Expanding the Range of Electron Spin Resonance Dating - American

Chapter 1

Expanding the Range of Electron Spin Resonance Dating 1

1,2

2

Anne R. Skinner , Bonnie A. B. Blackwell , Maysun M. Hasan , and Joel I. B. Blickstein 2

Downloaded by 80.82.77.83 on May 28, 2018 | https://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch001

1

Department of Chemistry, Williams College, Williamstown, MA 01267 R F K Science Institute, 75-40 Parsons Boulevard, Flushing, NY 11366

2

Archaeologists often turn to physical science for help in determining ages of sites and/or materials. Electron spin resonance (ESR) has proven a valuable tool for dating eutherian teeth from archaeological, paleoanthropological, and paleontological sites ranging in age from 10 ka to 5 M a . Because tooth enamel is the body's hardest tissue, teeth are frequent components of fossil assemblages. The E S R signal stability in enamel exceeds 100 M a . In eutherian teeth, the ESR signal does not depend on species. We have tested metatherian, reptilian and chondrichthyian teeth to see whether this method can be extended to additional environments and time periods. In diprotodontid marsupial teeth, the E S R signal appears identical to that in eutherian enamel. The crocodylian teeth tested here contained significant iron concentrations, which interfered with the E S R signal. Their fossilization in fluvial sediment may have caused the iron contamination. Reliability in other reptile teeth remains to be tested. Although shark teeth contain enameloid rather than enamel, the E S R signal morphology differs only slightly. However, the signal stability is reduced and calculated ages are much younger than expected.

© 2007 American Chemical Society

Glascock et al.; Archaeological Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

1

2 Archaeological and paleontological applications of chemistry predominantly take two forms. One is the direct determination of some archaeologically significant factor, perhaps the age, composition, or provenance of an artifact. The other is a methodological development, either a new technique or an improvement to an existing one, that enables scientists to analyze new objects. Electron spin resonance (ESR) has been very successful in dating teeth as young as a few thousand years and as old as several million years. Ages can, therefore, be cross-calibrated both with C and with ^ A r / ^ A r results. Most archaeological applications have used enamel from teeth of large placental mammals such as bovids and equids, in part because teeth, being the hardest tissues in the body, are often preserved (7). There are many environments and time periods where placental mammals are rare or non-existent. To be able to date teeth from a wider range of species would expand the physical and temporal range of this method. Therefore we have examined three other taxa: marsupials, crocodiles and sharks. Dating methods fall into three main groups. Radiometric methods measure the present radioisotope content of the sample and, by knowing the initial concentration and the decay rate, calculate an age that is largely independent of environment. Those most often part of archaeological and paleoanthropological investigations are C and A r / A r . These radiometric methods (like all methods) have limitations. Dating with C requires some calibration because the initial concentration has varied; A r / A r is useful only for volcanic rocks. Other chronometric methods including, amino acid racemization, uranium series and ESR require knowledge of the environment for accurate results, but can yield a numerical age. Relative methods such as stratigraphy simply enable one to tell which of two objects is older.


1 4

1 4

39

40

1 4

3 9

4 0

ESR Dating As one of the 'trapped charge' methods, E S R dates materials by measuring damage caused through environmental radiation, rather than by measuring radioisotope content. The general principles of the method have been covered in numerous reviews (2-4). ESR measures the number of unpaired electrons created when radiation cleaves a stable chemical bond (or knocks an electron out of a lone pair). To be used in dating, the ESR signal must grow in a reproducible fashion with radiation dose, whether natural or artificial. Most trapped charge methods, such as T L and OSL, as well as ESR, study a signal response as a function of this dose, creating a growth curve (Figure 1). T L , however, cannot study teeth, because the heating needed to produce luminescence chars the teeth. Saturation limits ESR dating of teeth; samples older than 5-8 million are saturated or so close to saturation that the difference cannot be detected with current technology.


