- Email: [email protected]
Q-band studies of the ESR signal in tooth enamel
Quaternary Science Reviews 20 (2001) 1027}1030
Q-band studies of the ESR signal in tooth enamel夽 Anne R. Skinner *, N. Dennis Chasteen, Junlong Shao, Bonnie A.B. Blackwell Department of Chemistry, Williams College, Williamstown MA, 01267, USA University of New Hampshire, Durham, NH, 03824, USA
Abstract Tooth enamel is one of the most promising materials for electron spin resonance (ESR) dating because the X-band signal is large, easy to measure, and extremely stable. The mean lifetime at ambient temperature has been measured greater than the age of the Earth! However, the X-band spectrum in fossil teeth is, in fact, a composite of two signals that can be resolved if the sample is examined in the Q-band region. The relative size of the two signals appears to be a function of degree of fossilization; older teeth have a better-de"ned second signal. A study of the dependence of these signals on radiation dose, microwave power, and temperature strongly suggests that both signals are located in the hydroxyapatite crystal structures. As such, then, the X-band spectrum, measured at moderate modulation amplitude, is suitable for determining the age of fossil teeth. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron spin resonance (ESR) dating depends on measuring a radiation-sensitive signal. In nature the signal is induced by environmental factors such as sedimentary radioisotopes. In the laboratory, samples are irradiated arti"cially in order to determine the total dose acquired by the sample over time, expressed as the gamma-equivalent dose, or ED. Then by measuring the environmental dose rate, the sample age can be found. Radiation damage signals can be quite complex, and for con"dence in the quoted results, one should study the fundamental parameters of the signal. Normally ESR dating utilizes the X-band region (frequency around 9.5 GHz; magnetic "eld strength around 300 mT). Q-band ESR spectroscopy, in comparison, requires high magnetic "elds (near 1250 mT) and high frequencies (near 35 GHz). These conditions permit the resolution of peaks with very similar g-values. Sample size is quite small (tubes are typically 2 mm in diameter). While reproducibility can be good for individual samples, quantitative comparisons between di!erent samples is limited by the e!ects of di!erent sample weights and tube positions. The greatest utility of this technique is, therefore, to explain di$cult spectra observed in other ESR modes, and to indicate promising lines for further study.
夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (A.R. Skinner).
The hydroxyapatite (HAP) signal in tooth enamel has long been known to exhibit variability (e.g., Ikeya, 1993). At relatively low modulation amplitude ((0.1 mT) the peak shows signs of underlying complexity, ranging from asymmetry in the high-"eld dip in Fig. 1 to a visible double peak on the low-"eld side of the HAP signal. Initially GruK n et al., 1987 suggested that such samples be rejected for dating. Careful examination of fossil teeth, however, suggests that this problem, if it is one, exists in the majority of samples, which would imply that ESR dating of teeth was unlikely to become a widespread technique. By increasing the modulation amplitude to 1 mT, irregularities can be forced to disappear. Line broadening and smoothing, most often by increasing microwave power rather than modulation amplitude, has been suggested for other systems (Lyons, 1996; Molodkov, 1988), but it should be justi"ed by studies that show the derived equivalent doses (ED's) and ages are accurate. Occasionally side bands can be seen (Fig. 1). Ikeya (1981), and Hennig and GruK n (1983), among others, attributed these features to the presence of alanine in the tooth enamel. The sidebands do suggest a multiplet structure similar to that of organic radicals, and given the presence of organic material in tooth enamel, such radicals are to be expected. The multiplet itself does not generally overlap the HAP signal and hence was not considered in this work. If the &extra' features in the HAP signal in Fig. 1 in fact also result from a contaminant, as suggested in Rink (1997), then ESR dating of teeth becomes problematic, since no contaminant would have the same response to
0277-3791/01/$ - 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 6 6 - 4
1028
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
2. Materials and methods
Fig. 1. X-band spectrum of hydroxyapatite signal in tooth enamel. The peak at H"336.4 (g+2.0) has two unresolved components. A quintet, centered at H"336.0 and probably attributable to organic radicals, can also be seen.
