Nuclear and Chemical Dating Techniques - American Chemical Society

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Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 21, 2017 | http://pubs.acs.org Publication Date: January 29, 1982 | doi: 10.1021/bk-1982-0176.ch016

Preliminary Studies on Dating Polar Ice by Carbon-14 and Radon-222 E. L . FIREMAN and T. L . NORRIS Smithsonian Astrophysical Observatory, Cambridge, M A 02138

The Antarctic and Greenland ice sheets originate from snow and are vast storehouses of information on the history of the earth's atmosphere and ecology. At 60-m depth, snow at Byrd Station is compressed into ice and air is occluded in the form of gas bubbles under pressure [1,2] . The ice then slowly moves toward the continental margin, where most of i t goes into the ocean. The travel distance for ice originating in central Antarc tica is at least 2000 km. Since the average travel speed is a few meters per year at the surface and slower below the surface, samples of million-year-old ice may exist. The radioactive dating of polar ice would give important information on the history of the region and aid searches for ancient ice. Interesting ice samples from Antarctica and Greenland have been and are being recovered. We studied samples of the Byrd core, which is a 12-cm-diameter core that extended to bedrock at 2100-m depth [1]. This core is presently kept at the Central Ice Core Storage Facility at S.U.N.Y. Buffalo (C. C. Langway, Jr., Curator). Its age-depth relationship has been calculated on the basis of rheological models [3,4,5], and comparisons of the σ O variations of the core with those in the Camp Century (Greenland) core. The age calculated for the bottom ice is between 50 x 10 and 100 x 10 years. 1

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3

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The ô 0 i n Byrd core melted i c e as a f u n c t i o n o f depth has been measured by mass spectrometry [4,5]. Since t h e 6 0 s c a l e depends on the temperature o f the ocean water t h a t developed i n t o snow f l a k e s [ 6 ] , accurate d a t i n g o f t h e core i t s e l f i s necessary to reveal t h e temperature h i s t o r y o f the ocean surface water. Oeschger e t a U , [ 7 ] measured the C contents o f C 0 e x t r a c t e d from 3 tons o f i c e melted i n s i t u , a t depths o f 100, 175, 270, and 380 m near the Byrd s i t e . ""Their C ages f o r 270- and 380-m depths are 1300 ± 700 and 3000 ± 500 y e a r s , r e s p e c t i v e l y . 1 8

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F i g u r e s i n b r a c k e t s r e f e r t o the l i t e r a t u r e r e f e r e n c e s a t the end of t h i s paper.

0097-6156/82/0176-0319$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S 14

I t i s necessary t o measure C i n s m a l l e r i c e samples t o date specimens from the core i t s e l f . The u n c e r t a i n t i e s i n the c a l c u l a t e d age-depth r e l a t i o n increase w i t h depth because i t i s d i f f i c u l t t o measure C a t l a r g e depths. Another r a d i o a c t i v e d a t i n g method i s d e s i r a b l e f o r the bottom h a l f of the c o r e ; R n , can be e x t r a c t e d and counted by e s s e n t i a l l y the same procedure as C, and when combined w i t h the measurement of T h , i s a promising d a t i n g method f o r the core bottom. We analyzed C and R n i n Byrd core samples from 270-, 362-, 1070-, and 1496-m depths. We a l s o analyzed surface and subsurface (5- t o 25-cm depth) i c e samples from the A l l a n H i l l s meteorite s i t e [ 8 ] , where more than 1000 meteorites have been recovered from an area of approximately 100 km [ 9 ] . A l l a n H i l l s i s a region near the c o n t i n e n t a l margin where the i c e movement i s blocked by h i l l s p r o t r u d i n g above the surface and i c e i s r a p i d l y d i s s i p a t e d by wind a b l a t i o n . The t e r r e s t r i a l ages of more than a score of l a r g e m e t e o r i t e s , unmoveable by wind, have been measured and range from (Ί1±1) χ ΊΟ years t o (700±100) χ 10 years [10, 11,12]. The recovery l o c a t i o n s of these specimens are known r e l a t i v e t o a network of stakes. A recovery l o c a t i o n - t e r r e s t r i a l age p a t t e r n i s beginning t o emerge t h a t i n d i c a t e s a continued upflow of i c e f o r ^700 χ 10 years a t one l o c a t i o n and e s s e n t i a l l y stagnant i c e ^ f o r t h i s p e r i o d of time a t another l o c a t i o n [13]. The ages of €he i c e from these two l o c a t i o n s should be determined by r a d i o m e t r i c d a t i n g . Figure 1 i s a map of A n t a r c t i c a showing the l o c a t i o n s of Byrd S t a t i o n , A l l a n H i l l s , and other well-marked s i t e s . 14

