On the radioactivity of fossil bones

NUCLEAR

INSTRUMENTS

AND M E T H O D S

142 ( 1 9 7 7 )

581-588

; Q

NORTH-HOLLAND

PUBLISItlNG

CO.

ON THE RADIOACTIVITY OF FOSSIl, BONES STEF. CHARALAMBOUS and CONST. PAPASTEFANOU

Department o/Nuclear Physics, Aristotle UniversiO" ol 7hessaloniki, Greece Received 11 October 1976 A study was made of the radioactivity of fossil bones of various geological ages from the Axios Valley and from Cb,alkidiki, in Northern Greece. In many tbssils a high uranium concentration was found, reaching in some cases values as high as 3 ~ 10 4 g/g. The 238U content was measured either by direct 7-spectroscopy of the 63 keV line of its first daughter 234Th, or by activation analysis. The uranium concentration in fossils is dependent on their apatite content. It is found that fossils rich in apatite are also rich in uranium. Experiments of deposition of uranyl on fossils with various apatite contents, as well as on different minerals, confirm this observation. 226Ra, and 238U in fossils are in disequilibrium because of radium leaching. Also 214Bi and 214pb are in disequilibrium with 226Ra and of course with 238U, because of radon escaping. 232Th was searched for in /bssils by both direct y-spectroscopy and by activation analysis with negative results. Using the presented methodology, it is estimated that the 232Th content in fossils is less than 10 8g/g. 40K was also not found, and its concentration must be lower than 2 pCi/g.

i. Introduction An exchange of the organic components of bones by inorganic compounds takes place during their fossilization. These inorganic compounds are taken from the environment. On account of this, it is evident that in a fossil there may be found particular concentrations of some elements which are very different from their initial concentrations in the bone. This may be the result of either a high concentration of these elements in the liquid environment during fossilization, or to a preferential deposition of these elements onto the bone. The eclectic exchange-deposition of uranium, for example, could be demonstrated by the measurement of the radioactivity of the fossil. It has been found that fossils exhibit unusually high radioactivity. Jaffe and Sherwoodl), using chemical analysis, found a high concentration of uranium (1.6 × 10-~ g/g). This they associated with the carbonate apatite of the fossil bones. Bowie and Atkin 2) using nuclear emulsions observed an unusual radioactivity in a fish fossil from Thurso, Scotland. They found a uranium concentration of 2.7 x 10-3 g/g and a thorium concentration of 4.5 x 10-3 g/g. For the same specimen their results were in agreement with those of Davidson and Atkin 3) as far as uranium was concerned (they measured 4.2 x 10-3 g/g uranium) but they gave the impression that a large quantity of thorium was present. It was because of the uncertainty in the mea-

surements of Bowie and Atkin that another examination of the same fossil was made by Diggle and Saxon4). Their results showed that the radioactivity of the fossil fish was mainly due to the disintegrations of the nuclides of the 238U series with a small contribution from the nuclides of the 232Th series. Jaworowski and Penskob studied the radioactivity of fossil bones from the Gobi desert in Mongolia. Their samples were from big animals, such as dinosaurs etc, of various geological periods. The maximum concentration measured by them for uranium and its daughters in (secular) equilibrium was about 2.1 × 10-3 g/g. Otgonsuren et al. 6) used dielectric track detectors to examine samples of recent as well as fossil bones of dinosaurs and of giant turtles about 70 million years old, from the Gobi desert in Mongolia. Their results verified some of those of Jaworowski and Pensko. The present work is an extensive study, by different methods, of the radioactivity of fossils and particularly of the radioactivity from the uranium series. Studies were also made on the mechanism of deposition of uranium in fossil bones.

2. Experimental methods and procedures The fossils examined were of big mammals such as elephants, rhinoceroses and of small animals such as hipparia, various horned animals etc. They were found in the Axios valley and the Kassandra

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peninsula of the Chalkidiki region (Sani beach and Athytos beach) in Northern Greece. For most of the fossils there existed precise information on the area as well as the immediate environment in which fossilization took place. The bones were classified according to the kind of animal and their geological ages by Dr. I. Melentis, Professor of Palaeontology at the University of Thessaloniki. The fossils were of various geological periods, from the Pontium-Upper Miocene to the ttolocene. They had not conglomerated and their relief was quite clear showing all the details of the bone of the animal. This permitted the assumption that the fossilization took place at the same spot as that where the animal died and where the bones were deposited. Thus the mould of the bone and the environmental sailsand was subjected together with the bone to the same geological variations, from petrifaction to the discovery of the fossil. In all cases, the fossil was separated from the sand as much as was possible. Its cavities were also cleaned. The sample was then powdered in a mortar of checked low radioactivity. Using differential sieving the grains were selected with diameter near to 50 ~m. This powder was then placed in plastic containers of low radioactivity. The sam-

