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ESR signals in a variety of speleothem calcites and their suitability for dating
Nucl. Tracks, Vol. 10, Nos 4--6, pp. 837-844, 1985 Printed in Great Britain
0191-278X/8553.00+0.00 Pergamon Press Ltd.
ESR SIGNALS IN A VARIETY OF SPELEOTHEM CALCITES A N D THEIR SUITABILITY FOR DATING B. W. SMITH,P. L. SMART* and M. C. R. SYMON$ Department of Chemistry, University of Leicester, Leicester LEI 7RH, U.K.. (Received 2 November 1984; in revised form 30 January 1985)
Abstract--The natural ESR signals observed in speleothems collected from different geographical areas have been examined. A study was made of the best ways of obtaining measurement reproducibility and avoiding interference from signals induced by irradiation and sample preparation. Annealing studies showed that the only calcite ESR signal stable enough for dating the past few hundred thousand years, at g = 2.0005 + 0.0003, occasionally had a less stable component which may produce a young date. Finally, equivalent doses calculated from each radiation sensitive signal are compared with those expected from dosimetry measurements and U/Th dates on the same speleothems. 1. I N T R O D U C T I O N UNTIL recently, the majority of Electron Spin Resonance (ESR) dates from speleothems have been reported from a single site; Caune de l'Arago in France (De Lumley, in preparation). Comparatively little work has been undertaken at other sites (Ikeya, 1975; Apers et al., 1980; Hennig et al., 1981). It was established that at least one of the ESR signals (at g ~ 2.0005 and designated h3) found in the Caune de l'Arago samples was suitable for dating (Hennig and Griin, 1983; Skinner, 1983). However, the results of exhaustive studies on the stabilities, and applicability to dating, of the signals in Caune de l'Arago calcite cannot necessarily be extended to all cave speleothems showing the same ESR signals. We have examined a wide variety of samples of different age, purity, crystal size, minerology and geographical location to determine the range of natural ESR signals present. Annealing studies and equivalent dose (ED) measurements on a selection of these samples have shown that the stability of individual ESR signals can vary from sample to sample.
2. E X P E R I M E N T A L T E C H N I Q U E S All measurements have been made at room temperature using a Varian El09 X-band spectrometer. The speleothems were gently crushed and the grain
size 125-250 # m selected. Grains of diameter greater than 2 5 0 # m tended to produce variations in .ESR signal intensity when rotated due to crystallite orientation effects. Grains smaller than 125 g m were found to give a slightly decreased signal strength, although not as great as the ,-,30% decrease reported by Karakostanogiou (1982). The main reason for rejecting the < 125 g m fraction was the creation, after gamma-irradiation, of a sharp signal at g = 2.0001 (+0.0002, half-width ~ 3 x 10-~T). This signal became stronger as the grain size decreased, an effect which has also been observed by Grfin and De Canniere (1984). It is of particular importance as it is superimposed on the g = 2.0005 signal and may not be detected if the magnetic field modulation is too large. The g = 2.0001 signal appears to be due to surface defects caused by crushing and is related to a short-lived signal (,,,80m in mean life) at g = 2.0030 ( + 0.0002) which transfers a variable fraction of its spin to the g = 2.0001 signal when it decays (Fig. 1). For all ED measurements in this study the 125-250#m grains were etched for 2min in 10% acetic acid, which effectively removes the defects producing the g = 2.0001 signal. This is a more severe etching than has normally been recommended for calcite (Wintle, 1978). Quartz tubes of 2 m m diameter were used to distribute 100 + 1 mg of each sample through the full depth of the spectrometer cavity. This reduces spec-
*Department of Geography, University of Bristol, Bristol BS8 1SS, U.K. 837
838
B . W . SMITH et al.
tral resolution, as the magnetic field is not precisely uniform throughout the cavity, but considerably improves reproducibility. The individual sample tubes were placed in an outer quartz tube which was accurately centred in the cavity before each set of measurements. As the instrument needed to be retuned for each sample, care was taken to ensure that variables such as the detector current remained constant. The reproducibility of signal amplitude using these techniques was better than 3%. All measurements for an ED determination were made on the same day but if comparisons needed to be made on different days the natural sample was used as a standard to allow for small drifts in instrument sensitivity. All samples were initially observed with a microwave power of 1 mW and a magnetic field modulation of 4 × 10 -5 T. We found no variations in ED with microwave power as long as the calcite was well-etched. The EDs listed in Table 1 were therefore measured using a power of 5 m W which improved the signal-to-noise ratio. At higher powers there is an
onset of saturation effects in the microwave absorption of the spins producing the g =2.0005 signal. The spectrometer gave first derivative absorption spectra which could be integrated twice by an interfaced computer to yield the area under individual absorption peaks. However, it was found that this gave extremely variable results and depended critically on the initial base-line used. Peak-to-peak measurements on the first derivative spectra were much more reproducible but measurements on different subsamples are only comparable providing the absorption peak width does not change. None of the signals for which EDs have been calculated showed a measurable change in width with irradiation, so the latter technique was employed. For gamma-irradiation using a 6°Co source, 150 mg aliquots of each sample were placed in gelatin capsules. The capsules were held in the centre of a 20 mm diameter marble pot which gave approximate secondary electron equilibrium. The dose-rate to calcite in this configuration was close to 1 Gy s -L.
c .E
i "i j ! w ¢,1
o
s'o
t6o
t~o
2bo
Minutes after irradiation
FIG. 1. The change in intensity of the g = 2.0030 and g = 2.0001 signals in sample T34-79A after a gamma-irradiation of 240 Gy. After 1 day g = 2.0001 has increased to a maximum value at the dashed
line, but it usually then fades over a period of months.
