a 33 ma lithostratigraphic record of tectonic and ...a 33 ma lithostratigraphic record of tectonic...

(2006) 217–235www.elsevier.com/locate/margeo

Marine Geology 230

A 33 Ma lithostratigraphic record of tectonic andpaleoceanographic evolution of the South China Sea

Qianyu Li a,b,⁎, Pinxian Wang a, Quanhong Zhao a, Lei Shao a, Guangfa Zhong a,Jun Tian a, Xinrong Cheng a, Zhimin Jian a, Xin Su c

a State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, Chinab Department of Geology and Geophysics, University of Adelaide, Adelaide SA 5005, Australia

c School of Marine Science, China University of Geosciences, Beijing 100083, China

Received 12 January 2006; received in revised form 20 May 2006; accepted 31 May 2006

Abstract

The 853 m thick sediment sequence recovered at ODP Site 1148 provides an unprecedented record of tectonic andpaleoceanographic evolution in the South China Sea over the past 33 Ma. Litho-, bio-, and chemo-stratigraphic studies helpedidentify six periods of changes marking the major steps of the South China Sea geohistory. Rapid deposition with sedimentationrates of 60 m/Ma or more characterized the early Oligocene rifting. Several unconformities from the slumped unit between 457 and495 mcd together erased about 3 Ma late Oligocene record, providing solid evidence of tectonic transition from rifting/slowspreading to rapid spreading in the South China Sea. Slow sedimentation of ∼20–30 m/Ma signifies stable seafloor spreading inthe early Miocene. Dissolution may have affected the completeness of Miocene–Pleistocene succession with short-term hiatusesbeyond current biostratigraphical resolution. Five major dissolution events, D-1 to D-5, characterize the stepwise development ofdeep water masses in close association to post-Oligocene South China Sea basin transformation. The concurrence of local andglobal dissolution events in the Miocene and Pliocene suggests climatic forcing as the main mechanism causing deep watercirculation changes concomitantly in world oceans and in marginal seas. A return of high sedimentation rate of 60 m/Ma to the latePliocene and Pleistocene South China Sea was caused by intensified down-slope transport due to frequent sea level fluctuations andexposure of a large shelf area during sea level low-stands. The six paleoceanographic stages, respectively corresponding to rifting(∼33–28.5 Ma), changing spreading southward (28.5–23 Ma), stable spreading to end of spreading (23–15 Ma), post-spreadingbalance (15–9 Ma), further modification and monsoon influence (9–5 Ma), and glacial prevalence (5–0 Ma), had transformed theSouth China Sea from a series of deep grabens to a rapidly expanding open gulf and finally to a semi-enclosed marginal sea in thepast 33 Ma.© 2006 Elsevier B.V. All rights reserved.

Keywords: tectonics; paleoceanography; South China Sea; ODP Leg 184; Oligocene; Miocene; Pliocene; stratigraphy; basin evolution

⁎ Corresponding author. State Key Laboratory of Marine Geology,Tongji University, Shanghai 200092, China. Tel.: +86 21 65987968;fax: +86 21 65988808.

E-mail addresses: [email protected],[email protected] (Q. Li).

0025-3227/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.margeo.2006.05.006

1. Introduction

Ocean Drilling Program (ODP) Leg 184, the firstdeep-sea scientific drilling offshore of China in theSpring of 1999, recovered about 55 km of high-qualitysediment cores from 17 holes at 6 deepwater sites in the

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http://dx.doi.org/10.1016/j.margeo.2006.05.006

218 Q. Li et al. / Marine Geology 230 (2006) 217–235

northern and southern parts of the South China Sea (Fig.1). At the deepest site, ODP Site 1148, a sedimentsequence of 853 m was recovered, providing excellentmaterial for studying the long-term history of tectonicand climate variability in the region (Wang et al., 2000).

Due to few complete deep sea records exceeding20 Ma available so far, knowledge of the long-termrelationship between regional paleoceanographicchanges and global climate in the Cenozoic can onlybe attempted by piecing information together fromvarious sites. Zachos et al. (2001) compiled deep-seaoxygen and carbon isotope records from more than 40DSDP and ODP sites to reveal the global climatepatterns for the last 65 Ma. This composite recordsmoothed away the regional climate variability thatcould have been critical in linking the forcing mechan-ism of climate change on a regional or even a globalscale. The long sediment record from ODP Site 1148,

Fig. 1. Map showing ODP Leg 184 sites. Site 1148 is located on the lowe

therefore, is advantageous especially in providing aprecise time sequence from a single site for studies ofregional geohistory as well as paleoceanographic andclimate changes. A proper understanding of the SouthChina Sea history will add valuable insight into theevolution of the marginal seas in the western Pacific andtheir responds to global climatic changes through time(Wang, 2004).

By reassessing the completeness of this sedimentrecord in the light of postcruise results, we discuss inthis paper the possible causes by tectonics or dissolu-tion of ODP Site 1148 unconformities and how tectonicand paleoceanographic changes had affected localsedimentation and sediment composition since theearly Oligocene that could be viewed as evidence ofstepwise evolution in the sea basin and its circulation.In this summary we aimed to provide a long-termlithostratigraphic framework for further detailed studies

r slope of the northern South China Sea, in a water depth of 3294 m.

219Q. Li et al. / Marine Geology 230 (2006) 217–235

of the sediment record from the site and the SouthChina Sea region in general. We will emphasize theimportance and implications of this long sedimentrecord for deciphering the tectonic and paleoceano-graphic evolutionary history of the South China Sea.

2. ODP Site 1148 data sources

ODP Site 1148 (18°50.17′N, 116°33.94′E) is locatedon the lower continental slope in the northern part of theSouth China Sea, at a water depth of 3294 m (Fig. 1). Itis the most offshore site drilled during Leg 184.Precruise seismic survey revealed two closely packedreflectors (“double reflectors”) at about 5.0 two-waytravel time (TWT) (Fig. 2). The drilling conforms thatthe double reflectors blanket the entire upper Oligocenesection dominated by slumped sediments and corre-spond to great swings in all physical property andlogging curves (Fig. 3). Holes 1148A and 1148Bcombined unfold an 853 m thick Cenozoic sedimentsuccession (Wang et al., 2000) (Fig. 3).

In this synthesis, we mainly refer to the shipboardresults for lithostratigraphy (Wang et al., 2000).Postcruise nannofossil and planktonic foraminiferbiostratigraphic studies respectively by Su et al.(2004) and Li et al. (2004, 2005) used the old timescale of Berggren et al. (1995) that are updated here.Kuhnt et al. (2002) described Oligocene–middleMiocene benthic foraminifer biofacies, while Zhao(2005) discussed ostracod assemblages from the entire

Fig. 2. Seismic profile through ODP Site 1148, show

cored section. Measurements of carbonate percentageby weight in 1600 samples, and planktonic foraminiferfragmentation (by counting 8 broken pieces for anindividual test) and benthic foraminifer abundance inthe >150 μm fraction from 680 samples, were carriedout by Chen et al. (2002). Oxygen and carbon isotopesin nearly 1600 samples were measured on benthicforaminifer Cibicidoides wuellerstorfi and planktonicforaminifers Globigerinoides ruber and Globigeri-noides sacculifer using a MAT252 spectrometer in theState Key Laboratory of Marine Geology, TongjiUniversity (Zhao et al., 2001; Wang et al., 2003).Geochemical and grain size analyses were done by Li etal. (2003), Shao et al. (2004), and Wei et al. (in press),while Jia et al. (2003) reported their findings from astudy of black carbon δ13C. Clift (2006) presentedhematite and goethite mineral data in his discussion oferosion over continental Asia.

