- Email: [email protected]
New dating techniques for Quaternary sediments and their application on the Russian Plain
Quaternary Science Reviews 20 (2001) 875}878
New dating techniques for Quaternary sediments and their application on the Russian Plain夽 A.I. Shlukov*, M.G. Usova, L.T. Voskovskaya, S.A. Shakovets 36-9-111 Novatorov Street, Moscow 117421, Russia
Abstract This paper presents the basis of a simpli"ed TL dating procedure, the S}S method. Results are given for two sections from the Russian Plain, for which the geological source of the quartz is in Scandinavia. Results for a modi"ed technique, the S}T method, are given for one section from the region of Novgorod. Quartz from this area has a di!erent origin and behaves di!erently, thus needing a new dating procedure. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In previous studies we have found several problems with the generally used approaches for TL dating of quartz from Quaternary sediments. Our studies have led us to abandon the use of laboratory irradiation to reconstruct a TL growth curve (Shlukov and Shakhovets, 1987; Shlukov et al., 1993). We have demonstrated radiation, induced fading (R-fading) as a more signi"cant cause of signal loss than thermal fading (Shlukov and Shakhovets, 1987). Our experiments show that the ultraviolet component of sunlight bleaches the pre-genetic (i.e. prior to deposition) TL signal to an equilibrium level. Based on these observations, we have developed a new technique, named the S}S technique (Shlukov and Shakhovets, 1987). It is a rapid technique that enables a large number of ages to be determined from a section. Such an approach makes it possible to detect, and thus reject, anomalous dates, such as those that would be obtained from non-zeroed material. We have applied the S}S technique to more than 2,000 samples. It was found applicable to sediments in a limited geographical area, i.e. the Russian Plain, whose sediments are derived from Scandinavia. It did not give consistent results in the southern Ukraine, for which the source of the sediment is in the Carpathians and the Caucasus. It was also inapplicable in the Orenburg region, for which the source is in the Urals. To overcome 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (A.I. Shlukov).
this problem, we have modi"ed the analysis and developed the S}T technique. This has been tested on sediments from the Novgorod region.
2. S}S dating technique We use the basic dating equation as given by Shlukov et al. (1993). This equation is obtained by rewriting the simple exponential equation which describes the growth of the TL signal (S) towards a saturation level (S ) from an initial base level (S ) S"S (1!exp (!aD))#S exp (!aD). where S is the area of the natural TL peak, S the area of the TL peak at saturation, a a constant de"ning the dose sensitivity of the TL peak, S the area of the TL peak after exposure to non-"ltered light from a 120 W mercury lamp at a distance of 25 cm. This is the so-called `theoretical zeroa of Shlukov et al. (1993) and D the natural absorbed dose. Rearranging the above equation gives S !S , t"(aE)\ ln S !S where t is the sample's age and E the natural gamma dose rate measured at the site. Of the "ve parameters in the age equation, three (S, S and E) require direct measurement for each sample; S and a are measured indirectly, using a very old quartz and a calibration sample, respectively.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 3 1 - 7
876
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
Fig. 1. Glow curves of saturated samples from the Russian Plain, together with that from an Upper Cretaceous sand (in bold). The S}S technique is used for this type of quartz.
