Archaeological Chemistry IV - American Chemical Society

17 Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 24, 2016 | http://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch017

Radiocalcium Dating: Potential Applications in Archaeology and Paleoanthropology R. E . Taylor , Peter J. Slota, Jr. , Walter Henning , Walter Kutschera , and Michael Paul 1

1

2

3

4

Radiocarbon Laboratory, Department of Anthropology, Institute of Geophysics and Planetary Physics, University of California, Riverside, Riverside, C A 92521 Gesellschaft für Schwerionenforschung, D-6100, Darmstadt 11, Federal Republic of Germany Physics Division, Argonne National Laboratory, Argonne, I L 60439 Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem, Israel 1

2

3

4

The usefulness and feasibility of using a long-lived = about 100,000 years) isotope of calcium ( Ca) as a means of dating bone and other calcium-containing samples up to ca. 1 million years old are being evaluated. The Ca/ Ca ratio in natural terrestrial materials has been measured for the first time by using accelerator mass spectrometry. The Ca method could potentially provide an independent temporal scale for the period centering on the Middle Pleistocene that is comparable to that provided by K/Ar values for the early portion of the Pleistocene and C values for the terminal Pleistocene and Holocene. (t½

41

41

40

41

14

/Y.

VARIETY OF GEOCHRONOLOGICAL METHODS

have b e e n u s e d over the last 3 decades to p r o v i d e the t e m p o r a l frameworks that have p e r m i t t e d archaeologists a n d paleoanthropologists to reconstruct t e m p o r a l relationships a m o n g fossil h o m i n i d forms a n d to investigate rates of change i n the e v o l u t i o n of h o m i n i d b e h a v i o r (1). T h e reconstruction of basic chronological r e l a t i o n ships a m o n g the late M i o c e n e , P l i o c e n e , a n d E a r l y Pleistocene H o m i n i d a e has b e e n a c c o m p l i s h e d b y the analysis of K / A r age estimates a n d inferences from the oxygen isotope a n d paleomagnetic r e c o r d c o m b i n e d w i t h l i t h o 0065-2393/89/0220-0321$06.00/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

Allen; Archaeological Chemistry IV Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

322

ARCHAEOLOGICAL CHEMISTRY

stratigraphie a n d biostratigraphic studies. C h r o n o l o g i c a l frameworks are r e a sonably c o m p l e t e for the last 30,000 to 50,000 years because objects f r o m this t i m e span can be dated b y the C m e t h o d .

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 24, 2016 | http://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch017

1 4

A l t h o u g h m a n y p r o b l e m s r e m a i n , there are reasonably specific c h r o n ological frameworks for h o m i n i d e v o l u t i o n at b o t h ends of the Q u a t e r n a r y p e r i o d . B y contrast, the d a t i n g frameworks for the p e r i o d c e n t e r e d o n the M i d d l e Pleistocene are generally i m p r e c i s e a n d ambiguous. T h i s a m b i g u i t y was h i g h l i g h t e d m o r e than a decade ago w h e n G l y n n Isaac e m p h a s i z e d the critical n e e d for chronological resolution for the t i m e p e r i o d b e y o n d the c o n v e n t i o n a l C range a n d before the p e r i o d for w h i c h the K / A r m e t h o d can b e r o u t i n e l y u s e d . T h e r e is v e r y little c h r o n o m e t r i c data for this p e r i o d , w h i c h lasted from about 1 m i l l i o n to 60,000 B . P . , and m u c h of what exists is of questionable v a l i d i t y (2). 1 4