3. Stratigraphy

3.1. Sediment sequence and lithostratigraphy

The sediment sequence at ODP Site 1148 can bebroadly assigned to two main sections: a muchexpanded (mainly lower) Oligocene and the relativelyslower accumulated Miocene to Pleistocene, separatedby the slumped interval between 457 and 495 mcd(meter composite depth). The lower section representssyn-rift sediments filled during the later Paleogene

ing the “double reflectors” (Wang et al., 2000).

Fig. 3. Preliminary drilling results from ODP Site 1148 (modified fromWang et al., 2000). Note that the great swings in all physical property recordsfor Unit VI mark a period of intensive tectonic activities centered at ∼25 Ma.


rifting in half-grabens, whereas the upper sectioncorresponds to wider sedimentation during the broadsubsidence in the Neogene (Wang et al., 2000; Clift etal., 2001). Rapid sediment fill in the early Oligoceneindicates either the sink basin was narrow and of smallgraben-type or the availability of a large amount of

sediments was due to intensive weathering in theregion, or both. Paleobathymetric analysis based onostracod assemblages indicates three large stages ofdeepening for the Site 1148 locality: upper bathyal(<1500 m) in the early Oligocene, middle to lowerbathyal (1500–2500 m) in the late Oligocene to early


middle Miocene (26–14 Ma), and lower bathyal(2500–3300 m) since the late middle Miocene (∼14–0 Ma) (Zhao, 2005).

Seven lithostratigraphic units were identified basedon composition (especially clay versus nannofossils),depositional facies, and color variations (Wang et al.,2000). As summarized in Table 1, the Pliocene–Holocene are represented by Unit I, the Miocene isby Units II–V, and the Oligocene by Units VI andVII.

The Oligocene section exceeds 390 m, about 90% ofwhich, ∼350 m, are monotonous grayish to oliver-green, quartz-rich clay that accumulated during the earlyOligocene (Unit VII) at a sedimentation rate of over60 m/Ma (Figs. 3 and 4). Nannofossil chalk was mainlyconfined to the upper Oligocene in Core 1148A-50X,∼473–480 mcd (Fig. 5), although pieces of chalk mixedwith other sediments can be found up to about 420 mcd.The Oligocene/Miocene boundary coincides with theunconformity at the top of the slumped unit, andsediment-mixing due to slumps characterizes the upperOligocene (Unit VI) sediments that bracket the doubleseismic reflectors (Fig. 5) and signal a large-scaletectonic transition from rifting to spreading (Clift et al.,

Table 1Major features of Site 1148 lithostratigraphic units (data from Wang et al., 2

Litho unit Depth atbase (mcd)

Lithological characteristics

I 194.02 Well-bioturbated nannofossils clay with qupyrite concretions. Subunit Ia (0–146.02 mclay-rich, with more green clay layers andmottles, and common “iron sulfide” in theIb contains more frequent light intervals mof nannofossils.

II 328.82 Dominated by clay with frequent light-cololayers of nannofossils, few quartz grains ormicrofossils. Subunit IIa (194–260 mcd) isSubunit IIb is reddish brown colored.

III 360.22 Grayish green clayey nannofossil ooze witintercalations of dark reddish brown clayeyclay with nannofossils, with a sudden jumpthe base.

IV 412.22 Brownish nannofossil clay mixed sedimentamount of greenish gray nannofossil clay ia significant increase in the a⁎ value at the

V 457.22 Greenish gray nannofossil clay mixed sediwith nannofossil clay and very minor amounannofossils.

VI 494.92 Slumped clay-nannofossil mixed sedimentclay especially between 457 and 472 mcd;at 473–480 mcd.

VII 859.45 Intensely bioturbated sequence of monotonolive-green nannofossil clay.

mcd=meter composite depth.

2001). The lower Miocene (Units V and IV) consistsmainly of greenish to grayish brown nannofossil claymixed sediments, with common iron sulfide particles.Core recovery was complete in Miocene and youngersections, but decreased downhole especially in twointervals of the Oligocene: 475–560 mcd and 790–850 mcd (Wang et al., 2000) (Fig. 3).

3.2. Chronobiostratigraphy

In this paper, we updated the Site 1148 chronos-tratigraphy to the new geologic time scale of Gradsteinet al. (2004). The original chronologic framework wasmainly based on nannofossil and planktonic foraminiferbiostratigraphy according to the time scale of Berggrenet al. (1995). Postcruise nannofossil study by Su et al.(2004) identified 53 nannofossil datums, with theoldest, the last occurrence of Reticulofenestra umbilicus(32.50 Ma), found at 730.33 mcd. For the Oligocene–Miocene succession, Li et al. (2004) identified 34planktonic foraminifer datums, including 3 local eventsaffected mainly by dissolution. Table 2 lists the mostimportant stratigraphic events according to updatedages.

000) and sedimentation rates

Sedimentation rate(m/Ma)

Age

artz and abundantcd) is moreirregular green claymiddle part; Subunitarking the increase

∼60 (Ia), ∼15 (Ib) Pleistocene toPliocene

red, carbonate-richsiliceousolive-gray colored;

∼22 (IIa), ∼15 (IIb) Late to middleMiocene

h 10–50 cm thicknannofossil ooze orin the a⁎ value at

∼30 Middle Miocene

with a minorntercalations;base.

∼20 Early middle toearly Miocene

ment interbeddednts of clay with

∼30 Early Miocene

and nannofossila thin layer of chalk

<5 Late Oligocene

ous grayish >60 Early Oligocene

Fig. 4. A plot of bio- and chemo-stratigraphic datums (Table 2), showing calculated sedimentation rates, late Oligocene hiatuses, dissolution eventsD-1 to D-5, and various stages proposed as representing the stepwise evolution of the South China Sea.


A plot of the datums listed in Table 2, as shown in Fig.4, illustrates the basic patterns of ODP Site 1148chronostratigraphy, sedimentation rates, and unconfor-mities. The time resolution of these datums averages to0.4 Ma, becoming slightly finer for the late Miocene andyounger intervals.