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pies were of 40 g weight for measurements taken with the Ge-Li detector, while for measurements taken with the Ge detector their weight was 1 g. The radioactivity of samples was measured by various methods, mainly by gamma-ray analysis. The gamma spectroscopy was done using Ge detectors. For the high-energy region (0.1-2.8 MeV), a coaxial type Ge-Li detector was used. This detector had a volume of 40 c m 3, resolution of about 2 keV at the 1.33 MeV of 6°Co, peak-to-Compton ratio 25/1 and "efficiency" 6.3%. For the low energy region (from about 4 keV to 100 keV) an intrinsic Ge detector was used. This detector had an active area of 200 mm 2, depletion depth of 5 mm and resolution of about 180 eV at 5.9 keV (SSFe). A 4096-channel pulse-height analyser was used. Special studies were performed to find the efficiency of the detectors due to the particular size of the samples. The efficiency was known to an accuracy of better than 6%. Fig. 1 shows a typical " n e t " gamma spectrum, after the subtraction of the background, for a representative sample of " h o t " fossil. The spectrum was taken with the Ge-Li detector and covering the range from 100 keV to 2.7 MeV. In the spectrum we clearly see the important gamma transitions of the uranium series from 226Ra onwards.

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From each of these gammas it is possible to calculate the uranium concentration, assuming radioactive equilibrium. However, this hypothesis could lead to large discrepancies and inaccuracies, should for example the chain be open, as explained below. In the spectrum of fig. 1, as well as in all the spectra examined, the y-peaks of the thorium series and of 4°K (E~ = 1.46 MeV) are absent. The same sample was taken with the intrinsic Ge detector for the range of 5-100 keV as shown in fig. 2. Shown in the spectrum is the important 63 keV ~4ine of 234Th, first daughter of 23sU. This line was used for the direct determination of the 23sU concentration. To augment the confidence in the results of the direct y-spectroscopy, activation analysis was also applied in the cases of some representative specimen of fossil. The samples were activated in a 5 MW reactor with thermal neutrons with fluences of 10 ~4 and 10~6n/cm 2. We looked for the reaction: 238U(r/,~)239 U

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and uncovered gold foils. The first spectra of activated samples were taken about 24 h after activation using the Ge-Li detector. The measurements continued for a period of about a month with the intention on one hand of following the activity of 239Np and on the other of establishing whether 233pa was produced by neutron capture from 232Th (half-life 27 d). It must be clarified that direct gamma spectroscopy does not show the presence of thorium. A typical spectrum of an activated sample of hot fossil is shown in fig. 3. The 228 keV and 278 keV gammas of 239Np are present. In addition to the measurement of the radioactivity of the fossil bones, that of their immediate environment i.e. the sand, was also measured. The distribution of radioactivity of the fossil was also investigated, by separate measurements on the outer part of the bone and on the inner spongy part.

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It is evident that if 239Np could be found after irradiation, then the presence of 23sU in the fossils could be deduced and its concentration measured. Fossil samples of 1 g were irradiated, in each case using the standard activation procedure i.e. irradiating samples both covered with Cd and uncovered, and monitoring the activation by covered

3. Results and remarks The uranium concentration in each sample examined is deduced from the activities of the 63 keV line of Z34Th, from the 186 keV of 226Ra and from the 609 keV of 2t4Bi, assuming secular equilibrium. In the calculations of uranium content were properly included all the necessary parameters, such as the intensities of the lines, the

TABLE I U r a n i u m concentration in fossil bones Lab. no.