Q-band spectroscopy was performed at the University of New Hampshire, using a custom-built spectrometer. Most of the spectra were obtained at room temperature, but some work on power saturation was done at 100 K. Sample sizes were approximately 25 mg and conditions of measurement were: power, 2 mW, modulation amplitude, 0.1 mT; scan speed, 2.5 and 5 mT/min; time constant, 3 s and 1 s. Two sets of samples were used. To test the changes in the Q-band spectrum as a function of radiation dose, spectra were obtained for a suite of samples from a previously-dated tooth (QT59, St. Acheul, France) with added radiation doses up to 5 kGy. Samples of teeth from sites of di!erent ages were also examined to see how the spectrum varied with sample age (Table 1).
3. Results
Fig. 2. Q-band spectrum of the primary (g+2.0) portion of the same signal. Two components, A and B, can be identi"ed, with peak heights H and H as indicated.
radiation, microwave power, and thermal lifetime as the hydroxyapatite itself. Q-band spectroscopy was thus used to explore the properties of the complex HAP signal. Previous Q-band spectra of tooth enamel have been obtained for the most part using whole pieces and orienting them within the ESR cavity in order to measure the angular dependence of the signal (Bouchez et al., 1988; Rossi and Poupeau, 1990). When powdered tooth enamel is examined in the Q-band, two features can be clearly seen (Fig. 2). Rossi and Poupeau (1990) noted that the primary peak, which they termed &A ', was assymetric, suggesting it contained two or more elements. A , how ever, is too narrow to encompass both the "rst half of A and all of B in Fig. 2. Skinner et al. (2000) reported that both A and B features had the same radiation sensitivity, implying that they derive from the same center. However, experimental justi"cation, while helpful, is not conclusive. We report here additional information that provides a theoretical justi"cation for this conclusion, reinforcing the opinion that using higher modulation amplitudes (i.e. 1 mT) should not a!ect the accuracy of the derived ages.
The Q-band spectroscopy of QT59 showed clearly that the multiplet of the main peak derived in fact from a single type of defect. As seen in Fig. 2, the two features, A and B, have essentially identical g-values. Both grow with radiation (Fig. 3) and the growth pattern is identical, as shown by plotting the two peak heights, normalized to their values in the natural sample (Fig. 4). Other properties of the signal, such as power saturation, are also identical (Skinner et al., 2000). As one examines older samples, the broad peak (A) in Fig. 2 develops increasing complexity (Fig. 5). Speci"cally, on the low-"eld side it "rst broadens and then splits into multiple features. On the high-"eld side, it begins as a slight shoulder which deepens as the sample becomes older. A also increases in relative intensity. In Table 1, the simple ratio of peak heights, A/B, shows a marked increase between the youngest and the oldest samples. If, in Fig. 5, one examines the A peak for the intermediate sample, the peak broadening understates its contribution to the ratio. Ultimately, for the 2 Ma sample, a third peak Table 1 Q band height ratios Site
Species
Age (Ma)
A/B
France
Mammuthus
Hungary
Equus
Olduvai
Elephas
0.10 $0.05 0.33 $0.06 1.8 $0.2
0.38 $0.01 0.38 $0.01 0.44 $0.01 0.70 $0.01
Includes third peak
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
1029
Fig. 3. The growth of the Q-band spectrum with arti"cial irradiation.
Fig. 4. The growth of Q-band components with artificial radiation, normalized to the natural sample peak height.
appears, still broader than A (see Fig. 5). If one sums that peak height as well, the shift in populated defects from the major to the minor components becomes even more marked. However, there is no independent evidence as yet that this third peak is an hydroxyapatite-type defect.
arise from identical or closely related defects. The primary di!erence lies in the linewidth. Broad linewidths are most often associated with distorted defects that di!er slightly in crystal structure from the primary hydroxyapatite defect and from each other. In essence the broad line is the envelope of many closely spaced lines with the same or nearly the same g-values. The changes in the Q-band spectra with sample age reinforce this conclusion. As the sample becomes older, the crystal structure should increasingly break down. Complete recrystallization, of course, would lead to the loss of accumulated defect sites. Crystals may be deformed in many ways, however, without actually destroying the crystal structure entirely. One would expect, then, to see a shift to the less-well-de"ned defect sites as fossilization proceeds. This is in fact observed qualitatively, and even semi-quantitatively, as one goes from relatively young to quite old teeth. One concern might be that the variation in spectra with age depends on the species. While the three teeth are all from di!erent species, the oldest and the youngest are from closely related mammals. It seems unlikely, therefore, that speciation is responsible for the observed spectra.