3 In enamel the E S R signal arises from hydroxyapatite (HAP). The shape of the signal (Figure 2) reflects the existence of both g and gjj components. Teeth have several advantages as dating samples. The signal is extremely stable, with a mean lifetime exceeding 10 years (5). The signal is also unaffected by light, which means samples can be collected and prepared in ambient light. Museum samples can be dated without fear that the sample has been affected by storage. Another advantage is that measuring the signal does not destroy the signal, allowing samples to be remeasured repeatedly, perhaps using different measurement parameters. For E S R in teeth, enamel is separated from dentine and then powdered. Aliquots are irradiated to obtain a growth curve. The accumulated dose (AD) is then converted to an age by factoring in the external and internal dose rates. Standard parameters, used for all dates in all tables here, assume sedimentary water concentrations of 10 ± 5%, no radon loss, an initial U activity ratio, ( U/ U) =1.20 ±0.20, and an α-efficiency factor, k = 0.15 ± 0.02. While the external sample dose and our artificial doses are supplied by γ-radiation, uranium within the enamel delivers primarily α particles. A l l ESR dating, therefore, requires knowing the relative efficiency of these two types in inducing the measured signal. ESR ages generally show a precision of about ± 5%. ±


10

234

238

0

a

There are two caveats with this method. First, the dose to the sample derives in part from the environment around the sample during the entire deposition period. Numerous environmental factors must be measured or modeled in order to obtain a result that is both accurate and precise. With increasing time, our ability to do so becomes more problematic. This problem is, of course, common to all materials and all trapped charge methods. Second, teeth (and some other materials, e.g., bones and shells) are open systems with respect to uranium uptake, which means the internal dose rate changes with time. Traditionally researchers have simply assumed one of three models: early uptake ( E U ; all uranium enters the tooth shortly after deposition), linear uptake ( L U ; continuous uptake), and recent uptake (RU; most of the modern concentration of uranium entered the tooth, and in particular the enamel, shortly before excavation). It is now possible to use coupled T h / U dating to obtain a more accurate model. While U-series ages such as those from T h / U disequilibrium are even more problematic for open systems, by iterating the two results a reliable age can be obtained (6). Where the sample age exceeds the T h / U limit (-500 ka), or if the tooth does not contain enough uranium for a precise T h / U age determination, modeled ages are still reported. 2 3 0

2 3 4

2 3 0

2 3 4

2 3 0

2 3 4

2 3 0

2 3 4

Marsupial Teeth The Lake Eyre Basin in Australia contains the world's largest ephemeral lake. During the Quaternary the climate fluctuated between relatively arid and


4


Saturation Intensity

20

0

20

40

60

80

100

120

Figure 1. Typical ESR growth curve. Aliquots of enamel or other sample material are irradiated artificially and the increase in intensity measured. A minimum of 10-12 points is required to provide statistical accuracy. The extrapolation of the growth curve to the x-axis gives the accumulated dose, AD.

3310 3320 3330 3340 3350 3360 3370 3380 3390 3400 3410 3420 Magnetic field (Gauss)

Figure 2. Typical HAP ESR spectrum



5 relatively humid episodes. The wet phases correlate to Northern Hemisphere glaciations. Faunal correlations appeared to place the Katipiri Formation, the younger of the two fluvially deposited Quaternary formations in the basin, in Oxygen Isotope Stage (OIS) 5. Faunal correlations at best provide only relative ages, especially when temporally mixed assemblages are detected. While fauna adapted to arid conditions can coexist with those adapted to wetter environments, any fossil assemblage in a fluvial deposition could be reworked from older deposits and, therefore, not provide accurate correlations. Studying the ages of these fauna would clarify temporal variability in the Katipiri sands and help understand paleoenvironments during this period. In addition, at some point during the Quaternary megafauna including Diprotodon, Genyomis and other giant marsupials became extinct. Miller et al. (7) suggested that this extinction might be related to the arrival of hominins. Alternatively extinction might have resulted from environmental changes. Chronometric dates for the fauna would provide some answers to this problem. Several diprotodontid teeth from the Lake Eyre Basin were prepared by standard methods. While collected from seven different locations, all were expected to be the same age. Diprotodontid canines and incisors have a single enamel layer around a dentine core. The signal from diprotodontid enamel does not differ significantly from that in eutherian mammals. The results, however, showed that deposition in the basin was quite complex (Table I). K T 5 , with an L U age of 46.3 ± 2.2 ka, is probably in situ. The wide age range for the other teeth clearly indicates they have been reworked from older formations. Therefore, K T 5 provides a maximum age for the Katipiri Formation deposition and dates it to OIS 3 rather than OIS 5. It also shows that diprotodonids, at least, were not extinct before that time. Hominins are believed to have arrived in Australia between 50 ka and 60 ka (5), and at least some megafauna survived for more than 15 ka after this appearance in southeastern Australia (9). E S R dating shows their survival in southwestern Australia as well. Diprotodon, about the size of a bear, should have been an excellent food source. Although hominins could well have played a role in the extinction, the pace of their influence may have been slow in comparison to similar events in other parts of the world.

