Initiation of the Changjiang (Yangtze) delta and its response to the mid-Holocene sea level change

Palaeogeography, Palaeoclimatology, Palaeoecology 388 (2013) 81–97

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Initiation of the Changjiang (Yangtze) delta and its response to themid-Holocene sea level change

Bing Song a,⁎, Zhen Li a,⁎, Yoshiki Saito b, Jun'ichi Okuno c, Zhen Li a,d, Anqing Lu a, Di Hua a, Jie Li a,e,Yongxiang Li f, Rei Nakashima b

a State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, Chinab Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba 305-8567, Japanc National Institute of Polar Research, 10-3, Midoricho, Tachikawa, Tokyo 190-8518, Japand Graduate School of Environmental Studies, Nagoya University, Fur-cho, Chikusa, Nagoya 464-8601, Japane Yunnan Institute of Environmental Science, No. 23 Wang Jiaba, Kunming 650034, Chinaf School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China

⁎ Corresponding authors. Tel.: +86 21 62238235; fax:E-mail addresses: [email protected], imbingsong@gm

[email protected], [email protected] (Z. Li).

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.palaeo.2013.07.026

a b s t r a c t
a r t i c l e i n f o
Article history:Received 9 December 2012Received in revised form 14 July 2013Accepted 23 July 2013Available online 30 July 2013

Keywords:HoloceneChangjiang (Yangtze) deltaSedimentary faciesDelta initiationSea-level change

The Changjiang (Yangtze) delta in eastern China is one of the largest tide-dominated deltas in the world. To ob-tain amore detailed information on its initiation,we drilled three sediment cores, PK01, HG01 and XJ03, from theuppermost region of the delta plain. On the basis of sedimentary facies and AMS 14C dating, we identified threedepositional systems: (1) a fluvial system, (2) an estuarine system, and (3) a deltaic system including an initialdelta (estuary–delta transition) and a typical delta. The maximum flooding surface separates the estuarine anddeltaic systems and occurred at ca. 8.0 cal kyr BP, when brackish water intruded landward and reached theNanjing area. The Changjiang delta was initiated at the area between Nanjing and Yizheng when the sea-levelrise decelerated after the rapid rise in sea level during 9.0–8.2 cal kyr BP.The subsequent evolution of the Changjiang delta has been shown as a seaward development model of river-mouth bars with shoreline migration. The oldest river-mouth bar called Hongqiao sand body, which is locatedmost landward, was dated at 6.0–5.5 cal kyr BP in this study, nearly one thousand years younger than the previ-ous estimates. A tide-dominated, funnel-shaped estuary was formed at 8.0 cal kyr BP, and lasted until theHongqiao sand bar became exposed. This stage is regarded as an aggradation-dominated initial stage of theChangjiang delta, followed by a progradation-dominated stage that is characterized by the formation of a seriesof seaward migrating river-mouth bars. A relative sea level highstand in the mid-Holocene may be associatedwith glacio-hydro-isostatic tilting.

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1. Introduction

Estuaries and deltas develop at river mouths during transgressiveand regressive phases, respectively (Boyd et al., 1992; Hori et al., 2004).Holocene sea-level rise has significantly effected landward movementof estuaries and initiation of deltas (Stanley and Warne, 1994; Horiet al., 2004). Stanley and Warne (1994) showed that delta initiation oc-curredworldwide during 8.5–6.5 14C kyr BP and concluded that the initi-ation was principally controlled by decelerating sea-level rise. Detailedanalyses of sediment cores of Asian deltas provided evidence for thedelta initiation in relation to a sea level stillstand or a decelerated sea-level rise at around 8.0 cal kyr BP, just after a rapid rise in sea level duringthe period of 9.0–8.2 cal kyr BP (e.g., Bird et al., 2007; Hori and Saito,2007; Tamura et al., 2009; Bird et al., 2010; Nguyen et al., 2010; Li et al.,2012). This delta initiation was followed by the aggradation-dominated

+86 21 62546412.ail.com (B. Song),

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stage from 8–6 cal kyr BP to the progradation-dominated stage for thelast 6–4 cal kyr BP (e.g., Mekong River delta: Ta et al., 2005; Tamuraet al., 2009, Red River delta: Tanabe et al., 2006; Funabiki et al., 2007).

The Changjiang delta, one of the largest tide-dominated deltas in theworld, holds heavy population and is threatened by the global sea-levelrise. To knowhow the delta initiated and developed as sea level changescould be a goodmeans of estimating the potential impact by the sea levelrise. The Holocene sea-level changes recorded in the Changjiang deltaarea and the East China Sea have been reported previously (e.g., Chenand Stanley, 1998; Saito et al., 1998; Liu et al., 2004; Zong, 2004; Horiand Saito, 2007; Chen et al., 2008). All of these previous studies supportedrapid sea level rise from the latest Pleistocene to the middle Holocene.However, the sea level in the middle–late Holocene is still controversial.Several sea-level curves from the coastal area of Changjiang delta andthe East China Sea show a gradual rise in sea level during the mid–lateHolocene and no sea level highstand (Chen and Stanley, 1998; Liu et al.,2004; Zong, 2004; Chen et al., 2008). Others suggested a mid-Holocenesea level highstand (Saito et al., 1998; Hori et al., 2001a; Liu et al., 2004;Hori and Saito, 2007). Unfortunately, the published sea-level curves are

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82 B. Song et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 388 (2013) 81–97

very rough and mostly lack high-quality data to provide the direct evi-dences for the relative sea level change. In one case a mistake wasmade by putting the sea-level curve above a terrestrial peat (Liu et al.,2004). The previous work also lacks high-resolution records from theuppermost region where the mid-Holocene sea level position may bereflected directly, especially during the transition period from the estuaryto the delta.

To understand the evolution andmodern processes of the Changjiangdelta, stratigraphic,morphologic, and sedimentological studies have beencarried out since the late 1970s (e.g., Delta Research Group, 1978; Chenet al., 1979; Guo et al., 1979; Li et al., 1979; Milliman et al., 1985; Chenand Stanley, 1993a, 1993b; Li et al., 2000a, 2000b; Hori et al., 2001a,2001b, 2002a, 2002b; Yi et al., 2006; Liu et al., 2007; Li et al., 2009; Liuet al., 2010; Wang et al., 2010, 2012). Delta Research Group,Department of Marine Geology, Tongji University (1978) and Li et al.(1979) proposed a depositional and developmentmodel of the HoloceneChangjiang delta based on stratigraphic analysis of drill cores. They sug-gested that the main delta plain was formed by a step-like seaward mi-gration of the river-mouth sand bars from Zhenjiang to the presentriver mouth in the following six stages: Hongqiao stage (7.5–6.014C kyr BP), Huangqiao stage (6.5–4.0 14C kyr BP), Jinsha stage(4.5–2.0 14C kyr BP), Haimen stage (2.5–1.2 14C kyr BP), Chongmingstage (1.7–0.2 14C kyr BP), and Changxing stage (0.7 14C kyr BP–present)(Fig. 1). This model was widely accepted by succeeding investigationsand theHolocene evolution of the Changjiang deltawas studied in detailsfrom the coastal to subaqueous delta areas (e.g. Chen et al., 1979, 1985;Li et al., 1979; Chen et al., 1987; Chen and Stanley, 1993a, 1993b; Liet al., 2000a, 2000b, 2002). However, these studies did not includehigh-resolution age control by 14C dating and three-dimensional analy-ses of sedimentary facies of thewhole Changjiang delta. Further paleon-tological and sedimentological analyses and models clearlydemonstrate the environmental changes of the lower delta-plain areasince ca. 12 cal kyr (Hori et al., 2001a, 2001b; Liu et al., 2001; Horiet al., 2002a, 2002b; Yi et al., 2003; Liu et al., 2005; Yi et al., 2006;Wang et al., 2011). However, the transitional stage from transgressiveestuary to prograding delta in the upper delta-plain area is still notclear due to the lack of high-resolution core data.

This paper presents high-resolution AMS 14C ages and detailed sed-imentary facies analyses of three cores from the uppermost region of

Fig. 1. Previous model of the Changjiang delta evolution and the estimated paleo-sh

the Changjiang delta plain (Fig. 1). Based on the sedimentary facies,chronostratigraphic correlation,we identify themaximum flooding sur-face (MxFS) and clarify the Holocene initiation and primary stages ofevolution of the Changjiang delta relative to the sea level changes. Wealso studied hydro-isostatic effect on the relative sea-level highstand ap-plying glacio-hydro-isostatic adjustment models.

