Thermoluminescence dating of loess and palaeosols from the Lantian section, Shaanxi province, China

Quaternary Science Reviews, Vol. 7, pp. 245-250, 1988. Printed in Great Britain. All rights reserved.

0277-3791/88 $0.00 + .50 Copyright © 1988 Pergamon Press plc

THERMOLUMINESCENCE DATING OF LOESS AND PALAEOSOLS FROM THE LANTIAN SECTION, SHAANXI PROVINCE, CHINA L u Y a n c h o u , * Z h a n g J i n g z h a o a n d Xie J u n

Xian Laboratory of Loess and Quaternary Geology, Academia Sinica, Xian, China * Permanent address." Institute of Geology, State Seismological Bureau, Beijing, P.O. Box 634, China

Ten samples of loess and palaeosol collected from the upper part of Lantian section, Shaanxi, have been dated using the fine grain (4-11 ~m) TL technique. Palaeodose values were determined by the regeneration method (Readhead, 1982). The TL ages obtained ranged from 2.4 × 104 years for the upper Malan Loess L1 to 1.5 x 105years for the upper part of loess L2 of the Lishi Loess. Fine grain TL dating does not seem to be suitable for the samples older than 150 ka. The uncertainty of the TL age estimate, especiallyfor the palaeosol sample, may be primarily an effect of disequilibrium of the 23Sudecay chain and of water content variations since deposition of sediments.

FOREWORD

Depth (m)

The Lantian section where the Lantian man (Homo erectus Lantianensis) was discovered (Wu Rukang et al., 1964) at a palaeosol layer, depth of about 38 m from the top, has been recognised as the palaeosol $6 and estimated to be about 0.65 Ma old on the basis of magnetostratigraphic study (An Zhisheng et al., 1985). This is an important loess section, located at the Chenjiawo village in the north-east of Lantian County, Shaanxi Province, China. At this locality the Middle and Late Pleistocene loess-palaeosol sequence is well preserved with a thickness of about 50 m. To date the sequence, in particular the upper part of the section, may be useful for understanding environmental changes during the interglacial-glacial cycle. The thermoluminescence (TL) technique has proved to be an applicable method for dating Loess (Wintle, 1981; Li Huhou, 1985; Lu et al., 1987). Thirteen samples of loess and palaeosol for T L dating have been collected from the upper part of the Lantian section (Fig. 1). All samples were excavated at 30-40 cm behind the surface of the section, and sealed in small aluminium boxes.

SAMPLE PREPARATIONS Samples were first treated with H202 to remove organic material, then treated with 50% v/v HC1 to remove carbonates and to break up aggregates. The 4-11 p~m fraction was isolated by repeated Stokes settling and then deposited from acetone onto 1 cm aluminium discs. The 4-11 ixm fraction of all samples, except sample 86-01 which shows a peak T L at about 350°C, have a T L peak at about 285°C when the heating rate is 5°K/s (Figs 2 and 3). The standard deviation of six discs for each sample from the natural T L ranged from 1 to 4.8%. No significant anomalous fading was detected for the samples stored in the dark room for 6 weeks after beta irradiation.

0

-

~

I0

"':111";2121;i;21::;

15

Sit L 3 ~:'; ; ; . . . . . . . . .

Sw

...

FIG. 1. Loess section at Lantian, Shaanxi Province, China, with the location of samples shown.

MEASUREMENTS OF EQUIVALENT DOSE Lu et al. (1987) have suggested for Chinese loess that the equivalent dose (ED) values for younger ( < 4 0 - 5 0 ka) loess samples with linear T L growth curves could be obtained by three methods, i.e. partial bleaching (Wintle and Huntley, 1979, 1982), the additive dose (Singhvi et al., 1982) and the regeneration method (Readhead, 1982) and, that for the older ( > 4 0 - 5 0 ka) samples, with non-linear T L response with dose, only the regeneration method is suitable for the determination of E D values. Most of the samples, except sample 86-01, ha,)e a non-linear T L growth curve. The regeneration method has been applied to measure the E D values for all samples. In addition, the partial bleaching and the additive dose methods have also been used for sample 86-01 which showed a roughly linear response characteristic, as shown in Fig. 4.