W h e n accelerator mass spectrometry ( A M S ) was i n t r o d u c e d for d i r e c t C c o u n t i n g , it was i n i t i a l l y anticipated that the d a t i n g range of the C m e t h o d w o u l d e x p a n d r a p i d l y a n d extend back as far as 100,000 years (3, 4). A l t h o u g h i t is possible that this l i m i t m a y e v e n t u a l l y be r e a c h e d , c u r r e n t e x p e r i m e n t a l conditions r e d u c e this m a x i m u m to b e t w e e n 40,000 a n d 60,000 years. F o r c o n v e n t i o n a l decay c o u n t i n g , limitations are i m p o s e d b y the sample sizes generally available from archaeological contexts a n d the p r o b lems of r e m o v i n g c o n t a m i n a t i o n from sample preparations. T h e s e l i m i t a t i o n s reduce the m a x i m u m ages that can be o b t a i n e d practically to b e t w e e n 40,000 a n d 50,000 years. U n d e r special circumstances, w i t h samples that are larger than are usually available f r o m the t y p i c a l p r e h i s t o r i c archaeological sites, the m a x i m u m range can b e e x t e n d e d to about 60,000 years. W i t h isotopic e n r i c h m e n t , again u s i n g r e l a t i v e l y large samples, ages u p to 75,000 years have b e e n r e p o r t e d for a few samples (5-7). 1 4

1 4

O v e r the next decade, i f the stringent r e q u i r e m e n t s for the exclusion of m o d e r n carbon c o n t a m i n a t i o n i n sample preparations can be m e t , d e v e l o p m e n t s i n A M S may p e r m i t r o u t i n e extension of the C t i m e frame b e y o n d the c u r r e n t 40,000- to 60,000-year range for the t y p i c a l archaeological sample (8). H o w e v e r , the p o t e n t i a l of A M S to extend the C t i m e frame into the 70,000- to 100,000-year range for samples from archaeological c o n texts m a y be possible o n l y w i t h the d e v e l o p m e n t of practical methods of C isotopic e n r i c h m e n t that can be r o u t i n e l y u s e d for samples that contain less than 1 g of carbon. T h e p o s s i b i l i t y of u s i n g a laser-based approach for C e n r i c h m e n t before A M S analysis is b e i n g s t u d i e d (9, 10). 1 4

1 4

1 4

1 4

Significant deficiencies are present i n the p h y s i c a l d a t i n g methods u s e d to infer t e m p o r a l relationships for the paleoanthropological a n d archaeological r e c o r d for the w h o l e of the M i d d l e Pleistocene (about 730,000 to 125,000 B.P.) (II), a n d for the interval of t i m e u p to the initiation of the C t i m e scale. T h e r e are no r e c o g n i z e d major reversals of the earth's geomagnetic field after 730,000 years ago. M a n y i m p o r t a n t sites are not i n areas of v o l c a n i s m , a n d e v e n for those that are, there are often serious analytical p r o b 1 4


17.

TAYLOR ET AL.

Radiocalcium

323

Dating


lems i n w o r k i n g w i t h samples from the m o r e recent e n d of the K / A r t i m e scale. U r a n i u m series d a t i n g can b e used for sites w i t h calcareous s e d i m e n t s , b u t contamination p r o b l e m s are sometimes acute, a n d the g e o c h e m i c a l h i s tory o f the t y p i c a l terrestrial sample is often unclear (12). A t t e m p t s to a p p l y t h e r m o l u m i n e s c e n c e ( T L ) a n d e l e c t r o n s p i n resonance ( E S R ) data to date soil horizons, a n d to use E S R to date b o n e are u n d e r w a y . A s yet, there are no a g r e e d - u p o n c r i t e r i a o n w h i c h to evaluate the o v e r a l l r e l i a b i l i t y of age inferences based o n E S R data (13). It has b e e n suggested (14) that i n some circumstances the obsidian h y d r a t i o n m e t h o d can b e u s e d to infer chronological age o v e r i n excess of 100,000 years (14). H o w e v e r , o b s i d i a n is not a w i d e s p r e a d natural resource a n d some of the h y d r a t i o n rate structures appear to y i e l d p r o b l e m a t i c results. A m i n o a c i d racemization ( A A R ) values can b e u s e d u n d e r some conditions to infer accurate age values for b o n e , b u t seriously anomalous values can be o b t a i n e d (15, 16). T h e conditions u n d e r w h i c h A A R values can be u s e d to accurately infer age, p a r t i c u l a r l y for b o n e samples, c o n t i n u e to b e investigated. Some M i d d l e Pleistocene sites have b e e n dated b y assigning various occupation levels to one of the traditional major g l a c i a l - i n t e r g l a c i a l cycles. T h i s approach to d a t i n g is still u s e d e v e n t h o u g h oxygen isotope analysis of d e e p sea cores a decade ago (17) d e m o n s t r a t e d that the c l i m a t i c r e c o r d is e x t r e m e l y c o m p l e x a n d difficult to resolve for m a n y intervals. M a n y of the sites i n E u r a s i a are p l a c e d into the g l a c i a l - i n t e r g l a c i a l scheme o n the basis of faunal correlations, loess stratigraphie cycles, a n d soil formation cycles. T y p i c a l l y , sites c o n t a i n i n g a " w a r m " fauna are p l a c e d into an interglacial phase a n d sites w i t h a " c o l d " fauna are p l a c e d into a glacial phase. T h e percentage of extent fauna a n d various taxa are u s e d along w i t h the stone tool technology to d e t e r m i n e to w h i c h stadial phase or interstadial phase the site is to be assigned. T h e accuracy of the u n d e r s t a n d i n g of faunal successions, c u l t u r e change, a n d e n v i r o n m e n t a l variability has not b e e n tested b y such methods (18). A n isotopic d a t i n g t e c h n i q u e that can d i r e c t l y assign age to b o n e samples d a t i n g to the M i d d l e Pleistocene w o u l d b e of major significance i n the critical study of the processes i n v o l v e d i n the b i o c u l t u r a l e v o l u t i o n o f humans.

