the processes which govern t h e i r d i s t r i b u t i o n i n deep-sea sediments and ( i i ) the a p p l i c a t i o n of deep-sea sediments t o r e t r i e v e h i s t o r i c a l records of l a r g e s c a l e phenomena, e.g. long term changes i n the r a t e of production of n u c l i d e s by cosmic rays. Even w h i l e d i s c u s s i n g these aspects, our emphasis w i l l be mainly on the processes r a t h e r than on the d e t a i l s of the chronometric method. 1

D e l i v e r y of Radionuclides

t o the Ocean

The r a d i o n u c l i d e s which are commonly used f o r determining the chronology of deep-sea sedimentary processes are given i n Table 1. The p r i n c i p a l pathways of i n t r o d u c t i o n of these n u c l i d e s i n t o the f i g u r e s i n brackets of t h i s paper.

i n d i c a t e the l i t e r a t u r e references a t the


end


Be

Th

2 3 0

7.52 χ 1 0

3.25 χ 1 0

22.3

a

u

2.44 χ 1 0 6.6 χ 1 0

3

4

4

4

1000

in s i t u production fronT^U

atmospheric d e p o s i t i o n

40

i n s i t u production fronT^U

1 4

2

1000

40

6-60

1.0

8

P b ) and those i n j e c t e d i n t o t h e atmosphere through n u c l e a r weapon t e s t s ( F e and * Pu). Both t h e cosmic r a y produced and a r t i f i c a l l y i n j e c t e d n u c l i d e s (except C , Table 1) e x h i b i t seasonal and l a t i t u d i n a l v a r i a t i o n s i n t h e i r d e p o s i t i o n , w i t h peak f a l l o u t i n t h e m i d - l a t i t i d e s i n s p r i n g [11,12]. The d e p o s i t i o n p a t t e r n o f cosmic r a y produced n u c l i d e s i s i d e n t i c a l i n both t h e hemispheres because o f hemi s p h e r i c a l l y symmetric source f u n c t i o n s . The s i t u a t i o n i s q u i t e d i f f e r e n t i n case o f t h e f a l l o u t p a t t e r n o f t h e a r t i f i c a l l y i n j e c t e d n u c l i d e s which e x h i b i t s a much higher d e p o s i t i o n i n t h e northern hemisphere, s i n c e the weapon t e s t s were conducted i n t h i s hemisphere. C i s i n t r o d u c e d i n t o the oceans mainly through the exchange of C0 a t the air-sea interface. C once produced i n t h e atmosphere gets q u i c k l y o x i d i z e d t o C 0 and enters the exchange able carbon system. R i v e r r u n o f f and i n s i t u p r o d u c t i o n are the major sources o f U-Th s e r i e s n u c l i d e s (Table 1) t o the oceans. The c o n c e n t r a t i o n s of t h e v a r i o u s U-Th s e r i e s n u c l i d e s i n r i v e r s vary c o n s i d e r a b l y and depend upon several f a c t o r s prime among them being t h e i r chemical r e a c t i v i t y [ 1 3 ] , t h e chemistry o f r i v e r water, and t h e nature o f the r i v e r bed. Release o f uranium ( U ) t o r i v e r waters f o l l o w s the general t r e n d o f chemical weathering o f rocks as can be i n f e r r e d from the s t r o n g p o s i t i v e c o r r e l a t i o n between t h e abundances o f U and t o t a l d i s s o l v e d s a l t s (TDS) i n r i v e r s ( f i g u r e 1 [14,15,16,17]). The U-TDS r e l a t i o n s h i p y i e l d s an average annual U flux of 1.05 χ 1 0 g [ 1 7 ] t o t h e ocean corresponding t o mean U c o n c e n t r a t i o n o f about 0.3 p g / l i t e r of r i v e r water. U i s t h e granddaughter o f U . The 2 3 4 / 2 3 8 activity r a t i o i n s u r f a c e waters i s g e n e r a l l y >1 and average about 1.2 on a g l o b a l s c a l e [14,15,17]. The source o f the 'excess U activity i n r i v e r waters i s known t o be rocks through which t h e r i v e r s f l o w , however the exact mechanism o f p r o d u c t i o n o f t h i s 'excess' i s s t i l l not w e l l understood. Processes l i k e p r e f e r e n t i a l leach ing [18,19], i n s i t u decay o f r e c o i l e d T h [20] and l e a c h i n g through alpha r e c o i l t r a c k s [21] have been suggested as p o s s i b l e mechanisms t o produce t h e commonly observed s u p r a - e q u i l i b r i u m values o f U i n s u r f a c e waters. Whatever may be the mechanism of p r o d u c t i o n o f the excess U a c t i v i t y i n s u r f a c e waters, i t serves as a handle t o probe i n t o t h e geochemistry o f uranium isotopes i n t h e marine environment and as a d a t i n g t o o l f o r s e l e c t e d marine d e p o s i t s [22,23]. 1 4

1 0

2 6

2 2 2

3 2

2 1 0


5 5

2 3 9

2 4 0

1 4

14

1 4

1 4

2

1 4

2

2 3 8

2 3 8

238

2 3 8

1 0

2 3 8

2 3 4

2 3 8

U

U

1

2 3 4

2 3 4

2 3 4

2 3 4


18.

