Dating Flint Artifacts with Electron Spin Resonance - American

Chapter 4

Dating Flint Artifacts with Electron Spin Resonance: Problems and Prospects Anne F. Skinner and Mark N. Rudolph

Downloaded by YORK UNIV on July 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0625.ch004

Department of Chemistry, Williams College, Williamstown, MA 01267

Electron spin resonance (ESR) dating has been applied to flint artifacts of recent origin (less than 10,000 years ago). ESR is a dating technique developed over the last two decades, using the accumulation of radiation damage as a chronological marker. Using this technique to study archaeological materials requires demonstrating that the ESR signal clock was reset to zero by thermal treatment during manufacture of the artifacts. Under probable heating conditions for these flints, it appears that resetting the signal is considerably more difficult than has been suggested by previous work. Comparison of the results with dates for the site obtained by C-14 yields apparent ages that for the most part are substantially older than the C-14 values, presumably because of insufficient heating in antiquity. However, some of the flints do fall within the generally accepted age range.

Whatever other information archaeologists require, almost always they need to know the chronological age of the material with which they are working. While many dating methods have been developed, all of them are limited in their applicability, either in terms of the time frame or in terms of usable material, or both. Thus any new method may be useful to fill some niche in the overall time and material matrix. Electron Spin Resonance as a Dating Technique Electron spin resonance (ESR) was proposed as a dating method as early as 1967 (1), but its practical application began with the work of Ikeya (2) in 1975. Since then, there have been substantial contributions based on carbonate materials, bones, and quartz. Griin (3) has written a good general review of the subject. ESR, which detects the presence of unpaired electrons, is extremely sensitive to the electronic environment, and has been used for archaeological provenance studies (4) as well as for dating. For present purposes, the unpaired electrons of interest are those created as a result of radiation damage, which implies that ESR has many similarities as a dating technique to thermoluminescence (TL). It has some advantages over TL, primarily in the fact that the signal is not destroyed during measurement, and so it is easier to study a given sample under a variety of experimental conditions. However

0097-6156/96/0625-0037$12.00/0 © 1996 American Chemical Society

In Archaeological Chemistry; Orna, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


38

ARCHAEOLOGICAL CHEMISTRY

ESR is at least in some cases less sensitive than TL; so far, for instance, it is not a practical method for dating pottery. Any object is subject to radiation, but not all forms of radiation damage are sufficiently stable to serve as chronological markers. The general approach to establishing ESR as a dating tool has been to examine materials of interest to archaeologists and geologists to detect ESR signals related to radiation damage. If such a signal exists, it must meet the following criteria in order to be usable for dating studies. (1) The signal must be detectable. ESR spectra are often complex, and the "dating" signal may be obscured by interference from other radical species. The sensitivity of detection puts a lower limit on thetimeframe for which the method is applicable. One of the primary aims of this work was to establish whether flint artifacts less than 10,000 year old were ESR-datable. (2) The signal must be inducible by radiation, and must grow monotonically with the appliedradiationdose. Every material will eventually reach saturation, when an increase in appliedradiationno longer increases the signal, limiting thetimerange of ESR dating for that type of sample. However, saturation occurs at different levels in different materials, and so the age limit depends on the substance being studied. (3) The signal must be stable. Although corrections can be made to the age if the mean life of the signal is known, Arrhenius-plot experiments usually require extrapolation over wide temperature ranges, and thus result in great uncertainty in the value of the mean life. In the case of flint, recent experiments (5) have suggested that laboratory heating yields mean life values that are so low that flint should not be dateable either by ESR or TL, and yet the determined age of artifacts is well within the limits of other evidence. Thus forflintwe deduce that there exists a stable signal, but the exact limit of stability is not yet known. (4) The signal must be zero at the beginning of the time period of interest. In many cases this is the moment at which the material is formed, e.g., the formation of a coral reef, or a stalagmite. Clearly, however, the age of formation of flint is not of interest to archaeologists, and the issue of zeroing the signal at thetimeof artifact production represents one of the major problems met in extending ESR dating from carbonate materials to flint. Assuming that the material has been judged suitable for ESR dating, the actual analysis is virtually identical to that used for TL. Aliquots of the material are irradiated artificially and a growth curve of the ESR intensity is extrapolated to zero, yielding D ("equivalent dose" in Gy), or the amount ofradiation(usually in gamma-equivalent form) needed to produce the observed natural signal. A sample of such an extrapolation is found in Figure 1. From D the age of the material in years can be found by dividing by the sum of the known radiation doses both external ( D in Gy/yr) and internal (Dint Gy/yr) to the material, as shown in the relationship: e

e

ext

in

Age = D / ( D e

e x t

+ Dint).