Biostratigraphic results indicate several unconformi-ties in the upper Oligocene section, corresponding to thedouble reflectors on the seismic profile (Fig. 5) (Li et al.,2005). Two unconformities that respectively coincidewith the lower/upper Oligocene boundary at ∼488 mcdand Oligocene/Miocene boundary at ∼460 mcd aremarked by the concurrence of the last occurrences ofChiloguembelina cubensis (28.5 Ma) and Sphenolithusdistentus (27.3 Ma) near 488 mcd and Paragloborotaliakugleri (23.0 Ma) and Zygrhablithus bijugatus (23.8 Ma)close to 460 mcd. Two other unconformities were alsoidentified within the upper Oligocene section. The

missing section probably spans a time interval of about3 Ma, although all the slumped sediment could haveaccumulated in a short period of time centered on 25 Ma(Li et al., 2005).

Minor unconformities of 103–104 yr scales in thepost-Oligocene section, if there are any, cannot beconfirmed by current stratigraphic means. Therefore,the Neogene sediment section at ODP Site 1148 isconsidered largely intact, as indicated by the relativesmooth, orderly distributed age-diagnostic datums (Fig.4). However, a much condensed section between302.87 and 293.27 mcd is marked, respectively, bythe last occurrence of Cyclicargolithus floridanus(13.33 Ma) and the first occurrence of Discoasterkugleri (11.86 Ma) with a calculated sedimentation rateof only 6.5 m/Ma, indicating a possible short-termhiatus associated with dissolution event D-3 (Fig. 4).The existence of a short-term hiatus also explains the

Fig. 5. The late Oligocene unconformities, as defined by planktonic foraminifer and nannofossil datums and lithology, are suggested to cause the“double reflectors” (Fig. 2) (modified from Li et al., 2005).


absence of a δ13C maximum (CM6) at ∼13.7 Ma, soonafter ice sheet buildup on east Antarctica (Fig. 6).

3.3. Isotopic stratigraphy

Oxygen isotopic stratigraphy of ODP Site 1148 relieson the chronobiostratigraphic framework and correla-tion of the results with published standards such asZachos et al. (2001). This practice is more commonlyused especially for high-resolution Pleistocene recordsbecause many properly dated isotopic-climatic eventsfrom contemporary ice cores or caves are readilyavailable for correlation. Long isotopic records of over20 Ma with a time resolution of 104 to 105 yr are rare,

including northwest Pacific DSDP Site 289 (Savin et al.,1981) and southwest Pacific DSDP Sites 588 and 590(Kennett, 1986). Site 1148 isotopic results provide acontinuous record for the entire Neogene with 104 yrresolution (Fig. 6). These continuous isotopic sequencesare more advantageous over the composite curves ofZachos et al. (2001) in proving a single solid ground fordiscussing long-term paleoceanographic trends andevents albeit the isotopic signals from ODP Site 1148may contain more local information especially inrelating to the opening and closing of the South ChinaSea.

Several δ18O and δ13C events correlate well withtheir counterparts identified earlier from other

Table 2Selected age-diagnostic datums from Site 1148, including planktonic foraminifer datums (Wang et al., 2000; Li et al., 2004), nannofossil datums (Suet al., 2004), isotopic events (Zhao et al., 2001), and Brunhes/Matuyama boundary geomagnetic event (Wang et al., 2000), with ages adjusted to thetime scale of Gradstein et al. (2004)

Planktonic foraminifer datum mcd Ma Nannofossil datum mcd Ma

FO Globigerinoides ruber Pink 29.54 0.40 FO Emiliania huxleyi Acme 13.05 0.09FO Globorotalia truncatulinoides 130.63 1.93 LO Pseudoemiliania lacunosa 34.93 0.44LO Globoquadrina altispira 160.38 3.13 LO Reticulofenestra asanoi 59.75 0.91LO Globoturborotalita nepenthes 176.91 4.37 FO Reticulofenestra asanoi 78.20 1.14FO Sphaeroidinellopsis dehiscens 188.16 5.53 LO Calcidiscus macintyrei 102.52 1.61FO Globorotalia tumida 196.08 5.72 LO Discoaster brouweri 122.33 1.93FO Globigerinoides conglobatus 206.79 6.20 LO Discoaster pentaradiatus 136.60 2.39FO Globigerinoides extremus 244.26 8.93 LO Reticulofenestra pseudoumbilicus 169.67 3.70LO Globoquadrina dehiscens (local datum) 257.16 9.80 LO Amaurolithus spp. 175.11 4.80FO Neogloboquadrina acostaensis 259.70 9.83 LO Ceratolithus acutus 177.77 5.04LO Paragloborotalia mayeri 275.22 10.46 FO Ceratolithus rugosus 187.37 5.23FO Globoturborotalita nepenthes 283.78 11.63 LO Triquetrorhabdulus rugosus 190.37 5.28LO Globorotalia fohsi (local datum) 301.02 13.00 FO Ceratolithus acutus 190.37 5.35FO Globorotalia fohsi 303.28 13.41 LO Discoaster quinqueramus 193.31 5.58FO Globorotalia praefohsi 308.68 13.77 LO Nicklithus amplificus 198.57 5.98LO Praeorbulina glomerosa 312.38 13.80 FO Nicklithus amplificus 211.27 6.91FO Globorotalia praemenardii 317.98 14.38 FO Amaurolithus primus 219.37 7.36LO Globigerinatella insueta 320.97 14.66 FO Discoaster berggrenii 242.61 8.29FO Orbulina 320.37 14.74 FO Discoaster quinqueramus 242.61 8.29FO Praeorbulina glomerosa 344.18 16.27 FO Discoaster pentaradiatus 253.37 8.55FO Praeorbulina curva 352.98 16.40 LO Discoaster hamatus 256.37 9.40FO Praeorbulina sicana 355.39 16.97 LO Catinaster calyculus 261.81 9.67LO Catapsydrax dissimilis 364.88 17.54 LO Catinaster coalithus 267.47 9.69FO Globigerinatella insueta (local datum) 367.37 18.00 FO Discoaster hamatus 275.57 10.38LO Globoquadrina binaiensis 377.18 19.09 FO Catinaster calyculus 279.21 10.41FO Globorotalia praescitula 379.53 19.10 FO Catinaster coalithus 281.01 10.89FO Globigerinoides altiapertura 406.38 20.40 LO Discoaster kugleri 286.67 11.58LO Paragloborotalia kugleri 408.83 21.12 FO Discoaster kugleri 293.27 11.86FO Globoquadrina dehiscens 454.17 22.70 FO Triquetrorhabdulus rugosus 302.87 13.20FO Paragloborotalia kugleri 460.12 22.96 LO Cyclicargolithus floridanus 302.87 13.33FO Paragloborotalia pseudokugleri 475.77 25.40 LO Sphenolithus heteromorphus 308.81 13.53LO Paragloborotalia opima 478.52 27.10 LO Helicopontosphaera ampliaperta 331.87 14.91LO Chiloguembelina cubensis 487.77 28.50 FO Sphenolithus heteromorphus 370.17 17.71FO Globigerina angulisuturalis 601.66 29.40 LO Sphenolithus belemnos 371.57 17.95LO Turborotalia ampliapertura 634.46 30.30 FO Sphenolithus belemnos 390.97 19.03FO Paragloborotalia opima 663.32 30.60 FO Discoaster druggii 454.41 23.05

