The10Be method of dating marine sediments — Comparison with the paleomagnetic method

Earth and Planetary Science Letters, 37 (1977) 5 5 - 6 0

55

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[11

THE

1°Be METHOD

OF DATING MARINE SEDIMENTS - COMPARISON WITH THE PALEOMAGNETIC METHOD

S. TANAKA, T. INOUE and M. IMAMURA Institute for Nuclear Study, University o f Tokyo, Tanashi, Tokyo 188 (Japan) Received May 2, 1977 Revised version received July 19, 1977

Simultaneous study of lOBe activity, magnetic reversal, mineralogy and trace elements in a 7 m long core from ~20°S, 170°W in the Pacific Ocean has been made. The 10Be and magnetic reversal chronology are found to be in agreement, indicating validity of the lOBe method of dating. The possible sources for the variations of lOBe are also discussed.

1. Introduction The use of cosmic-ray-induced long-lived radionuclide 1°Be (T1/2 = 1.6 × 10 6 years) was first proposed by Peters [1] as a promising method of investigating the cosmic ray intensity variations and the sedimentation rates of deep-sea sediments up to a few million years in the past. Since Arnold [2] and Goel et al. [3] first detected the l°Be in marine sediments, several investigators [4-9] have measured the concentrations in Pacific, Indian and Arctic Ocean sediments. The usefulness of l°Be for dating sediments has been shown in the paper of Amin et al. [7] by an intercomparison of sedimentation rates based on the l°Be, 23°Th/232Th and magnetic methods. The average global rate of precipitation of 1°Be atoms in freshly deposited sediment has been estimated to be 1.8 × 10 - 2 atoms cm -2 s -1 [7]. Since the in-situ density of sediment is around 0.4 and the sedimentation rate of normal pelagic sediment is 1 - 1 0 mm/1000 yr, the specific activity of l°Be is expected to be 1 - 1 0 dpm/kg dry sediment for the top surface. Most of the measured activities are indeed in this range. If both the l°Be precipitation (P) and the sedimentation rate (S) are assumed to have not changed with time, the specific activity at a depth x [N(x)]

can be written as:

N(x) = (P/S) exp(-~x/S) or

In N(x) = -(X/S)x + ln(P/S)

(1)

Here, X is a decay constant of l°Be. A plot of the logarithm of the l°Be activity versus depth in a sediment core [InN(x) vs. x] should be a straight line; x]S = T(x) is the age at a depth x. However, most of the measured depth profiles of l°Be in the open ocean sediments exhibit an irregular pattern which does not permit drawing a straight line in a plot of In N(x) vs. x. Thus, the validity of relation (1) is not warranted. Merrill et al. [4] and Amin et al. [5,7] suggested that the irregularity may be due to some unknown physical disturbances at the ocean floor. Inoue and Tanaka [8] provided clear evidence for the idea that the irregularity is due to dilution of l°Be by volcanic eruptions in the past and that the sedimentation rates have varied with time. With an x-dependent sedimentation rate S(x), the specific activity should be represented in the following equation: P

N(x)= ~(~exp[-XT(x)] ,

dx

dT(X)-s(x)

56 or

PaIeomagnetic age

Depth(cm)

Visuat c h a r a c t e r i s t i c s

o

T(x) = -~-1 in[1

~XN(x)dx1

- ,PXl

Yi1

(2)

o

where P is assumed to have remained constant throughout the time interval considered. This idea to determine the age of a layer was suggested by Peters [10]. According the equation (2), the l°Be concentrations, in all successive layers along the core down to a depth x must be measured to evaluate the age, Since the measurement of successive layers is necessary for this dating, the previous measurements which were made at intervals are insufficient for this purpose. In this study, the l°Be concentrations in all successive layers of a sediment core have been measured and the ages calculated with equation (2) are compared with those obtained by the magnetic method for the same core [11].