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E x t r a c t i o n and P u r i f i c a t i o n Procedures We e x t r a c t the gas from the i c e by an evacuation f o l l o w e d by a s e r i e s of helium purges of the melt water. A crack-free s e c t i o n of the core i s s e l e c t e d . The i c e i s r i n s e d w i t h d i s t i l l e d water using the S.U.N.Y., B u f f a l o c l e a n i n g procedure t o remove surface contaminants, c a r r i e d t o our l a b o r a t o r y , and p l a c e d i n our e x t r a c t i o n system. Figure 2 i s a drawing of our gas e x t r a c t i o n system. Up t o 30 kg of i c e can be lowered i n t o the 6-inchdiameter g l a s s c y l i n d e r . The cap w i t h the l a r g e c o l d f i n g e r shown i n f i g u r e 2 i s b o l t e d t o the g l a s s c y l i n d e r . A i r i s then removed from the c y l i n d e r by an evacuation t o a pressure of 4 pm. The c y l i n d e r i s then purged by f i l l i n g i t w i t h He t o 1.3 atm and then evacuating the He. This He purge procedure i s continued u n t i l the i c e takes on a glazed appearance showing t h a t surface m e l t i n g occurred; during t h i s time 200-400 cc of melt water c o l l e c t s a t the bottom of the c y l i n d e r . This procedure not only removes remnant a i r from the c y l i n d e r but a l s o most of the a i r adsorbed on the i c e surface. The c y l i n d e r i s then f i l l e d w i t h He t o 1.3 atm and c l o s e d o f f u n t i l the i c e melts completely. The m e l t i n g of 10-20 kg of i c e takes between 8 and 16 hours.

Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

FIREMAN AND

NORRis

U

222

Dating Polar Ice by C and Rn


16.


321


ï

CHARCOAL

TJ

He IN

PUMP

Ϊ

48"

00

ο_

oo oo

6"

ICE

Figure 2.

4

Mg(C10 )

Extraction system.

#2

SIEVE #5A

r

CHARCOAL CHARCOAL

**


SIEVE #5A

FLOW ιοί METER ]

m

o

H W

I

H

r

P

α

>

>

cο r

16.

