Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea

Quaternary Science Reviews 22 (2003) 407–421

Luminescence dating of well-sorted marine terrace sediments on thesoutheastern coast of Korea

J.H. Choia,b, A.S. Murrayb, M. Jainc, C.S. Cheongd, H.W. Changa,*aSchool of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea

b The Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, University of Aarhus, Ris� National Laboratory,

DK-4000 Roskilde, Denmarkc Ris� National Laboratory, DK-4000 Roskilde, Denmark

d Isotope Research Team, Korea Basic Science Institute, Daejeon 305-333, South Korea

Received 4 March 2002; accepted 26 July 2002

Abstract

Along the southeastern coastline of the Korean peninsula, well-developed marine terraces are found at various elevations. The

ages of these terraces, and the time of deposition of the terrace sediments are important to our understanding of the geological

history of this area during the Quaternary period, and represent a unique record of the regional tectonic activity. Previous efforts to

establish a chronology using optically stimulated luminescence (OSL) methods have produced controversial results, particularly

because of stratigraphic inconsistency and poor reproducibility. In this paper, the application of OSL dating based on the single-

aliquot regenerative-dose (SAR) protocol for quartz is investigated. The dependence of equivalent dose on the preheat and cut-heat

temperatures (thermal treatment of the regeneration and test-doses, respectively) are examined. Linearly modulated luminescence

signals from chemically cleaned quartz samples are used to identify the presence of a thermally unstable component with a large

optical cross-section (component A0), which in part affects the ability to correct for sensitivity changes during measurements, and

thus the reliability of the equivalent dose estimates. In some samples, a higher heat treatment after the test-dose is shown to improve

our ability to measure a dose given in the laboratory before any heat treatment (dose recovery test). This higher temperature

treatment effectively removes component A0, and hence improves sensitivity correction. Furthermore, the samples were broadly

divided into poorly sorted and well-sorted, based on field evidence. The poorly sorted samples contain friable, weathered gravel

clasts, which is a likely post-depositional source of quartz grains. In general, these grains will not have been zeroed prior to

deposition, and so the poorly sorted samples are rejected from further age studies. Results obtained from the well-sorted samples are

reproducible at each sampling location, and give ages grouping broadly into 50–70 and 110–120 ka, but laterally discontinuous on a

scale of tens of km. Our OSL results for the younger group are supported by radiocarbon ages from overlying terrestrial deposits. It

is concluded that these results point to considerable tectonic activity in the southeast of Korea during the Late Pleistocene.

r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Marine terraces are geomorphic surfaces that havebeen exposed by a lowering of sea level or by tectonicuplift; they thus indicate the location of a palaeo-shoreline. Therefore the timing of terrace formation canprovide an understanding of past sea level fluctuations,

which are interlinked with global climate change and/orlocal tectonic movement.

Since the early 1900s, there have been reports of well-exposed marine terraces at different altitudes along theshoreline of the southeastern part of the Koreanpeninsula (e.g. Yamanari, 1925); subsequent researchon these marine terraces has focused on establishingchronological correlations as well as an understandingof the main geological/geomorphological processes oftheir formation. More recently, these marine terraceshave attracted considerable attention because of theirpotential as a record of the crustal stability of thesoutheastern Korean peninsula. Lee and Schwarcz(2001) describe several fault systems of unknown age

*Corresponding author. Tel.: +82-2-880-6734; fax: +82-2-872-

7643.

E-mail addresses: [email protected] (J.H. Choi),

[email protected] (A.S. Murray), [email protected]

(M. Jain), [email protected] (C.S. Cheong), [email protected]

(H.W. Chang).

0277-3791/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 3 7 9 1 ( 0 2 ) 0 0 1 3 6 - 1

in this area; in many places, these faults cut terracesediments and other Quaternary deposits. Both heavyindustry complexes and nuclear power plants are locatedon or near these fault systems, and so the estimation ofthe recurrence interval of crustal movement is veryimportant not only for tectonic studies, but also forengineering risk prediction.

Unfortunately, no marine fossils (such as corals) havebeen found in the terrace sediments, and so traditionalmethods of age estimation have not been applied.Because of this, we have begun to examine the potentialof luminescence dating using these sediments. Kwonet al. (1999) reported some early attempts to obtain thedepositional ages of one of the marine terrace sedimentsin southeastern Korea using optically stimulated lumi-nescence (OSL), and gave results of 32–58 ka. However,these ages are contentious because of their stratigraphi-cal inconsistency and age inversion compared to earlyunpublished ages measured by the present authors,which showed considerable variation (52–90 ka) for thesame horizon in the same outcrop.

In this paper, we first examine the applicability ofthe single-aliquot regenerative-dose (SAR) protocol(Murray and Wintle, 2000) to quartz from marineterrace sediments collected from the southeastern coastof the Korean peninsula. The optimum conditions formeasuring equivalent dose values of the samples arethen examined. Finally, OSL ages are presented for aselected group of samples. These OSL ages, in additionto answering important engineering questions, help todefine the history of terrace formation, and so giveinformation about differential crustal uplift rates andsea level fluctuation in this area.

2. Sampling sites

In the southeastern part of the Korean peninsula(Fig. 1), well-developed marine terraces of variouselevations have been known for some time, and manyattempts have been made to establish a chronology forthese terraces (e.g. Kim, 1973; Oh, 1977; Lee, 1987; Kim,1990). Lee (1987) identified five different terraces in thisarea (Fig. 2), and his classification was developedfurther by Kim et al. (1998). They suggested that thelowest terrace (#1; elevation of 3–4 m a.m.s.l.) wasformed in the Holocene, and the second terrace (#2; 8–18 m a.m.s.l.) was formed during the last interglacialperiod in the Late Pleistocene. They further dividedterrace #2 into #2-I (15–18 m) and #2-II (8–10 m), andsuggested these to have been formed in oxygen isotopesub-stages 5e and 5a, respectively. They assignedterraces #3 (20–42 m a.m.s.l.) and #4 (40–60 m a.m.s.l.)to the Middle Pleistocene, and #5 (the highest elevationat 70–80 m a.m.s.l.) to the Early Pleistocene. However,because of the lack of material suitable for absolute age

STUDY AREA

Gyeong Ju

Busan

Suryeom (SU)

0 30 km

36o

129o

35o

Kwanseong (KS)

Oryu (OR)

Yangnam (YN)

Weseong (WS)Nasa (NS)

Eupcheon (EP)

Bihag (BH)

Gi Jang

Jeong Ja

Gam Po

42o

38o

34o

130 o125 o

CHINA

JAPAN

EAST SEASEOUL

BUSAN

GYEONGJU

Fig. 1. Site location map. Samples were collected along the southeastern

coastline of the Korean peninsula. Open and filled squares, respectively,

represent the sites from which sediments on terrace #2 and #3 were

sampled. Solid lines indicate known tectonic faults of unknown age.