ESR SIGNALS IN SPELEOTHEM
839
CALCITES
Magnetic field (T) 0.3355
0.3360
0.3365
0.3370
I
I
I
I
h2
h 3
Sample T 4 0 - 7 9 A
'
2.0boo
'
2.0b40
'
' 2.0020 g-value
2.
Oi
O0
i
i
1.9980
i
i
1.9960
FIG. 2. The ESR spectra from three samples, showing between them all of the natural signals observed. All spectra were measured using a scan speed of 10 -3 T min-~ with a modulation amplitude of 4 x 10 -5 T and microwave power of 5 roW. The magnetic field values are typical for measurement frequencies of ~9.4 GHz. Sample T34-79A shows g = 2.0055 (h 0, g = 2.0036 (h2) and g = 2.0005 (h3). G01B gives the same signals superimposed on the broad g = 2.0040 signal. The small signal at g = 1.9994 + 0.0003 is also apparent. T40-79A shows additional signals at g = 2.0025 + 0.0003 and g = 1.9970 + 0.0003. 3. T H E S P E C T R A
OBSERVED
T h e r a n g e o f n a t u r a l signals present in the differential a b s o r p t i o n spectra was similar for samples from all g e o g r a p h i c a l locations. T h e signals are basically the same as those r e p o r t e d previously by
H e n n i g a n d Gr/.in (1983) for various c a r b o n a t e form a t i o n s a n d are illustrated in Fig. 2. T h e three signals, g = 2.0055 + 0.0003, p e a k - t o - p e a k w i d t h ~ 1 x 10-aT (commonly denoted h0, g =2.0036_+0.0002, ,,-8 x 1 0 - S T (h2) and g = 2.0005 + 0.0003, ~ 2 x 1 0 - 4 T (h3), h a v e been
840
B. W. S M I T H et al. Table 1. The equivalent doses measured from five calcite ESR signals E.D. (Gy) for signals at g-valt~_s sho%m
Sample
Origin
2.0040
SUI3-80
~
16 -+5
BATC2
MAJORCA
57 +-7
2SIC8
DX~ND
*
CWT-81A
SARA~LK
-
2STC6
ENGLAkD
T14-79
SPAIN
PHI-82B
~
T40-79A
2.0025
2.0036
7-+2 -
93 -+14
2.0005
1.9970
EXl:mCt(~
COZTL~"r.(~
~D Limits (Gy)
U/lh Date (ky)
1_+3
10_+7
3_+2
5.5+_0.8
6-+4
170 + 60
5+3
9.2+_0.6
18_+5
26 +-14
9+- 5
II.5 t 1.4
18+_6
24 +-12
14 -+ 5
I00 -+ 60
-
19.7
+I ,8 -I .7
81 -+6
38_+9
I00 -+ 50
250
+51 -35
2_+6
149 +- 86
179
+65 -39
Saturated
2_+1
>80
315 -93
133 _+12
186 _+22
87 -+21
>460
430_110
2_+2
>80
> 350
I0 ± i0
275 -+30
20_+6
>60
>350
176 ± 18
175 _+25
174 ± 12
40-+5
-
-
77 _+8
-
SPAIN
-
-
122 _+18
CW3-81A
~
-
-
GOIB
~
165 -+50
>300
T34-79C
SPAIN
*
*
226h3
~
-
150 ± 30
128 ± 25
+ oo >320
*Signal small and obscured by other signals. --Signal not present in the natural sample.
Table 2. The localities of the samples listed in Table I OKIGIlq
TYPE Stance
SUI3-80
Umah a n Tartair, Sutherland, Scotlaz~
BA~C2
Cova de Na Barxa, Majorca, Spain
Fl~tor~
2S~C8
St. Cuthberts Swallet, Somerset, England
Flowstor~
CW7-8L%
Clearwater C a ~ ,
sarawak, Malaysia
2STC6
St. Cuthberts Swallet, Somerset, England
T14-79
Cueva del Agua, Santander, Spain
Stalagmite
PHI-82B
Picken's Hole, Somerset, England
T40-79A
Tere, Santander, Spain
Flo~to~ Stalagmite
CW3-8L%
Clearwater Cave, Sarawak, Malaysia
Stala~mtee
GOIB
Great Oor~s Hole, Somerset, England
Flc~cstxme
T34-79C
Cueva del Agua, Santander, Spain
Sta~te
226h3
P o n ~ ,
Flowstone
Wales
176
+29 -23
ESR SIGNALS IN SPELEOTHEM CALCITES discussed in detail in the Caune de l'Arago literature (Yokoyama et al., 1983). There is a small signal superimposed on the h3 signal at g = 1.9994__+0.0003 which has not been reported previously but its intensity is usually too small to be used for ED calculations. Some samples show a significant up-swing in the differential spectra at g = 2.0025 _ 0.0003 which extends to h3 and almost all have a small signal at g = 1.9970 + 0.0003. The latter signal is in the same region as a forbidden Mn 2+ transition line but appears even in samples with very little Mn 2÷ and grows rapidly with irradiation, whereas the Mn 2+ lines do not. When the etched calcite is not a pure white colour, there is always the presence of a broad signal at g = 2.0040 _+ 0.0005, width ~ 6 x 10-4T, which has been attributed to humic acids by Grfin and De Canniere (1984). All of the signals discussed show an onset of saturation in microwave absorption at different powers, indicating that they derive from independent spins. Exposure of 125-250/~m calcite grains to ambient lighting inside the laboratory windows for periods up to 6 months had no effect on any signal except g = 2.0036. This signal was reduced by 80~o after 4 days in the three samples in which it was found, but
I
"5
g = 2.0040
I
I
g = 2.0036
I
I
841