N(x), T(x).

black streak (magnetite) pinkish tone brownish white mottle

5O

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100

"~

.... - - ~ - - ~ q t~

alter'nation of reddish brown a n d l i g h t r e d d i s h brown s t r e a k s

greyish streak solid g r e y i s h w h i t e

inclusions

o OE

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/

-

1

-

200

brown m o t t l e s

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2. Experimental - 450

A sediment core (KH68-4-25), 716 cm in length, was collected from a 5307-m deep-ocean basin at 19°59'S, l'/0°02'W, about 500 km south of the Samoan island arc [12]. Visual characteristics of the core are illustrated in Fig. 1. The sample core was cut into 72 sections of 10-cm intervals. In deeper depths, two or three sections were combined together because of the expected low radioactivity. Beryllium, aluminum and manganese were chemically separated and purified for l°Be, 26A1 and S3Mn measurements; the results for the latter two nuclides will be published elsewhere. Chemical and counting procedures employed are essentially the same as those described in our previous paper [8]. The final stage of chemical procedure and the sample mounting were carried out in a dust-free room (made of polyvinyl chloride) to prevent any intrusion of dust through air filters. A difficulty arose from contamination of potassium into the counting samples. 1 ~tg of potassium gives a count rate of 0.00084 cpm in our counting system. Because the measured activities of l°Be in our counting samples were quite low, the amount of potassium had to be kept below a few micrograms. Therefore, vessels made of teflon or quartz, and carefully purified

......

.,.o,..

]..-yo

...... .,..o ....... o

two manganese

~ 7 0 0 I / / 7 / ~ / A = 716

.... ,,

nodules

brownish w h , t e s a n d w i t h minerals

crystalline

Fig. 1. Description of visual characteristics o f sediment core KH68-4-25.

reagents were used in the final chemical treatments for most of the samples. Potassium content in each counting sample was measured by the atomic absorption method and its contribution was subtracted from the counting result to evaluate the net count rate in Table 1. The activity of each sample was counted with two different sets of a needle counter [13], One

57 o f these has a b a c k g r o u n d c o u n t rate o f 0 . 0 0 9 4 -+ 0.0003 cpm and the other 0.0057 + 0.0003 cpm.

3. Results a n d discussion T h e results o b t a i n e d for t h e specific activities o f 1°Be [ N ( x ) ] , t h e c a l c u l a t e d ages [T(x)] a n d differential s e d i m e n t a t i o n rates [S(x)] are t a b u l a t e d in T a b l e 1 a n d i l l u s t r a t e d in Fig. 2. T h e overall u n c e r t a i n t y in

the specific activities was e s t i m a t e d o n the basis o f t h e u n c e r t a i n t i e s in t h e c h e m i c a l analysis o f b e r y l l i u m a n d p o t a s s i u m , c o u n t i n g e f f i c i e n c y a n d c o u n t i n g statistics. T h e m a g n e t i c reversals at d e p t h s 180, 4 1 5 , 4 6 5 a n d 6 0 0 c m in this core were m e a s u r e d b y K o b a y a s h i et al. [1 I] ; t h e results are p r e s e n t e d in Fig. 2. T h e J a r a m i l l o e v e n t is n o t clearly seen in this core. T h e l°Be ages ( 0 . 6 1 , 1.61, 1.89 a n d 2.43 m . y . ) at the polarity changes agree q u i t e well w i t h t h e ages b a s e d o n

TABLE 1 Specific activities of lOBe, calculated ages and differential sedimentation rates in the core KH68-4-25 Depth interval (cm)

In-situ density* (g/cm 3)

Net count rate (10 - 4 cpm)

Specific activity (dpm/kg dry sediment)

Age (m.y.)