2. Regional settings

2.1. Geography and hydrology

The Changjiang River originates on the Qinghai-Tibet Plateau andflows into the East China Sea. The drainage basins of the ChangjiangRiver and its tributaries cover approximately 1.8 × 106 km2 (ColemanandWright, 1975; Coleman, 1981). The Changjiang River is the world'sfifth and fourth largest in terms of water discharge (921 km3/yr) andsuspended load (4.8 × 108 t/yr), respectively (Milliman and Meade,1983). Approximately 70% and 87% of the annual water volume andsediment load, respectively, are discharged during the flood season(May to October). Suspended sediments disperse offshore from theriver mouth to the western Taiwan Strait (Liu et al., 2006; Xu, 2006; Xuet al., 2009, 2012). Nearly 70% of the sediments are deposited in theriver mouth to prodelta region to form a series of river-mouth sandbars (Milliman et al., 1985; Shen, 1998).

The Changjiang delta, located in the eastern Asian monsoon region(Fig. 2), occupies an area of approximately 5.2 × 104 km2, of which2.3 × 104 km2 is subaerial and 2.9 × 104 km2 is subaqueous (Li, 1986).The elevation of the delta plain is generally b5 m above the mean sealevel in the coastal zone (Atlas of Jiangsu Province, 1978; Stanley andChen, 1996; Chen, 1999) (Fig. 2). The subaqueous delta is divided intothree parts: delta front platform (intertidal to subtidal flats), delta frontslope, and prodelta with water depths ranging from b5–10 m, 5–10 to15–30 m, to N15–30 m, respectively (Atlas of Jiangsu Province, 1978;Chen et al., 1987; Hori et al., 2001a). The delta area is significantlyinfluenced by monsoons, with southeasterly and northwesterly windsprevailing in summer and winter, respectively (Li and Wang, 1998).The delta is dominated by tides. The average tidal range is approx-imately 2.7 m in the river mouth, and the maximum tidal rangeapproaches approximately 4.6 m (Shen et al., 1988). The tidal

orelines (Delta Research Group, 1978; Wang et al., 1981 and Hori et al., 2001a).

image of Fig.�1

Fig. 2. Geographicmap showing the locations of the researched cores (cores of DT, JD01 and HQ98 from Li andMin, 1981; Li et al., 2009 and Hori et al., 2004, respectively). A. Topographymap of the research area (Modified from Atlas of Jiangsu Province, 1978); B. Topographic locations of the cores with SRTM altimetry at 0.5-m vertical intervals.

83B. Song et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 388 (2013) 81–97

influence reaches an average of 210 km upstream from the rivermouth (Shen, 1998).

2.2. Subsurface and coastal geology

The Changjiang delta is located adjacent to a tectonic uplift zone toits west and is experiencing subsidence. The subsidence rate ranges

from 1 to 3 mm/yr and increases seaward (Li et al., 2000a). The transi-tion zone between uplift and subsidence is the Zhenjiang and the Yang-zhou area. The depth of bedrock increases gradually from the west tothe east, less than 100 m in Zhenjiang to 350–400 m at the rivermouth. The thickness of theHolocene sediments also increases seaward.The Yangtze Delta was divided into the main delta and its northern andsouthern areas (Li et al., 2000a). The main delta is a zone in which the

image of Fig.�2

Table 2Molluscan shell types and their habitats from the cores PK01, HG01 and XJ03.

Sample no. Depth(m)

Elevation(m)

Genus/species Habitats

PK01PK-01-27-2-18.92 18.92 −13.32 Unionidae sp. FreshwaterPK-01-27-25-19.15 19.15 −13.55 Corbicula sp. Brackish or

freshwaterPK-01-27-6-19.16 19.16 −13.56 Corbicula sp. Brackish or

freshwaterPK-01-27-34-19.24 19.24 −13.64 Littoraria sp. Brackish

Corbicula sp. Brackish orfreshwater

HG01HG01-16-10.42 10.42 −2.92 Gyraulus albus FreshwaterHG01-16-10.80 10.80 −3.30 Assiminea cf. sculpta Brackish waterHG01-16-10.94 10.94 −3.44 Parafossarulus

striatulusFreshwater

HG01-17-11.69 11.69 −4.19 Parafossarulusstriatulus

Freshwater

XJ03XJ03-23-48 11.98 −7.18 Corbicula leana FreshwaterXJ03-41-33 22.00 −17.20 Parafossarulus


XJ03-41-43 22.13 −17.33 Bellamya sp. Freshwater


paleo-river mouth shifted and deposition was influenced by tides(Li et al., 2000a, 2000b). As a most important part of the Holocene sedi-ments, incised-valley fills consist of fluvial facies, tide-dominated estuaryfacies and tide-dominated deltaic facies (Hori et al., 2002a, 2002b). Thesediments offluvial facies are composed of graymedium-fine sands, con-taining a few gravels andmud clasts at the bottom, with a fining-upwardsequence. It is characterized by the presence of cross-bedding andparallel-bedding without marine microfossils (Li et al., 2000a). Fluvialfacies was predominant in the incised valley before 11 cal kyr BP, andits thickness ranges from10 to 30 mwith the boundary of underlying de-posits at the depth of 60–80 m (Stanley and Chen, 1993; Li et al., 2000a,Hori et al., 2001a, 2001b). The tide-dominated estuary occupied the in-cised valley approximately during 11–8 cal kyr BP and it is approximate-ly 30 m thick beneath the present-day delta plain (Li et al., 2002). Thesediments consist of gray sandy clay and clayey sand, or softmud in estu-ary front, containing rich plant fragments, with upward-fining succes-sion. Sand–mud couplets and laminations are common in this facies.The tide-dominated delta has developed after approximately 8 cal kyrBP with the thickness of 10–30 m, in a coarsening-upward sequence ofdelta front, and a fining-upward sequence from delta front to delta plain(Li et al., 2000a, 2000b; Hori et al., 2001a, 2001b, 2002a, 2002b; Li et al.,2002). Sediments of the delta are composed of gray and yellowish graysandy clay with cross or parallel laminations, containing great lenticularsand bodies which may be up to several tens of kilometers long (Liet al., 2000a, 2000b).

3. Materials and methods

Three 9 cm-diameter cores were drilled from the uppermost regionof the Changjiang delta plain in 2009–2010 (Table 1 and Fig. 2). CorePK01 is located at 32°00′51″ N, 118°36′13″ E, at an elevation of 5.6 m.Core HG01 is located at 32°17′21″ N, 118°48′09″ E at an elevation of7.5 m. Core XJ03 is located at 32°18′35″ N, 119°17′44″ E at an elevationof 4.8 m and is 37.2 m in length. The elevation ismeasuredwith respectto the 85 National Datum, which is 1.902 m lower than the WusongDatum (Starting point: Zhenjiang 308′).

These three cores were drilled at the floodplain to the north bank ofthe present Changjiang River. Core PK01 is near the third denudation–accumulation terrace and from denudation low hill to the west ofNanjing City (Fig. 2). Core HG01 is near an entrance of a tributary ofthe Changjiang River and third denudation–accumulation terrace tothe north of theNanjing City. Cores XJ03 and JD01 are near theHolocenesand spits eastwards to the cities of Yizheng and Jiangdou, respectively.Lengths of the cores PK01, GH01 and XJ03 are 50 m, 23.5 m and 37.2 m,respectively. For Core HG01, we analyzed the upper 18.0 m in this paper.The core recoverywas N90% for cores PK01 andHG01, and N80% for CoreXJ03 (Table 1).

These cores were split, described, photographed and subsampled inthe State Key Laboratory of Estuarine and Coastal Research, East ChinaNormal University, China. Of the total of 765 samples, 403, 130, and 232

Table 1List of the cores drilled from the uppermost region of the Changjiang delta plain.

Core no. Location Elevation(m)

Length(m)

Recovery(%)

Latitude(°N)

Longitude(°E)

85 NationalDatum

WusongDatum

PK01 32°00′51″ 118°36′13″ 5.7 7.6 50.0 96.2HG01 32°17′21″ 118°48′09″ 7.5 9.4 23.5 98.3XJ03 32°18′35″ 119°17′44″ 4.8 6.7 37.2 80.0JD01* 32°24′35″ 119°35′43″ 5.7 7.6 53.6 82.0DT* 32°10′ 119°32′ 7.1 9.0 38.0 –

HQ98* 32°15′ 120°14′ 5.9 7.8 60.0 80.0

Note: Cores JD01*, DT* and HQ98* are from Li et al. (2009), Li and Min (1981) and Horiet al. (2001a), respectively.Wusong Datum: Zhenjiang 308′ is the starting point of measuring elevation.

samples were obtained from cores PK01, HG01, XJ03, respectively, atapproximately 8 cm intervals. Each sample was 2 cm thick. Grain-sizeanalyses were conducted on all the samples using a Malvern 2000,then, mud contents (grain size ≤ 63 μm) were calculated.

