ESR signals in bones: Interference from Fe3+ ions and a new method of dating

Nucl Tracks Radmt Meas, Vol 18, No 1/2,pp 213-221, 1991 Int J Radlat Appl lnstrum, Part D

0735-245X/91 $3 00 + 00 Pergamon Pressplc

Prmted m Great Britain

ESR SIGNALS IN BONES: INTERFERENCE FROM Fe 3+ IONS A N D A NEW METHOD OF D A T I N G A D ODUVdOLE and K D SALES Chcmlstry Department, Queen Mary and Westfield College, Umverslty of London, Mile End Road, London El 4NS, U K

Abstract--A study of the fadmg of the electron spin resonance (ESR) dating signal induced by laboratory y irradiation In modern and ancient bone has shown that the rate of fading is related to the age of the bone The observed mmal rate of decay of the signal appears to be smaller the older the sample and the relative size of the final signal to be related to the age of the sample This new method of dating is complementary to the usual ESR method but has some advantages Other results show that the accumulation of Fes+ Ions in bone can affect the dating signal and therefore the apparent age of the bone A method of overcommg this problem ~s discussed

1. INTRODUCTION Tim PRINCIPLESand pracUce of the dating of archaeological and geoiogacal artefacts using electron spin resonance (ESR) spectroscopy are now well estabhshed (Hennmg and Grun, 1983, Ikeya, 1988) The ideas beband the technique are similar m many respects to those behind the thermolummescence (TL) technique of dating a sample showmg a signal Is subjected to laboratory doses of radiation which increase the signal A plot of the s~gnal intensity against dose ts extrapolated to zero signal to yield the so-called eqmvalent dose (ED), t e the dose the sample might be expected to have received m its natural state If the annual dose to which the sample has been exposed is known then the ED can be converted mto an age and whilst, m pnnctple, the annual dose rate can be estimated from measurements of the ra&atlon m the environment of the bone and of the uranmm uptake of the bone (Grun and Schwarcz, 1987), there are considerable dffficulUes both m the experiments and m their interpretation In many cases the annual dose rate ~s not known, as for all the samples used in this study, but it has been shown that m several instances (Sales et a l , 1985, Ikeya, 1977, 1978) a value of about 2 mGy gwes dates m accordance with other techmques The datmg of bones by ESR ts particularly important because of their widespread occurrence and because the sample preparation normally used for TL precludes the use of bone The hydroxyapaute lattice appears to be necessary for the storage of a trapped electron defect signal (the dating signal) m bone and the datmg of very ancient bones may be m error due to gradual loss of hydroxyapatlte over the burial period (Sales et a l , 1985), an effect which may be associated with the processes of fosslhzatton and sfllctficauon Fadmg of the labora-

tory enhanced dating signal may also seriously impair the ESR dating techmque, gavmg rise to questionable ages for the bone Finally, any signal which overlaps the datmg signal and which may or may not be enhanced by laboratory trra&atton gwes rise to problems Particular examples are the so-called carbon signal, caused by the charring of organic remnants m the bone, and paramagnetlc trans~tlon metal tons such as Fe 3+ and Mn 2+, which are easdy picked up by the bone from surrounding soil In practice, the Mn 2+ signals are often not a problem because they are clearly &street from the dating signal, indeed, with the assumpuon that the signal is not enhanced by laboratory trradlauon, the Mn 2+ hnes have been used to calibrate the spectrometer (Ikeya, 1975) (see the discussion m the experimental section concernmg problems associated with spectrometer calibration) Our results suggest that the Mn 2+ signal is, m fact, enhanced by laboratory irradiation and that results obtained using this signal as cahbrauon are m error In this paper we report the results of a detaded study of signal fadmg m y trra&ated hydroxyapattte samples, mcludlng modern and ancient bones, and outline how the signal decay curves may be used to give relatwe dates of bone samples As mentioned above, some ancient bone samples contain stgmficant quantmes of paramagnettc Fe 3+ tons, often picked up from the burial environment, and we discuss the particular problems encountered m dating such bones

2 EXPERIMENTAL Most of the bones used in this study have radiocarbon dates of between 800 and 25,000 yr, a hst of the samples and their sources IS given m Table 1 The