Crocodylian Teeth Crocodylians, which include crocodiles, gavials, alligators, and their immediate relatives, are poikilothermic quadrupedal reptiles that first evolved in the late Triassic. Today, crocodylians inhabit the subtropical to tropical zones on all continents. Their wide geographic distribution and evolutionary stability make them valuable dating samples (70). Three crocodylian teeth plus one elephant tooth from the Siwalik Group of India were dated by standard E S R methods. From oldest to youngest, the Upper


6

Table I. Mean ESR Ages for Teeth from the Katipiri Formation


Sample (subsamples) KT5 + (4)

Accumulated Dose, AD (Gray) 36.61 0.76

Weighted Mean Standard ESR Ages RU LU EU (ka) (ka) (ka) 48.8 46.3 41.6 2.2 2.5 1.8

KT4 (4)

±

221.1 6.7

51.7 2.9

85.2 4.5

173.6 8.8

KT7 (6)

+

529.1 8.1

125.0 4.4

210.8 7.1

376.7 12.9

±

551.4 12.1

155.6 6.7

254.6 10.6

597.0 30.2

KT6 (3) KT3 (4)

±

1716. 26.

320.7 15.6

562.8 25.0

1411. 58.

KT8 (2)

±

1793. 117.

369.1 41.4

636.5 66.7

1516. 147.

KT1 (4)

±

2810. 68.

559.6 29.2

973.3 46.7

2439. 113.

KT2 (4)

+

2280. 42.

767.3 33.1

1196. 41.

2337. 90.

±

4419. 216.

876.2 77.6

1316. 115.

3601. 282.

±

4943. 318.

2657. 211.

5096. 437.

KT10 (1) KT9 (2)

Abbreviations:

1671. 137.

assuming early U uptake assuming linear (continuous) U uptake assuming recent U uptake



7 Siwalik Group contains three main formations, Tatrot, Pinjore, and the Boulder Conglomerate. The teeth in this study came from the Pinjore Formation. From its magnetostratigraphy, the Pinjore Formation ranges from 2.48 to 0.63 M a (77). Based on their faunal associations, the teeth probably date from 1.5 to 2.5 M a , but the lower units in the formation have not been dated chronometrically. The Siwalik Group has long been famous for its abundant vertebrate fossils, among which are some early hominid ancestors. Its primates include Ramapithecus and Sivapithecus from the older beds, and younger specimens, cf. Homo erectus, from the Upper Siwalik Group, including the Pinjore Formation (72). Dates would therefore help establish the arrival time of hominins in the Indian subcontinent. Preparing these teeth was extremely difficult. Crocodylian teeth are small, and dark mineralization made it difficult to detect the boundary between dentine and enamel in order to separate the two tissues. To ensure an accurate E S R age, all dentine must be removed from the enamel, not because dentine affects the ESR signal, but because the uranium concentration in dentine is often as much as an order of magnitude greater than in enamel. Contamination of enamel by dentine, therefore, affects the calculation of the internal dose. Once prepared, the ESR spectra showed that the fossil enamel contained significant amounts of Fe , interfering with-almost concealing in some cases-the H A P peak. The presence of F e causes a sloping baseline, with the slope directly proportional to the iron(III) concentration (Figure 3A). Dating requires knowing the intensity of the peak, and unfortunately could not be calculated for most crocodylian subsamples. Signal subtraction demonstrates that an H A P dating peak exists (Figure 3B and 3C). The signal shape looks different, reflecting a decrease in resolution due to the subtraction process. Nonetheless, the signal grows with radiation in a manner similar to other enamel samples (Figure 4). Despite these problems, growth curves could be constructed for some samples, and ages calculated (Table II). The ages of these teeth appear to be too young, unless the R U , or recent uptake, model is invoked. This model has been shown to be more probable for teeth > l M a in age (73). This appears counterintuitive, but is related to changes in enamel crystallinity and porosity that allow more uranium to penetrate an older tooth than a younger one. Invoking the R U model also yields an age for the elephant tooth more in agreement with expectations (Table II). Even so, the crocodile teeth appear younger than the elephant tooth, perhaps because the F e interference has led to inaccurate accumulated dose estimates or possibly due to reworking of the crocodile teeth. Determining signal stability awaits the discovery of some crocodile teeth without contamination. A single aliquot was annealed for 9 hours at 200°C without significantly reducing the apparent peak intensity, suggesting that the signal stability resembles that in mammalian enamel. 3+

3+

3+


8


Si m

Figure 3. Crocodylian ESR spectra. A: Spectrum of natural sample. Β: Spectrum after 30 kGy artificial irradiation. C: Result of subtracting A from B. Note that the subtracted spectrum has only one minimum, compared to two in Figure 2. The overall peak width is the same in both this spectrum and the typical mammalian example.

m c C4 CC ft LU

-5.6

0.0 5.6 11.1 16.7 22.2 A b s o r b e d D o s e (x 10 G r a y )

27.Θ

Figure 4. Growth curve of subtracted crocodile spectra. The shape is normal for ESR growth curves. That the x-axis intercept (AD) is not zero is due to imprecision in reading the spectra.


