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Optical and TL dating on glaciofluvial sediments
Quaternary Science Reviews, Vol. 11, pp, 161-163, 1992.
0277-3791/92 $15.00 © 1991 Pcrgamon Prcss pie
Printed in Great Britain, All rights reserved.
OPTICAL
AND TL DATING
ON GLACIOFLUVIAL
SEDIMENTS
G. Hiitt* a n d H. J u n g n e r ?
*Laboratory of Palaeodosimetry, Institute of Geology, Academy of Sciences, Tallinn, Estonia ?Dating Laboratory, University of Helsinki, Finland Three glaciofluvialsediment samples from the Yukki area near Leningrad were studied in order to compare and combine results from optical and thermoluminescence (TL) dating. The formation of the sediments can be related to the end of the last deglaciation of the area and thus the age is around 13 ka. Two of the samples were collected from a kame ridge, where a rapid sedimentation process and consequently poor bleaching of the sediments can be assumed, and one sample was collected from a terrace with slower sedimentation. Poor TL plateaux reflect this situation and TL dating of the samples was not fruitful. The result from optical dating of the kame samples led to ages that were far too high, but for the terrace sample the optical age of 16 ka is quite realistic. It is proposed that when optical dating of rapidly accumulated glaciofluvial sediments is attempted, TL plateau tests should be used to estimate whether the duration of bleaching at deposition had been adequate to achieve a sufficiently low residual signal.
INTRODUCTION Three samples from the Yukki area near Leningrad were studied using infrared stimulated luminescence (IRSL) and thermoluminescence (TL). The I R S L measurements were made at the Laboratory of Palaeodosimetry, Geological Institute of the A c a d e m y of Sciences in Tallinn and the TL measurements at the Dating Laboratory at the University of Helsinki. The samples were collected during a field trip led by Professor I.I. Krasnov from the V S E G E I Institute, Leningrad. Samples 1 and 2 are both from the same section of a kame ridge, demonstrated during excursions in connection with the I N Q U A congress in 1982 and the International Geological Congress 1984 (Krasnov and Zarrina, 1982). Sample 3 is from a terrace where, according to Krasnov, the sediment accumulation was slower than at the site of samples 1 and 2. The measurements were made on feldspar grains (0.1-0.2 mm) separated from the samples. The content of the long-lived isotopes of U, Th and K are given in Table 1 as observed separately in the two laboratories. The Tallinn data (T) are based on gammaspectrometry whereas the Helsinki data (H) are based on neutron activation and flame photometry.
establish thermodynamical equilibrium (in trap filling). No additional thermal treatment was included because the stimulation wavelength used avoids sampling unstable traps. A linear or exponential fit to the data points was used to calculate the absorbed dose (Fig. 1). The E D values thus obtained for the three samples were 260, 230 and 74 Gy respectively. PL 250
1
f
2OO
150
100
50
f 500
0
i 1000
i 1500 DOSE (Gy)
i
i
2000
2500
PL 100
Sample 3
6O
O P T I C A L DATING The I R induced light signal was measured for 3 sec during illumination with light of 810--860 nm from a semiconductor laser. The intensity of the light beam on the sample was 7 mW/cm2; the decrease of signal during the 3 sec of illumination was only slight. The artificial dose was obtained from a Co 60 gamma source giving a dose-rate of 1 Gy/min. After irradiation the samples were stored at room temperature for three weeks in order to remove phosphorescence and to
Sample
t
2O
0
i
i
50
100
i
150 200 DOSE iG y)
u
L
i
250
300
350
FIG. ]. Growth curves for sample 1 (exponential fit) and sample 3
(linear fit) from IRSL measurements. 161
162
G. Htin and H. Jungner
THERMOLUMINESCENCE Following the p r o c e d u r e of M e j d a h l (1988) regeneration E D s were o b t a i n e d following various durations of bleaching (20 hr and shorter); exponential fitting to the growth-curves was employed. A f t e r g a m m a irradiation (at 0.33 Gy/min) the samples were stored for 2 days at 100°C; in addition the glow-oven t e m p e r a t u r e was held at 250°C for 10 sec during registration of the glow-curves. T h e E D values (in the t e m p e r a t u r e region 300400°C) following the 20 hr bleach were 800,600 and 200 G y for samples 1, 2 and 3 respectively. Plots of E D values versus t e m p e r a t u r e s h o w e d very p o o r plateaux for samples 1 and 2. For sample 3 plots for some of the bleaching times are shown in Fig. 2; it is only for short bleaching times that there is a flat plateau. This can be taken to indicate a very short exposure to light at time of sedimentation. N o exact determination of a correct E D value can, however, be m a d e based on the form of the plateau. T h e E D values o b t a i n e d in the temperature range 350-370°C are plotted against bleaching time in Fig. 3. If the age of this sample is assumed to be 13 ka the c o r r e s p o n d i n g E D should be 43 G y if the H dose-
150
AD(Gy) •
a
125 100 Q
75'
o
o
o
o
o
o
o
o
o
o
o ~
o o
o o
o
o
o
o
o
o
o
o
50' Q ~l'm m O ~ll.O Q ~ e
e glt o o e e o I m ~
~
4-*+..