Hennig and Griin (1983) have reported a variability in its light stability for a range of calcites. Samples were prepared under red light and kept in light-tight containers, but a small exposure to shorter wavelength light during ESR measurement was unavoidable. The natural calcite samples were annealed in order to obtain information about the stability of each signal. All of the natural signals decrease on annealing except g = 2.0055 (h0, which initially increases as spins are transferred from other signals. Yokoyama et al. (1981) have used an annealing technique to transfer the spins from h2 (g = 2.0036) and h3 (g = 2.0005) to h~ before obtaining the ED from hi. This was not used in the present study because double integration of the differential ESR spectra clearly shows that the increase in h~ spins is often greater than the total of the h2 and h3 spins. This implies that spin is being transferred from trapping sites other than h2 and h3. Skinner (1983) has also shown that there are problems with extrapolating the non-linear h~ growth curve, Samples were heated for 20 rain at temperatures progressively increasing by 20°C intervals to 300°C. Figure 3 shows the resulting plot of the change in
/t'
t-
/
n"
/
ILl
n"
Ix
g= 2 . 0 0 0 5 g=
2.0055
100
200
300
T e m p e r a t u r e =C
F1G.3. The change in signal intensities from G01B after annealingfor 20 min at temperatures progressively increasing by 20~C, The measurements were made with a microwave power of 1 roW, so the relative intensities of the signals are different from those shown in Fig. 2. The g = 2.0055 signal is difficult to measure at low intensities because it falls on the peak ofg = 2.0040. Not all of the data points are shown. The error bars indicate the la uncertainties at selected temperatures.
842
B . W . SMITH et al. 1.0~5
~
}~
}
""-.--
Sample T40-79A
Sample CW3-81A
(~
0.2-
110
2'0 30 Annealing time minutes
40
gO
FIG. 4. The decrease in g = 2.0005 signal intensity with annealing at 200°C. Sample CW3-81A has a component which is less stable than is usual for g = 2.0005. signal intensities for sample G01B. In all samples examined, the most stable signals were g = 2.0005 and the broad organic signal at g = 2.0040 but the stabilities of individual signals varied from sample to sample. This became clearer when annealing was carried out over increasing time periods at the same temperature to obtain Arrhenius plots. The g = 2.0005 signal results for two samples at 200°C, which were annealed and measured at the same time, are shown in Fig. 4. Sample T40-79A shows a first order decrease in g--2.0005 signal strength but CW3-81A has an initial component which decays rapidly before a more stable component is established. This second component has a stability similar to the single component of T40-79A. Arrhenius plots using these measurements and others at different temperatures (which showed the same trend) yielded a mean life for the g = 2.0005 signal in T40-79A of 6 × 107 years at 10°C. The second component of CW3-81A gave a mean life of 7 × 107 years at 10~C, with the first component an order of magnitude less. There is a large error in these calculations because of the considerable extrapolations necessary. However, as can be clearly seen in Fig. 4, the relative stability of g = 2.0005 varies from sample to sample. Samples with a significant short-lived component in the g = 2.0005 signal were isolated by use of a standard test. The decrease in the natural signal was measured after annealing for 15 min at 200°C and in only one other sample, (G01B), was it more than 30%. This suggests that the g = 2.0005 signal is, in
most cases, stable enough to be used to date at least the last few hundred thousand years. The real test is whether the EDs calculated from it agree with those expected from dosimetry measurements and U/Th dates on the same speleothems. All of the signals listed in Table 1 showed an increase with irradiation. Sometimes signals which were not present above noise level in the natural sample became evident after irradiation. The most troublesome signal was at g = 2.0025 which often had a greater sensitivity to gamma-radiation than the nearby g = 2.0005 signal, thus making accurate peakto-peak measurements on g = 2.0005 difficult. In some cases, the g = 2.0005 intensity was measured by subtracting the contribution by the g = 2.0025 signal, or by measuring only the g = 2.0005 down-swing. The resulting uncertainties contribute to the error in g = 2.0005 EDs listed in Table l, particularly when the g = 2.0025 signal is also present in the natural spectrum. Fading after irradiation was not observed for any of the signals present in the natural spectra except g = 2.0025. The fact that this signal decreases even for samples in which it is strong in the natural spectra suggests that either there is a hard component resistant to fading or that there are two separate signals being observed. Annealing at low temperatures ( < 100°C) after irradiation was found to reduce the g = 2.0025 intensity further and thus decrease the interference with the g = 2.0005 signal. This procedure was not used in the present study.
