Quaternary Science Reviews xxx (2012) 1e12

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The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the delta, southern

Yongqiang Zong a,*, Kangyou Huang b, Fengling Yu c, Zhuo Zheng b, Adam Switzer c, Guangqing Huang d, Ning Wang a, Min Tang a a Department of Earth Sciences, The University of Hong Kong, Hong Kong, China b Department of Earth Sciences, Sun Yet-san University, Guangzhou, China c Earth Observatory of Singapore, Nanyang Technological University, Singapore d Guangzhou Institute of Geography, Guangzhou, China article info abstract

Article history: The early Holocene history of the Pearl River delta is reconstructed based on a series of sediment cores Received 27 June 2011 obtained from one of the main palaeo-valleys in the basin. Sedimentary and microfossil diatom analyses Received in revised form combined with radiocarbon dating provide new evidence for the interactions between sea-level rise, 21 December 2011 antecedent topography and sedimentary discharge changes within the deltaic basin since the last glacial. Accepted 4 January 2012 These new records show that river channels of last glacial age incised down to c. 40 m into an older Available online xxx (possibly MIS5 age) marine sequence which forms the floor of the deltaic basin and exists primarily at c. 10 me15 m below present mean sea level. Rapid postglacial sea-level rise flooded the incised valleys by the Keywords: fi Coastal evolution beginning of the Holocene, and prior to c. 9000 cal. years BP, marine inundation was largely con ned fi Sea-level rise within these incised valleys. The con ned available accommodation space of the incised valleys combined Monsoonal discharge with strong monsoon-driven freshwater, high sediment discharge and a period of rapid rising sea level Palaeo-incised valleys meant that sedimentation rates were exceptionally high. Towards c. 8000 cal. years BP as sea level rose to Microfossil diatoms about 5 m, marine inundation spilled out of the incised valleys and the sea flooded the whole deltaic Pearl River delta basin. As a result, the mouth of the Pearl River was forced to retreat to the apex of the deltaic basin, water salinity within the basin increased markedly as the previously confined system dispersed across the basin, and the sedimentation changed from fluvial dominated to tidal dominated. Sea level continued to rise, albeit at a much reduced rate between 8000 and 7000 cal. years BP, and deltaic sedimentation was concentrated around the apex area of the basin. During the last 7000 cal. years BP, the delta shoreline moved seawards, and the sedimentary processes changed gradually from tidal dominated to fluvial dominated. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction are largely determined by the relative sea-level history of the region (e.g. Smith et al., 2011), the amount of sediment supply from their Deltas and estuaries are fast evolving landforms. It has long been respective rivers (e.g. Woodroffe, 2000), other coastal processes proposed that changes of these landforms during the postglacial including tides and waves (e.g. Tanabe et al., 2006), and human period have been largely controlled by sea-level movements (e.g. activity in the late Holocene (e.g. Zong et al., 2009a). Stanley and Warne, 1994). During the late Pleistocene, sea level rose Over the past decades, the evolutionary history of many large rapidly (e.g. Hanebuth et al., 2000; Siddall et al., 2003; Bassett et al., deltas has been investigated, e.g. the delta (Saito et al., 2005), and vast areas of now continental shelves were inundated by 2000, 2001), the delta (Li et al., 2000, 2002; Hori et al., the sea (e.g. Steinke et al., 2003). As a result of the continuous rise 2001), the delta (Zong, 1992), the Song delta in sea level in the early Holocene, marine water transgressed many (Tanabe et al., 2003a, 2006; Li et al., 2006a,b; Funabiki et al., 2007), coastal basins and palaeo-river valleys. In river mouth regions, the delta (Nguyen et al., 2000; Ta et al., 2001, 2002), the deltas or estuaries formed during the middle and late Holocene as GangeseBrahmaputra delta (Goodbred and Kuehl, 2000), the sea level stabilised. The characteristics of these coastal landforms Mississippi delta (Coleman, 1988), the Nile delta (Chen et al., 1992), and other deltas (Woodroffe, 2000; Tanabe et al., 2003b), with some * Corresponding author. Tel.: þ852 22194815; fax: þ852 25176912. studies expanded to the subaqueous deltas (e.g. Chen et al., 2000; Liu E-mail address: [email protected] (Y. Zong). et al., 2004; Wang et al., 2010). These investigations have revealed