213

214

A

D

O D U W O L E and K

Table 1 Bone samples studied No 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Type Jack-rabbit PO933 Anglo-Saxon PO935 Arjoune African elephant Les Eyzles PO450 Abn Pataud PO906 elephant Choukoutlen Pleistocene elephant Eohlppus Eohlppus Tyrannosaurus Hadrosaurus Human skull

Source Tucson, Arizona, U S A * Exeter, U K ~" t ++ § t Aveley't Chma[I r § § ** ** **

*Dr S Olsen, Institute of Archaeology, London, U K tRadlocarbon Accelerator Unit, Oxford, U K :[:Geography Dept, QMW, University of London, U K §Brltlsh Museum, London. U K IlProfessor Wu, Peking, China ¶Sample bone issued to various laboratories m 1982 (Hennmg et al, 1985), obtained by us m 1985 **Professor L Martin, Museum of Natural H~story, Kansas, U S A bones were crushed and then powdered in a mortar and pestle and portions were taken for laboratory 7 lrradmtion with a 6°Co source Several samples containing Fe 3÷ ions were prepared as follows (l) Calcium phosphate was prepared by adding a large excess of a m m o n i a to calcium chloride solution, this white sohd is almost certainly the basic phosphate 3Ca3(PO4) 2 Ca(OH) 2, l e hydroxyapatlte (Elsenberger et al, 1940) (although for clanty tt will be referred to as calcmm phosphate) and soaked in ferric alum, NH4Fe(SO4), 12H,O, solution for 24 h, filtered, washed with water and dried overnight at 150°C 01) Calcium phosphate was prepared as above but in the presence of ferric alum in an atomic ratio of Ca 2+ Fe 3+ of 8 1 The resulting sample showed a signal due to Fe 3+ (Ill) A modern bone was irradiated to a dose of 9 0 0 G y , powdered to homogenize the sample, and ahquots soaked in varying concentrations of ferric alum solution for 24h The samples were then filtered, washed with water and drted at 40°C for 3 days, this relatively low temperature for drying being chosen to minimize the decay of trapped electrons and the charring of any remaining organic material For comparison, an unsoaked ahquot was also washed with water and dried The ESR measurements were made on a Bruker 200D spectrometer employing 100 kHz modulation, magnetic field markers from an N M R gaussmeter, and an external microwave frequency counter The g values, where quoted In the text, generally have an error of + 0 0002, the hnewldths, AH, are the peakto-peak distances in gauss measured directly from the spectra (for the dating signal this IS ambiguous and the distance between the two low field peaks is

D

SALES

measured, being effectively A H for g±), and the intensities refer to the area under the absorption peak, in arbitrary units (although always normahzed to a fixed spectrometer gain), calculated by double Integration from the experimental spectra using a computer program The signal amplitude, in arbltrary units, is measured as the height of the signal from ItS lowest to its highest peak The dating signal is again different a typical example is shown in Fig I and the signal amplitude is taken as the distance between the peak maximum and the lower field minimum, corrected as above for spectrometer gain Each sample was given a known dose of Irradiation, thoroughly mixed to homogenize the sample and then loaded into a spectrosll tube to a length of about 5 cm The sample was then located in the microwave cavity so that a fixed length of sample could be monitored over a period of several days Spectra were recorded for each sample immediately after irradiation and then daily A standard sample was also measured on each occasion in order to check the spectrometer performance and to correct for any microwave power variation In addition, we checked the variation in the intensity of the signal for each sample as a funcaon of microwave power and found it slightly better to use this cahbratlon curve to correct for any microwave power v a n a t l o n The E D has been estimated using the standard method of ~, irradiation of samples explained above, the size of the signal being taken as the distance between the peak maximum and minimum in the spectrum The ESR stgnals observed for the bone samples are listed in Table 2