Basis of Radiocalcium Dating Method A l t h o u g h the i d e a of u s i n g C a for d a t i n g h a d b e e n suggested e a r l i e r (19), Raisbeck a n d Y i o u (20) p r o v i d e d the first d e t a i l e d o u t l i n e of a d a t i n g m o d e l for the r a d i o c a l c i u m m e t h o d . F i g u r e 1 outlines the m e t h o d a n d is based o n t h e i r discussion. Because it has a half-life of about 100,000 years, C a c o u l d potentially be u s e d to infer the ages of c a l c i u m - c o n t a i n i n g samples (e.g., b o n e a n d C a C 0 c o n t a i n e d i n soils) o v e r about the last m i l l i o n years. L i k e C , C a is p r o d u c e d b y cosmic ray n e u t r o n secondaries; h o w e v e r , the b u l k of C a is not m a d e i n the atmosphere l i k e C . C a is p r o d u c e d p r i m a r i l y 4 1

4 1

3

1 4

4 1

4 1

1 4

4 1



324


t,/2

= C O . 100,000 YEARS

Figure 1. Radiocalcium dating model: production, distribution, and decay of Ca. After Raisbeck and Yiou (20) and Taylor (21). 41

i n the u p p e r m e t e r of the soil profile b y n e u t r o n capture o n C a . T h e cosmogenic C a is m i x e d w i t h the other naturally o c c u r r i n g c a l c i u m isotopes into the surface soils t h r o u g h g r o u n d water action. C a l c i u m is taken u p into the plant tissue i n the form of C a t h r o u g h ion absorption b y the root system. R a d i o c a l c i u m is t h e n i n c o r p o r a t e d into b o n e t h r o u g h ingestion of plants (21). 4 0

4 1

2 +

A C a / C a e q u i l i b r i u m ratio is m a i n t a i n e d i n l i v i n g organisms b y exchange a n d metabolic processes. I n contrast to C dating, w h e r e the death of an a n i m a l or plant a n d the isolation of a sample from one of the carbon reservoirs constitutes the t = 0 event (0 B . P . ) , the t = 0 event i n the C a 4 1

4 0

1 4

4 1


17.

TAYLOR ET AL.