0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2012.01.002

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 2 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 the influence of different driving mechanisms for deltaic evolution, valley. With these new data, we explain the sedimentary processes in primarily highlighting the importance of sediment supply and the early Holocene and examine the complex interactions between sea-level change. A few recent studies have focused on a possible sea-level rise, monsoonal discharge and the palaeo-landscape which sea-level jump in the early Holocene around 8200 cal. years BP and took place in an important time period of contemporaneous rapid its effects on sedimentary infill in incised valleys and a widespread sea-level rising (Zong, 2007) and strengthened summer monsoon marine transgression (Yim et al., 2006; Hori and Saito, 2007; Tamura (Wang et al., 2005). et al., 2009). Such work has been rather controversial as there is a lack of direct (glacial) evidence for such sea-level jump at this 2. The Pearl River delta time. Furthermore, a reflection in sea-level rise was reported from Singapore, which indicates further details of sea-level history The Pearl River delta lies in the transitional zone between the between 8000 and 7000 cal. years BP (Bird et al., 2007, 2010). Despite upland landscape of the catchment basin and the deposition centre the amount of research carried out in many large deltas, there is of the northern continental shelf of the (Fig. 1a). a lack of sedimentary records that reveal the detailed response of the Since the collision between the Indian plate and the Eurasian plate landscape to rapid sea-level rise in the late Pleistocene and early around 34 Ma (Aitchison et al., 2007), the upland region gradually Holocene. uplifted, whilst the continental shelf subsided, the latter receiving In theory, the Pearl River should have incised into the older sediment from the Pearl River. During the Late Quaternary, major marine sequences when sea level was low during the last glacial. If faults were active (Huang et al., 1982), and the current deltaic such palaeo-valleys have incised down as far as to the bedrock, i.e. to basin was gradually formed through subsidence. Along some faults, c. 40 m (Zong et al., 2009b), these valleys may have been inundated several valleys were formed. The lower part of these valleys was as early as the beginning of the Holocene. Recent drilling in the Pearl filled with the MIS 5 marine sediment unit, and the surface of this River delta has revealed one possible palaeo-valley (e.g. Wu et al., unit is about 10 m to 15 m (Zong et al., 2009b). The Holocene 2007; Liu et al., 2008) that contains sediment of early Holocene deltaic formation lies on the older marine unit (Zong et al., 2009a). age. Subsequently, we carried out a drilling programme to identify The Pearl River catchment is under the climatic influence of palaeo-incised valleys within the Pearl River deltaic basin and the Asian Monsoon. The history of monsoon climate of the region is to obtain sediments for examination. This paper reports new revealed by the Dongge Cave record ( et al., 2004; Wang et al., high-resolution palaeo-environmental data collected from an incised 2005), which shows a period of rapid warming, albeit with

Fig. 1. (a) Locations of the Pearl River catchment, the Pearl River delta and the northern continental shelf of the South China Sea, and (b) Locations of the lithostratigraphic transects across the Pearl River delta plain and key sediment cores for this study.

