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An intercomparison of ESR and uranium series ages for quaternary speleothem calcites
Quaternary Science Reviews, Vol. 7, pp. 411-416, 1988. Printed in Great Britain. All rights reserved.
0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc
AN I N T E R C O M P A R I S O N OF ESR AND U R A N I U M SERIES AGES FOR Q U A T E R N A R Y S P E L E O T H E M CALCITES P.L. Smart,* B.W. Smith,*¶ H. C h a n d r a , t J.N. Andrews$ and M . C . R . S y m o n s t * Department of Geography, University of Bristol, Bristol BS8 1SS, U.K. t Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K. $ Department of Chemistry, University of Bath, Bath BA2 7A Y, U.K.
Equivalent dose (ED) estimates for speleothem calcite derived by ESR measurements of the g = 2.0005 signal have a typical precision of + 7 % (minimum +2%), and depend on the signal strength, radiation sensitivity and the presence of interfering signals. Underestimation of ED will occur if the crushing signal at g = 2.0001 is not removed by acid etching, while overestimation may be caused if the short-lived radiation induced interfering signal at g = 2.0020 is not removed by annealing. Quoted errors for ESR ages are rarely better than + 10% and are typically + 15% because of uncertainties in ED, calibration of dosimeters and gamma source, and in the a-value. Serial analyses along the growth axis of single speleothems, and between speleothems interbedded in a sequence demonstrate excellent stratigraphic concordance. Intercomparison of ESR and uranium series ages for samples derived from 11 geographically diverse sites showgood agreement with no indication of systematic error. Although validation is more difficult for samples beyond the limit of 23°Th/234U dating, the results appear encouraging, but recrystallisation and changes in the external radiation dose may cause problems.
INTRODUCTION
span the 300 ka range of uranium series dating (Table 1). A number of samples with infinite uranium series ESR (electron spin resonance) dating of speleothem ages were also included from sites where faunal, calcite was first described by Ikeya (1975), the tech- geomorphic or palaeomagnetic data suggested a much nique having been initially proposed by Zeller et al. greater age. Detritally contaminated speleothems were (1967). Briefly, the technique involves measurement of not included, as they cannot be reliably dated by the electron spin concentration in the crystal lattice, uranium series methods. this being proportional to the radiation dose that the The sample preparation and instrumental methods calcite has received since formation (the equivalent used have previously been described in detail (Smith et dose, ED). Measurement of the annual radiation dose al., 1986), but two important aspects of the methodolto the sample, and determination of the spin concen- ogy should be stressed. First, the calcite must be acid tration that this produces allows calculation of the age. etched (10% acetic acid for 2 min) to remove the outer In this paper we examine the validity of ESR dates surface of the grains which have been damaged by on speleothem calcite using two criteria: stratigraphic crushing. Failure to do this results in interference from concordance and agreement with an independent a crushing signal at g = 2.0001 + 0.0002, resulting in dating method, in this case uranium series dating. The an underestimation of the ED, as also reported by results of similar previous studies have not been very Hennig and Griin (1983). Furthermore, because of the encouraging (Yokoyama et al., 1983; Hennig et al., low stability of the crushing signal, an erroneously short 1983; Hennig and Grtin, 1983; Griin, 1985), but these mean life will be derived in any determination of the were limited to particular sites where specific problems stability of the g = 2.0005 signal. of signal stability occur (Griin, 1985), and with respect The second important aspect of methodology into development of reliable techniques. We now con- volves the use of annealing for several hours at 120°C sider the methods detailed previously by Smith et al. after irradiation to eliminate interference from a short(1986), to be reliable, reproducible and routine. Only lived (mean life several months), radiation-induced the ESR signal at g = 2.0005 (+0.0003) is suitable for signal at g = 2.0020 + 0.0003. This interferes with the dating (Hennig and Griin, 1983; Smith et al., 1985a). It dating signal and causes a systematic overestimation of has a mean life of 6 × 107 years at 10°C (2 samples), the ED (Smith et al., 1985b, 1986). It should be noted and grows linearly with dose to at least 400 Gy. that this procedure differs from the more severe annealing used by Yokoyama et al. (1983), which SAMPLE SELECTION AND ESR MEASUREMENT totally removes the g = 2.0005 dating signal. For 19 samples, the decrease in ED on annealing ranged from The samples for this study were derived from a 0 to 60% with a mean reduction just under 20% (Fig. number of diverse geographic locations, and selected to 1). This interference cannot be removed instrumentally and is not affected by the type of curve-fitting applied. ¶Present address: Research Laboratory for Archaeology, Oxford For some samples interference may persist (even with University, 6 Keble Road, Oxford OX1 3QJ, U.K. annealing) after laboratory irradiation in excess of 411
412
P.L. Smart et al.