Radiocalcium

325

Dating

m e t h o d occurs w h e n a sample is s h i e l d e d from the effect of the cosmic-rayp r o d u c e d n e u t r o n irradiation (e.g., b y the p l a c e m e n t of a sample d e e p e r than about 3 m of soil or rock overburden). T h i s s h i e l d i n g c o u l d b e a c c o m p l i s h e d e i t h e r b y b u r i a l or p l a c e m e n t i n a c a v e - r o c k shelter e n v i r o n m e n t . A g e inferences are made o n the basis of the m e a s u r e m e n t of the r e s i d u a l C a w i t h respect to the stable isotopes of C a . C a decays to Κ b y e l e c t r o n capture a n d the e m i s s i o n of a n e u t r i n o . To d i r e c t l y use changes i n C a / C a ratios to accurately infer an age for c a l c i u m - c o n t a i n i n g samples, c e r t a i n f u n d a m e n t a l assumptions m u s t be made. T h e s e assumptions i n c l u d e 4 i

4 1

41


4 1

1. that the i n i t i a l concentration of cosmogenic C a i n samples has r e m a i n e d essentially constant over the p r o j e c t e d C a t i m e scale (or that appropriate corrections can be made for d o c u m e n t e d variations) 4 1

4 1

2. that the C a / C a ratio i n a sample has not b e e n a l t e r e d except b y C a decay since the sample was s h i e l d e d f r o m the effects of n e u t r o n irradiation (i.e., no postdepositional e x c h a n g e contamination of the i n situ C a has o c c u r r e d or b u i l d u p of n e w C a b y neutrons from natural radioactivity [ u r a n i u m a n d thorium]) 4 1

4 1

4 1

4 1

3. that m i x i n g of cosmogenic

4 1

C a i n the c a l c i u m source for a

sample has o c c u r r e d over a r e l a t i v e l y short p e r i o d of t i m e i n c o m p a r i s o n to the half-life of 4. that the half-life of

4 1

4 1

Ca

C a is accurately k n o w n

5. that natural levels o f C a can b e m e a s u r e d w i t h i n reasonable levels of uncertainty. 4 1

F r o m a technological p o i n t of v i e w , the C a m e t h o d c u r r e n t l y stands at the same p o i n t i n its d e v e l o p m e n t as the C m e t h o d d i d i n about 1 9 4 6 - 4 7 . T h e favored m o d e of p r o d u c t i o n for C h a d b e e n k n o w n for some t i m e (thermal neutrons o n N ) a n d , o n the basis of this a n d several other c o n siderations, L i b b y f o r m u l a t e d a d a t i n g m o d e l . H o w e v e r , the half-life was still somewhat u n c e r t a i n a n d the i n i t i a l measurements of natural C c o n centrations w e r e just b e i n g made. R o u t i n e l o w - l e v e l c o u n t i n g was several years away. T h e r e w e r e no e x p e r i m e n t a l data to support several f u n d a m e n t a l assumptions o n w h i c h the practical use of C w o u l d d e p e n d . L i b b y h i m s e l f w o u l d later say (22) that he was i n i t i a l l y c o n c e r n e d that his " n o t i o n " of C dating was " b e y o n d reasonable c r e d e n c e " . F o r a l l its comparisons to C , the C a m e t h o d has p o t e n t i a l l y serious deficits that c o u l d easily p u t q u i t e r i g i d constraints o n the types of d e p o s i tional e n v i r o n m e n t s that c o u l d b e expected to give straightforward results. F o r example, the fact that l i t h o s p h e r i c rather t h a n atmospheric p r o d u c t i o n predominates raises the strong p o s s i b i l i t y that l o c a l i z e d m i x i n g a n d erosion 4 1

1 4

1 4

1 4

1 4

1 4

1 4

1 4

4 1


326


may have caused significant variations i n i n i t i a l C a / C a ratios i n m a n y e n v i r o n m e n t s . I n a d d i t i o n , because samples that have not b e e n b u r i e d d e e p l y e n o u g h w i l l c o n t i n u e to be subjected to C a formation from the cosmic r a y generated n e u t r o n secondaries, the b u r i a l histories of samples m a y affect C a concentrations. A l s o , it is reasonable to expect post-depositional ex change o f c a l c i u m i n some samples t h r o u g h g r o u n d water contact. T h e s e a n d other diagenetic factors suggest that samples from each site or locality m i g h t e x h i b i t u n i q u e i n i t i a l C a concentrations. A n advantage of the p r o j e c t e d C a m e t h o d is that studies can b e c a r r i e d out o n the samples t h e m selves a n d do not necessarily d e p e n d o n access to the actual b u r i a l e n v i r o n m e n t , as is the case w i t h T L , E S R , A A R , a n d o b s i d i a n h y d r a t i o n applications that r e q u i r e t e m p e r a t u r e a n d local radiation dose levels to b e m e a s u r e d to rather close tolerances to achieve reasonable accuracy. 4 I

4 1

4 1


4 1

4 1

Major Issues in the Development of the

4 1

C a Method.