Foraminifera analyses were performed on 31 samples from the keysediment layers. Nine samples from core PK01 and 22 samples fromCore XJ03 were treated in the laboratory using a standard method(Wang et al., 1988). All sampleswere dried at 40 °C in an oven, weightedand washed over a 63 μm sieve. After drying, foraminifera were concen-trated and separated using CCl4 flotation method. Species identificationswere conducted at ×40 amplification under a Motic Optical Stereoscope.The identifications were mainly based on Wang et al. (1988). The“foraminiferal abundance” parameter represents the number of fo-raminifera per gram sediments (Wang et al., 1988). Molluscan shellswere identified in the Geological Survey of Japan and Nanjing Insti-tute of Geology and Paleontology, Chinese Academy of Sciences(Table 2). Of the selected molluscan shells, plant materials or organicmud from the sediment cores, 34 accelerator mass spectrometry(AMS) 14C ages were measured by Beta Analytic Inc. (Table 3). Agedeterminations were based on a Libby half-life of 5568 years. Theconventional radiocarbon ages were calibrated using Calib 6.0 (http://calib.Qub.ac.uk/calib/calib.html) and the Reimer et al. (2009) data set.The data measured from marine shells were calculated with theMarine09 program. The data obtained from terrestrial plants andorganic materials were calibrated using the IntCal09 program. Be-cause of the lack of ΔR (Difference between the regional and global

XJ03-41-55 22.25 −17.45 Bellamya quadrata FreshwaterParafossarulusstriatulus

Freshwater

XJ03-42-14 22.84 −18.07 Parafossarulusstriatulus

Freshwater


Freshwater

XJ03-42-23.65 22.94 −18.14 Parafossarulusstriatulus

Freshwater

XJ03-42-73 23.45 −18.65 Parabithynialognicornis

Freshwater


Freshwater

XJ03-42-92 23.62 −18.82 Bellamya sp. FreshwaterXJ03-43-1 23.71 −18.91 Parabithynia

lognicornisFreshwater

XJ03-46-89 28.09 −23.29 Bellamya sp. FreshwaterXJ03-47-87 29.07 −24.27 Parafossarulus


http://calib.Qub.ac.uk/calib/calib.html

http://calib.Qub.ac.uk/calib/calib.html


marine 14C age) for the Changjiang delta area,we used a default value of0 ± 0 yr in common (Stuiver and Braziunas, 1993). The other ageswere calculated by linear interpolation. We also converted the conven-tional 14C age (14C yr BP) of the cores JD01 (Li et al., 2009) and HQ98(Hori et al., 2001a, 2001b) into calibrated years (cal yr BP) (Table 3).

The calculation of the glacio-hydro-isostatic adjustment (GIA) of theearth for the relative sea level estimation is based on a well-establishedtheory formulated by previous researchers (e.g., Nakada and Lambeck,1987). This model describes relative sea-level variations in response tochanges in surface loading derived by the last deglaciation. The modelincludes both an earth component describing the viscoelastic natureof the solid earth and an ice component documenting the history ofice melting reconstructed mainly from far-field sea-level observations(e.g. Yokoyama et al., 2000, 2001; Lambeck et al., 2002). Relative sea-level predictions are based on the Earth models with a lithosphericthickness of 50 km (H50) and 100 km (H100). In both models, theupper and lower mantle viscosities were specified at 2 × 1020 Pa sand 1022 Pa s, respectively. The ANU model of Lambeck et al. (2002)was used as the ice model after the last glacial maximum. Two types ofHolocene ice melting models were employed and all explain the ice-melt volume equivalent to a 3 meter sea-level rise during the last ca.7000 years as suggested by Nakada and Lambeck (1988). Continuousmelting of ice sheets throughout the mid- to late-Holocene was assignedas ESL model A, whereas major melting of ice sheet was completed by4000 years ago for ESL model B.

4. Results

4.1. Sedimentary facies

4.1.1. PK01 coreThe Core PK01 includes Pleistocene and Holocene sediments. The

lowermost 0.9-m thick sediments of the core are composed of relativelyconsolidated, reddish brown silty clay with several centimeters of cal-careous concretions (Figs. 3, 4a), corresponding to the Pleistocene ter-restrial deposits. Fifty-meter thick Holocene sediments overlay anerosional base at approximately 42.5 m below the present-day meansea level. According to the sediment composition, grain size, texture,and faunal content, the Holocene stratigraphy was divided into fourunits from the bottom to the top: P-a, P-b, P-c, and P-d (Fig. 3).

4.1.1.1. P-a: 49.0–32.3 m depth. Unit P-a is mainly composed of blackishgray clayey silt to silty sand with occasional thickly-interlaminated tothinly-interbedded sand and mud (Fig. 3). Sand–mud couplets are ob-served (Fig. 4b). The thickness of a single couplet ranges from 2 mmto 30 mm. The mud contents vary greatly from approximately 40% to100% throughout the unit. The lower part from 49.0 m to 45.0 mdepth shows coarsening upward. The upper part above 45.0 m depthshows several abnormal cycles from clayey silt–silt to sandy silt–siltysand (Fig. 3). Plant fragments, scour and fill, and cross-, ripple- and len-ticular laminations are observed in this unit. However, no foraminiferawere found. Three 14C ages at 46.78 m, 45.64 m and 40.26 m depthswere measured, and the calibrated medium ages are 11,485 cal yr BP,11,897 cal yr BP, and 11,795 cal kyr PB, respectively (Table 3).

Sand–mud couplets indicate that the facies is influenced by tide(Tessier and Gigot, 1989; Shi, 1991; Dalrymple et al., 1992; Hori et al.,2001b, 2002a, 2002b). Overbank or floodplain sediments comprisefloodplain fines, crevasse splay sand and thin channel sand bodies(Collinson, 1996). The cycles from mud to sand are perhaps associatedwith levees, crevasse splay and distributary channel sands coveringfloodplain silts. A fining upward succession from 36 to 32 m and the ab-sence of marine fossils might be because the site is away from the chan-nel, or due to retrogressive aggradation (Li et al., 2002). Makkaveev(1960) conducted flume experiments to examine the extent of backwa-ter and retrogressive aggradation caused by base level rise, and foundthat the extent of retrogressive aggradation could reach beyond that

of the backwater.Moreover, the core site is near the channel of the pres-ent Changjiang River (Fig. 2), where water table could be affected bytides. Therefore, we interpret this unit to be tidal-river facies.

4.1.1.2. P-b: 32.3–18.4 m depth. The sediments of unit P-b consist of clayto silt, and themud content is N95%. The lower part (32.3–20.0 mdepth)mainly consists of dark-gray clayey silt and contains abundant plant frag-ments, organic materials and some calcareous concretions (Fig. 4c). Noforaminifera were found. The upper part (20.0–18.4 m in depth) mainlyconsists of grayish-brown clayey silt and yields somemolluscan shells ofUnionidae sp., Corbicula sp., and Littoraria sp. on the top part near the ero-sional boundary (Table 2, Fig. 4d). However, plant fragments and organicmaterials are less abundant in the lower part. Nine AMS 14C ages weremeasured in this unit, and the calibrated ages span from 10.3 cal kyr BPto 9.0 cal kyr BP (Table 3).

The abundant plant fragments and organic materials in this unit in-dicate that the deposits are predominantly influenced by terrestrial con-ditions. Although the molluscan shells of Corbicula sp. and Littoraria sp.indicate a brackish-water environment, near an erosional boundary, it isalso possible that the molluscan shells were reworked and not deposit-ed in situ.

The characteristics of the lower part are analogous to the floodplain,as described by Miall (1992), Collinson (1996), Hori et al. (2001b) andTamura et al. (2009). This unit can be interpreted to be a floodplain tocoastal plain environment.

4.1.1.3. P-c: 18.4–2.9 m depth. The P-c unit covers the P-b unit with asharp boundary at 18.4 m. The unit consists of dark gray clayey silt tofine sand with thickly interlaminated to thinly interbedded sand andmud (Fig. 4e and f). Sand–mud couplets were observed (Fig. 4f). Themud content increases from 20% to 80% in general and in the lower part(18.4–5.5 m in depth) shows a fining upward succession. The upperpart (5.5–2.9 m in depth) consists of an upward-coarsening successionand has dark gray silt and brown clayey silt couplets. Ripple, cross, andlenticular laminations and cut-and-fill structures were observed in theunit. Two 14C ages at 18.22 m and 5.86 m were measured and the cali-brated medium ages are 4459 cal yr BP and 4109 cal yr BP, respectively(Table 3).

The sand–mud couplets clearly show that tides played a significantrole in deposition (Tessier and Gigot, 1989; Shi, 1991; Dalrymple et al.,1992; Hori et al., 2001b, 2002a, 2002b). And the main part (18.4–5.5 min depth) shows a fining upward succession with the couplets analogousto a tidal-river sediment facies described by Hori et al. (2001b).