J.H. Choi et al. / Quaternary Science Reviews 22 (2003) 407–421408

determination, this chronological stratigraphy is mainlybased on weathering features, palaeosols and geomor-phological characteristics such as the continuity andextent of the terraces. It must therefore be regarded astentative.

Unfortunately, terrace #1 is inaccessible in our studyarea, because of extensive coastal building. For thispreliminary study, we decided to concentrate on terraces#2 and #3 (Fig. 2), because these were expected to be inthe current age range of the luminescence technique.

Fig. 3 identifies the sampling sites, presents a simplifiedstratigraphy, and identifies the locations of the lumines-cence samples (open and filled circles).

Twenty-two samples were collected from terrace #2between 7 and 26 m (a.m.s.l.) and 4 from terrace #3between 40 and 47 m (a.m.s.l.). All samples werecollected by driving stainless steel pipe, 25 cm in lengthand 7 cm in diameter, into a cleaned section, to providesediment which had not been exposed to daylight sincedeposition.

Fig. 2. Distribution of marine terraces in the southeastern coastal area of Korean peninsula (Lee, 1987).

Fig. 3. Schematic view of the stratigraphy of terrace sediments, showing location; filled and open circles are indicative of samples classified as poorly

sorted and well-sorted in the field. OSL ages of the well-sorted terrace sediments (shown in ka, with sample code) were obtained using a preheat

temperature of 2601C for 10 s and cut-heat temperature of 2201C. The elevation of sampling points indicates the minimum elevation of the palaeo-

shoreline.

J.H. Choi et al. / Quaternary Science Reviews 22 (2003) 407–421 409

From field observation, we classify the sediments intotwo broad types: poorly sorted and well-sorted (openand filled circles, respectively, in Fig. 3). The poorlysorted sediments usually contain undulating or inter-leaved gravel and/or pebble bands separated by thinsand layers (o15 cm), sometimes containing gravel.These gravel clasts are derived from granites, sandstonesand tuffaceous volcanic rocks. The presence of themixture of rock types rules out the possibility that thisdeposit results from ‘‘in situ’’ weathering. In contrast,the well-sorted sediments usually consist of a relativelythick fine-sand layer (>30 cm) with no particles>1 mm. Our field observation confirms the conclusionof previous authors that these are marine deposits,rather than aeolian or fluvial. At one site (SU), weidentified marine shell fragments in the deposits, and nosignificant palaeo-river channels are known in this area.We selected 4 samples (two from the poorly sortedsediments; PNA and PYN, and two from the well-sortedsediments; WYN, WKR-5B) for the detailed studiesdescribed in Sections 4 and 5. Sample PNA was takenfrom terrace #3, whereas both PYN and WYN werecollected from an archaeological site that had beenexcavated in terrace #2; PYN was taken 3.5 m below thelayer from which WYN was taken. WKR-5B was alsocollected from terrace #2 but about 30 km southwest ofthe PNA, PYN and WYN sites (Fig. 3).

Two further samples from near Kwanseong (KS) andBihag (BH) (Fig. 1) were collected from just below thesurface of the modern beach, to provide analogues forthe bleaching condition of terrace sediments at the timeof deposition.

3. Sample preparation, experimental facilities and

methods

In the laboratory, 90–250 mm quartz grains wereextracted from the sediments by sieving, and cleaningin 10% H2O2, 10% HCl and 40% HF. All OSLmeasurements were carried out with a Ris� TL/OSLautomatic reader; the stimulation light source was ablue-LED (470730 nm) array which delivers40 mW cm�2 to the sample. The reader was alsoequipped with a 90Sr/90Y beta source delivering0.11 Gy s�1. Photon detection was through 7mm ofHoya U-340 filter. Doses were measured using the SARprotocol (Murray and Wintle, 2000). With the exceptionof the linearly modulated OSL measurements (Section5.1), the sample temperature during blue-light stimula-tion was 1251C.

3.1. The single-aliquot regenerative-dose (SAR) protocol

Over the last decade, various methods for theestimation of the equivalent dose (De) from single

aliquots of feldspar and quartz have been developed(e.g. Duller, 1991; Murray et al., 1997). Wintle (1997)reviewed these methods and others, and pointed out theinherent superiority of such single aliquot protocols (inwhich all measurements required to estimate the De aremade on a single sub-sample or aliquot).

More recently, Murray and Wintle (2000) haveproposed the SAR protocol, and this approach is nowwidely used to obtain high precision estimates of De (e.g.Bailey et al., 2001; Banerjee et al., 2001; Gautier, 2001).We outline the SAR protocol in Table 1, and discuss itbriefly below.