This of course requires that samples of the matrix in which an artifact was found be available for isotopic analysis. It also implies that the doseratehas been constant with time. For flint, this latter assumption is generally valid, since uranium and its daughter isotopes are in secular equilibrium. The Archaeological Setting Extensive stoneworking activities have been identified at a number of sites in northcentral Florida The known time periods documented for the area range from about



4.

SKINNER & RUDOLPH

Dating Flint Artifacts with Electron Spin Resonance 39

1000-10,000 years ago, placing these sites within the range of carbon-dating. However, artifacts are often recovered from contexts that contain no carbonaceous material, making the implementation of a second dating technique valuable. Johnson Lake is a small pond in the extreme northwestern part of Marion County, Florida, between Gainesville and Ocala. On a bluff overlooking the south shore of the pond is a workshop area first excavated in 1959 (6). The soil of the bluff is welldrained sand, quite dry for Florida. There is noflintoutcrop on the bluff, but approximately one mile to the east is a quarry site, presumed to be the source of most of the numerous specimensrecoveredfrom the workshop site. However, there is considerable variability in the composition of some of the samples, raising the possibility that material may have been brought to Johnson Lake from other areas as well. Most samples show definite evidence of weathering. The samples used in this study were collected by Dr. Barbara Purdy during the late 1980's. They were found at depths from 30-50 cm below the surface. The only criterion used in selecting them was that they appeared to have been heated, judging by color and surface appearance. In addition to the flint samples, samples of the freshwater snail viviparus georgianus were collected from a shell midden at another Florida site expected to be roughly contemporaneous with Johnson Lake. Lake Monroe, in Volusia County, Florida, has been dated by radiocarbon to 4-6000 years Before Present (BP). The Johnson Lake site is estimated to have been occupied between 5-8000 years BP. Comparison of the ESR ages of the two materials should provide further cross checking on the validity of the dating method. Archaeological Use of Flint Hint (or chert, there being no significant geochemical or pétrographie differences between the two terms) is generally described as fine-grained silica. However, samples of flint may differ widely due both to the presence of various silica polymorphs and to trace elements in the non-siliceous matrix between grains. Since ESR is extremely sensitive to small changes in electronic environment, it is not surprising that a variety of ESR spectra can be obtained under the generic heading of "flint". However, there are several radiation-induced signals that are characteristic of siliceous material, and those are the ones being studied for use in dating. Heating flint makes it easier to work into usable artifacts (7-9). However, flint such as that from northern Florida must be heated gradually ("baked") or it will explode and leave no usable fragments. Therefore heating by early peoples would have required a technique such as the one described by Griffiths, et al. (10) of burying flints under campfires. This heating has the potential to anneal any radiation damage experienced by the flint since its formation, thereby setting the dating "clock" back to zero. Thus initial work in this area focused on methods of determining whether or not particular flint samples had been heated in antiquity (to be referred to as "archaeologically heated"). Physical appearance, such as luster, is one criterion. ESR can itself indicate the thermal history of the material. However, the extent of resetting of the ESR signal depends on both the temperature and the duration of heating, something much more difficult to establish. ESR Spectra of Flint The ESR spectra offlintare complex and can vary greatly depending on the source of the flint. However, there are two primary signals derived from the silica fraction of flint that have been studied as chronological markers. Signals are defined by their g-




Saturation Intensity

Radiation Dose (Gy) Relative to Natural Sample Figure 1. Example of growth curve for ESR signal in flint D is given by the x-intercept In this case the natural signal corresponds to approximately 23 Gy of gamma-equivalent radiation. e

I

g-value

2.015

2.010

i

2.005

i

2.00

— I

1.995

Figure 2. ESR spectrum of Florida flint before artificial irradiation, taken at room temperature, showing E' signal. The intensity (I) of the signal is taken as the E* peak height, as shown by the lines.