F-la F-lb S-I F-17 F-16 F-13 F-10 F-14 F-7 F-2 F-I 5 F-18 F-22 F-5 F-4

Kind of fossil

Rhinoceros tibia (outer part) Rhinoceros tibia (inner spongy) Sand Elephant shoulder Elephant femur Elephant pelvis Rhinoceros Jaw Bos h u m e r u s Tragocerus h u m e r u s Tragocerus bone E q u u s femur E q u u s falanx E q u u s fibula Hipparium tibia Hipparium shuttle

Region of finding

Uranium concentration ( x 10 4 g/g)

Geological period

from 6 3 k e V

from 186KeV

from 609keV

Apatite concentration ('.~,)

Axios Valley

Pliocene

3.00_'0.13

2.24_'0.24

2.10+0.11

84-2

Axios Valley

Pliocene

2.51 : 0.12

1.88__T0.24

1.76±0.09

81 _-_2

Axios Valley San± San± Athytos San± Athytos San± Axios Valley San± Sani San± Axios Valley Axios Valley

-Upper Miocene Upper Miocene Upper Miocene Upper Miocene Holocene Upper Miocene Upper Miocene Pleistocene Pleistocene Pleistocene Upper Miocene Upper Miocene

-2.75!0.25 1.325-0.08 2.93 ~:0.12 2.47 ~-0.10 0.00 ±0.10 2.82 ~"0.12 1.82+0.09 0.954 0.07 0.3520.11 0.28i0.10 1.26-+ 0.12 1.11 ±0.09

0.00 ~__0.24 1.952-0.21 0.97"__0.11 2.30±0.24 1.71J 0.18 0.00+0.24 2.02 ! 0.22 1.34±0.21 0.70±0.19 0.26±0.24 0.21 ~0.24 0.93--0.19 0.82 " 0.19

0.02:30.32 1.72-' 0.09 0.68±0.04 1.97±0.10 0.88 _40.05 0.01 '__'0.16 1.19_--0.11 0.65:_'.0.04 0.35._.'_0.03 0.17./0.02 0.14 i 0 . 0 3 0.55±0.03 0.48 t 0.03

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efficiency for these lines of the detector-sample combination and the branching ratios of all isotopes of the series up to the nuclide responsible for the line. For the last parameters, as well as for the energy calibration, the tables of isotopes of Lederer e t al. 7) and Marion 8) were mainly used. The results on the radioactivity of fossil bones which were studied by direct gamma spectroscopy are summarised in table 1. The following general remarks could be made on the results shown: 1) Most of the fossil bones examined were radioactive. The radioactivity was only due to the radionuclides of the uranium series. 2) In a given fossil the sample of the outer part of the bone shows a different uranium concentration (column 5) than the inner spongy part. Typical example are lines 1 for F-la and 2 for F-lb. This may be due to the fact that the two parts have a different apatite concentration (column 8) as explained below. 3) The big animals, such as elephants and rhinoceroses (lines 1-7 of table 1), have a higher uranium concentration than the small animals, such as hipparia (lines 14 and 15). A possible explanation is that the tubular bones of big animals are richer in Haversian canals than those of small

OF t. OSSIL

BONES

animals and also have a higher apatite content than those of small animals. 4) In the same region (Sani beach, lines 4, 6 and 8), fossil bones of various geological periods were found to have a different uranium concentration. It should be noted that the older fossils are "hotter" than the recent ones. Assuming that the fossilization is a relatively rapid phenomenon 9) and the deposition of uranium takes place mainly during the time of fossilization, we attribute the different uranium concentration to the different amounts of uranyl ions present in the aqueous environment of fossilization. 5) The sand of the nearest environment, i.e. the mould of the bone, is not radioactive and consequently the radioactivity of fossil bones is due to a preferential deposition of uranium in the bones and not to a kind of "pollution". In the table we present only one set of measurements for sand (line 3). We would like to mention however that we examined most of the sands surrounding the fossils and have not observed any unusual amounts of radioactivity. 6) Shown in columns 5-7 are the activities of z38U in various fossils, as inferred from the 63 keV line of 234Th, from the 186 keV line of 226Ra and