TABLE 1. Speleothem sample locality, uranium series age, ESR ED and ESR age for samples discussed in text. Errors are +1 standard deviation. In all uranium series analyses the 23°Th/232Th ratio was greater than 20 except where marked
23°Th/234U age (ka)
Sample
Locality
UEA780527-3/5 SU13-80 CW7-81A T31-79B T31-79A SP5-84A KC4-83C T3-84D T3-84A 226h21 226h3 211e2 211e8 211f2 211f3 T14-79 CW3-81A GO1-B T34-79C CW28-81A VC2-84 T25-79C T22-79I SP3-82A SP1-82 CFPM8-85 CFPM5-85 CFPM10-85
GB, Cave, England Uamh an Tartair, Scotland Clearwater Cave, Sarawak Cueva del Agua, Spain Cueva del Agua, Spain Simpson's Pot, England Kents Cavern, England Cueva del Agua, Spain Cueva del Agua, Spain Pontnewydd Cave, Wales Ponmewydd Cave, Wales Caune de l'Arago, France Caune de l'Arago, France Caune de l'Arago, France Caune de l'Arago, France Cueva del Agua, Spain Clearwater Cave, Sarawak Great Oone's Hole, England Cueva del Agua, Spain Clearwater Cave, Sarawak Victoria Cave, England Cueva del Agua, Spain Cueva del Agua, Spain Simpson's Pot, England Simpson's Pot, England Cova de Fum, Spain Cova de Fum, Spain Cova de Fum, Spain
10.5 + 0.6 5.5 + 0.8 19.7 + 1.8 36.3 + 2.8 104.0 + 9.2 85.0 + 3.0 99.5 ~983 84.3 +8_7175 116~ 1 205+3425* 205~* 178 +1~ ~1 251 ~4 sl >254* >254* 255~256 >320 >3505 >350 >350 >350 >350:~ >350:~ -->267 >267 >267
ESR ED (Gy) 2.1 0.4 9 22.3 32.3 27 18.0 13.7 24.6 176 183 69 108 184 191 116 226 245 201 77 421 512 430 147.7 206 100 105 99
+ 0.5 + 0.6 + 4 __+_2.2 +__ 1.0 _+ 8 + 3.0 _+ 1.2 + 0.5 + 20 + 10 + 12 _+ 6 __+ 12 + 16 + 22 + 8 + 16 + 27 + 7 + 24 + 12 + 30 +_ 4.4 + 6 + 136 + 2 + 3
ESR age (ka) 1.5 2.7 15 34.8 188 86 125 91 103 183 190 232 185 280 365 237 81 1060 940 850 290 3700 2400 286 424 163 256 228
+ 0.5 + 4.2 + 7 + 5.2 + 17 + 31 + 26 _+ 13 + 16 + 21 + 27 _+_+54 + 31 + 41 _+ 55 + 53 + 43 + 160 + 160 + 210 + 37 + 440 + 620 + 38% + 57% + 222t + 31~; + 29t
Key: *Average age and standard deviation for multiple analyses of this horizon. %First estimate with internal dose calculated from analysis of adjacent sample. ~:23aTh/232Th ratio less than 20.
Average e r r o r in E D ,
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Change in ED on annealing
-60
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FIG. 1. Percentage change in the measured ED for samples annealed at 120°C for several hours after gamma irradiation. The systematic error introduced by interferencb from the g = 2.0020 signal is much greater than the average error in fitting the growth curves to the data.
200 Gy. The apparent non-linearity of signal growth with dose in these samples is not due to saturation of the dating signal, but is an artefact of the interference. In these few samples only the linear portion of the growth was considered. The ED was measured by the additive gamma dose method using an error weighted linear fit to the data, the precision being derived from the combined least
squares uncertainties in the intercept and slope parameters. The best precision achieved was +2% (Fig. 2) with a median of +7%, but for some samples the uncertainty was so high that they were effectively undateable. The precision is predominantly sample dependent being affected by two main factors; the signal strength (controlled by both the natural ED and the sensitivity to gamma irradiation) and the presence
413
Intercomparison of ESR and Uranium Series Ages 150
32 136
tt
2£ C3 W
"6
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• o~%,__,g =2.0005 Low "6 sensitivity
a. 10
•
•
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440
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FIG. 2. The precision of the ED determination expressed as a function of its magnitude. The arrows indicate four samples with very large percentage errors.
of interfering signals (for instance that at g = 2.0025) which are not removed by annealing (Smith et al., 1985a) (Fig. 2). DOSIMETRY AND ESR AGE DETERMINATION
tration and the 234U/238U and 23°Th/234U activity ratios (from alpha spectrometry) using the method of Wintle (1978) and the uranium series age. For infinite uranium series ages and those for which the internal dose contributed more than a few percent, an iterative procedure was employed, calculating an initial ESR age and then using this to recalculate the internal dose and an improved ESR age. The gamma component of the internal dose was corrected for the difference between the actual speleothem size and the infinite matrix dose. The major uncertainty in calculation of the internal dose arises from uncertainty in the a-value. We have used a value of 0.3 + 0.1 for all samples, which covers the range derived using a thick sample irradiation technique (Smith et al., 1985b) and is consistent with the mean value of 0.34 + 0.08 obtained by Gr/in (1985). Although the a-value has a large uncertainty, this does not carry through significantly to the ESR age because the internal alpha dose is typically only a few per cent of the total dose. The quoted error for the ESR age was calculated by combining the random and systematic errors outlined above for the ED, external dose and internal dose. The systematic errors are particularly significant in any intercomparison of dating methods, and a +6% error was also included to cover that arising from calibration of the gamma source used for the laboratory irradiations. The uncertainty in the ESR ages derived was typically + 15%, and was rarely better than + 10%. This does not compare favourably with uranium series dates less than 200 ka, which have uncertainties generally between 5 and 10%, but is at least as good as older uranium series dates, for which the errors become very large as equilibrium is approached.
