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Global and Planetary Chang

Geochemical composition of inner shelf Quaternary sediments in thenorthern South China Sea with implications for provenance

discrimination and paleoenvironmental reconstruction

Shouye Yang a,⁎, Wyss W.-S. Yim b, Guangqing Huang c

a Department of Marine Geology, State Key Laboratory of Marine Geology, Tongji University, 1239 Siping Road, Shanghai 200092, PR Chinab Department of Earth Sciences, The University of Hong Kong, Hong Kong SAR, Chinac Guangzhou Institute of Geography, 100 Xian Lie Road, Guangzhou 510070, China

Received 6 October 2006; received in revised form 1 February 2007; accepted 7 February 2007Available online 15 March 2007

Abstract

Sediment samples were collected from a borehole in the northern South China Sea with the depositional age back to 400 ka BP,for grain size and geochemical analyses to constrain the sediment provenance and paleoenvironmental variability. Geochemicalindices of Th/Sc, Ti/Nb and Th/Nb ratios suggest that the Zhujiang (Pearl River) was the main provenance of the inner shelfsediments of Hong Kong deposited during interglacial periods, whereas the locally-derived granitoids contributed significantly tothe exposed inner shelf through the incision of local streams during glacial periods. Furthermore, the influence of the Zhujiang-derived sediments on the inner shelf of Hong Kong varied spatially and temporally with different sea-level changes during the past400 kyr. Chemical weathering indices suggest hot and wet climate conditions were dominant in South China during interglacialperiods of marine isotope stages (MIS) 7, 9 and 11 whereas a dry and cold paleoclimate prevailed during glacial periods of MIS 6which accounts for weak chemical weathering and coarse-grained deposition on the inner shelf. The Holocene and last interglacialperiod did not see more intense chemical weathering in the Zhujiang drainage basin than other interglacial periods. Although thehigh resolution paleoenvironmental changes can not be easily reconstructed due to ubiquitous unconformity in the sedimentarystrata and weak age controls compared to the deep sea sedimentation, the present study sheds new lights on the understanding ofthe transport process of the Zhujiang sediment in the deep ocean and provides a teleconnection of East Asian palaeomonsoonactivity between South China, the inland and open sea areas.© 2007 Elsevier B.V. All rights reserved.

Keywords: geochemical composition; provenance discrimination; paleoenvironmental change; South China Sea; Hong Kong; Quaternary;continental shelf; East Asian Monsoon

1. Introduction

Flux and fate of terrestrial sediment in marginal seashave become the main research goals of the Interna-

⁎ Corresponding author. Tel.: +86 21 6598 9130; fax: +86 21 65982208.

E-mail address: [email protected] (S. Yang).

0921-8181/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.gloplacha.2007.02.005

tional Geosphere and Biosphere Program (IGBP) and itscore project, Land Ocean Interaction in the Coastal Zone(LOICZ), and the MARGINS Program (Syvitski, 2003;Syvitski et al., 2005). East Asia fosters several of largestrivers in the world including the Huanghe (YellowRiver), the Changjiang (Yangtze River) and theZhujiang (Pearl River) in China and is surrounded bytypical marginal seas, such as the Yellow Sea, East

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208 S. Yang et al. / Global and Planetary Change 60 (2008) 207–221

China Sea and South China Sea. Huge supplies ofterrestrial sediment into the marginal seas from theselarge rivers and small streams in the coastal highlands,including Taiwan Island, has global significance forstudying sediment and chemical flux budgets in theglobal ocean and understanding the source to sinktransport process of terrestrial carbon in continentalmargins (Milliman and Syvitski, 1992; Ludwig andProbst, 1998; Jia et al., 2002; Yim et al., 2002a; Yang etal., 2003; Thomas et al., 2004).

In the last two decades the South China Sea hasincreasingly attracted many research interests in paleo-ceanographic and paleoenvironmental reconstructionsin relation to East Asian monsoon evolution during theCenozoic (Wang et al., 1999; Lüdmann et al., 2001;Wehausen and Brumsack, 2002; Tamburini et al., 2003;Wang et al., 2003). In recent years, a strong link betweensediment supply from the Zhujiang and East Asianmonsoon activity during the Quaternary has beenestablished on the basis of paleoceanographic, mineral-ogical and geochemical studies on the South China Seasediments (Wang et al., 1999; Clift et al., 2002;Wehausen and Brumsack, 2002; Liu et al., 2003). Yetfew studies put emphasis on the inner continental shelfof the northern South China Sea, although the inner

Fig. 1. A sketch map of the study area showing the Zhujiang Estuary, the innsurface sediment samples from the Zhujiang Estuary. The dispersal patternsindicated. The tidal pattern is modified from Watts (1973).

shelf plays a significant role in directly linking the SouthChina continent and the South China Sea and thuscontrolling the source to sink process of terrestrialsediment. Thus far, the transport process and fluxvariability of the Zhujiang river sediment to the northernSouth China Sea during the Quaternary remain unclear.

Hong Kong lies on the northern coast of the SouthChina Sea at the eastern entrance of the Zhujiang Estuary(Fig. 1). The siliciclastic-dominated continental shelf ofHong Kong was tectonically stable during the lateQuaternary and the Quaternary offshore stratigraphy hasbeen widely investigated with extensive engineeringinvestigations on the inner shelf in the past 20 yrs (Yim,1994; Owen et al., 1998; Yim et al., 1998; Sewell, 1999).Consequently, the inner shelf of Hong Kong, serving asan important sediment sink and inevitable pathway of theZhujiang sediment transported to the open South ChinaSea, is considered to be the best site to investigate thenature of source-to-sink processes and dispersal patternsof the Zhujiang sediment in the northern South ChinaSea during the Quaternary. Previous studies suggestedthat most of the Quaternary inner shelf sediments ofHong Kong are derived from the Zhujiang although theriver sediment supply varies spatially and temporally(Yim, 1994; Owen et al., 1995, 1998; Yim et al., 1998;

er continental shelf of Hong Kong, the locations of Borehole BH2 andof the suspended sediments in the estuary and the tidal pattern are also

209S. Yang et al. / Global and Planetary Change 60 (2008) 207–221

Davis, 1999; Fyfe et al., 2000; Yim et al., 2002a; Owen,2005). However, this recognition is primarily basedintuitively on the huge sediment discharge of theZhujiang to the estuary and lacks substantial evidencesto support the distribution of the Zhujiang riversediments in the northern South China Sea. Furthermore,apart from the huge sediment supply from the Zhujiang,whether and to what extent small streams in the HongKong highlands and Taiwan Island supplied terrestrialsediments into the inner shelf of Hong Kong during theQuaternary awaits clarification. Previous investigationshave shown that small Taiwanese rivers can transport alarge quantity of sediment (about 6.96×108 ton/yr) intothe surrounding continental margins (Milliman andSyvitski, 1992; Ren, 2003).