4.1.1.4. P-d: 2.9–0 m. The unit is characterized by dull reddish brown tobrown clayey silt and silt (Fig. 4g), having a transitional contact withUnit P-c. Abundant plant rootlets, organic matters, worm burrows,scattered charcoals and calcareous concretion are observed (Figs. 3and 4G). Themud content is N90% in themost of the unit. These charac-teristics should be lacustrine environment to surface soils on floodplain.

4.1.2. HG01 coreHG01 core consists of Pleistocene and Holocene sediments with a

boundary at the depth of 17.16 m (Figs. 4H and 5). The sediment below17.16 m in depth is hard clay, in which a grayish yellow color at the bot-tomgradually changes into a grayish green color upward. This “hard clay”is typical for Pleistocene sediments in the Changjiang delta area (Chenet al., 1991). The overlying Holocene sediments can be divided intothree deposition units: H-a, H-b and H-c, according to sediment compo-sition, grain size, texture and faunal content (Fig. 5).

4.1.2.1. H-a: 17.16–5.38 m depth. Unit H-a consists of heavily-bioturbated gray clay to silt, containing abundant plant fragmentsand gastropods (Fig. 4i). Themud content is generally N87%. Themediangrain size is mainly about 7–6 ø. There are some granules in the lowerpart. Freshwater gastropods of Parafossarulus striatulus and Gyraulus

Table 3Summary of radiocarbon dates obtained from cores PK01, HG01, XJ03, JD01 and HQ98.

Sample no. Elevation(m)

Material Measured 14C age(14C yr BP)

13C/12C(‰)

Conventional 14C age(14C yr BP)

Calibrated 14C age(cal yr BP)

Beta ID(Bate-)

2σ range Median age

PK01PK01-11-36-5.96 −0.26 Plant fragments 3710 ± 40 −20.6 3780 ± 40 4094–4123 4109 270253PK01-25-42-18.22 −12.52 Plant fragments 4100 ± 40 −28.7 4040 ± 40 4444–4474 4459 270255PK01-26-36-18.66 −12.96 Molluscan shell 9080 ± 70 −11.2 9310 ± 70 10,130–10,204 10,167 270256PK01-27-2-18.92 −13.22 Molluscan shell 9150 ± 60 −7.5 9440 ± 60 10,219–10,354 10,287 270257PK01-27-26-19.16 −13.46 Molluscan shell 9180 ± 50 −9.7 9430 ± 50 10,210–10,341 10,276 270258PK01-27-34-19.24 −13.54 Molluscan shell 8980 ± 50 −8.1 9260 ± 50 10,065–10,176 10,120 270259PK01-27-18-20.1 −14.40 Plant fragments 8150 ± 50 −27.5 8110 ± 50 9008–9031 9020 291706PK01-30-68-22.63 −16.93 Plant fragments 8220 ± 50 −25.7 8210 ± 50 9123–9263 9193 270262PK01-31-35-23.30 −17.60 Plant fragments 8420 ± 60 −28.5 8360 ± 60 9399–9453 9426 270263PK01-33-94-25.89 −20.19 Charred plant 8370 ± 50 −15.7 8520 ± 50 9496–9533 9515 270264PK01-37-15-29.10 −23.40 Charred plant NA NA 8660 ± 60 9547–9630 9589 270266PK01-48-31-40.26 −34.56 Plant fragments 10,190 ± 60 −27.2 10,150 ± 60 11,754–11,835 11,795 270267PK01-53-51-45.64 −39.94 Plant fragments 10,240 ± 60 −28.4 10,180 ± 60 11,807–11,986 11,897 270268PK01-54-83-46.78 −41.08 Plant fragments 10,060 ± 60 −25.8 10,050 ± 60 11,404–11,565 11,485 270269

HG01HG01-9-25-4.75 2.75 Plant fragments 5040 ± 40 −26.9 5010 ± 40 5713–5749 5731 291707HG01-10-40-5.40 2.10 Plant fragments 6070 ± 40 −12.8 6270 ± 40 7154–7273 7214 287364HG01-11-37-5.87 1.63 Plant fragments 6150 ± 50 −12.5 6350 ± 50 7258–7308 7283 270343HG01-13-75-7.75 −0.25 Plant fragments 6750 ± 40 −26.1 6730 ± 40 7575–7609 7592 270345HG01-14-35-8.35 −0.85 Plant fragments 6740 ± 50 −12.3 6950 ± 50 7728–7828 7778 270346HG01-15-58-9.58 −2.08 Plant fragments 7050 ± 50 −27.2 7010 ± 50 7826–7870 7848 270347HG01-16-58-10.58 −3.08 Plant fragments 7090 ± 50 −24.5 7100 ± 50 7931–7961 7946 270348HG01-18-55-12.55 −5.05 Plant fragments 7340 ± 50 −27.2 7300 ± 50 8042–8168 8105 270349HG01-19-90-13.90 −6.40 Plant fragments NA NA 7460 ± 50 8206–8262 8234 270350HG01-20-61-14.61 −7.11 Plant fragments 7720 ± 50 −27.4 7680 ± 50 8418–8462 8440 270351HG01-17.10 −9.60 Charred plant 8050 ± 40 −23.7 8070 ± 40 8997–9013 9005 287365

XJ03XJ03-15-24-7.64 −2.84 Plant fragments 3910 ± 40 −28.1 3860 ± 40 4237–4298 4268 283511XJ03-20-4-10.04 −5.24 Molluscan shell 4280 ± 40 −8.6 4550 ± 40 4704–4822 4763 287368XJ03-24-7-12.07 −7.27 Molluscan shell 4090 ± 40 −8.8 4360 ± 40 4422–4555 4489 287369XJ03-29-20-14.7 −9.90 Molluscan shell 5140 ± 40 −9.8 5390 ± 40 5700–5847 5774 287370XJ03-36-34-18.94 −14.14 Plant fragments 6360 ± 40 −24.3 6370 ± 40 7268–7315 7292 283512XJ03-39-5-20.25 −15.45 Plant fragments 7870 ± 40 −25.1 7870 ± 40 8599–8648 8624 283513XJ03-43-47-24.17 −19.37 Charred plant NA NA 8420 ± 40 9437–9480 9459 283514XJ03-47-53-28.73 −23.93 Plant fragments 8610 ± 40 −24.6 8620 ± 40 9540–9555 9548 283515XJ03-49-76-30.96 −26.16 Organic 10,230 ± 50 −14.9 10,400 ± 50 12,203–12,359 12,281 287371

JD01*JD01-3-7.07 −1.37 Molluscan shell 5340 ± 50 −9.7 5590 ± 50 5908–6032 5970 254176JD01-5-12.09 −6.39 Plant material 6620 ± 50 −28 6570 ± 50 7433–7486 7460 254177JD01-6-14.14 −8.44 Molluscan shell 5300 ± 50 −8.4 5570 ± 50 5900–5991 5946 254178JD01-11-16.42 −10.72 Molluscan shell 4920 ± 40 −9.9 5170 ± 40 5479–5578 5529 254179JD01-13-22.38 −16.68 Molluscan shell 5020 ± 40 −9.6 5270 ± 40 5579–5666 5623 254180JD01-16-23.82 −18.12 Molluscan shell 5180 ± 40 −9.8 5430 ± 40 5741–5874 5808 254181JD01-17-24.38 −18.68 Molluscan shell 5340 ± 40 −10.4 5580 ± 40 5904–6009 5957 254182JD01-18-25.99 −20.29 Plant material 5340 ± 40 −28.5 5280 ± 40 6078–6117 6098 254183JD01-19-27.21 −21.51 Plant material 5950 ± 40 −27.4 5910 ± 40 6676–6749 6713 254184JD01-22-28.20 −22.50 Plant material 5320 ± 40 −28.5 5260 ± 40 5988–6017 6003 254185JD01-24-29.00 −23.30 Molluscan shell 5450 ± 40 −9.3 5710 ± 40 6050–6194 6123 254186JD01-26-30.05 −24.35 Molluscan shell 5240 ± 40 −9.4 5500 ± 40 5849–5927 5888 254187JD01-28-31.07 −25.37 Molluscan shell 5470 ± 40 −9.9 5720 ± 40 6081–6206 6144 254188JD01-29-31.79 −26.09 Plant material 6120 ± 40 −27.4 6080 ± 40 6898–6979 6939 254189JD01-33-32.85 −27.15 Plant material 5770 ± 50 −27.6 5730 ± 50 6482–6557 6520 254190JD01-33-33.82 −28.12 Plant material 5820 ± 40 −27.3 5780 ± 40 6543–6637 6590 254191JD01-39-40.15 −34.45 Molluscan shell 5740 ± 40 −10.9 5970 ± 40 6312–6431 6372 254192JD01-40-41.94 −36.24 Molluscan shell 5450 ± 40 −9.9 5700 ± 40 6025–6181 6103 254193JD01-40-44.90 −39.20 Molluscan shell 5870 ± 40 −12.1 6080 ± 40 6433–6568 6501 254194