O"
.~
d
+
25 ~
250
j t
+
+ +
300
J- +
+
+
+
350 Temperature
+, + +
400 ('C)
+
+
bl
++ j
450
500
FIG, 2. ED plateau plots for sample 3 from TL measurements when regeneration from TL levels obtained after different times of bleaching is applied. (a) Total ED; (b) residual after 200 hr UV; (c) ED after subtraction of residual for 4 hr UV; (d) ED after subtraction of residual for 20 min UV.
rate in Table 1 for this sample is used. A n accumulated dose of this value is reached in Fig. 3 for a bleaching time of about 8 min only. CONCLUSIONS F r o m this small exercise a few conclusions may be drawn. The well k n o w n geological fact that the sediments in the kames have been deposited very quickly can be observed in the results f r o m T L m e a s u r e m e n t s as very p o o r plateau behaviour. N o realistic T L dating of these sediments was therefore possible. H o w e v e r , the E D values obtained for sample 3 in the t e m p e r a t u r e region a r o u n d the glow-peak m a x i m u m at 370°C s h o w e d that this sample has b e e n bleached to some extent. F r o m the experimental results the depositional bleaching of this sample corresponds to about 10 min laboratory exposure of etched grains, using a U V lamp having an intensity of the same o r d e r as daylight on a sunny day. A c c o r d i n g to Hiitt 11989) a bulk sample bleaching time of 60 min is e n o u g h to reduce the residual I R S L to less than 10% of its natural level, and this bulk sample bleaching time is approximately equal to 5 rain bleaching of etched grains. T h e optical data o b t a i n e d for sample 3 are in reasonable a g r e e m e n t with geological prediction and this confirms the validity of these estimates. T h e E D values from I R S L m e a s u r e m e n t s were about a quarter of those obtained with T L w h e n no residual was subtracted. The ratio between the E D values for the three samples was a b o u t the same for I R S L and TL. T h e I R S L dating leads to an age of about 18 ka for sample 3, which should be c o m p a r e d to an expected age of 13 ka. T h e E D s o b t a i n e d by this m e t h o d for samples 1 and 2 were far too high. W e did not attempt to exclude t h e m by the shine plateau test ( H u n t l e y et al., 1985) because we are reluctant to use p r o l o n g e d illumination; we consider that this is liable to disturb TABLE 1. Annual dose and ED obtained for the three samples studied (T = Tallinn laboratory,* H = Helsinki laboratory?) Sample 1 rF H
Sample 2 T H
Sample 3 T ft
2.41 9.16 2.42
1.69 7.37 2.61
A D(Gyl
200
U (ppm) Th (ppm) K (%) Km~ (%) Dose rate (mGy/a) ED (Gy)
1011
5O
0
I
i
i
2
4 i.(t)
6
8
FIG. 3. The dependence of ED from TL measurements on time of artificial bleaching of sample 3. Note the logarithmic time scale. Time in minutes.
