Holocene Delta Evolution and Sediment Flux of the Pearl River

JOURNAL OF QUATERNARY SCIENCE (2016) 31(5) 484–494 ISSN 0267-8179. DOI: 10.1002/jqs.2873

Holocene delta evolution and sediment ﬂux of the Pearl River, southern China

XING WEI,1* CHAOYU WU,2 PEITONG NI3 and WENYUAN MO2 1State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Science, Guangzhou 510301, China 2Center for Coastal Ocean Science and Technology Research, Sun Yat-sen University, Guangzhou 510275, China 3Guangdong Research Institute of Water Resources and Hydropower, Guangzhou 510610, China Received 19 January 2016; Revised 19 April 2016; Accepted 14 May 2016

ABSTRACT: Delta progradation and sediment flux of the Pearl River Delta (PRD), southern China, during the Holocene are presented based on analyses of borehole data on the delta plain. Results indicate that the delta prograded into the drowned valley because of early Holocene inundation from 9 to 6 cal ka BP, as sea-level rise decelerated. The sea level reached its present level at about 6 cal ka BP and, as a consequence, a large portion of the drowned valley was covered by the estuary, with more than 160 rock islands and platforms. The scattered landmasses promoted active deposition and acted as deposition nuclei during deltaic evolution. Consequently, apart from exhibiting a general tendency towards progression, PRD development occurred less regularly over time and space because of deposition around island boundaries. During the last 2 ka, mainly because of significantly increased human activities, which have trapped sediments in the encircled tidal flats along the front of delta plains, the shoreline has advanced rapidly. Estimated sediment fluxes for the three periods (9–6, 6–2 and 2–0 cal ka BP), based on the sediment volume analysis, were 17–25, 22–30 and 44–58 million t a 1, respectively. Copyright # 2016 John Wiley & Sons, Ltd.

KEYWORDS: delta evolution; Holocene; Pearl River; sea-level change; sediment flux.

Introduction estuary of about 1740 km2 (Zong et al., 2009; Wu et al., 2010). Because the receiving basin is semi-closed and no Deltas are recognized as discrete bulges in the shoreline offshore sediment dispersal to the deep ocean has occurred where the associated rivers deliver more sediment than (Huang et al., 1982; Wei and Wu, 2014), by quantifying the marine processes can redistribute (Elliot, 1978). Recent deltaic sediments of the Pearl River, we can calculate its studies have shown that on a millennial time scale, coastal Holocene sediment flux. hydrodynamics and past sediment discharges are the key In this paper, we present new data from three newly factors controlling delta morphology and progradation rates collected drill cores taken from the PRD. We then compile during the middle to late Holocene. For example, the data on sedimentary facies and radiocarbon dates from 19 sedimentary facies of the delta front became coarser grained, previously described sediment cores taken from the delta and the progradation rate of the Mekong Delta decreased at (Fig. 1). Based on the sedimentary facies data and radiocar- 3.0–2.5 cal ka BP, in large part owing to changes in the bon dates from all the sediment cores, the palaeogeography coastal oceanographic setting from a tide-dominated bay and evolution of the PRD and sediment flux during the last head to a wave-influenced open coast (Ta et al., 2002; 9 ka are discussed. Tanabe et al., 2003). The morphology of the Yellow River Delta also changed from straight to lobate because of an increase in river sediment discharge caused by human Study area activities in its drainage basin during the last 1000 years The Pearl River system, 2044 km long, is one of the most (Saito et al., 2001). An increase in sediment discharge during important river systems in China. The Pearl River catchment the late Holocene also created deltaic protrusions of the is located between 102˚150–114˚530E and 21˚500–26˚490N. shorelines at the mouths of the Po, Tevere, Krishna and Song The total area of the catchment within China is 44.21 104 Hong rivers (Rao et al., 1990; Bellotti et al., 1994; Cencini, km2. Its delta, the PRD, is located in Guangdong province, 1998; Amorosi and Milli, 2001; Tanabe et al., 2006). Many South China. After entering the delta area, the West River, factors control the morphodynamics and sedimentary facies North River, East River, Liuxi River and Tang River, which are of deltas. Therefore, comprehensive, quantitative analyses of the main tributaries of the Pearl River, bifurcate continuously variable effects such as coastal hydrodynamics, sediment flux and form the extremely complicated network system of the and sea-level fluctuation are required to better understand Pearl River. The Pearl River discharges into estuarine bays delta evolution. through eight major outlets. The network and the estuarine The Pearl River delta (PRD), located in southern China, is bays are affected by tidal rise and freshwater dilution; thus the third-largest estuarine delta in China after the Yangtze they are both ‘estuaries’ by definition (Samoylov, 1958; and Yellow River Deltas, in terms of delta plain area. Pritchard, 1967). Following the terminology of Galloway (1975), the PRD can The hydraulic features of the Pearl River are summarized be classified as a tide-dominated system. For the last 6 ka, in Table 1. Water discharge levels from the Pearl River during the Holocene sea-level highstand, the delta has vary seasonally because most of the drainage area exists prograded more than 160 km seaward because of the volumi- within a subtropical monsoon climate regime. Runoff and nous supply of sediment from the Pearl River. At present the sediment discharge during the flood season (April–September) receiving basin is not completely filled and drains into a large account for 74–84 and 91–95% of the total annual amount, Correspondence to: X. Wei, as above. respectively. Suspended sediments consist primarily of silt E-mail: [email protected] and clay (Huang et al., 1982; Zhao, 1990). Although bedload