3 THE INTRINSIC ORGANIC RADICAL S I G N A L IN M O D E R N BONES The ESR spectra of unheated and unlrradlated modern bones often show an intrinsic organic radical signal with a g value and llnewtdth of 2 0048 and 8 0 G respectively A typical spectrum is shown in Fig 2(a) for a recent Jack-rabbit bone The fact that the g and A H values of this hne are very similar to those observed for other irradiated organic materials such as cereal grains, wood, paper, gelatine and plant resms confirms that it IS due to an organic radical derived from carbonaceous material Figure 2(b)

f

/ /

FIG 1 A typical spectrum of the dating signal, also referred to as the trapped electron signal

ESR SIGNALS IN BONES

215

Table., 2 ESR stgnals observed m the bone samples i

L

,

,

rl

,

rl

i

Dating Number I

2 3 4 5 6 7 8 9 10 11 12 13 14

Age/kyr 0

08 10 12 15 22 96 600 Unknown 10,000 10,000 100,000 100,000 Unknown

Fe 3+ g=2 g=4

A

B

Carbon

n

y

y

1

7

n

n y ~ y y y y y y n n n n

y y y y y y y y y n n n y

y y y y y ~ n ~ n n n n n

1 1 2 5 2 54 4 46 74 278 95 91 128

10 27 32 38 55 148 16 260 202 831 1092 1239 638

n n n

Mn 2+

7 n 221 n 25 23 457 334 114

(y) Indicates the presence, and n the absence, of a pamcular ssgnal, (7) indicates that it Is difficult to decide whether a signal is there or not, column (A) under the datmg signal refers to the natural hne, and column (B) to a laboratory reduced hne, the numbers m the Fe 3+ and Mn 2+ columns are the intensity of the signal relatwe to the intensity of the g = 2 Fe 3+ signal in the Jack-rabbit bone shows the spectrum of the same bone sample after ~rrad~auon (to a dose of 180 Gy) and ~t is apparent that the mlUal signal perturbs the defect signal which has been generated This dose of 180 Gy is about the upper hrmt usually given to samples m the addluve dating method and it should be apparent from the spectrum that an attempt to date the bone by the addltwe dose method would be difficult Irradlauon of a sample with U V hght does not affect the defect signal m bone but It does enhance the mtnnslc orgamc radical signal m modern bones After t e r m m a t m n of the irradiation the enhanced signal slowly decays An example is shown m Fig 3 for sample 2 (about 800 yr old), older samples show

a slmdar but reduced effect It is reasonable to expect a slmdar behavlour for the mtnnslc orgamc signal after laboratory 7 Irradiation, although the effect Is, m pracuce, masked by the presence of the defect signal which has also been reduced To show that this effect does m fact occur, the spectrum m Fig 2(a) has been subtracted from that of Fig 2(b) (after allowing for the different spectrometer gain settings) and shows a residue of the o n g m a l signal stdl prsent [Fig 2(c)] Even after two subtractmns, a small residue remains [Fig 2(d)], suggesting that there ts at least a 100% ~, irradlaUon enhancement of the m t n n sic orgamc radical present m the Jack-rabbit bone

4. E S R S I G N A L S IN I R R A D I A T E D C A L C I U M PHOSPHATE AND MODERN BONE SAMPLES The spectra of trradlated calcmm phosphate showed signals due to hydrogen atoms and to trapped electrons, a typical example being dlustrated m Ftg 4(a) The two small hnes of equal intensity are 506 6 G apart, compared to the hterature value of 5 0 6 8 G for hydrogen atoms ( C a m n g t o n and 100

9O Signal Amplitude 8O

70

60

i

0

FIG 2 (a) Spectrum of modern .lack-rabbit bone, (b) as (a) but after a 7 dose of 180 Gy, (c) result of subtracting (a) from (b), (d) result of twice subtracting (a) from (b)

100

~

•

i

200 Minutes 300

i

400

FIG 3 Change m the amphtude of the orgamc radmcal signal for sample 2 wtth UV trradtauon, the lamp was swstched off after 65 mm

216

A D O D U W O L E and K D SALES

FIG 4(a) A wide sweep spectrum of lrradtated calcmm phosphate showing the H atom and trapped electron (dating) signals 10 I

