10Be in marine sediments

t:'arth and Planetao, Science Letters, 29 (1976) 155- 16(I © I'.lsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands

155

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1°Be IN MARINE SEDIMENTS TERUO INOUE and SHIGEO TANAKA Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo (Japan) Received September 19. 1975

Revised version received November 29, 1975

The depth profile of the long-lived radionuclide l°Be in a marine sediment recovered in the vicinity of the Samoan Islands has bcen precisely assayed with a highly sensitive needle-type gas counler. The obtained irregular pattern of I°Be concentrations with depths ranging from 4.7 to 0.3 dpm/kg dry sediment is interpreted as being due to dilution of l°Be by volcanic eruptions in ttle past.

i. Introduction

2. Experimental

The long-lived radionuclide 1°Be (T1/2 = 1.5 X l 0 6 years) is continuously produced by cosmic rays in the upper atmosphere and accumulated in marine sediments. The global rate of precipitation of ~°Be atoms in freshly deposited sediment has been estimated by Amin et al. [1] to be 1.8 X 10 -2 ]°Be atoms cm -2 sec -~ based on the revised values of half-life and core sections. Several investigators [ 1 - 6 ] attempted to develop the '°Be method of dating sediments, because its halflife is suitable for chronological studies up to several million years. They have reported an irregular pattern of t°Be concentrations ranging mostly 1 - 1 0 dpm/kg in sediment cores with depth. The scarcity of cores having an uniform '°Be concentration at different depths suggests that the sedimentation at the ocean floor in most regions has not been uniform but disturbed by some geophysical events in the past. In this study, the depth profile of ~°Be in a sediment core has been precisely assayed with a low-level needle counter developed by the present investigators [7]. Again an irregular pattern has been obtained in the depth profile of ~°Be ranging from 4.7 dpm/kg dry sediment to 0.3 dpm/kg dry sediment. The result will be discussed in connection with terrigenous sediment influx in the past by several subsidiary experimental facts.

The sample core of 316 cm in length was collected during the KH68-4 cruise of the R.V. "Hakuho Maru" [8] by a piston corer (i.d. = 136 mm) from a 5110-m deep-ocean basin at 10°57'S, 169°59'W, about 500 km north of the Samoan island arc. The ~°Be concentrations were analysed in seven samples at different depths in the core. Visual characteristics of the core and the sampling depths are shown in Fig. 1. Sample number

SAM-N-24

i

Depth (cm)

~.__~.O

SAM-N-58 ~ S.~M - k - 78 | =, 3AM-N-,O0 ~-~ /

[-50

- coarse muddy fine groined dark reddish brown cloy sandstone slob coarse reddish brown cloy

I

( ~[O0

sandstone slabs with redosn , . . . . Ks

brown

* J

SAM-N-154 /l ¢°o A"

-smal

reda,sn brown mot'~es

. / I "? I-2oo

i sAM-~,-2,5

VISLIOI characteristics

f

° I e o

SAM -N-299 ~

°* 0 300 316

Fig. 1. Description of visual characteristics of the core KII68-422. In sample notation. SAM-N means that the core has been taken from the north of the Samoan Islands and tile last figures indicate the average depth in em of the samples.