Differential sedimentation rate (mm/1000 yr)

0 - 10 1 0 - 20 2 0 - 30 3 0 - 40 4 0 - 50 5 0 - 60 6 0 - 70 7 0 - 80 8 0 - 90 90-100 100-120 120-130 130-160 160-170 170-180 180-190 190-220 220-250 250-270 270-290 290-310 310-340 340-360 360-390 390-415 415-465 465-500 500-540 540-570 570-600 600-630 630-660 660-690

0.59 0.51 0.55 0.41 0.43 0.46 0.42 0.48 0.41 0.46 0.48 0.46 0.48 0.49 0.48 0.46 0.47 0.44 0.45 0.43 0.38 0.41 0.44 0.39 0.42 0.46 0.48 0.44 0.46 0.50 0.48 0.48 0.54

176 22 18 62 39 64 57 101 90 100 403 129 178 78 183 303 370 345 60 71 340 88 63 504 353 624 171 336 123 96 367 194 5

1.27 0.86 0.57 2.10 1.45 1.78 2.74 1.03 2.29 2.30 4.04 1.38 1.44 0.85 2.84 1.97 3.27 1.54 2.42 1.63 2.63 0.80 0.72 1.89 1.43 1.70 1.06 0.84 1.13 0.99 0.91 1.07 0.05

0.026 0.041 0.051 0.081 0.103 0.132 0.174 0.192 0.226 0.266 0.42 0.44 0.53 0.55 0.61 0.65 0.86 0.97 1.08 1.16 1.27 1.33 1.37 1.51 1.61 1.89 2.03 2.15 2.29 2.43 2.56 2.72 2.73

3.8 6.7 10.0 3.3 4.5 3.4 2.4 5.6 2.9 2.5 1.3 4.0 3.5 5.6 1.7 2.5 1.4 2.9 1.8 2.6 1.8 5.1 5.1 2.2 2.5 1.8 2.5 3.2 2.2 2.1 2.3 1.9 30

(10) ** (13) (11) (13) (11) (15) (14) (15) (17) (15) (33) (14) (18) (16) (24) (30) (25) (32) (16) (15) (15) (19) (17) (34) (19) (49) (18) (31) (15) (11) (30) (14) (4)

± 0.14 ± 0.52 ± 0.34 ± 0.50 ± 0.53 -+ 0.45 -+ 0.74 +- 0.19 ± 0.46 +- 0.41 ± 0.48 ± 0.21 ± 0.20 ± 0.20 -+ 0.45 ± 0.27 ± 0.43 ± 0.22 ± 0.70 ± 0.39 +- 0.26 ± 0.19 ± 0.22 ± 0.23 -+ 0.16 ± 0.24 ± 0.15 ± 0.12 ± 0.16 ± 0.14 ± 0.12 ± 0.13 ± 0.04

* ln-situ density is defined as the ratio of the dry weight of sediment to the in-situ volume occupied by the sediment. ** The figure in parenthesis denotes the statistical error (1 o).

58 o

BRUNHES

8 .--

MATUYAMA

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GAUSS

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=.

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200

300 DEPTH

400

500

600

700

(cm)

Fig. 2. Depth profiles of 10Be concentrations and differential sedimentation rates in sediment core KH68-4-25.

the magnetic stratigraphy (0.69, 1.65, 1.85 and 2.43 m.y.). Thus, the validity of equation (2) has been verified. In this l°Be age calculation, the value of 1.14 X 10 -2 t°Be atoms cm -2 s -1 was adopted for the rate of precipitation (P) at latitude 20°S, which is 37% lower compared to the estimated global value by Amin et al. [7]. The specific activity of 1°Be does not decay uniformly with depth, or, differential sedimentation rates at different depths vary largely by a factor of 4 from ~1.5 mm/1000 yr to ~6 mm/1000 yr with two extreme exceptions of 10 and 30 mm/1000 yr (Table 1 and Fig. 2). This is apparently not consistent with the statement in the paper by Kobayashi et al. [11]: "the rate of sedimentation for this core is nearly uniform throughout and approximately 2.4 ram/1000 years". This is not surprising if we consider that the magnetic method gives only the ages at polarity changes and does not give any age information during the period between each polarity change. The average sedimentation rate derived from both methods is the same: approximately 2.4 mm/1000 yr. It should be noted here that an extrapolation of sedimentation rate derived from the 14C or 23°Th/232Th method to ages beyond the limit of detection of its own method could be quite erronous because of a possible large variation of sedimentation rate with depth. A rapid sedimentation or a dilution of l°Be occur-