In order to reduce errors derived from uncertainties in the internal dose from the speleothem (see below), samples in close association with sediments were selected for this study. The external gamma dose, which is dominant in such situations, was measured by STRATIGRAPHIC CONCORDANCE OF ESR DATES in situ dosimetry using CaSO4:Dy and CaF2 thermoluminescence dosimeters exposed for at least one year. Stratigraphic concordance, both within a single At some sites a small cosmic ray contribution was also speleothem and between speleothems which are interpresent (Prescott and Hutton, 1988). To correct for bedded with other sediments, provides a simple and differences in the radiation sensitivity of quartz and robust test of the precision of ESR ages. Figure 3 calcite, the 'quartz equivalent' dose was multiplied by 1.05 (Fain et al., 1985). Corrections were also made for Hiatus 1 35gamma attenuation in the outer layers of speleothem calcite (which were removed to eliminate external "t " r\f alpha and beta radiation) using the measurements of 30Debenham and Aitken (1984). Radon plate-out on the I speleothems was assumed to be negligible. The dosiI I meters gave very good internal consistency, but a 3 I systematic error of + 10% was assigned to the external I 25gamma dose to account for inaccuracies in calibration I I and dose rate corrections. In a few cases radiometric I assays of the sediment were used to calculate infinite matrix dose rates (Bell, 1979; Nambi and Aitken, 20I , - F F F '1, 1986), which were corrected for layering (Aitken, 1985) I i I 510 i i and attenuation. The agreement between these esti0 100 150 200 mm TOD Base mates and those derived from in situ dosimetry was satisfactory, but the latter were preferred. FIG. 3. ED measurements along the growth axis of a single The internal alpha, beta and gamma doses were ,stalagmite, T31-79 from Cueva del Agua, Santander, Spain. The stalagmite has three pronounced growth hiatuses, as indicated. calculated from measurements of uranium concen-
--4
',
414
P.L. Smart et al.
presents values of ED for small samples taken from along the growth axis of a single stalagmite. As would be expected there is a decrease in ED up the axis, with the most significant differences being across the growth hiatuses indicated by breaks in crystal deposition. Deviations from the general pattern may relate to a pocket of sandy sediment preserved adjacent to the basal unit, and to variations in the uranium concentration between samples (work in progress). For three speleothems, T31-79, T3-84 and 226h21/226h3, ESR ages have been obtained for sequential samples (Table 1) and are found to be stratigraphically concordant. A more severe test arises when the speleothems are interbedded with other deposits, which give rise to different external doses and therefore also provide a test of the dosimetry. At Simpson's Pot, samples were taken from a complex interbedded flowstone sequence. The basal date on bedrock was 424 + 57 ka, that for an intermediate horizon 286 + 38, while that for a second flowstone deposited after erosion of the earlier deposits was 85 + 31 ka. In the case of samples from the lowest stalagmite floor in Caune de l'Arago, kindly provided by N. Debenham, samples 211e8, 211f2 and 211f3 give ages in the correct stratigraphic sequence, as does the
overlying sample 211e2 if account is taken of the +1 standard deviation errors (Table 1). Preliminary ages for samples from Cova de Fum, Mallorca, where a 4 m sequence of flowstones overlies a bone breccia with the Plio-Pleistocene mammal Myotragus antiquus (Gines and Antoni Fiol, 1981), show similar ages for the middle and base of the sequence (CFPM5-85 and CFPM10-85), and are also much younger than would be expected (CFPM8-85). This is probably due to recrystallisation of the samples which appears to be a major limitation for ESR datings of speleothems, as also reported by Griin (1985). Recrystallisation in older samples is often indicated by very low sensitivities for the g = 2.0005 signal (giving the poor precision for CFPM8-85) and in some cases by the presence of a nonradiation sensitive signal at g = 1.9994 + 0.0003. INTERCOMPARISON OF ESR, URANIUM SERIES AND GEOLOGICAL AGES Intercomparison with the well established radiometric 23°Th/234Utechnique provides a useful check for the reliability and bias of ESR ages. The data are presented both in Table 1 and graphically in Fig. 4. All
400-
300-
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Uncorrected for I exCtheang~°ne I
,IJ II
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1;o
2;0
Uranium series age ka
FIG. 4. Comparisonof ESR and uraniumseries ages. Infiniteuraniumseries agesare shownarrowed (as are ESR ages greater than 400 ka) with ESR ages plotted at the lower limit. Errors are + 1 standard deviation.
Intercomparison of ESR and Uranium Series Ages
415
uranium series results are uncorrected for contami- values. This problem also affected the ages for T31-79, nation by detrital thorium, but the 23°Th/232Th ratio is and is a significant limitation in ESR dating, which may greater than 20 in all but 2 cases. For ages less than only be partially overcome by careful geomorphic 10 ka the two ESR ages obtained are underestimates, reconstruction and more general in situ dosimetry. Two other noteable old dates were obtained in this and there is a considerable uncertainty due to the limited signal strength and small ED. Thereafter the study for GO1B (1.06 + 0.16Ma) and CW28-81A agreement between the two techniques is good, (0.85 + 0.21 Ma). Geomorphic control is available for especially when the substantial variation reported for the latter (from uranium series ages and remnant multiple uranium series dates at Pontnewydd and palaeomagnetism of sediments) and the agreement Caune de l'Arago is considered (see discussion in between the predicted date of 0.83 Ma and the ESR Debenham and Aitken, 1984). The gradient of an error age is excellent. This suggests that where recrystallisweighted regression line fitted to the 14 points with ation has not occurred ESR offers considerable potenfinite uranium series dates did not significantly differ tial for dating the interval between 100 ka and 1.0 