Trappeo Electron

6

S~gnal

Amphtude 4

2

~

.~ Hatom

0 10 Days 20

30

FIG 4(b) Decay curves for the signals from the H atom and

the trapped electron

McLachlan, 1967, p xlx) and the g value ~s about 2 The point midway between these two hnes does n o t correspond to the g value because the large hyperfine couphng makes second order effects qmte large (Carrmgton and McLachlan, 1967, p 18), we have not attempted to measure the g value more accurately the signal is obviously due to hydrogen atoms Similar decay curves are observed for both signals over a period of about 15 days [see Fig 4(b)], although the hydrogen atoms appear to decay more quickly and after about 20 days become unobservable Migration of the trapped electrons and the hydrogen atoms leading to destrucuon of the radlcat species ts presumably the dominant reason for the loss of the signals m thts materml The spectra of irradiated modern bones do not show signals due to hydrogen atoms although the? always contain a trapped electron s~gnal and a hne with a peak maximum at an apparent g value of 2 0084, some of them also show an addmonal hne with an apparent g value at ~ts peak maximum of 2 0126 (see Fig 5) The signal at 2 0084 is probably due to the Intrinsic orgamc radical now d~storted by the overlapping defect s~gnal It is clear from this figure that the s~gnal at 2 0126 is radmtlon reduced and, in all probability, the other one as radmtlon enhanced as suggested m Section 3 All three signals

decay over a period of days after the trradmtlon The decay curves observed for the trapped electron s~gnal for a variety of samples are shown m Fig 6(a) The curves for the more modem samples can clearly be seen to consist of two components a rapid decay part and a slower decay part The fast decay component ts probably due to the relatively rapid decay of the radiation enhanced mtnnslc organic radical signal which overlaps the dating signal There does not appear to be any correlation between the absolute size of the observed trapped electron signal amphtude after lrradtatlon and the age of the bone and the observed fading would seem to be unrelated to m~tlal signal size However, for whatever reason, there ~s an apparent fading of the dating signal which ts more pronounced the younger the bone The reason for this fadmg may be a combination of a slight overlap between the dating signal and the line at g = 2 0 0 8 4 and an unobserved orgamc radical signal hidden beneath the dating signal, as suggested by the results in Section 3 The fading results are reproducible provided that spectra are recorded soon after sample Irradiation (ideally within an hour), this is particularly Important for modern bones because of the mltml rapid signal decay as shown in F~g 6(a) In this figure, the amplitude of the dating signal measured immediately after 7 Irradtauon (that ~s, less than 2 h after lrradmtlon) Is taken as 100%, and subsequent measurements are related to this The figure shows that both the lmtlal and the overall decay of the datmg signal m these bones are related to their ages

~¢

¢

i

/

¢

FIG 5 The spectrum of Jrradmted modern bone showing the additional hne at an apparent g value of 2 0126 indicated by the arrow Spectrum (a) zs from umrradtated bone, (b) is from the same bone after a / dose of 180 Gy, and (c)-(g) show the decay of all the signals over a period of 10 days

ESR SIGNALS IN BONES 100 ~

&

I~-N~_~

-

-

,I, =

m, Re,tire

S=gnal Ampktude

90

] \

~

1 ~

.~_ . ~

'

n

A

100 "~

& 10 n s

6

Rg S iealnW ale

5

:

=

1

_-

;

~, -" s

217

95'

4

i

Arnpl~ude 90

75 I 0

.

, 4

•

, 8

C~aum p h o s p h a t e . , • , Days 12 16

85

F=G 6 (a) Decay curves of the dating signal for vanous samples after a 7 dose of 180 Gy, the sample number from

Table I being gnven by each curve

0

!

i

4

8

Days

l

!