Geochemical approaches have been proven as beingpowerful in identifying sediment provenances in theEast Asian marginal seas (Clift et al., 2002; Li et al.,2003; Yang et al., 2003; Yang et al., 2004a; Clift et al.,2006; Yan et al., 2007). Previous geochemical studies onthe offshore sediments of Hong Kong primarily focusedon organic carbon and stable isotopes (Owen et al.,1995; Yim et al., 2002a; Owen and Lee, 2004; Owen,2005), and heavy metals (Yim and Fung, 1981; Tanneret al., 2000; Li et al., 2000, 2001). Remarkablecontaminations of organic matter and heavy metals onthe surface sediments are primarily resulted from rapidurbanization, industrialization, and reclamation ininland and coastal areas. A geochemical proxy,however, has not been established to study the transportprocess of the Zhujiang sediment into the Hong Kongoffshore area and to reconstruct Quaternary paleoenvir-onmental changes around Hong Kong waters.

This paper presents geochemical analyses of theQuaternary sediments collected from a vibrocore takenfrom Hong Kong offshore area. After examining thevariations of geochemical compositions of the boreholesediment, we develop geochemical proxies to identifythe origins of the Quaternary inner shelf sediments,especially to discriminate the Zhujiang-derived sedi-ment from those other sediment provenances. Further-more, geochemical indices are also applied to decipherthe Quaternary paleoenvironmental changes on the innershelf of Hong Kong.

2. Regional setting and the Quaternary stratigraphyof Borehole BH2

2.1. Geographic, geological and hydrological settings

Hong Kong is characterized by a subtropical climateregime with an annual temperature of about 22 °C and

rainfall of 2300 mm. The present-day climate is typicalof the East Asian monsoon, and the summer time is hot(averaging 28.8 °C in July) and humid caused by thesouthwest monsoon. The winter is mild and dry due tothe northeast monsoon. Typhoons occur often in thesummer and autumn and play an important role in thesediment redistribution of the Zhujiang Estuary andinner shelf area (Huang and Yim, 2001; Yim et al.,2002a). Geologically, Middle Jurassic to Lower Creta-ceous volcanic rocks with andesitic and rhyoliticcomponents and granitic rocks occupy about 85% ofthe land area of Hong Kong (Sewell and Campbell,1997; Fyfe et al., 2000). A sedimentary succession ofDevonian, Carboniferous, Permian, and Lower Jurassicsedimentary-dominant rocks is overlain by igneousrocks, and Lower Cretaceous to Lower Tertiary sedi-mentary rocks overly the volcanic and granitic rocks.The solid rocks onshore are unevenly mantled bymiddle-late Pleistocene to Holocene colluvial andalluvial deposits (Sewell, 1999). A regolith, or mantleof deeply weathered rocks, especially those of andesiteand granodiorite, occurs over most of Hong Kong landarea (Shaw, 1995, 1997; Sewell, 1999; Fyfe et al., 2000).

Present-day streams entering the northern SouthChina Sea from Hong Kong are all small in size andhave low water and sediment discharges relative to theZhujiang River (Yim et al., 2002a). The Zhujiang is thethird largest river flowing into the South China Sea, anddischarges approximately about 300–370×109 m3 offreshwater and 69–100×106 tons of suspended sedi-ment annually into the estuary (Wang and Aubrey, 1987;Zhang et al., 1999). In the present day, the Zhujiangfreshwater primarily occupies the west part of theestuary while the saline shelf water mass invades theestuary from the western and eastern sides of the LantauIsland of Hong Kong and dwells on the southern partand adjacent outside area. The strong ebb tidal currentstransport Zhujiang-derived suspended sediments intothe Hong Kong inner shelf via the deep tidal channelswhich previously served as the incised valleys duringglacial periods with lowstand of sea level (Watts, 1973;Fyfe et al., 2000; Fig. 1).

The oceanic and coastal currents in the South ChinaSea also have a great influence on the hydrology andsediment redistribution of Hong Kong waters. TheSouth China Sea current pattern changes from summerto winter in response to the monsoonal wind circulationpatterns. During the winter, both the Taiwan Current andthe Kuroshio Current affect Hong Kong coastal waters(Morton and Wu, 1974). The Taiwan Current carryingcolder water of relatively low salinity southwest fromthe East China Sea mixes with the high-salinity warm


waters of the Kuroshio Current sourced from the Pacificinto the South China Sea via the Luzon Strait. Duringthe summer, the Hainan Current has a dominantinfluence, carrying highly saline warm water up to thenortheastern coast of the South China Sea (Morton andWu, 1974; Fyfe et al., 2000; Parry, 2000).

2.2. Quaternary stratigraphy of Borehole BH2

Early in 1961 Niino and Emery identified relict sandysediments on the middle-outer shelf of the northernSouth China Sea (Niino and Emery, 1961), which wereformed during the last glacial maximum and are nowexposed on the sea floor. In contrast, the inner shelf ofHong Kong is primarily floored by featureless muddysediment, except for some tidal channels where bedrockis exposed and receives no deposition. In the pastdecade, the Quaternary strata on the inner shelf of HongKong has been widely investigated using a wealth ofgeophysical and borehole data (Yim, 1994; Fyfe et al.,1997; Owen et al., 1998; Yim et al., 1998; Fyfe et al.,1999; Davis, 1999). In the present study we adopt theoffshore Quaternary geological model proposed by Yim

Fig. 2. Lithofacies and grain size compositions of the Borehole BH2. The Quathe Quaternary offshore geological model by Yim (1994). Mz: mean grain s

(1994) to identify the depositional sequence of a 45 mlong research core, Borehole BH2, which was drilled byFugro Geotechnical Services Hong Kong Limited in2001. The borehole was taken from the West LammaChannel east of Cheung Chau Island (Fig. 1) at a waterdepth of 12.2 m below Principal Datum (the Hong Kongreference datum which is approximately 1.23 m belowmean sea level). The borehole comprises five marinesedimentary units (M) and three terrestrial units (T),which are dated back older than 400 ka BP, followingYim (1994)'s model (Fig. 2).