4. Discussion Given that both parts of the central multiplet hydroxyapatite peak behave identically with respect to radiation dose and other experimental factors, they must
5. Conclusions The primary conclusion that can be drawn from this preliminary work is that ESR dating of mammal teeth is
1030
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
Fig. 5. Comparison of the Q-band spectra for teeth of di!erent ages. As the sample age increases, the size and complexity of the A component increases. For the oldest sample, the `third peaka consists of a low-"eld component at 12,335 G and a high-"eld component at 12,374 G.
not limited by the presence of minor components in the standard X-band hydroxyapatite peak because these components are related to the same HAP defect structure. The normal practice, increasing modulation amplitude to obtain a smooth spectrum, is justi"ed by the Q-band data. The variation in line shape with age reinforces the hypothesis that the minor component, A, may relate to variability in the HAP lattice, since such variability would be associated with crystallographic breakdown during fossilization. If the HAP crystal structure changes with time, the mechanism of U-uptake is likely to change also as the permeability of the crystal lattice increases, complicating Electron spin resonance (ESR) dating. Further research in this area should focus on con"rming the trends seen in Fig. 5 by examining samples from other species and other time periods, and also on crystallographic studies of tooth enamel to look for direct correlations between changes in the ESR spectrum and changes in crystallinity.
Acknowledgements Funding for this work was provided to Williams College by NSF (ILI 9151111, SBR 9709912, SBR 9896289) and to UNH by NIH (R37 GM20194), Irradiations were done at the Wadsworth Laboratories, New York Dept. of Public Health, Albany, NY, courtesy of Dr. Ulrich Rudofsky, and at McMaster University Nuclear Reactor.
Haibo Gu, Williams College, did calculations on the Q-band spectra. Teeth were provided by Dr. J. Hooker, British Museum of Natural History; Prof. L. Kordos, the Hungarian National Geological Museum; and Prof. I. Tattersall, American Museum of Natural History. References Bouchez, R., Cox, R., Herve, A., Lopez}Carranza, E., Ma, J.L., Piboule, M., Poupeau, G., Rey, P., 1988. Q-band ESR studies of fossil teeth: consequences for ESR dating. Quaternary Science Reviews 7, 497}501. GruK n, R., Schwarcz, H.P., Zymela, S., 1987. ESR dating of tooth enamel. Canadian Journal of Earth Sciences 24, 1022}1037. Hennig, G., GruK n, R., 1983. ESR dating in Quaternary geology. Quaternary Science Reviews 2, 157}238. Ikeya, M., 1981. Paramagnetic alanine molecular radicals in fossil shells and bones. Naturwissenschaften 68, 474}475. Ikeya, M., 1993. New Applications of Electron Spin Resonance: Dating, Dosimetry and Microscopy. World Scienti"c, Singapore. Lyons, R.G., 1996. Back to basics: qualitative spectral analysis as an investigatory tool, using calcite as a case study. Applied Radiation and Isotopes 47, 1385}1391. Molodkov, A., 1988. ESR dating of Quaternary shells: recent advances. Quaternary Science Reviews 7, 477}484. Rink, W.J., 1997. Electron Spin Resonance (ESR) dating and ESR applications in Quaternary science and archaeology. Radiation Measurements 27, 975}1025. Rossi, A.M., Poupeau, G., 1990. Radiation damage in bioapatites: the ESR spectrum of irradiated dental enamel revisited. Nuclear Tracks and Radiation Measurements 17, 537}545. Skinner, A.R., Blackwell, B.A.B., Chasteen, N.D., Shao, J., Min, S.S., 2000. Improvements in dating tooth enamel by ESR. Applied Radiation and Isotopes 52, 1337}1344.