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trons yields a uranium concentration of about (3.00+0.13)× 10-4 g/g, the same as given by direct y-spectroscopy. 7) Given in column 8 is the apatite content of fossil bones. This was examined by X-ray analysis and was measured chemically, namely by the method of quinoline. The two methods gave roughly the same results. Comparison of columns 5 and 8 shows that fossil bones rich in apatite exhibit high uranium activity. Plotted in fig. 4 is the uranium concentration versus apatite content. The dotted line is traced between the experimental points, and although the curve is not given by a simple analytical law, we remark on its regularity. Assuming that the uranium concentration as a function of apatite content will reach saturation, the solid curve is fitted to the experimental points using a law of the type {1-exp(-k[C-Co])}, where k and Co are constants and C is the apatite concentration. We found k - - 0 . 0 7 and Co = 16. 232Th was searched for in all the samples of fossil bones studied. The search was made by direct ~,-spectroscopy, i.e. looking for the 2614 keV line of 2°8T1, as well as by activation analysis with thermal neutrons, namely the (n,),) reaction

from the 609 keV line of 2'4Bi respectively. There is a clear decrease of activities from columns 5-7. The differences of activities are real and well beyond experimental error. Assuming that the fossilization-deposition of uranium was done a long time ago and that an equilibrium must therefore exist, we attribute the low activity of column 7 mainly to the escape of radon. The low activity of 226Ra is explained by the leaching of radium during the period that the bone was buried in the soil. This is in accordance with the results of Lindeken et al.t°) who observed smaller than expected radium concentration in phosphate (apatite-derived) fertilizers. Radium dilution was probably caused by calcium sulfate. More details on this will be given in another paper. The above observation dictates that if the uranium concentration in fossils, or anywhere else, should be determined by direct gamma spectroscopy, this must be done only via the measurement of the 63 keV line. This is also confirmed by the results of activation analysis. The results of measurements of uranium concentration by activation analysis, (r/, 7) reactions with thermal neutrons, confirmed those of column 5 in table 1. As a typical example consider sample F-la in this table, for which activation by neu-

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followed by the examination of the main 2~3Pa line of 312 keV. This line would be present in the spectra several days after the end of activation. Although we examined in great detail all the spectra taken, we could not detect the 312 keV line. The smallest thorium concentration which could be measured by our ),-spectroscopy system was of the order of 10-7g/g when using the 2614keV line, while using activation analysis (312 keV line), the detection limit is of the order of 10-s g/g. An attempt was also made to measure the thorium content by the "pair method"ll). The method consists of counting pairs of alphas coincident within a time interval At of the order of a fraction of a sec. This is done because in the chain of 232Th there exist two successive alpha emissions (22°Rn ~ 216po ~ ) having a time difference between them which is determined by the half-life of 216po (T,/2=0.145s). We used different At's between 0.150 and 0.250 s. Using this technique, it was found for example, that 232Th if present in sample F-la, must have been in a concentration lower than 10-~ g/g. By considering the results of all methods used, it is concluded that the thorium concentration in the fossils studied was lower than the above mentioned limits, i.e. of 10-8 g/g. 4°K ( E r " 1460 keV) was also searched for. In this case, the lowest content which could be measured by our F-spectroscopy system, was estimated to be about 2 pCi/g. For all samples which were examined, 4°K was not detected. It is concluded that 4°K in the fossils studied was in concentrations lower than this value.

4. Uranium deposition on the apatite crystal surface of fossil bones The results indicate that fossil bones rich in apatite are also rich in uranium. We therefore examined the possibility that uranium is deposited as bivalent uranyl ion, U O i ' , on the apatite crystal surface. A series of experiments were made under strictly identical conditions using pure uranyl nitrate (without daughter isotopes). These consisted of examining the action of: 1) uranyl nitrate on " h o t " fossil sample, rich in apatite, 2) uranyl nitrate on " c o l d " fossil sample, poor in apatite and rich in calcite, 3) uranyl nitrate on apatite mineral sample, and finally of

FOSSIl.

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587

4) uranyl nitrate on calcite mineral sample. The experimental procedure followed for the purification of uranium from its daughters is that described by Overman and ClarkU). The chemical procedure was the following: a quantity of 5 ml uranyl nitrate, purified as above, was rarefied in 100 ml of distilled water. Two grams of powdered sample of diameters about 50/~m were added to the above solution and allowed to interact for about 24 h, assisted by vigorous stirring. Finally the solution was filtered and the filtrate, on a planchet, was counted using a Ge-Li detector. Our intention was to detain in the filter only the uranyl which interacted with the sample used. For this purpose the filter was then washed up to 20 times so as to remove all the quantity of soluble uranyl which had no interacted with the sample. The samples were counted regularly for a period of about 100d to observe the build-up of the 63 keV line of 234Th (Tl/2 = 24 d). Each experiment was followed by a blank one of the same quantity of purified uranyl nitrate. The blank experiment was used as an indicator of the efficiency of purification. The build-up of the 63 keV line of the blank samples was also followed. The measurements of build-up were performed alternately for the samples and their indicators. Shown in fig. 5 are the build-up curves of the 63 keV line for the samples prepared by the uranyl deposition experiments. The solid lines [ ( 1 - e -~') build-up law] were fitted to the experimental points using the MINUIT program of James and Roosl3). The •2 of the fitting of these curves was about 3. The results of uranium deposition experiments are presented in table 2. Uranium and apatite conTABLE 2