red at depths 0 - 3 0 cm (0-0.05 m.y.), 7 0 - 8 0 cm (0.17-0.19 m.y.), 120-170 cm (0.42-0.55 m.y.), 310-360 cm (1.27-1.37 m.y.), 500-540 cm (2.03215 m.y.) and 660-690 cm (2.72-2.73 m.y.). The visual characteristics of the core (Fig. 1), the depth profiles of chemical elements (Fig. 3; [ 14]) and the distribution of glassy fragments (Fig. 4) indicate that the dilution of 1°Be by volcanic eruptions occurred at the depths or ages given above; this is especially clear at the top 30 cm or up to 0.05 m.y. The plausibility of drawing this conclusion appears in our previous paper [8]. The Samoan island arc on the Pacific plate is considered to be emerging under the Indian plate by tectonic motion [15] and the volcanic activities of the arc would have occurred in recent times. Further, it is interesting to see that the cycle of volcanism seems to correlate rather well with that of the ice ages measured to 2 m.y. ago (e.g. [16]). However, the correlation needs to be confirmed in other sediment cores on a global scale. At the bottom of the core, the 1°Be concentration is highly depleted and the sampling corer hits a sandy zone with crystalline minerals (Fig. 1). This suggests that the Samoan islands would have been formed 2.7. m.y. ago. The present 1°Be method is based on the assumption that the rate of 1°Be precipitation (P) has been constant with time. The agreement between the l°Be dates and the magnetic dates measured in this core is

59 10

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° NO

°CL hi

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700

DEPTH ( cm ) Fig. 3. Depth prorfles of certain chemical elements in sediment core K1t68-4-25. in favor of this assumption, at least in the sense that the average rates of l°Be ~precipitation during the time intervals between each adjacent reversal have been constant up to the Matuyama-Gauss boundary (2.4 m.y.). However, the secular variation of P in a shorter time interval cannot be ruled out in principle.

The precipitation rate can be affected by cosmis ray variations and/or changes of the influx of 1°Be into the ocean water; the possibility o f these effects has been discussed, for example, in a recent paper by Somayajulu [17]. From the results of this study it is very likely that

i

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50

~ ;

2;0

' 300

~ ' 400

' 500

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DEPTH ( c m )

Fig. 4. Depth profile of volcanic glassy fragments in sediment core KH68-4-25.

760

60 the rates of sedimentation have changed with time by a variety of events. The plausible event is the influx of volcanic eruptions discussed in this and our previous paper [8]. The volcanic eruptions contain practically no l°Be causing the dilution of ~°Be concentrations in the sediment. The sedimentation rate will change when the chemical environment such as the acidity at the interface between ocean water and sediment changes. By such an environmental change the l°Be concentration in the time interval of more than 10,000 years may not change in a measurable amount, because the residence time of beryllium in the ocean water is only about 500 years [4]. The physical disturbances caused by events such as high-speed current flows near the Antarctic or avalanches of sediments near ocean ridges will result in an irregular pattern in the depth profile of l°Be. The 1°Be method is not applicable in such cases. However, events drastic enough to influence the rate of precipitation (P) can be recognized by this method. This is not the case in core KH68-4-25. In conclusion, the ages up te late Pliocene time in a 7 m long deep-sea sediment core have been determined accurately by the l°Be method on the basis of the assumption of constant 1°Be precipitation. Changes in the sedimentation rates with time have been discussed in terms of volcanism and other processes. The measurable ages by this method can be extended to 5 m.y. by further improvement of beryllium chemistry and l°Be counting, and it will become possible to measure l°Be in thinner layers of sediments and to analyse events in shorter time intervals.