Q-band studies of the ESR signal in tooth enamel夽 Anne R. Skinner *, N. Dennis Chasteen, Junlong Shao, Bonnie A.B. Blackwell Department of Chemistry, Williams College, Williamstown MA, 01267, USA University of New Hampshire, Durham, NH, 03824, USA
Abstract Tooth enamel is one of the most promising materials for electron spin resonance (ESR) dating because the X-band signal is large, easy to measure, and extremely stable. The mean lifetime at ambient temperature has been measured greater than the age of the Earth! However, the X-band spectrum in fossil teeth is, in fact, a composite of two signals that can be resolved if the sample is examined in the Q-band region. The relative size of the two signals appears to be a function of degree of fossilization; older teeth have a better-de"ned second signal. A study of the dependence of these signals on radiation dose, microwave power, and temperature strongly suggests that both signals are located in the hydroxyapatite crystal structures. As such, then, the X-band spectrum, measured at moderate modulation amplitude, is suitable for determining the age of fossil teeth. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron spin resonance (ESR) dating depends on measuring a radiation-sensitive signal. In nature the signal is induced by environmental factors such as sedimentary radioisotopes. In the laboratory, samples are irradiated arti"cially in order to determine the total dose acquired by the sample over time, expressed as the gamma-equivalent dose, or ED. Then by measuring the environmental dose rate, the sample age can be found. Radiation damage signals can be quite complex, and for con"dence in the quoted results, one should study the fundamental parameters of the signal. Normally ESR dating utilizes the X-band region (frequency around 9.5 GHz; magnetic "eld strength around 300 mT). Q-band ESR spectroscopy, in comparison, requires high magnetic "elds (near 1250 mT) and high frequencies (near 35 GHz). These conditions permit the resolution of peaks with very similar g-values. Sample size is quite small (tubes are typically 2 mm in diameter). While reproducibility can be good for individual samples, quantitative comparisons between di!erent samples is limited by the e!ects of di!erent sample weights and tube positions. The greatest utility of this technique is, therefore, to explain di$cult spectra observed in other ESR modes, and to indicate promising lines for further study.
夽
Paper published in December 2000. * Corresponding author. E-mail address: [email protected] (A.R. Skinner).
The hydroxyapatite (HAP) signal in tooth enamel has long been known to exhibit variability (e.g., Ikeya, 1993). At relatively low modulation amplitude ((0.1 mT) the peak shows signs of underlying complexity, ranging from asymmetry in the high-"eld dip in Fig. 1 to a visible double peak on the low-"eld side of the HAP signal. Initially GruK n et al., 1987 suggested that such samples be rejected for dating. Careful examination of fossil teeth, however, suggests that this problem, if it is one, exists in the majority of samples, which would imply that ESR dating of teeth was unlikely to become a widespread technique. By increasing the modulation amplitude to 1 mT, irregularities can be forced to disappear. Line broadening and smoothing, most often by increasing microwave power rather than modulation amplitude, has been suggested for other systems (Lyons, 1996; Molodkov, 1988), but it should be justi"ed by studies that show the derived equivalent doses (ED's) and ages are accurate. Occasionally side bands can be seen (Fig. 1). Ikeya (1981), and Hennig and GruK n (1983), among others, attributed these features to the presence of alanine in the tooth enamel. The sidebands do suggest a multiplet structure similar to that of organic radicals, and given the presence of organic material in tooth enamel, such radicals are to be expected. The multiplet itself does not generally overlap the HAP signal and hence was not considered in this work. If the &extra' features in the HAP signal in Fig. 1 in fact also result from a contaminant, as suggested in Rink (1997), then ESR dating of teeth becomes problematic, since no contaminant would have the same response to
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 6 6 - 4
1028
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
2. Materials and methods
Fig. 1. X-band spectrum of hydroxyapatite signal in tooth enamel. The peak at H"336.4 (g+2.0) has two unresolved components. A quintet, centered at H"336.0 and probably attributable to organic radicals, can also be seen.