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SKINNER & RUDOLPH

Dating Flint Artifacts with Electron Spin Resonance

value, determined by studies on pure quartz. Figure 2 shows the room temperature spectrum of one of theflintsused in this study. Visible on the high-field (right hand) side of the spectrum is a peak labeled E\ attributable to an unpaired electron in an oxygen vacancy. The method of production of this defect is believed to be the extraction of an oxygen ion, O", from an S1O4 tetrahedron. E defects can be annealed by heating at temperatures of approximately 350 °C, and if the sample is heated above 500 °C the ability of radiation toregeneratethe signal is much reduced (12). The number of E' defects in most flint appears to be small, and this signal saturates rapidly with artificial irradiation. Also worth noting in this spectrum are the two small peaks immediately to the high-field side of the E signal. These peaks appear to be absent from never-heated flint, but present in heatedflintboth from this site and from at least one other. Therefore their presence or absence in the natural spectrum can serve as a quick screening test for suitable samples. The ESR spectrum can also indicate when theflintbeing studied is not microcrystalline quartz. Figure 3 shows a sample that is superficially indistinguishable from the one used to obtain Figure 2. However, it varied significantly in trace element composition, and this isreflectedin the ESR spectrum. This sample proved completely unsuitable for dating. f


1

Materials and Methods The sample group described here consists offlintartifacts that were judged to have been archaeologically heated, plus some raw flint from the same region. On heating to 350 °C for 15 minutes, this flint changes color. However, prolonged heating is required to reduce the geological signal to a minimum. While it is likely that prolonged heating did occur (given the previously noted need to bake Ocalaflint),the first observation is that color alone will not tell whether the sample is suitable for chronological study. The artifacts range in length from 5.5 cm to 11 cm, in maximum thickness from 10 mm to 15 mm, and in weight from 20 g to 109 g. Allflintswere sanded toremovethe outermost surface, a step that eliminates the surface effects of alpha radiation from the environment The largerflintswere sampled in two ways: "edge" samples were taken from sections whose thickness was less than half the maximum value, and "center" samples were taken from thicker sections. Our intent was to see whether heating had been uniform; our assumption was that center samples would appear older because of greater residual signal. However, within the accuracy of measurement, no difference could be seen between the two types of sample. Samples of both the archaeologically heated and natural flint were powdered in air, and the size fraction between 90-250 μτη was washed with water and dilute hydrochloric acid toremovesurface effects. The powdered sample was then divided and approximately half was heated as noted in the following section (for archaeologically-heated samples, results from this portion will be referred to as "reheated"). Irradiation of 8-10 aliquots each of heated and unheated powder was performed with a Cs-137 source at a rate of approximately 3 Gy/min. Post-irradiation treatment varied from none at all, to annealing at room temperature for two weeks, to annealing at 100 °C for one week. Annealing did not have a significant effect on the results. The range of artificial irradiation has been shown to affect ESRresultsin other materials (11). Large artificial doses may not truly mimic the effect of thousands of years of natural aging — experiments have shown that the signal of saturated


42


signals in TL. In theory these signals should yield the same value for De, and in fact one criterion for reporting these particular samples is that the D values for the two methods do agree within experimental uncertainty (+15%/-25%). Both analytical techniques can be criticized, however. The first relies heavily on the value determined for the reheated natural sample; the second relies heavily on the value determined for the natural archaeologically-heated sample. The second technique has an additional difficulty for these samples. The saturation of the E' signal inflintresults in a sharply curved growth curve, as shown in Figure 1. Thus the interpolation of the natural signal onto the regenerated curve has more uncertainty than is reported when this technique is used in TL. The reported values of D in Table I are derived from extrapolation of the reduced growth curves. The R-values for the fit of these curves is * 0.98. e


e

Table I. Results of ESR Analysis F14 (Gy) UjP (ppm) Thffi (ppm) Kfl" (ppm) Dim (mGy/a) Dext

(mGy/a)

F17

F18

Shell

87.8

185

21.8

3.6

0.71

0.68

0.97

2.35

0.219

0.136

0.208

0.147

164

81

123

91

600

482

698

1416

8

745

745

745

745

468

16.0

72

128

10.1

7.65

21.5

F16

0.1 —

—

c

Age

a

Ufl, Thfl, and Kfl refer to isotope concentrations within the sample. ^Dfext forflintwas calculated from the following information: Used - 0.8 ppm; Thsed - 1.5 ppm; Ksed - 200 ppm; cosmic dose 193 mGy/a. Efext for shell was calculated from the following information: Used - 0.7 ppm; Thsed - 1.2 ppm; Ksed -150 ppm; cosmic dose 210 mGy/a. Water content of shell matrix: 12% Age uncertainties on ages are +15%/-25%. c

In converting D to age, a number of assumptions had to be made. The factor κ, which describes the relative efficiency of alpha radiation in inducing ESR-detectable damage with respect to the efficiency of beta and gamma radiation, was set at 0.1 in agreement with most other work (15). However, this value is derived from TL work on flint There is no experimental evidence for this value for ESR measurements, and values from 0.05 to 0.2 can be found in the literature. Using a value of 0.05 would increase the reported age by approximately 10%; using a value of 0.2 would decrease the age by approximately 20%. The moisture content of the soil was assumed to be e