9 There is another potential explanation for the young ages. Oduwole and Sales (14) previously observed in bones that F e interferes with the H A P signal. They proposed several possible explanations, ranging from the possibility that the dating signal, always weak in bones, simply could not be seen, to the more intriguing possibility that F e reacts with electrons that otherwise would have been trapped in the H A P , reducing F e to F e and also, of course, reducing the 3+

3+

3+

2+


Table II. Preliminary E S R Ages for Teeth from Devni-Khadri, India. Accumulated Sample Dose, AD (Gray) (subsamples) a. Crocodylian teeth FT46 5098. (2) ± 334. FT47 (2) FT48 (3)

b

129. 10.

237. 18.

0.79 0.06

1.30 0.09

6052. 438.

171. 15.

305. 26.

0.85 0.07

1.17 0.09

±

4394. 145.

173. 9.

307. 16.

0.80 0.03

1.06 0.04

162. 6. 3.8%

290. 11. 3.8.%

0.81 0.03 3.4%

1.13 0.04 3.5%

336. 15. 4.4%

639. 27. 4.3%

2.59 0.09 3.4%

-

b. Elephantid tooth FT38 (8) ±

c

Mean Standard ESR Ages RU" LU (Ma) (Ma) (ka)

±

Crocodylian mean

a

EU (ka)

3693. 81. 2.2%

Abbreviations: EU = assuming early U uptake LU =assuming linear (continuous) U uptake RU =assuming recent U uptake Calculated using U uptake parameter, ρ = 10. Calculated using U uptake parameter, ρ = 20

dating peak intensity. The process of creating the growth curve for one subsample demonstrated some direct evidence for the latter. The slope of the Fe baseline decreased exponentially with increasing applied radiation, suggesting that the F e concentration was decreasing (Figure 5). Not all samples in this study showed this phenomenon, however. The explanation may well lie in the same factor that generally accounts for the signal stability in enamel - the 3+

3+


10 large crystal size. Radicals formed in the interior of such a crystal would normally be sheltered from destruction by reaction with water, or oxygen. If, in these teeth, the F e was incorporated into the enamel at the time of crystallization, due to iron in ground water, it could easily react with other radicals formed by radiation. Other samples, where the F e was perhaps adsorbed on the surface of the crystal, would not show this effect, explaining why it was not universally observed. Studies presently underway into the uptake of F e into crocodylian and other teeth may differentiate these possibilities or even suggest other explanations. 3+

3+


3+

Shark Teeth Clearly environments hospitable to sharks are not likely to be occupied by humans. Dating shark teeth will thus not provide information on hominins directly, but when combined with other studies the dates can reveal paleoenvironmental information, which could include factors affecting hominid behavior. Shark teeth are sometimes of archaeological interest. People have been known to use shark teeth as decorative items. In principle, i f the teeth were mined from a deposit rather than collected from the shore, the date of the teeth might indicate provenance, and hence elucidate such evidence of human mobility as trade patterns. This experiment, however, simply assesses whether shark teeth might yield a signal and, therefore, a reasonable age. As with the marsupials, in looking at a population the youngest tooth would provide a maximum age for the formation, with older samples indicating the extent of reworking. Given that shark teeth are easy to prepare, analyzing a population would not be excessively challenging. The shark teeth in this study came from a museum collection and were catalogued as Pleistocene or Pliocene. The structure of these teeth differs from the other taxa in this chapter in that the material around the dentine core is enameloid, not enamel. The ESR signal in shark teeth (Figure 6) also differs from that in mammalian enamel. The minor peaks in Figure 6 can be attributed to organic matter. Mammalian enamel contains less than 2% organic material; dentine contains around 20% and its ESR spectrum shows some of the same organic structure peaks. The H A P signal in Figure 6 also exhibits different contributions of gy and g components when compared to Figure 2. The calculated ages for shark teeth are younger than predicted from their provenance (Table III). The signal might have faded due to reduced signal stability, suggested by annealing experiments. Signal stability is expected to ±



11

Added Ctos# (bm0 Figure 5. Effect of artificial irradiation on ESR intensity for FT48en3. The intensity appears to decrease exponentially with added dose.