LO Sphenolithus capricornutus 458.57 23.11LO Sphenolithus delphix 458.57 23.11LO Reticulofenestra bisectus 461.57 23.20

Other events mcd Ma LO Zygrhablithus bijugatus 461.57 23.80B/M boundary 55.0 0.78 LO Sphenolithus ciperoensis 461.57 25.20Mi3 309.28 13.8 LO Sphenolithus distentus 476.10 27.30Mi2 (=CM3) 337.23 15.7 FO Sphenolithus ciperoensis 618.58 29.90CM-O/M (top) 432.38 22.0 FO Sphenolithus distentus 673.41 31.80CM-O/M (base) 458.98 23.0 LO Reticulofenestra umbilicus/

Reticulofenestra hillae730.33 32.50

mcd=meter composite depth.


localities, including Mi (Miocene oxygen isotopicmaxima) events and CM (carbon maxima) events(Miller et al., 1987, 1991; Woodruff and Savin, 1991;Zhao et al., 2001), providing extra points in tighteningthe ODP Site 1148 chronostratigraphy (Table 2; Fig.4). The absence of carbon maximum event CM6 from

∼13.7 Ma confirms the dissolution impact. A detailedanalysis of planktonic and benthic δ18O curves fromODP Site 1148 reveals findings similar to those fromthe neighboring ODP Site 1146 (Holbourn et al.,2005) especially in relation to orbital forcing andinfluence of monsoons in the middle Miocene (Tian et

Fig. 6. Standard chronostratigraphy (Gradstein et al., 2004; Berggren et al., 1995), sequence stratigraphy (1—Haq et al., 1987; 2—Hardenbol et al., 1998), deep-sea hiatuses (3—Keller and Barron,1983, 1987; 4—Spencer-Cervato, 1998), and ODP Site 1148 results for CaCO3%, CaCO3 MAR, planktonic foraminifer (PF) fragments, benthic foraminifer (BF) abundance, and benthic δ18O andδ13C with major Miocene glacial (Mi) and carbon maximum (CM) events. MMCT=mid-Miocene climate transition; EAIS=east Antarctic ice sheet. Benthic foraminifer assemblages 1 to 5 are fromKuhnt et al. (2002). Also marked are dissolution events D-1 to D-5 and the missing of CM6 (short dash line).

225Q.Liet

al./Marine

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230(2006)

217–235


al., submitted for publication). Compared to Zachos etal. (2001), the benthic isotopic results from ODP Site1148 show a shift at 16–15 Ma, characterized bylighter δ18O values and lighter δ13C values before andafter 15 Ma respectively. We interpret this isotopicshift at ∼15 Ma reflects the impact from thetermination of seafloor spreading in the South ChinaSea. Lighter δ18O values from ∼23 to 15 Ma appearto support the paleobotanic evidence for an earlyMiocene development of the East Asian monsoon(Sun and Wang, 2005) and relative humid conditionssurrounding ODP Site 1148 (Clift, 2006). In contrast,lighter δ13C values after ∼15 Ma could have beencaused by bottom water ventilation or even thedevelopment of a relative isolated local bottom watersince the end of spreading.

3.4. Summary of stratigraphy and sedimentation rates

Combined litho-, bio- and chemo-stratigraphies forODP Site 1148 indicate that the sediment sequencerecovered at this northern South China Sea site spans thelast 33 Ma, with the upper Oligocene truncated byunconformities up to ∼3 Ma duration (Li et al., 2005).The Neogene succession is relatively complete,although short-term hiatuses due to dissolution cannotbe ruled out (see next section).

The calculated sedimentation rates easily separate thesediment sequence into 4 major sections (Fig. 4). Thelower Oligocene section (Unit VII) and the younger,Pleistocene section (Subunit Ia) both register fastsedimentation rates of 60 m/Ma or more. The Miocenesection (Subunit Ib to Unit V) has slower average ratesof about 15–20 m/Ma. The upper Oligocene (Unit VI)shows the slowest sedimentation rate of <10 m/Ma if theslumped sediment can be similarly averaged forsedimentation rate calculation. Changes in sedimentcomposition and sedimentation rates may haveresponded to various evolutionary stages of the SouthChina Sea over the past 33 Ma, as discussed below.

4. Carbonate content and dissolution events

Postcruise analyses indicate strong variations in thecarbonate content and carbonate mass accumulation rate(MAR) (Fig. 6). Carbonate peaks (>45%) at 126, 202,240, 300, 410–425, and 460–476 mcd are followed byits decline especially between 260 and 410 mcd and anoverall low CaCO3 accumulation above 200 mcd. Verylow, <10% CaCO3 values occur at three intervals,respectively, 0–100 mcd, 130–160 mcd, and 310–335 mcd, all after the end of seafloor spreading at

∼16 Ma. A jump to over 70% CaCO3 (Fig. 6) and arapid increase in the median grain size to >10 (Fig. 7)immediately above the Oligocene/Miocene boundarysignify a significant change in the depositional regimeprobably relating to high carbonate production during anew phase of seafloor spreading.

The ODP Site 1148 locality now lies between themodern lysocline (∼3000 m) and the carbonatecompensation depth (CCD; ∼3500 m) (Wang et al.,2000). All calcareous components in the sedimentrecovered are affected at least partially by dissolution.The <150 μm residue contains more fragmentedplanktonic foraminifer tests than in the coarser fraction.When few tests are left in the >150 μm residue, theentire finer fraction often contains 100% finelyfragmented pieces or even totally barren, hamperingproper biostratigraphic and other analyses. The mostseverely affected intervals are found at 360 mcd, 347–349 mcd, 272–295 mcd, and 180–200 mcd. Theplanktonic foraminifer assemblage is dominated bysuch solution-resistant taxa as Sphaeroidinellopsis,Globoquadrina and Paragloborotalia found mostlyliving in deeper waters, also indicating preferentialdissolution effects (Li et al., 2004). From the >150 μmresidue, 10% fragmentation values of planktonicforaminifers first occur at ∼21 Ma, and those withover 20% fragments at 16–15 Ma and 12–11 Ma. From10Ma to∼8Ma and from 5Ma to 3 Ma and younger aretwo intervals characterized by assemblages with over50% fragments (Fig. 6). These high fragment values areaccompanied by low CaCO3% and MAR and highbenthic foraminifer abundance. Therefore, theCaCO3%, CaCO3 MAR, fragmentation and benthicforaminifer abundance together convey a clear messagethat 5 periods of intensive dissolution had occurred inthe Miocene–Pliocene South China Sea. Respectively at∼21 Ma (D-1), 16–15 Ma (D-2), 13–11 Ma (D-3), 10–9 Ma (D-4), and 5–3 Ma (D-5) (Fig. 6). Thesedissolution events indicate major steps in the changesof South China Sea deeper waters.