Figure 1. (A) The drainage basin of the Pearl River. (B) Map showing the Pearl River delta plain, estuary and bathymetry. Water-depth contour interval is 5 m. Open circles indicate the borehole sites. A–A’, B–B’, C–C’ and D–D’ are the locations of the proﬁles shown in Fig. 5.

sediments consist of medium and coarse sand, these sedi- average ﬂood tide volumes reach as high as 73 500 m3 s 1, ments are thought to account for <10 % of the total sediment which is nearly 13 times the average freshwater discharge load (Li et al., 1990). (Zong et al., 2009). Tides in the Pearl River estuary are irregular and semi- Wind-driven waves and currents have a minimal impact on diurnal with amplitudes that range from approximately sediment transport within the estuary relative to tidal currents 0.8–0.9 m near the Wanshan Islands to approximately 1.7 m (Owen, 2005). Protected from storm waves by offshore near Humen (Zhao, 1990). Despite this small tidal range, rocky islands, wave energy levels within the estuary are low,

Table 1. Flow and sediment discharge of the Pearl River (averaged from 1954 to 2000).

Flow Suspended load

Tributary Annual amount (108 m3) Mean discharge (m3 s 1) % Concentration (kg m 3) Annual amount (104 t) %

West River 2220 7020 73.29 0.32 7100 80.03 North River 413 1310 13.63 0.13 647 7.29 East River 233 737 7.69 0.13 296 3.34 Sui River 68.4 217 2.26 0.16 109 1.23 Zeng River 38.2 121 1.26 0.14 56.1 0.63 Liuxi River 18.7 59.4 0.62 0.06 10.2 0.11 Tan River 20.7 65.5 0.68 0.11 23.0 0.26 The rest 17.1 0.56 630.7 7.11 Total 3029.1 9584.3 100 8872 100

Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) 486 JOURNAL OF QUATERNARY SCIENCE although not during the passage of typhoons. Under typhoon deltaic sediments). In ascending order, these consist of two conditions, wave heights in excess of 1.5 m are common (channel-fill and floodplain sediments), two (tide-influenced (Zong et al., 2009). channel-fill and sub- to intertidal flat sediments) and five (tide-influenced channel-fill, delta front platform, tidal flat, Materials and methods abandoned channel-fill sediments and floodplain sediments) 0 00 0 00 sedimentary facies, respectively (Fig. 2). Each sedimentary Three borehole cores, PRD04 (22˚29 23 N, 113˚11 38 E), unit is characterized by a combination of its lithology, colour, PRD10 (22˚4302200N, 113˚1404200E) and PRD15 (22˚5404900N, 0 00 sedimentary structures, textures, contact character, lithologi- 113˚31 02 E), were obtained from the present delta plain cal succession, fossil components, grain size, and radiocar- during 2004–2005. The cores were split, described and bon dates. Details of the sedimentary units are described photographed. Radiographs of slab samples were taken below. throughout all split cores. Sand and mud (silt and clay) contents were measured every 20 cm throughout the core Unit 0 (late Pleistocene undifferentiated sediments) using 5-cm-thick sand and 2-cm-thick silt and clay samples. Fossils and microfossils such as diatoms, foraminifers and This unit is the lowest part of the section, approximately mollusc species were identified. Twenty-seven samples were below 19.52 m (below present sea level) at PRD10 and collected for 14C dating; analyses were undertaken on plant approximately 17.79 m at PRD15; the unit was not recov- fragments and molluscan shells by conventional 14C dating at ered at the PRD04 site. It is mainly composed of mottled, the Key Laboratory of Isotope Geochronology and Geochem- slightly oxidized, grey stiff silt, silty clay and sandy silt istry, Guangzhou, Chinese Academy of Sciences. Radiocar- (Fig. 3A), suggesting a period of subaerial exposure. This unit bon ages were calibrated using the IntCal13 calibration curve is unconformably overlain by the Holocene estuarine sedi- (Reimer et al., 2013) and the CALIB 7.0.4 calibration program ments, with a sharp contact in lithology and colour. This unit 14 (calib.qub.ac.uk/calib/). is dated to 35 610 480 C a BP at the PRD15 core. Another 19 representative core records were chosen from the literature to complement the study as these core records Unit 1 (fluvial sediments) provide sedimentary, microfossil and macrofossil information, This unit is present in the lowest part of the PRD04 core from and radiocarbon dates (Huang et al., 1982; Li et al., 1990; 29.66 to 25.36 m. It consists of pebbly sand (Facies 1.1) Long et al., 1997; Zong et al., 2009). The core records and mottled clay (Facies 1.2) in ascending order. The sedi- provided information for the construction of a series of cross- ments display an overall fining-upward succession (Fig. 2). sections across the deltaic plains. The unit contains abundant organic materials composed of 14 All ages in this paper are reported as calibrated C ages plant fragments but no shell fragments. Mud clasts are 14 (cal a BP) unless otherwise noted as C a BP. common. This unit is interpreted as fluvial sediments, because the sediments are not bioturbated and the abundant Results plant fragments indicate that the sediment was deposited Sedimentary units and facies under predominantly terrestrial conditions. Furthermore, a fining-upward succession is characteristic of lateral accretion The PRD04, PRD10 and PRD15 core sediments can be in a meandering river system (Allen, 1963; Visher, 1965; divided into four sedimentary units, 0 (late Pleistocene Miall, 1992). undifferentiated sediments), 1 (latest Pleistocene fluvial sedi- Details of facies 1.1 and 1.2 composing this unit are ments), 2 (Holocene estuarine sediments) and 3 (Holocene described below.

Figure 2. Sedimentary columns of cores PRD04, PRD10 and PRD15 cores.

Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) DELTA EVOLUTION AND SEDIMENT FLUX OF THE PEARL RIVER, SOUTHERN CHINA 487

Figure 3. Photographs of typical sedimentary facies of cores PRD04, PRD10 and PRD15. Top is at the upper left in all ﬁgures. Scale bar ¼ 10 cm (A) Unit 0, core PRD10, 21.80–22.88 m depth. Tan-coloured silty sand overlain by black clay. (B) Facies 1.1, core PRD04, 27.76–28.94 m depth. Yellow gravelly sand. (C) Facies 1.2, core PRD04, 26.84–27.72 m depth. Grey organic clay. (D) Facies 2.1, core PRD04, 25.72–26.69 m depth. Sand–mud alternation. (E) Facies 2.2, core PRD10, 18.67–20.66 m depth. Bioturbated sandy clay. (F) Facies 3.1, core PRD10, 5.78–7.35 m depth. Sand–mud alternation. (G) Facies 3.2, core PRD04, 6.12–7.68 m depth. Medium sand interbedded with black organic clay. (H) Facies 3.3, core PRD04, 4.09–6.01 m depth. Interlaminated sand and mud with rootlets. (I) Facies 3.4, core PRD15, 2.10–3.45 m depth. Grey mottled clay. (J) Facies 3.5, core PRD04, 1.0–2.0 m depth. Yellow grey silty clay.