The basic assumption of the S}S method is the consistency of the saturation level S found for early}middle Quaternary sediments from across the Russian Plain. The weight-normalised natural TL signals for these samples are shown in Fig. 1. The main peak at about 3003C is the one used for dating. Also shown is the response of an Upper Cretaceous quartz ('70 million years). The data in Fig. 1 led us to conclude that (a) quartz from the Russian Plain had a common origin, namely the Scandinavian Massif, and (b) the TL for this quartz has an identical saturation level. As a laboratory standard for S we use the Upper Cretaceous quartz. S and S are the TL parameters measured on each sample and E is a c-dose rate measured in the "eld when each sample is collected. The constant a is obtained using a sample for which the above measurements are made, but which has a known age. The age for this calibration sample is based on a 34,000 year radiocarbon date on fossil plant material. a is thus derived to have a value of 1.3;10\ Gy\. The c-dose rate is measured with a scintillation radiometer placed in a hole in a freshly exposed surface, such that the natural water content is retained. No measurements are made of a- or b-activity. The former is irrelevant since the surface layers of the quartz grains are removed by etching with hydro#uoric acid. The latter is considered unnecessary, since the b- to c-dose rate ratio was found to be constant (within 15%) for 200 samples reported in the literature. In the laboratory, quartz grains of 0.1}0.25 mm diameter are extracted with a "nal purity of not (98%. This is achieved by dispersing in water and wet sieving to select the appropriate grain size. These grains are then treated with HCl and then concentrated HF for 40 min. The quartz fraction is then separated from heavy minerals using bromoform or an electromagnetic separator. The 98}99% pure quartz thus obtained has a clear TL peak at 3003C, with smaller peaks at both lower and
Fig. 2. Quartz glow curves for sand from a region requiring S}T technique, showing systematic trend of peak position with changing light sum.
higher temperatures (see Fig. 2). Further details are given by Shlukov et al. (1999). The TL analysis is carried out using an apparatus designed and built by Shlukov. For dating, the TL is measured from 20 to 5503C at a heating rate of 103C/s. For each sample, the peak areas (light sums) S and S are measured, along with S for an aliquot of the Upper Cretaceous quartz. An error analysis, based on instrumental errors (standard deviation) is carried out, and the uncertainity in the age t is given as
*p #* p#*p 1 , * "E\ tp #a\ 1 R # * * where * "S!S , * "S !S , *"S !S, p , 1 # p ,p and p are instrumental errors of E, S , S and S , 1 respectively. If the peak height S of the sample coincides with the peak height S within a 1.5 fold error, i.e. S !S(1.5(p #p ), we consider these results to give 1 us a limiting value; we then calculate the age by the formula: t*(aE)\ ln
S !S . 1.5(p #p ) 1
3. S}T technique In the more recent S}T technique, both the peak area (light sum) and peak temperature are measured. This is necessary for minerals that exhibit second-order kinetics (Antonov-Romanovsky, 1966). The TL peak is displaced to a lower temperature as the signal increases. The extent of displacement of the peak position depends upon the saturation level. For these samples, for which S may not be the same, S and the peak temperature can be used to calculate S for the sample.
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
877
Between 1984 and 1992 we analysed more than 2000 samples from over 50 sections. These were published in 1999 (Shlukov et al., 1999) and results using both the S}S and S}T techniques were reported. In this paper we reproduce results for three Sections (Fig. 3). The section at Seroglazovka is from the Astrakhan Region of the Lower Volga River; it is site 8 in Shlukov et al. (1999). Here the Blake Event has been found in the palaeomagnetic record. The S}S technique was applied and gave ages consistent with the age inferred for the Blake Event from the ocean sediment record. The section at Latishi is in West Belorussia and is site 11 in Shlukov et al. (1999). This region is also part of the Russian Plain, and hence the S}S technique was employed. Here there is a comparison with several radiocarbon dates. The section from the Volga River in the Novgorod Region is section 55 in Shlukov et al. (1999). Novgorod is east of the region where the S}S technique works, and there the S}T technique had to be applied. No ge control was available, but the ages obtained at this and four other sections were in stratigraphic order.
4. Conclusions
Fig. 3. Examples of Quaternary sections from the Russian Plain, illustrating the use of the S}S and S}T techniques.
The age equation is more complex than in the S}S technique, and is given as
S S !S t"(bS E)\ arth #0.5 ln S S #S as derived in the appendix of Shlukov et al. (1999). Once again, the normalised dose sensitivity b"4.52;10\ Gy\ is obtained using a sample of known age. For each sample to be dated, S is derived from the light sum, according to the following equation: S "(SS exp((e/k)(¹\!¹\)), where S and ¹ are light sum of peak maximum of our laboratory standard LtQ, e"1.7 eV is the activation energy of the electron trap and k the Boltzmann's constant.