Chemically cleaned aliquots of quartz are firstchecked for the absence of feldspar contamination usinginfrared stimulation. Further aliquots are preheated at acertain temperature for 10 s and then the natural OSL ismeasured (Lo) at 1251C. A test-dose (Dt) is given(usually about 10% of the expected De), the sampleheated again, and the OSL (T0) measured to provide ameasurement of sensitivity (the heating of the samplefollowing the test-dose is called the ‘‘cut-heat’’ through-out this paper, because the sample is cooled immediatelyafter reaching temperature). This measurement cycleprovides the sensitivity-corrected natural OSL(R0 ¼ L0=T0). The complete measurement cycle isusually then repeated three times, each time precededby one of three regeneration doses (Di; i ¼ 1; 3) chosento bracket the natural dose. Corrected regenerated OSLsignals (Ri) are calculated by dividing each regeneratedOSL signal (Li; measured following Di) by the OSLfrom the fixed test-dose given in each cycle, such thatRi ¼ Li=Ti: A fourth regeneration dose (0 Gy) is thengiven to test whether the curve of Ri against Di passesclose to the origin, and finally a fifth regeneration dose(chosen to be the same as the first regeneration dose) isapplied to test whether the sensitivity correction using

Table 1

The single-aliquot regenerative-dose protocol (after Murray and

Wintle, 2000)

Step Treatmenta Observedb

1 Give dose, Di —

2 Preheatc (160–3001C for 10 s) —

3 Stimulated for 40 s at 1251C Li

4 Give test dose, Dt —

5 Cut-heatc to 1601C —

6 Stimulate for 40 s at 1251C Ti

7 Return to step 1 —

aFor the natural sample, i ¼ 0; and D0 is the natural dose.b Li and Ti were derived from the initial 0.8 s OSL signal minus a

background estimated from the last part of the stimulation curve (In

this experiment, the last 4 s was used as a background).cAliquot cooled to less than 601C after heating. In step 5, the TL

signal from the test dose can be observed, but it is not made use of in

routine applications.dThe stimulation time depends upon the stimulation light intensity

and wavelength. In this experiment, blue-LEDs were used.


the test-dose signal is successful. The ‘‘recycling ratio’’,R5=R1; should be close to unity.

An illustrative diagram of the results from thismeasurement sequence (filled circles) is given in Fig. 4using a dose response curve of one of the samples takenin the study area; interpolation of R0 onto the growthcurve gives an estimate of the De for this aliquot.Further details, and justification for the parametervalues (temperature, etc.) used here, are given in Murrayand Wintle (2000).

3.2. Dosimetry

The radionuclide concentrations of the samples weremeasured using low-level high resolution gamma spec-trometry at both Ris� National Laboratory, Denmarkand the Korea Basic Science Institute, South Korea, toallow intercomparison (Table 2). Conversion to doserates used the data presented by Olley et al. (1996). Alldose rates were modified using the water contents ofTable 2, and the attenuation factors given by Zimmer-man (1971). Cosmic ray contributions were calculatedusing the equations given in Prescott and Hutton (1994).

4. SAR measurement using a test-dose cut-heat of 1601C

4.1. Dependence of equivalent dose on preheat

temperature

We first examined the dependence of the equivalentdose on preheat temperature using the four samplesselected for detailed study (PNA, PYN, WYN and

WKR-5B). Different aliquots were preheated between1801C and 3001C (and held at raised temperature for10 s) in 201C steps. Twenty-four aliquots were measuredfor each sample (three aliquots at each preheattemperature), using laboratory regeneration doses (Di;i ¼ 1; 3) of 100, 200 and 300 Gy, and a test-dose ofabout 10 Gy. Following administration of the test-dose,each aliquot was heated to 1601C followed by immediatecooling (i.e. cut-heat of 1601C). The measurements wererepeated using a cut-heat of 2201C and these results arepresented as filled symbols; these results will bediscussed in Section 5.3.

Fig. 5 presents the De values obtained using the 1601Ccut-heat temperature as open circles, and the recyclingratio and recuperation (i.e. response to 0 Gy dose) aredepicted by open triangles and open squares, respec-tively. Sample PNA shows a continuous increase in De

as a function of preheat temperature (Fig. 5a). Recyclingratios are far from the desired value of unity with amaximum value of 1.4 at 2601C. Recuperation alsoshows an increasing pattern from 8% of R0 at 1601C to18% at 3001C. De values of PYN (Fig. 5b) show nomarked systematic variation with temperature and thereis a poorly defined De plateau from 1801C extending to3001C; the average value of De is 132712 Gy. Recyclingratios in the low preheat temperature region (180–2401C) are about 1.1, but above 2601C these increase tomore than 1.5. This variation in recycling ratios is alsoreflected in the increase in recuperation which reachesabout 15% in the temperature region 260–3001C. It isnoticeable that the values of De are relatively low in therange 280–3001C; this is also the range in whichrecycling ratios deviate significantly from unity and thedegree of recuperation increases.

In Fig. 5c (sample WYN), we can identify a preheatplateau from 1601C to 3001C with a mean value of135710 Gy. Although there is some scatter in values ofDe; the plateau is more clearly defined than those ofPNA and PYN, and the recycling ratios are closer tounity (except for the recycling ratio measured at 2001Cpreheat temperature). For sample WKR-5B, the valuesof De in the low preheat temperature region (180–2201C)are lower, but there is a well-defined preheat plateaufrom 2201C up to 3001C (Fig. 5d). The recycling ratiosobtained are between 0.9 and 1.1 over the wholetemperature range and the maximum recuperation isabout 8%.

From these data, we consider samples WYN andWKR-5B to be suitable for analysis using the conven-tional SAR protocol. However, the poorly sortedsample PYN has a weakly defined preheat plateau withpoor recycling ratios, and no preheat plateau can beidentified for the poorly sorted sample PNA. Weconclude that the De values obtained from these samplesare unreliable. The next section tests the generality ofthis conclusion.

Regeneration Dose (Di)

0 150 300 450 600

Cor

rect

ed O

SL

(Ri =

Li /

Ti)

0

1

2

3

4

5

6

7

(D1 , R1)

(D2 , R2)

(D3 , R3)R0

(D4 , R4)De

(D5 , R5)

Fig. 4. The SAR protocol (Murray and Wintle, 2000). The growth

curve was obtained using a single aliquot of sample WKS5. The first

three regeneration doses were given in increasing order, followed by

the point at the origin, and the repeat point at (D5; R5). Thereafter, the

remaining data were measured, including a further repeat for 310Gy

(open triangle).