Facies 1.1 (River-channel sediments) sandy silt and fine sand (Fig. 3E). Shell fragments and mica flakes are common. A large portion of this facies is bioturbated, This facies is found from 29.66 to 26.37 m at the PRD04 but very fine sand and clay laminations remain as faint lenses site. It is composed of pebbly sand (Fig. 3B) and poorly sorted 3–8 mm thick. This facies is interpreted as subtidal to intertidal medium to fine sands with cross-bedding. This facies is flat sediments because of the lenticular bedding, which is characterized by a fining-upward succession with an ero- characteristic of those environments (Reineck and Singh, 1980). sional surface at the bottom. No marine fossils or tidal Sand is transported and deposited by currents, whereas mud in sedimentary structures were found. The river channel envi- suspension settles onto the sand during slack-water periods ronment is interpreted on the basis of the bottom erosional (Reineck and Wunderlich, 1968). Shelly layers of P. laevis and surface, the fining-upward succession and the lack of marine Balanus sp. indicate a brackish-water environment. fossils. Unit 3 (deltaic sediments) Facies 1.2 (floodplain sediments) This unit in cores PRD04, PRD10 and PRD15 is characterized This facies is found from 26.37 to 25.36 m at the PRD04 by laminated sand containing cross-lamination and abundant site. It is composed of grey silty clay (Fig. 3C). Sand lenses, shell fragments (Fig. 2). It consists of a coarsening-upward plant debris and peat layers were found in the lower part the lithological succession from massive clay to interlaminated facies. The silty clay is intercalated with well-sorted very fine sand and mud or massive sand. Molluscan shells vary upward to fine sand beds in the interval from 26.30 to 25.40 m. from those with a shallow-marine habitat, including Scap- The sedimentary environment of this facies is interpreted as hander idae sp., to those with an intertidal habitat, including floodplain sediments based on its gradational contact with Potamocorbula sp. The number of burrows and intensity of the underlying channel sand and the occurrence of tidal bioturbation decreases upward, whereas the number of wood laminae. The sediments were dated to 9975 and 8472 cal a and plant fragments increases upward. This unit is interpreted BP at 26.22 and 25.20 m, respectively. as deltaic sediments because an upward-coarsening and shallowing lithological succession is typical of prograding Unit 2 (estuarine sediments) deltaic deposits (Scruton, 1960; Visher, 1965; Coleman and This unit is characterized by thinly and rhythmically inter- Wright, 1975). laminated or bedded sand and mud, cross-lamination and Details of facies 3.1–3.5 composing this unit are described paired mud drapes. Abundant wood and plant fragments and below. molluscan shells of Balanus sp., Nassarius sp. and Arca sp. occur in the sediments. The unit yielded 14C dates ranging Facies 3.1 (tide-influenced channel-fill sediments) from 9.1 to 6.6 cal ka BP. The rhythmically interlaminated/ This facies can be found in all the three cores. It consists of bedded sand and mud, cross-lamination and paired mud well-sorted fine to medium sand partly interlaminated/bedded drapes of this unit indicate that the sediments were deposited with clay and silt (Fig. 3F). The medium sand contains in environments strongly influenced by tides (Reineck and abundant shell fragments of Potamocorbula sp. andCorbicula Singh, 1980; Nio and Yang, 1991; De Boer, 1998). The sp., which are mostly broken into thin pieces <5mm in wood/plant fragments and the molluscan shells, respectively, diameter. The occurrence of Potamocorbula sp. and Corbic- indicate terrestrial input and a brackish-water environment. ula sp. also indicates a brackish-water environment, which The mixture of terrestrial input with brackish water and tidal supports the interpretation that the unit was deposited in a influences suggests that this sediment was deposited in a tide-influenced environment. Sedimentary structures such as mixed zone with marine and fluvial influences such as in an cross-lamination and lenticular bedding are found. This facies estuarine environment. is interpreted as tide-influenced channel-fill sediments. Details of facies 2.1 and 2.2 composing this unit are described below. Facies 3.2 (delta front platform sediments) Facies 2.1 (tide-influenced channel-fill to coastal This facies was found at the PRD04 site. It consists marsh sediments) of interbedded/laminated sand and mud (Fig. 3G). Cross- lamination and bivalve shells of Potamocorbula sp. from an This facies occurs at the PRD04 and PRD15 sites (Fig. 2), and intertidal to shallow-marine habitat are common throughout it shows an overall fining-upward succession from medium this facies. The sedimentary structures suggests that this sand to laminated clay (Fig. 3D). Sedimentary structures such facies was deposited in a tide-influenced environment. From as parallel lamination and lenticular bedding are found. There the lithological succession from Facies 3.1, this facies is an intermixture of marine plankton and marine–brackish corresponds to a delta front platform environment. and freshwater diatom species, suggesting a marine, in particular a tidal, influence. This facies is interpreted as tide- Facies 3.3 (tidal flat sediments) influenced channel-fill to coastal marsh sediments. An overall fining-upward lithological succession and the occurrence of This facies can be found in all three cores. It is generally grey mud drapes and bidirectional ripple cross-lamination are to yellowish grey mud and muddy sand with 30–60% fine common in tidal creek and tidal flat sediments (Reineck and sand, 20–50% silt and 10–20% clay (Fig. 3H). A fining-upward Singh, 1980; Dalrymple, 1992). Peaty laminated clay or clay trend is typical of this facies. Iron and manganese nodules and with roots, which occurs at the top of this facies, is a common plant debris are common near the top. The sedimentary feature of floodplain and coastal marsh environments (Frey structure is characterized by cross- and flaser bedding in the and Basan, 1985; Miall, 1992; Collinson, 1996). lower portion, lentoid and wavy bedding in the middle and horizontal bedding variously upward with horizontally lami- Facies 2.2 (sub- to intertidal flat sediments) nated beds, where bimodal cross-bedding and deformation bedding exist occasionally. The combination of sedimentary This facies is found from 19.62 to 11.37 m at the PRD10 structures indicates a tidal flat depositional environment, site and is characterized by dark grey, intensively bioturbated similar to those described by Reineck and Singh (1980).

Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) DELTA EVOLUTION AND SEDIMENT FLUX OF THE PEARL RIVER, SOUTHERN CHINA 489

Table 2. List of 14C ages from the cores taken from the Pearl River Delta.

Calibrated age (cal a BP) (1s)

Core Altitude (m) Material Conventional 14C age (a BP) Intercept Range Sample code Ref.

PK4 4.30 Plant fragment 5940 300 6791 6479–7157 KWG-5 B K1 5.00 Bulk organic matter 5900 320 6747 6405–7030 C PK16 0.90 Bulk organic 2670 85 2794 2725–2876 KWG-100 E 12.20 Plant fragment 6150 160 7032 6879–7248 GC-520 E K5 5.30 Plant fragment 6300 330 7151 6841–7497 KWG-8 B D20 1.1 Bulk organic matter 5813 140 6625 6449–6755 D D24 1.50 Shell 3997 190 4472 4235–4709 D 4.90 Bulk organic matter 6174 140 7061 6912–7248 D D02 6.00 Oyster shell 5440 120 6247 6172–6322 D 8.40 Oyster shell 6550 150 7447 7320–7573 D 11.40 Oyster shell 6620 170 7538 7414–7662 D ZK18 4.40 Wood 2250 90 2241 2151–2347 C 13.00 Bulk organic matter 6150 110 7041 6893–7172 C A23 4.70 Bulk organic matter 1610 80 1501 1404–1570 KWG-62 B 21.80 Oyster shell 5360 160 6140 5987–6292 KWG-55 B D04 3.70 Bulk organic matter 2270 130 2283 2110–2464 D JL2 8.1 Oyster shell 3950 150 4368 4153–4582 GC-687 C D10 2.6 Bulk organic matter 2560 120 2626 2481–2770 D 10.4 Bulk organic matter 3390 150 3652 3470–3831 D 17.1 Wood 4600 200 5261 5038–5483 D ZK08 11.2 Bulk organic matter 3710 110 4065 3897–4164 B 19.36 Bulk organic matter 5660 130 6464 6309–6566 B 24.70 Bulk organic matter 8930 220 10 008 9683–10 250 B GK2 12.9 Bulk organic matter 4710 120 5427 5319–5429 KWG-99 C DZ1 0.20 Bulk organic matter 3180 95 3400 3327–3494 D 8.90 Bulk organic matter 6040 130 6905 6734–7026 D 19.50 Bulk organic matter 8920 129 9995 9886–10 222 D ZK3 33.00 Shell 7340 140 8121 8021–8221 B D6 2.20 Bulk organic matter 1390 70 1309 1263–1375 KWG-48 D 18.20 Oyster shell 2350 90 2401 2306–2496 KWG-49 D 54.20 Bulk organic matter 8050 200 8941 8636–9140 D ZK2 13.00 Bulk organic matter 502 91 530 474–566 GZ8901 B 29.00 Bulk organic matter 7580 520 8475 7924–9022 GZ8902 B PRD04 0.62 Shell 2230 180 2225 1989–2463 GC-05-108 A 6.82 Bulk organic matter 2665 100 2785 2707–2928 GC-05-115 A 7.75 Bulk organic matter 3050 100 3237 3141–3375 GC-05-124 A 13.78 Bulk organic matter 3710 110 4065 3897–4164 GC-05-125 A 14.68 Bulk organic matter 4320 115 4922 4809–5054 GC-05-126 A 15.84 Bulk organic matter 5495 125 6284 6179–6438 GC-05-127 A 19.06 Shell 6210 115 7116 6978–7254 GC-05-123 A 20.89 Shell 6505 100 7409 7317–7500 GC-05-131 A 22.12 Shell 6780 115 7648 7559–7736 GC-05-132 A 25.20 Bulk organic matter 7650 180 8472 8289–8635 GC-05-135 A 26.22 Wood 8885 100 9975 9886–10 183 GC-05-136 A PRD10 4.17 Shell 2685 150 2848 2696–2999 GC-06-112 A 6.46 Wood 2820 140 2966 2778–3078 GC-06-110 A 9.02 Shell 3540 120 3833 3686–3980 GC-06-108 A 10.12 Shell 4100 130 4668 4514–4821 GC-06-107 A 10.99 Shell 4820 90 5559 5466–5652 GC-06-106 A 12.26 Bulk organic matter 4920 150 5668 5578–5770 GC-06-105 A 14.27 Shell 6760 130 7607 7493–7721 GC-06-102 A 15.67 Shell 8130 145 9124 8952–9295 GC-06-101 A 17.62 Shell 8280 300 9235 8927–9543 GC-06-99 A 19.05 Shell 8530 130 9544 9403–9685 GC-06-97 A PRD15 0.01 Wood 1030 160 957 781–1086 GC-06-28 A 1.29 Bulk organic matter 1140 140 1066 931–1184 GC-06-27 A 4.21 Bulk organic matter 1230 180 1141 965–1298 GC-06-24 A 8.57 Plant fragment 1315 95 1226 1172–1314 GC-06-21 A 9.64 Shell 7110 105 7925 7824–8025 GC-06-19 A 14.12 Shell 8010 135 8834 8644–9024 GC-06-14 A