5. Geochemical characteristics

Similar to physical property records (Wang et al.,2000), a sudden change in geochemical elements andNd isotopes has been documented from the slumpedinterval of ODP Site 1148 (Li et al., 2003). Across 472–457 mcd, Sr, Zr, Nb, Ce, Th, Ti, Ga and Rb all decreaseand then increase sharply to a maximum, accompaniedby rapid increase in Th/La and Th/Cr ratios but decreasein Al/Ti, Al/K, Rb/Sr and La/Lu ratios (Fig. 7). The εNdvalue also decreased from about −10 to −13 and


remained at more negative values in younger intervals.These elemental changes appear to record tectonicdistortion and a rapid shift in sediment sources in the lateOligocene from earlier, mainly southwestern (Indochinaand Sunda Shelf) to later, mainly northern provenances(China mainland) (Li et al., 2003) (Fig. 7).

Further geochemical studies reveal other majorperiods of changes in elemental ratios in associationwith median grain size variations (Shao et al., 2004)(Fig. 7). For example, the distribution of rare earthelements (REE) was similar to the post-Archaean

Fig. 7. Geochemical results, showing changes in various elemental ratios (mvalues from the Oligocene section are indicative of a southern sediment sou

average Australian shale standard for the 3–0 Maperiod, but their values declined stepwise for olderintervals likely due to different sediment sources (Fig.8). However, unusual high heavy REE valuesoccurred in the 33–30 Ma and 28.5–23.5 Ma periods,when Ce from the light REE group also increasedsignificantly (Fig. 8). These elemental variationsmatch well with grain size changes (Fig. 7),apparently corresponding to various sediment produc-tion and transport dynamics at different evolutionarystages of the South China Sea.

odified from Li et al., 2003 and Shao et al., 2004). Less negative εNdrce from the paleo-Sunda region.

Fig. 8. Variations of rare earth elements (REE) after removing all carbonates are shown according to 7 different time intervals at ODP Site 1148 (Shaoet al., 2004). Together with less negative εNd values shown in Fig. 7, the elevated distribution of heavy REE in an overall low REE background fromsediments older than 23 Ma indicates (1) a sediment source from the south during the Oligocene, and (2) a change of source area to the north after theOligocene. PAAS=post-Archaean average Australian shale standard.


Together with clay mineral results, these elementalchanges may also imply intensive chemical weatheringat about 29.5 Ma for the south source region and atabout 23 Ma for the north source region (Wei et al., inpress). Based on the hematite/goethite ratio, Clift(2006) asserted that warm periods were associatedwith summer monsoons, particularly during the early toearly middle Miocene and from Pliocene onward. Thewhole South China Sea was relative humid during theearly Miocene before becoming drier in the north due tostrengthened winter monsoons (Sun and Wang, 2005),resulting in stronger N–S paleoceanographic contrastsince the late Miocene–Pliocene (Li et al., 2004).

6. Discussion

6.1. Completeness of the Site 1148 record

From various stratigraphic data mentioned above, itis apparent that the sediment sequence recovered atSite 1148 provides a unique archive of the SouthChina Sea history. The existence of a 32.5 Ma marinesediment about 120 m above the terminated depth(850 mcd) indicates that the first marine sedimentationat the site was at 34–33 Ma, close to the Eocene/Oligocene boundary. This 33-Ma-old marine sedimentmay signify the timing of the initial breakup of the seabasin, providing evidence of rapid rifting in the earlygeohistory of the South China Sea (Su et al., 1989;

Clift and Lin, 2001). High sedimentation rates of>60 m/Ma from the much expanded, ∼350 m lowerOligocene section characterize rapid subsidence andrapid sedimentation after the initial breakup. Theexpanded, ∼33–28.5 Ma lower Oligocene sectionshows no sign of large unconformities, and istherefore considered as complete although corerecovery is poor (Fig. 3) and age-diagnostic paleobio-logical datums are rare at certain intervals (Fig. 4).Featuring Nothia and Rhabdammina (respectivelybiofacies 5 and 4 of Kuhnt et al., 2002), the earlyOligocene benthic foraminifer biofacies show a strongaffinity to Tethys assemblage, indicating a continuousinfluence by deep water from the southwest up toprobably the late Oligocene (Fig. 6). A southwesternsource of Oligocene sediments mainly from Indochinaand Sunda Shelf is also endorsed by geochemistrydata (Li et al., 2003; Shao et al., 2004).

The upper Oligocene at ODP Site 1148 isdominated by displaced and distorted sediments,from which four major unconformities have beenidentified in the slumped lithostratigraphic Unit VI(457–495 mcd) (Fig. 5). The mid-Oligocene uncon-formity at ∼28.5 Ma coincides with many recordsfrom other parts of the Indo-West Pacific region,mainly responding to a significant uplift of theHimalayan–Tibetan Plateau to the west (Tapponnieret al., 2001) and the initial collision between Indonesiaand Australia in the south (Kuhnt et al., 2004; Li et


al., 2005 and references). A narrowed Indonesianseaway may have accounted for the brief lateOligocene warming and chalk deposition in thenorthern South China Sea including ODP Site 1148(473–480 mcd) (Fig. 5). The unconformities andslumps near the Oligocene/Miocene boundary indicatea very unstable tectonic regime (Clift et al., 2001).Centered at ∼25 Ma, these unconformities and slumpsapparently attribute to the great swings in all physicalproperty records and geochemical curves (Figs. 3 and5). The four unconformities together may have erasedabout 3 Ma late Oligocene record, and represent aperiod of rapid tectonic changes in the South ChinaSea and the SE Asia region at large. One possibility isthat they resulted from various stages of interactionand coupling between different land blocks and theirrotation (Hall, 2002) and a change in the seafloorspreading ridge from near E–W to NE–SW, asrecognized earlier at magnetic Anomaly 7 (Briais etal., 1993). In responding to these changes, mainsediment source area for Site 1148 also shifted fromthe south to the north after the end of the Oligocene(Li et al., 2003; Wei et al., in press).

For the Miocene and Pliocene succession, significantunconformities cannot be detected at least biostratigra-phically, especially those dissolution-induced hiatusesof <104 yr duration. However, an extremely lowsedimentation rate of only 6.5 m/Ma recorded for thesection between 302.87 and 293.27 mcd is considered asa distortion by dissolution event D-3 (Fig. 4). Thisundefined short-term hiatus caused the missing of aδ13C maximum (CM6) at ∼13.7 Ma from the isotopicrecord (Fig. 6).

6.2. Implications of Neogene dissolution events

The dissolution events described above combine therecords of CaCO3%, CaCO3 MAR, planktonic forami-nifer fragmentation and benthic foraminifer abundanceand, therefore, may imply the existence of minorhiatuses and the section not 100% complete (Fig. 6).The 5 major dissolution events at ∼21 Ma, 16–15 Ma,13–11 Ma, 10–9 Ma, and 5–3 Ma can be correlatedrespectively to deep-sea hiatuses NH1a, NH2, NH3–4,NH5, and NH7–8 identified from other oceaniclocalities (e.g. Keller and Barron, 1983, 1987; Ramsayet al., 1994) and to some major sequence boundariesmarking sea level low-stands (Spencer-Cervato, 1998;Hardenbol et al., 1998) (Fig. 6). The oldest dissolutionevent D-1, for example, can be viewed as a localmanifestation of global CaCO3 reduction at 23–20 Ma(Lyle, 2003). Periodic invasion of corrosive bottom

waters after a shift of bottom water source from a Tethystype in the early Oligocene to a Pacific type since theearly Miocene most likely caused the dissolution eventsidentified at ODP Site 1148. The concurrence of localdissolution events with open oceanic and continentalmargin records (Fig. 6) suggests a common climaticforcing. However, while the global cooling since thedawn of the Miocene contributed to continuous heavierδ18O values toward the Pleistocene (Zachos et al.,2001), the stepwise decline in the local CaCO3% recordand relative lighter δ18O values from ODP Site 1148(Fig. 6) may suggest the coupled impact by changes inglobal climate and regional water masses and monsoonvariability.