Ma. from 1 at the 95% confidence level. There is therefore CONCLUSIONS no significant systematic error in the ESR ages as, for example, would be the case if the mean life of the Because of intrinsic limitations in dosimetry, the g = 2.0005 signal was much less than the measured precision of ESR ages is generally not better than value of 6 x 10 7 years at 10°C. The results for samples with infinite uranium series _+15%. However, this technique offers a useful alternaages are also encouraging. Only 1 sample, CW3-81A tive to uranium series dating in the case of detritally from Sarawak, yielded a much younger ESR age than contaminated calcite (Smith et al., 1985b) and for older expected. In this case the g = 2.0005 signal contained dates approaching the uranium series limit. Despite an unstable component, which was indicated by a problems of recrystallisation in some samples, ESR decrease of more than 30% in the signal strength after offers the potential for dating speleothems as old as annealing for 15 min at 200°C (Smith et al., 1985a). 1 Ma. In these and other cases, however, care is This may also be a result of recrystallisation. The needed to establish that removal or deposition of validation of these older ESR dates remains problem- sediment adjacent to the speleothem has not signifiatic. Samples were collected from a site where speleo- cantly changed the external dose determined by present them was associated with the Brunhes/Matuyama day dosimetry, and that the calcite and sediment has boundary, but the dating signal was absent. A geo- remained closed to radionuclide migration. morphological approach was therefore adopted. Incision of the major rivers draining karst areas ACKNOWLEDGEMENTS causes the progressive abandonment of higher cave We would like to thank Martin Aitken and Nick Debenham for passages and the development of new drainage routes to the lower base-level. The rate of incision can be help with source calibration and contribution of samples from Pontnewydd and Caune de l'Arago. Martin Aitken has also permitted estimated by determining the age of the earliest the continuing use of laboratory facilities for the measurement of TL speleothems deposited in cave passages at a particular dosimeters. This study was supported by Research Grant GR3/5049 height above the resurgence. For the Cueva del Agua from the Natural Environment Research Council. in Northern Spain the rate of incision was estimated as REFERENCES 0.3 m/ka using this technique (Smart, 1986). Table 2 compares the predicted ages of the earliest speleothems deposited at 3 sites where ESR ages have been Aitken, M.J. (1985). Thermoluminescence Dating. Academic Press, London. obtained, using this long term rate. In all 3 cases the Bell, W.T. (1979). Thermoluminescence dating: Radiation dose-rate ESR ages calculated using current site dosimetry are data. Archaeometrv, 21,243-245. much too old. However, for both T22-79C and T25- Debenham, N.C. and Aitken, M.J. (1984). Thermoluminescence dating of stalagmitic calcite. Archaeometrv, 26, 155-170. 79A there is evidence of burial of the samples in a Fain, J., Erramli, H., Miallier, D., Montret, M. and Sanzelle, S. sediment fill which has now been removed. Recal(1985). Environmental gamma dosimetry using TL dosimeters: Efficiency and absorption calculations. Nuclear Tracks, 10, 639culation of the ages with external dose rates derived 646. from in situ dosimetry in similar sediment fills gives Gines, A. and Antoni Fiol, L. (1981). Estratigraphia del yacmiento ages in much better agreement with the predicted de la Cova des Fum. Endins, 8, 25-42. "FABLE 2. Comparison of ESR ages using present dosimetry with those assuming burial in sediment, and with ages predicted using long-term rates of base-level incision derived from uranium series dating: Cueva del Agua, Spain. Figures are M a and errors are + one standard deviation Sample
Present dosimetry
Sediment burial
Predicted
T22-79I T25-79C T34-79C
2.40 + 0.62 3.70 + 0.44 0.94 + 0.16
0.60 4- 0.09 0.62 + 0.07 0.28 _+ 0.05
0.57 0.60 0.37
Grtin, R. (1985). Beitrage zur ESR-Datierung. SonderverOffentlichungen des Geologischen Instituts der Universitiit zu K6ln, 59, 1-157. Hennig, G.J. and Griin, R. (1983). ESR dating in Quaternary geology. Quaternary Science Reviews, 2, 157-238. Hennig, G.J., Herr, W. and Weber, E. (1983). Electron spin resonance dating of speleothem. In: de Lumley, H. and Labeyrie, J. (eds), Absolute Dating and Isotope Analysis in Prehistorv -Methods and Limits, (proceedings in press). Ikeya, M. (1975). Dating a stalactite by electron paramagnetic resonance. Nature, 255, 48-50. Nambi, K.S.V. and Aitken, M.J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometry, 28, 202-205.
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Prescott, J.R. and Hutton, J.T. (1988). Cosmic ray and gamma dosimetry for TL and ESR. Nuclear Tracks and Radiation Measurements, 14, 223-227. Smart, P.L. (1986). Origin and development of glacio-karst closed depressions in the Picos de Europa, Spain. Zeitschrift fiir Geomorphologie, 30, 423-443. Smith, B.W., Smart, P.L. and Symons, M.C.R. (1985a). ESR signals in a variety of speleothem calcites and their suitability for dating. Nuclear Tracks, 10, 837-844. Smith, B.W., Smart, P.L., Andrews, J.N. and Symons, M.C.R. (1985b). ESR dating of detritally contaminated calcites. In: Ikeya, M, and Miki, T. (eds), ESR Dating and Dosimetry, pp. 49-59. Ionics, Tokyo. Smith, B.W., Smart, P.L. and Symons, M.C.R. (1986). A routine
ESR technique for dating calcite speleothems. Radiation Protection Dosimetry, 17, 241-245. Wintle, A.G. (1978). A thermoluminescence dating study of some Quaternary calcite: Potential and problems, Canadian Journal of Earth Sciences, 15, 1977-1986. Yokoyama, Y., Quaegebeur, J.P., Bibron, R., Leger, C., Chappal, N., Michelot, C., Shen, G.J. and Nguyen, H.V. (1983). ESR dating of stalagmites of the Caune de l'Arago, Grotte du Zazaret, Grotte du Vallonet and Alan Pie Lombard: A comparison with U-Th method. PACT, 9, 381-389. Zeller, E.J., Levy, P.W. and Mattern, P.L. (1967). Geologic dating by electron spin resonance. In: Proceedings of the Symposium on Radioactive Dating and Low-Level Counting, pp. 531-540. IAEA, Wein.