245

246

Y.-C. Lu et aL 5 . 0 0 -E6

86-TL-OI

o) +J .=_ _J

S

I ~ / / I O0

=.-.-~r

I

200

300

Temperoture

(*C)

I

I

400

500

FIG. 2. The TL glow curves of fine grains for sample 86-01 at a heating rate of 5°Ks 1. Line a is natural TL; b, c and d are the TL after 90 minutes bleaching followed by 60.5, 121 and 181.5 Gy 13-irradiation; line e is the TL of natural plus 60.5 Gy 13irradiation. I.OO E7

86-TL-05

4J E .J j-

Ioo

200

300

Temperoture

400

500

(*C)

FIG. 3. The TL glow curves of fine grains from sample 86-05 at a heating rate of 5°Ks 1. A thick line refers to the natural TL. The dashed line is the TL of natural plus 162 Gy 13-irradiation. The thin lines are the TL regenerated after bleaching for 20 hours and 13, 26, 52, 104, 208, 312 and 416 Gy 13-irradiation respectively.

Residuot 8 6 - T L - O

R-Dose

1/370"C

86-TL-OI/370°C

c: .j I--

.J

2

f-

2 b

J ED= 112+ - 8 . 5 6

I00

Beto-dose

I

Gy

ED = 1 1 5 + - 9 , 4 2

I

I

I

IO0

200

500

(Gyl

0

100

0 Beto-dose

Gy

I

I

I

i00

200

500

(Gy)

FIG. 4. The TL growth curve for a temperature of 370°C for sample 86-01, showing the equivalent dose (ED) determination by the partial bleach method (Wintle et al., 1979) and the residual method (Singhvi et al., 1982).

Comparing the residual TL fractions of the surface samples, taken from the surface of the profile scarp with adhesive tapes, with those of the sample after bleaching for various time, it appears that the TL remaining after bleaching for about 20 hr under our

laboratory conditions is very close to that of the surface sample taken at the same layer of the sample to be dated (cf. Lu et al., Fig. 4, 1988). The remaining TL of the surface sample might be considered to be the residual TL level of the same layer sediments at the

247

TL Dating of Loess and Palaeosols

time when it was deposited (Readhead, 1982; Lu et al., 1987). Therefore for regeneration of the TL growth curve, a set of the prepared fine grain discs was bleached under a sunlamp for about 20 hr, thus reducing its TL to close to that of the surface sample, a range of beta (9°Sr) irradiation was then given to these discs, up to approximately twice the natural accumulated equivalent dose. A beta TL growth curve was established for each sample at 10°C intervals along the temperature plateau region between 340 and 400°C by providing a computer fit to the regenerated TL data sets. In order to provide a check on TL sensitivity changes, due to differing means of TL reduction and rate of irradiation in the field and the laboratory, the TL data of the natural added beta irradiation was also fitted to the regenerated TL growth curve, as shown in Fig. 5. Most of the TL data points of the natural added beta irradiation fitted well to the regenerated TL growth curve for each sample to have been dated in this study, that implys no significant change of TL sensitivity of these samples. The ED value is taken from the dose co-ordinate of the point at which the natural TL and the regenerated TL growth curve match, as also shown in Fig. 5.

86-TL 0 8/350"C

ESTIMATES OF ANNUAL DOSE RATE

The contributions of the uranium and thorium decay chains to the environmental dose rate were calculated from the uranium and thorium contents that were obtained by neutron activation analysis. The 4°K contribution was calculated from the total potassium content as determined by atomic absorption spectrophotometry. The dose rate conversion factors of Aitken (1985) were used. The cosmic ray contribution was taken into account (Prescott and Stephan, 1982). Corrections to these values for the water content of the sample were made according to the equations given by Fleming (1979). The water content of all samples as collected was measured to be less than 8% dry weight. However, these values are thought not to be representative of the water content for the geological period concerned. On the basis of generalization of moistures of loess core in the adjacent area, the water content of these samples was estimated as 10 + 5% for the upper samples 86-01 and 86-02, 15 + 5% for all other samples. In fact, the moisture of loess layers and palaeosol layers probably varied during burial, which results in an uncertainty in the annual dose rate. The sample TL age was derived from the mean of the values at 10°C intervals along the TL age versus temperature plateau region. The typical plots of this type are shown in Fig. 6.