Q-band spectroscopy was performed at the University of New Hampshire, using a custom-built spectrometer. Most of the spectra were obtained at room temperature, but some work on power saturation was done at 100 K. Sample sizes were approximately 25 mg and conditions of measurement were: power, 2 mW, modulation amplitude, 0.1 mT; scan speed, 2.5 and 5 mT/min; time constant, 3 s and 1 s. Two sets of samples were used. To test the changes in the Q-band spectrum as a function of radiation dose, spectra were obtained for a suite of samples from a previously-dated tooth (QT59, St. Acheul, France) with added radiation doses up to 5 kGy. Samples of teeth from sites of di!erent ages were also examined to see how the spectrum varied with sample age (Table 1).
3. Results
Fig. 2. Q-band spectrum of the primary (g+2.0) portion of the same signal. Two components, A and B, can be identi"ed, with peak heights H and H as indicated.
radiation, microwave power, and thermal lifetime as the hydroxyapatite itself. Q-band spectroscopy was thus used to explore the properties of the complex HAP signal. Previous Q-band spectra of tooth enamel have been obtained for the most part using whole pieces and orienting them within the ESR cavity in order to measure the angular dependence of the signal (Bouchez et al., 1988; Rossi and Poupeau, 1990). When powdered tooth enamel is examined in the Q-band, two features can be clearly seen (Fig. 2). Rossi and Poupeau (1990) noted that the primary peak, which they termed &A ', was assymetric, suggesting it contained two or more elements. A , how ever, is too narrow to encompass both the "rst half of A and all of B in Fig. 2. Skinner et al. (2000) reported that both A and B features had the same radiation sensitivity, implying that they derive from the same center. However, experimental justi"cation, while helpful, is not conclusive. We report here additional information that provides a theoretical justi"cation for this conclusion, reinforcing the opinion that using higher modulation amplitudes (i.e. 1 mT) should not a!ect the accuracy of the derived ages.
The Q-band spectroscopy of QT59 showed clearly that the multiplet of the main peak derived in fact from a single type of defect. As seen in Fig. 2, the two features, A and B, have essentially identical g-values. Both grow with radiation (Fig. 3) and the growth pattern is identical, as shown by plotting the two peak heights, normalized to their values in the natural sample (Fig. 4). Other properties of the signal, such as power saturation, are also identical (Skinner et al., 2000). As one examines older samples, the broad peak (A) in Fig. 2 develops increasing complexity (Fig. 5). Speci"cally, on the low-"eld side it "rst broadens and then splits into multiple features. On the high-"eld side, it begins as a slight shoulder which deepens as the sample becomes older. A also increases in relative intensity. In Table 1, the simple ratio of peak heights, A/B, shows a marked increase between the youngest and the oldest samples. If, in Fig. 5, one examines the A peak for the intermediate sample, the peak broadening understates its contribution to the ratio. Ultimately, for the 2 Ma sample, a third peak Table 1 Q band height ratios Site
Species
Age (Ma)
A/B
France
Mammuthus
Hungary
Equus
Olduvai
Elephas
0.10 $0.05 0.33 $0.06 1.8 $0.2
0.38 $0.01 0.38 $0.01 0.44 $0.01 0.70 $0.01
Includes third peak
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
1029
Fig. 3. The growth of the Q-band spectrum with arti"cial irradiation.
Fig. 4. The growth of Q-band components with artificial radiation, normalized to the natural sample peak height.
appears, still broader than A (see Fig. 5). If one sums that peak height as well, the shift in populated defects from the major to the minor components becomes even more marked. However, there is no independent evidence as yet that this third peak is an hydroxyapatite-type defect.
arise from identical or closely related defects. The primary di!erence lies in the linewidth. Broad linewidths are most often associated with distorted defects that di!er slightly in crystal structure from the primary hydroxyapatite defect and from each other. In essence the broad line is the envelope of many closely spaced lines with the same or nearly the same g-values. The changes in the Q-band spectra with sample age reinforce this conclusion. As the sample becomes older, the crystal structure should increasingly break down. Complete recrystallization, of course, would lead to the loss of accumulated defect sites. Crystals may be deformed in many ways, however, without actually destroying the crystal structure entirely. One would expect, then, to see a shift to the less-well-de"ned defect sites as fossilization proceeds. This is in fact observed qualitatively, and even semi-quantitatively, as one goes from relatively young to quite old teeth. One concern might be that the variation in spectra with age depends on the species. While the three teeth are all from di!erent species, the oldest and the youngest are from closely related mammals. It seems unlikely, therefore, that speciation is responsible for the observed spectra.