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SKINNER & RUDOLPH


materials can appear to be increased by additional irradiation. Thus the upper limit used for analysis of these samples was 100 Gy. ESR spectra were taken on a JEOL RE1X spectrometer. The E spectra were recorded using a modulation amplitude of 0.5 millitesla (mT), a scan speed of 0.3 mT/min and at 0.1 mW of power. This signal is extremely sensitive to power, largely because it is not a single line. Artificial irradiation induces an additional signal at g=2.002. Annealing experiments suggest that this signal is short-lived, and it is only a minor component in the natural sample. At low power this signal can be distinguished from the "dating" peak at relatively low levels of artificial irradiation. Above approximately 200 Gy of artificial irradiation, measurement of the dating signal becomes problematic even at low power. However, since upper limit of additional dose was 100 Gy, this did not limit our ability to measure these samples.


1

Results Results of Heating Experiments. Studies on Florida flint (8) indicate that improvement in flint working properties requires heating within a fairly narrow temperature range of 350-400 °C. Our heating experiments, therefore, were performed at 350 °C. Previous work (12) had suggested that heatingflintto 350 °C for 30 minutes was sufficient to anneal completely the E signal. The samples in that case were considerably older than the ones reported here. In contrast, we found that even after heating for 24 hours, there was a perceptible residual E' signal. In some cases this irreducibly minimum signal level was greater than 50% of the original signal. In itself this is not surprising. As noted, these are relatively young samples and therefore the signal due to regeneration since manufacture would be expected to be small. In older samples, where the archaeological signal is much larger (since it has been accumulating for a longer period of time), the residual signal is a small percentage of the original and might be overlooked considering the numerous other uncertainties of measurement The question then becomes: what happens when an archaeologically heated sample is reheated and subsequently irradiated with similar doses (i.e., total artificial dose approximately 100 Gy) to regenerate the growth curve? Does the residual signal show a different sensitivity from the original one, or does the regenerated signal depend on the size of the residual signal? Comparison of the growth curves of archaeologically heated and reheated artifact samples generally suggests that irradiation of reheated samples reproduces the shape of the archaeologically heated samples (see Figure 4). f

f

Dating Results Using the E Signal. Most of the artifacts in this sample set were not dateable. As noted, one was not siliceous flint. Others had not been heated at all, on the evidence of their natural signal and on the evidence of the rapid heating test suggested by Toyoda et al. (13). Two samples had been heated, but when irradiated artificially they saturated almost immediately. The interpretation of this result is that the archaeological heating was insufficient to decrease the signal. Parenthetically, the "dating" of natural flint produced an "age" of approximately 500,000 years, but the uncertainty of this measurement is over 100% due to the saturation of the E' signal. We were most interested in four other samples where the results were more ambiguous. Two different growth curves were constructed for each of these samples. In one, the intensity of the residual signal was subtracted from all aliquots of the artifact signal, creating a reduced growth curve. This subtraction was based on the observation that the sensitivity was unchanged by further heating. In the other, a regeneration technique was used, analogous to the treatment of unreachable



44


g-value

2.01

1.99

Figure 3. ESR spectrum of anomalous Florida flint, taken at room temperature.

-100

-50

0

50

100

150

200

Artificial Dose (Gy) Figure 4. Comparison of growth curves of archaeologically-heated (Unheated) and reheated (Heated) flint The dose values for the unheated sample have been shifted by an amount equivalent to D in order to show the superposition of the two curves. e


4.

SKINNER & RUDOLPH


the present-day value of 5%. Calculation of cosmic dose assumed a uniform burial depth of 40 cm. Table I shows thatflintsfrom the same site do not necessarily have the same apparent age. The older ages are presumably due to heating that was insufficient to reset the E signal from its geological value to its irreducibly minimum value. However, the youngest ages are not inconsistent with the other evidence of the age of the site, and are sufficiently promising to justify continued study of flints. The last entry in Table I is the set of results for the freshwater snails from Monroe Lake. Non-marine snails are very sensitive dosimeters (14) and thus young samples can be dated. The datereportedis in good agreement with radiocarbon results from the site (Purdy, Β. Α.; private communication, 1994), although at the upper limit of those results. While not directly relevant to the age of the Johnson Lake site, the results suggest that most assumptions about the external dosimetry in northern Florida are reasonable, but that a part of the reason for theflintages being "too high" might lie in uncertainties in dosimetry.


