Thermoluminescence measurements of a calcite shell for dating

Quaternary Science Reviews, Vol. 7, pp. 367-371, 1988. Printed in Great Britain. All rights reserved.

THERMOLUMINESCENCE

0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc

MEASUREMENTS

OF A CALCITE

SHELL FOR DATING

Kiyotaka Ninagawa,* Nobu~uke Takahashi,* T o m o n o r i W a d a , t Isao Y a m a m o t o , N o b u h i k o Y a m a s h i t a t and Yoshihiko Y a m a s h i t a t * Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan t Okayama University, 3-1-1 Tsushima-naka, Okayama 700, Japan

For thermoluminescence (TL)-dating of fossil calcite shells of Pecten, a new procedure is introduced. A supralinear function fitted to the 'first-glow growth' of the fossil shell makes it possible to evaluate the natural dose. From the natural doses obtained, we can estimate TL ages of fossil shells from about 5 x 105 years ago to more recently.

INTRODUCTION In TL-dating of fossil calcite shells of Pectinidae Pecten (Notovola) albicans (SCHROTER), which will be abbreviated as Pecten hereafter, it is most important to evaluate precisely the natural dose of the specimen. For the case of a calcite shell, however, chemi-thermoluminescence (chemi-TL) due to organic comPOunds and a supralinear behaviour (Ninagawa et al., 1985) in radio-thermoluminescence (radio-TL) growth prevent us from doing precise TL dosimetry. In early work (Johnson and Blanchard, 1967) on the natural radiation dosimetry of post-Pliocene calcite shells, these essential problems were disregarded. In the present investigation, some attempts are made to evaluate the natural dose of fossil shells from radioTL studies. The first attempt is to separate the radioTL from the chemi-TL (spurious glow) by a 'threedimensional (3-D) display of thermoluminescent emission' (Levy et al., 1971; Akber and Prescott, 1985; Imaeda et al., 1985; Ninagawa et al., 1986) of the calcite shell. The second attempt is to express the first-glow growth for the fossil shell by using a common function which fits to data of the second-glow growth for the fossil shell and of the first- and the second-glow growth for the modern shell. Here the first-glow growth means TL growth of the natural shell additionally irradiated by artificial ~/-rays, while the second-glow growth means TL growth of shells artificially irradiated after annealing. These attempts make it possible to evaluate precise natural doses of fossil shells and to estimate their TL ages. SAMPLES AND EQUIPMENT

Pecten consists of a single calcite morphology except muscle scar (Kobayashi, 1971). The margin part, therefore, was used for the present investigation. Shells were first etched by 5% acetic acid to remove mud and the surface portion which suffer from et- and [3-ray irradiation in the soil. Samples were gently ground with a mortar and pestle. The grains of 105-297 ~m (145-048 mesh) were separated out by sieving.

The 3-D displays of TL emission (Figs 1 and 2) were measured by a time-resolving spectroscopy system, which is constructed from a spectroscope, an image intensifier, a CCD-TV camera, a video image processor and a computer system. Details of the system are described in the references (Imaeda et al., 1985; Ninagawa et al., 1986). The glow curves (Fig. 3) were measured by a photon counting system (Yamamoto et al., 1984) using a photomultiplier R550 (Hamamatsu Photonics Co. Ltd.) with a multialkali photocathode, which has high sensitivity at longer wavelengths. A short wave pass filter with 50% transmission at 620 nm was used to cut out the incandescent radiation emitted from the sample at higher temperatures. 3-D TL DISPLAY OF CHEMI- AND RADIO-TL For the precise evaluation of the natural dose, the TL glow due to the radiation must be distinguished from various spurious ones. In order to make the nature of each TL glow clear, three-dimensional (3-D) measurements of TL emission, namely measurements of the TL intensity as a function of both wavelength and temperature, were done for modern and fossil shells. Figure 1 shox+s the 3-D display of TL emission of a non-irradiated modern shell measured in air. Figure l(a) is the gradation display of the 3-D glow curve, in which high emission of TL is shown by aggregated points. Figure l(b) is the TL emission spectrum obtained by the integration from 50-410°C. Figure l(c) is the conventional glow curve. The TL emission spectrum (b) consists of a wide band around 550 nm and the glow curve (c) has a peak at 350°C. It must be noted that this TL glow was suppressed in a nitrogen atmosphere. This kind of glow, therefore, must not be radio-TL but chemi-TL associated with the combustion of organic compounds. Figure 2 shows the 3-D display of TL emission of the fossil calcite shell irradiated additively by a dose of 160kR. The TL emission spectrum (b) has a peak at 600 nm and the glow curve (c) has two peaks at 190° and 240°C. The chemi-TL treated in Fig. 1 is suppressed in this figure, because the measurement was