To de

v e l o p a practical m e t h o d of u s i n g C a concentrations i n c a l c i u m - c o n t a i n i n g samples to infer age a n d to d e t e r m i n e the degree of general a p p l i c a b i l i t y , a series of studies m u s t be u n d e r t a k e n that, i n b r o a d o u t l i n e , w o u l d p a r a l l e l the i n i t i a l set o f e x p e r i m e n t s that established the general u t i l i t y a n d accuracy of the C m e t h o d . A l l of the experiments that w o u l d be n e e d e d to d e m onstrate the usefulness of the p r o p o s e d C a m e t h o d w o u l d r e q u i r e a practical a n d effective means of r o u t i n e l y m e a s u r i n g natural C a values i n terrestrial samples. A practical m e a s u r e m e n t technology a n d sample p r e p a r a t i o n m e t h odology is b e i n g d e v e l o p e d against the b a c k d r o p of the f o l l o w i n g c o n s i d erations a n d e x p e r i m e n t a l data. 4 1

1 4

4 1

4 1

Natural

4 1

C a Equilibrium Concentrations in Terrestrial Sam

ples. T h e e q u i l i b r i u m concentration (or " s a t u r a t i o n " values) of C a i n terrestrial (near-surface) samples was i n i t i a l l y estimated b y u s i n g the r e l a t i o n s h i p expressed i n equation 1 as g i v e n b y Raisbeck a n d Y i o u (20) 4 1

(1)

w h e r e / is the t h e r m a l n e u t r o n flux, σ is the cross section of the reaction, ti/2. is the half-life o f C a , a n d t is the p e r i o d of exposure. I f it is assumed that t is m u c h greater than ty (ca. 1 0 years), σ = 4.4 Χ 1 0 " c m , a n d / = 3 X 10~ neutrons c m " s" i n the u p p e r m e t e r o f soil (23), t h e n C a / C a is about 1 0 " (24). T h i s value indicates that for m a x i m u m usefulness i n d a t i n g applications, it w o u l d b e necessary to d e t e r m i n e C a / C a ratios i n the range of 1 0 to 1 0 . 4 1

5

z

3

2

4 0

25

1

2

4 1

14

4 1

1 4

4 0

1 5

Half-life of C a . P u b l i s h e d values for the half-life of C a range from 0.7 Χ 1 0 to 1.7 Χ 1 0 years w i t h stated uncertainties r a n g i n g as h i g h 4 1

4 1

5

5


17.

Radiocalcium

TAYLOR ET AL.

Dating

327

as 3 0 % (see the references i n references 25 a n d 26). Raisbeck a n d Y i o u (20, 27) u s e d 1.3 Χ 1 0 years, whereas other researchers have u s e d values of about 1.0 Χ 1 0 y. T h e most recent value of t = 1.03 ± 0.04 Χ 1 0 years was d e t e r m i n e d from the relative yields of C a ( n , 7 ) C a to C a ( n , y ) C a b y M a b u c h i et a l . (26). U n f o r t u n a t e l y , no d i r e c t m e a s u r e m e n t of the halflife v i a specific activity has b e e n p e r f o r m e d . T h i s m e a s u r e m e n t w o u l d i n v o l v e a mass spectrometric d e t e r m i n a t i o n o f the C a i n a h i g h l y e n r i c h e d sample a n d an a c t i v i t y m e a s u r e m e n t . T h e activity m e a s u r e m e n t poses the greater p r o b l e m because the o n l y detectable radiation i n the decay of C a is soft X - r a y s a n d A u g e r electrons o f a r o u n d 3 - k e V energy. Efforts to achieve a d i r e c t m e a s u r e m e n t of the half-life of C a b y m e a s u r i n g its specific activity are u n d e r w a y . 5