A new approach to dating has been presented. It involves no laboratory irradiation, just the measurement of the natural TL and the signal from a laboratory-bleached portion of each sample. The peak areas (light sums) are compared with that from a standard upper Cretaceous quartz that is in saturation. The response to radiation is calibrated using a sample of known age (34 ka). The c-dose rate for the calibration sample, and that being dated, are measured in the "eld. The approximation of a "rst-order exponential formula was applied in the S}S procedure. This formula is not correct but is also that used in more traditional TL-dating procedures. This approximation is suitable for sediments for which the source is the Scandinavian Massif; for these sediments the quartz has an identical saturation level. However, the S}T technique is used for samples showing second-order behaviour and hence we used a more appropriate tangential formula, rather than the exponential one. These procedures have been compared previously (Shlukov et al., 1993).
Acknowledgements We would like to thank all investigators for their active assistance and support in sample collection and chronostratigraphic interpretation. We also wish to thank Prof. A.G. Wintle, Dr. H. Jungner and the late Prof. V. Mejdahl for discussions.
878
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
References Antonov-Romanovsky, V.V., 1966. Kinetics of Photoluminescence of Crystallophosphors. Nauka, Moscow. Shlukov, A.I., Shakhovets, S.A., 1987. Kinetic studies of quartz thermoluminescence as applied to sediment dating. Ancient TL 5, 11}15. Shlukov, A.I., Shakovets, S.A., Voskovskaya, L.T., Lyaschenko, M.G., 1993. A criticism of standard TL-dating technology. Nuclear Instruments and Methods in Physics Research 73, 373}381.
Shlukov, A.I., Usova, M.G., Voskovskaya, L.T., Shakovets, S.A., 1999. New absolute dating techniques for Quaternary sediments and their application on the Russian Plain. In: Bruns, P., Hass, H.C. (Eds.), Determination of Sediment Accumulation Rates, GeoResearch Forum, vol. 5, Trans Tech Publications, Switzerland, pp. 145}168. Patents Shlukov, A.I., Shakhovets, S.A. 1984. USSR Certi"cate of invention No. 1250041. Shlukov, A.I., 1989. USSR Certi"cate of invention No. 155038.
New dating techniques for Quaternary sediments and their application on the Russian Plain夽 A.I. Shlukov*, M.G. Usova, L.T. Voskovskaya, S.A. Shakovets 36-9-111 Novatorov Street, Moscow 117421, Russia
Abstract This paper presents the basis of a simpli"ed TL dating procedure, the S}S method. Results are given for two sections from the Russian Plain, for which the geological source of the quartz is in Scandinavia. Results for a modi"ed technique, the S}T method, are given for one section from the region of Novgorod. Quartz from this area has a di!erent origin and behaves di!erently, thus needing a new dating procedure. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In previous studies we have found several problems with the generally used approaches for TL dating of quartz from Quaternary sediments. Our studies have led us to abandon the use of laboratory irradiation to reconstruct a TL growth curve (Shlukov and Shakhovets, 1987; Shlukov et al., 1993). We have demonstrated radiation, induced fading (R-fading) as a more signi"cant cause of signal loss than thermal fading (Shlukov and Shakhovets, 1987). Our experiments show that the ultraviolet component of sunlight bleaches the pre-genetic (i.e. prior to deposition) TL signal to an equilibrium level. Based on these observations, we have developed a new technique, named the S}S technique (Shlukov and Shakhovets, 1987). It is a rapid technique that enables a large number of ages to be determined from a section. Such an approach makes it possible to detect, and thus reject, anomalous dates, such as those that would be obtained from non-zeroed material. We have applied the S}S technique to more than 2,000 samples. It was found applicable to sediments in a limited geographical area, i.e. the Russian Plain, whose sediments are derived from Scandinavia. It did not give consistent results in the southern Ukraine, for which the source of the sediment is in the Carpathians and the Caucasus. It was also inapplicable in the Orenburg region, for which the source is in the Urals. To overcome 夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (A.I. Shlukov).
this problem, we have modi"ed the analysis and developed the S}T technique. This has been tested on sediments from the Novgorod region.