367

368

K. Ninagawa et al. xlO s 5 (b) Integrated spectrum (50-410*C)

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FIG. 1. The 3-D display of the TL emission of a non-irradiated modern calcite shell measured in air. (a) The gradation display, where high emission of TL is shown by aggregated points, (b) the integrated spectrum and (c) the glow curve.

done in a nitrogen atmosphere. The two peaks in Fig. 2(c) are due to radio-TL, since they arise from an additional radiation. The TL emission spectrum (b) suggests that the emission of the radio-TL of the calcite shell is due to Mn (Medlin, 1968; Down et al., 1985). The 3-D second-glow curve of the fossil shell, which had been irradiated by a dose of 160kR after annealing, was also measured in a nitrogen atmosphere. The shape of the TL emission spectrum and the glow temperature were the same as those shown in Fig. 2. This means that the mechanism of the radio-TL is not varied by annealing and that the TL glow shown in Fig. 2 is due to the radio-TL and is free from the chemi-TL. RADIO-TL GLOW CURVE Figure 3 shows the conventional TL glow curves of a natural fossil shell measured using the photon counting system. The glow curve 'a' was measured in air and the curve 'b' in a nitrogen atmosphere. In curve 'b', the spurious glow pointed out in Fig. 1 is suppressed well. The glow peak of the fossil shell is at 240°C. To suppress the spurious glow more completely, we introduced two methods: (1) By etching the grained shell with 5% acetic acid, we could improve the glow curve 'b' to 'c' in Fig. 3. The

difference would be tribo-thermoluminescence[triboTL] associated with the pressure during grinding (Zeller et al., 1955). (2) By using a long wave pass filter with 50% transmission at 580 nm, we could suppress the spurious glow more completely because the spectrum of the spurious glow of calcite shells consists of a wide band around 550 nm while that of the radio-TL is around 600 nm as seen in Figs lb and 2b. The result is shown as the glow curve 'd' in Fig. 3. In conclusion, the spurious glow is reduced to a minimum when the TL glow curves are measured by using a long wave pass filter in a nitrogen atmosphere for etched grain samples. The glow peak at 240°C of the radio-TL thus measured (curve 'd') was used for TLdating in the present work. RADIO-TL GROWTH Solid circles in Fig. 4 show the growths of the TL intensity at 240°C. The first-glow growth of the modern shell exhibits a supralinear behavior at lower doses (Ninagawa et al., 1985). The curve shows a best fit function to the experimental data. The function derived from the 'competing trap model' (Camelon et al., 1968) is

369

TL Measurements of a Calcite Shell for Dating

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FIG. 2. The 3-D display of the TL emission of a fossil calcite shell measured in a nitrogen atmosphere. The shell is irradiated to a dose of 160 kR by 6°Co "y-rays. (a) The gradation display, (b) the integrated spectrum and (c) the glow curve.

x103 15 Fossil

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FIG. 3. TL glow curves of a natural fossil calcite shell observed through a short wave pass filter with 50% transmission at 620 nm, by which the incandescent radiation from the sample is subtracted. The rate of temperature rise was 1.7*C/see. The grain size is 105-297 ~m. (a) Measured in air, (b) measured in a nitrogen atmosphere, (c) measured in a nitrogen atmosphere after etching by acetic acid, (d) measured in a nitrogen atmosphere after etching by acetic acid with an additional long wave pass filter with 50% transmission at 580 nm.