2.24 1.62 10.60 8.23 3.10 2.99 i2.51) 4.96 4.?,3 260
4.22 230
2.12 9.14 2.48 12.80 4.114
4./18
1.07 5.4t 2.20 13.20 3.33
74
*For gamma spectrometry the samples were used 'as found', directly after collection, thus assuming that the water content on that occasion was representative of the average humidity conditions during burial; no additional correction was made for the enhanced influence of water on the beta dose. The internal dose-rate from potassium in the feldspar grains was assumed to be 0.353 Gy/ka, which corresponds to 70% of the dose-rate from the maximum theoretical content of potassium in K-feldspar. ?The Helsinki dose-rates were calculated using the conversion factors given by Aitken (1985). Correction was made for water content using a wet-dry mass ratio of 1.1 : l.
Optical and TL Dating the equilibrium charge distribution between traps. Instead we prefer to use the T L evidence as an indicator of sufficiency of bleaching at deposition. As mentioned above the T L plateaux obtained for samples 1 and 2 are poor. A n o t h e r observation made in respect of sample 1 was that the natural IRSL signal is about one-third of the saturation level obtained with laboratory irradiation. From investigation of very old samples from the same palaeoclimatic region one of us ( G H ) has observed that the ratio is in the range 0.3--0.4. Based on this one must conclude that the natural IRSL for sample 1 is very close to natural saturation (i.e. equilibrium) and consequently the sample cannot be dated. Sample 2 gave poor T L plateaux (indicating that the bleaching time was less than that needed for the residual to reach a low level); consequently the age obtained was excessively high. So only the age of
163
sample 3 can be accepted, though even here correction is necessary for the residual left after bulk sample bleaching. We conclude that in the dating of poorly bleached sediments it is advantageous to employ a combination of optical and T L measurements.
REFERENCES Aitken, M.J. (1985). Annual dose evaluation. In: Thermoluminescence Dating (Appendix G). Academic Press, London. Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W. (1985). Optical dating of sediments. Nature, 313, 105--107. Htitt, G. (1989). Infrared stimulated photoluminescence dating of sediments. Ancient TL, 7(3), 48-51. Krasnov, I.I. and Zarrina, E.P. (eds) (1982). Guidebook for Excursions A-15 and C-15, XI Inqua Congress, Moscow. Mejdahl, V. (1988). The plateau method for dating partially bleached sediments. Quaternary Science Reviews, 7, 347-348.
0277-3791/92 $15.00 © 1991 Pcrgamon Prcss pie
Printed in Great Britain, All rights reserved.
OPTICAL
AND TL DATING
ON GLACIOFLUVIAL
SEDIMENTS
G. Hiitt* a n d H. J u n g n e r ?
*Laboratory of Palaeodosimetry, Institute of Geology, Academy of Sciences, Tallinn, Estonia ?Dating Laboratory, University of Helsinki, Finland Three glaciofluvialsediment samples from the Yukki area near Leningrad were studied in order to compare and combine results from optical and thermoluminescence (TL) dating. The formation of the sediments can be related to the end of the last deglaciation of the area and thus the age is around 13 ka. Two of the samples were collected from a kame ridge, where a rapid sedimentation process and consequently poor bleaching of the sediments can be assumed, and one sample was collected from a terrace with slower sedimentation. Poor TL plateaux reflect this situation and TL dating of the samples was not fruitful. The result from optical dating of the kame samples led to ages that were far too high, but for the terrace sample the optical age of 16 ka is quite realistic. It is proposed that when optical dating of rapidly accumulated glaciofluvial sediments is attempted, TL plateau tests should be used to estimate whether the duration of bleaching at deposition had been adequate to achieve a sufficiently low residual signal.
INTRODUCTION Three samples from the Yukki area near Leningrad were studied using infrared stimulated luminescence (IRSL) and thermoluminescence (TL). The I R S L measurements were made at the Laboratory of Palaeodosimetry, Geological Institute of the A c a d e m y of Sciences in Tallinn and the TL measurements at the Dating Laboratory at the University of Helsinki. The samples were collected during a field trip led by Professor I.I. Krasnov from the V S E G E I Institute, Leningrad. Samples 1 and 2 are both from the same section of a kame ridge, demonstrated during excursions in connection with the I N Q U A congress in 1982 and the International Geological Congress 1984 (Krasnov and Zarrina, 1982). Sample 3 is from a terrace where, according to Krasnov, the sediment accumulation was slower than at the site of samples 1 and 2. The measurements were made on feldspar grains (0.1-0.2 mm) separated from the samples. The content of the long-lived isotopes of U, Th and K are given in Table 1 as observed separately in the two laboratories. The Tallinn data (T) are based on gammaspectrometry whereas the Helsinki data (H) are based on neutron activation and flame photometry.