Table

2

OSL

age

resu

lts

and

sum

mary

ofdosim

etry

Sam

ple

238U

(Bq

kg�

1)

226R

a

(Bq

kg�

1)

232T

h

(Bq

kg�

1)

40K

(Bq

kg�

1)

Dry

bet

aa

(Gyka�

1)

Dry

gam

ma

a

(Gyka�

1)

Wate

r

conte

nt

(wt%

)

Tota

ldose

rate

b

(Gyka�

1)

De

(1601C

CH

c)

(Gy)

(n)

De

(2201C

CH

c)(G

y)

(n)

Age

(1601C

CH

c)

(ka)

Age

(2201C

CH

c)(k

a)

WY

N21.77

4.8

18.77

0.8

25.27

0.7

7407

30

2.1

67

0.1

01.0

37

0.0

315.6

2.7

87

0.1

01337

415

487

2

WY

N1

20.87

4.6

18.77

0.8

24.07

0.8

7357

31

2.1

37

0.1

01.0

27

0.0

315.4

2.7

57

0.1

01447

99

527

4

WY

N2

39.17

5.2

41.87

1.0

31.67

0.8

7307

29

2.3

77

0.1

11.2

77

0.0

417.0

3.1

27

0.1

22097

27

12

677

9

WK

S1

20.27

4.9

17.47

0.8

29.47

0.8

7797

31

2.2

77

0.1

11.1

07

0.0

311.3

3.0

67

0.1

12047

920

1817

16

12

677

4597

6

WK

S2

27.17

3.7

24.17

0.6

32.77

0.6

7757

27

2.3

57

0.1

01.1

97

0.0

38.7

3.3

07

0.1

02017

11

9617

4

WK

S3

32.47

6.4

27.97

1.1

35.37

1.0

7857

34

2.4

37

0.1

21.2

67

0.0

48.9

3.4

37

0.1

31967

12

20

2337

11

12

577

4687

4

WK

S4

21.27

4.4

18.97

0.7

29.27

0.7

8317

31

2.4

17

0.1

11.1

57

0.0

39.7

3.2

87

0.1

12227

89

687

3

WK

S5

36.77

4.0

30.97

0.4

41.57

0.5

8147

15

2.5

77

0.0

91.3

87

0.0

36.9

3.7

47

0.0

91177

21

10

2677

19

10

317

6717

5

WK

R-5

A31.67

4.8

33.57

0.9

35.17

0.8

3537

18

1.4

07

0.0

70.9

57

0.0

320.3

1.9

97

0.0

82327

10

15

2227

10

91177

71127

7

WK

R-5

B33.77

4.8

33.17

0.9

35.27

0.8

4707

21

1.6

97

0.0

81.0

47

0.0

319.2

2.3

17

0.0

92457

16

15

1067

8

WK

R-8

A15.57

3.6

15.07

0.6

24.57

0.6

7787

28

2.2

17

0.1

01.0

27

0.0

322.4

2.6

47

0.1

01627

915

1767

711

617

4677

4

WK

R-8

B10.57

3.6

10.57

0.6

22.97

0.6

9157

30

2.4

97

0.1

11.0

87

0.0

322.4

2.9

07

0.1

11797

13

11

627

5

WK

R-9

A27.27

5.0

21.97

0.8

30.97

0.8

6737

28

2.0

77

0.1

01.0

77

0.0

320.8

2.6

17

0.1

01437

69

557

3

WK

R-9

B17.97

4.4

18.87

0.7

28.87

0.7

7267

29

2.1

47

0.1

01.0

67

0.0

316.9

2.7

57

0.1

01217

715

1567

11

12

447

3577

5

aD

ata

from

hig

h-r

esolu

tion

gam

ma-s

pec

trom

etry

wer

eco

nver

ted

toin

finite

matr

ixdose

rate

susing

the

conver

sion

data

pre

sente

dby

Olley

etal.

(1996).

bThe

natu

raldose

rate

wasca

lcula

ted

using

the

wate

rco

nte

ntatten

uation

fact

or(H

)giv

enby

Zim

mer

man

(1971),

and

contr

ibutionsfr

om

cosm

icra

ysw

ere

incl

uded

using

the

equationsgiv

enby

Pre

scott

and

Hutt

on

(1994).

cC

ut-

hea

tte

mper

atu

re.


4.2. Testing the ability of the SAR protocol to recover

known doses

In this test of performance, samples are given aknown laboratory dose (given dose) after an extendedoptical bleaching at room temperature. The dose is thentreated as an unknown and the SAR protocol is used toestimate the dose, but with a single preheat (namely 10 sat 2601C). We selected 12 samples for this test. Newaliquots were bleached at room temperature with blueLEDs using the Ris� reader for 1000 s, stored for10,000 s at room temperature to allow the 1101C TLpeak (resulting from optical transfer from the OSL trap)to decay, and bleached again for 1000 s. The sampleswere then beta irradiated to give a dose (158 Gy) of thesame order as the natural dose. Fig. 6 shows thenormalised doses measured with the SAR protocol

(open symbols; 1601C cut-heat temperature). Thefield division of samples into poorly sorted and well-sorted sediments is clearly reflected in the ability torecover the known dose; in general, the given dose isrecovered moderately well for all well-sorted samples(except WKS2 and WKR-9B), but all the poorly sortedsamples underestimate the given dose, by 20% onaverage.

The dependence of the dose recovery on the preheattemperature was also investigated. New aliquots of thesamples used in Fig. 5 were optically bleached as beforeand then irradiated with a known dose. The measuredDe values derived using the poorly sorted samples(Fig. 7a and b) underestimate the given dose at allpreheat temperatures. However, measurements usingthe well-sorted samples recover the given dose well(Fig. 7c and d). It should be noted that although the

Preheat Temperature ( oC)

160 200 240 280 3200

25

50

75

100

Preheat Temperature (oC)

160 200 240 280 320

Rec

yclin

g

0.0

0.5

1.0

1.5

Recuperation (%

of R

o)

0

25

50

75

100

Rec

yclin

g

0.0

0.5

1.0

1.5

De (

Gy)

0

100

200

300

PNA

(a)160oC220oC

(b)

PYND

e(G

y)

0

100

200

300 (c)

WYN

Recuperation

(% of

Ro

)

WKR-5B

(d)

Fig. 5. The dependence of De on preheat temperature. (a) PNA and (b) PYN; poorly sorted, (c) WYN and (d) WKR-5B; well-sorted. De values

obtained using a cut-heat temperature of 1601C are shown as open circles (open triangle; recycling ratio, open square; recuperation). De values,

recycling ratios and recuperation obtained using a 2201C cut-heat temperature are shown as filled symbols.


measured doses using PNA and PYN show no moredependence on preheat temperature than samples WYNand WKR-5B, the value obtained is an underestimate.