12

16

FIG 6 (c) Dependence of the decay curves upon total 7 dose

A = 180 Gy, B = 720 Gy, and C = 900 Gy, the unpnmed letters refernng to sample 2 and the primed letters to

100

sample 3 100 ~

95 Relatwe Signal Amphtude

i0

95 ! Relative S~jnal 90 Ampl~u~e

90

B *' =

B

--

_m

~_

85

85

B IA • A

80 ,

0

4

8

Days 12

16

1

75 8

FIG 6 (b) Dependence of the decay curves for a 7 dose of 180Gy upon the heat treatment of the sample A--unheated and B = heated to 105°C for 90 mm before lrradlatmn, the unpnmed letters refernng to sample 2 and the primed letters to sample 3 the older bones show httle signal fading whereas more recent bones show an mmal rapid decay followed by a residual dating signal which is apparently stable This Is m accord with the suggestions above the quantity of orgamc components m the bone can be expected to dechne wtth the age of the bone and consequently the signal Induced or enhanced by the ~ irradiation will be smaller for older bones with, of course, a smaller decrease relative to the datmg signal The lmphcatlon of the observed signal fading, for modern bones m particular, is that spectra recorded lmmed~ately after laboratory irradiation of samples would give very low apparent ED values and hence very low ages for the samples For example, consider the data for sample 3 the spectra recorded 20 h after irradiation of the samples gave an ED value of 2 56 + 0 64 Gy, whereas those recorded after delays of 3 weeks, l y r and 4yr gave 792_+088Gy, 7 4 7 + 0 8 2 G y and 6 7 3 + 0 5 G y respectwely, the last three values being essentially the same to within the experimental error The corresponding ages are 3 8 4 + 0 9 6 , 11 8 8 + 1 32, 11 2 1 + i 23 and 10 I + 0 75kyr, assuming an annual dose rate of 2mGy These observations are not m themselves new, and several authors (Henmng and Grun, 1983) have reported delaying recording spectra for about 14

12

Days FIG 6 (d) Decay curves slmdar to Fig 6(a) but for a y dose of 900 Gy days after sample irradiation m order to obtam conststent and rehablc results What is new, however, is the observation that the dating signal fading curves [Fig 6(a)] appear to be related to the ages of the bones the older the bone the slower the rate of decay of the signal and the larger the final (constant) signal relatwe to the ongmal signal Obviously, wc now have a method for relatwe age deterrmnatlon The advantages of this new method of ESR dating are as follows (1) It reqmrcs no knowledge of the annual and the laboratory dose rates (the former is often not known, when its value must be assumed) (11) It is useful when there is too httle sample for it to be divided into several ahquots for irradiation (111) The method might be capable of Identifying ancient heating of bone if unheated bones from the same site were avadable (or if the age of the bone had been estabhshed by other methods) m that a heated bone would have a comparatively low orgamc content and a different dating signal fading curve would be observed relatwe to the other bones That this is possible is shown for samples 2 and 3 m Fig 6(b), there is a lower mmal decay rate and a higher final signal for the heated bone It may be possible to extend the ~dca to be able to obtain absolute dates for bone although two problems wdl inevitably comphcatc the Issue the

218

A

D

O D U W O L E and K

annual dose rate must be known (or assumed) and the original organic content of the bones being assessed must be assumed to be similar and to have decayed at a similar rate Nevertheless, the emplncat evidence adduced below is worthy of consideration Firstly we note that the decay curves depend on the total / dose given to the sample [Fig 6(c)] the larger the dose, the slower the initial decay rate and the higher the final signal Furthermore, as might be expected, the decay curve for a bone depends also upon whether the sample has been previously irradiated Samples re-irradiated about 6 months after a previous dose show a reduced initial decay and a higher final signal, consistent with a decreased initial contribution from the organic radical It should be possible b) a statable choice of parameters to develop the method to give absolute dates To this end we repeated the experiments summarized in Fig 6(a), but with a b~gger : dose of 9 0 0 G y in order to accentuate the effect [see Fig 6(d)] There are two obvious parameters to try to correlate with the age of the bone the initial slope of the decay and the final signal level We ha~e not yet attempted the former, but the final signal level appears approximately to be related to the logarithm of the age

5 E F F E C T O F Fe ~+ O N T H E S T A B I L I T Y O F T H E D A T I N G S I G N A L IN B O N E S Some of the bones listed in Table 1 were not ongmallx included m the dating signal fading study because of their relatively high concentrations of Fe 3+ and other transition metal ions which we knew could complicate the interpretation of the results Some ancient bones rich m Fe ~+ do not show a dating signal exen after laboratory irradiation although the results from Jr and X-ray diffraction studies show that they contain hydroxyapame (Sales et a l , 1985) This suggests that a large concentration of Fe ~ ~ons can destabilize the dating signal in bones A detailed studx was therefore carried out to examine the effect of Fe ' . ions on electron trapping in bones and in h~droxyapatlte There are three digtmct posslbihtles for the non-appearance of a dating signal