370

K. Ninagawa et al.

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FIG. 4. The first- and second-glow growths of thermoluminescence for (a) a modern shell and (b) a Shimoda-jobu fossil shell. The curves show the best fit of the function (1) to the experimental data. The arrow shows the total dose TD of the fossil shell.

Y= Cxf(TD

(1)

+x),

and f(TD + x) = N x {1 - exp[-a(TD + x)]

(2)

N~ [1 - e x p ( - b ( T D + x))]} N where Y is the TL intensity, C the constant of the TL sensitivity, N the maximum number of traps to be filled, Nc the maximum number of competing traps, x an artificial dose and TD a total dose (= natural dose). The values of parameters a, b and NJN were determined from the first-glow growth of the modern shell to be 7.74 x 10-3/kR, 4.3 x 10-2/kR and 0.167, respectively. The first-glow growth of the modern shell is well fitted to the function (1).

The second-glow growth of the modern shell also exhibits the supralinear behavior and is well fitted to the same function (1) with the same values of parameters written above. In another expression, we can superimpose the first-glow growth on the secondglow growth by shifting downward. The constant of the TL sensitivity C is reduced by annealing but the mechanism of the radio-TL is not varied by annealing. The second-glow growth of the fossil shell is also well fitted to the same function (1) with the same values of parameters written above, and we can therefore superimpose the first-glow growth of the modern shell on the second-glow growth of the fossil shell. This proves that in the radio-TL growths for Pecten there is no individual difference between the modern and fossil shell. This makes it possible to extrapolate the first-glow

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FIG. 5. Correlation between TL ages and ages estimated by other methods (see text) of the fossil shells picked up from various shell beds in Japan.

TL Measurements of a Calcite Shell for Dating

growth of the fossil shell to the natural dose region by fitting the function (1) with the same values of parameters a, b and Nc/N. The total dose TD of the fossil shell can be evaluated as 38.3 kR (337 Gy) in the case of Fig. 4 by this fitting method. TL-DATING

The ages of the fossil shell can be estimated by dividing the total dose TD by the natural radiation rate (dose/year). TL dating was tried for the fossil shells picked up from seven shell beds (Ninagawa, 1987) under the following careful handling procedures of the specimens. (1) The shells were picked up from shell beds without sun light exposure. (2) The shells were divided into three parts; margin part, hinge and centre part and muscle scar part. (3) Samples were gently ground with a mortar and pestle, and the grains of 105-297 ~m (145-048 mesh) were separated out by sieving. (4) Annual doses were estimated including the correction of moisture effect (Wintle and Huntley, 1980). In Fig. 5, the TL ages of the fossil shells are plotted against ages of formations estimated by other dating methods [14C (Matsushima, 1984), ESR for aragonite shell (Ikeya and Ohmura, 1984), 23°Th/234U (Ohmura, 1980), fission track (Sugihara et al., 1978; Suzuki, 1978) and stratigraphy (Mori, 1980)]. They are consistent with each other within the present measurement accuracy. TL-dating is fundamentally useful for fossil calcite shells from about 5 x 105 years ago to more recently. ACKNOWLEDGEMENTS The authors greatly thank Prof. Hasegawa and Dr Nakamura for helpful discussions. The authors are greatly indebted to Prof. Chinzei, Prof. Takaoka, Dr Kaneoka, Dr Matsushima, Dr Maeda and Dr Sekimoto for assistances in sampling.