The Holocene unit M1 (0–10.0 m) is mostlycomposed of very soft to soft, grey clayey silt withshells and shell fragments. Sandy sediments occur at thebottom. The mean grain size is 5.44 Φ (Fig. 2). TheAMS carbon-14 dating results measured on paphiaundulate yield uncalibrated ages of 6370±40 aBP at7.2–7.4 m and 6590±45 aBP at 9.2–9.4 m respectively.

Marine unit M2 (10.0–13.3 m) contacts the overlyingHolocene sediment with a distinct unconformity andmainly consists of soft to firm, grey, homogeneousclayey silt with few shell fragments and humus layersand mottles. An upward-increasing trend of sand

ternary stratigraphic classification and depositional ages are based uponize (unit: Φ).


content is apparently present in this marine unit, with amean grain size of 5.22 Φ (Fig. 2). This unit wasinterpreted to be estuarine and littoral facies formedduring the last interglacial period of Marine IsotopeStage 5 (MIS 5, about 80–140 ka BP; Yim, 1994) basedupon the absolute uranium-series dates and thermolu-minescence ages of similar stratigraphic units in HongKong offshore area (Yim et al., 1990, 2002b).

Terrestrial unit T2 (13.3–16.6 m) comprises uncon-solidated, yellowish, sub-rounded to sub-angular sandand gravel. This featureless medium to coarse-grainedsand and gravel has a mean grain size of 0.66 Φ andsand contents higher than 90% (Fig. 2). The sedimentscontain abundant quartz and potassium feldspar. Shellfragments and plant debris are absent. The distinctlithofacies of this terrestrial sediment are diagnostic ofits fluvial-colluvial origins and it was deposited duringthe second last glacial period (MIS 6, about 150–180 kaBP; Yim, 1994). Similar sandy layers occur often on theinner shelf of Hong Kong and this unit is distinguishedfrom other depositional units by its characteristicsedimentary facies and mineralogical compositions(Yim, 1994; Owen et al., 1998).

Marine unit M3 (16.6–19.8 m) contacts sharply withthe overlying Unit T2 and primarily consists of firm,mottled grey and brown clayey silt, with a mean grainsize of 6.17 Φ. Plant debris and humus mottles areoccasionally present, whereas shell fragments areabsent. A desiccated crust occurs in the upper 1 m ofthe unit, which can also be observed in nearby BoreholeWB7 (Yim and Li, 2000; Tovey and Yim, 2002; Yim etal., 2002a). The sediments are considered as littoral toneritic facies formed during MIS 7 (about 190–240 kaBP; Yim, 1994).

Terrestrial unit T3 (19.8–25.8 m) is mainly composedof brown sandy silt, with gravels occurring at the top,constituting an upward-coarsening sequence (Fig. 2).Leaf and root debris occurs occasionally in the lowerpart. The silty sediments are of fluvial origin and have amean grain size of 5.36Φ, obviously finer than the upperunit T2. An equivalent depositional unit from a boreholeclose to the Hong Kong International Airport yieldsthermoluminescence dates of MIS 8 (about 250–300 kaBP; Yim, 1994; Price et al., 1996; Yim et al., 2002b).

Marine unit M4 (25.8–31.2 m) is primarily com-posed of firm, mottled grey and brown clayey silt at thetop 1 m, grey silt in the middle-lower part, and dullyellow to bluish grey sandy silt at the bottom. Humuslayers and plant debris are well preserved at the top,indicative of a desiccated crust. The unit sediments havea mean grain size of 5.69 Φ, similar with the overlyingmarine units. This unit is interpreted as estuarine-littoral

deposition during third last interglacial period (MIS 9,about 310–340 ka BP; Yim, 1994).

Marine unit M5 (31.2–40.0 m) comprises a distinctdesiccated crust at the top, which is characterized bystiff, mottled yellowish brown clayey silt with humuslayers. The middle-lower part is composed of stiff, greysilt interbedded with several thin find sand layers, andwave laminations occur occasionally. Fossils are absentin this unit. These marine sediments were formed underan estuarine–intertidal environment during MIS 11(about 380–420 ka BP; Yim, 1994).

The bottom unit T5 (40.0–41.8 m) consists of firm,locally mottled yellowish brown and white–grey clayeysilt. Quartz, feldspar and kaolinitic minerals are abundant,whereas plant debris and humus layers are absent. Thedistinct sedimentary facies characterizes a fluvial depo-sitional environment. The bottom of the borehole isunderlain disconformably by regolith comprising com-pletely weathered granite, which constitutes the base ofthe Quaternary strata on the inner shelf of Hong Kong.

3. Samples and methods

A total of 185 subsamples from the Borehole BH2were collected for grain size measurements. For majorand trace elemental determination, a total of 74subsamples were selected from the core according tothe sediment character. Meanwhile, 20 surface sedimentsamples were collected from the Zhujiang Estuary by agrab sampler in 2004 for geochemical analysis.