0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc
AN I N T E R C O M P A R I S O N OF ESR AND U R A N I U M SERIES AGES FOR Q U A T E R N A R Y S P E L E O T H E M CALCITES P.L. Smart,* B.W. Smith,*¶ H. C h a n d r a , t J.N. Andrews$ and M . C . R . S y m o n s t * Department of Geography, University of Bristol, Bristol BS8 1SS, U.K. t Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K. $ Department of Chemistry, University of Bath, Bath BA2 7A Y, U.K.
Equivalent dose (ED) estimates for speleothem calcite derived by ESR measurements of the g = 2.0005 signal have a typical precision of + 7 % (minimum +2%), and depend on the signal strength, radiation sensitivity and the presence of interfering signals. Underestimation of ED will occur if the crushing signal at g = 2.0001 is not removed by acid etching, while overestimation may be caused if the short-lived radiation induced interfering signal at g = 2.0020 is not removed by annealing. Quoted errors for ESR ages are rarely better than + 10% and are typically + 15% because of uncertainties in ED, calibration of dosimeters and gamma source, and in the a-value. Serial analyses along the growth axis of single speleothems, and between speleothems interbedded in a sequence demonstrate excellent stratigraphic concordance. Intercomparison of ESR and uranium series ages for samples derived from 11 geographically diverse sites showgood agreement with no indication of systematic error. Although validation is more difficult for samples beyond the limit of 23°Th/234U dating, the results appear encouraging, but recrystallisation and changes in the external radiation dose may cause problems.
INTRODUCTION
span the 300 ka range of uranium series dating (Table 1). A number of samples with infinite uranium series ESR (electron spin resonance) dating of speleothem ages were also included from sites where faunal, calcite was first described by Ikeya (1975), the tech- geomorphic or palaeomagnetic data suggested a much nique having been initially proposed by Zeller et al. greater age. Detritally contaminated speleothems were (1967). Briefly, the technique involves measurement of not included, as they cannot be reliably dated by the electron spin concentration in the crystal lattice, uranium series methods. this being proportional to the radiation dose that the The sample preparation and instrumental methods calcite has received since formation (the equivalent used have previously been described in detail (Smith et dose, ED). Measurement of the annual radiation dose al., 1986), but two important aspects of the methodolto the sample, and determination of the spin concen- ogy should be stressed. First, the calcite must be acid tration that this produces allows calculation of the age. etched (10% acetic acid for 2 min) to remove the outer In this paper we examine the validity of ESR dates surface of the grains which have been damaged by on speleothem calcite using two criteria: stratigraphic crushing. Failure to do this results in interference from concordance and agreement with an independent a crushing signal at g = 2.0001 + 0.0002, resulting in dating method, in this case uranium series dating. The an underestimation of the ED, as also reported by results of similar previous studies have not been very Hennig and Griin (1983). Furthermore, because of the encouraging (Yokoyama et al., 1983; Hennig et al., low stability of the crushing signal, an erroneously short 1983; Hennig and Grtin, 1983; Griin, 1985), but these mean life will be derived in any determination of the were limited to particular sites where specific problems stability of the g = 2.0005 signal. of signal stability occur (Griin, 1985), and with respect The second important aspect of methodology into development of reliable techniques. We now con- volves the use of annealing for several hours at 120°C sider the methods detailed previously by Smith et al. after irradiation to eliminate interference from a short(1986), to be reliable, reproducible and routine. Only lived (mean life several months), radiation-induced the ESR signal at g = 2.0005 (+0.0003) is suitable for signal at g = 2.0020 + 0.0003. This interferes with the dating (Hennig and Griin, 1983; Smith et al., 1985a). It dating signal and causes a systematic overestimation of has a mean life of 6 × 107 years at 10°C (2 samples), the ED (Smith et al., 1985b, 1986). It should be noted and grows linearly with dose to at least 400 Gy. that this procedure differs from the more severe annealing used by Yokoyama et al. (1983), which SAMPLE SELECTION AND ESR MEASUREMENT totally removes the g = 2.0005 dating signal. For 19 samples, the decrease in ED on annealing ranged from The samples for this study were derived from a 0 to 60% with a mean reduction just under 20% (Fig. number of diverse geographic locations, and selected to 1). This interference cannot be removed instrumentally and is not affected by the type of curve-fitting applied. ¶Present address: Research Laboratory for Archaeology, Oxford For some samples interference may persist (even with University, 6 Keble Road, Oxford OX1 3QJ, U.K. annealing) after laboratory irradiation in excess of 411
412
P.L. Smart et al.