From a global database, Spencer-Cervato (1998)noted a higher number of hiatuses at high latitudes andon continental margins, suggesting various and morecomplex causes than a simple effect by climate coolingand/or corrosive deep water development. We havedemonstrated that the late Oligocene unconformities atSite 1148 (Fig. 5) were related to regional tectonicactivities (Li et al., 2005), but a similar tectonic forcingcannot be applied to most Neogene dissolution eventsidentified although their relationship with circulationchanges caused by newly altered geomorphology duringdifferent stages of basin evolution remains unclear.Here, we consider all Neogene dissolution eventsmainly affected by global climate coupled with bottomwater changes at various stages of South China Seabasin development because (1) they concurred withdissolution-induced hiatuses worldwide, (2) they con-strained the development of benthic communities suchas benthic foraminifers and ostracods, and (3) theirlithostratigraphic record varied through time. Forexample, Kuhnt et al. (2002) reported two early tomiddle Miocene benthic foraminifer assemblages,including subassemblages 2a and 2b, that apparentlyresponded to deep water changes in the northern SouthChina Sea region. Marking the first occurrence of somemodern species, the “Planulina wuellerstorfi” assem-blage (1) from 15.7 to 10 Ma is associated with deepwater ventilation (Fig. 6), as indicated also by a markedchange from greenish gray to reddish sediment color inthe uppermost part of Unit III (Wang et al., 2000) (Fig.3). Interestingly, the local deep water became ventilatedonly after the South China Sea ceased its seafloorspreading at ∼16 Ma (Briais et al., 1993) and CaCO3

sharply dropped to <5% at ODP Site 1148, ordissolution event D-2 (Fig. 6). Since then, the sitelocality reached a deeper bathyal setting of >2500 m,according to ostracod evidence (Zhao, 2005). Therefore,carbonate reduction and dissolution occurred when


different deepwater masses developed or their propertieschanged, largely in responding to the production of theproto-Antarctic bottom water (AABW) and northerncomponent water (NCW) (Ramsay et al., 1994). Anyimpact from the AABW or NCW would be possiblewhen bottom water exchange between the South ChinaSea and the deep ocean remained effective. Therefore,any changes in bottom depth and topography of the seabasin and sea-ocean passages must have been critical forlocal carbonate dissolution. An increase of paleodepth atthe ODP Site 1148 locality to over 2500 m since∼14 Ma (Zhao, 2005) was equal to or even deeper thanthe present Bashi Strait, the only passage with a silldepth of 2600 m now connecting the South China Seaand open west Pacific Ocean. This increase in basindepth would encourage more free exchange of deepwater if passages including the then Bashi Strait were

Fig. 9. Six major stages marking the stepwise tectonic–paleoceanographic evolithostratigraphic evidence, and their relationship with global climatic recordintervals of seafloor spreading during the early and middle Miocene, but subdMiocene production patterns of the Northern Component Water (NCW), Antaare modified from Ramsay et al. (1994).

also deeper than 2500–2600 m. Unlike most Site 1148dissolution events that often fall in periods of increasedproduction of the corrosive AABW (Fig. 9), dissolutionevent D-2 from 16 to 15 Ma appears to correspondmainly to the proto-Antarctic intermediate water(AAIW) (Ramsay et al., 1994) (Fig. 9). Since 16–15 Ma, the overall low CaCO3 and more frequent severedissolution at ODP Site 1148 are interpreted as resultingfrom local deepwater circulation change or even theinitial development of a local bottom water after thecessation of South China Sea spreading.

Lasting from∼13Ma to 11Ma at ODP Site 1148 wasdissolution event D-3 with CaCO3 <20% closelymatching with a temporary carbonate reduction on theOntong Java Plateau (Berger et al., 1993). Thisdissolution event D-3 was probably affected at leastpartly by the AABW about 1 Ma after the initial

lution of the South China Sea over the past 33 Ma are recognized on thes is suggested. Stage 3 can be subdivided further to represent variousivision of Stage 2 is hampered by the slumped upper Oligocene deposit.rctic BottomWater (AABW) and Antarctic Intermediate Water (AAIW)


establishing of the Antarctic ice sheet at about 14 Ma(Flower and Kennett, 1994; Zachos et al., 2001), whenglobal sea level dropped significantly at the middle/lateMiocene boundary (Haq et al., 1987; Hardenbol et al.,1998) (Fig. 6). Shevenell et al. (2004) suggested thatorbital paced ocean circulation changes, rather thanperiodic pCO2 drawdown, altered meridional heat/vapor transport and subsequently triggered ice growthand global cooling at ∼14 Ma. Holbourn et al. (2005)identified the importance of orbital forcing but also thedeclining atmospheric carbon dioxide levels both actingon the thermal isolated Antarctica as possible drivingmechanism for this 14 Ma global event. It is noteworthythat better CaCO3 preservation after Antarctic ice sheethas been observed in eastern subtropical Pacific (Lyle,2003; Holbourn et al., 2005), but not in the central orwestern Pacific (Lyle, 2003), suggesting a morecomplex cause of deep sea dissolution than a simplefactor of deep water production driven by ice sheetgrowth.

Nevertheless, the subsequent “carbonate crash” at∼11–10 Ma was mainly a dissolution event (Lyle, 2003)influenced by an intensified NCW production thatsubsequently led to a reorganization of global deepwater circulation and the decoupling of Atlantic fromPacific deep water systems (Roth et al., 2000).Corresponding to this “carbonate crash” there wasdissolution event D-4 at 10–9 Ma from ODP Site 1148marked by a sudden increase in planktonic foraminiferfragments to over 90% or even 100% (Fig. 6). Althoughthe relationship between dissolution events D-3 and D-4is not clear, D-3 as a precursor to D-4 in respectivelymarking the initial and first maximum deepwatercooling in the post-spreading South China Sea was notimpossible.