Grain size of the core sediments was measured bylaser particle size analyzer (Coulter LS 230) in the StateKey Laboratory of Marine Geology, Tongji University,after removing organic matter and biogenic carbonatefrom the samples with 10% H2O2 and 1N HCl,respectively. The b63 μm fine-grained fraction wasused in this study for geochemical analysis in order tominimize grain size effect on the chemical compositionof the sediment (Yang et al., 2002, 2004b). The b63 μmfraction was separated from bulk sediments in deionizedwater by a wet sieving method, dried at 50 °C in a cleanoven, and ground in an agate mortar. Sediment powder of0.125 g powdered subsamples was digested withconcentrated HF, HNO3, and HClO4 in an airtight Teflonvessel, following the method by Yang et al. (2002,2004b). Concentrations ofmajor and trace elements weremeasured by Inductively Coupled Plasma-AtomicEmission Spectrometry (ICP-AES, JY-38S) in the StateKey Laboratory of Mineral Deposit Researches, NanjingUniversity. Geostandards of GSS-5 were analyzed withthe sample sets in order to determine analytical precisionand accuracy. Relative deviations between measured and


certified values are less than 5% for most elements withthe exception Ca, Cr and V (5–9%).

4. Results and discussions

4.1. Compositions of major and trace elements

Average concentrations of major and trace elementsin the Borehole BH2 sediments are given in Table 1.Major and trace elements show variable trends inconcentrations in different depositional units (Figs. 3and 4). Most elements with the exception of K aregreatly depleted in sandy sediments of Unit T2 relativeto other depositional units and moreover, Na, Ca, Mg,and Sr yield higher concentrations in the Holocene unitM1 than in the underlying sediments (Figs. 3 and 4).Overall, the terrestrial units, especially Unit T2, havelarger compositional variations for most elements thanthe marine units and, particularly, Unit M1 has smallvariations with the coefficients of variations mostly lessthan 20% (Table 1). Compared to the Zhujang estuarinesediments, the Borehole BH2 sediments are character-ized by lower concentrations of most transition elementssuch as Fe, Ti, Mn, Co, Ni, Cr, V, Zn, Pb, and Nb, but arerelatively enriched in K and Na (Table 1). Otherelements such as Th and Sc exhibit similar concentra-tions between the core and the Zhujiang sediments.

Granitoids are dominant in the land surface area ofHong Kong and may be the potential provenance ofcoastal sediments, as suggested bymany previous studies.In general, the Hong Kong granite is enriched in K, Na,Al,Mn, Sr, and Th, but depleted inmost transitionmetals,particularly Fe, Co, Ni, Ti, Cr, V, Sc, Cu, Pb, and Zn,relative to the core sediments (Table 1). Geochemistry ofmore than 2200 sediment samples from Hong Kongstreams of various scales has been documented by Sewell(1999). Comparatively, the Hong Kong stream sedimentsare characterized by higher concentrations ofMn, Cu, Zn,Nb, and Th, and lower of Fe, Ti, Sr, Co, Ni, and Crcontents than the Borehole BH2 sediments (Table 1).However, detailed comparison of elemental compositionsbetween the core sediments, Hong Kong granite andstream sediments can not bemade in the present study dueto insufficient geochemical data from granitoids andstream sediments from Hong Kong in previous studies(Sewell and Campbell, 1997; Sewell, 1999).

4.2. Provenance discrimination of the Borehole BH2sediments

Many factors such as chemical weathering, dynamicsorting during transport, post-depositional diagenesis

and anthropogenic activity are responsible for compo-sitional deviation of detrital sediments from their parentrocks. The seasonally hot and humid climate regime ofHong Kong promotes active chemical weathering of therocks. Consequently, chemical weathering may have agreat influence on the compositional deviation ofweathered material from residual parent rocks. Previousstudies revealed that alkaline and alkaline earth elementsincluding Na, Ca, Mg, and K can be considerablyleached from weathered materials in Hong Kong(Sewell, 1999; Fyfe et al., 2000; Guan et al., 2001),and even ferrous iron and silicon can be attacked duringintense chemical weathering (Guan et al., 2001). Inaddition, the sediment from the inner shelf of HongKong may be readily contaminated from agriculture,industry, sewage treatment, domestic sources and illegaldumping of materials since Hong Kong is highlydeveloped and densely populated in coastal areas (Yimand Fung, 1981; Yim and Leung, 1987; Sewell, 1999;Tanner et al., 2000; Li et al., 2000, 2001; Zhou et al.,2004). Transition elements such as Cu, Zn, Pb, Mn, Cr,V, Fe, and Ni exhibit very large variations in concentra-tions in the sediments of Hong Kong streams and ofoffshore areas with the variation magnitude up to fiveorders for specific elements (Yim and Fung, 1981; Yimand Leung, 1987; Sewell, 1999). Although thesetransition elements are primarily bound in a minerallattice (residual fraction) in sediment they are notsuitable for provenance discrimination because they canbe easily contaminated during sediment transport anddeposition (Li et al., 2001). In contrast, elementsincluding REE, Sc, Ti, Co, Zr, Hf, Th, and Nb havebeen widely used for provenance discrimination of riverand marine sediments because they generally behaveconservatively in hypergene environments and residemostly in mineral lattices and are resistant to chemicalalteration (Taylor and McLennan, 1985; Rollinson,1993; Yang et al., 2003). Secular compositional changesof the above trace elements in the Zhujiang riversediments are considered to be small and negligiblebecause the large drainage basin of the Zhujiang isdominated by granitoids and carbonate rocks, whichhave relatively stable compositions for these conserva-tive elements (Zhang and Wang, 2001; Zhou et al.,2004). Therefore, we consider that the modern fine-grained seabed sediments (b63 μm) from the ZhujiangEstuary can represent the basic chemical compositionsof the Zhujiang sediment into the continental shelf of thenorthern South China Sea.