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FIREMAN AND NORRis

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Dating Polar Ice by C and

Rn

The gas i s removed from the melt water i n three e x t r a c t i o n s : (1) the gas i n the volume above the melt water i s c o l l e c t e d ; (2) the melt water i s purged w i t h He; (3) the melt water i s a c i d i f i e d w i t h H S 0 t o pH=l, heated t o 55 °C, and then again He purged. We r e c e n t l y s t a r t e d t o a c i d i f y w i t h HN0 which g i v e s more repro d u c i b l e Rn r e s u l t s . During the e x t r a c t i o n s , the c o l d f i n g e r above the melt water i s maintained a t -70 °C t o remove most o f the H 0 vapor ( t h e remnant vapor i s removed by a Mg ( C 1 0 ) t r a p ) . The C0 i s trapped on a f i n e g l a s s s p i r a l a t -196 °C and the a i r (gas minus C 0 ) i s c o l l e c t e d by a molecular s i e v e a t -196 °C. The C 0 i s removed from the C0 t r a p a t room temperature, measured v o l u m e t r i c a l l y , and stored. The a i r i n the s i e v e i s then removed a t 100 °C measured v o l u m e t r i c a l l y and stored. Approximately 90 percent of the a i r and h a l f the C0 i s c o l l e c t e d from the volume above the melt water; approximately 10 percent o f the a i r and h a l f of the C0 i s c o l l e c t e d i n the f i r s t He purge. Very l i t t l e a i r and C0 i s c o l l e c t e d i n the second He purge a f t e r a c i d i f y i n g and heating the water. The C0 i s accompanied by radon, which i s d i f f i c u l t t o separate. The C0 i s t h e r e f o r e o f t e n s t o r e d before counting i t s C a c t i v i t y . A small amount o f radon i s i n the bubble f r a c t i o n ; l a r g e r amounts of radon are i n the purged e x t r a c t i o n s . The C0 i s p u r i f i e d before c o u n t i n g by a s e r i e s o f f r e e z i n g and u n f r e e z i n g steps. A f t e r the t h i r d e x t r a c t i o n , we measure the radon and i t s y i e l d by adding several ce STP of c a r r i e r Ar t o the melt water w i t h He t o 1.3-atm pressure and a l l o w the radon t o b u i l d up f o r 4 days. We a l s o replace the s p i r a l g l a s s C0 t r a p w i t h a charcoal t r a p t o i n s u r e the c o l l e c t i o n o f the c a r r i e r Ar w i t h the radon. We then He-purge the water and c o l l e c t the Rn w i t h the c a r r i e r Ar on the charcoal a t l i q u i d a i r temperature. The Ar p l u s Rn i s recovered from the charcoal a t 300 °C; p u r i f i e d over hot T i and counted i n a p r o p o r t i o n a l counter. R n (3.8-day h a l f - l i f e ) i s the daughter of R a (1600year h a l f - l i f e ) . I f R a i s i n s o l u t i o n , R n should accumulate w i t h a 3.8-day h a l f - l i f e a f t e r the He purge; furthermore, the l o s s of R a by h a n d l i n g and storage the melt water should be n e g l i g i b l e . A f t e r removal of the melt water from the e x t r a c t i o n u n i t and s t o r a g e , a l i q u o t s of the melt water are sometimes returned f o r r e p e t i t i v e R n analyses. A l i q u o t s of the melt water can a l s o be evaporated t o dryness f o r R a and Th analyses by « p a r t i c l e - c o u n t i n g and neutron a c t i v a t i o n s t u d i e s . Measurements of R n purged from a l i q u o t s o f the water r e i n t r o duced i n t o the e x t r a c t i o n u n i t a f t e r several months storage showed a s e r i o u s l o s s of R a , which we a t t r i b u t e t o i n s o l u b l e RaS0 l o s t on the w a l l s of the storage v e s s e l . To e l i m i n a t e t h i s problem, we now a c i d i f y w i t h HN0 r a t h e r than w i t h H S0 . 2

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N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S