D

SALES

shows a general increase in the hnewidth of the trapped electron signal with increase m the Fe 3+ content of the samples It is possible that concentration broadening due to a large number o f trapped electrons may be the cause of an increase in hnewldth, but no such effect was observed when modern bones were exposed to prolonged y irradiation equivalent to an age of about 13 mdhon yr ( l e 2 6 k G y ) The extraneously large hnewidth observed for sample 9 may be due to additional line broadening caused by V 4+ and other transition metal ions (detected m plasma analysis results) which are relatively more abundant in this sample than in the others The results for samples doped with Fe 3+ are given m Table 3 and the overall increase in A H indicates that relaxation of the trapped electron spins by Fe 3+ ions is occurnng An Increase in hnewidth can cause an apparent loss of signal intensity because, for a line of Lorentzlan shape, a significant part of the intensity resides in the wings of the line and in practice the (double) integration is performed on spectra of fixed field sweep This might be happening but not, we think, to any appreciable extent The apparent loss of the dating signal m Fe3+-nch bones is almost certainly due to some other mechanism most likely a partial quenching of the signal as suggested in (n) above The radiation sensitivity of different bones is different, I e the increase In the size of the signal generated by a given I: dose depends upon the bone This has sometimes been interpreted as due to the approach of electron trap saturation, by analogy with TL methods However, bones generally pick up iron from the environment w~th the iron content being greater the older the bone The possible effect of Fe 3+ ions on the radiation sensitivity of bones and similar materials appears not to have been considered previously, on the contrary, it has been suggested that Fe 3+ ions act as stable electron traps in calcite (Hennlng and Grun, 1983) The new method proposed above for the relative dating of bones must be limited to samples containing low concentrations of Fe 3+ and other transition metal ions until their effects, if any, are known In order to 7

0) The paramagneuc Fe ~ ions might cause fast relaxation and ~ery broad spectral lines (ll) The Fe '" ions might react w~th the electrons, which would otherwise be trapped to form the source of the signal and be reduced to diamagnetic Fe z+ ions The effect on the size of the Fe 3+ signal is not likely to be detected because of the very low concentration of electrons relative to Fe ~+ ions (m) The intense very broad Fe a+ signal near g = 2 might s~mply mask the much weaker dating signal

~H/G

// S

I:1

4

200

Case (m) might easily occur in those samples for which no dating signal is observed but it is rather difficult to investigate the problem experimentally However, support for Q) is illustrated in Fig 7 which

400 600 Relat)ve Fe3+ S)gnal Intenslly

800

FIG 7 The variation of the llnewtdth of the dating signal w~th Fe ~+ concentration, the sample number from Table 1 being given by each point

ESR SIGNALS IN BONES

219

Table 3 Lmewldths, inten=Ues, and EDs for samples nnpreipaated wlth Fe3* Ca3 (PO,)2~; Co-preclp Fe3+ m Ca3 (PO,)2 Ca3 (PO,)2§ Ca3 (1'O,)2+ 10%11 Ca3 (PO,)2 + 100%11 Modern bone¶ Modern bone + 10%11 Modern bone + 100%11

AH*

Intenntyt

ED

35 40 42 48 52 41 43 48

58 14 48 20 l2 14 9 14 2 90

9206+248 9019±169 5639±97

*The error m thts hnewtdth is probably + 0 3 G because of the dh~culty m locaung accurately the peak nummum, winch is rather broad tThe mtenuty, m arintrary umts but corrected to a standard spectrometer gain, of the daung signal after basehne correction to ehrmnate the drift caused by the Fe3+ background :~Made as described m the experimental section and irradmted wlth a 7 dose of 180 Gy §General purpose reagent grade pre,~pltated talcum phosphate purchased from Hopkln and Wdhams, and ~rradmtedwith a y dose of 180 Gy before soalang In fernc alum tlTins refers to either a 10 or 100% soluUon of ferric alum m winch the sample was soaked ¶Irradmted with a ~ dose of 900 Gy before soaking m ferric alum solutmn