In the present study element ratios of Ti/Nb, Th/Nb,and Th/Sc were selected for identification of sedimentsource of the Borehole BH2, given that the ratios of these

Table 1Comparison of major and trace elemental concentrations between the sediments of Borehole BH2 and the Zhujiang River and the granite in Hong Kong (units: wt.% for K, Na, Ca, Fe, Mg, and Al; μg/gfor other elements). Avg denotes average value; CV is coefficient of variation; n is sample numbers. HK-granite data are from Sewell and Campbell (1997) and Sewell (1999); HK-river data arefrom Sewell (1999)

Units n K Na Ca Fe Mg Al P Ti Mn Ba Sr Co Ni Cr V Cu Zn Pb Nb Sc Be Li Th

M1-Avg 20 1.96 1.06 1.43 3.50 1.47 5.96 525 3939 456 341 112 11.8 24.9 88.4 72.6 69.5 106 136 19.7 8.98 1.99 74.9 22.5M2-Avg 8 2.00 0.77 0.97 3.62 1.21 4.70 623 4053 381 310 82.9 13.6 26.1 89.0 74.9 85.4 87.8 95.6 20.0 7.24 2.19 76.3 20.6T2-Avg 6 2.14 0.21 0.09 2.03 0.15 2.73 195 848 165 290 27.6 4.13 9.85 36.0 16.4 89.8 58.7 81.9 6.6 2.22 0.86 28.0 10.5M3-Avg 8 2.04 0.58 0.13 3.74 0.72 6.90 366 4646 105 502 51.9 12.9 29.2 83.5 90.4 59.9 111 145 18.3 10.5 2.28 73.2 20.2T3-Avg 5 1.89 0.51 0.18 2.80 0.58 5.48 300 2918 178 282 44.1 10.0 22.0 58.0 63.4 43.4 88.2 140 13.9 8.45 1.55 50.3 17.7M4-Avg 9 1.86 0.39 0.22 3.66 0.66 6.63 327 4640 190 293 49.3 11.5 22.0 70.3 79.8 49.7 80.1 148 18.2 9.90 1.96 76.2 20.2M5-Avg 15 1.92 0.54 0.35 3.91 0.80 5.99 434 4627 219 352 59.1 14.4 25.3 72.8 78.4 49.5 86.9 146 17.1 9.52 1.98 63.4 18.0T5-Avg 3 1.75 0.28 0.21 4.56 0.60 7.61 364 4641 187 343 49.5 10.6 23.7 91.9 86.7 50.8 83.3 145 18.2 12.4 2.08 70.5 21.0M1-CV 5 10 12 8 9 22 12 5 18 14 19 11 6 7 5 37 17 23 11 15 6 7 12M2-CV 4 14 68 15 15 35 81 8 16 17 58 15 13 8 12 38 19 28 11 29 15 13 19T2-CV 27 25 54 133 56 34 31 45 116 30 22 50 40 32 39 52 56 29 30 44 26 36 35M3-CV 8 21 34 26 26 33 22 10 52 77 27 45 38 15 14 41 53 31 14 35 35 16 16T3-CV 6 22 40 52 36 30 38 30 121 20 23 45 46 27 28 21 48 19 24 35 24 32 19M4-CV 11 27 34 36 29 14 29 16 97 28 19 21 29 22 20 20 11 15 10 22 16 14 11M5-CV 7 36 63 21 28 22 34 11 68 13 32 22 17 7 6 9 12 9 7 24 16 13 11T5-CV 10 57 13 31 14 9 27 14 25 13 19 17 9 10 11 8 14 6 9 12 9 7 17Zhujiang 20 0.81 0.24 0.58 4.73 0.85 4.68 1034 6339 662 367 52.5 16.5 49.1 107 103 84.1 240 176 25.2 7.84 2.58 63.0 18.5HK-granite 372 4.39 2.39 0.93 1.60 0.18 7.19 381 1544 528 429 151 2.6 8.6 20.6 4.1 2.9 29 39 18.1 b2 29.5HK-river 2246 0.48 2.88 3600 1007 317 35 6 11 49 43 102 141 104 38 9.4 44

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Fig. 3. Downcore variations of major elements in the Borehole BH2 sediments. Note the distinct variations of Na and Ca concentrations occurbetween the M1 unit and other depositional units.


elements can better offset the grain size effect than theabsolute concentrations of these elements. Discrimina-tion plots of these ratios distinctly show that most of thecore sediments have similar compositions to theZhujiang estuarine sediments, whereas all of Unit T2and part of the T3 sediments have more scattereddistributions and do not fall in the compositional range ofthe Zhujiang estuarine sediments (Fig. 5). In contrast,

Fig. 4. Downcore variations of selected trace elements in the Borehole BH2 ssignificantly low concentrations of most elements in the T2 unit.

Unit T2 sediments are more similar to the Hong Konggranite and stream sediments in chemical compositions.The chemical similarity of the Hong Kong streamsediments and the HongKong granite is evident, becausemost of the streams in Hong Kong drain the granite area.The chemical compositions of most sediments from themarine units M3, M4 and M5 and terrestrial unit T5 aremore similar to those of the Zhujiang sediments. In

ediments. Note a sharp decrease of Sr concentration in the M2 unit and

Fig. 5. Discrimination plot of Th/Sc vs Ti/Nb and Th/Nb vs Ti/Nb inthe core sediments and the comparisons with those Zhujiang estuarinesediments, the Hong Kong granites (Sewell and Campbell, 1997), andthe Hong Kong stream sediments (Sewell, 1999).


contrast, the chemical compositions of the marine unitsM1 and M2 sediments are just located between those ofUnit T2 and the other marine units (M3, M4 andM5) butmore approximate to the latter (Fig. 5).

Elements such as Ti, Sc, Cr, and Co are primarilyenriched in mafic rocks, whereas Zr, Nb, Th, Hf, and Taare more concentrated in felsic rocks. Therefore, higherTh/Sc ratios and lower Ti/Nb ratios in sedimentsindicate more felsic compositions of source rocks. Theplots of Ti/Nb vs Th/Sc and Th/Nb vs Ti/Nb suggest thatthe sandy sediments of the T2 unit have more felsicsource rock compositions, e.g. granitic rocks, than theother depositional units (Fig. 5).

Consequently, the discrimination plots suggest thatmost of the Borehole BH2 sediments were derived fromthe Zhujiang, especially those fine-grained estuarine-shelf sediments deposited during the MIS 7, 9 and 11

(units M3, M4 and M5), whereas the sandy fluvial–colluvial sediments of Unit T2 with the MIS 6 age weremostly sourced from the local provenance of granitoidsin Hong Kong. The marine units M1 and M2 formedduring MIS 1 and 5 respectively were largely sourcedfrom the Zhujiang and partly from local and/or othersources in terms of their chemical compositions.Geochemical and mineralogical studies of the deep-seacores from Ocean Drilling Program ODP Leg 184 havealso suggested that the Zhujiang could be one dominantsource of terrigenous sediments into the South ChinaSea (Clift et al., 2002; Li et al., 2003; Liu et al., 2003).