5

5

V2

4 0

4 1

4 1

4 5


4 1

4 1

4 1

Measurement of Natural Ca Concentrations in Terrestrial Samples 41

W i t h the d e v e l o p m e n t of A M S methods of isotope d e t e c t i o n , i t has b e c o m e feasible to examine the p o t e n t i a l of C a d a t i n g o f b o n e a n d other c a l c i u m c o n t a i n i n g samples. C o u n t i n g C a at natural levels (i.e., at l o w activities) is v e r y difficult because, i n a d d i t i o n to the l o n g half-life a n d c o n s e q u e n t l y l o w specific a c t i v i t y , decay is b y electron capture that emits a l o w - e n e r g y X - r a y a n d A u g e r - e l e c t r o n . H o w e v e r , b y u s i n g A M S t e c h n i q u e s , it is possible to measure a C a i o n d i r e c t l y , e v e n i n the presence o f a stable e l e m e n t of the same atomic mass ( K , w i t h 6 . 7 % natural abundance). A M S m e a s u r e ments i n v o l v e the acceleration of ions to sufficiently h i g h energies that t h e i r atomic n u m b e r can b e d e t e r m i n e d . T h i s acceleration u s u a l l y involves a final energy of at least 5 M e V / a t o m i c mass u n i t (amu), so that ions w i t h adjacent values of Ζ have appreciably different energy losses i n a t h i n detector, or that a l l the electrons can be r e m o v e d from the ions so that t h e y can b e separated electromagnetically. F o r C a , the presence of the r e l a t i v e l y a b u n dant stable isobar means that separation at the final energy m u s t b e c a r r i e d out v e r y efficiently. 4 1

4 1

4 1

4 1

4 1

S i n c e 1980, several groups have e x a m i n e d various approaches to the m e a s u r e m e n t o f C a . U s i n g the A l i c e accelerator facility at O r s a y , R a i s b e c k a n d Y i o u (27) accelerated C a ions to about 1 M e V / a m u i n a l i n e a r accelerator, s t r i p p e d off e n o u g h electrons to m a k e C a ions, a n d t h e n accelerated these ions to 7.5 M e V / a m u i n a c y c l o t r o n . T h e s e ions w e r e f u r t h e r s t r i p p e d to Ca , a n a l y z e d i n a magnetic spectrometer, a n d m e a s u r e d i n a detector telescope that measures b o t h t h e i r specific energy loss (ΔΕ) a n d total e n e r g y (E). A s expected, the p r i n c i p a l b a c k g r o u n d was f r o m K . T h e s e e x p e r i m e n t s d e m o n s t r a t e d that energy levels c o u l d be attained that c o r r e s p o n d to C a / ^ C a = 1 0 " i n e n r i c h e d samples. T h i s m e t h o d was not efficient e n o u g h to measure natural C a concentrations. I n an alternative approach, R a i s b e c k et a l . (28) d e t e r m i n e d that b y starting w i t h C a H " , w h i c h is an easily f o r m e d 4 1

1 4 +

2 0 +

4 1

4 1

12

4 1

3


328


negative m o l e c u l a r i o n , it was possible to obtain e n o r m o u s l y i m p r o v e d suppression of the K b a c k g r o u n d , because K H ~ does not form r e a d i l y , i f it forms at a l l . T h e C a H ~ i o n c o u l d t h e n b e i n j e c t e d into a t a n d e m e l e c trostatic accelerator to achieve a h i g h e n o u g h final energy that magnetic a n a l y s i s - d e t e c t i o n c o u l d separate C a f r o m the r e m a i n i n g K . 4 1