esttmate the effect of Fe 3+ tons on datmg results obtained by the usual method of ahquot trradtatmn, we used the modem bone sample prewously trradtated and doped wtth Fe 3+ The complete spectrum ts shown m Ftg 8, the dating s~gnal appeanng as the apparently sharp hne m the middle of the broad hne centred on g = 2 due to Fe 3+, the small hne at low field is also due to Fe 3+ Note that when the dating s~gnal ts observed m greater detad, that is the spectral width is decreased, the broad Fe 3+ signal wdl appear as a slopmg basehne, a point we refer to again below Three years after the first experiment we considered the sample to be anctent, m the sense that ~t had had a large ~ dose and all fading must have occurred, and determined tts ED m the usual way, the spectra bemg recorded 21 days after the new trradtatton of the ahquots The results m Table 3 show a decrease m apparent ED w~th increased Fe 3. content of the sample Th~s suggests that datmg results obtamed for samples that have accumulated large amounts of

Fe 3÷ are m error and that th~s error mcreases w~th Fe 3+ content and must be corrected for As menttoned above, the spectra for samples w~th large amounts of Fe 3+ appear, for the sweep w~dths usually employed, to suffer from bad basehne drift and the slope of the basehne should be an approxtmate measure of the concentratmn of the Fe 3÷ (because ~t will be an mdlcauon of the stze of the Fe 3÷ signal) If no quenchmg of the datmg signal by Fe 3. tons occurred, then the correct signal amphtudes would be obtamed by a basehne correcuon of the spectra That this does not work is shown by the data m Table 3 A possible method to try to correct for the presence of Fe 3÷ ts to take ~ as the slope of the basehne measured m arbttrary umts per gauss (and adjusted for spectrometer g i n ) and plot 6 agamst the apparent ED values Tlus appears to gwe a stratght hne (Fig 9), although we do not have many pomts on the plot The mtercept at zero 6 should be equal to the true ED A correcUon for the Fe 3+ content of an unknown bone sample can be esumated by measunng 6 for the bone, usmg Fig 9 to find the ED correspondmg to th~s 6, takmg the ratm of the ED at 1000900ED/Gy • 800 700 6OO 500 0

FIo 8 A rode sweep spectrum for the final sample m Table 3 showmg the Fe3+ and dating signals

100

&xl08 G 1

200

FIG 9 A plot of basehne slope, 6, asamst apparent ED for the final three samples m Table 3

220

A D O D U W O L E and K D SALES

zero 6 to this ED, and multiplying the apparent ED of the unknown sample by this ratm For example, consider the case of a Eohlppus bone which dates to 10 Myr Our previous dating result gave an ED of 1006 7Gy, corresponding to an age of 1 51 Myr, assuming an annual dose rate of 2 mGy The slope, 6, is 158 3 x 10 -s G -~ which, with the corresponding ED raUo, gives a corrected ED value of 1609 Gy and an age of 2 41 Myr a considerable ~mprovement on the previous result, but still too low Obviously, the Fe 3÷ content cannot, m th~s case, account for all the d~screpancy and perhaps our mmal suggesuon (Sales et al, 1985) that some of ~t ~s due to fossd~zatmn is stdl correct The ESR spectrum of sample 9 shows an intense Fe TM s~gnal which ~s comparable w~th that observed for the Eoh~ppus bone d~scussed above (see also Table 2) and a weak but d~scermble natural dating s~gnal [F~g 10(a)] The problem of dating the bone from the s~gnal m F~g 10(a) ~s obvious and the results have not previously been reported (Hennmg et al, 1985) The dating signal can be seen more clearly by correcting for the basehne drift of the dating s~gnal, an apparent drift caused by the supenmposed broad Fe 3+ s~gnal [F~g 10(c)] Even clearer are the other spectra tn Fig 10, namely that obtained by accumulating the spectra from 50 sweeps [F~g 10(b)] and the basehne corrected version of this [F~g 10(d)] Both of the basehne corrected spectra show a broadening on the low field s~de due to the fact that the basehne draft has been corrected as ~f~t were a straight hne whereas,