2.0 N to o

1.5

,-I I--

1.0

o N + UV (20 hrs)÷/9-dose • N + B - dose

--

~

86-09

--t

86 - 05

150

0.5 ED - 544.3 -t- 77.7 Gy

I

I

I

i

I

I

I

I

I

I

20

40

60

80

I00

120

140

160

180

200

(6.48

Gy)

Beta-dose

--

A

,= 0 0 I00

on v

w 86-TL

I0/350"C

Om 0 _1 I-

to o

50

c

o N + UV (20hrs)

f• N +

+ 19-dose

÷

.B-dose

-,-

--I I-.-

+

-+--

÷

+---f----~4t--

--~t-

-~----~-

86-03

*----4.~--

--~- --

86-02

-~t-- 8 6 - 0 1

ED • 615.6 +_ 58.2 Gy

I

I

I

20

40

60

i---T Beta

80

I00

- dose

i

I

I

I

I

120

140

160

180

200

( x 6.4.8 Gy)

0

I 350

I 360

I 370

Temperoture

FIG. 5. The regenerated TL growth curves for a temperature of 35&C for sample 86-08 and 86-10, showing the equivalent dose (ED) determination. The points and cycles are data point of N + UV + 13 and of N + 13, respectively. N is the mean value of the sample natural TL.

I 380

I 390

I 400

(*C)

FIG. 6. The plots of TL age versus temperature showing the TL age plateau for temperature of 350°C-400°C for samples 86-01, -02, -03, -05 and -09.

248

Y.-C. Lu et al.

DISCUSSION

For 10 samples of the Lantian section, the weighted mean TL ages, for a plateau temperature of about 340-400°C, are given in Table 1 and Fig. 7. The TL dates of fine grains at the Zhaitang section near Beijing (Lu et al., 1987) are also shown in Fig. 7. Comparison of the TL age estimates of the Malan Loess (LI) at the Lantian, the Zhaitang (Lu et al., 1987) and the Luochuan section (Li, 1985) show that the TL age of sample 86-04 taken from the top of palaeosol (S~) at Lantian may appear to be too young. The sample has a high content of uranium and a higher annual dose rate as a result (see Table 1). Field observation has demonstrated that there are many secondary calcium carbonate nodules along vertical fractures in the palaeosol profile. The ~4C dates of these nodules ranged from 24 ka BP to 37 ka BP (Zhou Mingfu and J. Head, pers. commun.). The uranium content of the palaeosol might be also increased as a result of leaching from the upper layers into the palaeosol with time since its formation, which could

cause an increase of the dose rate. Hence the TL age estimate should be younger than the true age. The palaeosol $1, which is one of the key layers for the stratigraphic correlation in the Loess Plateau, has been identified as partly produced during the last interglaciation (An and Lu, 1984). On the basis of magnetostratigraphy (Heller and Liu, 1982) and soil stratigraphy (Lu and An, 1979). Liu et al. (1985) has suggested that the palaeosol $1, $2 and the interbedded loess L2 could be respectively correlated with the oxygen isotope stage 5, 7 and 6 (Shackleton and Opdyke, 1976). Therefore, the palaeosol $1, $2 and loess L2 might be formed during the periods of 75 ka-125 ka (Lu et al., 1987) or 104 ka-145 ka (Liu et al., 1985), about 210 ka-244 ka and about 125 ka or 145 ka-210 ka, respectively. The TL ages of 121.2 + 10.2 ka and 144.0 + 11.5 ka f o r sample 86-05 at the basement of palaeosol S1 and sample 86-06 at the top of loess L2 in the Lantian section are consistent with the estimates as mentioned above. The TL age estimate of 150.2 + 12.7 ka for sample 86-07 at the upper part (L2_3) of the loess L2

T A B L E 1. T h e r m o l u m i n e s c e n c e result from the fine grain and radioactivity data for the Lantian section

No.

Sample

01 02 03 04 05

Loess Palaeosol Loess Palaeosol Weatherad Loess Loess Loess Palaeosol Loess Palaeosol

06 07 08 09 10

Depth (m)

U (ppm)

Th (96)

K (96)

2.0 3.1 4.1 5.5 6.8

1.97 4.19 3.45 3.73 2.42

13.3 15.3 9.8 15.8 11.4

2.01 2.09 1.56 2.34 1.88

10 10 15 15 15

+ + + + +

5 5 5 5 5

110.0 198.9 170.4 287.3 506.0

+ + + + +

6,4 12.0 10.6 13.5 27.1

0.062 0.078 0.063 0.080 0,073

+ + + + +

0.002 0.005 0.003 0.004 0.012

4.527 5.942 4.17 5,749 4.175

24.3 33.5 41.9 50.0 121.2

+ + + + +

1,7 2.4 2.6 3.7 (?) 10.2

7.8 10.3 10.9 11.7 13.0

2.79 2.77 3.38 3.57 2.71

11.1 13.5 12.0 15.9 13.9

1.82 1.73 1.62 1.86 2.00

15 15 15 15 15

+ + + + +

5 5 5 5 5

586.0 678.7 579.3 761.8 627.3

+ + + + +

24.4 57.4 63.7 80.0 31.2

0,062 0.080 0.076 0.076 0.065

+ + + + +

0.002 0.003 0.008 0.004 0.002

4.071 4.519 4.434 4.875 4.575

144.0 150.2 130.7 156.3 137.1

+ + + + +

11.8 12.7 14.0 (?) 15.9 (?) 13.6 (?)