3

3

4 1

4 1

T h e approach j u s t d e s c r i b e d has b e e n evaluated b y several groups. A d e t a i l e d study was made b y F i n k et a l . (29), w h o i n j e c t e d b o t h C a O " a n d C a H - from artificially e n r i c h e d samples ( C a / C a = 2 Χ 10" ) into the R e h o v o t P e l l e t r o n t a n d e m accelerator (30). A l t h o u g h a final C a e n e r g y of o n l y 130 M e V was o b t a i n e d , the K interference was strongly suppressed b y the C a H ~ i n j e c t i o n . F i n k et a l . a c h i e v e d sensitivities of C a / C a = 1 X 1 0 " for C a O " i n j e c t i o n a n d 5 X 10" for C a H ~ . T h i s sensitivity was still not good e n o u g h for natural terrestrial samples because of the l o w negative i o n y i e l d a n d the presence of C a H ~ and C a H ~ i n the i n j e c t e d i o n beams that, i n the absence of a velocity-sensitive e l e m e n t i n the analysis system, r e s u l t e d i n h i g h b a c k g r o u n d count rates. M o r e r e c e n t l y , Ca/Ca ratios i n the range o f 10 ~ have b e e n m e a s u r e d i n meteorites at the A M S t a n d e m facilities of Rehovot(3I) a n d Rochester (32). 4 1


4 1

4 1

3

4 0

12

4 1

4 1

4 1

4 1

3

11

12

4 2

4 0

3

4 3

2

4 1

12

T h e first d i r e c t m e a s u r e m e n t of the terrestrial C a concentration i n natural samples was a c c o m p l i s h e d at the A r g o n n e N a t i o n a l L a b o r a t o r y w i t h the A r g o n n e t a n d e m linac accelerator system ( A T L A S ) f o l l o w i n g the p r e e n r i c h m e n t of the C a w i t h a C a l u t r o n isotope separator at the O a k R i d g e N a t i o n a l L a b o r a t o r y (33). A calibration sample w i t h a k n o w n C a c o n c e n tration (artificially e n r i c h e d i n C a b y n e u t r o n irradiation) was i n c l u d e d to c h e c k o n the accuracy of the e n r i c h m e n t factors d e t e r m i n e d from the c o n centration of ^ C a c o l l e c t e d d u r i n g the p r e e n r i c h m e n t process. O n the basis of the data f r o m the the c a l i b r a t i o n sample, it was d e t e r m i n e d that the p r e e n r i c h m e n t factors calculated o n the basis of C a data w e r e accurate to ±15%. 4 1

4 1

4 1

4 1

4 2

C a l c i u m extracted from a m o d e r n b o v i n e b o n e a n d f r o m surface a n d b u r i e d l i m e s t o n e was p r e e n r i c h e d b y about 2 orders of m a g n i t u d e . T h e c a l c i u m as C a H ~ was accelerated i n a negative-ion sputter source for i n j e c t i o n into the t a n d e m accelerator. A g a i n , C a H " ions w e r e chosen to greatly r e d u c e isobaric interference from K . V e r y efficient suppression of n e i g h b o r i n g stable C a isotopes was a c h i e v e d t h r o u g h the c o m b i n e d filtering action of the velocity-focussing l i n a c a n d the magnetic beam-transport sys t e m . A n E n g e split-pole magnetic spectrometer filled w i t h n i t r o g e n was u s e d for p a r t i c l e identification. T h i s m e t h o d v e r y efficiently separated C a from the i n t e r f e r i n g stable K isobar. 4 1

3

3

4 1

4 1

4 1

T a b l e I lists the results of this e x p e r i m e n t . A m e t a l l i c c a l c i u m sample was p r e p a r e d from l i m e s t o n e f r o m an 11-m d e p t h and was not p r e e n r i c h e d . T h e sample was assumed to contain no cosmogenic C a at the i n s t r u m e n t a l d e t e c t i o n l i m i t of the A M S system u s e d , because of its geological age ( M e s ozoic) a n d b u r i a l d e p t h . N o counts w e r e o b s e r v e d for a t i m e a p p r o x i m a t e l y three times longer than that for the bone sample. F r o m the m e a s u r e m e n t 4 1


17.

TAYLOR ET AL.

Table I.

4 1

C a / C a Concentrations in Natural Preenriched Terrestrial Samples

Sample

Preenrichment Factor* 1 151

Inferred Original CaICa _ 2.0 ± 0.5 x 1 0

-13

116

7.6

±

4.5 x

-13

117

3.4

±

2.1 x 1 0

Observed Ca/Ca 41

Limestone (11 m) Modern bone Limestone (surface) Limestone (11m) rf


329

Radiocalcium Dating

a

Archaeological Chemistry IV - American Chemical Society

Recommend Documents