The Zhujiang is the largest river emptying into theSouth China Sea and has been considered to exert a greatinfluence on the sedimentation of the northern SouthChina Sea (Yim, 1994; Owen et al., 1995, 1998; Yim etal., 1998; Davis, 1999; Fyfe et al., 2000; Yim et al.,2002a; Owen, 2005). Borehole BH2 is situated southeastof the Zhujiang Estuary and in the central–western partof the Hong Kong offshore area. Therefore, it isreasonable to suggest that the Borehole BH2 sedimentsweremostly sourced from the Zhujiang although the coresite does not directly face the estuary because of theshelter of the Lantau Island (Fig. 1). The study on tidalcirculation pattern in the Zhujiang Estuary clearly showsthat strong ebb tidal currents can transport suspendedsediment via tidal channels from the Zhujiang innerestuary to the western and central parts of Hong Kongoffshore area (Watts, 1973; Fyfe et al., 2000; Owen,2005; Fig. 1). Nevertheless, the influence of the Zhujiangon the inner shelf sediment of Hong Kong might varyspatially and temporally with sea-level changes since theMiddle Pleistocene climatic transition. Sea-level fluctua-tions caused the extension and retreat of the ZhujiangEstuary during interglacial and glacial periods respec-tively. During interglacial periods with high sea level, thewestern and central parts of Hong Kong, such as theBorehole BH2 location, were dominated by estuarine,tidal flat and/or open shelf depositional environments.Deposition there was strongly controlled by the dispersalpatterns of the Zhujiang sediments through directdeposition of suspended particles from the dilutedfreshwater or through transport via tidal pathwayspreviously formed during the low sea-level stands withthe river incision (Owen, 2005). Furthermore, strongsouthwest summer monsoon during interglacial periodsprevailed in South China, resulting in abundant rainfalland intense chemical weathering in the Zhujiangwatershed, and consequently, causing high fluvialdischarge into the marginal sea (Wang et al., 1999;Wehausen and Brumsack, 2002). Therefore, the marinesediments from Borehole BH2 deposited in interglacial


periods were primarily derived by the Zhujiang, albeitwith different extents of sediment supply.

In contrast, the Zhujiang extensively incised thecontinental shelf and the river channel extendedsouthwards to the middle and outer shelf of the northernSouth China Sea during glacial periods, when sea-levelwas low (Owen et al., 1998; Fyfe et al., 2000; Owen andLee, 2004). Correspondingly, most of the Zhujiangsediments might be expected to have directly depositedin the middle-outer shelf of deep waters, and bypassedthe exposed inner shelf, including the study area, withonly part of the Zhujiang sediment being trapped. Incontrast, the local rivers might contribute significantly tothe exposed inner shelf. Sewell (1999) suggested that thenatural deep water harbor, tidal channel and steep,rugged topography in Hong Kong is largely the productof a drowned river valley system which developedduring glacial periods with the falling of sea level. In thepresent day there is no large drainage system in HongKong, similar to the Zhujiang scale. Streams on the steephillslopes are usually closely spaced with short and steepcourses, resulting in intense fluvial gulling and associ-ated soil erosion, particularly around the headwaters ofstreams in granite areas (Fyfe et al., 2000). The erodedmaterials can be readily transported by these streamsdirectly down hillsides to the coastal areas of HongKong. Likewise, the local streams extended and incisedthe exposed inner shelf with the falling of regional baselevel during glacial periods and deposited a largequantity of proximal sediments on the inner shelf.

Middle Jurassic to Early Cretaceous granitoids rocksare exposed over more than 50% of the present-day'sland area of Hong Kong (Sewell and Campbell, 1997;Fyfe et al., 2000). The site of Borehole BH2 issurrounded by granitic terrane of Lantau and KowloonIslands. The strong, northeastward trend of the majorvalleys and strike ridges across these major islands,therefore, account for the transport of weatheredgranitoid material into the research core site throughthe incised river valleys and/or via episodic landslideprocesses. Geochemical and mineralogical composi-tions, as discussed above, clearly demonstrate that theterrestrial sandy sediments of units T2 and part of T3 arecharacterized by granitic source rock compositionsindicative of proximal provenance. Units T2 and T3were formed during two glacial periods, MIS 6 and8 respectively. However, the sediment characters andchemical compositions are quite different between them.The T2 unit sediments are of the coarsest grain size inthe core and have more felsic source compositions thanthe T3 and T5 units, explicitly suggesting the domi-nance of local provenance at those times.

Local provenance of Unit T2 is also evidenced fromhigher K concentrations in the unit sediments than in theother units (Table 1; Fig. 3). Kaolin as a majormineralogical component of weathered rock masses inHong Kong is widely observed in weathering profileswithin rhyolitic tuffs and granitoid rocks (Parry, 1999;Fyfe et al., 2000). The kaolin primarily comprisespotassium-residing kaolinite and halloysite which varywidely in relative amounts. During glacial periods, theincised streams drained the weathering profile and thuscould have a high content of kaolin in the erodedsediment, which might account for higher K concentra-tions in Unit T2. In contrast, the less sandy sediments ofUnits T3 and T5 received a large quantity of fine-grainedsediments from the Zhujiang probably because theincision of the local streams during MIS8 and olderglacial/interglacial periods were not very intense asduring MIS6.

The small streams in Taiwan can be another sedimentprovenance of the northern South China Sea. It issuggested that the sediment yield of Taiwanese streamsmay reach 6.96×108 t/yr, in which about 45.8%sediments directly empty into Okinawa Trough, 36.6%into the East China Sea, and 17.6% into the South ChinaSea (Ren, 2003). Our preliminary research resultsindicate that the sediments originated from Taiwancontributed insignificantly to the inner shelf of HongKong compared to the Zhujiang and local riversediments. Furthermore, the Taiwan Current of colderwater and low salinity, which transports the sedimentfrom Taiwan into the Hong Kong offshore area can bemuch variable in current pathway during glacial andinterglacial periods. Hence, although a large quantity ofterrestrial sediments from Taiwan is presently trans-ported into the adjoining continental margins (Millimanand Syvitski, 1992; Ren, 2003), this did not exert a greatinfluence on the inner shelf deposition of Hong Kong.

