Trace Element Analysis in the Characterization of Archaeological Bone

6 Trace Element Analysis in the Characterization of Archaeological Bone G. WESSEN, F. H . RUDDY, C. E. GUSTAFSON, and H . IRWIN 1

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Downloaded by UNIV LAVAL on May 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0171.ch006

Departments of Chemistry and Anthropology, Washington State University, Pullman, WA 99163

Trace element analysis using neutron activation has been used to characterize archaeological bone. The alkaline earth elements strontium and barium appear to be reliable indicators of bone origin. Studies of recently killed specimens suggest that these elements are homogeneous throughout the skeletal matrix so that small samples may be regarded as representative of the entire organism. Alterations of elemental concentrations resulting from interactions of the sample with the depositional environment have been examined empirically by analyzing various samples in contact with contrasting depositional environments for different time periods. The results of the analysis of over 350 morphologically distinct specimens have provided identification criteria for archaeological artifacts made from bone of unknown origin.

Hprace element analysis of selected archaeological mateials has been an expanding of enquiry since the early 1950s ( J ). The ability to characterize materials i n terms of distinctive trace element content has provided the opportunity for detailed reconstruction of prehistoric economies and technologies i n many parts of the world. I n particular, such studies have been especially informative i n the examination of trade and exchange networks and the growth of associated economic centers. A number of inorganic materials have provided useful trace element data including obsidian (2,3,4,5,6), ceramics ( 7 , 8 ) , coins (9), and Present address: Hanford Engineering Development Laboratory ( Westinghouse Hanford), P.O. Box 1970, Richland, WA 99352. Deceased April 1978. 1

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0-8412-0397-0/78/33-171-099$05.00/l © 1978 American Chemical Society Carter; Archaeological Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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other metal objects ( I ) . However, the application of trace element studies to organic materials such as bone has been particularly limited (10,11,12,13). The analysis of archaeological bone presents several problems which are encountered to a much lesser degree i n the analysis of inorganic substances where often a discrete source is involved and where the matrix is not as open to contamination b y the depositional environment. Nevertheless, bone is one of the most commonly found archaeological materials, and any inferences made from its trace elemental composition would certainly be useful. W e relate herein our experiences with the trace element analysis of archaeological bone, the problems encountered i n these analyses, and some of the conclusions that w e have reached as the result of our measurements. Experimental Considerations Trace element data may be useful for characterization purposes provided that the following criteria are met: (1) The concentrations measured are homogeneous throughout the entire matrix and are characteristic of the organism studied. (2) The concentrations measured are not altered by contamination. (3) Trace elements have not been lost through interaction with the depositional environment. (4) The variations of trace element content may be reasonably and reliably interpreted. In order to satisfy the first condition, trace element concentrations were measured i n samples from various portions of single bones and throughout the skeletons of recently killed specimens. Conditions two and three seemed to be met best by choosing elements that were chemically similar to the major elements i n the matrix. Calcium is a major constituent of bone, and elements from the alkaline earth group were chosen for study. Attempts to measure magnesium were unsuccessful, and attention was focused on barium and strontium which were measureable in most samples. Neither barium nor strontium has a demonstrated nutritional or metabolic role so they both may be regarded as biologically inert. Both are calcium analogs and are are found mainly i n bone. The fact that environmental availabilities of strontium and barium are variable (14,15), lead to optimism that the choice of these elements might satisfy condition four. The method of analysis used is neutron activation. The samples are irradiated for 10 min i n a thermal neutron flux of 6 X 10 n / ( c m sec) i n the Washington State University T R I G A Mark III reactor. After 10 min have been allowed for decay of short-lived activities, the samples are assayed for gamma-ray activity using a G e ( L i ) detector whose signals are monitored by a 1024-channel ND-160 analyzer. The analyzer is inter12