TABLE 1. Speleothem sample locality, uranium series age, ESR ED and ESR age for samples discussed in text. Errors are +1 standard deviation. In all uranium series analyses the 23°Th/232Th ratio was greater than 20 except where marked
23°Th/234U age (ka)
Sample
Locality
UEA780527-3/5 SU13-80 CW7-81A T31-79B T31-79A SP5-84A KC4-83C T3-84D T3-84A 226h21 226h3 211e2 211e8 211f2 211f3 T14-79 CW3-81A GO1-B T34-79C CW28-81A VC2-84 T25-79C T22-79I SP3-82A SP1-82 CFPM8-85 CFPM5-85 CFPM10-85
GB, Cave, England Uamh an Tartair, Scotland Clearwater Cave, Sarawak Cueva del Agua, Spain Cueva del Agua, Spain Simpson's Pot, England Kents Cavern, England Cueva del Agua, Spain Cueva del Agua, Spain Pontnewydd Cave, Wales Ponmewydd Cave, Wales Caune de l'Arago, France Caune de l'Arago, France Caune de l'Arago, France Caune de l'Arago, France Cueva del Agua, Spain Clearwater Cave, Sarawak Great Oone's Hole, England Cueva del Agua, Spain Clearwater Cave, Sarawak Victoria Cave, England Cueva del Agua, Spain Cueva del Agua, Spain Simpson's Pot, England Simpson's Pot, England Cova de Fum, Spain Cova de Fum, Spain Cova de Fum, Spain
10.5 + 0.6 5.5 + 0.8 19.7 + 1.8 36.3 + 2.8 104.0 + 9.2 85.0 + 3.0 99.5 ~983 84.3 +8_7175 116~ 1 205+3425* 205~* 178 +1~ ~1 251 ~4 sl >254* >254* 255~256 >320 >3505 >350 >350 >350 >350:~ >350:~ -->267 >267 >267
ESR ED (Gy) 2.1 0.4 9 22.3 32.3 27 18.0 13.7 24.6 176 183 69 108 184 191 116 226 245 201 77 421 512 430 147.7 206 100 105 99
+ 0.5 + 0.6 + 4 __+_2.2 +__ 1.0 _+ 8 + 3.0 _+ 1.2 + 0.5 + 20 + 10 + 12 _+ 6 __+ 12 + 16 + 22 + 8 + 16 + 27 + 7 + 24 + 12 + 30 +_ 4.4 + 6 + 136 + 2 + 3
ESR age (ka) 1.5 2.7 15 34.8 188 86 125 91 103 183 190 232 185 280 365 237 81 1060 940 850 290 3700 2400 286 424 163 256 228
+ 0.5 + 4.2 + 7 + 5.2 + 17 + 31 + 26 _+ 13 + 16 + 21 + 27 _+_+54 + 31 + 41 _+ 55 + 53 + 43 + 160 + 160 + 210 + 37 + 440 + 620 + 38% + 57% + 222t + 31~; + 29t
Key: *Average age and standard deviation for multiple analyses of this horizon. %First estimate with internal dose calculated from analysis of adjacent sample. ~:23aTh/232Th ratio less than 20.
Average e r r o r in E D ,
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+10
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f
r
-40
-20
Change in ED on annealing
-60
%
FIG. 1. Percentage change in the measured ED for samples annealed at 120°C for several hours after gamma irradiation. The systematic error introduced by interferencb from the g = 2.0020 signal is much greater than the average error in fitting the growth curves to the data.
200 Gy. The apparent non-linearity of signal growth with dose in these samples is not due to saturation of the dating signal, but is an artefact of the interference. In these few samples only the linear portion of the growth was considered. The ED was measured by the additive gamma dose method using an error weighted linear fit to the data, the precision being derived from the combined least
squares uncertainties in the intercept and slope parameters. The best precision achieved was +2% (Fig. 2) with a median of +7%, but for some samples the uncertainty was so high that they were effectively undateable. The precision is predominantly sample dependent being affected by two main factors; the signal strength (controlled by both the natural ED and the sensitivity to gamma irradiation) and the presence
413
Intercomparison of ESR and Uranium Series Ages 150
32 136
tt
2£ C3 W
"6
g
f
Interference from g=2.0025
• o~%,__,g =2.0005 Low "6 sensitivity
a. 10
•
•
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High "~ sensitivity No interference
2~)0
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440
I
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FIG. 2. The precision of the ED determination expressed as a function of its magnitude. The arrows indicate four samples with very large percentage errors.
of interfering signals (for instance that at g = 2.0025) which are not removed by annealing (Smith et al., 1985a) (Fig. 2). DOSIMETRY AND ESR AGE DETERMINATION
tration and the 234U/238U and 23°Th/234U activity ratios (from alpha spectrometry) using the method of Wintle (1978) and the uranium series age. For infinite uranium series ages and those for which the internal dose contributed more than a few percent, an iterative procedure was employed, calculating an initial ESR age and then using this to recalculate the internal dose and an improved ESR age. The gamma component of the internal dose was corrected for the difference between the actual speleothem size and the infinite matrix dose. The major uncertainty in calculation of the internal dose arises from uncertainty in the a-value. We have used a value of 0.3 + 0.1 for all samples, which covers the range derived using a thick sample irradiation technique (Smith et al., 1985b) and is consistent with the mean value of 0.34 + 0.08 obtained by Gr/in (1985). Although the a-value has a large uncertainty, this does not carry through significantly to the ESR age because the internal alpha dose is typically only a few per cent of the total dose. The quoted error for the ESR age was calculated by combining the random and systematic errors outlined above for the ED, external dose and internal dose. The systematic errors are particularly significant in any intercomparison of dating methods, and a +6% error was also included to cover that arising from calibration of the gamma source used for the laboratory irradiations. The uncertainty in the ESR ages derived was typically + 15%, and was rarely better than + 10%. This does not compare favourably with uranium series dates less than 200 ka, which have uncertainties generally between 5 and 10%, but is at least as good as older uranium series dates, for which the errors become very large as equilibrium is approached.