4.3. Chemical weathering and paleoenvironmentalimplication

The degree of chemical weathering recorded insediments can be estimated by the chemical index ofalteration (CIA) and the diagram of Al2O3–(CaO⁎+Na2-O)–K2O (A–CN–K) (Nesbitt and Young, 1982). CIArepresents Al2O3/(Al2O3+CaO⁎+Na2O+ K2O)×100(molar contents, CaO⁎means CaO content in the silicatefraction of the sample). A higher CIA value indicatesstronger chemical weathering of the sediment during itsformation. A CIAvalue of 100 indicates strong chemicalweathering because all the alkali and alkaline earthelements are completely removed from the weathered


sediment. Average upper continental crust (UCC) andunaltered granite rocks have CIA values of about 50 and47, respectively. Plagioclase and potassium feldsparhave a CIA value of 50. Since chemical weathering istightly related to climate condition, these geochemicalproxies have been widely used to reconstruct thepaleoenvironmental changes of various marine andterrestrial sediments (Nesbitt and Young, 1982; Nesbittet al., 1996; Vital and Stattegger, 2000; Yang et al.,2004a,b; Singh et al., 2005).

The CIA in Borehole BH2 sediments displays asystematic variation with an obvious shift between theM3 and T2 units (Fig. 6). The upper part of UnitsM1,M2and T2 have lower CIA values than the underlyingdepositional units, suggesting the upper sedimentsexperienced relatively weaker chemical weathering thanthe lower sediments. Unit T5 sediments have the highestCIA values on average in the core, coinciding with itssediment character and mineralogical compositions. Thisunit mainly consists of firm to stiff, locally mottledyellowish brown and grey silt and sandy silt withabundant quartz grains. These features, as well asgeochemical compositions, suggest that Unit T5 sedi-ments are derived largely from the Zhujiang and partlyfrom local provenance with almost completely weatheredsource rocks. Although there is no definite age control ofthis unit (probably N440 ka BP; Yim, 1994), we infer thatstrong and continuous chemical weathering imprinted inthe sediments mainly because of long and recycled

Fig. 6. Downcore variation of the chemical index of variation (CIA,Nesbitt and Young, 1982) in the Borehole BH2 sediments.

exposure history of the source rocks. Overall, Units M3,M4 andM5 have higher CIAvalues than Unit T3 (Fig. 6),indicating that degrees of chemical weathering arestronger during interglacial periods (MIS 7, 9, and 11)than in glacial periods (MIS 8). The significantly higherCIA values in these marine units compared to theoverlying Units M1 and M2 indicate the chemicalweathering was more intense during MIS 7, 9 and 11than in MIS 1 and MIS 5. In comparison, the terrestrialunit T2, with local sediment dominant, has the lowestCIAvalue on average in the core (Fig. 6), implying that arelatively dry and cold climate dominated in Hong KongduringMIS 6 and caused weak chemical weathering. TheCIA values of the M3, M4 and M5 sediments varybetween those of the Changjiang (Yangtze River)sediments (ranging of 64–75, averaging 69, Yang et al.,2004a) and the Zhujiang estuarine sediments (rangingfrom 66 to 89 with an average of 81). This means that thepaleoclimate in South China during the MIS 7, 9, and 11was somewhat hotter and wetter than the present-day'sclimate in the Changjiang drainage basin in the humidsubtropic zone. However, the southern Chinese climatedid not reach the condition of today's Zhujiang drainagebasin in the subtropic to tropic zone with an annualtemperature of 22 °C and precipitation of 1600 mm. TheCIA of the terrestrial T2 unit yields an average of 58.8,close to the averaged value (55) of the Huanghe (YellowRiver) sediments, suggesting that the paleoclimate duringthe MIS 6 was comparable with today's Huanghedrainage basin in the arid to subarid temperate zonewith an annual temperature of 13 °C and rainfall of476 mm (Yang et al., 2004a).

The Quaternary paleoenvironmental changes re-corded in Borehole BH2 are tightly related to thevariability of Asian paleomonsoon activity in SouthChina, as suggested by many previous studies in thenorthern South China Sea (Sarnthein and Wang, 1999;Wang et al., 1999; Jia et al., 2002; Wehausen andBrumsack, 2002; Jian et al., 2003; Liu et al., 2003;Tamburini et al., 2003; Wang et al., 2003). Based onthese preliminary results, we infer that during the glacialperiods, such as MIS 8 the winter Asian winter monsoonwas not so severe and the summer monsoon wasrelatively strong, although weaker than in the intergla-cial stages, such as MIS 7, 9 and 11, which broughtabout abundant rainfall for chemical weathering inSouth China. A similar case has also been observed inthe lower Changjiang drainage basin based upon theXiashu Loess study (Yang et al., 2004b). Based on abroad multiproxy-data set obtained from the cores in thenorthern South China Sea Wang et al. (1999) alsosuggested the glacial periods during the late Quaternary

Fig. 7. Pair diagram of CIAvs Ti/Nb ratio in the core sediments and thecomparisons with those Zhujiang estuarine sediments and the HongKong granites.


were characterized by a strongly intensified wintermonsoon and weakened summer monsoon with conti-nental aridity in subtropical South China, whereas theinterglacial periods were dominated by strengthenedsummer monsoon circulation which caused extremecontinental wetness in South China. Nevertheless, ourstudy further revealed that during some glacial periods,such as MIS 8, the winter monsoon was not so strongand the summer monsoon was strong enough to produceconsiderable precipitation for chemical weathering.