Build-up activities of 63 keY line of uranium deposition on fossil bones, apatite and calcite Kind of sample (Lab. no)

Apatite

Uranium concentration ( × 1 0 4g/g)

Apatite concentration (%)

Activity at saturation of 63keV line

--

--

0.59 ±0.09

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3.00A 0.13

84*_2

1.20!0.15

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0.28 ~0.10

16±1

0.16±0.03

--

--

0.00 + 0.15

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centrations of the samples are listed in columns 5 and 6, respectively. The counting rates of the 63 keV line at saturation, which are normalized by the saturation values of their indicators are given in column 4. For the case of fossils (lines 2 and 3), the activity of the 63 keV line is the net activity, i.e. after subtraction of the natural activity of the fossils. From table 2 and fig. 5 we can deduce the following: for fossil bones which are rich in uranium ( " h o t " fossils) as well as in apatite (line 2), the deposition of uranium is stronger than it is in " c o l d " fossil bones which are poor in uranium and in apatite (line 3). There is a ratio of about 8 between the two saturation activities. The same ratio is found to exist between the uranium and apatite concentrations, columns 2 and 3. This shows that the deposition of uranium of fossil bones depends on their apatite content. The above results would lead one to expect that the activity of the sample pure apatite would be greater than that of any fossil sample. We see however that its activity is about half that of a hot fossil (see lines 1 and 2 of table 2). We explain this experimental fact as follows: the grains of all samples used in this experiment were selected using differential sieving. They all had about the same apparent density and approximately the same apparent surface. The grains of fossil bones must present however a larger active surface for deposition than those of pure apatite. This is due to the Haversian canals present in bones and preserved in fossils as well as to their porous structure. Microscopic examination of both apatite and fossil confirms the above comments. Fig. 6 shows

photographs of grains of (a) fossil bones and (b) apatite. The grains of the apatite mineral are compact and have fairly smooth surfaces, while those of fossil bones show clearly the Haversian canals and the porous structure which add larger active surfaces. The above results of the deposition experiments of uranium in fossils are in agreement with those of Neuman et al.L4), who worked on uranium deposition on recent bones. The authors would like to thank Prof. I. Melentis for his advice on palaeontology and Dr. C. Christodoulides for stimulating discussions and Miss E. Karayanni for her technical assistance. References I) E. B. Jaffe and A. M. Sherv, ood, US. Geol. Survey Rept. TEM 149, 19p. 2) S. 1t. U. Bowie and l). Atkin, Nature 177 (1956) 487. 3) C. F. Davidson and D. Atkin, C. R. XIX lnt. Geol. Congr. Algiers II (1952) 13. 4) W. R. Diggle and J. Saxon, Nature 208 (1965) 400. 5) j. Jaworowski and J. Pensko. Nature 214 0967) 161. 6) O. Otgonsuren, V. P. Perelygin and D. Chultem, At. Energy 29 0970) 301. 7) C. M. Lederer, J. M. ttollander and I. Perlman. Table o[ isotopes (J. Wiley, New York, 1968). 8) j. B. Marion, Report ORO 2098-58, University of Maryland (1967). 9) K. K. Turekian, D. P. Kharkar, J. Funkhouser and O. A. Schaeffer, Earth Planet. Sci. Lett. 7 (1970) 420. 10) C. L. Lindeken, D. G. Coles and J. W. Meadows, Report UCRL-75659, University of California (1974). II) R. C. Turner, J. M. Radley, W. V. Mayneord, Brit. J. Radiol. 31, no. 368 (1958) 397. 12) R. T. Overman and H. M. Clark, Radiotsotope techniques (McGraw-Hill, New York, 1960) p. 326. 13) F. James and M. Roos, CERN Computer 7600 Interim Program Library: MINUIT D506 Version 1.74. 14) W. F. Neuman, M. W. Neuman, E. R. Main and B. J. Mulryan, J. Biol. Chem. 179 (1949) 325.