\

/

FIG 10 Spectra obtained from sample 9 (a) the natural signal, (b) the natural signal accumulated over 50 sweeps, (c) the baseline-corrected version of (a) (d) the basehnecorrected version of (b)

as can be seen from Figs 10(a) and 10(c), ~t ~s actually shghtly curved The ED values obtained m the usual way by measunng the amphtudes of the basehne corrected spectra of the natural dating signal and of the laboratory 7 lrradmtmn enhanced signal depend markedly on the magneuc field sweeps used to obtam the spectra For example, EDs of 423 + 9 3 G y and 1205 + 307 Gy are obtained for 60 and 100 G sweeps respectwely (corresponding to ages of 211 + 47 and 603 + 154kyr ff an annual dose rate of 2 m G y is assumed) The comphcatlons due to the overlapping signal(s), and perhaps to the effect of transmon metal ions on the size of the dating signal (as mentioned above), are obvmusly very severe and this bone ts a rather difficult case for an rater-laboratory ESR dating comparison project Techmques to try to overcome these comphcatmns include a more general basehne correction program and further studies of the effect of Fe 3÷ ions on the signal size, these are currently bemg undertaken

6 CONCLUSION This study shows how dating signal fading curves observed for bones after laboratory ? irradiation can be used for a relatwe datmg of a collecuon of bone samples and suggests a potentml of the method for absolute dat,ng and for assessing previous heat treatment The method would seem to presume a slmdar orgamc composmon of samples of the same age and would therefore be unsmtable for dating a mixed collection of bones, teeth, and tusks since these would have widely different orgamc contents even for samples taken from the same place The practical problem of spectrometer stabdlty over the period of time reqmred for measurements on a sample is a difficulty which can be surmounted as suggested Problems are encountered both m the present method and m the more conventional sequential ~rradmtmn techmque when quantities of Fe 3+ runs are present m the bone A method of correcting for errors due to these ~ons is suggested, ~t revolves esumatmg the amount of Fe 3+ present m the sample by measuring the slope of the basehne and then using a standard graph to obtain a correction ratio Nevertheless, consistent ED results for sample 9 (the original sample for the rater-laboratory ESR dating comparison project) are difficult to obtain because of the very large background signal given by this sample Acknowledgements--We would like to thank Dr S Olsen, Professor Wu, Professor L Martin Dr Hedges of the Radiocarbon Accelerator Unit at Oxford, and the British Museum for providing us with bone samples We gratefully acknowledge the assistance of David Clarke with the ), ~rradlatlon The ESR measurements were carried out using the Umversity of London Intercollegiate Research Service

E S R S I G N A L S IN B O N E S

REFERENCES Carnngton A and McLachlan A D (1967) Introduction to Magnetic Resonance Harper and Row, New York Elsenberger S, Lehrman A and Turner W D (1940) The basic calcmm phosphates and related systems Some theoretical and practical aspects Chem Rev 26, 257-296 Grun R and Schwarcz H (1987) Some remarks on ESR dating of bones Anctent TL 5, 1-9 Hennmg G J , Geyh M A and Grnn R (1985) The first rater-laboratory ESR companson project Phase II evaluation of eqmvalent doses (ED) of calcites Nucl Tracks 10, 945-952

221

Hennlng G J and Grfin R (1983) E S R daung m Quaternary geology Quat Set Rev 2, 157-238 Ikeya M (1975) Daung a stalacuteby electron paramagneUc resonance Nature 225, 46-48 Ikeya M (1977) Electron spin resonance dating and fisslon track detection of Petralona stalagmite Anthropos 4, 152-168 Ikeya M (1978) Electron spin resonance as a method of daUng Archaeometry 20, 147-158 Ikeya M (1988) Dating and radlatlon doslmetry w~th electronspin resonance Magnet Res Rev 13, 91-134 Sales K D, Oduwole A D , Robins G V and Olsen S (1985) The radlatlonand thermal dependence of E S R signals in ancient and modern bones Nucl Tracks 10, 845-851