Carter; Archaeological Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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faced to a PDP-15 computer which is used to find the peaks in the gamma ray spectra and to determine the intensities by numerical peak integrations. The amounts of the elements present are determined by comparison with known standards containing a measured amount of the elements. A typical standard consisted of 1.114 mg of calcium, 802 f i g of strontium, and 650/Ag of barium as the nitrate salts evaporated to dryness from standard solutions i n the bottom of an irradiation vial. Seven samples and one standard are irradiated simultaneously. Immediately after cooling the samples and standards are counted for 2 min each to determine the activity of 8.80-min C a (Ey = 3084.4 k e V ) . The calcium concentrations are roughly constant i n the range 15-25% by weight and indicate metabolic imbalance or other abnormalities i n a sample. After the calcium counts a longer count (8-16 min) at a higher amplifier gain setting determines the activities caused by 170-min S r ( E y = 388.5 k e V ) and 82.8-min B a ( E y = 165.8 k e V ) . Special consideration has been paid to sample preparation to minimize problems resulting from loss of trace elements to or contamination by the depositional environment. The isotopes M n and N a have been particularly troublesome as background activities. According to the x-ray microprobe work of Parker and Toots (16) sodium appears to be associated with the apatite part of the bone (probably substituting for calcium in apatite), whereas manganese tends to occur i n voids and fractures. It was hoped that sample pretreatment might lower the concentration of elements such as manganese that are probably present from contamination by the depositional environment. W e found that all treatments short of washing with strong mineral acids d i d not reduce substantially the manganese content of the samples. Although mineral acids d i d reduce the manganese content, the bone matrix deteriorated so that this procedure could not be used. W e adopted a procedure that involves repeated ultrasonic agitation of the bone samples i n demineralized water to remove contamination that might adhere to the surface of the bone and rejected any harsher chemical pretreatments.


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The inadvertent presence of manganese i n our samples prevented accurate measurement of the magnesium content of the bone since the 846.7-keV gamma ray of 2.58 hr M n cannot be resolved from the 843.8keV gamma ray from 9.46 min M g . The 1014.5-keV gamma ray from M g is subject to a large background correction owing to the intense Compton continuum from the 1811.2-keV y of M n . M g can only be measured in the presence of M n when the concentration of magnesium is many times greater than the concentration of manganese i n the matrix. This was not the case i n most of our samples. Compton interference from the 846.7-keV M n gamma ray provides background interference at the S r and B a peak positions, occasionally completely obscuring the 6 5

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peak. N a is also a source of background interference. Chemical separation of the elements of interest would eliminate these interference problems, but a major advantage of the method chosen is the large number of samples that can be processed i n a short time. W e are able to obtain calcium, barium, and strontium concentrations on seven samples per irradiation. Complete reduction of the data can be accomplished within hours of the irradiation, and several irradiations may be performed i n a day. The use of chemical separations would lead to delay and would reduce the rate of sample analysis considerably. The Poisson statistics of radioactive decay provide the major source of error in these analyses. Where the background interference was least, the standard deviation of the measured quantity of alkaline earth element is approximately 15%. In many cases, particularly when the alkaline earth metal concentration is low and the background is high, the standard deviation can be as much as 40%. Concentrations with greater uncertainties were reported as upper limits. A 10-min irradiation followed by two counts of each sample appears to be the optimum choice for obtaining sensitivity for the elements of interest (Ba, C a , Sr) while keeping the activity of the interfering elements ( M n , N a ) as low as possible. Although strontium and barium are often determined by neutron activation analysis in much longer irradiations (8 hr or more), i n the present application a longer irradiation could lead to more drastic interferences from other elements such as sodium and phosphorus because of the nature of the matrix being analyzed. Relatively small sample sizes are used (10-20 mg) which can be an advantage in the analysis of archaeological specimens.


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Results and Discussion In order to check the uniformity in the skeletal matrix for the elements studied, samples of a recently killed deer taken at Potlach, I D and a recently killed fur seal from Cape Alava, W A were analyzed. A total of 34 samples from different portions of the two skeletons yielded the results shown in Figures 1 and 2 (13,17). The average concentrations and ranges for calcium, strontium, and barium are shown in Table I. The concentrations of calcium and strontium showed little variation within the two skeletons. However, the barium concentrations were an average of 30 times higher in the deer than i n the fur seal. This variation was evident (but to a lesser degree) in samples analyzed at three different Northwest coast archaeological sites (13). O n the basis of these results, we conclude that a single bone specimen is representative of the entire skeleton in trace element concentrations and that, at least i n the case of barium, characteristic variations i n concentrations do exist.