156 Each sample was dissolved in I-IF without adding any carriers and beryllium was separated and purified using the method [9] of the combination of hydroxide precipitation and acetylacetortate extraction into carbon tetrachloride. About a half o f the hydroxide precipitate was reserved for 2 6 A 1 measurement in another project. The acetylacetonate extraction was repeated four or five times in each run. Finally the beryllium was dried up from nitric acid solution on a platinum plate under an infrared lamp and immediately covered with a collodion tilm. The thicknesses of the motmtcd sources were measured to be 4 6 mg/cm 2 in a diameter o f about 8 ram. The blank run was carried out for the ~°Be chemistry and the net t3 count rate was found to be 0.0005 -+ 0.0019 cpm, which is small compared to the sample count rates and no correction was applied. -l'he ~°Be was assayed by measuring its 555-keV i3-ra}' with a low-level needle-type gas counter [71 which had a sharpened needle tip (anode) against a 10.5 mm diameter end-window (cathode) of goldplated mylar fihn and was operated in GM mode by flowing a gas mixture of helium (98.7%) and isobutane (1.3%). The counter was heavily shielded with a well-type Nal (T1) anticoincidence guard and massive shields. The sample and the background counting runs were alternated every 1 - 5 days. The efficiency calibration and the plateau check were performed with a ~37Cs standard after each counting run. The background count rate was 0.0092 --4"0.0005 cpm for 32889 min counting. The counting efficiency for ~°Be in these samples was determined with a ~37('s standard (/f~/max = 0.514 MeV) to be 44 (-+3)%. The error in efficiency determination was estimated on the basis of uncertainties in the original 4rrl3 standardization, in the source dispensing and in the estimation of efficiencies for ~-rays to ground state (f"~max = 1.17 MeV), 624keV conversion electrons and 662-keV 7-ray from tire ' 3"~Cs. The corrections due to self-absorption effects were made by muhiplying by a factor from 0.89 -+0.05 to 0.92 + 0.05 for each counting sample. The factors were derived from the absorption experiment described below. The contents of beryllium in the core samples and in the counting samples were determined by the atomic absorption method. The former was performed after the separation of beryllium. A sample aliquot of ~ 1 - 2 g was dissolved after Na2CO3 fusion and beryllium was co-precipitated with ahlminum

hydroxide followed by the extraction into carbon tetrachloride as acetylacetonate. The chemical yield and its reproducibility of these steps were determined by several 7Be tracer experiments to be 95 (+5)%.

3. Results

The obtained results are shown in Table 1 and illustrated in Fig. 2. The overall uncertainty in the specif ic activities was estimated on the basis of uncertainties in chemical analysis of beryllium (8.4%), counting efficiency (6.8%), self-absorption correction (5.5%) and counting statistics ( 2.7 - 13.8%). To ascertain the measured activities are certainly due to ~°Be, several checks were made. (1) The absorption curve of the sample SAM-N-24 was measured with aluminum absorbers. The half: thickness of the sample is 21 ± 3 mg/cnfl which agrees with the reference values of 21.2 -+ 0.3 mg/cm 2 by Arnold [2] and 22.0 + 0.7 mg/cm 2 by M611er [10]. (2) The energy spectrum of the ~3-rays from the sample SAM-N-24 was measured with a 3 cm 2 × 2 mm Si(Li) detector (Nuclear Semiconductor Co. Ltd.). The result is shown in Fig. 3. The spectral shape is reasonable as 1°Be ( b , ' m a x = 555 keV). The disintegration rate of this sample has been calculated by taking the total counts from 230 to 560 keV. The result based

I0 "10

b C~

E Q. "10

I

0.5

O

O.I

I O

I IOO DEPTH

L

i 200 (crn)

Fig. 2. Depth profile of l°Bc in the core K1168-4-22.

|

3O0

157 'FABLE 1 Specific activities of l°Be and differential sedimentation rates in the core KII 68-4-22 Sample

l)epth interval (cm)

Dry weight * (g)

ln-situ density ** (g/cm 3 )

Net count rate (cpm)

Specific activity (dpm/kg dry sedimen t)

l)ifferential sedimentation rate (ram/l(13 yr)

SAM-N-24 SAM-N-58 SAM-N-73 SAM-N-100 SAM-N-154 SA M-N-215 SAM-N-299

20-28 56.5-59.5 70-75 92-102 150-157 212-217 296-301

257 162 375 678 283 193 163

0.33 0.55 0.77 1.22 0.41 0.40 0.34

0.1615143) *** 0.0260(20) 0.0087(12) 0.0245116) t).0255( 21 ) I).0231 (14) 0.0118115 )