References: A, this study; B, Huang et al. (1982); C, Li et al. (1990); D, Long et al. (1997); E, Zong et al. (2009).

Facies 3.4 (abandoned channel-fill sediments) 15–20 m in the depocentre. In the mouth area of the basin, the valleys incise to 25–30 m. Comparatively, the palaeo-basin is This facies is found at the PRD10 and PRD15 sites. It shows much shallower and more complex than those in the Yangtze an overall fining-upward succession from well-sorted medium (60–100 m, Li et al., 2000), the Mekong (ca. 70 m, Ta et al., sand to grey, mottled silt (Fig. 3I). Mud clasts and parallel 2002) and the Song Hong River (ca. 40 m, Tanabe et al., lamination occur in the sand. Burrows and in situ jointed 2006). The shallow nature of the receiving basin has resulted Corbicula sp. are common in the greyish clay at the top of in it being initially inundated by the sea as late as 9 cal ka BP this facies. This suggests that the sediments were influenced when relative sea level rose to 25 m (Zong, 2004). by brackish water. Therefore, this facies is interpreted as The sequence boundary is observed as an unconformity channel-fill sediments. between latest Pleistocene–Holocene fluvial–estuarine–deltaic Facies 3.5 (floodplain sediments) sediments (units 1, 2 and 3) and the underlying late Pleistocene sediments (Unit 0) (Wei and Wu, 2011). Overlying sediments This facies occurs at the PRD04 site and is about 3 m thick. It consisting of fluvial, estuarine and deltaic sediments compose consists of yellow grey silty clay, containing yellow clay a single depositional sequence reflecting one regression– briquette, iron nodules and plant roots with massive and transgression–regression cycle since the Last Glacial Maximum horizontal bedding (Fig. 3J). Radiocarbon dating places this (LGM). at <2 cal ka BP. Holocene transgression reached a maximum at 6 ka BP, when the sea level was equivalent to that at present (Li et al., Radiocarbon dates 1990; Long et al., 1997; Zong, 2004). The maximum transgressive surface of the PRD also occurred in this phase In all, 60 14C dates were obtained and calibrated. All the data (He et al., 2006). In various boreholes, the maximum are shown in Table 2. 14C dates obtained from units 1, 2 and transgressive surface was generally located in the soft marine 3 all fell within the latest Holocene. Individually, units 1, 2 silty layer. However, due to tidal scour, the maximum and 3 date roughly to 10–7, 7–4 and 4–0 cal ka BP, transgressive surfaces were shown as erosion surfaces in respectively. some boreholes, and as unique equivalent time horizons of Figure 4 shows age–depth plots with accumulation curves the PRD in Holocene sedimentary layers. for cores D02, D10, ZK08, PRD04, PRD10 and PRD15. The accumulation curves do not take into account sediment Delta evolution compaction effects. The accumulation curves indicate that the incised valley of the palaeo-Pearl River was rapidly buried Based on analyses of the borehole sedimentary facies, at the PRD10 and PRD15 sites during the marine transgres- sedimentary sequences, dating results, the sequence isochron sion. The average accumulation rate of the estuarine sedi- of the cross-sections in the PRD and Wei and Wu (2011), the ments was 2.9 mm a 1 from around 9 to 6 cal ka BP. In the evolution of the PRD since the Holocene can be divided into deltaic system, accumulation rates seem to increase upward. three stages: Stage I (9–6 cal ka BP), Stage II (6–2 cal ka BP) At the PRD04 site, the rate of accumulation of the prodelta and Stage III (2–0 cal ka BP). Figure 6 shows the palae- and shelf sediments was 1.0 mm a 1, which is less than the ogeography of the PRD since 9 cal ka BP. The palaeogeog- 5.9 mm a 1 for the delta front sediments. A similar pattern raphy and sedimentary processes characterizing these stages was also recognized in PRD15. are described in detail below.