Acknowledgements We thank the crew of the R.V. "Hakuho Maru" for collecting the sample core. This work could not have been completed without the help of Prof. K. Kobayashi, Ocean Research Institute of the Tokyo University. We thank Dr. S. Yanagita for his valuable discussion and cooperation throughout. Thanks are also due to Miss. C. Kimura and to Mr. M. Hosoda for their collaboration in the microscopic observation of glassy fragments and the maintenance of electronic circuits, respectively.

References 1 B. Peters, Radioactive beryllium in the atmosphere and on the earth, Proc. Indian Acad. Sci. Sect. A, 41 (1955) 67. 2 J.R. Arnold, Beryllium-10 produced by cosmic rays, Science 124 (1956) 584. 3 P.S. Goel, D.P. Kharkar, D. Lal, N. Narasappaya, B. Peters and V. Yatirajam, Beryllium-10 concentrations in deep-sea sediment, Deep-Sea Res. 4 (1957) 202. 4 J.R. Merrill, E.F,X. Lyden, M. Honda and J.R. Arnold, Sedimentary geochemistry of beryllium isotopes, Geochim. Cosmochim. Acta 18 (1960) 108. 5 B.S. Amin, D.P. Kharkar and D. Lal, Cosmogenic 10Be and 26A1in marine sediments, Deep-Sea Res. 13 (1966) 805. 6 S. Tanaka, K. Sakamoto, J. Takagi and M. Tsuchimoto, Aluminum-26 and beryllium-10 in marine sediment, Science 160 (1968) 1348. 7 B.S. Amin, D. Lal and B.L.K. Somayajulu, Chronology of marine sediments using the lOBe method: intercomparison with other methods, Geochim. Cosmochim. Acta 39 (1975) 1187. 8 T. Inoue and S. Tanaka, lOBe in marine sediments, Earth Planet. Sci. Lett. 29 (1976) 155. 9 R. Finkel, S. Krishnaswami and D.L. Clark, lOBe in Arctic Ocean sediments, Earth Planet. Sci. Lett. 35 (1977) 199. 10 B. Peters, l]ber die Anwendbarkeit der 10Be.Methode zur Messungkosmischer Strahlungsintensit/it under tier Ablagerungsgeschwindigkeityon Tiefseesedimenten yon einigen Millionen Jahren, Z. Phys. 148 (1957) 93. 11 K. Kobayashi, K. Kitazawa, T. Kanaya and T. Sakai, Magnetic and micropaleontological study of deep-sea sediments from the west-central equatorial Pacific, Deep-Sea Res. 18 (1971) 1045. 12 K. Kobayashi and K. Kitazawa, Piston coring, in: Preliminary Report of the Hakuho Maru Cruise KH68-4, Y. Horibe, ed. (Ocean Research Institute, University of Tokyo, Tokyo, 1970) 36. 13 Y. Fujita, Y. Taguchi, M. Imamura, T. Inoue and S. Tanaka, A low-level needle counter, Nucl. Instrum. Methods 128 (1975) 523. 14 S. Tanaka, S. Shibata, P.Y. Chen, C.H. Ke and S.J. Yeh, Depth profiles of chemical elements in pelagic clay sediments, Geochem. J. (in press). 15 J.W. Hawkins, Jr. and J.H. Natland, Nephelinites and basanites of the Samoan linear volcanic chain: their possible tectonic significance, Earth Planet. Sci. Lett. 24 (1975)427. 16 B.D. Ericson and G. WoUin,Pleistocene climates and chronology in deep-sea sediments, Science 162 (1968) 1227. 17 B.L.K. Somayajulu, Analysis of causes for the beryllium10 variations in deep sea sediments, Geochim. Cosmochim. Acta (in press).