This implies that, in some cases, De values obtainedfrom otherwise acceptable preheat plateaus may beerroneous.

Sample

PSUPNA

POR2

POR4PYN

WYN

WKS1

WKS2

WKR-5

A

WKR-5

B

WKR-8

A

WKR-9

B

Mea

sure

d D

ose

(nor

mal

ised

to g

iven

dos

e)

0.0

0.6

0.8

1.0

1.2

Poorly-sorted Well-sorted

Fig. 6. After bleaching with blue light at room temperature, 12 samples were given a known dose (158Gy) in the laboratory (dashed line). This given

dose was measured with the SAR protocol (2601C preheat for 10 s and 1601C cut-heat; open squares), and the measured doses were normalised to the

given dose of 158Gy. The results obtained using a 2201C cut-heat are depicted as filled squares.

PNAMea

sure

d D

ose

(Gy)

0

100

200

300


160 200 240 280 3200

100

200

300

WYN

given dose = 183Gy

given dose = 148Gy

given dose = 167Gy

PYN


160 200 240 280 320

WKR-5B

given dose = 190Gy

(a) (b)

(c) (d)

Mea

sure

d D

ose

(Gy)

Fig. 7. After bleaching with blue light at room temperature, samples were given a known laboratory dose approximately equal to the De (dashed

line). These given doses were then measured using preheat temperatures increasing from 1801C to 3001C in 201C steps and the cut-heat temperature

was fixed at 1601C. (a) PNA and (b) PYN; poorly sorted, (c) WYN and (d) WKR-5B; well-sorted.


5. Investigation of the importance of the test-dose cut-

heat temperature

5.1. LM-OSL characteristics

To identify the causes of De underestimation using thepoorly sorted samples and some of the well-sortedsamples, we investigated the thermal stabilities ofdifferent OSL components using the linear modulationtechnique (Bulur, 1996). If the stimulating light islinearly increased in power from zero to some valueover time (rather than being held constant throughoutthe measurement), then the OSL signal will appear as aseries of peaks; each linearly modulated OSL (LM-OSL)peak represents a component of the OSL signal with aparticular optical cross-section. The degree of separa-tion of these components depends on the size of thedifferences in cross-sections, and the rate of increase ofthe light power. Bulur et al. (2000) identified fourcomponents (A–D) in the quartz OSL signal bydeconvolution of the LM-OSL curve. From their studyof the thermal stability of these four components, theyconcluded that the fastest component (A) was related tothe 3251C TL peak, and that it corresponds to the fastcomponent of continuous-wave OSL (CW-OSL) decaycurve (Bailey et al., 1997). This fast component isassumed to be the dominant signal in the initial part ofthe decay curve, and this is usually the signal used for De

estimation.In our study, the samples were first optically

bleached (40 mW cm�2 of blue light for 100 s at roomtemperature) and irradiated in the laboratory(B100 Gy). Then, the samples were heated to differenttemperatures (801C, 1601C, 2201C and 2601C) followedby immediate cooling. As a precaution against anypreviously undetected OSL contribution to minorcomponents from feldspars, the samples were stimulatedusing an IR laser diode for 150 s at room temperatureprior to the LM-OSL measurements. Finally, theintensity of the blue-LEDs was increased linearly from0 to 40 mW cm�2 over 500 s at room temperature, togenerate a LM-OSL signal.

The LM-OSL characteristics of the four samples areshown in Fig. 8. The results for sample WKR-5B(Fig. 8d) are similar to those published previously forquartz (B�tter-Jensen et al., 2000; Bulur et al., 2000;Kuhns et al., 2000; Larsen et al., 2000). There is awell-defined peak early in the curve, and the peakposition is not significantly dependent on prior heatingbetween 801C and 2601C. We identify this componentwith component A of Bulur et al. (2000), and the fastcomponent (FC) of Bailey et al. (1997). There are alsostrong signals at longer stimulation times (higherpower); these presumably relate to the componentsB–D of Bulur et al. (2000), but we have not examinedthese in detail. It is interesting, however, that at least

some of these components are very sensitive to priorheating.

Contrast these results with those of the other threesamples (Fig. 8a–c). There is a pronounced peakshift with prior heating in all samples, and the peaksimilar to the component A of Fig. 8d is only clearlyvisible after heating to at least 2201C. This component Ais also very much weaker than in Fig. 8d. At lowertemperatures a previously unidentified componentdominates the signal, peaking at 4 s of stimulation(0.7% of full power), compared to the 32–39 s (6.4–7.8%) for component A. We call this previouslyunobserved signal component A0. This component isthermally unstable; it becomes smaller with increasingtemperature, and is completely removed above 2201C.The A0 signal has not been reported in previous studies(B�tter-Jensen et al., 2000; Bulur et al., 2000; Kuhnset al., 2000; Larsen et al., 2000), but recent unpublishedwork by Jain et al. (2001) suggests that it is not uniqueto these samples. From the point of view of our study,the presence of the A0 signal can be expected to affectthe accuracy of the sensitivity correction, if it does notsensitise in the same manner as the component A usedfor dating. This suggests that it may be necessary toincrease the cut-heat temperature in these samples. Sincethe A0 signal is not present in the natural OSL signal,this also sets a lower limit to the preheat temperature ofabout 2201C. It should also be noted that the relativeintensity of the slower components, compared to thecomponent A, and after heating to 2601C, is muchstronger in these three samples.