Both dissolution events D-4 and D-5 have a similarmagnitude of increase in planktonic fragments to over90% or even 100%, but dissolution event D-5 at 5–3 Ma appears to have lasted for a much longer period(Fig. 6), suggesting more intensive dissolution prob-ably in responding to the first formation of modernlysocline (∼3000 m) and CCD (∼3500 m) in thestudy area. Its association with a contemporary CaCO3

reduction at other central and western equatorialPacific localities (e.g. ODP Sites 803, 804, 806,Berger et al., 1993) implies another major change inregional water masses. Longitudinal contrast betweenthe west and east Pacific at the time also increased, asindicated by planktonic foraminifer assemblages(Kennett et al., 1985; Chaisson and Ravelo, 2000)and a more significant increase of CaCO3 accumula-tion in the eastern Pacific (Lyle, 2003).

6.3. Stepwise evolution of the South China Sea basin

It is apparent that the lithostratigraphic and geo-chemical evidence from ODP Site 1148 points to severalperiods of large-scale changes that characterize thestepwise pattern in the South China Sea evolution overthe last 33 Ma. A preliminary analysis suggests that 6such periods are most significant.

(1) 33–28.5 Ma. Rapid sedimentation of abundantclastic sediments at ODP Site 1148 (lithostrati-graphic Unit VII) represents very active rifting-related activities in the initial phase of seafloorspreading in the South China Sea (Su et al., 1989;Briais et al., 1993; Clift and Lin, 2001) and theavailability of a large amount of sediments due tointensive weathering in surrounding regions (Cliftet al., 2001; Clift, 2006). The earliest South ChinaSea basin was likely composed of a series of deepgrabens before evolving to an elongate gulf whenspreading actually started (Briais et al., 1993;Hall, 2002). Coarser grain size and high heavyREE basalt composition also indicate a near-source provenance (Figs. 7 and 8) with asignificant portion of weathered material fromthe southeast – the paleo-Sunda area (Li et al.,2003). The role of the then Pearl River in thissedimentary phase is not clear but its influenceappears to have been very limited. Neither do weknow any passages connecting with the true“Tethys” through the paleo-Sunda Shelf at thetime, although the then ∼1500 m deep SouthChina Sea water was indeed supporting a benthicforaminifer fauna of “Tethys” affinities (Kuhnt etal., 2002). By summarizing available tectonic andmodeling data, Kuhnt et al. (2004) demonstratedthe existence of a deep basin once to the east ofBorneo, which separated Borneo from westSulawesi, and which may have linked the CelebesSea to the Indian Ocean. The northern extensionof this deep basin is not known because thetopography has been completely altered byPhilippine islands after ∼12 Ma. Toward∼28.5 Ma, seafloor spreading appeared to stopin the northern South China Sea and the spreadingridge started to propagate southeastward to followa N–S spreading direction (Briais et al., 1993).This change of spreading direction culminating at∼25 Ma must have interacted with what happenedin the Indonesian archipelago area (Hall, 2002;Kuhnt et al., 2004)) because opening wider to thesouth could have encouraged a circulation change


with more warm water influx into the then SouthChina Sea.

(2) 28.5–23 Ma. The subsequent deposition of chalkbetween 473 and 480 mcd manifested a regionalwarming at ∼27 Ma that could have responded toa period of global warming and coincided alsowith a first connection between the Eurasian and(Indo-)Australian plates (Hall, 2002 and refer-ences; Li et al., 2005 and references) whichsubsequently stopped deep water exchangebetween the Pacific and Indian oceans by∼25 Ma (Kuhnt et al., 2004). Apart from thethin chalk layer, almost the entire upper Oligocenesection from ODP Site 1148, or lithostratigraphicUnit VI, was slumped, indicating an episode ofintensive tectonic activities in the South ChinaSea. Evidence of a great change is also revealedby strong variations in all physical property andgeochemical records (Wang et al., 2000; Li et al.,2003; Shao et al., 2004) (Figs. 3 and 7). Theunconformities identified from the slumped inter-val between 457 and 472 mcd indicate the missingof ∼3 Ma record, and provide additional evidenceof extremely powerful, large-scale tectonic activ-ities that could only be accounted for from astepwise change in seafloor spreading direction,or “ridge jump” (cf. Briais et al., 1993). Althoughthe Site 1148 record alone cannot fully testify theexact timing of tectonic zenith, paleobiologicaldating of the slump base suggests an age of∼25 Ma (Li et al., 2005) when major tectonic-induced geomorphologic changes in and aroundIndonesia archipelago also occurred (Hall, 2002;Kuhnt et al., 2004).

(3) 23–15 Ma. Compared to the previous period, amajor decrease in SiO2 and εNd values, plus rapidincrease in La and slow increase in Rb, mayindicate a shift of sediment provenance to thenorth (mainland China) (Li et al., 2003) and thenortheast (via the then Taiwan Strait) (Shao et al.,2004). This period also saw the progressivegrowth of C4 plants on the Asia continent dueto the development of monsoons and humidclimate, as indicated by black carbon (Jia et al.,2003), palynology (Sun and Wang, 2005), andmineralogy data (Clift, 2006). The inclusion oflithostratigraphic Units V–III also indicateschanges of the sedimentary regime in varioussubstages of seafloor spreading southward, atwater depths of 1500–2500 m (Zhao, 2005).Characterizing these substages is an “S”-shapedgrain size curve although grain size increase near

16 Ma was not as robust as during 23–21 Ma (Fig.7), suggesting a possible subdivision. However,the overall changes in other geochemical recordsand sedimentation rates between 20 and 30 m/Maare relative minor, implying a period of overallrather consistent tectonic and sedimentary varia-tions perhaps as a result of the site localitybecoming remote from the active spreadingmargin now lying hundreds of kilometers to thesouth. The elemental and grain size changes near16 Ma likely reflect a big change, again, in thesedimentary regime when the South China Seaended its spreading due to more intensive push ofthe Philippine Plate (Briais et al., 1993) andweakening and ending of motion along the RedRiver Fault Zone (Briais et al., 1993; Tapponnieret al., 2001; Gilley et al., 2003). Among others,dissolution event D-1 at ∼21 Ma represents ashort interval within this period influenced bysignificant colder deep water invasion and anelevated CCD, making possible the separation ofsubstages 3a and 3b that include lithostratigraphicUnits V and IV–III, respectively (Fig. 9). Thesubsequent dissolution event D-2 was muchstronger and lasted much longer from ∼16 to14.5 Ma (Fig. 9), suggesting a more intensivebottom water cooling after spreading stopped andthe onset of a new circulation mode in the watercolumn of the mid-Miocene South China Sea nowreaching over 2500 m water depths (Zhao, 2005).