HQ98*HQ-2-70 5.21 Snail shell 180 ± 50 −4.6 520 ± 50 522–540 531 130653HQ-4-470 1.21 Molluscan shell 4120 ± 50 −7.6 4410 ± 50 4499–4639 4569 130654HQ-5-550 0.41 Molluscan shell 5230 ± 30 −9.0 5490 ± 30 5836–5924 5880 132941HQ-5-615 −0.24 Molluscan shell 5580 ± 60 −7.1 5870 ± 60 6244–6339 6292 130655HQ-10-1330 −7.39 Molluscan shell 4310 ± 50 −9.1 4570 ± 50 4767–4837 4802 130656HQ-17-2320 −17.29 Molluscan shell 5310 ± 60 −6.4 5620 ± 60 5936–6095 6016 130657HQ-20-2860 −22.69 Molluscan shell 6830 ± 70 −4.6 7170 ± 70 7584–7674 7629 130658HQ-21-3012 −24.21 Plant fragments 8130 ± 70 −28.4 8080 ± 70 8999–9019 9009 130659HQ-30-4275 −36.84 Plant fragments 9080 ± 120 −26.9 9050 ± 120 10,205–10,234 10,220 130660HQ-34-4832 −42.41 Molluscan shell 10,250 ± 80 −10.3 10,490 ± 80 11,403–11,745 11,574 130661HQ-36-5265 −46.74 Molluscan shell 10,450 ± 70 −9.3 10,710 ± 70 11,974–12,120 12,047 130662HQ-37-5380 −47.89 Molluscan shell 10,500 ± 90 −7.8 10,780 ± 90 12,159–12,327 12,243 130663


Table 3 (continued)

Sample no. Elevation(m)

Material Measured 14C age(14C yr BP)

13C/12C(‰)

Conventional 14C age(14C yr BP)

Calibrated 14C age(cal yr BP)

Beta ID(Bate-)

2σ range Median age

HQ-38-5550 −49.59 Molluscan shell 10,250 ± 70 −9.2 10,510 ± 70 11,604–11,887 11,746 130664HQ-39-5700 −51.09 Molluscan shell 10,150 ± 70 −8.4 10,420 ± 70 11,331–11,650 11,491 130665HQ-40-5795 −52.04 Molluscan shell 10,240 ± 40 −10.2 10,480 ± 40 11,403–11,729 11,566 132942

Note: 13C/12C and conventional 14C were measured by Beta Analytic Inc. using an accelerator mass spectrometer (AMS). Calibrated 14C ages were all calculated using Calib 6.0 (http://calib.qub.ac.uk/calib/calib.html) and the Reimer et al. (2009) data set. NA: The original sample was too small to provide a 13C/12C ratio on the original material. However, a ratioincluding both natural and laboratory effects was measured during the 14C detection to calculate the true conventional radiocarbon age.

HQ98*


albus are scattered in most sediments of this unit. The brackish watertype, Assiminea cf. sculpta, only can be observed at 10.8 m depth(Fig. 5). A 5 mm-diameter calcareous concretion occurs at 14.33 m.Partial silt–clay couplets are observed at depths of 12.70–12.40 m,11.37–11.00 m, and 7.60–7.40 m. Ten 14C ages were measured, and thecalibrated age spans from 9.0 cal kyr BP to 7.2 cal kyr BP (Table 3).

Abundant plant fragments, freshwater gastropods and other organicmaterials in this unit indicate terrestrial sediment source, whereas themuddy sediments and weak couplets indicate weak flow conditions.Apart fromwindblown dust, most floodplain sedimentation occurs dur-ing floods and is mostly from suspension (Collinson, 1996). Thus, it isprobably of floodplain/lake environment. Brackish-water gastropodsof A. cf. sculpta at 10.8 m shows a tidal influenced condition. Accordingto Miall (1992), Collinson (1996), and Tamura et al. (2009), permanentlakes, flooding lakes, flood basin, and marshes are all found in a flood-plain environment. Therefore, this unit could be a lake or marsh envi-ronment and was intruded by brackish water at about 8.0 cal kyr BP.

4.1.2.2. H-b: 5.38–3.50 m depth. Unit H-b is characterized by a fining up-ward succession. The mud content ranges from 40% to 60%, less thanthat in other units of the core (Fig. 5). The median grain size is mainlyabout 5–4 ø. The sedimentsmainly consist of graded beds from laminat-ed silty sand to silt upward (Fig. 4j). Sand–mud couplets were found.Cross laminations are found in silt to sand parts. The boundary withthe underlying unit is sharp. Calibrated medium age at the depth of4.75 m is 5731 cal yr BP (Table 3).

The characterization of a fining upward succession reflects a gradu-ally weakening hydrodynamic force. Having sand–mud couplets and asharp contact with underlying unit, this unit will be interpreted astidal-influenced channel fills, which is similar to what was describedby Tanabe et al. (2003). Considering the core's location at the entranceof a tributary of the Changjiang River (Fig. 2), we interpret this unit astidal-influenced channel fills of a tributary on the floodplain.

4.1.2.3. H-c: 3.50–0 m depth. Unit H-c is characterized by dull reddishbrown to brown clayey silt and sandy silt with a fining upward succes-sion. The unit has a gradual contact with the underlying unit. The mudcontent is N60% andmottled by yellowish brown silty claywith scatteredangular gravel. The median grain size is mainly about 6–5 ø. Abundantplant rootlets and fragments and burrows are found (Fig. 4k). This unitis interpreted to be a floodplain to surface soil.

4.1.3. XJ03 coreThe Core XJ03 also consists of Pleistocene and Holocene sedi-

ments. 6.7 m thick sediments in the lowermost part are composedof relatively-consolidated silty clay, and scattered calcareous concre-tions (Fig. 6). The sediment color changes gradually from reddishbrown to grayish green upward (Fig. 4l and m), which is the characterof the Pleistocene sediments in the Changjiang delta area (Chen et al.,1991). The Pleistocene sediments are overlain by 30.5-m thick Holocenesediments with a gradual contact. 14C age at the depth of 30.96 m wasmeasured and the calibrated medium age is about 12.3 cal kyr BP.According to sediment composition, grain size, texture and faunal

content, the Holocene sediments were divided into four units: X-a,X-b, X-c, and X-d (Fig. 6).

4.1.3.1. X-a: 30.5–19.2 m depth. The X-a unit is divided into two upward-coarsening sections. The lower parts of the sections (30.5–27.0 m and24.2–22.0 m in depth) both mainly consist of dark gray clayey silt withmud content N90%, containing abundant plant fragments (Fig. 4n). Gas-tropods of Parafossarulus striatulus, Bellamya sp. and Parabithynialognicornis were found (Table 2) (Fig. 4n and p). Both upper parts(27.0–24.2 m and 22.0–19.2 m in depth) consist mainly of dark graysandy silt with the sand content of 5–40%. Sand–mud couplets arefound. The single couplet is approximately 1 to 4 mm thick (Fig. 4o andq). Foraminifera were found in the uppermost part of the upper subunit(Fig. 6). Carbonate concretions are scattered in the unit. Three 14C agesat the depths of 28.73 m, 24.17 mand 20.25 maremeasured and the cal-ibrated medium ages are 9548 cal yr BP, 9459 cal yr BP, and 8624 cal yrBP, respectively (Table 3).

This unit is characterized by high mud content and abundant plantfragments, which shows back-swamp or flood-plain deposits, analo-gous to floodplain described by Miall (1992) and Collinson (1996),which is also supported by the occurrence of gastropods Parafossarulusstriatulus, Bellamya sp., and Parabithynia lognicornis. The interactive ofclayey siltwith veryfine sand is interpreted as crevasse splay sediments.Floodplain also includes various sub-environments such as back-swamp and crevasse splay (Collinson, 1996). Therefore, this unit be-longs to a floodplain. The plant fragments, laminations and gastropodssuggest that this unit was deposited in a fluvial-dominant condition. Afew foraminifera in the uppermost part reflect an intrusion by brackishwater. Thus, this unit were deposited under a condition affected by bothriver and sea water.