establish thermodynamical equilibrium (in trap filling). No additional thermal treatment was included because the stimulation wavelength used avoids sampling unstable traps. A linear or exponential fit to the data points was used to calculate the absorbed dose (Fig. 1). The E D values thus obtained for the three samples were 260, 230 and 74 Gy respectively. PL 250
1
f
2OO
150
100
50
f 500
0
i 1000
i 1500 DOSE (Gy)
i
i
2000
2500
PL 100
Sample 3
6O
O P T I C A L DATING The I R induced light signal was measured for 3 sec during illumination with light of 810--860 nm from a semiconductor laser. The intensity of the light beam on the sample was 7 mW/cm2; the decrease of signal during the 3 sec of illumination was only slight. The artificial dose was obtained from a Co 60 gamma source giving a dose-rate of 1 Gy/min. After irradiation the samples were stored at room temperature for three weeks in order to remove phosphorescence and to
Sample
t
2O
0
i
i
50
100
i
150 200 DOSE iG y)
u
L
i
250
300
350
FIG. ]. Growth curves for sample 1 (exponential fit) and sample 3
(linear fit) from IRSL measurements. 161
162
G. Htin and H. Jungner
THERMOLUMINESCENCE Following the p r o c e d u r e of M e j d a h l (1988) regeneration E D s were o b t a i n e d following various durations of bleaching (20 hr and shorter); exponential fitting to the growth-curves was employed. A f t e r g a m m a irradiation (at 0.33 Gy/min) the samples were stored for 2 days at 100°C; in addition the glow-oven t e m p e r a t u r e was held at 250°C for 10 sec during registration of the glow-curves. T h e E D values (in the t e m p e r a t u r e region 300400°C) following the 20 hr bleach were 800,600 and 200 G y for samples 1, 2 and 3 respectively. Plots of E D values versus t e m p e r a t u r e s h o w e d very p o o r plateaux for samples 1 and 2. For sample 3 plots for some of the bleaching times are shown in Fig. 2; it is only for short bleaching times that there is a flat plateau. This can be taken to indicate a very short exposure to light at time of sedimentation. N o exact determination of a correct E D value can, however, be m a d e based on the form of the plateau. T h e E D values o b t a i n e d in the temperature range 350-370°C are plotted against bleaching time in Fig. 3. If the age of this sample is assumed to be 13 ka the c o r r e s p o n d i n g E D should be 43 G y if the H dose-
150
AD(Gy) •
a
125 100 Q
75'
o
o
o
o
o
o
o
o
o
o
o ~
o o
o o
o
o
o
o
o
o
o
o
50' Q ~l'm m O ~ll.O Q ~ e
e glt o o e e o I m ~
~
4-*+..
O"
.~
d
+
25 ~
250
j t
+
+ +
300
J- +
+
+
+
350 Temperature
+, + +
400 ('C)
+
+
bl
++ j
450
500
FIG, 2. ED plateau plots for sample 3 from TL measurements when regeneration from TL levels obtained after different times of bleaching is applied. (a) Total ED; (b) residual after 200 hr UV; (c) ED after subtraction of residual for 4 hr UV; (d) ED after subtraction of residual for 20 min UV.