4. Discussion Given that both parts of the central multiplet hydroxyapatite peak behave identically with respect to radiation dose and other experimental factors, they must
5. Conclusions The primary conclusion that can be drawn from this preliminary work is that ESR dating of mammal teeth is
1030
A.R. Skinner et al. / Quaternary Science Reviews 20 (2001) 1027}1030
Fig. 5. Comparison of the Q-band spectra for teeth of di!erent ages. As the sample age increases, the size and complexity of the A component increases. For the oldest sample, the `third peaka consists of a low-"eld component at 12,335 G and a high-"eld component at 12,374 G.
not limited by the presence of minor components in the standard X-band hydroxyapatite peak because these components are related to the same HAP defect structure. The normal practice, increasing modulation amplitude to obtain a smooth spectrum, is justi"ed by the Q-band data. The variation in line shape with age reinforces the hypothesis that the minor component, A, may relate to variability in the HAP lattice, since such variability would be associated with crystallographic breakdown during fossilization. If the HAP crystal structure changes with time, the mechanism of U-uptake is likely to change also as the permeability of the crystal lattice increases, complicating Electron spin resonance (ESR) dating. Further research in this area should focus on con"rming the trends seen in Fig. 5 by examining samples from other species and other time periods, and also on crystallographic studies of tooth enamel to look for direct correlations between changes in the ESR spectrum and changes in crystallinity.
Acknowledgements Funding for this work was provided to Williams College by NSF (ILI 9151111, SBR 9709912, SBR 9896289) and to UNH by NIH (R37 GM20194), Irradiations were done at the Wadsworth Laboratories, New York Dept. of Public Health, Albany, NY, courtesy of Dr. Ulrich Rudofsky, and at McMaster University Nuclear Reactor.
Haibo Gu, Williams College, did calculations on the Q-band spectra. Teeth were provided by Dr. J. Hooker, British Museum of Natural History; Prof. L. Kordos, the Hungarian National Geological Museum; and Prof. I. Tattersall, American Museum of Natural History. References Bouchez, R., Cox, R., Herve, A., Lopez}Carranza, E., Ma, J.L., Piboule, M., Poupeau, G., Rey, P., 1988. Q-band ESR studies of fossil teeth: consequences for ESR dating. Quaternary Science Reviews 7, 497}501. GruK n, R., Schwarcz, H.P., Zymela, S., 1987. ESR dating of tooth enamel. Canadian Journal of Earth Sciences 24, 1022}1037. Hennig, G., GruK n, R., 1983. ESR dating in Quaternary geology. Quaternary Science Reviews 2, 157}238. Ikeya, M., 1981. Paramagnetic alanine molecular radicals in fossil shells and bones. Naturwissenschaften 68, 474}475. Ikeya, M., 1993. New Applications of Electron Spin Resonance: Dating, Dosimetry and Microscopy. World Scienti"c, Singapore. Lyons, R.G., 1996. Back to basics: qualitative spectral analysis as an investigatory tool, using calcite as a case study. Applied Radiation and Isotopes 47, 1385}1391. Molodkov, A., 1988. ESR dating of Quaternary shells: recent advances. Quaternary Science Reviews 7, 477}484. Rink, W.J., 1997. Electron Spin Resonance (ESR) dating and ESR applications in Quaternary science and archaeology. Radiation Measurements 27, 975}1025. Rossi, A.M., Poupeau, G., 1990. Radiation damage in bioapatites: the ESR spectrum of irradiated dental enamel revisited. Nuclear Tracks and Radiation Measurements 17, 537}545. Skinner, A.R., Blackwell, B.A.B., Chasteen, N.D., Shao, J., Min, S.S., 2000. Improvements in dating tooth enamel by ESR. Applied Radiation and Isotopes 52, 1337}1344.