4.1.3.2. X-b: 19.2–10.0 m depth. The X-b unit consists primarily of well-sorted sands (Fig. 4r) with a slightly coarsening upward-succession(Fig. 6). Mud content is from 10% to 50%. The sediments contain manyforaminifera, such as Ammonia beccarii var., Cribrononion porisuturalis,Florilus cf. atlanticus, and Elphidiella asiaticum. Foraminifera abundanceshows an upward decrease with peaks in the middle part (Fig. 6).Sand–mud couplets consist of clayey silt to silt and silty sand to finesand, and the thickness ranges from2 to 30 mm(Fig. 4s). Cross and par-allel laminations and fragments of molluscan shells are found in thisunit (Fig. 6). Four 14C ages at the depths of 18.94 m, 14.70 m, 12.07 mand 10.04 m were measured and the calibrated medium ages are7292 cal yr BP, 5774 cal yr BP, 4489 cal yr BP, and 4763 cal yr BP,respectively (Table 3).

Foraminifera assemblage and sand–mud couplets indicate a saline-water and tide-influenced environment (Tessier and Gigot, 1989; Shi,1991; Dalrymple et al., 1992; Hori et al., 2001b, 2002a, 2002b). Thisunit with a coarsening-upward succession is interpreted as a tide-dominated deltaic environment (Coleman and Wright, 1975; Coleman,1981; Hori et al., 2001a).

4.1.3.3. X-c: 10.0–2.5 m depth. Unit X-c displays a fining-upward succes-sion, consisting of dark gray silty sand to sandy silt (Fig. 6). The mud

http://calib.qub.ac.uk/calib/calib.html

http://calib.qub.ac.uk/calib/calib.html

Fig. 3. Lithological column and sediment units of Core PK01 (age is the median of the calibrated age).


image of Fig.�3

Fig. 4. Photographs presenting the major sedimentary structures of cores PK01, HG01 and XJ03. a: Reddish brown silty clay and calcareous concretion; b: Silt-fine sands with 1–3 mm-thicksand–mud couplet; c: Dark gray clay containing abundant plant fragments; d: Gray brown clayey silt yielding somemolluscan shells; e: Thin laminations of silt-fine sand and clayey silt; f: Thinsand layers (thedarker) and sand–mudcouplets inmuddy layers (the lighter); g: Grayish yellowsilt to silty claywithburrows, charcoals; h:Dark gray clay overlaying the green-grayhard clay;i: Dark gray clay containing abundant plant fragments and gastropods; j: Clayey silt and silty sandwith cross laminations and ripples; k: Grayish yellow silt and claywith abundant plant root-lets and bioturbation; l: Yellow brown to grayish brown clayey silt upward;m: Grayish green hard clay containing abundant plant fragments and organicmaterials; n: Grayish dark clayey siltcontaining plant fragments and gastropods; o: Clayey silt with very fine sand–mud couplets, cross laminations, ripples, and abundant plant fragments; p: Grayish dark clayey silt containingplant fragments and gastropods; q: Clayey silt with very fine sand couplets, cross laminations and bi-directional current structures; r: Grayish dark fine sands; s: Grayish dark fine sands withsilt-clay couplets, abundant plant fragments and shell fragments; t: Grayish dark clayey silt and clayey silt with very fine sand–mud couplets and parallel laminations; u: Grayish dark clayeysilt with very fine sand–mud couplets; v: Grayish yellow and grayish dark very fine sand to silt, containing sand–mud couplets, thin rhythmic laminations and abundant plant fragments; w:Grayish dark clayey silt mottled by gray yellow clayey silt.


content ranges from 24% to 85%. Sand–mud couplets and rhythm lami-nations are found in this unit (Fig. 4t and u). Some plant fragments andforaminifera are also found. This unit overlies the X-b unit with a transi-tional contact. The median of calibrated 14C age at the depths of 7.64 mis 4268 cal yr BP (Table 3).

Sand–mud couplets are typical structures in muddy tidal flats ininter-tidal to sub-tidal environments, where the sediments alsoshow a fining upward-succession (Tessier and Gigot, 1989; Shi, 1991;Dalrymple et al., 1992; Hori et al., 2001b, 2002a, 2002b). Similar to the in-tertidal to subtidal sedimentary environments of the present Changjiang

delta reported by Li et al. (2000a, 2000b) and Hori et al. (2001a), thisunit is interpreted as an intertidal to subtidal flat.

4.1.3.4. X-d: 2.5–0 m depth. The X-d unit predominantly consists of gray-ish yellow and dark gray silt. The mud content is N80% and containsabundant plant rootlets/fragments. The unit can be divided into twosections. The lower section (2.5–1.5 m depth) contains thin rhythmiclaminations (Figs. 4V and 6). The thickness of laminations is b2 mm.Rootlets are found. The upper section (1.5–0 m depth) predominantly

image of Fig.�4

Fig. 5. Lithological column and sediment units of Core HG01 (age is the median of the calibrated age).


consists of dull reddish brown silt at 1.5–1.0 m and grayish dark clayeysilt at 1.0–0.5 m (Figs. 4W and 6).

Thinly rhythmic laminated sediments are reported from the upperpart of the intertidal zone of the present tidal flats of the ChangjiangRiver delta (Li et al., 2000a, 2000b; Hori et al. 2001a). Overlying anintertidal to subtidal flat (unit X-c), this unit is interpreted as asupra-tide (floodplain) to surface soil.

4.2. Modeling result of the relative sea level

To understand the regional background of the relative sea levelchange, local corrections for glacio-hydro-isostasy have been calculated.GIA predictions since 7.0 cal kyr BP, based on the two earthmodels andHolocene melting models described in Section 3, were obtained. Weused the numerical scheme to calculate the past sea level representedby Lambeck et al. (2003). The model result shows that the relative sealevel recorded at the core sites of PK01, HG01, XJ03, DT, JD01 andHQ98 were all at ~2 m above the present sea level at about 7.0 cal kyrBP and gradually declined till the present level, shown by the curve ofthe relative sea level versus time (Fig. 7). The modeling result showsthat the relative sea level at 7.0 cal kyr BP was different between theupper delta region and the coast or East China Sea, and it was variablefrom ~+0.5 m to ~+2.4 m above the present sea level (Fig. 8).

5. Discussion

5.1. Stratigraphic correlation

The latest Pleistocene sediments below the Holocene form a con-tinental sequence deposited during the Last Glacial Period in theChangjiang delta area. The Pleistocene sediments appear as grayishgreen or yellowish green “hard clay” that is widely distributed in the

delta plain (Wang et al., 1981; Li, 1986). Eroded by river, the “hardclay” was not completely preserved, partly replaced by Holocene fluvialsediments or paleo-channel filling sequence in the Changjiang paleo-channel area (Guo et al., 1979). The Holocene sediments cover the latestPleistocene sequence with an unconformable contact (Wang et al., 1981;Li and Li, 1983; Hori et al., 2002a, 2002b). Therefore, the top of the “hardclay” is regarded as an indicator for the boundary between the Holoceneand the latest Pleistocene sequences (Guo et al., 1979;Wang et al., 1981;Chen et al., 1991; Qin et al., 2004). In this research, the top of the “hardclay” is recognized at depths of 17.16 m, 19.2 m and 49.0 m in coresHG01, XJ03 and PK01, respectively.

The Holocene sediments in the Changjiang delta were divided intothree formations: the lower formation (Q4

1), the middle formation (Q42)

and the upper formation (Q43) (Fig. 9) (Guo et al., 1979). The sequence

is characterized by a fining upward to a coarsening upward trend fromsilty fine sand and clayey silty sand of Q4

1, to clay and silty clay of Q42,

then to silt and clayey silt of Q43 (Guoet al., 1979). TheHolocene sediments

are composed of a transgressive depositional sequence and a regres-sive progradational deltaic depositional sequence. The former in-cludes retrogradational estuarine and fluvial depositional system.The deltaic depositional sequence includes progradational systemof prodelta, delta platform and delta plain (Wang et al., 1981; Li and Li,1983; Hori et al., 2002a, 2002b). Units P-a and P-b of Core PK01 and X-a of Core XJ03 are correlated to the transgressive retrogradational se-quence, H-a of CoreHG01 correlated to the sequence from retrogradationto progradation sequence, and other units of P-c, P-d, H-b, H-c, X-b, X-cand X-d correlated to the deltaic progradational sequence (Fig. 9).