4.73 0.67 0.32 0.29 1.39 0.44 1.91

3 12 19 13 8 26 7

Background

t 0.60 ~ 0.10 : 0.06 t 0.04 ~ 0.21 -* 0.06 ± 0.33

11.0092(5)

* About 3/4 portion of the core was used for this work. ** In-situ density is defined as the ratio of the dry weight of sediment to the in-situ volume occupied by the sediment. *** ] h e figure in parenthesis denotes the statistical error (1o).

on the Si(Li) d e t e c t o r is c o m p a r e d with that by the needle c o u n t e r in Table 2. C o u n t i n g rates o b t a i n e d

chemical p r o c e d u r e s applied are proved to work completely for beryllium purification.

using two d i f f e r e n t d e t e c t o r s were c o n s i s t e n t within the e x p e r i m e n t a l errors.

(4) Sample SAM-N-58 was c o u n t e d in 1971 using a d i f f e r e n t t y p e o f gas c o u n t e r , the c o u n t e r being a

(3) The recycle o f chemical purification was applied to sample SAM-N-154. The specific activity c o u n t e d after recycle was 0.85 +- 0.15 ( d p m / m g Be) and is in

16 m m d i a m e t e r Lal-Schinck-type GM c o u n t e r [ 11 ].

good a g r e e m e n t w i t h the one b e f o r e recycle. The FABLI- 2 Comparison of the counting result by the Si(Li) detector and that by the needle counter OOIO

SAM-N-24

Background

Length of counting time (min)

7148

4196

Cuunt rate (cpm)

0.115 + 0.004

Disintegration rate * (dpm)

0.47

.+- 0.06

0.40

± 0.04

~lj_l ~0 key l

550

Si(l,i) detector

key

0 (D 0

0

I0

20

30

40

50

CHANNEL

60

70

80

90

O0

NUMBER

Fig. 3. Energy spectrum (background subtracted) of the 3-rays from the sample SAM-N-24 measured by a 3 cm 2 x 2 mm Si(Li) detector.

Needle counter Disintegration rate ** (dpm)

0.059 +- 0.004

* The counting efficiency was measured to be 12.9 +- 1.(If~,. and a factor of 0.92 :* 0.115 was applied for self-absorption correction. ** From Table 1.

158

The result obtained by the Lal-Schinck-type counter is compared with that by the needle counter in Table 3. Both countings are in excellent agreement. Since the time lag between the two different countings is about four years, the possibility of contamination with shortlived radioactive impurities is ruled out. The various checks above prove that the measured activities are indeed due to '°Be.

1ABI.E 3 ( ' o m p a r i s o n of the c o u n t i n g data by two d i f f e r e n t c o u n t e r s Background

SAM-N-58 Lal-Schinck-t.vpe counter L e n g t h of c o u n t i n g 14004 t i m e ~min)

7O36

( ' o u n t rate t c p m )

0 . 0 9 5 ± 0.003

0.072 : 0.003

D i s i n t e g r a t i o n rate * (dpm)

0 . 0 6 7 .~ 0.013

;\:ecdh, cottager l ) i s i n t e g r a t i o n rate ** (dpm)

4. Discussion

As clearly seen in Fig. 2, the specific activities of t°Be at different depths are scattered beyond experimenial errors by an order of magnitude difference. How can this zig-zag behavior in the 1°Be concentrations with depth be explained'?. The answer would be drawn by considering the following facts and subsidiary experiments.

0.064 -* I).008

* The c o u n t i n g e f f i c i e n c y was m e a s u r e d to be 37 ,- 3%, and a factor of 0.92 r 0.05 was a p p l i e d for s e l f - a b s o r p t i o n correction. ** F r o m l a b l e 1.