KRISHNASWAMI A N D L A L

As i s common w i t h a l l m a t e r i a l s introduced i n t o the oceans through r i v e r r u n o f f , uranium isotopes a l s o enter the open ocean through the f r e s h and s a l t water mixing zone. The behaviour o f uranium isotopes i n t h i s domain was s t u d i e d r e c e n t l y [17,24,25, 26]. These s t u d i e s reveal t h a t the d i s t r i b u t i o n o f uranium isotopes i n e s t u a r i e s beyond g C l ~ / L are c o n t r o l l e d only by the mixing o f f r e s h and s a l t water ( f i g u r e 2) and t h a t these regions o f e s t u a r i e s ( i . e . , >0.1 g C1"7L) are n e i t h e r a source nor a sink for U and U . The geochemistry o f uranium isotopes i n the very low c h l o r o s i t y region ( ω, i . e . , when t h e removal r e s i dence time o f t h e n u c l i d e from sea water i s l e s s than t h e p e r i o d i n the v a r i a t i o n o f cosmic ray i n t e n s i t y . Once these n u c l i d e s d e p o s i t on t h e ocean f l o o r they a r e l i k e l y t o be s u b j e c t e d t o p a r t i c l e mixing processes. I n t h e f o l l o w i n g we d i s c u s s a t t e n u a t i o n due t o a simple case o f m i x i n g , i n which t h e sedimentary p a r t i c l e s are mixed t o a constant depth, L, from t h e sediment-water i n t e r f a c e [72,73]. F o r such a case the temporal v a r i a t i o n i n t h e standing crop (atoms/cm ) C, o f the r a d i o n u c l i d e i n t h e mixed l a y e r i s given by: 2

χ

x

2

§

= -C (λ • f ) • Q ( t ) d

(

1

5

)

where λ i s t h e r a d i o a c t i v e decay c o n s t a n t , S the sediment accumu l a t i o n r a t e (cm/time), L i s t h e mixed l a y e r t h i c k n e s s (cm) and Q ( t ) i s the d e p o s i t i o n f l u x on the ocean f l o o r . S o l v i n g equation d

(15) by s u b s t i t u t i n g f o r Q |(t) from r e l a t i o n (14) and assuming S C

and L t o be c o n s t a n t , we o b t a i n : (, ,

11 +

C(t) where Λ

a cos (u)t - α - β)

2

layer, Λ

T72

1/2

i s t h e sum removal ( o f(1 + m /A r)a t e

2

)

constants tJh e mixed (1 + ω / Λ from )

2

2

t

2

2

1

2

J

= (λ + ^ ) , and β = t a n - (ω/Λ ). 2

(

1 0

For B e ,

2 6

1

6

)

A1,

Λ - ( S / L ) , and t h e r e f o r e t h e f l u x o f these n u c l i d e s out o f t h e mixed l a y e r o f t h e sediments i n t o t h e h i s t o r i c a l l a y e r , Q ( t ) would be 2

s

Q (t) = Q s

0

j l

'~

%l ' - »


)

m


382

N U C L E A R A N D C H E M I C A L DATING

TECHNIQUES

H

Λ (yrs ) Figure 9. Calculated attenuation factors for various values of Τ (period in the sinusoidal cosmic ray intensity variations) and Λ (the total rate constant). See section on deep-sea sediments and historical records for discussion.



18.

KRISHNASWAMI AND LAL

Deep-Sea Sedimentation

383

The forms o f equations 14 and 17 d e s c r i b i n g the d e p o s i t i o n of t h e t r a c e r on the ocean s u r f a c e , ocean f l o o r and i n t o t h e h i s t o r i c a l l a y e r s o f the sediments a r e a l l s i m i l a r . However, t h e amplitude v a r i a t i o n s i n t h e h i s t o r i c a l l a y e r s o f sediment a r e attenuated c o n s i d e r a b l y compared t o v a r i a t i o n s i n d e p o s i t i o n on the ocean s u r f a c e , i . e . , a t i n p u t , the a t t e n u a t i o n being governed by t h e e f f e c t i v e residence times o f n u c l i d e s i n sea water and i n the mixed l a y e r of the sediments. In Table 2 we present t h e expected a t t e n u a t i o n f a c t o r s o f Be. A residence time o f 200 years (ψ = 5 χ 10"* y r " ) f o r B e i n sea water and a mixed l a y e r t h i c k n e s s o f 10 cm have been assumed i n the c a l c u l a t i o n s . 10

3

Table 2. C a l c u l a t e d A t t e n u a t i o n Factors f o r Ocean F l o o r and W i t h i n Sediments.