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 3

fluctuations, after the last glacial maximum. Strongest monsoon sequences lies a layer of weathered crust which formed during the occurred between 9000 and 7000 cal. years BP (Fig. 2a), since which last glacial when the older marine sequence was exposed to oxida- the monsoon became weakened towards the last 500 years. The tion. Based on their stratigraphic records, Huang et al. (1982), Li et al. diatom data and organic carbon isotope ratios from the mouth (1990) and Yim (1994) believed the postglacial marine inundation region of the Pearl River (Fig. 2b, c) are in agreement with the only started by c. 8000 years BP and the Holocene deltaic sediment stalagmite records, suggesting that freshwater discharge from the overlies directly on the older marine sequence. Other investigations Pearl River was strong before c. 7000 cal. years BP (Zong et al., 2006) suggested the onset of postglacial deltaic or marine sedimentation and gradually declined since (Zong et al., 2010b,c; Yu et al., in press). took place in palaeo-incised channels as early as c. 9500 years BP In contrast, eustatic sea level rose rapidly since the last glacial (e.g. Owen et al., 1998). Despite some seismic profiles indicating maximum (Hanebuth et al., 2000; Siddall et al., 2003; Bassett et al., small incised channels in the Hong Kong area, there is little sedi- 2005) in the South China Sea region. Importantly, sea level reached mentary evidence to reveal major palaeo-incised valleys in the Pearl its postglacial maximum height by c. 7000 years BP (Fig. 2d) (Bird River deltaic basin (e.g. Zong et al., 2009a). et al., 2007, 2010; Zong, 2007), i.e. 3000 years later than the initia- tion of a period of increased monsoon strength. 3. Methods A number of Late Quaternary stratigraphic models have been put forward by various authors. Yim (1994) proposed a model with up to In order to investigate the early Holocene stratigraphy of the five marine sequences possibly corresponding to the last five periods palaeo-valleys drill cores in three series (the SS, SDZK and MZZK of high sea level, whilst others confirmed two (e.g. Huang et al.,1982; series) were collected from the deltaic basin. The drill holes were Fyfe et al., 1997) or three marine sequences which were related to put down using a push corer to capture undisturbed sediment. Core the interglacial high sea levels in marine isotopic stages 1, 5 and 7 samples were stored with PVC tubes in a refrigerator. Lithostrati- (e.g. Owen et al., 1998; Zong et al., 2009b). Between the two marine graphic transects were reconstructed (Fig. 3) using the new cores

a

b

c

d

Fig. 2. (a) The stalagmite records from Dongge Cave (Yuan et al., 2004; Wang et al., 2005), (b and c) the reconstructed water salinity and strength of monsoonal freshwater discharge from core UV1 at the mouth area of the Pearl River estuary (Zong et al., 2010c; Yu et al., in press), and (d) Sea-level curve from Singapore (Bird et al., 2007, 2010). The sea-level curve inserted is from Zong (2007), based on data compiled from the Baneparte Gulf, north Australia (Yokoyama et al., 2000), the Sunda Shelf (Hanebuth et al., 2000), and the Strait of Malacca (Geyh et al., 1979). MWP stands for melt water pulse.

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 4 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12

Fig. 3. Stratigraphic transects from the Pearl River delta showing the palaeo-incised valleys and the older Quaternary sedimentary sequences. Locations of these transects are indicated in Fig. 1b. Details of the radiocarbon dates for cores SS0901, SS0904, SDZK01, SDZK14 and MZZK04 are listed in Table 2. Details of stratigraphy and radiocarbon dates are from Zong et al. (2009a,b) for cores JT81, PK14, A23, GK2 and PK19. Dates for PRD05 are calibrated based on the original dates from Wu et al. (2007).