1

Conclusions Future work must concentrate in two areas. Use of the E' signal requires more information about the irreducible signal — whether the behavior of the Florida samples can be generalized to otherflints,or whether it is related in some way to the microstructure of flint In that case there might be other sites where reheating of the archaeological samples was not necessary to determine ages with a room-temperature technique. Another aspect of the E signal that needs further study is the dependence of the signal on microwave power. If a power level of 0.6 mW is used with theseflints,D decreases substantially, bringing the calculated ages into very good agreement with other data. Ikeya (15) has noted that there seem to be good reasons not to use power levels above 0.1 mW for the E signal, a conclusion supported by some preliminary work in this study. However, like so many other aspects of the study of flint, it may be that the optimum power level varies with the type offlint,and that more work might demonstrate a justification for higher power levels. The second possible approach is to use a different marker. The signal shown in Figure 5 is attributable to the substitution of A l * for silicon. Clearly one cannot 1

e

1

3

Al

g-value

2.01

2.00

1.99

Figure 5. ESR spectrum of Floridaflint,taken at 77K, showing Al signal. Note that the intensity (I) of the signal is measured from the peak of the high-g line to the trough of the low-g line. In Archaeological Chemistry; Orna, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

46


rely on the presence of this signal, but in fact it is found in most flints. This signal must be measured at 77 Κ in order to localize the unpaired electron. It appears to be annealed more readily than the E' signal. Therefore an initial test for adequate heating in antiquity would be to determine if both signals give the same apparent age. However, in flint there appears to be an interfering signal (12) and signal subtraction was used to obtain Figure 5. This may introduce as much uncertainty in the A l results as the irreducible signal does for the E results. Clearly this dating method is not ready for routine use on young samples. The uncertainties of measurement alone are too large for accurate determination in cases where 1000 years is a major time period. However, the present work has shown that small signals can be isolated and measured. And, although it is always desirable to find identical ages for samples from a given site, if a series of ages is found, this work indicates a theoretical basis for selecting the youngest as the most probable date. Downloaded by YORK UNIV on July 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0625.ch004

3 +

1

Acknowledgments Neutron activation analyses were performed at the Oregon State University Radiation Center with funding from the U.S. Department of Energy's University Reactor Sharing Program. The JEOL spectrometer was purchased with the assistance of NSF I U grant number 915111. Sample irradiations were performed at the Wadsworth Laboratories of the New York State Department of Health with the kind assistance of Dr. Ulrich Rudofsky. Data analysis used the FIT-SIM program of Dr. Rainer GrUn. The authors would also like to express their gratitude to Dr. Barbara Purdy, professor emerita at the University of Florida, not only for supplying the samples but for discussions on a wide range of archaeological topics. Literature Cited 1. Zeller, E. J.; Levy, P. W.; Mattern, P. L. Proceedings of the Symposium on Radioactive Dating and Low Level Counting; International Atomic Energy Agency: Vienna, 1967; pp 531-540. 2. Ikeya, M. Nature 1975, 255, 48-50. 3. Grün, R. Quaternary International 1989,1,65-109. 4. Lloyd, R. V.; Smith, P. W.; Haskell, H. W. Archaeometry 1985, 27, 108116. 5. Porat, N. and Schwarcz, H. P. 1995, Appl. Rad. Isotopes, in press. 6. Bullen, R. P.; Dolan, Ε. M. The Florida Anthropologist 1959,12,77-94. 7. Crabtree, D.; Butler, B. Tebiwa 7, 7, 1-6. 8. Purdy, Β. Α.; Brooks, Η. K. Science 1971, 173, 322-325. 9. Domanski, M.; Webb, J. Α.; Boland, J. Archaeometry 1994, 36, 177208. 10. Griffiths, D. R.; Bergman, C. Α.; Clayton, C. J.; Ohnuma, K; Robins, G. V.; Seeley, N.J. In The Human Uses of Flint and Chert, Sieveking, G. G.; Newcomer, M. H., Eds.; Cambridge University Press: Cambridge, 1987; pp 43-52. 11. Barabas, M.; Walther, R.; Wieser, Α.; Radkte, U.; Grün, R. Appl. Rad. and Isotopes 1993, 44, 119-130. 12. Porat, N ; Schwarcz, H. P. Nuclear Tracks Radiation Measurement 1991, 18, 203-212. 13. Toyoda, S.; Ikeya, M. Appl. Rad. and Isotopes 1993, 44, 227-231. 14. Skinner, A. F.; Mirecki, J. Appl. Rad. and Isotopes 1993, 44, 139-144. 15. Ikeya, M. New Applications of Electron Spin Resonance: Dating, Dosimetry and Microscopy; World Scientific: Singapore, 1993;p279. RECEIVED October 9, 1995


Dating Flint Artifacts with Electron Spin Resonance - American

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