Depth Zhoitong sect ion (rn) O-

Water content (96)

TLoge(lO ayr) (fine groin)

ED (Gy)

a-value

Lontion section

Dose rate (Gy/ka)

T L age (ka)

TL oge(lOayr) (fine groin)

!i ::i::!ii! -24.3

I.:!iiiii!ib 27-8.-4.7" I::::i:::::l . . . V/////I--32.7

5 -

-

.

+ - 1.7

-33.5 + -2.4

[ ~ / - J ~, ~ ' , /

!ii!!!!i!!|

!iiii!!iii 41.1 .-4.2

i!!14,!it I0

+ -3.5

L:.:. ::::::: r/~/,~,

-41.9

+-2.6

-50.0

+-3.7

(?)

~ 121.2 + - 10.2 !

lii !!???',

144.0+ -I

1.3

150.2+ - 12.7 1 3 0 . 7 + -- 14.0 ( ? )

156.3+-- 1 5 . 9 ( ? ]

FIG. 7. C o m p a r i s o n of the T L dates of the loess and palaeosols at the Lantian section, Shaanxi Province, with those at the Zhaitang section near Bcijing (from Lu et al., 1986).

_3

Q'~

Q~-~

Q~-3

Holocene

Epoch

~ o

Coarser layer

:~

(L I-I) Drab-reddish palaeosol (S,)

(Lt_2)

(S,)

layer (LI-I) Drab-reddish and red clay

:~

Lower 3;

(S,)

Lower layer (LI-I) Drab-reddish palaeosol The third marine bed

Fluvio-lacustrine deposit

The second marine bed

Drab-grey palaeosol (L i_ s)

Drab-grey palaeosol (LI_s)

Finer layer o "

(LI-3)

(L,- 3)

o -J

Upper layer F luvio-lacustrine deposit

The first marine bed

Slope wash soil(so)

Slope wash soil(so)

Black loessial soil(so) Coarser layer Upper layer (L t-3 )

Plain (Hebel Plain) Zhao and Chen, 1982)

Intermontane basin (Lu et a l . , 1987)

Lantian (this work)

Maritime

Piedmont

Luochuan (An and Lu,1984"

Loess plateau

Last

_1

(_9

interglacial age

Early substage

Interstadial

Late substage

Post glacial age

age

Glacial

Climatic

Dry - cold Warm -moist

-

Cool moist

Dry -cold

Warm -moist

Climatic phase

stage

TABLE 2. A preliminary geochronological scale of the last interglacial-glacial cycle in north China

~" 70--85

40--45

25--30

10

(ka)

Age

4~

O

r~

O

t"

"H

250

Y.-C. Lu et al.