2. S}S dating technique We use the basic dating equation as given by Shlukov et al. (1993). This equation is obtained by rewriting the simple exponential equation which describes the growth of the TL signal (S) towards a saturation level (S ) from an initial base level (S ) S"S (1!exp (!aD))#S exp (!aD). where S is the area of the natural TL peak, S the area of the TL peak at saturation, a a constant de"ning the dose sensitivity of the TL peak, S the area of the TL peak after exposure to non-"ltered light from a 120 W mercury lamp at a distance of 25 cm. This is the so-called `theoretical zeroa of Shlukov et al. (1993) and D the natural absorbed dose. Rearranging the above equation gives S !S , t"(aE)\ ln S !S where t is the sample's age and E the natural gamma dose rate measured at the site. Of the "ve parameters in the age equation, three (S, S and E) require direct measurement for each sample; S and a are measured indirectly, using a very old quartz and a calibration sample, respectively.
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 3 1 - 7
876
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
Fig. 1. Glow curves of saturated samples from the Russian Plain, together with that from an Upper Cretaceous sand (in bold). The S}S technique is used for this type of quartz.
The basic assumption of the S}S method is the consistency of the saturation level S found for early}middle Quaternary sediments from across the Russian Plain. The weight-normalised natural TL signals for these samples are shown in Fig. 1. The main peak at about 3003C is the one used for dating. Also shown is the response of an Upper Cretaceous quartz ('70 million years). The data in Fig. 1 led us to conclude that (a) quartz from the Russian Plain had a common origin, namely the Scandinavian Massif, and (b) the TL for this quartz has an identical saturation level. As a laboratory standard for S we use the Upper Cretaceous quartz. S and S are the TL parameters measured on each sample and E is a c-dose rate measured in the "eld when each sample is collected. The constant a is obtained using a sample for which the above measurements are made, but which has a known age. The age for this calibration sample is based on a 34,000 year radiocarbon date on fossil plant material. a is thus derived to have a value of 1.3;10\ Gy\. The c-dose rate is measured with a scintillation radiometer placed in a hole in a freshly exposed surface, such that the natural water content is retained. No measurements are made of a- or b-activity. The former is irrelevant since the surface layers of the quartz grains are removed by etching with hydro#uoric acid. The latter is considered unnecessary, since the b- to c-dose rate ratio was found to be constant (within 15%) for 200 samples reported in the literature. In the laboratory, quartz grains of 0.1}0.25 mm diameter are extracted with a "nal purity of not (98%. This is achieved by dispersing in water and wet sieving to select the appropriate grain size. These grains are then treated with HCl and then concentrated HF for 40 min. The quartz fraction is then separated from heavy minerals using bromoform or an electromagnetic separator. The 98}99% pure quartz thus obtained has a clear TL peak at 3003C, with smaller peaks at both lower and
Fig. 2. Quartz glow curves for sand from a region requiring S}T technique, showing systematic trend of peak position with changing light sum.
higher temperatures (see Fig. 2). Further details are given by Shlukov et al. (1999). The TL analysis is carried out using an apparatus designed and built by Shlukov. For dating, the TL is measured from 20 to 5503C at a heating rate of 103C/s. For each sample, the peak areas (light sums) S and S are measured, along with S for an aliquot of the Upper Cretaceous quartz. An error analysis, based on instrumental errors (standard deviation) is carried out, and the uncertainity in the age t is given as
*p #* p#*p 1 , * "E\ tp #a\ 1 R # * * where * "S!S , * "S !S , *"S !S, p , 1 # p ,p and p are instrumental errors of E, S , S and S , 1 respectively. If the peak height S of the sample coincides with the peak height S within a 1.5 fold error, i.e. S !S(1.5(p #p ), we consider these results to give 1 us a limiting value; we then calculate the age by the formula: t*(aE)\ ln
S !S . 1.5(p #p ) 1
3. S}T technique In the more recent S}T technique, both the peak area (light sum) and peak temperature are measured. This is necessary for minerals that exhibit second-order kinetics (Antonov-Romanovsky, 1966). The TL peak is displaced to a lower temperature as the signal increases. The extent of displacement of the peak position depends upon the saturation level. For these samples, for which S may not be the same, S and the peak temperature can be used to calculate S for the sample.