4000

Figure 6. ESR Spectrum of shark enameloid. Arrows indicate position of organic radical peaks.


12 Table III. Mean ESR Ages for Shark Teeth Accumulated Dose, AD (Gray)

Sample

Weighted Mean Standard ESR Ages"

(subsamples) ±


RST3 (2)

±

RU (Ma)

(ka)

(ka)

RST1A (2)

LU

EU

2980.9

265.2

435.1

1.7

314.2

25.1

47.2

0.08

10.5%

9.5%

11.4%

4.96%

1661.3

277.4

506.4

1.61

129.1

27.8

51.0

0.17

7.8%

10.0%

10.1%

10.28%

"Assuming: κ = 0.15 (as is true for enamel), and enameloid density is the same as that of enamel α

decrease with crystal size. Both enameloid and dentine have much smaller H A P crystals than does enamel. S E M photos illustrate clearly the order in enamel compared to the disorder and small crystal size in enameloid (Figures 7 and 8). Alternatively, radiation may affect enameloid and enamel in different ways. Consider, specifically, the α-efficiency factor, κ . As noted earlier, in enamel it has been measured as 0.15 ± 0.02. Experiments in our laboratory suggest this factor is substantially less in dentine, although a precise value has not yet been determined. If the same were true for enameloid, the calculated ages would be considerably greater. α

Conclusions Methodological developments are essential to improving the ability of physical science to solve archaeological problems. In this case, we have moved ESR dating forward slightly. The marsupial results open up archaeological possibilities on the Australian continent, for example dating aboriginal settlements beyond the C limit. The F e in crocodylian teeth requires further study. Modern crocodylians do not have this impurity. Investigating F e scavenging in these teeth can illuminate the mechanism of radical formation and decay in teeth. Also, of course, clearly not all crocodylian teeth will be contaminated with iron, so their dating potential merely awaits discovery of , 4

3+

3+



13

Figure 7. SEMmicrograph

of mammalian enamel. Note smooth surface.

Figure 8. SEM micrograph of enameloid at the same scale. Note small crystals and random orientations.


14 appropriate samples. While shark teeth are not suitable dating material, at least with current technology, the SEM results reinforce other observations, such as the weak and unstable signals in dentine and bone, both of which, like enameloid, contain only very small HAP crystals.

References


1. 2.

3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14.

Falguères, C. Quatern. Sci. Rev. 2003, 22, 1345-1351. Blackwell, Β. A. B. In Tracking Environmental Change Using Lake Sediments: Basin Analysis, Coring, and Chronological Techniques; Last, W. M.; Smol, J. P., Eds.; Kleuwer: Dordecht, 2001; Vol. 1, pp 283-368. Grün, R. Quatern. Int. 1989, 1, 65-109. Rink, W. J. Radiat. Meas. 1997, 27, 975-1025. Skinner, A. R.; Blackwell, B. A. B.; Chasteen, D. E.; Shao, J. M.; Min, S. S. Appl. Rad. Isot. 2000, 52, 1337-1344. Grün, R. Ancient TL 2000, 18, 1-4. Miller, G. H.; Magee, J. W.; Johnson, B. J.; Fogel, M. L.; Spooner, Ν. Α.; McCulloch, M . T.; Ayliffe, L.K. Science (Washington, DC) 1999, 283, 205-208. Bowler, J. M.; Johnston, H.; Olley, J. M.; Prescott, J. R.; Roberts, R. G.; Shawcross, W.; Spooner, N. A. Nature (London) 2003, 421, 837-840. Trueman, C. N. G.; Field, J. H.; Dortch, J.; Charles, B.; Wrœ, S. Proc. Natl. Acad. Sci. 2005, 102, 8381-8385. Naish, D. Geol. Today 2001, 17, 71-77. Nanda, A. C. J. Asian Earth Sci. 2002, 21, 47-58. Patnaik, R. pers. comm., 2005. Skinner, A. R.; Chasteen, D. E.; Shao, J. M.; Goodfriend, G. A; Blackwell, Β. A. B. Quatern. Int. 2005, 135, 13-20. Oduwole, A. D.; Sales, K. D. Nucl. Tracks 1991, 13, 213-221.


Expanding the Range of Electron Spin Resonance Dating - American

Recommend Documents