ESR SIGNALS IN SPELEOTHEM CALCITES The g = 2.0005 signal generally showed the start of intensity saturation after a total dose of 200-300 Gy. However, this value varied from 150 Gy for sample 2STC8 to over 600 Gy for T34-79C. The other signals in Table 1 showed a similar trend, with saturation effects usually appearing after 200-400 Gy. In a few cases g = 2.0040, g = 1.9970 and g = 2.0025 were linear beyond 400 Gy. Saturation with dose is likely to limit the datable range to a greater extent than the inherent stability of the g = 2.0005 signal. The uncertainty in extrapolating growth curves of old samples can be significantly reduced if the same speleothem contains younger layers which can be used to define the saturation characteristics (assuming that they are the same in each layer). 4. THE EQUIVALENT DOSES
MEASURED Table 1 shows the EDs measured from five of the natural signals. All samples have been U/Th dated but the amount of dosimetry information available varies. The approximate alpha and beta doses can be calculated but many of the samples were selected for a comparison of different geographical regions and a return to the site to place gamma dosimeters was not practical. Table 1 therefore gives only expected limits in the ED, calculated from the U/Th dates and the dosimetry measurements available. The preliminary dose-rates were calculated using the method and values given by Wintle'(1978) and the Debenham and Aitken (1984) determinations of gamma-attenuation in calcite. The major uncertainty arises from the doses from radon and the soil at the base of the stalagmites. The a-value was taken as 0.2 _+ 0.1 after examination of the literature on ESR values for calcite (Yokoyama et al., 1981; Hennig et al., 1981; Karakostanoglou, 1982). Where gamma dosimetry information was not available the only external dose was assumed to be from the soil at the base of the speleothems, at which point the gamma dose was taken as 1 + 0.6 mGy a - ~. Sample 226h3 was kindly supplied by Debenham and Aitken (1984) who have accurately dated it by thermoluminescence (TL). In this case, the ED measured during the TL dating is quoted, together with their U/Th date. Even though the expected ED limits are large, it can be seen that g = 2.0005 is the only signal which is consistently in the correct range. However, it was not present in .samples 2STC6 or PHI-82B. Not.
843
enough measurements have been made on g = 2.0036 to draw conclusions but annealing studies (e.g. Fig. 3) and work by others (Hennig and Grfin, 1983; Yokoyama et al., 1983) suggest that it is not stable enough for dating. The broad signal at g = 2.0040 usually gave EDs which were too large, a result not unexpected from an organic signal unrelated to the calcite lattice. The signals at g =2.0025 and g = 1.9970 gave EDs which were sometimes far too small. The variability in the stability of these signals is obvious from their strong presence in the natural spectra of some samples and complete absence in others--even when showing up strongly after irradiation. The samples CW3-81A and G01B had shown components in the g -- 2.0005 signal which were less stable than usual. Gamma dosimetry measurements have been made at the CW3-SIA site, so the lower limit on the expected ED (from the U/Th data) is reasonably well-defined. It can be seen that the ED measured from the g = 2.0005 signal is considerably less than this limit (which is high due to a U concentration of 3.3 #g g-l). Hence, assuming the U/Th date to be correct, the g = 2.0005 signal will, in this case, produce a young date. The g = 2.0005 signal in G01B shows substantial dose-saturation effects, so its stability cannot be established. In summary, the only signal suitable for dating is at g = 2.0005 _ 0.0003 (h3). In a few cases, it contains a thermally unstable component which may be expected to produce a young date for older samples. It is suggested that the results from any sample which shows a decrease in the g = 2.0005 signal of more than 30~o after a 15 min annealing at 200°C be treated with care. Acknowledgements--M. Aitken and N. Debenham are thanked for many fruitful discussions and help with source calibration. J. N. Andrews, D. C. Ford, J. Hess and M. Ivanovich carded out the uranium series determinations quoted. The project was supported by The Natural Environment Research Council, research grant GR3/5049.
REFERENCES Apers D. J., Debuyst R., Dejehet F. and Lombard E. (1980) A propos d'un essai de datation par RPE de concretions calcaires originaires de grottes Beiges. Radiochem. RadioanaL Lett. 45, 427-440. Debenham N. C. and Aitken M. J. (1984) Thermoluminescence dating of stalagmitic calcite. Archaeometrv 26, 155-170.
844
B . W . S M I T H et al.
Griin R. and De Canniere P. (1984) ESR-dating: Problems encountered in the evaluation of the naturally accumulated dose (AD) of secondary carbonates. 3". Radioanal. nucl. Chem. Lett. 85, 213-226. Hennig G. J. and Griin R. (1983) ESR dating in Quaternary geology. Quat. Sci. Rev. 2, 157-238. Hennig G. J., Herr W., Weber E. and Xirotiris N. I. (1981) ESR-dating of the fossil hominid cranium from Petralona Cave, Greece. Nature, Lond. 292, 533-536. Ikeya M. (1975) Dating a stalactite by electron paramagnetic resonance. Nature, Lond. 255, 48-50. Karakostanoglou I. (1982) ESR isochron dating of speleothems. Unpublished M.Sc, Thesis, McMaster University. Skinner A. F. (1983) Overestimate of stalagmitic calcite
ESR dates due to laboratory heating. Nature, Lond. 304, 152-154. Wintle A. G. (1978) A thermoluminescence dating study of some Quaternary calcite: potential and problems. Can. J. Earth Sci. 15, 1977-1986. Yokoyama Y., Quaegebeur J. P., Bibron R., Leger C., Nguyen H. V. and Poupeau G. (1981) Electron spin resonance (ESR) dating of stalagmites of the Caune de l'Arago at Tautavel. In Datations Absolues et Analyses Isotopiques en Pr~histoire, Methodes et Limites, pp. 507-532. Colloque international du C.N.R.S. Yokoyama Y., Quaegebeur J. P., Bibron R. and Leger C. (1983) ESR dating of Palaeolithic calcite: thermal annealing experiment and trapped electron lifetime. PACT 9, 371-379.