WESSEN E T A L .

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Ba(ppm) Figure 1. (top) Strontium vs. barium concentrations for a recently killed Cape Alava, WA fur seal. All concentrations are reported on a weight ppm (fig/g) basis. Figure 2. (bottom,) Strontium vs. barium concentrations for a recently killed Potlatch, ID deer


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Table I.

Barium, Calcium, and Strontium Concentrations from Recently Killed Specimens of Fur Seal and Deer

Species

Element

F u r seal (Callorhinus ur sinus)

Ca Sr Ba

Deer (Odocoileus hemionus)

Ca Sr Ba

Average Concentration

° Elemental concentrations are 0*g/g).


II

Low

High

19.3% 258 ppm 20.5 ppm

14.7% 174 ppm 8.6 ppm

25.8% 433 ppm 139 ppm

15.9% 302 ppm 601 ppm

13.8% 217 ppm 494 ppm

21.9% 392 ppm 780 ppm

0

reported as percent by weight or weight

ppm

N o matter how carefully prior contamination from the depositional environment is removed or contamination of the samples during analysis is avoided, altered concentrations could still be observed. Therefore, experiments were conducted to determine empirically the nature and extent of variation i n concentrations that could be attributed to postmortem conditions related to the depositional environment. In the first experiment samples of elk and bison bones from two archaeological sites with different ages were studied. The first site was W a w a w i i (45-Ga-17), a late prehistoric site near Central Ferry, W A with associated radiocarbon dates of approximately 2000 B.P. The second site examined was L i n d Coulee (45-Gr-92), an early postglacial site near Warden, W A with associated radiocarbon dates of approximately 9000 B.P. The results of the analyses for strontium and barium are shown i n Figures 3 and 4. These two populations represent the same types of organisms i n similar environments, the main difference between the two sites being age. The W a w a w i i results show a variation i n both strontium and barium concentrations that is consistent with data from several archaeological sites (13,16,17). The L i n d Coulee results, on the other hand, show extreme internal variations i n both strontium and barium concentrations. A t present neither elk nor bison occur i n the immediate vicinity of these sites. The W a w a w i i elk are consistent with modern elk from nearby Northern Idaho (13,16). However, only the lower end of the L i n d Coulee distribution is similarly consistent. In order to examine the possibility of a time-related depositional effect, we analyzed a series of samples from throughout a stratigraphic column at Marmes Rockshelter (45-Fr-50), a site located approximately halfway between W a w a w i i and L i n d Coulee. A t Marmes we were able to sample deer and pronghorn antelope throughout a period of more than 7000 years. The results appear in Figures 5 and 6. N o significant time-related variation was indicated for either strontium or barium.


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Figure 3. Strontium vs. barium concentrations for archaeological bone spedmens taken from the Wawawii site. (A) bison; (O) elk.

1000

O

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Ba Figure 4.

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Strontium vs. barium concentrations for archaeological bone specimens taken from the Lind Coulee site. (A), bison; (O) elk.


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5 0 0 -


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Figure 5. (top) Strontium concentration vs. age for terrestrial mammal bone samples from the Marmes Rockshelter. Each point represents a single sample. The error bars represent the standard deviation based on the statistics of radioactive decay. Figure 6.