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DEPTH (cm)

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DEPTH (cm)

l i g . 4. l.)epth profiles o f certain e l e m e n t s in the core KTI68-4-22 (rectangles d e n o t e the c o n t e n t in the S a m o a n v o l c a n i c rocks).

159

(1) Visual characteristics of the core. Very impressively, the two sample sections which give the low specific activities of 1°Be (SAM-N-73 and SAM-N-100) correspond to the broad solid sandy zone in the core illustrated in Fig. I. It suggests that a dilution of '°Be occurred by a sediment influx foreign from pelagic origin. A similar correspondence can be seen in the density variation with depth in Fig. 4. (2) The paleomagnetic studies of thi.~ core have been made by K. Kobayashi (private communication) in the Ocean Research Institute, University of Tokyo. According to him, directions of the natural remanent magnetization are too scattered to identify the reversversed epoch. It may partly be due to magnetic unstability of the sediment, but the influence of flow-in of foreign ed epoch. It may partly be due to magnetic unstability about 75 cm. At depths (70-- 102 cm, ~220 cm) corresponding to the low specific activities of ~°Be, the directions of magnetization appear to be scattered compared to those at the other depths. Flow-in of foreign sediment may also be suggested by the fact that a sudden change in magnetic intensity has been observed at about 75 cm from 1.3 X 10 -.4 emu/g to 2 X 10 -s emu/g. (3) Chemical composition. The analysis of chemical elements in the sediment samples was made by means of the atomic absorption method for Be, Mg, Ca and Fe, of the neutron activation method for Na, AI, C1 and Mn and of the non-destructive 7-ray spectroscopy with a Ge(Li) detector for K and Th. The results are illustrated in Fig. 4, together with the chemical compositions of the Samoan volcanic rocks reported by Hawkins and Natland [12]. In Fig. 4, a sharp drop can be seen in the depth profiles of Na, C1, K, Mn, Th and Be at depths of 70--102 cm (samples SAM-N-73 and SAM-N-100) and inversely a rise in those of Fe, A1, Mg and Ca at the same depths. Especially, the depth profiles of Na, C1, Mn and Ca correlate sharply to the depth profile of ~°Be at the above depths. And so does the density. At ~220 cm depth a drop of lesser extent can be seen in the depth profiles of K, Mn and Be. The concentration of Mn remains at a low value of about 0.15% except in the top sample. By comparing the content of these elements in the sediment samples with those in the Samoan volcanic rocks, it is suspected that the change of elemental abundances at 70-102 cm depth is caused by the influx of volcanic eruptions probably from nearby w)lcanoes similar to tile Samoan

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I

150

I

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200

250

300

(cm)

Fig. 5. Depth distribution o f volcanic glass concentratiun in

the core KH68-4-22,

Islands. tlawkins and Natland [I 2] have suggested that the Samoan linear volcanic chain has been volcanically active relatively recent in 1 or 2 m.y. from their morphological and petrological investigations. (4) Glassy fragments. The distribution of glass shards and spherules in the sediment core was determined by microscopic observation. The results are shown in Fig. 5. The increase of glasses at depths of 75- 102 cm and 210--300 cm may reflect the influx of volcanic eruptions. The increase in concentration of glass fragments is quite marked in 210-300 cm section, but the changes in visual characteristics, magnetic intensity and content of chemical elements are not prominent in this section. The evidence in the above subsidiary experiments supports the idea that the irregular pattern in the depth profile of 1°Be can be explained by dilution due to the eruptions from nearby volcanoes in the past. However, the volcanic activities corresponding to the two different depths (70 102 cm and ~220 cm) seem to be different in many respects. At the present time the