(4) 15–9 Ma. Strong fluctuations in CaCO3%, slightincrease in Nb/Ta and rather steady contents in allother elements within lithostratigraphic SubunitsIIb may imply a period of strong climate-relatedcirculation changes with a stable sediment source.The major contributor to these changes wasapparently the global climate variability accom-panied by the development of eastern Antarcticice sheet (EAIS) and its subsequent waning andwaxing (Fig. 9). Regionally, a shrinking Indone-sian seaway due to continuous collision betweenthe Eurasian, Indo-Australian and Pacific (Philip-pine) plates could have brought about a series ofrelated geomorphologic and paleoceanographicchanges sufficient to impact the regional or evenglobal climate. These changes include furtheruplift of Tibetan plateau (Tapponnier et al., 2001),the ending of South China Sea spreading (Briais etal., 1993), a narrowing Indonesian seaway (Hall,2002; Kuhnt et al., 2004), and intensifiedlatitudinal thermal gradient because of pileup ofwarm water in the western Pacific and the


development of a late Miocene warm pool(Kennett et al., 1985; Li et al., in press). Theselow latitude geomorphologic and paleoceano-graphic changes could have exerted a moresignificant role in the development of the EAISthan previous anticipated. More specifically,dissolution events D-3, D-4, and D-5 recorded atODP Site 1148 likely evince stepwise local deepsea cooling influenced partly by AABW or NCWproduction concomitant with the EAIS expansion.The periodic production of the AABWand AAIWalso caused many contemporary dissolution-induced deep-sea hiatuses in world oceans(Ramsay et al., 1994; Lyle, 2003) (Fig. 9).

(5) 9–5 Ma. This period was characterized by muchreduced paleoceanographic activities with mostlithological and geochemical proxies becomingless variable in lithostratigraphic Subunit IIa.However, slow increase in grain size, stepwisedecline in La/Sm and Nb/Ta ratios and a smallincrease in CaCO3% flux at ∼8 Ma may suggeststronger weathering (Wei et al., in press) andactive sediment transport (Shao et al., 2004),although hematite/goethite ratio was too low toreflect a period of strong erosional flux (Clift,2006). These variations could have responded toan intensification of the East Asian monsoon (Anet al., 2001) due to rapid uplift of the Tibetanplateau at∼8 Ma (Molnar et al., 1993; Clark et al.,2005), coinciding at least partially with the lateMiocene “biogenic bloom” (e.g. Grant andDickens, 2002). The ODP Site 1148 black carbonrecord also indicates a period of C4 plantexpansion in close association with monsoonsover the continent (Jia et al., 2003). It isnoteworthy that background dissolution at Site1148 was becoming more frequent, indicating theexistence of a much cooler bottom water com-pared to previous periods (Fig. 6).

(6) 5–0 Ma. Dissolution intensified from 5 Ma to∼3 Ma due to stronger bottom water cooling and amore elevated CCD apparently in responding tothe onset and intensification of northern hemi-sphere glaciations (Zachos et al., 2001) and theinitial development of the modern local lysoclineand CCD. It is not clear, however, how thestronger dissolution event D-5 recorded at Site1148 can be related to the emergence of Taiwanand the present Bashi Strait as a result of LuzonArc/Eurasian Plate collision beginning ∼6.5 Maago (Huang et al., 1997). Most likely, theunderwater part of the Bashi Strait, now at

∼2600 m, could have since acted as a barrierblocking Pacific deep waters flowing into theSouth China Sea such that the isolated SouthChina Sea bottom water became colder and morecorrosive, subsequently causing the dissolutionevent D-5 at ODP Site 1148 (3294 m). Since∼3 Ma, rapid increase in sedimentation rates toabout 60 m/Ma from lithostratigraphic Unit I (Fig.4) and resilient elemental ratios above 120 mcd(Fig. 7) together suggest a more variable sedi-mentary regime in tune with glacial–interglacialcycles. Another phase of plateau uplift andassociated neotectonic activities (Ludmann andWong, 1999), plus stronger monsoon windsduring glacial times (Sun and Wang, 2005), mayhave caused significant increases in weathering(Wei et al., in press). Intensified down-slopetransport due to frequent sea level fluctuations andexposure of a large shelf area during sea level low-stands directly contributed to the high sedimenta-tion rates since ∼3 Ma in the South China Sea.

7. Summary and conclusions

The 853 m thick sediment sequence recovered atODP Site 1148 from the northern South China Seaprovides the longest record to-date to study regionalpaleoceanographic changes and their impacts on globalclimate. Although distorted by unconformities (mainlyin the late Oligocene) and dissolution (mainly since theearly Miocene), it represents one of the most completerecords from a single site in the western Pacific to studythe regional geohistory over the past 33 Ma. A synthesisof preliminary litho-, bio- and chemo-stratigraphicresults has revealed that local paleoceanographicpatterns were mainly responding to coupled impacts ofstepwise evolution of the South China Sea as a result ofregional geomorphologic change and global climatevariability. Six periods of lithostratigraphic changes,marking six major evolutionary stages of the SouthChina Sea, are characterized by different lithostrati-graphic units with distinct sedimentary, biofacies andelemental variations. These results lay a solid founda-tion for further exploring the detailed climate cyclicityhistory in the region.

Rapid deposition with sedimentation rate of 60 m/Maor more characterized the early Oligocene rifting. Thestrongly condensed upper Oligocene between 457 and495 mcd is dominated by slumped deposits truncated byunconformities to a total duration of about 3 Ma. Shownas closely packed “double reflectors” on seismic profile,the slumped deposits provide hard evidence of South


China Sea tectonic transition from rifting to spreading.Geochemical data indicate that part of the Oligocenesediments was transported from the southwest paleo-Sunda region, when the deep water was supportingbenthic foraminifers of “Tethys” affinities. Slow sedi-mentation of 20–30 m/Ma since the early Miocenemarked a progressive moving away of the site localityfrom sediment source after seafloor spreading southwardaccelerated, accompanied by a shift of sedimentprovenance to the north and northeast. The completenessof Miocene–Pleistocene succession is clouded byfrequent dissolution but present biostratigraphy cannotresolve any short-term hiatuses by dissolution. Fivemajor dissolution events, D-1 to D-5, record the stepwisedevelopment of deepwater masses in close association tobasin transformation in the post-Oligocene South ChinaSea. The concurrence of local and global dissolutionevents suggests mainly a climatic forcing that causeddeep water circulation changes concomitantly in worldoceans andmarginal seas. A return of high sedimentationrate of ∼60 m/Ma to the late Pliocene and PleistoceneSouth China Sea can be accounted from intensifieddown-slope transport due to frequent sea level fluctua-tions and exposure of a large shelf area during sea levellow-stands. The six paleoceanographic stages, respec-tively corresponding to rifting (∼33–28.5Ma), changingspreading southward (28.5–23 Ma), stable spreading toend of spreading (23–15 Ma), post-spreading balance(15–9 Ma), further modification and monsoon influence(9–5 Ma), and glacial prevalence (5–0 Ma), hadtransformed the South China Sea from a series of deepgrabens to a rapidly expanding open gulf and finally to asemi-enclosed marginal sea in the past 33 Ma.

Acknowledgments

This research used samples and data provided by theOcean Drilling Program (ODP). ODP is sponsored bythe U.S. National Science Foundation (NSF) andparticipating countries under management of JointOceanographic Institutions (JOI), Inc. Funding for thisresearch was provided by grants from the NationalNatural Science Foundation of China (40476030,40576031 and 40321603). The manuscript wasreviewed by Peter Clift and Wolfgang Kuhnt whosecomments greatly improved its final presentation.

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a 33 ma lithostratigraphic record of tectonic and ...a 33 ma lithostratigraphic record of tectonic...

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