rate in Table 1 for this sample is used. A n accumulated dose of this value is reached in Fig. 3 for a bleaching time of about 8 min only. CONCLUSIONS F r o m this small exercise a few conclusions may be drawn. The well k n o w n geological fact that the sediments in the kames have been deposited very quickly can be observed in the results f r o m T L m e a s u r e m e n t s as very p o o r plateau behaviour. N o realistic T L dating of these sediments was therefore possible. H o w e v e r , the E D values obtained for sample 3 in the t e m p e r a t u r e region a r o u n d the glow-peak m a x i m u m at 370°C s h o w e d that this sample has b e e n bleached to some extent. F r o m the experimental results the depositional bleaching of this sample corresponds to about 10 min laboratory exposure of etched grains, using a U V lamp having an intensity of the same o r d e r as daylight on a sunny day. A c c o r d i n g to Hiitt 11989) a bulk sample bleaching time of 60 min is e n o u g h to reduce the residual I R S L to less than 10% of its natural level, and this bulk sample bleaching time is approximately equal to 5 rain bleaching of etched grains. T h e optical data o b t a i n e d for sample 3 are in reasonable a g r e e m e n t with geological prediction and this confirms the validity of these estimates. T h e E D values from I R S L m e a s u r e m e n t s were about a quarter of those obtained with T L w h e n no residual was subtracted. The ratio between the E D values for the three samples was a b o u t the same for I R S L and TL. T h e I R S L dating leads to an age of about 18 ka for sample 3, which should be c o m p a r e d to an expected age of 13 ka. T h e E D s o b t a i n e d by this m e t h o d for samples 1 and 2 were far too high. W e did not attempt to exclude t h e m by the shine plateau test ( H u n t l e y et al., 1985) because we are reluctant to use p r o l o n g e d illumination; we consider that this is liable to disturb TABLE 1. Annual dose and ED obtained for the three samples studied (T = Tallinn laboratory,* H = Helsinki laboratory?) Sample 1 rF H
Sample 2 T H
Sample 3 T ft
2.41 9.16 2.42
1.69 7.37 2.61
A D(Gyl
200
U (ppm) Th (ppm) K (%) Km~ (%) Dose rate (mGy/a) ED (Gy)
1011
5O
0
I
i
i
2
4 i.(t)
6
8
FIG. 3. The dependence of ED from TL measurements on time of artificial bleaching of sample 3. Note the logarithmic time scale. Time in minutes.
2.24 1.62 10.60 8.23 3.10 2.99 i2.51) 4.96 4.?,3 260
4.22 230
2.12 9.14 2.48 12.80 4.114
4./18
1.07 5.4t 2.20 13.20 3.33
74
*For gamma spectrometry the samples were used 'as found', directly after collection, thus assuming that the water content on that occasion was representative of the average humidity conditions during burial; no additional correction was made for the enhanced influence of water on the beta dose. The internal dose-rate from potassium in the feldspar grains was assumed to be 0.353 Gy/ka, which corresponds to 70% of the dose-rate from the maximum theoretical content of potassium in K-feldspar. ?The Helsinki dose-rates were calculated using the conversion factors given by Aitken (1985). Correction was made for water content using a wet-dry mass ratio of 1.1 : l.
Optical and TL Dating the equilibrium charge distribution between traps. Instead we prefer to use the T L evidence as an indicator of sufficiency of bleaching at deposition. As mentioned above the T L plateaux obtained for samples 1 and 2 are poor. A n o t h e r observation made in respect of sample 1 was that the natural IRSL signal is about one-third of the saturation level obtained with laboratory irradiation. From investigation of very old samples from the same palaeoclimatic region one of us ( G H ) has observed that the ratio is in the range 0.3--0.4. Based on this one must conclude that the natural IRSL for sample 1 is very close to natural saturation (i.e. equilibrium) and consequently the sample cannot be dated. Sample 2 gave poor T L plateaux (indicating that the bleaching time was less than that needed for the residual to reach a low level); consequently the age obtained was excessively high. So only the age of
163
sample 3 can be accepted, though even here correction is necessary for the residual left after bulk sample bleaching. We conclude that in the dating of poorly bleached sediments it is advantageous to employ a combination of optical and T L measurements.
REFERENCES Aitken, M.J. (1985). Annual dose evaluation. In: Thermoluminescence Dating (Appendix G). Academic Press, London. Huntley, D.J., Godfrey-Smith, D.I. and Thewalt, M.L.W. (1985). Optical dating of sediments. Nature, 313, 105--107. Htitt, G. (1989). Infrared stimulated photoluminescence dating of sediments. Ancient TL, 7(3), 48-51. Krasnov, I.I. and Zarrina, E.P. (eds) (1982). Guidebook for Excursions A-15 and C-15, XI Inqua Congress, Moscow. Mejdahl, V. (1988). The plateau method for dating partially bleached sediments. Quaternary Science Reviews, 7, 347-348.