In order to reduce errors derived from uncertainties in the internal dose from the speleothem (see below), samples in close association with sediments were selected for this study. The external gamma dose, which is dominant in such situations, was measured by STRATIGRAPHIC CONCORDANCE OF ESR DATES in situ dosimetry using CaSO4:Dy and CaF2 thermoluminescence dosimeters exposed for at least one year. Stratigraphic concordance, both within a single At some sites a small cosmic ray contribution was also speleothem and between speleothems which are interpresent (Prescott and Hutton, 1988). To correct for bedded with other sediments, provides a simple and differences in the radiation sensitivity of quartz and robust test of the precision of ESR ages. Figure 3 calcite, the 'quartz equivalent' dose was multiplied by 1.05 (Fain et al., 1985). Corrections were also made for Hiatus 1 35gamma attenuation in the outer layers of speleothem calcite (which were removed to eliminate external "t " r\f alpha and beta radiation) using the measurements of 30Debenham and Aitken (1984). Radon plate-out on the I speleothems was assumed to be negligible. The dosiI I meters gave very good internal consistency, but a 3 I systematic error of + 10% was assigned to the external I 25gamma dose to account for inaccuracies in calibration I I and dose rate corrections. In a few cases radiometric I assays of the sediment were used to calculate infinite matrix dose rates (Bell, 1979; Nambi and Aitken, 20I , - F F F '1, 1986), which were corrected for layering (Aitken, 1985) I i I 510 i i and attenuation. The agreement between these esti0 100 150 200 mm TOD Base mates and those derived from in situ dosimetry was satisfactory, but the latter were preferred. FIG. 3. ED measurements along the growth axis of a single The internal alpha, beta and gamma doses were ,stalagmite, T31-79 from Cueva del Agua, Santander, Spain. The stalagmite has three pronounced growth hiatuses, as indicated. calculated from measurements of uranium concen-
--4
',
414
P.L. Smart et al.
presents values of ED for small samples taken from along the growth axis of a single stalagmite. As would be expected there is a decrease in ED up the axis, with the most significant differences being across the growth hiatuses indicated by breaks in crystal deposition. Deviations from the general pattern may relate to a pocket of sandy sediment preserved adjacent to the basal unit, and to variations in the uranium concentration between samples (work in progress). For three speleothems, T31-79, T3-84 and 226h21/226h3, ESR ages have been obtained for sequential samples (Table 1) and are found to be stratigraphically concordant. A more severe test arises when the speleothems are interbedded with other deposits, which give rise to different external doses and therefore also provide a test of the dosimetry. At Simpson's Pot, samples were taken from a complex interbedded flowstone sequence. The basal date on bedrock was 424 + 57 ka, that for an intermediate horizon 286 + 38, while that for a second flowstone deposited after erosion of the earlier deposits was 85 + 31 ka. In the case of samples from the lowest stalagmite floor in Caune de l'Arago, kindly provided by N. Debenham, samples 211e8, 211f2 and 211f3 give ages in the correct stratigraphic sequence, as does the
overlying sample 211e2 if account is taken of the +1 standard deviation errors (Table 1). Preliminary ages for samples from Cova de Fum, Mallorca, where a 4 m sequence of flowstones overlies a bone breccia with the Plio-Pleistocene mammal Myotragus antiquus (Gines and Antoni Fiol, 1981), show similar ages for the middle and base of the sequence (CFPM5-85 and CFPM10-85), and are also much younger than would be expected (CFPM8-85). This is probably due to recrystallisation of the samples which appears to be a major limitation for ESR datings of speleothems, as also reported by Griin (1985). Recrystallisation in older samples is often indicated by very low sensitivities for the g = 2.0005 signal (giving the poor precision for CFPM8-85) and in some cases by the presence of a nonradiation sensitive signal at g = 1.9994 + 0.0003. INTERCOMPARISON OF ESR, URANIUM SERIES AND GEOLOGICAL AGES Intercomparison with the well established radiometric 23°Th/234Utechnique provides a useful check for the reliability and bias of ESR ages. The data are presented both in Table 1 and graphically in Fig. 4. All
400-
300-
&
_._Z/ 200 -
Uncorrected for I exCtheang~°ne I
,IJ II
JC |,
!
/I,'
100Unstable component ,; present in : ...... ~g =2.0005 signal
1;o
2;0
Uranium series age ka
FIG. 4. Comparisonof ESR and uraniumseries ages. Infiniteuraniumseries agesare shownarrowed (as are ESR ages greater than 400 ka) with ESR ages plotted at the lower limit. Errors are + 1 standard deviation.
Intercomparison of ESR and Uranium Series Ages
415
uranium series results are uncorrected for contami- values. This problem also affected the ages for T31-79, nation by detrital thorium, but the 23°Th/232Th ratio is and is a significant limitation in ESR dating, which may greater than 20 in all but 2 cases. For ages less than only be partially overcome by careful geomorphic 10 ka the two ESR ages obtained are underestimates, reconstruction and more general in situ dosimetry. Two other noteable old dates were obtained in this and there is a considerable uncertainty due to the limited signal strength and small ED. Thereafter the study for GO1B (1.06 + 0.16Ma) and CW28-81A agreement between the two techniques is good, (0.85 + 0.21 Ma). Geomorphic control is available for especially when the substantial variation reported for the latter (from uranium series ages and remnant multiple uranium series dates at Pontnewydd and palaeomagnetism of sediments) and the agreement Caune de l'Arago is considered (see discussion in between the predicted date of 0.83 Ma and the ESR Debenham and Aitken, 1984). The gradient of an error age is excellent. This suggests that where recrystallisweighted regression line fitted to the 14 points with ation has not occurred ESR offers considerable potenfinite uranium series dates did not significantly differ tial for dating the interval between 100 ka and 1.0 Ma. from 