5.2. Environmental changes of each core site

Since the earlyHolocene, the site of the PK01 coremaintained afluvialdepositional system inwhich a tidal river andfloodplainwere considered

image of Fig.�5

Fig. 6. Lithological column and sediment units of Core XJ03 (age is the median of the calibrated age).


to be an aggrading fluvial system (Fig. 10) (Tamura et al., 2009). At thepresent Changjiang river mouth, a tidal fluvial system including a tidalriver and its floodplain, of which the water table is influenced by tides,is also classified as an estuary in view of geomorphology. Therefore,Core PK01 recorded an aggrading fluvial system that belonged to a land-ward region of an estuary. An erosional surface at 18.4-m indicates ahiatus between approximately 8.8 cal kyr BP and 4.5 cal kyr BP from14C ages. Abundant brackish water or freshwater shells and shell frag-ments, such as Littoraria sp. andCorbicula sp., are foundnear the erosionalsurface at the age span of 10.0–10.3 cal kyr BP (Table 2) (Fig. 3). This agereversal indicates that these shells were re-worked and not deposited insitu. Below and above this erosional surface, neither foraminifera norbrackish water shells were observed, although laminations caused bythe river and tides existed. The erosional part, spanning from 8.8 cal kyr

BP to 4.5 cal kyr BP,might include themost landwardbrackish sedimentsduring the Holocene. The top layer of brackish water sediments in CorePK01, which might be at the same elevation as that of Core HG01, waseroded by strong fluvial currents after 4.5 cal kyr BP, and was replacedby younger fluvial sediments (Fig. 10).

In Core HG01, an erosional contact above the elevation of 4.0 m atapproximately 7.2 cal kyr BPwas found. Above and below the erosionalsurface, the sediment facies represent channel fills after approximately6.0 cal kyr BP, and a floodplain environment between 9.0 cal kyr BPand 7.2 cal kyr BP, respectively (Figs. 6 and 10). Thefloodplain sedimentscontained abundant freshwater gastropods whereas brackish gastropodsonly appeared at the depth of 10.8 m. At this depth, the 14C age is mea-sured at approximately 8.0 cal kyr BP. Tidal effects are indicated byweak couplets at depths of 7.40–7.60 m and 12.40–12.70 m at ages of

image of Fig.�6

Fig. 7. Predicted relative sea level curves at core sites of PK01, HG01, XJ03, DT01, JD01 and HQ98 during the last 7000 years. In these calculations, we employ two earth models (H50 andH100) and two ESL (Holocenemelting)models (A and B). The upper and lowermantle viscosities for these earthmodels are assumed to be 5 × 1020 and 1022 Pa s, respectively. The ESLmodels are shown in this figure.


about 7.6 cal kyr BP and 8.1 cal kyr BP, respectively. No erosional contactor a hiatus is observed. Thus, we consider the sediment with brackishwater gastropods as the maximum flooding surface at approximately8.0 cal kyr BP (Fig. 10).

A hiatus between 8.4 cal kyr BP and 7.4 cal kyr BP is reflected by anerosional surface at the depth of 19.0 m in the XJ03. The sedimentsbelow the erosional surface showafloodplain spanning from12.0 cal kyrBP to 8.4 cal kyr BP with abundant plant fragments and freshwater gas-tropods Parabithynia lognicornis, P. striatulus, Bellamya sp., B. quadrataand occasional foraminifera (Table 1) (Fig. 6). Above the erosional sur-face, more foraminifera were observed from 7.4 cal kyr BP to 4.8 cal kyrBP.

5.3. Maximum flooding surface (MxFS)

The maximum flooding surface (MxFS) is defined as the marineflooding surface that separates the retrogradational systems from theprogradational systems. Below the MxFS, sedimentary environmentsindicate a fining-upward succession, whereas a coarsening upward

succession developed above the MxFS. Thus, we can identify themaximum-landward shoreline position according to the isochron lineof MxFS (Fig. 10). Hori et al. (2002a, 2002b) estimated the MxFS be-tween 9.0 cal kyr BP and 7.6 cal kyr BP based on core successionsfrom an upward-deepening sedimentary environment to an upward-shallowing environment in the Core HQ98 from Huangqiao area(Fig. 10). The brackishwater gastropods in the CoreHG01 indicatemarineinfluence in themost landward at only 8.0 cal kyr BP (Fig. 10), when a hi-atus occurred in cores XJ03 and HQ98 with a sharp contact. Therefore,the MxFS is regarded at about 8.0 cal kyr BP in the Changjiang Delta(Fig. 10). Brackish water influence at the maximum flooding timereached Nanjing area (Core HG01), which is more landward than theestimates of Wang et al. (1981) and Hori et al. (2002a, 2002b)(Fig. 10). However the Core JD01 did not record such a contact and itsmarine sediments were only deposited during the period of 7.0–6.0 cal kyr BP. Based on cores in the same area as that of the CoreJD01, Li et al. (2002) reported an erosional surface between riverchannel facies and deltaic facies at about 30 m depth. Thus, it is pos-sible that the older sediments in the Core JD01 were eroded due to

image of Fig.�7

Fig. 8. Predicted relative sea level at 7 cal kyr BP in the Changjiang delta and the East China Sea. The earth model for H50 and the ESL (Holocene melting) model for B are used.


very strong tidal forcing and replaced by marine sediments deposit-ed after 7.0 cal kyr BP (Li et al., 2009).

The age of the MxFS in the Changjiang delta is within the period ofworldwide delta initiation between 9.5 and 7.4 cal kyr BP as suggestedby Stanley andWarne (1994).Moreprecise ages of theMxFS forHolocene

Fig. 9. Schematic diagram showing the stratigraphic correlation of

deltas have recently been reported from the Song Hong (Red River) delta(Tanabe et al., 2003) and Mekong River delta (Tamura et al., 2009;Nguyen et al., 2010) at ~8.0 cal kyr BP as well. These are correlatedwith a short sea-level stillstand or relatively abundant sediment accu-mulation at a river mouth when rates of sea-level rise decelerated

the cores from the uppermost region of the Changjiang delta.

image of Fig.�8

image of Fig.�9

Fig. 10. Stratigraphic correlation and the initiation of Changjiang (Yangtze) delta.


remarkably before a further 3–5 m sea-level rise until 7–6 cal kyr BP(Bird et al., 2010).

The very low sediment accumulation rate of prodelta sedimentsin the Core HQ98 might result from the fact that most sedimentswere trapped in a fluvial-delta plain to an estuary–delta transitionalenvironment around Nanjing (Core HG01) to Yizheng (Core XJ03)(Figs. 2, 10). The sea-level rise from the time of MxFS to 7–6 cal kyrBP created enough accommodation space for delta initiation.

5.4. Holocene relative sea-level changes

Our data from sediment cores indicate a possibility of a relative sea-level highstand at 6–4 cal kyr BP (Saito et al., 1998; Hori et al., 2001a;Liu et al., 2004; Hori and Saito, 2007) rather than a sea level 2–5 mlower than the present one which is supported from the lower Yangtzedelta region by Chen and Stanley (1998), Zong (2004) and Chen et al.(2008) (Fig. 11). The upper limit of the upper intertidal sediments isat the elevations of +2.3 m (2.5 m in depth) in the Core XJ03,+3.2 m (2.5 m in depth) in the Core JD01 and +4.4 m (1.5 m indepth) in the Core HQ98, respectively (Hori et al., 2001a; Li et al.,2009). Li and Min (1981) also reported coastal saltmarsh facies withmaximum elevation of about +2.1 m (DT in Table 1 and Fig. 10). Thepresent average tidal range is approximately 2.7 m in the river mouth(Shen et al., 1988). Not considering the compaction effect, we estimatethat the mid-Holocene sea level stood nearly 1–2 m above the present(Fig. 11). However, these apparent highstand records also could becaused by stronger-than-present tides. Li et al. (2009) reported a strong

tide-influenced sedimentary regime during 6–5 cal kyr BP in the upperdelta plain controlled by lots of 14C dating (Li et al., 2009). Numericalsimulation of tides at the maximum transgression during the Holocenealso shows strong tides in the Paleo-Changjiang estuary, which is evenstronger than that of the present Qiantangjiang Estuary (Uehara et al.,2002; Uehara and Saito, 2003, 2012).

On the other hand, those local records of the relative sea level couldalso be influenced by the variable tectonic movements and complexhydro-isostatic processes. Hori et al. (2001a) andWang et al. (2012) as-cribed the sea-level highstand in the mid-Holocene in the Changjiangdelta region to the result of hydro-isostatic effects. Ourmodeling resultsfrom glacio-hydro-isostatic adjustment (GIA) show the relative sealevel change in the last 7000 years (Fig. 7). The relative sea-level curvesat the core locations in the upper region of the delta (100–200 km land-ward from the present shoreline) obviously reflect a similar highstandat about 2 m above the present sea level during about 7.0–4.0 cal kyrBP (Fig. 7). This sea level highstand in the mid-Holocene results fromhydro-isostasy, similar to the Mekong River delta (Nguyen et al., 2010;Hanebuth et al., 2011). The difference of the maximum sea-level heightrecorded in the sediment cores from the modeling result might becaused by compaction effects and/or topographic effects in numericalsimulation, which neglected the 200-km shoreline migration seawardfor the last 7000 years. The glacio-hydro-isostatic adjustment alsoresulted in differences of the relative sea level spatially, which variedfrom 2.4 m above the present sea level in the upper region to 0.5 m inthe coast or East China Sea (Fig. 8). This could be the main reason ofthe difference between our mid-Holocene sea level highstand on the

image of Fig.�10

Fig. 11. Schematic diagram of accumulation curve for each core and sea-level curves. Thickness of the sea level curve in this study is vertically limited by 4 m according to the present tidalrange (Li and Min, 1981; Chen and Stanley, 1998; Hori et al., 2001b; Zong, 2004; Chen et al., 2008; Li et al., 2009).


upper delta area and those lower sea level records in the coast or EastChina Sea (Figs. 8, 11) (Chen and Stanley, 1998; Zong, 2004; Chenet al., 2008).