To examine whether component A0 has a detrimentaleffect on the sensitivity correction, new aliquotsof all four samples were optically bleached as beforeand given a known regeneration dose of about 100 Gy.The SAR measurement cycle was then repeated eighttimes, using this constant regeneration dose (i.e.Di ¼ 100 Gy; i ¼ 1; 8). The corrected OSL values(regenerated OSL/test dose OSL) are shown in Fig. 9a(poorly sorted) and Fig. 9b (well-sorted). One aliquot ofeach sample was measured using 1601C cut-heat (opensymbols), and one aliquot using 2201C cut-heat (filledsymbols). The higher temperature cut-heat results in amuch more accurate sensitivity correction for the twopoorly sorted samples, and there is a small improvementfor one well-sorted sample (WYN); this is the well-sorted sample which showed a significant component A0

(Fig. 8c).We conclude that the A0 signal probably does

contribute to the poor recycling of some samples,and the sensitivity correction can be improved byremoving this component using the higher cut-heattemperature (2201C). It is interesting to note thatBailey (2000) also recommends a higher tempera-ture cut-heat, although not for the reason presentedhere.


5.2. Dependence of dose recovery on cut-heat

temperature

From the preceding discussion, it can be inferred thatsome samples, mainly from the poorly sorted sediments,do not behave well with the conventional SAR protocolusing preheat at 2601C for 10 s and a cut-heat at 1601C,at least in part because of the presence of a componentA0 with a larger optical cross-section than the compo-nent A. To determine whether removing the A0 signalmight improve our dose estimates, especially from thepoorly sorted samples, we repeated the dose recovery

test of Section 4.2 (De dependence on heat treatment),but using a preheat temperature of 2601C (for 10 s) anda cut-heat temperature increasing in 201C steps from1601C up to 2601C (Fig. 10).

In the lower cut-heat temperature region, the givendoses from poorly sorted samples PNA and PYN areunderestimated by between 60 and 100 Gy (32–60%).However, above a cut-heat temperature of 2201C, thegiven doses are successfully recovered by the SARprotocol; the recycling ratios are also closer to unity inthe high cut-heat temperature region (Fig. 10a and b).We conclude that an inadequate sensitivity correction at

Component A'

LM-O

SL

(cou

nts

/ 0.2

5s)

103

104

Stimulation time (s)

102

Component A'

Component A

PNA

(a)

80 oC

160 oC

220 oC

260 oC

0 150 300 450

102

103

104

LM-O

SL

(cou

nts

/ 0.2

5s)

(c)

WYN

80oC

160 oC

220 oC

260 oC

Component A

10 2

10 380oC

160 oC

220 oC

260 oC

(b)

Component A

0 150 300 450

103

104

Stimulation time (s)

Component A

(d)

PYN

160 oC

220 oC

260 oC

80oC

WKR-5B

0 100 1000Power (%) Power (%)

Component A'

Fig. 8. LM-OSL curves obtained as stimulation power is increased. The unusual fast component (component A0) can be observed at 0.7% of the full

stimulation power ((a) PNA, (b) PYN and (c) WYN). This component becomes smaller with increasing temperature, and is efficiently removed by

heating to 2201C. (d) In WKR-5B, component A0 is not observed. Note the logarithmic OSL axes.


the lower cut-heat temperatures was the main cause ofthe underestimation of the given dose. In contrast, therecycling ratios of WYN and WKR-5B have valuesclose to 1.0 and the measured doses are indistinguishablefrom the given doses, irrespective of the cut-heattemperature (Fig. 10c and d).

To help confirm that the revised cut-heat temperatureis likely to improve the accuracy of the De estimates, the

dose recovery tests of Section 4.2 were repeated usingthe cut-heat of 2201C. As shown in Fig. 6 (filledsymbols), there is a marked improvement in the abilityto measure the given dose. The observed dose from thepoorly sorted samples (which previously underestimatedthe 158 Gy given dose by more than 20% using cut-heatof 1601C) is now in good agreement with the given dosewhen using the 2201C cut-heat (155.471.3 Gy weightedaverage, filled squares). Significant improvement in doserecovery can also be seen in the well-sorted samples.Two of these (WKS2 and WKR-9B) previously under-estimated the given dose by about 20%. However, usingthe 2201C cut-heat, the measured dose is in goodagreement with the expected values; the overall weightedaverage of the measured dose from the nine well-sortedsamples was 14872 Gy using 1601C cut-heat, but withthe 2201C cut-heat, the weighted average is156.871.1 Gy. Therefore, it seems likely that theestimation of De from both the poorly sorted and thewell-sorted samples should be improved by applying a2201C cut-heat after the test-dose, rather than the 1601Ccut heat temperature used earlier.

5.3. Equivalent dose preheat plateau using test-dose

cut-heat of 2201C

The variations in De values with increasing preheattemperature were measured using a 2201C cut-heattemperature and the results are shown in Fig. 5 (filledsymbols). It should be noted that no data were collectedfor preheat temperatures less than the 2201C cut-heattemperature. When the 1601C cut-heat was applied(open symbols), De values of PNA (Fig. 5a) increase asthe preheat temperature rises without defining a preheatplateau. However, by applying the 2201C cut-heat, aconstant De region from 2401C to 3001C can beidentified. Both recycling ratios and recuperation arealso improved at the higher cut-heat temperature. InPYN (Fig. 5b), only a weak preheat plateau (200–2601C) could be defined with the 1601C cut-heattreatment. However, by applying the 2201C cut-heat,there is a much better-defined preheat plateau from2201C to 3001C, and a significant increase in De:Recycling ratios are again markedly improved usingthe higher cut-heat. As expected, preheat plateaus forthe 2201C cut-heat for samples WYN and WKR-5B(Fig. 5c and d) are more similar to those obtained with a1601C cut-heat, although the recycling ratios are closerto unity using the 2201C cut-heat (especially for sampleWYN).