0191-278X/8553.00+0.00 Pergamon Press Ltd.
ESR SIGNALS IN A VARIETY OF SPELEOTHEM CALCITES A N D THEIR SUITABILITY FOR DATING B. W. SMITH,P. L. SMART* and M. C. R. SYMON$ Department of Chemistry, University of Leicester, Leicester LEI 7RH, U.K.. (Received 2 November 1984; in revised form 30 January 1985)
Abstract--The natural ESR signals observed in speleothems collected from different geographical areas have been examined. A study was made of the best ways of obtaining measurement reproducibility and avoiding interference from signals induced by irradiation and sample preparation. Annealing studies showed that the only calcite ESR signal stable enough for dating the past few hundred thousand years, at g = 2.0005 + 0.0003, occasionally had a less stable component which may produce a young date. Finally, equivalent doses calculated from each radiation sensitive signal are compared with those expected from dosimetry measurements and U/Th dates on the same speleothems. 1. I N T R O D U C T I O N UNTIL recently, the majority of Electron Spin Resonance (ESR) dates from speleothems have been reported from a single site; Caune de l'Arago in France (De Lumley, in preparation). Comparatively little work has been undertaken at other sites (Ikeya, 1975; Apers et al., 1980; Hennig et al., 1981). It was established that at least one of the ESR signals (at g ~ 2.0005 and designated h3) found in the Caune de l'Arago samples was suitable for dating (Hennig and Griin, 1983; Skinner, 1983). However, the results of exhaustive studies on the stabilities, and applicability to dating, of the signals in Caune de l'Arago calcite cannot necessarily be extended to all cave speleothems showing the same ESR signals. We have examined a wide variety of samples of different age, purity, crystal size, minerology and geographical location to determine the range of natural ESR signals present. Annealing studies and equivalent dose (ED) measurements on a selection of these samples have shown that the stability of individual ESR signals can vary from sample to sample.
2. E X P E R I M E N T A L T E C H N I Q U E S All measurements have been made at room temperature using a Varian El09 X-band spectrometer. The speleothems were gently crushed and the grain
size 125-250 # m selected. Grains of diameter greater than 2 5 0 # m tended to produce variations in .ESR signal intensity when rotated due to crystallite orientation effects. Grains smaller than 125 g m were found to give a slightly decreased signal strength, although not as great as the ,-,30% decrease reported by Karakostanogiou (1982). The main reason for rejecting the < 125 g m fraction was the creation, after gamma-irradiation, of a sharp signal at g = 2.0001 (+0.0002, half-width ~ 3 x 10-~T). This signal became stronger as the grain size decreased, an effect which has also been observed by Grfin and De Canniere (1984). It is of particular importance as it is superimposed on the g = 2.0005 signal and may not be detected if the magnetic field modulation is too large. The g = 2.0001 signal appears to be due to surface defects caused by crushing and is related to a short-lived signal (,,,80m in mean life) at g = 2.0030 ( + 0.0002) which transfers a variable fraction of its spin to the g = 2.0001 signal when it decays (Fig. 1). For all ED measurements in this study the 125-250#m grains were etched for 2min in 10% acetic acid, which effectively removes the defects producing the g = 2.0001 signal. This is a more severe etching than has normally been recommended for calcite (Wintle, 1978). Quartz tubes of 2 m m diameter were used to distribute 100 + 1 mg of each sample through the full depth of the spectrometer cavity. This reduces spec-
*Department of Geography, University of Bristol, Bristol BS8 1SS, U.K. 837
838
B . W . SMITH et al.
tral resolution, as the magnetic field is not precisely uniform throughout the cavity, but considerably improves reproducibility. The individual sample tubes were placed in an outer quartz tube which was accurately centred in the cavity before each set of measurements. As the instrument needed to be retuned for each sample, care was taken to ensure that variables such as the detector current remained constant. The reproducibility of signal amplitude using these techniques was better than 3%. All measurements for an ED determination were made on the same day but if comparisons needed to be made on different days the natural sample was used as a standard to allow for small drifts in instrument sensitivity. All samples were initially observed with a microwave power of 1 mW and a magnetic field modulation of 4 × 10 -5 T. We found no variations in ED with microwave power as long as the calcite was well-etched. The EDs listed in Table 1 were therefore measured using a power of 5 m W which improved the signal-to-noise ratio. At higher powers there is an
onset of saturation effects in the microwave absorption of the spins producing the g =2.0005 signal. The spectrometer gave first derivative absorption spectra which could be integrated twice by an interfaced computer to yield the area under individual absorption peaks. However, it was found that this gave extremely variable results and depended critically on the initial base-line used. Peak-to-peak measurements on the first derivative spectra were much more reproducible but measurements on different subsamples are only comparable providing the absorption peak width does not change. None of the signals for which EDs have been calculated showed a measurable change in width with irradiation, so the latter technique was employed. For gamma-irradiation using a 6°Co source, 150 mg aliquots of each sample were placed in gelatin capsules. The capsules were held in the centre of a 20 mm diameter marble pot which gave approximate secondary electron equilibrium. The dose-rate to calcite in this configuration was close to 1 Gy s -L.
c .E
i "i j ! w ¢,1
o
s'o
t6o
t~o
2bo
Minutes after irradiation
FIG. 1. The change in intensity of the g = 2.0030 and g = 2.0001 signals in sample T34-79A after a gamma-irradiation of 240 Gy. After 1 day g = 2.0001 has increased to a maximum value at the dashed
line, but it usually then fades over a period of months.