P e r i o d i n Q(t) 200 y r s .

10

5

B e a t Deposition on

1570

1.02

yrs.

1 0

A t t e n u a t i o n Factor W i t h i n Sediments A t D e p o s i t i o n S=0.2 cm/ky S=2 cm/ky 6.4

7000 y r s .

1 0

1

157

45

1.0

4.6

3.3

1.05 1 0

The c a l c u l a t e d values i n Table 2 show t h a t f o r B e even i n f a s t d e p o s i t i n g sediments (S=2 cm/10 y r s . ) the imposed s i g n a l i s a p p r e c i a b l y attenuated f o r Q(t) < 1 0 y r s . The major a t t e n u a t i o n i n t h i s case i s due t o p a r t i c l e mixing which c o n s t i t u t e s an apparent decay constant, S/L, 2 χ 1 0 " y r " f o r L=10 cm. The obvious c o n c l u s i o n which emerges from these simple c a l c u l a t i o n s i s t h a t f o r studying t h e p e r i o d i c i t i e s o f cosmic r a y i n t e n s i t y , one should s e l e c t samples from regions o f minimal o r zero p a r t i c l e reworking, e.g., t h e anoxic sediments d e p o s i t i n g i n the Arabian Sea, Black Sea, e t c . The B e s t u d i e s i n p e l a g i c sediments from o x i c b a s i n s , can a t best y i e l d i n f o r m a t i o n on t h e long-term i o y r s . ) changes i n Q ( t ) . ( I n the preceding d i s c u s s i o n , we have c a l c u l a t e d the attenua t i o n f a c t o r s f o r B e f o r three p e r i o d s , 200, 7 χ 1 0 and 1 0 years. Of these t h e 200 and 7000 year periods a r e w e l l estab l i s h e d and have been a s c r i b e d t o s o l a r c y c l e v a r i a t i o n s and earth's magnetic f i e l d e x c u r s i o n s , r e s p e c t i v e l y . For d e t a i l e d c a l c u l a t i o n s on t h e e f f e c t o f these v a r i a t i o n s on t h e production r a t e s o f isotopes by cosmic rays reference i s made t o C a s t a g n o l i and L a l , 75.) Short-term changes i n the cosmic ray i n t e n s i t y (T * 200 y r s . ) can be best s t u d i e d through the analyses o f S i and C i n r e l a t i v e l y f a s t accumulating sediments (S > 10 cm/10 y r . ). Both these n u c l i d e s a r e introduced i n surface waters, from where they are t r a n s p o r t e d t o sediments by s i n k i n g remains o f organisms 3

5

4

1

1 0

5

1 0

3

3 2

5

1 4

3


384

N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S

w i t h i n 1-2 years. Hence these n u c l i d e s are expected t o provide high r e s o l u t i o n data on cosmic ray i n t e n s i t y v a r i a t i o n s . Synthesis 2 2 6

Since the d i s c o v e r y o f R a enrichment i n deep sea sediments n e a r l y a century ago, s e v e r a l new methods based on U-Th s e r i e s n u c l i d e s and cosmic ray produced isotopes have been developed to 'date deep-sea sediments. Recent advances i n sample c o l l e c t i o n , a n a l y s i s and modeling have r a d i c a l l y r e v o l u t i o n i z e d t h e p r e v a i l i n g concepts o f r a d i o n u c l i d e t r a n s p o r t t o the oceans, t h e i r behavior i n the water column and t h e i r d i s t r i b u t i o n i n s e d i ments. Studies o f r a d i o n u c l i d e d i s t r i b u t i o n s have y i e l d e d f a r more i n f o r m a t i o n than r e l a t i v e chronology; they have c o n t r i b u t e d s i g n i f i c a n t l y t o t h e general understanding o f various processes o c c u r r i n g i n the marine environment. The aim o f t h i s a r t i c l e has been t o present an o v e r a l l view o f t h e modern ideas on t h e t r a n s p o r t o f r a d i o n u c l i d e s t o sediments and t h e i r d i s t r i b u t i o n w i t h i n the sediment p i l e . One o f the more s i g n i f i c a n t advances o f recent times has been t o b r i n g i n t o sharper focus t h e e f f e c t o f p a r t i c l e reworking processes on t h e d i s t r i b u t i o n o f p r o p e r t i e s i n the sediment column and t h e i r e f f e c t s on the h i s t o r i c a l records. These s t u d i e s have demonstrated t h a t sediments a c t as a 'low pass f i l t e r ' and t h a t the preserved records are a smudgy v e r s i o n o f t h e i r d e l i v e r y p a t t e r n on the ocean f l o o r . F o r t u n a t e l y there e x i s t s an e x p l i c i t i n t e r r e l a t i o n between the imposed and the preserved s i g n a l s and one can be d e r i v e d from the other through a p p r o p r i a t e mathematical modeling. In s p i t e o f a l l the c o m p l e x i t i e s governing t h e t r a n s p o r t and d i s t r i b u t i o n o f t r a c e r s i n the sediments, these d e p o s i t s are s t i l l one o f the best a v a i l a b l e storehouses to retrieve earth's h i s t o r i c a l records.