From the results of Section 5, it is concluded that thepresence of the A0 signal significantly affects thereliability of the SAR estimates of De using a cut-heatof 1601C. The remaining data to be presented in thispaper were obtained using a cut-heat of 2201C. Notethat the cut-heat of 2201C is still significantly below

Number of measurements

0 2 4 6 8

Cor

rect

ed O

SL

0.6

0.8

1.0

1.2

1.4160oC (WYN)

220oC (WYN)

160oC (WKR-5B)

220oC (WKR-5B)

0 2 4 6 8

Cor

rect

ed O

SL

0.6

0.8

1.0

1.2

1.4

160 oC (PNA)

220 oC (PNA)

160 oC (PYN)

220 oC (PYN)

(a)

(b)

Fig. 9. For a single aliquot of each sample, the SAR cycle was

repeated eight times with a constant regeneration dose. All corrected

OSL signals (the regenerated OSL divided by the test-dose OSL) are

normalised to the first values. With a 1601C cut-heat, the corrected

OSL increases with repeated measurement in both poorly sorted

samples ((a) PNA; open circles and PYN; open squares). The corrected

OSL becomes much closer to unity using 2201C cut-heat temperature

(filled symbols). The well-sorted samples show only small differences in

the corrected OSL with the two cut-heats ((b) WYN; triangles, WKR-

5B; diamonds).


the preheat temperature of 2601C, and thus shouldnot cause significant additional sensitisation ofcomponent A.

6. OSL ages of the marine terrace sediments

Before presenting the OSL ages of the samples,another unusual feature observed in some of the poorlysorted sediments must be discussed.

Heavily weathered and friable gravel clasts are foundin some poorly sorted samples, such that these gravelclasts can easily be broken by hand (Jeong et al., 2002).Except for the small portion of grains on the surface ofindividual clasts, most quartz is presumed to be insidethe individual pieces of gravel clasts, and thus not

exposed to sunlight. It is, therefore, likely that the greatmajority of any quartz grains derived from theweathered gravel clasts after deposition should havehigh natural doses, probably in the saturation region ofthe growth curves. Prior to, during sampling in the fieldand during sieving in the laboratory, it is clearly possiblethat unbleached quartz grains from weathered gravelclasts could have contaminated the bleached sedimen-tary quartz, giving average De values greater than the De

resulting from the burial period.As mentioned in Section 2, there is a lack of

independent age controls to support the OSL ages. Itis thus very important that the samples used to provideOSL ages are as secure as possible, in terms of sourceand bleaching prior to deposition. We therefore decidednot to analyse the poorly sorted samples further at this

Cut-Heat Temperature (oC)

160 200 240 2800

25

50

75

100

Cut-Heat Temperature ( oC)

160 200 240 280

Rec

yclin

g

0.0

0.5

1.0

1.5

Recuperation

% of

R0

0

25

50

75

100

Rec

yclin

g

0.0

0.5

1.0

1.5

PNAMea

sure

d D

ose

(Gy)

0

100

200

300

Mea

sure

d D

ose

(Gy)

0

100

200

300

given dose =189Gy

given dose = 139Gy

WYN

given dose = 167Gy

PYN

(a) (b)

(c)

given dose = 190Gy

WKR-5B

Recuperation

% of

R0

(d)

Fig. 10. Aliquots were bleached at room temperature, and then given a known laboratory dose approximately equal to De: They were then measured

using increasing cut-heat temperatures from 1601C to 2601C in 201C steps, with a fixed preheat of 2601C for 10 s. In the poorly sorted samples, the

given doses are measured successfully with a cut-heat temperature of 2201C and above ((a) PNA and (b) PYN). For the well-sorted samples ((c)

WYN and (d) WKR-5B), the given dose is measured successfully irrespective of the cut-heat temperature.


stage of our dating program, although Fig. 4 shows thatacceptable De plateaus can be obtained using the 2201Ccut-heat.

All the De measurements used for calculating ages(derived only from well-sorted samples) were obtainedusing a preheat temperature of 2601C for 10 s and thecut-heat of 2201C, and the temperature during blue lightstimulation (40 s) was held at 1251C. Values of De; doserates and derived OSL ages of the samples aresummarised in Table 2.

The De of the two analogue samples collected fromthe modern beach sediments near Kwanseong (KS) andBihag (BH) were also measured, to provide an indica-tion of the likely degree of bleaching of the terracesediments at the time of deposition. The De values ofthese samples are 0.470.2 and 0.470.1 Gy, respectively.If we assume that sedimentation environments in thepast were not very different from those of the present,these De values (close to 0 Gy) from modern depositsimply that it is very unlikely that bleaching wasincomplete at the time of deposition of the terracesediments.

In view of the relatively large values of De found inthese samples, it is important to demonstrate that theobserved values do not lie close to the saturation regionof the growth curve. This is shown in Fig. 4 (opencircles), where the routine SAR growth curve describedin Section 3.1 was extended to over 500 Gy. During theconstruction of this curve, sensitivity corrections werechecked by re-measuring the OSL signals for theregeneration dose of 77 Gy (1st recycling, grey diamond)and 310 Gy (2nd recycling, open triangle); these pointswere measured after the OSL measurements followingthe regeneration doses of 232 and 464 Gy, respectively.The curvature is such that natural doses of more than400 Gy could be measured using this growth curve; thelargest average value of De in Table 2 is about 290 Gy(WNJ-B).

The OSL ages of the samples collected from theterrace #2 sediments (Yangnam (YN), Kwanseong(KS), Nasa (NS) and Bihag (BH)) all lie in the range71–48 ka. These ages are supported by the 14C agesobtained from peat layers in terrestrial deposits whichoverly terrace #2 sediments near JeongJa (Fig. 1);301707160, 327307220, and 357307300 yr BP, fromtop to bottom (KIGAM, 2000). The ages of the samplescollected from Weseong (WS) are 10678 and 11277 ka(Fig. 3).