The sea-level curve shows a slow rise during 12–9 cal kyr BP, a rapidrise during 9–8 cal kyr BP, and another slow rise during 8–6 cal kyr BP.The sea-level rise after 8 cal kyr BP is correlated with a 4–5 m sea-levelrise shown by Bird et al. (2007, 2010) and Yu et al. (2007). The rapidsea-level rise during 9–8 cal kyr BP was associated with an earlyHolocene sea-level jump reported from many regions, for example,the Red River incised-valley in Vietnam, coastal area of the Changjiangincised-valley in China, Kiso River incised-valley in Japan, a gap of coraldata in the Huon Peninsula in Papua New Guinea (Hori and Saito,2007), and the Mekong River incised-valley in Cambodia and Vietnam(Tamura et al., 2009; Nguyen et al., 2010).

5.5. Initial stage of the Holocene Changjiang delta

Most of previous researches estimated that the Changjiang delta ini-tiated at about Hongqiao area during 7.5–6.0 14C kyr BP (Fig. 1) (Chenet al., 1959; Delta Research Group, 1978; Li et al., 1979; Wang et al.,1981; Li and Li, 1983; Hori et al., 2001a, 2002a, 2002b). However, ourstudy shows that the apex of the Holocene Changjiang delta is locatedbetween sites of Core HG01 from Nanjing and XJ03 from Yizheng, land-ward of the Hongqiao area (Figs. 1 and 10). Two cores (XJ03 and JD01)from theHongqiao area indicate that the sand body formedmainly dur-ing 6–5 cal kyr BP (Fig. 8). The age of the Hongqiao sand body is muchyounger than that in themodel of the Delta Research Group (1978). Anelevated isochron line at 5 cal kyr BP might be associated with high ac-cretion by the overlap sand bodies of Hongqiao and Huangqiao, whichdeveloped together (Fig. 10).

Consistent with the initiation of the worldwide deltas, theChangjiang delta began to prograde seaward under the condition ofthe decelerated sea level rise after 8.0 cal kyr BP. Progressive depositionoccurred after themaximum flooding time and the delta has initiated inYizheng area since 7.4 cal kyr BP. The isochron lines of 8.0, 7.0 and6.0 cal kyr BP from the site of Core HG01 to the site of Core HQ98are approximately parallel, which means aggradation prevailing overprogradation. This might reflect a continuous sea-level rise lasting to6.0 cal kyr BP (Fig. 10), which is well consistent with other sea-levelcurves in the Changjiang area (Chen and Stanley, 1998; Zong, 2004)and other regions (Bird et al., 2007, 2010; Yu et al., 2007). The initiationof the delta was also regarded as the formation of a tidal plainwithin anestuarine system created byfluvial infill because there is no clear typicaldeltaic sedimentary sequence in a funnel-shaped estuary, as from thefirst two stages of Hongqiao and Huangqiao (Li and Fan, 2009). The ini-tial delta shows an upward aggradation prevailing over progradation,and it can also be regarded as an estuarine environment (Li et al.,2000a), or a transition from retrogradational estuary to progradationaldelta.

When the sea level kept stable after 6.0 cal kyr BP, the deltaicdepocentermoved seaward, and sand bodies of Hongqiao andHuangqiaowere rapidly deposited (Figs. 1, 10 and11). A typical tide-dominateddeltabegan to develop after the Hongqiao sand body had grown above thewater table when amounts of sediments filled the inlet at about 5.5–5.0 cal kyr BP. According to isochronal lines and sediment facies, theupper sediments of the Core XJ03 were deposited much later than thesediments of the Core JD01 (Fig. 10). This might result from the landwardor lateral development of the Hongqiao sand body. Similar evolution ofriver-mouth bars is reported from the present Changjiang river moutharea (Chen et al., 1979; Wang et al., 1981; Meng and Cheng, 2005;Zhang and Meng, 2009).

image of Fig.�11


6. Conclusions

Three cores PK01, HG01 and XJ03 were drilled from the uppermostregion of the present Changjiang delta plain to study the initiation of theChangjiang delta relative to sea level changes. As an indicator of theboundary between the Holocene and Pleistocene sediments in the re-search area, the top of the Pleistocene hard clay is observed at depthsof 17.16 m, 19.2 m and 49.0 m in cores HG01, XJ03 and PK01, respec-tively. The Holocene sediments cover the hard claywith an unconform-able contact. On the basis of the sediment composition, grain size,texture, faunal content, and comparison with previous research onCore JD01 (Li et al., 2009) and Core HQ98 (Hori et al., 2001b), threesediment systems were reconstructed in the Holocene sediments:(1) a fluvial depositional system, (2) an estuarine depositional sys-tem, and (3) a deltaic depositional system.

Core PK01 records two sets of fluvial sediments, a transgressive suc-cession in 12.0–8.8 cal kyr BP and a progradational succession after4.5 cal kyr BP with a hiatus from 8.8 cal kyr BP to 4.5 cal kyr BP sepa-rating the two sequences. Core HG01 records a floodplain environmentin the period 9.0–7.2 cal kyr BP and tide-influenced channel fills tosurface soil after 6.0 cal kyr BP. In Core XJ03, a deltaic progradationalsystem including an estuarine–deltaic transition and typical deltasubsystemhad covered the estuarine depositional system (transgression)after 7.4 cal kyr BP.

MxFS is recognized at 8.0 cal kyr BP, when saline-water intruded tothe site HG01 in Nanjing area, much more inland than the estimates ofWang et al. (1981) and Hori et al. (2002a, 2002b). The MxFS about8.0 cal kyr BP can be correlated with a short stillstand or a deceleratedrise of sea level before a further 3–5 m sea-level rise until 7–6 cal kyrBP (Bird et al., 2010). Results from both observations of drilling coresand a glacio-hydro-isostatic adjustment (GIA) model imply that relativesea level at 6–4 cal kyr BP stood about 2 m above the present. The differ-ence of the relative sea level between the landward region of theChangjiang delta and the East China Sea could result from hydro-isostatic tilting.

The Changjiang delta began to prograde seaward under the condi-tion of the decelerated sea level rise after 8.0 cal kyr BP of the MxFS.This initial stage of Changjiang delta was characterized by a degradationprevailing over progradation until 6.0 cal kyr BP. When sea level keptstable after 6.0 cal kyr BP, Hongqiao and Huangqiao sand bodies weredeveloped with a prograding deltaic depocenter (Figs. 1, 10). The apexof the Holocene Changjiang delta is located between Nanjing andYizheng, more landward from the Hongqiao area. The Hongqiaosand body formed mainly during 6.0–5.0 cal kyr BP, about one thou-sand years younger than previously suggested. Overlapping sandbodies of Hongqiao and Huangqiao resulted in a high accumulationrate until 5.0 cal kyr BP, as indicated by an elevation isochron line.The upper sediments of the Core XJ03 were deposited much laterthan the sediments of the Core JD01. This might be due to the landwardor lateral migration of the Hongqiao sand body, similar to the develop-ment pattern of present-day river-mouth bars of the Changjiang deltaarea.

Acknowledgments

Thisworkwas supported by the ChineseNatural Science Foundation(No. 41076034), the Creative ResearchGroups of China (No. 40721004),908 special project of the State Oceanic Administration (No. 908-02-01-04), the 111 project, and theMinistry of Science and Technology (Grant,No. SKLEC-009KYYN02). Mapping Technical Support Center of JiangsuProvince helped to measure and correct elevations. We thank Prof. Q.Q.Chen, Ms. F. Zhang, Dr. L. Ye, Ms. Y. Liu, Dr. Y. Yang and Dr. Z. Yang fromthe State Key Laboratory of Estuarine and Coastal Research, East ChinaNormal University for their help in the field work and subsampling. Wegreatly appreciate Senior Editor Dr. Derek Booth, Editor Dr. Paul Hesse,

Prof. Karl Stattegger and another anonymous reviewer for their usefulsuggestions and comments.

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Initiation of the Changjiang (Yangtze) delta and its response to the mid-Holocene sea level change