Discussion Stage I (9–6 cal ka BP) Stratigraphy During this stage, the drowned incised valley of the Pearl River (Fig. 6A, B) was in-ﬁlled, mainly by Pearl River riverine Figure 5 shows the stratigraphy of the postglacial transgressive sediments, and then the large area of the present delta plain sedimentary cycle in the PRD region. The depth of the was inundated by the sea because of the sea-level rise after receiving basin varies between 5 m near the apexes and the LGM (Fig. 6AC). The drowned valley may have been

Figure 4. Accumulation curves (age–depth plots) from cores D02, D10, ZK08, PRD04, PRD10 and PRD15. The accumulation curves do not take into account sediment compaction. The sea-level curve (broad grey line) for the last 10 ka is illustrated based on Zong (2004).

Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) DELTA EVOLUTION AND SEDIMENT FLUX OF THE PEARL RIVER, SOUTHERN CHINA 491

Figure 5. The stratigraphic profiles for the Pearl River Delta plain. SB, sequence boundary. Location of the profiles is shown in Fig. 1. completely filled by 6 cal ka BP (Fig. 6C). The relatively thick such as the Wugui, Xijiao and Shiqiao mountains (Fig. 6C). nature of the channel-fill sediments (Facies 2.1) in the DT The scattered landmasses promoted active deposition and core may have been caused by aggradation because of the acted as deposition nuclei during deltaic evolution. Conse- overall rise in sea level during this stage. quently, apart from exhibiting a general tendency towards progression, PRD development occurred less regularly over Stage II (6–2 cal ka BP) time and space because of deposition around island The maximum Holocene transgression occurred at around boundaries (Fig. 6D, E). 6 cal ka BP, when sea level was equivalent to that at Stage III (20 cal ka BP) present (Zong, 2004). Except for the surrounding margin areas that acted as accumulation terraces during this stage, During this stage, the previous open water was narrowed into a large portion of the drowned valley was covered by the the channel with the further siltation of deposition bodies estuary, with more than 160 bedrock island and platforms, (Fig. 6E). The basic framework of the Pearl River network was

Figure 6. Palaeogeographical map illustrat- ing the evolution of the Pearl River Delta during the past 9 ka. Palaeo-shorelines and depth distribution are estimated on the basis of Huang et al. (1982), Zong et al. (2009), Wei and Wu (2011, 2014), core data, surface geological data and archaeological data. formed gradually. According to Zheng et al. (2003) and Zong mean tide levels, the tidal flat was fully reclaimed through the et al. (2006), between 4 and 3 cal ka BP, a shift from hunting construction of an earth bank or sea wall along the stone ridge. and gathering to wet rice farming occurred in the PRD area. These active land reclamation activities had two effects. First, By the arrival of the Han Dynasty (206 BC–AD 220), large shoreline advances were accelerated. Second, the large sections of the exposed delta plain were available for cultiva- volumes of sediment trapped along the edges of the delta plain tion. Over the past 2000 years, humans have employed reduced sediment loads in the estuary. Hence, the evolution of various techniques for reclaiming newly exposed areas of delta the PRD was not driven by purely natural processes over the plain for agriculture. Primitive sea walls can be found in last 2 ka. several localities where farmers had built rows of gravel and stones along the low tide marks of a tidal flat, raising their height each year. As a result, growing levels of sediment Variation of sediment flux were deposited behind the ridge of stones, and the altitude of The variation in Pearl River sediment flux during the last 9 ka the tidal flat increased. As the tidal flat land surface rose above can be roughly estimated based on the shoreline positions on

Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) DELTA EVOLUTION AND SEDIMENT FLUX OF THE PEARL RIVER, SOUTHERN CHINA 493

Table 3. Sediment volumes and sediment discharges of the Pearl undifferentiated sediments), Unit 1 (fluvial sediments), River during the past 9 ka. Unit 2 (estuarine sediments) and Unit 3 (deltaic sediments), in ascending order. Units 1, 2 and 3 are dated Age (cal Land area Depth Volume Sediment discharge < 2 3 1 to 10 cal ka BP and consist of two (channel-fill and ka BP) (km ) (m) (km ) (million t a ) floodplain sediments), two (tide-influenced channel-fill to coastal marsh and sub- to intertidal flat sediments) and five 9-6 7100 5–7 36–50 17–25 (tide-influenced channel-fill, delta front platform, tidal flat, 6-2 7400 8–11 59–81 22–30 abandoned channel-fill sediments and floodplain sedi- 2-0 6400 9–12 58–77 44–58 ments) sedimentary facies, respectively. The boundary Total – – 118–208 27–37 between the Units 2 and 3 was interpreted as a maximum Values were determined using a dry bulk density of 1.3 g cm 3. flooding surface. 2. The Holocene evolution of the PRD was divided into three stages within the context of the sea-level changes. During Stage I (9–6 cal ka BP), the incised valley of the palaeo- the palaeogeographical maps (Fig. 6) and the thicknesses of Pearl River was rapidly buried during marine transgression. sediments constituting the PRD plain and the subaqueous During Stage II (6–2 cal ka BP), as relative sea level delta. The bulk density of the deltaic sediments is estimated stabilized, the delta started to grow. Mainly because of the as 1.3 g cm 3 (He et al., 2006). The thickness of the deltaic presence of an irregular coastline and hundreds of rocky sediments trapped during 9–6 cal ka BP is regarded as 5–7 m islands scattered in the upper palaeo-estuarine bay, sedi- on the basis of the shape of the Pearl River incised valley mentation processes in the PRD have been highly irregular (Huang et al., 1982). The thickness of the deltaic sediments both in time and in space. During Stage III (2–0 cal ka BP), trapped between the palaeoshorelines from 6 to 2 cal ka BP the shoreline advanced rapidly due to a drastic increase in is 8–11 m, and that of the sediments between the palae- sediment discharge and human activities. Based on sedi- oshorelines from 2 to 0 cal ka BP is regarded as 9–12 m on ment volume analysis, the sediment discharge of the three the basis of the thickness of the deltaic sediments and stages were estimated to have been 17–25, 22–30 and the present offshore limit of the prodelta. Usually in tide- 44–58 million t a 1, respectively. dominated coastal areas, sediment dispersal offshore is common. However, for most of the Holocene the Pearl River has been trapping sediment to fill its extensive estuarine Acknowledgments. This work was supported by the Science and waters (Liu et al., 2014; Gao et al., 2015). Li et al. (1990) and Technology Program of Guangzhou, China (No. 201505061627540), He et al. (2006) have calculated that about 85–90% of the Self-research Program of the State Key Laboratory of Tropical sediment from the Pearl River was deposited in the palaeo- Oceanography (LTOZZ1503), Public Science and Technology estuarine bay during the Holocene. Here, we consider that Research Funds Projects of Ocean (No. 201205015), and the National 85% of the sediment was deposited in the paleo-estuarine Natural Science Foundation of China (No. 41206071). bay. So, the sediment flux for the three time periods (9–6, 6–2 and 2–0 cal ka BP) can be roughly estimated as 17–25, Abbreviations. LGM, Last Glacial Maximum; PRD, Pearl River Delta; 1 22–30 and 44–58 million t a , respectively (Table 3). PRFS, Pearl River Fault System. The resulting calculated palaeo-sediment flux shows varia- tions during the past 9 ka. The sediment flux was 17–25 References million t a 1 during 9–6 cal ka BP, 22–30 million t a 1 during 1 Allen JRL. 1963. 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