160 origin of this difference cannot be solved. The dates of the w~lcanic activities could not be determined exactly, because of the lack of dating evidence such as paleomagnetic reversals. However, approximate dates can roughly be estimated from the ~°Be concentrations if the global production rate of ~°Be is assumed to be constant with time. The differential sedimentation rates at different depths were calculated by adopting the value of 1.8 X I0 -2 ~°Be atoms cm -2 sec -l proposed by Amin et al. [I] for the rate of ~°Be precipitation. The calculated differential sedimentation rates are tabulated in Table 1 ; in this calculation the decay of ~°Be was neglected. On this basis it is estimated front these differential rates of sedimentation that the event at the depth of 7 0 - 1 0 2 cm occurred ~(1 1.5) X l0 s years ago in the interval of about 2 × 104 years and that at the depth around 220 cm occurred ~ ( 2 3) × 10 s years ago. In conclusion, the depth profile of ~°Be in marine sediments can be used to ascertain any changes in the sedimentation rate due to geophysical disturbances. The 1°Be method is also promising for dating events in Quaternary times, preferably when the palc~magnetic method is simultaneously applied to the same core. In our laboratory, the study is under way on the depth profile of 1°Be in another core, whose chronology has been determined by the paleomagnetic method up to 2.5 m.y.

Acknowledgements We thank the crew of the R.V. " l l a k u h o Maru" for collecting the sample core. Thanks are also due to Dr. M. Imamura, Dr. K. Komura and Miss. C. Kimura of tiffs Institute for their collaborations in the analysis of chemical elements, the non-destructive 7-ray measurements and the microscopic observation of glasses, respectively. Mr. lt. Daidoji of the Institute for Solid

State Physics is thanked for the analysis of beryllium and Mr. M. Hosoda for cooperation in the design and maintenance of electronic circuits. Prof. K. Kobayashi of the Ocean Reasearch Institute made helpful suggestions which improved the manuscript.

References 1 B.S. Amin. 1). I.al and I..K. Somayajulu. Chronology of marine sediments using the lOBe method: intercomparison ~ith other methods, Geochim. Cosmochim. Acta 39 (1975) 1187. 2 J.R. Arnold. Beryllium-I O produced by cosmic-rays, Science 124 (1956) 584. 3 P.S. (.;oel. I).P. Kharkar. D. Lal, N. Narasappaya, B. Peters and V. Yatirajam, Bcryllium-I 0 concentrations in deep-sea sediment, l)ecp-Sea Res. 4 (1957) 202. 4 J.R. Merrill. t'.F.X. Lyden, M. Honda and J.R. Arnold, Sedimentary geochemistry of the beryllit, m isotopes, Gcochim. Cosmochim. Acta 18 (1960) 108. 5 B.S. Amin, I).P. Kharkar and D. Lal, ('osmogcnic l°t3e and 26A1 in marine sediments. Deep-Sea Rcs. 13 (1966) 805. 6 S. Tanaka, K. Sakamoto, J. Takagi and M. "l'suchinmto, Aluminum-26 and bcryllium-I 0 in marine sediment. Science 160 (1968) 1348. 7 Y. I:ujita. Y. Taguchi, M. hnamura, T. Inoue and S. Tanaka. A low-level needle counter. Nucl. Instrum. Met hods (1975) in press. 8 K. Kobayashi and K. Kitazawa, Piston coring, in: Preliminary Report of the Itakuho Maru Cruise K1t68-4, Y. ltoribe, cd. f()cean Research Institute, University of l'okyo, 1970) 36. 9 J.R. Merrill, M. Honda and J.R. Arnold, Methods for separation and determination of beryllium in sediments and natural waters, Anal. ('hem. 32 (1960) 1420. l0 P. M611er, Isolierune yon Radionuklidreinen 7Be und lOBe aus Regenwasser und Bodenproben, J. Inorg. Nucl. ('hcm. 32 (1970) 2473. 11 1). Lal and I). Schinck, Rcv. Sci. lnstrum. 31 {1960) 395. 12 J.W. llawkins, Jr. and J.H. Natland, Nephelinites and basanites of the Samoan linear volcanic chain: their possible tectonic significance, l:.arth Planet. Sci. Lett. 24 11975) 427.