1

Literature Cited [1] Joly, J., Phil. Mag., 6, 196 (1908). [2] Petterson, Η., Das Verhaltnis Thorium Zu Uran, In: den Gesteinen Und im Meer. Sitzleer Akad, Wiss. Wien, Mathnaturw K. 127, Mitt. Inst. Radium forsch wien Nr., 400 a (1937). [3] Piggot, C. S., Urry, W. D., Am. J. Sci., 240, 1-12 (1942). [4] Broecker, W. S., Isotope Geochemistry and the Pleistocene Climatic Record, In: Wright, H. E. and Grey, D. G., ed., The Quartenary of the United States, Princeton University Press, New Jersey (1965). [5] Goldberg, E. D., Bruland, Κ., Radioactive Geochronologies, In: Goldberg, E. D., ed., The Sea, Wiley Interscience, New York, 5, 451-489 (1974).



18. KRISHNASWAMI AND LAL Deep-Sea Sedimentation 385

[6] Burton, J. D., Radioactive nuclides in the marine environ ment, In: Riley, J. P., Skirrow, G., eds., Chemical Oceanog raphy, Academic Press, London, 3, 92-191 (1975). [7] Ku, T. L., Ann. Rev. Earth Planet. Sci., 4, 347-379 (1976). [8] Krishnaswami, S., Lal, D., Radionuclide Limnochronology, In: Lerman, Α., ed., Lakes - Chemistry, Geology, Physics, Springer-Verlag, New York, 153-173 (1978). [9] Robbins, J. Α., Geochemical and Geophysical Applications of Radioactive Lead, In: Nriagu, J. O., ed., Biogeochemistry of Lead, Elsevier Scientific Publishers, Netherlands, 285393 (1978). [10] Turekian, Κ. Κ., Cochran, J. Κ., Determination of marine chronologies using natural radionuclides, In: Riley, J. P., Chester, R., eds., Chemical Oceanography, Academic Press, London, 7, 313-360 (1978). [11] Lal, D., Peters, Β., Cosmic ray produced radioactivity of the earth, In: Handbuch der physik, Springer-Verlag, Berlin, 46, 551-612 (1967). [12] Joseph, Α. Β., Gustafson, P. F., Russel, I. R., Schuert, E. Α., Volchok, H. C., Tamplin, Α., Sources of Radioactivity and their characteristics in Radioactivity in the marine environ ment, U. S. National Academy of Sciences, Washington, D.C., 6-41 (1971). [13] Goldberg, E. D., Minor elements in seawater, In: Riley, J. P., and Skirrow, G., eds., Chemical Oceanography, Academic Press, London, 1, 163-196 (1965). [14] Bhat, S. G., Krishnaswami, S., Proc. Ind. Acad. Sci., 70, 117 (1969). [15] Turekian, Κ. Κ., Chan, L. Η., The Marine Geochemistry of the Uranium Isotopes, Th and Pa, In: Activation Analysis in Geochemistry and Cosmochemistry, Universitetsforlaget, Oslo, 311-320 (1971). [16] Borole, D. V., Radiometric and trace elemental investigations on Indian estuaries and adjacent seas, PhD Thesis, Gujarat University, Ahmedabad (1980). [17] Borole, D. V., Krishnaswami, S., Somayajulu, B. L. Κ., submitted to Geochim. Cosmochim. Acta 1980. [18] Rosholt, J. Ν., Shields, W. P., Garner, E. L., Science, 139, 224 (1963). [19] Hussain, Ν., Krishnaswami, S., Geochim. Cosmochim. Acta, 44, 1287-1292 (1980). [20] Kigoshi, Κ., Science, 173, 47-48 (1971). [21] Fleischer, R. L., Science, 207, 979-981 (1980). [22] Thurber, D. L., Broecker, W. S., Blanchard, R. L., Portraz, Η. A., Science, 149, 55-58 (1965). [23] Veeh, Η. Η., J. Geophys. Res., 71, 3379-3386 (1966). [24] Borole, D. V., Krishnaswami, S., Somayajulu, B. L. Κ., Est. Coast. Mar. Sci., 5, 743-754 (1977). 230

231



386

NUCLEAR AND CHEMICAL DATING TECHNIQUES

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