ESR SIGNALS IN SPELEOTHEM
839
CALCITES
Magnetic field (T) 0.3355
0.3360
0.3365
0.3370
I
I
I
I
h2
h 3
Sample T 4 0 - 7 9 A
'
2.0boo
'
2.0b40
'
' 2.0020 g-value
2.
Oi
O0
i
i
1.9980
i
i
1.9960
FIG. 2. The ESR spectra from three samples, showing between them all of the natural signals observed. All spectra were measured using a scan speed of 10 -3 T min-~ with a modulation amplitude of 4 x 10 -5 T and microwave power of 5 roW. The magnetic field values are typical for measurement frequencies of ~9.4 GHz. Sample T34-79A shows g = 2.0055 (h 0, g = 2.0036 (h2) and g = 2.0005 (h3). G01B gives the same signals superimposed on the broad g = 2.0040 signal. The small signal at g = 1.9994 + 0.0003 is also apparent. T40-79A shows additional signals at g = 2.0025 + 0.0003 and g = 1.9970 + 0.0003. 3. T H E S P E C T R A
OBSERVED
T h e r a n g e o f n a t u r a l signals present in the differential a b s o r p t i o n spectra was similar for samples from all g e o g r a p h i c a l locations. T h e signals are basically the same as those r e p o r t e d previously by
H e n n i g a n d Gr/.in (1983) for various c a r b o n a t e form a t i o n s a n d are illustrated in Fig. 2. T h e three signals, g = 2.0055 + 0.0003, p e a k - t o - p e a k w i d t h ~ 1 x 10-aT (commonly denoted h0, g =2.0036_+0.0002, ,,-8 x 1 0 - S T (h2) and g = 2.0005 + 0.0003, ~ 2 x 1 0 - 4 T (h3), h a v e been
840
B. W. S M I T H et al. Table 1. The equivalent doses measured from five calcite ESR signals E.D. (Gy) for signals at g-valt~_s sho%m
Sample
Origin
2.0040
SUI3-80
~
16 -+5
BATC2
MAJORCA
57 +-7
2SIC8
DX~ND
*
CWT-81A
SARA~LK
-
2STC6
ENGLAkD
T14-79
SPAIN
PHI-82B
~
T40-79A
2.0025
2.0036
7-+2 -
93 -+14
2.0005
1.9970
EXl:mCt(~
COZTL~"r.(~
~D Limits (Gy)
U/lh Date (ky)
1_+3
10_+7
3_+2
5.5+_0.8
6-+4
170 + 60
5+3
9.2+_0.6
18_+5
26 +-14
9+- 5
II.5 t 1.4
18+_6
24 +-12
14 -+ 5
I00 -+ 60
-
19.7
+I ,8 -I .7
81 -+6
38_+9
I00 -+ 50
250
+51 -35
2_+6
149 +- 86
179
+65 -39
Saturated
2_+1
>80
315 -93
133 _+12
186 _+22
87 -+21
>460
430_110
2_+2
>80
> 350
I0 ± i0
275 -+30
20_+6
>60
>350
176 ± 18
175 _+25
174 ± 12
40-+5
-
-
77 _+8
-
SPAIN
-
-
122 _+18
CW3-81A
~
-
-
GOIB
~
165 -+50
>300
T34-79C
SPAIN
*
*
226h3
~
-
150 ± 30
128 ± 25
+ oo >320
*Signal small and obscured by other signals. --Signal not present in the natural sample.
Table 2. The localities of the samples listed in Table I OKIGIlq
TYPE Stance
SUI3-80
Umah a n Tartair, Sutherland, Scotlaz~
BA~C2
Cova de Na Barxa, Majorca, Spain
Fl~tor~
2S~C8
St. Cuthberts Swallet, Somerset, England
Flowstor~
CW7-8L%
Clearwater C a ~ ,
sarawak, Malaysia
2STC6
St. Cuthberts Swallet, Somerset, England
T14-79
Cueva del Agua, Santander, Spain
Stalagmite
PHI-82B
Picken's Hole, Somerset, England
T40-79A
Tere, Santander, Spain
Flo~to~ Stalagmite
CW3-8L%
Clearwater Cave, Sarawak, Malaysia
Stala~mtee
GOIB
Great Oor~s Hole, Somerset, England
Flc~cstxme
T34-79C
Cueva del Agua, Santander, Spain
Sta~te
226h3
P o n ~ ,
Flowstone
Wales
176
+29 -23
ESR SIGNALS IN SPELEOTHEM CALCITES discussed in detail in the Caune de l'Arago literature (Yokoyama et al., 1983). There is a small signal superimposed on the h3 signal at g = 1.9994__+0.0003 which has not been reported previously but its intensity is usually too small to be used for ED calculations. Some samples show a significant up-swing in the differential spectra at g = 2.0025 _ 0.0003 which extends to h3 and almost all have a small signal at g = 1.9970 + 0.0003. The latter signal is in the same region as a forbidden Mn 2+ transition line but appears even in samples with very little Mn 2÷ and grows rapidly with irradiation, whereas the Mn 2+ lines do not. When the etched calcite is not a pure white colour, there is always the presence of a broad signal at g = 2.0040 _+ 0.0005, width ~ 6 x 10-4T, which has been attributed to humic acids by Grfin and De Canniere (1984). All of the signals discussed show an onset of saturation in microwave absorption at different powers, indicating that they derive from independent spins. Exposure of 125-250/~m calcite grains to ambient lighting inside the laboratory windows for periods up to 6 months had no effect on any signal except g = 2.0036. This signal was reduced by 80~o after 4 days in the three samples in which it was found, but
I
"5
g = 2.0040
I
I
g = 2.0036
I
I
841