(bottom) Barium concentration vs. age for terrestrial mammal bone sample from the Marmes Rockshelter


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Nonetheless, we still find the differences between the W a w a w i i and L i n d Coulee distributions disturbing. The fact that depositional alteration does not appear to occur at one or even many localities does not preclude its occurrence elsewhere. It appears to us that depositional alteration, if it occurs, is more likely to be a factor with older materials and/or when represented material covers an extended time interval. W e do believe, however, that this matter warrants further study, and we are continuing to examine it. A basic precaution i n our archaeological studies has been to work within single stratigraphic units or at least within single localities. Differences can exist i n the range of concentrations for the same species at different localities (18,17,18). Caution must be used when comparing distributions between localities. Summary and Conclusions Regardless of the seemingly insurmountable difficulties associated with extracting meaningful trace element data from samples taken from archaeological sites, useful results have been obtained. W e have determined that barium is a useful indicator of environment and possibly diet but that strontium seems to be much less useful i n this regard (18). The characteristic differences i n barium concentration have already proven useful i n an archaeological context. Artifactual bone material consisting of harpoon valves from the Ozette archaeological site (45-Ca24) on the northwest Washington coast have been analyzed for strontium and barium content. It was shown (IS) on the basis of the observed barium concentrations that seven of the 12 harpoon valves analyzed represented terrestrial mammal bone (deer or e l k ) , but only two had concentrations consistent with marine mammal bone (seal, sea lion, walrus, whale). Since more than 9 0 % of the bone at Ozette represents sea mammal (19), a clear selection process is evident. Reasons for such selection w i l l doubtless require a broader range of experiments, but w e believe that trace element studies have a real potential i n this area. The present method of analysis offers several distinct advantages. The first of these is speed; calcium, barium, and strontium concentrations are determined simultaneously i n a short irradiation, and analysis of up to seven samples can be completed within hours. Neutron activation analysis is highly sensitive to the elements of interest compared with other methods such as x-ray fluorescence and atomic absorption techniques. Additionally, the required sample size is small (10-20 m g ) , thus resulting in little alteration of archaeological specimens. W e have analyzed over 350 samples of recent and archaeological bone and have found patterned relationships i n the barium and strontium concentrations. Provided that suitable caution is exercised to exclude


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effects leading to portmortem alterations i n the concentrations, such relationships may be used to investigate problems concerning prehistoric economy and technology.


Literature Cited 1. Perlman, I., Asaro, F., Michel, H. V., Ann. Rev. Nucl. Sci. (1972) 22, 283. 2. Renfrew, C., Cann, J. R., Dixon, J. E., Ann. Br. School Archaeol. Athens (1965) 60, 225. 3. Weaver, J. R., Stross, F. H., Contr. Univ. Calif. Archaeol. Res. Facil. (1965) 1, 89. 4. Gordus, A. A., Wright, G. A., Griffen, J. B., Science (1968) 161, 382. 5. Bowman, H. R., Asaro, F., Perlman, I., Archaeometry (1973) 15, 1. 6. Nelson, D. E., D'Auria, J. M., Bennett, R. B., Archaeometry (1975) 17, 1. 7. Perlman, I., Asaro, F., Archaeometry (1969) 11, 21. 8. Harbottle, G., Archaeometry (1970) 12, 23. 9. Ambrisino, G., Pindrus, P., Rev. Mett. (1953) 50, 136. 10. Brown, A. B., Contr. Geol. (1974) 13, 47. 11. Brown, A. B., Thesis, University of Michigan, Ann Arbor, MI, 1973. 12. Gilbert, R., Thesis, University of Massachusetts, Amherst, MA, 1975. 13. Wessen, G., Ruddy, F. H., Gustafson, C. E., Irwin H., Archaeometry (1977) 19, 2. 14. Hill, M. N., "The Sea," Wiley-Interscience, New York, 1963. 15. Mitchel, R. L., "Trace Elements in Soil" in "Chemistry of the Soil," F. E. Bear, Ed., Reinhold, New York, 1964. 16. Parker, R. B., Toots, H., Geological Soc. Am. Bull. (1970) 81, 925. 17. Wessen, G., Thesis, Washington State University, Pullman, WA, June 1975. 18. Wessen, G., Ruddy, F. H., Gustafson, C. E., Irwin, H., unpublished data. 19. Gustafson, C. E., Science (1968) 161, 49. RECEIVED September 15, 1977.


Trace Element Analysis in the Characterization of Archaeological Bone

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