1 at the 95% confidence level. There is therefore CONCLUSIONS no significant systematic error in the ESR ages as, for example, would be the case if the mean life of the Because of intrinsic limitations in dosimetry, the g = 2.0005 signal was much less than the measured precision of ESR ages is generally not better than value of 6 x 10 7 years at 10°C. The results for samples with infinite uranium series _+15%. However, this technique offers a useful alternaages are also encouraging. Only 1 sample, CW3-81A tive to uranium series dating in the case of detritally from Sarawak, yielded a much younger ESR age than contaminated calcite (Smith et al., 1985b) and for older expected. In this case the g = 2.0005 signal contained dates approaching the uranium series limit. Despite an unstable component, which was indicated by a problems of recrystallisation in some samples, ESR decrease of more than 30% in the signal strength after offers the potential for dating speleothems as old as annealing for 15 min at 200°C (Smith et al., 1985a). 1 Ma. In these and other cases, however, care is This may also be a result of recrystallisation. The needed to establish that removal or deposition of validation of these older ESR dates remains problem- sediment adjacent to the speleothem has not signifiatic. Samples were collected from a site where speleo- cantly changed the external dose determined by present them was associated with the Brunhes/Matuyama day dosimetry, and that the calcite and sediment has boundary, but the dating signal was absent. A geo- remained closed to radionuclide migration. morphological approach was therefore adopted. Incision of the major rivers draining karst areas ACKNOWLEDGEMENTS causes the progressive abandonment of higher cave We would like to thank Martin Aitken and Nick Debenham for passages and the development of new drainage routes to the lower base-level. The rate of incision can be help with source calibration and contribution of samples from Pontnewydd and Caune de l'Arago. Martin Aitken has also permitted estimated by determining the age of the earliest the continuing use of laboratory facilities for the measurement of TL speleothems deposited in cave passages at a particular dosimeters. This study was supported by Research Grant GR3/5049 height above the resurgence. For the Cueva del Agua from the Natural Environment Research Council. in Northern Spain the rate of incision was estimated as REFERENCES 0.3 m/ka using this technique (Smart, 1986). Table 2 compares the predicted ages of the earliest speleothems deposited at 3 sites where ESR ages have been Aitken, M.J. (1985). Thermoluminescence Dating. Academic Press, London. obtained, using this long term rate. In all 3 cases the Bell, W.T. (1979). Thermoluminescence dating: Radiation dose-rate ESR ages calculated using current site dosimetry are data. Archaeometrv, 21,243-245. much too old. However, for both T22-79C and T25- Debenham, N.C. and Aitken, M.J. (1984). Thermoluminescence dating of stalagmitic calcite. Archaeometrv, 26, 155-170. 79A there is evidence of burial of the samples in a Fain, J., Erramli, H., Miallier, D., Montret, M. and Sanzelle, S. sediment fill which has now been removed. Recal(1985). Environmental gamma dosimetry using TL dosimeters: Efficiency and absorption calculations. Nuclear Tracks, 10, 639culation of the ages with external dose rates derived 646. from in situ dosimetry in similar sediment fills gives Gines, A. and Antoni Fiol, L. (1981). Estratigraphia del yacmiento ages in much better agreement with the predicted de la Cova des Fum. Endins, 8, 25-42. "FABLE 2. Comparison of ESR ages using present dosimetry with those assuming burial in sediment, and with ages predicted using long-term rates of base-level incision derived from uranium series dating: Cueva del Agua, Spain. Figures are M a and errors are + one standard deviation Sample
Present dosimetry
Sediment burial
Predicted
T22-79I T25-79C T34-79C
2.40 + 0.62 3.70 + 0.44 0.94 + 0.16
0.60 4- 0.09 0.62 + 0.07 0.28 _+ 0.05
0.57 0.60 0.37
Grtin, R. (1985). Beitrage zur ESR-Datierung. SonderverOffentlichungen des Geologischen Instituts der Universitiit zu K6ln, 59, 1-157. Hennig, G.J. and Griin, R. (1983). ESR dating in Quaternary geology. Quaternary Science Reviews, 2, 157-238. Hennig, G.J., Herr, W. and Weber, E. (1983). Electron spin resonance dating of speleothem. In: de Lumley, H. and Labeyrie, J. (eds), Absolute Dating and Isotope Analysis in Prehistorv -Methods and Limits, (proceedings in press). Ikeya, M. (1975). Dating a stalactite by electron paramagnetic resonance. Nature, 255, 48-50. Nambi, K.S.V. and Aitken, M.J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometry, 28, 202-205.
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Prescott, J.R. and Hutton, J.T. (1988). Cosmic ray and gamma dosimetry for TL and ESR. Nuclear Tracks and Radiation Measurements, 14, 223-227. Smart, P.L. (1986). Origin and development of glacio-karst closed depressions in the Picos de Europa, Spain. Zeitschrift fiir Geomorphologie, 30, 423-443. Smith, B.W., Smart, P.L. and Symons, M.C.R. (1985a). ESR signals in a variety of speleothem calcites and their suitability for dating. Nuclear Tracks, 10, 837-844. Smith, B.W., Smart, P.L., Andrews, J.N. and Symons, M.C.R. (1985b). ESR dating of detritally contaminated calcites. In: Ikeya, M, and Miki, T. (eds), ESR Dating and Dosimetry, pp. 49-59. Ionics, Tokyo. Smith, B.W., Smart, P.L. and Symons, M.C.R. (1986). A routine
ESR technique for dating calcite speleothems. Radiation Protection Dosimetry, 17, 241-245. Wintle, A.G. (1978). A thermoluminescence dating study of some Quaternary calcite: Potential and problems, Canadian Journal of Earth Sciences, 15, 1977-1986. Yokoyama, Y., Quaegebeur, J.P., Bibron, R., Leger, C., Chappal, N., Michelot, C., Shen, G.J. and Nguyen, H.V. (1983). ESR dating of stalagmites of the Caune de l'Arago, Grotte du Zazaret, Grotte du Vallonet and Alan Pie Lombard: A comparison with U-Th method. PACT, 9, 381-389. Zeller, E.J., Levy, P.W. and Mattern, P.L. (1967). Geologic dating by electron spin resonance. In: Proceedings of the Symposium on Radioactive Dating and Low-Level Counting, pp. 531-540. IAEA, Wein.