REFERENCES does not seem to be much different from the geological observation. However, the TL age estimates of samples 86-08 (L2_s), 86-09 (L2_1) and 86-10 at the top of the Aitken, M.J. (1985). Thermoluminescence Dating, pp. 66-76, 221-226. Academic Press, London. palaeosol $2 appear too young and show no consistent An Zhisheng and Lu Yanchou (1984). A climatostratigraphic increase in age with depth down the profile. The subdivision of Late Pleistocene strata named by Malan formation in North China. Kexue tongbao, 29, 1239-1241. possible reason for these phenomena may be conAn Zhisheng, Gao Wanyi, Zhu Yizhi, Kan Xiaofeng, Wang Junda sidered as follows: and Sun Jianzhong (1985). Loess-paleosol sequence and chronol1. The natural TL of these samples was close to ogy at Lantian man localities. In: Liu Tungsheng (ed.), Aspects of Loess Research, pp. 192-203. China Ocean Press, Beijing. saturation, hence the reliable palaeodose was hardly Debenham, N.C. (1985). Use of UV emissions in TL dating of determined. sediments. Nuclear Tracks, 10, 717-724. 2. Various degrees of the anomalous fading might have Fleming, S.J. (1979). Thermoluminescence Techniques in Archaeology, p. 31. Clarendon Press, Oxford. occurred for these samples during the geological period concerned, although 'no significant fading' Heller, F. and Liu Tungsheng (1982). Magnetostratigraphical dating of loess deposits in China. Nature, 300, 431-433. after storage in a dark room for 6 weeks after beta Li Huhou (1985). Thermoluminescence dating of loess. In: Liu Tungsheng (ed.), Loess and Environment, pp. 231-238. China irradiation has been found and the fine grain Ocean Press, Beijing. fraction (4-11 Ixm) of Chinese loess is mainly Liu Tungsheng, An Zhisheng, Yuan Baoyin and Han Jiamao (1985). composed of quartz. The loess-paleosol sequence in China and climatic history. Episodes, 8, 21-28. 3. The TL signal of these samples is finding an equilibrium against a decay with a mean life of 105 Liu Tungsheng et al. (1985). Loess and the Environment, pp. 166. China Ocean Press, Beijing. years which was suggested by Debenham (1985). Lu Yanchou and An Zhisheng (1979). The quest for series of natural environmental changes during the Brunhes Epoch. Kexue Tong4. The equilibrium state of the uranium and thorium bao, 24, 221-244 (in Chinese with English abstract). decay chains for these sediment layers, particular in Lu Yanchou, Prescott, J.R., Robe~tson, G.B. and Hutton, J.T. the palaeosols, has been changed, as well as the (1987). Thermoluminescence dating of the Zhaitang section of the Malan loess, China. Geology, 15, 603-605. water content changed, as discussed above. All these problems, however, are still open Lu Yanchou, Prescott, J.R. and Hutton, J.T. (1988) Sunlight bleaching of the thermoluminesccnce of Chinese loess. Quaternary questions. But TL dating of the fine grains appears Science Reviews, 7,335-338. suitable for Chinese loess produced during the last Prescott, J.R. and Stephan, L.G. (1982). The contribution of cosmic radiation to the environmental dose for thermoluminescent dating. interglacial-glacial cycle. Latitude, attitude and depth dependences. Council of Europe From Fig. 7 one can see that the TL results from the Journal PACT, 6, 17-25. Lantian section are generally in good agreement with Readhead, M.L. (1982). Extending thermoluminescence dating to geological sediments. In: Ambrose, W. and Duerden, P. (eds), those in the Zhaitang section. On the basis of the Archaeometry, An Australian Perspective, pp. 276-281. Australian geological records and geochronological data of the National University, Canberra. Lantian section and Luochuan section (An and Lu, Shackleton, N.J. and Opdyke, N.D. (1976). Oxygen isotope and palaeomagnetic stratigraphic of equatorial Pacific core V28_~38: 1984) and the Zhaitang section (Lu et al., 1987), as well Oxygen isotope temperature and ice volumes on a 105 and 10 6 year as of the Nanpai core in the Maritime Plain near the scale. Quaternary Research, 3, 39-55. Bohai sea (Zhao and Chen, 1982), a preliminary Singhvi, A.K., Sharma, Y.P. and Agrawal, D.P. (1982). Thermoluminescence dating of sand dunes in Rajasthan, India. Nature, geochronological scale of the last interglacial-glacial 295, 313-315. cycle in north China has been suggested, as given in Wintle, A.G. (1981). Thermoluminescence dating of late Devensian loesses in southern England. Nature, 289, 479-480. Table 2. ACKNOWLEDGEMENTS This study was financed by an Academia Sinica Foundation Grant. We thank Dr Yang Shouzhen at the Institute of High Energy Physics, Academia Sinica, for providing the U, Th and Rb content analysis; Dr Zhang Guangyu and Mr Zhang Xiaoyie for the K content analysis; and Mr Gao Wanyi for all figures drawing. Thanks are due to J.R. Prescott for a critical reading of the manuscript.

Wintle, A.G. and Huntley, D.J. (1979). Thermoluminescence dating of a deep-sea ocean core. Nature, 279, 710-712. Wintle, A.G. and Huntley, D 3. (1982). Thermoluminescence dating of Sediments. Quaternary Science Reviews, 1, 31-53. Wu Rukang et al. (1964). Vertebrata Palasiatia, Sinica, 8, 1-12. Zhao, S.-L. and Chen, Y.-S. (1982). Transgressions and sea-level changes in the eastern coastal region of China in the last 300,000 years. In: Liu, T.-S. and Wang, Y.-Y. (eds), Quaternary Geology and Environment of China, pp. 147-154. China Ocean Press, Beijing.