7. Discussion

There are no closely associated independent agecontrols for these sample sites, and so it is importantto summarise the reasons why the OSL ages of Table 2should be given credence. Firstly, the analogue samples

collected from the modern beach have values of De closeto zero, indicating that it is unlikely that incompletebleaching is a problem in these terrace sediments.Secondly, the values of De observed from all thesediments are below 300 Gy; this is well below thepractical dose limit of >400 Gy deduced from thegrowth curve of Fig. 4. Thirdly, dose recovery testsshowed that laboratory doses given after room-tem-perature bleaching can be successfully measured using a2201C cut-heat (Fig. 6 and 7); this accuracy is attributedmainly to the removal of the A0 component observed inthe LM-OSL experiment (Fig. 8). Fourthly, using a2201C cut-heat, all preheat plateaus showed a well-defined region where the observed values of De wereindependent of preheat temperature (Fig. 5). Finally, theOSL ages obtained at each site have good internalconsistency (Fig. 3). At the Kwanseong (KS) site, theuse of a 2201C cut-heat led to a marked improvement inthe internal consistency; the OSL ages obtained using a1601C cut-heat were, from top to bottom, 6774 ka(WKS1), 5774 ka (WKS3), and 3176 ka (WKS5), i.e. astratigraphic inversion in depositional order (Table 2).This age inversion appears mainly to be due to anunderestimation of De for sample WKS5 (117721 Gy).However, after applying a 2201C cut-heat, there was aconsiderable increase in the estimate of De (to267719 Gy), and a significant improvement in reprodu-cibility. This increase in De gave rise to a correspondingincrease in age, and so the OSL ages from this site arenow stratigraphically consistent and reproducible; theages are, from top to bottom, 5976, 6874, and7175 ka.

The OSL ages suggest that Weseong (WS) sedimentsprobably formed in the last interglacial period (OIS 5e),which corresponds to the suggested age of formation ofKim et al.’s (1998) terrace #2-I (see Section 2). In Fig. 3,the terrace sediments from sites YN and KS seem tohave very similar depositional ages to those from NSand BH, but their elevations are different by about 15–20 m. There are two potential explanations of thisvertical discontinuity.

The first possibility is that the terrace sediments mightnot have been formed in marine environments. How-ever, the terraces located along the eastern coastline ofthe Korean peninsula, including those investigated inthis research, have long been recognised as marineterraces and are well documented (e.g. Lautensach,1945; Lee, 1985, 1987; Kim et al., 1990). Geologicalevidence in support of this identification includes thedirection and arrangement of terrace surfaces and cliffs;palaeo-shoreline indicators (e.g. notch, shore bars,beach ridges); an absence of large river channels nearthe shoreline; and fossil marine sedimentary micro-structures on the terrace surfaces. Kim et al. (1990) alsoexamined the mean roundness and maximum projectionsphericity (MPS) of gravels collected from modern


beaches and rivers in this area, and found that thegravels from the two environments could be discrimi-nated using these parameters. They then used thisapproach to identify the gravels from terraces alongthe eastern Korean coastline as being deposited in amarine environment. From these previous studies, weare confident that our terrace sediments are marine inorigin, and thus terraces of a similar age can be expectedto have been deposited at the same elevation.

The other possibility is that the regions YN to KS andNS to BH have experienced different uplift histories,with the creation or evolution of tectonic faults betweenthe sites, only some of which have been identified in thefield (see Fig. 1). In a Quaternary fluvial deposit in thisarea (near Oryu in Fig. 1), a vertical offset of about 15 mat a single location on a thrust fault has been observedby Cheong et al. (2002) and so a differential uplift of 15–20 m between the YN, KS and NS, BH sites is clearlypossible. Furthermore, this conclusion of differentialuplift is supported by the greater density of fault lines inGamPo–JeongJa region than in JeongJa–GiJang region(Figs. 1 and 3). Nevertheless, it is important to examinethe field evidence again for the presence of faultsbetween two sites.

Based on the eustatic sea level record obtained fromthe coral terraces of the Huon Peninsula, Papua NewGuinea (Chappell and Shackleton, 1986), the global sealevel at 50–70 ka should have been about 30 m below thepresent sea level, and a considerable net verticaldisplacement at these sites is implied—at least 40 mduring the Late Pleistocene if the sea level changesobtained from Huon Peninsula are taken as reference.However, it must be remembered that the geoidal levelsof the ocean vary from place to place (Lowe andWalker, 1998), and so the assessment of the precisedisplacement and average uplift rate in this area willonly be possible when OSL age data are combined withthe elevation of palaeo-shorelines from a detailedgeological survey now underway. Nevertheless, it isclear from our ages that this region has been tectonicallyactive in the last 50 ka.

8. Conclusion

Previous OSL ages from these sites (measured by thepresent authors using a 1601C cut-heat) were obviouslyinconsistent with each other; the work presented hereclearly shows that more reliable results can be expectedusing a 2201C cut-heat. This conclusion is supported bythe internal consistency of the revised OSL agesobtained at each site.

On the basis of geomorphological description, pre-vious research identified the various terrace deposits inthis region as marine, and in order to derive theirstratigraphic chronology, they assumed that terraces of

similar present-day elevation should be of similar age.This in turn implicitly assumed that tectonic uplift in theregion had not been significant over the relevant timeperiod (e.g. Choi, 2001). Our OSL ages lead us toconclude that this is not true. Two terrace sedimentationperiods have been identified, one at about 110 ka (siteWS) and one between 50 and 70 ka (sites YN, KS, NS,BH). Despite the chronological consistency of the lattergroup, the elevations vary considerably, from 7 to 25 m(a.m.s.l.). We agree with the description of these terracesas marine (Lautensach, 1945; Lee, 1985, 1987; Kim et al.,1990, 1998), but we recognise that there is nounambiguous independent control on our OSL agesexcept for several 14C ages from terrestrial deposits.Nevertheless, we have no reason to doubt the reliabilityour OSL ages, and conclude that it is likely that therehas been considerable tectonic activity during the LatePleistocene.

Acknowledgements

K.S. Lee and D.G. Hong are thanked for theircomments and discussions throughout this work. Theauthors also thank to A.G. Wintle for her reviewing themanuscript. This work is financially supported by KoreaResearch Foundation Grant through the ResearchInstitute of Basic Sciences, Seoul National University(No. KRF-2001-015-DP0604).

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Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea

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Transcript of Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea