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Quaternary International 183 (2008) 23–39

Stratigraphy and optically stimulated luminescence dating of subaeriallyexposed Quaternary deposits from two shallow bays

in Hong Kong, China

W.W.-S. Yima,�, A. Hilgersb, G. Huangc, U. Radtkeb

aDepartment of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, ChinabDepartment of Geography, University of Cologne, Albertus-Magnus-Platz, D-50923 Cologne, Germany

cGuangzhou Institute of Geography, 100 Xian Lie Road, Guangzhou 510070, China

Available online 19 July 2007

Abstract

Episodes of subaerial exposure recorded in Quaternary sedimentary sequences on continental shelves are indicators of sea-level

change. In this study, the Quaternary superficial deposits from two shallow bays in Hong Kong are examined in order to identify the

timing of such episodes. Two rotary boreholes in Tai O Bay penetrated a succession of marine and terrestrial deposits formed during the

last four interglacial-glacial cycles. The interglacial periods and the glacial periods are found to be represented by siliciclastic-dominated

shallow marine–estuarine deposits and colluvial–alluvial fan deposits, respectively. An excavation of a coastal land reclamation in Deep

Bay revealed Holocene intertidal–subtidal estuarine deposits overlying aeolian deposits of the last glacial age. Optically stimulated

luminescence dating carried out has confirmed the last glacial age of the terrestrial unit unconformably overlain by the Holocene marine

unit but only minimum ages can be obtained from the pre-last glacial terrestrial units. In one of the Tai O boreholes studied, the density

and moisture content distribution is found to reflect the episodes of subaerial exposure of the pre-Holocene interglacial marine units. In

the palaeo-desiccated crusts of these units, density increases through iron cementation resulting from acid-sulphate soil development

while moisture content decreases as a consequence of desiccation. Quaternary terrestrial deposits occurring on continental shelves are

therefore datable to a greater degree of certainty than their subaerial counterparts.

r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Terrestrial deposits of Quaternary age above the presentday sea level are often found to be difficult to date reliably.The use of radiocarbon dating for such deposits isproblematic not only because of the age limit of themethod, but also because of the presence of old and newsources of carbon (Bird et al., 1999) and large variations ofatmospheric radiocarbon concentration during the lastglacial period (Beck et al., 2001). Consequently, the pre-Holocene radiocarbon ages obtained are often found toshow a young age bias (e.g. Yim et al., 1990; Yim, 1999a).Furthermore, in the dating of the age of tin placer depositsformed by alluvial processes, the true age may exceed the

e front matter r 2007 Elsevier Ltd and INQUA. All rights re

aint.2007.07.004

ing author. Tel.: +852 2859 2829; fax: +852 2517 6912.

ess: wwsyim@hku.hk (W.W.-S. Yim).

radiocarbon age of the plant remains contained within it(Yim, 2000).Terrestrial deposits of Quaternary age below the present

sea level are in contrast ‘easier’ to date because of theirassociation with marine deposits. Since the marine depositsand terrestrial deposits in shelf sequences were formedduring the interglacial and glacial periods, respectively, theage may be inferred by count back and correlation with themarine isotope record of deep-sea cores. Additionally,marine deposits may contain fossils suitable for age deter-mination using other dating methods, including U-seriesdating (e.g. Yim et al., 1990) while the erosional boundariesbetween the marine and terrestrial deposits are readilyidentifiable within the siliciclastic-dominated shelf se-quences (Yim, 1994). For obtaining a complete picture ofterrestrial environments during glacial periods, Quaternaryterrestrial deposits formed during the different episodes ofsubaerial exposure on continental shelves are also needed.

served.

ARTICLE IN PRESSW.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3924

The present study is aimed at drowned Quaternaryterrestrial deposits formed by subaerial exposure since theMiddle Pleistocene climatic transition.

Tectonically stable sites located far from the former ice-covered regions are regarded to be the best indicators ofeustatic sea-level changes during the Quaternary period.Examples include the Sunda Shelf (Hanebuth et al., 2000)and Bonaparte Gulf in northwestern Australia (Yokoyamaet al., 2001). The northern South China Sea continentalshelf off the coast of Hong Kong appears to fit into thesame category (Yim et al., 2006).

Based on the study of excavations and offshore bore-holes from the inner continental shelf of Hong Kong, anoffshore geological model represented by the last fiveinterglacial–glacial cycles was identified by Yim (1994). Upto five interglacial marine units (M1–M5) and five glacialterrestrial units (T1–T5) in agreement with the Vostok icecore (Petit et al., 1999) bounded by hiatuses have beenidentified. In addition to the presence of terrestrialdeposits, terrestrial environments during episodes ofsubaerial shelf exposure are indicated by acid-sulphate soildevelopment in the pre-Holocene marine units to formpalaeo-desiccated crusts (Yim and Tovey, 1995; Tovey andYim, 2002). The offshore geological model of Yim (1994) issupported by sedimentological evidence (Yim, 1984, 2001);palaeontological evidence (Yim et al., 1988, 1990, 1997;Yim and He, 1991; Yim and Li, 2000); moisture content(Yim et al., 2002); magnetic susceptibility (Davis et al.,1999; Yim et al., 2004); sequence stratigraphy (Davis, 1999;Bahr et al., 2005) and dating including radiocarbon(Kendall, 1976; Yim, 1986, 1999a; Bahr et al., 2005; Yimet al., 2006), U-series (Yim et al., 1990; Yim and Choy,2000) and thermoluminescence (TL) (Yim et al., 2002).

In the present study, the stratigraphy of Quaternaryterrestrial deposits in two shallow bays in Hong Kong is

New Territories

Hong Kon

New Hong KongInternational Airport

Tin Shui Wai

Deep Bay

LantauBH6BH9

+

Guangdong

Tai O Bay

Pearl RiverEstuary

Fig. 1. Map of Hong Kong showing the location of boreholes BH6 and BH9 i

investigated and selected samples from the deposits weredated using the optically stimulated luminescence (OSL)method assisted by the measurement of density andmoisture content distribution in one borehole. Theimplications of the findings are then examined.

2. Background on the study areas

Hong Kong, located near the mouth of the Pearl RiverEstuary on the coast of southern China in the northernpart of the South China Sea (Fig. 1) has a subtropicalmonsoonal climate. The region falls into the category of apassive continental margin (Taylor and Hayes, 1980) butneotectonic fault activity is known to be present (Lee, 1981;Ding and Lai, 1997; Lee et al., 1997). A 1–2m high sea-level stand during the Middle Holocene is identified alongthe southeast coast of China where there is tectonic uplift(Zong, 2004).A map of the two localities selected for the present study

is shown in Fig. 1. The Tai O Bay site was chosen becauseof ground investigation carried out for coastal develop-ment, including the construction of a sheltered boatanchorage area. Out of the available offshore rotaryboreholes penetrating bedrock, two boreholes (BH6 andBH9) located near the middle of the Bay were selected forstudy. The Deep Bay site was chosen because of theexposure of superficial deposits provided by excavationinto the former subtidal area for the construction of theTin Shui Wai New Town.Tai O Bay is a shallow bay no deeper than about 3m

below present mean sea level located on the northwesternpart of Lantau Island (Fig. 1). The surrounding onshorearea is predominantly of resistant acid crystal tuff ofMesozoic age, giving rise to steep and rugged rocky slopeswhich are common near the hilltops, while the lower slopes

g

Hong Kong

23 ½ °N

0

km

Key -

Borehole

Excavation+

SouthChinaSea

5

n Tai O Bay and the location of the Tin Shui Wai excavation in Deep Bay.

ARTICLE IN PRESSW.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 25

are littered with colluvial boulders. Based on the age of theabandoned lime kilns found along the coast (W. Meacham,personal communication), the area probably becameimportant for sea-salt manufacturing by the time of theTang Dynasty (AD 618–907). This resulted in thereclamation of the intertidal and subtidal parts of the bayto create ponds protected from the sea by dykes for saltmaking. It was not until the late 1960s that sea-saltmanufacturing was abandoned and some of the salt pondswere converted into fish ponds. The recently completedcoastal development involved the construction of anoffshore breakwater to provide anchorage facilities forfishing boats and the conversion of some of the abandonedsalt ponds into a mangrove area for attracting migratorybirds. This development is regarded by the Hong KongSAR Government to be a means of compensation for theloss of natural coastline through the construction of theNorth Lantau Expressway and the new Hong KongInternational Airport (Fig. 1).

Deep Bay is a shallow bay no deeper than about 4mbelow the present mean sea level located in the north-western part of the New Territories (Fig. 1). The bayreceives runoff from the Sham Chung River and the ShekKong River. Evidence for a palaeoshoreline about 2–3 kminland of the present coast was put forward by Brimicombe(1986) but it is uncertain whether a higher MiddleHolocene shoreline or a last interglacial shoreline wasrepresented. Grant (1962) reported that parts of the baywere reclaimed for sea-salt manufacturing during an earlystage of development. In the late nineteenth century, due toa decline in salinity of the coastal waters through theincreasing influence of freshwater discharge from thesoutherly prograding Pearl River Delta, the salt pondswere converted into fish ponds. Unlike Tai O Bay, thecoastal area fringing the bay around Tin Shui Wai is lowlying and is susceptible to flooding during typhoon-inducedstorm surges and rainstorms (Peart and Yim, 1992). Inorder to prepare the ground for the construction of the TinShui Wai New Town, two channels were excavated tocreate an eastern outfall and a western outfall for drainageimprovement. The excavation site chosen for the presentstudy was located on the western outfall on land reclaimedoriginally from the intertidal and subtidal zone of the bay.

3. Materials and methods

The location of boreholes BH6 and BH9 in Tai O Bay isshown in Fig. 1. Borehole BH6 is from a seabed depth of�0.3m Principal Datum (PD—approximately 1.23mbelow the mean sea level) and is 65.53m in length.Borehole BH9 is from a seabed depth of �0.04m PDand is 57.55m in length. For logging, both disturbedsamples and selected core sections including 1-m longpistons, 0.45-m long U76s and 1-m long maziers (to a lesserextent in borehole BH9) were used. The metal liner samples(pistons and U76s) were extruded hydraulically and allsamples were logged by visual examination using the

descriptive scheme for rock and soil of the GeotechnicalControl Office (1988). For assisting environmental inter-pretation, the palaeontological, sedimentological, miner-alogical, chemical and engineering features for marine andterrestrial deposits of Yim (1994) were used.In the follow-up study, up to a maximum of 450 samples

in the core sections of boreholes BH6 and BH9 were alsomeasured for wet and dry bulk densities, moisture content,magnetic susceptibility and p-wave velocity. Further detailsare given in Bahr et al. (2005). In borehole BH6, the samplespacing used for the measurement of density and moisturecontent was at an average interval of approximately 50 cm.In order to determine the absolute ages of the pre-Holocene terrestrial deposits of the two boreholes, OSLdating was carried out. Two standard penetration test linersamples and four standard penetration test liner samples(W1–W6) collected from the sand-bearing sections ofborehole BH6 and borehole BH9, respectively, wereselected. With the exception of sample W6, ‘as found’moisture contents are available through laboratory testingcarried out in Hong Kong. Previously Yim et al. (2002)have found that such samples are suitable for TL dating.The location of the Tin Shui Wai excavation selected for

study is shown in Fig. 1. In the field the exposed profile waslogged by visual examination using the soil descriptionscheme of the Geotechnical Control Office (1988). Threesamples of the Pleistocene terrestrial deposits werecollected from the same depth by driving 30 cm-long7.5 cm-diameter PVC pipes horizontally into the exposurebefore covering both ends with tight fitting rubber caps.One of the three samples (W7) was selected for OSL dating.In the laboratory of the University of Cologne, all

samples were measured for ‘as found’ moisture contentfollowed by drying and sieving. They were then treatedwith hydrochloric acid, sodium oxalate and hydrogenperoxide in order to remove carbonates, clay and organicmaterial, respectively. For fine-grain dating, the polymin-eral 4–11 mm size fraction of samples W1–W6 was extractedfrom the bulk samples by dry sieving and settling. Theparticles were deposited as a thin layer on aluminium discsafter suspension in acetone. For coarse-grain dating, the100–200 mm quartz fraction was separated from samplesW1–W3 and W7 using sodium polytungstate solutions withspecific gravities of 2.68 and 2.62 g/cm3 and further purifiedby subsequent etching in 40% HF for 40min. The aliquotsfor the measurements were prepared by mounting about200 grains on stainless-steel discs using silicone oil.All luminescence measurements were carried out on

automated TL/OSL readers equipped with 90Sr/90Y betasources for irradiation. Infra-red (880780 nm) and blue-emitting diodes (470730 nm) were used for stimulationand EMI 9235 photomultiplier tubes were used forluminescence detection. The protocols of OSL dating haverecently been reviewed by Aitken (1998) and Bøtter-Jensenet al. (2003).In order to estimate the annual dose (D0) derived from

the decay of lithogenic radionuclides, the concentration of

ARTICLE IN PRESSW.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3926

U, Th and K in the sediment samples was measured byneutron activation analysis (NAA) at the BecquerelLaboratories in Australia. The cosmic-ray dose rate wasdetermined for each sample as a function of samplingdepth and geographical position according to Prescott andHutton (1994). The influence of variations of the over-burden thickness is considered to be negligible because ofthe small contribution of the cosmic dose rate to the overallannual dose. This is caused predominantly by the highradionuclide concentrations.

4. Stratigraphy of the localities studied

A simplified stratigraphy of boreholes BH6 and BH9 inTai O Bay and of the Tin Shui Wai excavation in Deep Bay(Fig. 1) and their possible correlation is shown in Fig. 2.

Although boreholes BH6 and BH9 are located approxi-mately 250m from each other near the middle of Tai O Bay(Fig. 1), differences have been found in the sedimentarysequence of the two boreholes. In borehole BH6 (Fig. 3),intertidal–subtidal marine deposits containing abundantcomplete shells and shell fragments 10.5m in thickness ofthe Holocene age (M1 unit) rest on top of 4-m thickupward-fining fluvial deposits of the last glacial age (T1unit). The T1 unit is underlain by 17.5-m thick undiffer-entiated marine deposits representing shallow marine toestuarine deposits of the last interglacial (M2 unit) and thesecond last interglacial (M3 unit) age. Because of thediscontinuous nature of the core samples, the preciselocation of the boundary between the two units cannot beidentified. Subaerial exposure of the M2 unit during thelast glacial period is indicated by the development of a5.5m-thick stiff and mottled palaeo-dessicated crust. TheM3 unit is underlain by 3-m thick upward-fining sequenceof soil and distal colluvial–alluvial fan deposits (T3 unit)and by 6.2-m thick shallow marine–estuarine deposits witha firm and mottled palaeo-dessicated crust (M4 unit). Thepre-Holocene marine deposits differ from the M1 unit inthat their physical properties indicate subaerial exposure,including an increase in stiffness, mottled appearance,lower moisture contents and the absence of complete shellsand shell fragments. The M4 unit is underlain by 9.1-mthick upward-fining subangular to subrounded gravels toboulder, representing proximal colluvial–alluvial fan de-posits (T4 unit) and 15.23-m thick residual soil andbedrock of completely decomposed to slightly decomposedlapilli tuff of Mesozoic Lantau Formation age (Langfordand Kirk, 1994). Borehole BH9 (Fig. 4) shows three maindifferences from borehole BH6. First, upward-coarseningsilty sand with shell fragments representing intertidaldeposits is present in the top 1.5m of the M1 unit. Second,the T1 unit is absent, and third, a 2-m thick T2 unit withupward-fining fluvial deposits representing distal colluvia-l–alluvial fan deposition of the second last glacial age ispresent between the M2 and M3 units. Three stiff andmottled palaeo-dessicated crusts formed by subaerialexposure during the last glacial period, the second last

glacial period and the third last glacial period are presenton top of the M2–M4 units, respectively. The differences inthe sedimentary sequence observed between borehole BH6and borehole BH9 may be accounted for by the geomor-phologic conditions of the subaerially exposed Tai O Bayduring the different glacial periods. During the last glacialperiod, deposition of the T1 unit was restricted to the innerpart of the bay near the palaeomountain slopes whileduring the older glacial periods, deposition of the T2–T4units were further out into the bay because of thedownward shift of palaeomountain slopes. Whether theT1 and the T2 units are absent or present would begoverned by the occurrence of braided river channels (Bahret al., 2005). Furthermore the increase in thickness in theolder T units is confirmative of colluvial–alluvial fansedimentation activated during the glacial periods.The lithological description and interpretation of the

profile exposed at the Tin Shui Wai location (Fig. 1) isshown in Fig. 5. Only the M1 unit and the T1 unit arerecognized. The sequence from top to bottom is anthro-pogenic fill deposits, the M1 unit and the T1 unit. The T1unit is identified to be an aeolian deposit due to the well-sorted nature, the dominant particle size ranges frommedium silt to fine sand size and the blanket nature of thedeposit. It is possible that the original deposit may haveincluded shell fragments of a last interglacial age but thesefragments were destroyed by leaching during the lastglacial period.

5. Density and moisture content distribution in borehole

BH6

Previous work on an offshore borehole from the newHong Kong International Airport has shown that moisturecontent measurements are useful in providing chronologi-cal information within a Quaternary sequence (Yim et al.,2002). Moisture content not only helps to distinguishbetween Holocene marine deposits from their pre-Holo-cene counterparts but also in assisting the identification ofpalaeo-desiccated crusts formed by subaerial exposure ofthe pre-Holocene marine units. In the present study, bothmoisture content and density have been measured inselected samples of borehole BH6 (Fig. 6).The distribution of density and moisture content in

borehole BH6 is shown in Fig. 6. Within the M1 unit, amaximum moisture content of 80% and a minimum densityof 1.54 g/cm3 were found. This is consistent with marinedeposits never having been subaerially exposed. In samplesfrom the top 1.5m, moisture content is the lowest at32.6–46.7% while density is the highest at 1.79–1.87 g/cm3.This is consistent with the presence of tidal and subtidaldeposits containing significant amounts of medium tocoarse sand. Within the T1 unit, a minimum moisturecontent of 25.7% and a maximum density of 2.1 g/cm3 havebeen found. This is consistent with terrestrial deposits ofgravelly sand formed mainly by fluvial deposition. Withinthe undifferentiated M2 and M3 units, moisture content

ARTICLE IN PRESS

?

?

M1

T1

M2

/

M3

T3

M4

T4

RS

B

M1

M2

M3

T2

T3 s

M4

T4

M1

T1 s

F

RS

B

0

-10

-20

-30

-40

-50

-60

Borehole

BH6

Borehole

BH9

Tin Shui Wai

excavation

Key-

F

M1 Holocene

T1 Last glacial

Fill

M2 Last interglacial

T2 2nd Lastglacial

M3 2nd Last interglacial

T3 3rd Last glacial

M4 3rd Last interglacial

T4 4th Last glacial

RS Residual soil

B Bedrock

S Sample dated

Sealevel

s

-0.3

-10.8

-14.8

-32.3

-35.3

-41.5

-50.6

-58.3

-65.83

-57.59

-48.64

-46.54

-39.04

-36.04

-34.54

- 28.04

-26.04

-11.54

-0.04

+3.4+2.62+1.04

-2.0

Lapilli

tuff

Lapilli

tuff

Ele

vation (

m)

s

s

s

Fig. 2. Simplified stratigraphy of boreholes BH6 and BH9 in Tai O Bay and of the Tin Shui Wai excavation in Deep Bay and their possible correlation.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 27

ranges 35.9–51% while density ranges 1.57–1.91 g/cm3. Incomparison to the M1 unit, the lower moisture content isconsistent with desiccation caused by subaerial exposurewhile the higher density is consistent with cementation and/or consolidation. The lowest moisture content is foundin the palaeo-desiccated crust samples of the M2 unitat a core depth of 16–18m while the palaeo-desiccated crustof the M3 unit is indicated by the decrease in moisturecontent at a core depth of between 26 and 28m. Within theT3 unit, the moisture content ranges 27.8–32.8% while thedensity ranges 1.81–2.01 g/cm3. This is consistent withterrestrial deposits of gravely sand formed by distalcolluvial–fluvial fan deposition. Within the M4 unit,moisture content and density are found to show increasesand decreases, respectively, in comparison to the T3 unit.The presence of a palaeo-desiccated crust is reflected by adecrease in moisture content at a core depth of 35–37mto 35.7–36.4% as compared to 37.8–44.5% at a core depthof 37–40.1m.

Based on the density and moisture content distributionin borehole BH6, three main conclusions can be drawn:

(1)

The M1 unit possesses the lowest densities and thehighest moisture content because it has not beenaffected by subaerial exposure. Intertidal depositspresent within the unit containing significant amountsof sand fraction are found to increase in density anddecrease in moisture content.

(2)

The pre-Holocene marine units M2–M4 show higherdensities and lower moisture content in comparison tothe M1 unit because of the influence of subaerialexposure. In the palaeo-desiccated crusts, moisturecontent show decreases while density may show increasesdue to cementation or decreases if the location is affectedby bioturbation such as through plant rootlets.

(3)

The terrestrial units T1–T4 show higher densities andlower moisture content in comparison to the marineunits. The presence or absence of the T1 and T2 units

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Fig. 3. Lithological description and interpretation of borehole BH6 from Tai O Bay.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3928

appear to be related to the occurrence of alluvialchannels on the subaerially exposed continental shelfduring the last glacial and the second last glacial periods,respectively. In contrast, the T3 and T4 units are bothgreater in thickness than the T1 and T2 units, andrepresent distal to proximal colluvial–alluvial fan depos-its formed during the subaerial exposure of Tai O Bay.

6. OSL dating

A summary of the OSL dating results is shown inTable 1. Three finite ages were obtained for samplesW1, W3 and W7; sample W1 was from a depth of�12.8 to 13.25m PD (core depth 12.5–12.95m) in bore-hole BH6, sample W3 was from a depth of �12.54 to 13m

ARTICLE IN PRESS

Fig. 4. Lithological description and interpretation of borehole BH9 from Tai O Bay.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 29

PD (core depth 12.5–12.95m) in borehole BH9 and sampleW7 was from a depth of �1.1m PD in the Tin Shui Waiexcavation. The remaining four samples from boreholesBH6 and BH9 yielded only minimum ages. In this section,the protocols used for coarse-grain and fine-grain datingare described in more detail and the problems whichoccurred in the course of the dating procedure will bediscussed.

6.1. Coarse-grain quartz dating

Luminescence measurements were carried out followingthe single-aliquot regenerative dose (SAR) protocol asdescribed by Murray and Wintle (2000) for sand-sizedquartz.The number of aliquots measured for each sample is

summarized in Table 2. Conditions include preheating of

ARTICLE IN PRESS

Fig. 5. Lithological description and interpretation of the Tin Shui Wai excavation in Deep Bay.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3930

the samples for 10 s at 260 1C, a cutoff temperature of160 1C (heating rate 5 1C/s) and optical simulation for 50 swith the sample kept at 125 1C constant throughout theillumination with blue light emitting diodes at a wavelengthrange of 470730mm. Several aliquots of each sample weremeasured in order to determine the equivalent dose De

which represents the amount of radiation dose accumu-lated within the crystal lattice of a mineral grain since itwas shielded from sunlight. After excluding outliers, the De

of each sample was calculated from the remaining valuesstatistically concordant with a normal distribution at a 5%level of significance.

Because of the mode of transport and deposition,sunlight exposure of the mineral grains of fluvial sediments(samples W1–W6) might not be long enough to release thestored energy completely from the crystal lattice prior todeposition. In addition, saturation of the luminescencesignal due to the high annual dose could prove problematichere, as reported in previous studies of similar sedimentarysequences (Yim et al., 2002). This problem is illustrated inFigs. 7(a)–(d) and Table 2. Some aliquots of the samplesW2, W3 and W7 show saturated growth curves and aretherefore unsuitable for dating. But most measurementsyielded growth curves showing sufficient increase of thesignal intensities with increasing irradiation doses forequivalent dose estimation.

In order to further investigate the presence of residualsignals due to incomplete bleaching, measurements werecarried out on small aliquots of about 200 grains. Based onthe work of Fuchs and Wagner (2003), smaller aliquots

should better reflect heterogeneities in the equivalent dosedistribution. The shape and width of the data distribution,the latter either expressed as coefficient of variation (n) orrelative standard deviation, are often used to discriminatebetween well and insufficiently bleached samples. A highn-value and a skewed distribution with a tail towardshigher dose values may be interpreted to indicate poorbleaching conditions. Histograms of De distributions of thesand-sized quartz samples W1–W3 and W7 are shown inFigs. 8(a)–(d). The most asymmetric histogram is observedfor sample W2, but the weighted mean of 184Gy is still agood representation of the peak of the distribution. In spiteof the broad distribution found in the samples, withcoefficients of variation ranging between 15% and 22%, weconclude that poor bleaching is most likely not the majorcause of the data spread. A possible source of scatter in thequartz is the proximity of the natural luminescenceintensities to the signal saturation level observed withinthese samples, thus limiting a precise De estimation (Figs.7(a)–(d)). Sufficient resetting of the luminescence signalsduring transport and deposition could be further deducedby comparing the data spread observed in the aeoliansample W7 with the fluvial samples W1–W3. Thecoefficients of variation of between 17% and 22% foundfor these three samples are not so different from the 15%obtained for sample W7, which is the sample assumed to bemost effectively bleached at the time of deposition. Theobservation of good agreement between TL and OSL agesobtained for Hong Kong sea-floor sediments by Owenet al. (1995) further supports the conclusion of sufficient

ARTICLE IN PRESS

Fig. 6. Distribution of density and moisture content in borehole BH6 from Tai O Bay.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 31

resetting of luminescence signals in such depositionalenvironments, otherwise an age overestimation caused bythe less bleachable TL signal would be expected.

6.2. Polymineral fine-grain dating

For equivalent dose determination of the polymineralfine-grain materials, multiple-aliquot regenerative dose

(MAR) and SAR protocols were used. MAR measure-ments were carried out on samples W1–W6 using tenaliquots per sample to measure the intensity of thenatural luminescence signal accumulated during the burialperiod. Twenty-five aliquots per sample were artificiallybleached for 16 h and then irradiated in groups offive aliquots using five 60Co-gamma-ray doses between44.5 and 445Gy. The aliquots were then stored at room

ARTICLE IN PRESS

Table 2

Coefficients of variation (v) calculated for the initial data sets of sand-sized

quartz (uall, n ¼ number of all measured aliquots) and for the normally

distributed data (un.d.)

Sample ref. uall un.d. n Excluded values (Gy)

W1 20 17 31 4235, 224

W2 18 18 28 4344 (n ¼ 2), saturated (n ¼ 2)

W3 27 22 27 253, 4258, saturated (n ¼ 2)

W7 22 15 18 53, 469, saturated (n ¼ 1)

Those values significantly deviating from the normal distribution were

excluded as outliers and are shown here together with other excluded De

values.

Table 1

Summary of OSL dating results for the seven samples from Tai O Bay and Tin Shui Wai, Hong Kong

Sample

ref.

Laboratory

code

Elevation

(m PD)

Equivalent

dose

(Gy)

Uranium

(ppm)

Thorium

(ppm)

Potassium

(%)

Moisture content (weight %) Dose

rate

(Gy/ka)

OSL age (ka)

Measured Assumed

W1-FG C-L1055 �12.8 to 13.25 183735 4.8770.24 36.871.8 1.7870.09 21 20–40 6.4470.90 FG-MAR 2877

17579 FG-SAR 2774

W1-Q 12877 4.1470.27 Q 3173

W2-FG C-L1056 �34.6 to 35.1 4445 3.9570.20 24.271.2 1.3770.07 39 30–50 4.2270.60 FG-MAR 4105

W2-Q 184710 2.7170.18 Q 6876

W3-FG C-L1057 �12.54 to 13 204745 4.8170.24 37.371.9 1.4870.07 25 30–50 5.7370.84 FG-MAR 3679

183721 FG-SAR 3276

W3-Q 12676 3.6070.24 Q 3573

W4-FG C-L1058 �28.04 to 28.5 457732 4.2470.21 24.071.2 1.3470.07 29 30–50 4.2770.61 FG-MAR 107717

W5-FG C-L1059 �34.04 to 35 4445 4.9770.25 24.771.2 1.5170.08 38 40–60 4.3270.62 FG-MAR 4103

W6-FG C-L1060 �41.04 to 41.5 4445 3.5970.18 23.971.2 1.9470.10 ND 40–60 4.1470.55 FG-MAR 4107

W7-Q C-L1061 �1.1 3072 1.1670.13 3.5770.18 0.4270.03 ND 10–30 0.9170.06 Q 3373

ND ¼ not determined.

Radionuclide concentrations were measured by neutron activation analysis (NAA). The dose rate (D0) values shown for polymineral fine-grain (FG) and

coarse-grain quartz (Q) samples were calculated assuming secular equilibrium for the U and Th-decay chains and an alpha-efficiency factor of 0.0770.02

(Preusser et al., 2005) for the former. The equivalent dose (De) estimates are derived from multiple-aliquot (MAR) or single-aliquot regenerative dose

(SAR) measurements with each SAR-De value representing the error weighted mean with the error including the standard error and a 5% uncertainty in

the beta source calibration. Uncertainties in the equivalent doses, dose rates and age determinations are expressed at the 1s confidence level.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3932

temperature for 4 weeks before MAR measurements werecarried out.

For samples W2, W5 and W6, two problems wereencountered. First, the natural signal intensities were foundto exceed the highest dose point. Consequently, anequivalent dose determination by interpolation of thenatural signal onto the growth curves was not feasibleeven though the growth curves showed no evidence ofsaturation up to 445Gy. A repetition of the MARmeasurements using higher laboratory doses might there-fore be promising. Second, the rather broad disc-to-discscatter in the MAR measurements, particularly in thegroup of natural subsamples results in large uncertaintiesin the equivalent dose estimates. A normalization proce-dure could be carried out to minimize the effect of suchscattering but was not applied here. Nevertheless, forsamples W1 and W3, the polymineral fine-grain ages are ingood agreement with the OSL ages obtained for the sand-sized quartz fraction (Table 3).

Instead of repeating the MAR measurements for allsamples, which would have been time consuming due to anecessary repetition of the sample preparation to obtainsufficient aliquots and the storage time, SAR measure-ments were carried out on the remaining natural discs. Asdemonstrated by Banerjee et al. (2001) for SAR measure-ments only a few subsamples are needed for De determina-tions. However, in general one critical disadvantage of theregenerative approach is the possibility of sensitivitychanges, which would result either in an over- or under-estimation of the palaeodose. In contrast to the MARmethod, the SAR method corrects for such changes insensitivity, which is one of the major advantages of theSAR protocol (Murray and Wintle, 2000).The SAR equivalent dose values were determined using

either the protocol for fine grains described by Banerjee etal. (2001) and Roberts and Wintle (2001), or the SARtechnique described by Wallinga et al. (2000) for sand-sizedK-rich feldspar extracts. Table 3 provides a summary of theparameters used for De measurements of the fine-grainsamples, number of aliquots included in the De calculationand the resulting De values and ages.Single-aliquot measurements were also carried out on the

younger samples, which already yielded reliable MAR agesin order to provide a data base for evaluating the mostappropriate method for dating samples with unknownages. The SAR procedure described by Banerjee et al.(2001) provides two De values because it is based on twoluminescence measurements. Both the signal induced byinfra-red stimulation and the signal induced by blue-lightstimulation were measured in the ultra-violet range. Thisprotocol was applied for samples W1, W2, W4 and W6,and in each case the IRSL resulted in a significantly lower

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Fig. 8. (a)–(d) Histograms of equivalent dose distribution of sand-sized quartz in samples W1, W2, W3 and W7. The histograms and the normal

distribution curves are based on all aliquots included in the equivalent dose calculation. Sample W2 shows the most asymmetric distribution with a tail

towards higher De values possibly indicating poor bleaching. Furthermore, the natural luminescence signals are already close to the saturation level, giving

rise to additional De scatter and inaccuracies in De determination.

Fig. 7. (a)–(d) Some growth curves of sand-sized quartz from single-aliquot regenerative (SAR) dose measurements of samples W1, W2, W3 and W7. The

problem of saturation is clearly demonstrated in case of samples W2 and W3, a.u. ¼ arbitrary unit.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 33

De value than the blue light-stimulated luminescence(BLSL) (Table 2). The effect of signal recuperation whichis monitored routinely by a ‘zero-dose’ cycle (Murray and

Wintle, 2000; Banerjee et al., 2001), was found to benegligible for both signals (Table 2) and is not the cause forsuch a marked and systematic difference.

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Table 3

Parameters used for equivalent dose (De) measurements of the fine-grain samples, number (n) of aliquots included in the De calculation and the resulting

equivalent dose values and ages

Sample ref. Protocol Thermal pre-treatment Stimulation Stimulation Detection n Recuperation (%) De (Gy) Age (ka)

Regenerative dose Test dose

W1 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – 183735 2877

SAR-IRSL (blue) 10 s@ 270 1C 10 s@ 270 1C 350 s@ 50 1C IR LED Blue 4 2.1 12476 1973

SAR-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C IR LED UV 4 3.1 11276 1773

SAR-post-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C Blue LED UV 4 1.0 17579 2774

W2 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – 4445 4105

SAR-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C IR LED UV 3 2.2 263713 6279

SAR-post-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C Blue LED UV 3 1.3 374722 89714

W3 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – 204745 3679

SAR-IRSL (blue) 10 s@ 270 1C 10 s@ 270 1C 350 s@ 50 1C IR LED Blue 3 5.3 11479 2073

SAR-IRSL (blue) 60 s@ 270 1C 60 s@ 270 1C 350 s@ 50 1C IR LED Blue 3 1.6 183721 3276

W4 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – 457732 107717

SAR-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C IR LED UV 3 1.6 280714 65710

SAR-post-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C Blue LED UV 3 1.1 375719 88713

W5 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – b445 b103

SAR-IRSL (blue) 10 s@ 270 1C 10 s@ 270 1C 350 s@ 50 1C IR LED Blue 4 3.4 391721 91714

W6 MAR-IRSL 16 h@ 150 1C – 70 s@ 50 1C IR LED Blue 35 – b445 b107

SAR-IRSL (blue) 10 s@ 270 1C 10 s@ 270 1C 350 s@ 50 1C IR LED Blue 4 2.9 300716 72710

SAR-IRSL (blue) 60 s@ 270 1C 60 s@ 270 1C 350 s@ 50 1C IR LED Blue 3 1.5 500729 121717

SAR-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C IR LED UV 3 2.0 343717 83712

SAR-post-IRSL (UV) 10 s@ 250 1C 200 1C@ 5 1C/s 200 s@ 125 1C Blue LED UV 3 1.5 471725 114716

MAR-IRSL: multiple-aliquot regenerative dose infra-red stimulated luminescence. SAR-IRSL (blue): single-aliquot regenerative dose infra-red stimulated

luminescence following in general the protocol described for sand-sized K-feldspars by Wallinga et al. (2000). SAR-IRSL (UV): single-aliquot regenerative

dose, infra-red stimulated luminescence following in general the protocol described by Banerjee et al. (2001). SAR-post-IRSL (UV): single-aliquot

regenerative dose blue light stimulated luminescence following in general the protocol described by Banerjee et al. (2001); 4 or b natural luminescence

signal exceeding the highest dose point significantly.

Stimulation was either conducted by diodes emitting in the wavelength range of 880780 nm ( ¼ IR) or 470730 nm ( ¼ blue). Detection windows were in

the ultra-violet (290–370nm) or in the violet to blue (390–450nm, ‘blue’) wavelength range. The recuperation signal is shown as a percentage of the natural

signal. OSL ages in bold are referred to in Table 1.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–3934

Shine-down curves of the natural luminescence signalsand growth curves obtained from SAR dose measurementsof the polymineral fine-grain samples W1 and W6 areshown in Figs. 9(a)–(h). The results obtained for sampleW1 using different protocols is found to show similar IRSLintensities (Figs. 9(a) and (c)) but a completely differentshape of the growth curves (Figs. 9(e) and (g)) anddifferences in the resulting equivalent dose values. Mostevident is the significantly higher luminescence output inthe case of the post-IRSL signal (Fig. 9(b)). Sample W6(Fig. 9(h)) shows no sign of saturation even up to 1000Gy.As the ultra-violet signal from IRSL measurements ofpolymineral fine-grain material is known to be insuffi-ciently stable for dating (Krbetschek et al., 1997), theobserved underestimation compared to the post-IRSLsignal measured during blue light stimulation is explain-able. As there is a good agreement for the SAR post-IRSLage with the fine-grain MAR age and the OSL age of thesand-sized quartz fraction (Tables 2 and 3), this protocolappears to be suitable for dating sample W1. However theSAR-post-IRSL ages of the older samples W2, W4 and W6were significantly lower than the expected ages from thestratigraphy and the MAR measurements. In the case ofsample W4, the De ratio SAR-post-IRSL to MAR is only0.82. In order to further investigate the significance of this

difference, a so-called ‘dose recovery test’ (DRT) wascarried out by fully bleaching four aliquots in the labo-ratory followed by irradiation with a beta-dose of 312Gy.In the subsequent SAR measurement, the same protocolwas applied as for the natural subsamples. Both the signalsfrom infra-red and blue-light stimulation were found toyield similar equivalent dose values of 247 and 252Gy,respectively. But with the De ratios (recovered/given dose)of 0.79 and 0.81, both values clearly underestimate theadministered laboratory dose. Without follow-up testingof the various measurement parameters, the SAR-post-IRSL signal appears to be unsuitable for dating depositsolder than 100 ka regarding the sediments investigated inthis study.In order to determine whether the violet–blue (referred

to as ‘blue’) emission from IRSL measurements of thepolymineral fine grains results in more reliable ageestimates, SAR-IRSL measurements were carried out onsamples W1, W3, W5 and W6. The SAR protocol for sand-sized K-rich feldspars of Wallinga et al. (2000) was adoptedbased on the assumption, that the IRSL signal of thepolymineral fine-grain fraction is strongly influenced by theemissions known from K-feldspars (Krbetschek et al.,1997). However, the ages obtained by the SAR-IRSL(‘blue’) protocol (Table 3) are clearly lower than those

ARTICLE IN PRESS

Fig. 9. (a)–(h) Shine-down curves of the natural luminescence signals and growth curves obtained from single-aliquot regenerative dose (SAR)

measurements of the polymineral fine-grain samples W1 and W6.

W.W.-S. Yim et al. / Quaternary International 183 (2008) 23–39 35

obtained with the other protocols. This underestimationcould be the result of an inappropriate pre-heat procedurewhich was insufficiently strong to release any unstablesignals. Because of this, samples W3 and W6 weremeasured again using a 60 s pre-heating instead of 10 s.Higher equivalent dose values were obtained now. In thecase of sample W3, the resulting SAR-IRSL age is in goodagreement with the MAR age and the OSL age of thequartz fraction. For the older sample W6, this measure-ment protocol seems to fail. Even though the signalintensity is good and the growth curve shows a lumines-cence intensity increasing with irradiation doses even up to1000Gy (Fig. 9(h), the resulting IRSL age of about 121 kaunderestimates the expected ages severely. A DRT onsample W6 using a pre-heating at 270 1C for 60 s appears tobe much less promising with a De ratio (recovered/givendose312GY) of only 0.79. In conclusion, the SAR approach

seems to work for the younger samples of the last glacialage but not for deposits older than 100 ka.

7. General discussion

The offshore geological model of Yim (1994) has beenfound to be applicable to the two localities studied. In TaiO Bay borehole BH6, in spite of the absence of the T2 unit,four interglacial–glacial cycles are evident from the bore-hole log (Fig. 3). The absence of the T2 unit at the locationcan be explained by the absence of a river channel incisionat the locality. In Tai O Bay borehole BH9 which is locatedapproximately 250m west of borehole BH6, again fourinterglacial–glacial cycles are evident from the borehole log(Fig. 4). The absence of the T1 unit at the BH9 location canalso be explained by the absence of a river channel incisionat the locality. In the Tin Shui Wai excavation, only one

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interglacial–glacial cycle is evident from the log of theexcavation (Fig. 5).

Based on the present day topography of the twolocalities studied and the characteristics of the marineand terrestrial deposits found, a number of inferences maybe drawn on the palaeoenvironment during the episodes ofsubaerial exposure. At both Tai O Bay and Deep Bay,erosion rates during the present and early interglacialperiod are relatively slow in comparison to the glacialperiod(s). Tai O Bay in the present day and during the pastthree interglacials represents a drowned highland coastlineof the ria type. The deposits evident from the boreholes aremainly shallow marine to estuarine in nature. Duringglacial periods, following sea-level lowering and exposureof the continental shelf, erosion rates increased. This isevident from the terrestrial deposits T1–T4 units foundpreserved in boreholes BH6 and BH9. These deposits areidentified to represent colluvial–alluvial fans associatedwith boulders, cobbles, gravels and sands. Proximaldeposits are recognized in the T4 unit grading into distaldeposits in the T1–T3 units. This inference is alsosupported by other boreholes located further inland inTai O Bay (Tam, 2000) and in other localities in HongKong (Yim, 1999b). A reduction in vegetation coveraccompanying an increase in dryness is seen to favour thedevelopment of colluvial–alluvial fans.

The Tin Shui Wai locality in Deep Bay differs from TaiO Bay in that it is located on the fringe of the Yuen Longcoastal plain. Because of the low gradient of thesurrounding area, it represented an area with aeoliandeposition during the last glacial period. It is possible thatthe coastal deposits of the last interglacial period werereworked by wind under ‘dry’ conditions to give rise tothese deposits. These aeolian deposits were subsequentlyburied by shallow marine deposits during the post-8.2 kaevent transgression (Yim et al., 2006), reaching present sea-level elevation by around 6000 yr BP (Yim, 1999a).

The preservation of marine deposits of four interglacialperiods in Tai O Bay is indicative that the highstandtransgressive marine deposits are preserved without sig-nificant erosion during the following glacial episode. This isseen to be a difference from shelves with glacial impactwhere repeated occurrences of fast-moving ice streamsduring glacial periods were strong, e.g. northwesternEuropean continental margin (Sejrup et al., 2005). In orderfor Quaternary deposits from multiple interglacial–glacialcycles to be preserved, subsidence of the inner shelf part ofthe continental margin through sediment loading andhydro-isostasy are likely.

OSL dating is found to work only for the T1 unitsamples W1 and W7. In the case of sample W1, goodagreement was found using different protocols for fine-grain dating and coarse-grain quartz dating. The ages ofabout 30 ka are within the expected age range of 8.1–70 kafor the last glacial period. In contrast, the OSL agesobtained for sample W2 for deposits of the T3 unit clearlyunderestimate the expected age of 250–300 ka (Marine

Isotope Stage 8). At this stage of the study no explanationcan be given for the differences between the fine-grainresults (4105 ka) and the significantly younger quartz ageof 6876 ka. Because the equivalent dose determination inthe case of quartz fraction worked reasonably well, apossible explanation for the observed age underestimationcould be the result of erroneous dose rate estimations, forexample, by an underestimation of the true moisturecontent variability. The OSL ages are calculated assuminga constant dose rate since deposition, but disequilibria inthe U-decay series are a well-known problem in water-laiddeposits (Krbetschek et al., 1994). The marine andterrestrial deposits investigated here tend to behave asaquicludes and aquifers, respectively, thus enhancing theprobability for the occurrence of disequilibria. Further-more, the iron cementation in the palaeo-desiccated crustcould also cause disequilibria in the U-decay series due topost-sedimentary enrichments of iron oxide, resulting in anU excess. If this is the case, than the U content measured byNAA overestimates the initial U content, causing over-estimation of dose rates and age underestimation. Thesamples could not be tested for the occurrence ofdisequilibria because NAA does not yield the necessaryinformation about the concentration of the variousdaughter nuclides of the U-decay series. Further investiga-tions using low-level high-resolution gamma-spectrometryhave not been carried out yet. In the present stage, wecannot exclude the possibility of an erroneous dose rateestimation being the reason for the observed age under-estimation.A further reason for the shortfall in ages could be a loss

of the luminescence signal during the burial time. This isthe so-called fading phenomenon, which, if present, causesan underestimation of the equivalent dose in the case offeldspar-dominated signals of the polymineral fine grainsand would result in age underestimation. For example, theexpected age range of sample W3 is 90–140 ka (MarineIsotope Stage 5), whereas the OSL dating results yieldedages of between 32 and 36 ka. However, because of thegood agreement with the quartz results, which are ingeneral not supposed to be affected by fading problems, thepolymineral fine-grain dating results of samples W1 andW3 support the assumption that fading is unlikely to causesuch a severe age underestimation.Sample W5, one of the oldest samples was chosen for a

fading test. Several subsamples were artificially bleachedand irradiated with a known beta-dose of 23Gy andsubsequently stored at room temperature for 6 monthsbefore the SAR measurements. The ratio of the recoveredto the administered dose was 0.68. Consequently, thepresence of fading could not be ruled out, but as alreadydiscussed above, the DRTs based on the same SARprotocol also underestimated the given laboratory dosesignificantly. In sample W6 the ratio found, for example,was 0.79.Attenuation of ionizing radiation varies with the

moisture content of sediments and is more effective in

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sediments with water-filled interstices. The ‘as found’moisture content of the samples was determined by weightloss after drying, but these values (Table 1) are hardlyrepresentative of the changes in sediment moisture over thewhole burial time span. In order to take this into account,water content variations were included into the doserate calculations in the range as shown in Table 1.Nevertheless, the uncertainty related with this parameterrepresents a fundamental limitation in improving the OSLage error limits.

A major problem found is in the moisture content usedfor the final determination of sediment ages as waspreviously pointed out by Yim et al. (2002). Terrestrialdeposits within continental shelf sequences already possessan important advantage over their continental propercounterparts. This is because their moisture contents areless likely to be affected by post-depositional changesduring the interglacial period(s) when they were submergedbelow sea level. Nevertheless during glacial periods, due tohigh permeability of colluvial–alluvial fan deposits, theywould act as either as confined or unconfined aquifers andwould be affected by groundwater flow. Thus aeoliandeposits from arid and semi-arid environments where themoisture content is nearly zero yield the best results inluminescence dating. In view of this shortcoming, it isessential to carry out moisture content measurements ofoffshore sequences prior to carrying out luminescencedating.

8. Conclusions

Episodes of subaerial exposure of continental shelvesduring the Quaternary period are indicated by drownedterrestrial deposits and palaeosols developed on the pre-Holocene marine deposits. At the Tai O Bay site, fourinterglacial–glacial cycles with each cycle represented bymarine and terrestrial deposits have been identified. Themarine deposits were formed under shallow marine toestuarine conditions while the terrestrial deposits wereformed by colluvial–alluvial sedimentation. At the DeepBay site, only one interglacial–glacial cycle represented bymarine and terrestrial deposits have been found. Themarine deposits were mainly formed in a tidal mudflatwhile the terrestrial deposits were identified to representaeolian deposits.

OSL dating is found to provide valid ages for the T1 unitbut only minimum ages for the pre-T1 units. Luminescencedating of these deposits proved to be problematic,especially for those samples with expected depositionalages of more than 100,000 years. From the comparison ofvarious measurement protocols used for dating the fine-grain polymineral fraction, it is concluded that the SARprotocol seemed to work for both fluvial and aeoliandeposits of the last glacial age but failed for the olderfluvial deposits. The most profound data base regardingour fine-grain samples seemed to be produced by the MARprotocol, but the necessity of normalization procedures to

reduce the large errors in the De determination has to beemphasized. As long as the occurrence of disequilibria inthe radionuclide decay chains remains uncertain, aconclusive explanation for the significant age underestima-tion cannot be given. At this stage of the study, most of theages obtained have to be interpreted as minimum ages.Although it is not possible to obtain reliable absolute

OSL ages on terrestrial units older than the last glacial age,density and moisture content distribution together withstratigraphy are found to be useful in providing indicativeages consistent with the geological history. Episodes ofsubaerial exposure of the continental shelf would lead post-depositional palaeoenvironmental changes affecting thepre-Holocene interglacial marine units while the age ofterrestrial units when present may be determined bycountback. Terrestrial deposits present within continentalshelf sequences are therefore concluded to possess advan-tages over their continental proper counterparts in theirsuperior dating.

Acknowledgements

W.W.-S.Y. is funded by the Research Grants Council ofthe Hong Kong Special Administrative Region, China(Project nos. HKU 7126/00P and HKU 7024/03P). Thispaper is a contribution to the International GeoscienceCorrelation Programme Project no. 464 ‘Continentalshelves during the last glacial cycle: knowledge andapplications’ and the Commission on Coastal and MarineProcesses of the International Union for QuaternaryResearch. We are particularly grateful to Dr. L.S. Chan,W. Ng, Prof. W.N. Ridley Thomas, Prof. A.G. Wintle andProf. Dr. H.K. Wong for their assistance. We would alsolike to thank two anonymous reviewers whose constructivecomments helped to improve the paper.

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