Hennig and Griin (1983) have reported a variability in its light stability for a range of calcites. Samples were prepared under red light and kept in light-tight containers, but a small exposure to shorter wavelength light during ESR measurement was unavoidable. The natural calcite samples were annealed in order to obtain information about the stability of each signal. All of the natural signals decrease on annealing except g = 2.0055 (h0, which initially increases as spins are transferred from other signals. Yokoyama et al. (1981) have used an annealing technique to transfer the spins from h2 (g = 2.0036) and h3 (g = 2.0005) to h~ before obtaining the ED from hi. This was not used in the present study because double integration of the differential ESR spectra clearly shows that the increase in h~ spins is often greater than the total of the h2 and h3 spins. This implies that spin is being transferred from trapping sites other than h2 and h3. Skinner (1983) has also shown that there are problems with extrapolating the non-linear h~ growth curve, Samples were heated for 20 rain at temperatures progressively increasing by 20°C intervals to 300°C. Figure 3 shows the resulting plot of the change in
/t'
t-
/
n"
/
ILl
n"
Ix
g= 2 . 0 0 0 5 g=
2.0055
100
200
300
T e m p e r a t u r e =C
F1G.3. The change in signal intensities from G01B after annealingfor 20 min at temperatures progressively increasing by 20~C, The measurements were made with a microwave power of 1 roW, so the relative intensities of the signals are different from those shown in Fig. 2. The g = 2.0055 signal is difficult to measure at low intensities because it falls on the peak ofg = 2.0040. Not all of the data points are shown. The error bars indicate the la uncertainties at selected temperatures.
842
B . W . SMITH et al. 1.0~5
~
}~
}
""-.--
Sample T40-79A
Sample CW3-81A
(~
0.2-
110
2'0 30 Annealing time minutes
40
gO
FIG. 4. The decrease in g = 2.0005 signal intensity with annealing at 200°C. Sample CW3-81A has a component which is less stable than is usual for g = 2.0005. signal intensities for sample G01B. In all samples examined, the most stable signals were g = 2.0005 and the broad organic signal at g = 2.0040 but the stabilities of individual signals varied from sample to sample. This became clearer when annealing was carried out over increasing time periods at the same temperature to obtain Arrhenius plots. The g = 2.0005 signal results for two samples at 200°C, which were annealed and measured at the same time, are shown in Fig. 4. Sample T40-79A shows a first order decrease in g--2.0005 signal strength but CW3-81A has an initial component which decays rapidly before a more stable component is established. This second component has a stability similar to the single component of T40-79A. Arrhenius plots using these measurements and others at different temperatures (which showed the same trend) yielded a mean life for the g = 2.0005 signal in T40-79A of 6 × 107 years at 10°C. The second component of CW3-81A gave a mean life of 7 × 107 years at 10~C, with the first component an order of magnitude less. There is a large error in these calculations because of the considerable extrapolations necessary. However, as can be clearly seen in Fig. 4, the relative stability of g = 2.0005 varies from sample to sample. Samples with a significant short-lived component in the g = 2.0005 signal were isolated by use of a standard test. The decrease in the natural signal was measured after annealing for 15 min at 200°C and in only one other sample, (G01B), was it more than 30%. This suggests that the g = 2.0005 signal is, in
most cases, stable enough to be used to date at least the last few hundred thousand years. The real test is whether the EDs calculated from it agree with those expected from dosimetry measurements and U/Th dates on the same speleothems. All of the signals listed in Table 1 showed an increase with irradiation. Sometimes signals which were not present above noise level in the natural sample became evident after irradiation. The most troublesome signal was at g = 2.0025 which often had a greater sensitivity to gamma-radiation than the nearby g = 2.0005 signal, thus making accurate peakto-peak measurements on g = 2.0005 difficult. In some cases, the g = 2.0005 intensity was measured by subtracting the contribution by the g = 2.0025 signal, or by measuring only the g = 2.0005 down-swing. The resulting uncertainties contribute to the error in g = 2.0005 EDs listed in Table l, particularly when the g = 2.0025 signal is also present in the natural spectrum. Fading after irradiation was not observed for any of the signals present in the natural spectra except g = 2.0025. The fact that this signal decreases even for samples in which it is strong in the natural spectra suggests that either there is a hard component resistant to fading or that there are two separate signals being observed. Annealing at low temperatures ( < 100°C) after irradiation was found to reduce the g = 2.0025 intensity further and thus decrease the interference with the g = 2.0005 signal. This procedure was not used in the present study.
ESR SIGNALS IN SPELEOTHEM CALCITES The g = 2.0005 signal generally showed the start of intensity saturation after a total dose of 200-300 Gy. However, this value varied from 150 Gy for sample 2STC8 to over 600 Gy for T34-79C. The other signals in Table 1 showed a similar trend, with saturation effects usually appearing after 200-400 Gy. In a few cases g = 2.0040, g = 1.9970 and g = 2.0025 were linear beyond 400 Gy. Saturation with dose is likely to limit the datable range to a greater extent than the inherent stability of the g = 2.0005 signal. The uncertainty in extrapolating growth curves of old samples can be significantly reduced if the same speleothem contains younger layers which can be used to define the saturation characteristics (assuming that they are the same in each layer). 4. THE EQUIVALENT DOSES
MEASURED Table 1 shows the EDs measured from five of the natural signals. All samples have been U/Th dated but the amount of dosimetry information available varies. The approximate alpha and beta doses can be calculated but many of the samples were selected for a comparison of different geographical regions and a return to the site to place gamma dosimeters was not practical. Table 1 therefore gives only expected limits in the ED, calculated from the U/Th dates and the dosimetry measurements available. The preliminary dose-rates were calculated using the method and values given by Wintle'(1978) and the Debenham and Aitken (1984) determinations of gamma-attenuation in calcite. The major uncertainty arises from the doses from radon and the soil at the base of the stalagmites. The a-value was taken as 0.2 _+ 0.1 after examination of the literature on ESR values for calcite (Yokoyama et al., 1981; Hennig et al., 1981; Karakostanoglou, 1982). Where gamma dosimetry information was not available the only external dose was assumed to be from the soil at the base of the speleothems, at which point the gamma dose was taken as 1 + 0.6 mGy a - ~. Sample 226h3 was kindly supplied by Debenham and Aitken (1984) who have accurately dated it by thermoluminescence (TL). In this case, the ED measured during the TL dating is quoted, together with their U/Th date. Even though the expected ED limits are large, it can be seen that g = 2.0005 is the only signal which is consistently in the correct range. However, it was not present in .samples 2STC6 or PHI-82B. Not.
843
enough measurements have been made on g = 2.0036 to draw conclusions but annealing studies (e.g. Fig. 3) and work by others (Hennig and Grfin, 1983; Yokoyama et al., 1983) suggest that it is not stable enough for dating. The broad signal at g = 2.0040 usually gave EDs which were too large, a result not unexpected from an organic signal unrelated to the calcite lattice. The signals at g =2.0025 and g = 1.9970 gave EDs which were sometimes far too small. The variability in the stability of these signals is obvious from their strong presence in the natural spectra of some samples and complete absence in others--even when showing up strongly after irradiation. The samples CW3-81A and G01B had shown components in the g -- 2.0005 signal which were less stable than usual. Gamma dosimetry measurements have been made at the CW3-SIA site, so the lower limit on the expected ED (from the U/Th data) is reasonably well-defined. It can be seen that the ED measured from the g = 2.0005 signal is considerably less than this limit (which is high due to a U concentration of 3.3 #g g-l). Hence, assuming the U/Th date to be correct, the g = 2.0005 signal will, in this case, produce a young date. The g = 2.0005 signal in G01B shows substantial dose-saturation effects, so its stability cannot be established. In summary, the only signal suitable for dating is at g = 2.0005 _ 0.0003 (h3). In a few cases, it contains a thermally unstable component which may be expected to produce a young date for older samples. It is suggested that the results from any sample which shows a decrease in the g = 2.0005 signal of more than 30~o after a 15 min annealing at 200°C be treated with care. Acknowledgements--M. Aitken and N. Debenham are thanked for many fruitful discussions and help with source calibration. J. N. Andrews, D. C. Ford, J. Hess and M. Ivanovich carded out the uranium series determinations quoted. The project was supported by The Natural Environment Research Council, research grant GR3/5049.
REFERENCES Apers D. J., Debuyst R., Dejehet F. and Lombard E. (1980) A propos d'un essai de datation par RPE de concretions calcaires originaires de grottes Beiges. Radiochem. RadioanaL Lett. 45, 427-440. Debenham N. C. and Aitken M. J. (1984) Thermoluminescence dating of stalagmitic calcite. Archaeometrv 26, 155-170.
844
B . W . S M I T H et al.
Griin R. and De Canniere P. (1984) ESR-dating: Problems encountered in the evaluation of the naturally accumulated dose (AD) of secondary carbonates. 3". Radioanal. nucl. Chem. Lett. 85, 213-226. Hennig G. J. and Griin R. (1983) ESR dating in Quaternary geology. Quat. Sci. Rev. 2, 157-238. Hennig G. J., Herr W., Weber E. and Xirotiris N. I. (1981) ESR-dating of the fossil hominid cranium from Petralona Cave, Greece. Nature, Lond. 292, 533-536. Ikeya M. (1975) Dating a stalactite by electron paramagnetic resonance. Nature, Lond. 255, 48-50. Karakostanoglou I. (1982) ESR isochron dating of speleothems. Unpublished M.Sc, Thesis, McMaster University. Skinner A. F. (1983) Overestimate of stalagmitic calcite
ESR dates due to laboratory heating. Nature, Lond. 304, 152-154. Wintle A. G. (1978) A thermoluminescence dating study of some Quaternary calcite: potential and problems. Can. J. Earth Sci. 15, 1977-1986. Yokoyama Y., Quaegebeur J. P., Bibron R., Leger C., Nguyen H. V. and Poupeau G. (1981) Electron spin resonance (ESR) dating of stalagmites of the Caune de l'Arago at Tautavel. In Datations Absolues et Analyses Isotopiques en Pr~histoire, Methodes et Limites, pp. 507-532. Colloque international du C.N.R.S. Yokoyama Y., Quaegebeur J. P., Bibron R. and Leger C. (1983) ESR dating of Palaeolithic calcite: thermal annealing experiment and trapped electron lifetime. PACT 9, 371-379.