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371

the 2nd Conference on Luminescence Dosimetry, CONF-680920, pp. 332-340. Down, J.S., Flower, R., Strain, J.A. and Townsend, P.D. (1985). Thermoluminescence emission spectra of calcite and iceland spar. Nuclear Tracks, 10, 581-589. Ikeya, M. and Ohmura, K. (1984). ESR age of Pleistocene shells measured by radiation assessment. Geochemical Journal, 18, 11-17. Imaeda, K., Kitajima, T., Kuga, K., Miono, S., Misaki, A., Nakamura, M., Ninagawa, K., Okamoto, Y., Saavedra, O., Saito, T., Takahashi, N., Takano, Y., Tomiyama, T., Wada, T., Yamamoto, I. and Yamashita, Y. (1985). Spatial distribution readout system of thermoluminescence sheets. Nuclear Instruments and Methods, A241, 567-571. Johnson, N.M. and Blanchard, R.L. (1967). Radiation dosimetry from the natural thermoluminescence of fossil shells. The American Mineralogist, 52, 1297-1310. Kobayashi, I. (1971). Internal shell microstructure of recent bivalvian molluscs. Science Reports of Niigata University Series E, 2, 27-71. Levy, P.W., Mattern, P.L. and Lengweiler, K. (1971). Three dimensional thermoluminescent analysis of minerals. Modern Geology, 2, 295-297. Matsushima, Y. (1984). Shallow marine molluscan assemblages of postglacial period in the Japanese island - - its historical and geographical changes induced by the environmental changes. Bulletin of the Kanagawa Prefectural Museum, 15, 37-109. Medlin, W.L. (1968). The nature of traps and emission centers in thermoluminescent rock materials. In: Mcdougall, D.J. (ed.), Thermoluminescence of geological materials, pp. 193-223. Academic Press, New York. Mori, S. (1980). Oiso kyuryou no chishitsu 1. Hiratsuka-shi Hakubutsukan shiryou, 24, 1-70 (in Japanese). Ninagawa, K. (1987). Thermoluminescence dating of fossil calcite shells. Japanese Journal of Applied Physics, 26, 2127-2133. Ninagawa, K., Yamamoto, I., Yamashita, Y., Wada, T., Sakai, H. and Fujii, S. (1985). Comparison of ESR with TL for fossil calcite shells. ESR Dating and Dosimetry, (Ionics, Tokyo), 105-114. Ninagawa, K., Yamamoto, I., Wada, T., Yamashita, Y. and Takaoka, N. (1986). Application of a spatial distribution readout system of thermoluminescence to meteorites. Memoirs of National Institute of Polar Research, Special Issue, 41,328-337. Ohmura, A. (1980). Uranium-series age of the Hiradoko and Uji shell beds Noto Peninsula, Central Japan. Transactions and Proceedings of the Palaeontological Society of Japan, 117,247-253. Sugihara, S., Arai, F. and Machida, H. (1978). Tephrochronology of the Middle to Late Pleistocene sediments in the northern part of Boso Peninsula, central Japan. Journal of the Geological Society of Japan, 84, 583-600. Suzuki, M. (1978). Fission track ages of obsidians in some markertephras in South Kanto (I). The report of the 'Japanese Ministry of Education' foundation in 1976, 1977 [Syouwa 51, 52 nendo kagakukenkyuhi hojokin sogokenkyu (daihyousya: Hiroshi MACHIDA) kenkyuseika houkokusyo, tefura kenkyusiryo, 78-2 (in Japanese)]. Wintle, A.G. and Huntley, D.J. (1980). Thermoluminescence dating of ocean sediments. Canadian Journal of Earth Sciences, 17, 348-360. Yamamoto, I., Tomiyama, T., Miyai, H., Wada, T. and Yamashita, Y. (1984). Screening out obstructive radiation from thermoluminescence sheets (BaSO4:Eu) exposed to cosmic rays. Nuclear Instruments and Methods, 224, 573-575. Zeller, E.J., Wray, J.L. and Daniels, F. (1955). Thermoluminescence induced by pressure and by crystallization. Journal of Chemical Physics, 23, 2187.