324

Counter D e s c r i p t i o n and Counting

Procedure

Figure 3 shows a photograph and a drawing o f one o f our mini-counters. The counters are o f Davis design [14]. Counters of 0.70 cm volume were used f o r a l l t h e samples l i s t e d i n Table 1 except the A l l a n H i l l s s u r f a c e sample from stake 12, A.H. s u r . , and f r o z e n d i s t i l l e d water sample; f o r these 2.5-cm volume counters were used. The e f f i c i e n c i e s o f t h e m i n i counters were determined w i t h a C 0 standard having 2.15 dpm o f C per cm (STP). This standard has been counted i n a large counter o f known e f f i c i e n c y . The mini-counter i s evacuated over n i g h t w h i l e heated t o approximately 150 °C. The p u r i f i e d C 0 together w i t h an equal amount o f p u r i f i e d Ar i s put i n t o the counter. The counter i s removed from t h e f i l l i n g l i n e and placed i n a l o w - l e v e l u n i t l o c a t e d i n t h e basement o f the Observatory P l a t e Stacks. The u n i t has a s h i e l d w i t h 14-inch t h i c k n e s s o f Pb, l i n e d w i t h 1-inch t h i c k n e s s o f OHFC copper. The counter i s placed i n a copper can surrounded by a 2-inch t h i c k n e s s o f Hg and a double l a y e r o f a n t i c o i n c i d e n c e p r o p o r t i o n a l counters o f 42-inch length. The energy r e s o l u t i o n o f t h e counter i s determined w i t h an e x t e r n a l F e source. Unless the 5.8-keV F e l i n e has a r e s o l u t i o n o f b e t t e r than 20 percent, the counter i s rebaked and r e f i l l e d w i t h r e p u r i f i e d C 0 and Ar. The counts above 1.0 keV are recorded w i t h a 100-channel a n a l y z e r and those above 7.5 keV with scalers. S u p e r i o r backgrounds r e l a t i v e t o e f f i c i e n c i e s f o r C are achieved by u s i n g the counts above 6.8 keV. For counts above t h i s energy t h e background r a t e i n the 0.70 cm counters i s approximately 2 counts d a y " w i t h 1 atm pressure o f C0o from a petroleum source and has an e f f i c i e n c y o f 45 percent f o r * C. The 2.5-cm counters g i v e a background o f 5 counts/day and 55 percent efficiency. Several months o f counting together w i t h p e r i o d i c F e s t a b i l i t y checks are necessary t o o b t a i n C ages t o ^ 1 0 years w i t h such small samples. The counting o f R n i s r e l a t i v e l y simple; no attempt was made t o achieve backgrounds l e s s than 0.1 counts/min. I f the a c t i v i t y was higher than 0.2 counts/min, the sample was counted f o r approximately two weeks t o ensure t h a t the a c t i v i t y had the 3.8-day h a l f - l i f e . The problem w i t h R n i s t h a t i t i s d i f f i c u l t to o b t a i n r e p r o d u c i b l e r e s u l t s , p a r t i c u l a r l y a f t e r the sample i s removed from and r e i n s e r t e d i n t o the e x t r a c t i o n l i n e . This type of r e p r o d u c i b i l i t y i s necessary i f T h and U are t o be r e l i a b l y determined on a l i q u o t s o f melt water. 3

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Results Table 1 g i v e s t h e amounts o f gas minus C 0 obtained i n the f i r s t two e x t r a c t i o n s and i n the heated water e x t r a c t i o n , the percentage o f C 0 i n t h e gas, t h e s p e c i f i c a c t i v i t y o f the C 0 , C age, and R n content o f t h e i c e . The amounts o f gas minus 2

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16. FIREMAN AND NORRis

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Dating Polar Ice by C and

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Rn

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Figure 3. Photograph and drawing of a mini-counter used for the C measurements.



24 >C, 5.5 55 ' C , 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 1 55 ° C , 1

24 ' C , 1 55 •C 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 1 55 ° C , 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 5.5 55 ° C , 1

24 ° C , 5.5 55 ° C , 1

Byrd 271 m 7.6 kg

Byrd 272 m 8.8 kg

Byrd 362 m 6.2 kg

Byrd 363 m 8.0 kg

Byrd 1068 m 9.2 kg

Byrd 1071 m 6.2 kg

Byrd 1469 m 9.5 kg

A . H . , sur. 31.0 kg

A . H . , sur. 16.6 kg

A . H . , 5-25 cm 12.9 kg

Frozen water 7.8 kg

1 8

1 2

Extraction (temp, pH)

l i f

1

69.3 1.3 ,

74.3 0.5

57.5 0.1

66.8 0.1

Gas

20.4 0.06

46.0 1.5

20.1 0.4

47.0 0.9

115.1 0.4

101.2 7.5

117.8 0.2

2 2

2

0.034

1.35

0.054

0.129

0.187

0.0237

0.0356

0.0216

0.051

0.0336

%

C0

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3

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C/C0 dpm/cm3)

8.7 ± 0.5

7.0 ± 3.0

25 ± 6

27 ± 3

8.0

2.0 ± 0.7

1.0 ± 1.0

y r

i * c age (ip3 )

Nuclear and Chemical Dating Techniques - American Chemical Society

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