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
877
Between 1984 and 1992 we analysed more than 2000 samples from over 50 sections. These were published in 1999 (Shlukov et al., 1999) and results using both the S}S and S}T techniques were reported. In this paper we reproduce results for three Sections (Fig. 3). The section at Seroglazovka is from the Astrakhan Region of the Lower Volga River; it is site 8 in Shlukov et al. (1999). Here the Blake Event has been found in the palaeomagnetic record. The S}S technique was applied and gave ages consistent with the age inferred for the Blake Event from the ocean sediment record. The section at Latishi is in West Belorussia and is site 11 in Shlukov et al. (1999). This region is also part of the Russian Plain, and hence the S}S technique was employed. Here there is a comparison with several radiocarbon dates. The section from the Volga River in the Novgorod Region is section 55 in Shlukov et al. (1999). Novgorod is east of the region where the S}S technique works, and there the S}T technique had to be applied. No ge control was available, but the ages obtained at this and four other sections were in stratigraphic order.
4. Conclusions
Fig. 3. Examples of Quaternary sections from the Russian Plain, illustrating the use of the S}S and S}T techniques.
The age equation is more complex than in the S}S technique, and is given as
S S !S t"(bS E)\ arth #0.5 ln S S #S as derived in the appendix of Shlukov et al. (1999). Once again, the normalised dose sensitivity b"4.52;10\ Gy\ is obtained using a sample of known age. For each sample to be dated, S is derived from the light sum, according to the following equation: S "(SS exp((e/k)(¹\!¹\)), where S and ¹ are light sum of peak maximum of our laboratory standard LtQ, e"1.7 eV is the activation energy of the electron trap and k the Boltzmann's constant.
A new approach to dating has been presented. It involves no laboratory irradiation, just the measurement of the natural TL and the signal from a laboratory-bleached portion of each sample. The peak areas (light sums) are compared with that from a standard upper Cretaceous quartz that is in saturation. The response to radiation is calibrated using a sample of known age (34 ka). The c-dose rate for the calibration sample, and that being dated, are measured in the "eld. The approximation of a "rst-order exponential formula was applied in the S}S procedure. This formula is not correct but is also that used in more traditional TL-dating procedures. This approximation is suitable for sediments for which the source is the Scandinavian Massif; for these sediments the quartz has an identical saturation level. However, the S}T technique is used for samples showing second-order behaviour and hence we used a more appropriate tangential formula, rather than the exponential one. These procedures have been compared previously (Shlukov et al., 1993).
Acknowledgements We would like to thank all investigators for their active assistance and support in sample collection and chronostratigraphic interpretation. We also wish to thank Prof. A.G. Wintle, Dr. H. Jungner and the late Prof. V. Mejdahl for discussions.
878
A.I. Shlukov et al. / Quaternary Science Reviews 20 (2001) 875}878
References Antonov-Romanovsky, V.V., 1966. Kinetics of Photoluminescence of Crystallophosphors. Nauka, Moscow. Shlukov, A.I., Shakhovets, S.A., 1987. Kinetic studies of quartz thermoluminescence as applied to sediment dating. Ancient TL 5, 11}15. Shlukov, A.I., Shakovets, S.A., Voskovskaya, L.T., Lyaschenko, M.G., 1993. A criticism of standard TL-dating technology. Nuclear Instruments and Methods in Physics Research 73, 373}381.
Shlukov, A.I., Usova, M.G., Voskovskaya, L.T., Shakovets, S.A., 1999. New absolute dating techniques for Quaternary sediments and their application on the Russian Plain. In: Bruns, P., Hass, H.C. (Eds.), Determination of Sediment Accumulation Rates, GeoResearch Forum, vol. 5, Trans Tech Publications, Switzerland, pp. 145}168. Patents Shlukov, A.I., Shakhovets, S.A. 1984. USSR Certi"cate of invention No. 1250041. Shlukov, A.I., 1989. USSR Certi"cate of invention No. 155038.