The lowest CIA values of the Holocene unit M1occur at the bottom of this section (Fig. 6), implying thatwith the onset of the Holocene, the paleoclimate wascold and dry relative to the middle and late Holocene.According to the absolute CIA values, we infer that thepaleoclimate at that time was similar with today'sclimate condition in the Huanghe drainage basin. Thenear-global “8.2-kyr-BP” cold event in the earlyHolocene has been well documented in the low latitudesof Asia continent and marginal seas, which is suggestedto strongly link with global climate variability (Gupta etal., 2003; Rohling and Pälike, 2005; Wang et al., 2005).The AMS carbon-14 dating of the BH2 sediments gavean uncalibrated age of 6590±45 yrs BP at 9.2–9.4 m,which indicates that this cold climate is not matchedwell with the global “8.2-kyr-BP” cold event. Due to thelack of more reliable age constraints and of more corecorrelations, we are not sure whether this Holocene coldevent in the Borehole BH2 happened locally or hasregional meaning.

The pair diagram of CIA vs Ti/Nb plots the BoreholeBH2 sediments into three clusters (Fig. 7). Most of theM3, M4 and M5 sediments have similar values of CIAand Ti/Nb ratios, close to those of the Zhujiangsediments, which further suggests that these marinesediments are predominantly sourced from the Zhujiangand experienced intense chemical weathering under ahot and humid paleoclimate. The units M1 and M2sediments are largely derived from the Zhujiang andpartly from the local provenance of Hong Kong buthowever, experienced relatively weaker chemicalweathering compared to those units M3, M4 and M5sediments. The fluvial–colluvial sandy sedimentsformed during MIS 6 (unit T2) have the lowest valuesof CIA and Ti/Nb ratio, similar with those of the HongKong granitic rocks, clearly suggesting the localprovenance and very weak chemical weathering.

Likewise, the A–CN–K diagram shows that differentsediments from Borehole BH2 exhibit variable chemicalweathering trends (Fig. 8). The sediments that approachthe Al2O3 apex indicate more intense chemical weath-ering. Unit T2 sediments are paralleled with the Al2O3–

K2O line whereas most of the other sediments are moreapproximately linear with the Al2O3–(CaO+Na2O) line,suggesting that the T2 sediments have different sourcerock compositions and underwent variable weatheringdegrees. The fluvial–colluvial unit T2 sediments areultimately sourced from local and weakly weatheredgranitoids, but they deflect from the parent material ofthe Hong Kong granite in the diagram (Fig. 8). This ismainly due to abundant kaoline in the sedimentaccounting for the higher K concentrations. The marinesediments of units M3, M4, and M5 are closer to theAl2O3 apex than those of units M1 and M2, whichimplies that these sediments experienced strongerchemical weathering with significant leaching of Ca-and Na-bearing silicate minerals. Furthermore, theweathering trends of the core sediments, especiallythose of units M3, M4 and M5 sediments, suggestremarkable loss of K from the source rocks, probablyrelating to the dissolution of K-feldspar under humid andhot climate weathering conditions. The weatheringdegrees of units M3, M4 and M5 sediments are similarwith those of the present-day Zhujiang estuarinesediments (Fig. 8), again, suggest that the paleoclimateof South China during MIS 7, 9 and 11 was comparablewith the present-day's climate in the Zhujiang drainagebasin.

5. Conclusions

A continuous borehole with a depositional history ofabout 400 kyr was taken from the Hong Kongcontinental shelf and examined for grain size and

Fig. 8. The Al2O3–(CaO+Na2O)–K2O diagram showing the chemical weathering trends of difference sediments in the core. UCC: upper continentalcrust (Taylor and McLennan, 1985). Ksp: K-feldspar; Pl: plagioclase; Gi: gibbsite; Chl: chlorite; Kao: kaolinite.


chemical compositions. Five marine units and threeterrestrial units constitute the depositional sequences ofthe core, which is primarily composed of silt and clayeysilt.

Geochemical discrimination study suggests that theZhujiang is the main provenance of the sediments on theinner shelf of Hong Kong since the Middle Pleistoceneclimatic transition, especially for those fine-grainedestuarine-shelf sediments deposited during interglacialMIS 7, 9 and 11, whereas the local granitoids contributedsignificantly to the sandy fluvial–colluvial sedimentsformed during glacial MIS 6. During glacial periods withlow sea level, the Zhujiang incised the inner shelf and theestuary extended southwards to the middle and outershelf of the northern South China Sea, and contributedlittle to the sedimentation on the inner shelf of HongKong. In contrast, the small streams in Hong Kongsupplied a large quantity of local sediments to the innershelf areas. Furthermore, the sediment supply from theZhujiang and local rocks of Hong Kong to the northernSouth China Sea and even to the deep sea variedconsiderably with the shifts of the Zhujiang river channelon the shelf and the weathering intensity onshore.

Chemical weathering indices clearly suggest that hotand wet climate conditions prevailed in the Zhujiangdrainage basin during interglacialMIS 7, 9 and 11, whichled to abundant precipitation in the continent and intensechemical weathering of source rocks. In contrast, a dryand cold paleoclimate dominated in Hong Kong during

glacial MIS 6 and caused weak chemical weatheringonshore. The paleoclimate conditions during the Holo-cene and the last interglacial period were probably not ashot and wet as in MIS 7, 9 and 11, while glacial MIS8 was characterized by a relatively warm and humidclimate. The variability of the paleoclimate during thepast 400 kyr in South China is strongly related to thechanges of the East Asian monsoon activity. Thereconstruction of high-resolution Quaternary paleoen-vironmental changes on the inner shelf of Hong Kongwas greatly restrained due to extensive dissolution ofmicrofossils in the Quaternary sediments during subaer-ial exposure and the lack of continuous and reliableaging dates. However, geochemical indicators used inthe present study raise a possibility of reconstructing thepaleoenvironmental variability in the northern SouthChina Sea, where a strong land–sea interaction anddrastic sedimentary environmental changes during theQuaternary are recognized. More high-resolution re-search cores from the northern South China Sea arerequired for better understanding the transport process ofterrigenous sediments into the open ocean and recon-structing the paleoenvironmental changes, especiallyduring the Holocene period.

Acknowledgements

This work was supported by research grants awardedby the Hong Kong Research Grants Council (Grant Nos.


7024/03 and 7051/04), the National Science Foundationof China (Grant No. 40476029; 40676031), and by theShanghai Rising-Star Program (04QMX1430). We aregrateful to the suggestions and comments from twoanonymous reviewers and A. Chivas during thepreparation of the manuscript. We also thank L.W. Qiufor analytic work.

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