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 5 together with previous core records. These transects run across and freshwater e F) according to Denys (1991e1992). They were further down the deltaic basin (Fig. 1b) and reveal a palaeo-landscape divided into two sub-groups, planktonic and benthic. Such classi- that includes two incised valleys. Several cores captured sediment fication helps understand the relative strength of marine water and sequences from incised valleys (Table 1), and they were selected for freshwater influence as well as water depth at the time of deposi- additional laboratory analyses. tion, albeit qualitatively (Vos and de Wolf, 1993). The sediment In this study, four new cores (SS0901, SDZK01, SDZK14 and samples were selected at various intervals for analysis using a laser MZZK04) from the deltaic plain were analysed, and one core (BVC) particle size analyser, and classified into clay, silt and sand fractions. from the estuarine mouth region was re-examined, because they all contain the early Holocene sediment sequence which was not 4. Results well understood in previous studies. A total of 32 radiocarbon dates are reported in Table 2. All these dates are calibrated using the CALIB 4.1. Sedimentary chronology 5.10 software (Stuiver et al., 1998) with the Intcal04 programme for terrestrial samples and the Marine04 programme for marine Deltas are a dynamic landform which involves considerable samples with a correction factor, DRas128 40 years according to contrast in sediment deposition and reworking. Adding to this Southon et al. (2002). Whilst the details of the radiocarbon dates complexity is the influx of terrestrial organic carbon matter which are given in Table 2, the central calibrated ages to nearest decade are may affect the precision of age determination by radiocarbon plotted on diagrams for easy reading. dating methods (Raymond and Bauer, 2001). This problem appears Sediment samples from cores SS0901, SDZK01 and BVC more severe for the early Holocene when sea level rose fast and were chosen for diatom analysis in order to infer the palaeo- deltaic sedimentation was rapid (Stanley and Chen, 2000). This has environmental conditions. For each sediment sample, a minimum resulted commonly in age reversals from the early Holocene of 300 diatom valves were counted and identified to species level sediment sequences of the Mekong (Tamura et al., 2009), The Song according to Van der Werff and Huls (1958e1966). They were then Hong (Tanabe et al., 2003a) and the Yangtze (Wang et al., 2010). In grouped into two categories (marine and brackish water e MB and our core SS0901, this situation occurs too (Table 2). It is clear that

Table 1 Lithological details of key new sediment cores.

Depth (m) Altitude (m) Description Core SS0901 (1125003000E, 231000500N, ground altitude: 4.8 m) 0.00e0.41 4.80 to 4.39 Filled material 0.41e1.41 4.39 to 3.39 Greenish light grey clay, with organic matter increasing downwards 1.41e1.67 3.39 to 3.13 Blackish grey clay with rich organic matter and large pieces of Glypstostrobus sp. 1.67e5.83 3.13 to 1.03 Dark grey clay, with organic matter increasing from 4.10 m upwards 5.83e14.83 1.03 to 10.03 Dark grey clay, with shell fragments in various depths and plant roots at the base 14.83e15.60 10.03 to 10.80 Brownish grey clayey peat 15.60e15.98 10.80 to 11.18 Yellowish grey sand and gravel 15.98e16.25 11.38 to 11.65 Reddish weathered crust of sandstone Core SS0904 (1125600300E, 230501500N, ground altitude: 4.4 m) 0.00e0.41 4.40 to 3.99 Filled material 0.41e1.00 3.99 to 3.40 Yellowish grey clay 1.00e2.14 3.40 to 2.26 Grey silt with a thin band of peat 2.14e3.21 2.26 to 1.19 Grey find sand 3.21e4.28 1.19 to 0.12 Grey clay with organic rich bands 4.28e9.40 0.12 to 5.00 Grey clay with thin bands of silt 9.40e9.63 5.00 to 5.23 Yellow coarse sand Core SDZK01 (1131202200E, 225500800N, ground altitude: 3.5 m) 0.0e2.8 3.5 to 0.7 Filled material 2.8e7.8 0.7 to 4.3 Yellowish grey clay 7.8e15.4 4.3 to 11.9 Dark grey clay, with fragments of oyster shell and sea shell 15.4e36.4 11.9 to 32.9 Yellowish grey clay 36.4e39.9 32.9 to 36.4 Yellowish grey sandy clay 39.9e41.0 36.4 to 37.5 Reddish weathered crust of sandstone bedrock Core SDZK14 (1132103500E, 224402500N, ground altitude: 2.5 m) 0.0e1.8 2.5 to 0.7 Filled material 1.8e12.5 0.7 to 10.0 Grey clay with oyster shells and shell fragments increasing towards the top 12.5e15.1 10.0 to 12.6 Dark grey clay with plant fragments 15.1e19.2 12.6 to 16.7 Grey to dark grey clay with laminations and occasional plant fragments 19.2e43.7 16.7 to 41.2 Light grey clay with laminations and thin silt and sandy layers 43.7e49.0 41.2 to 46.5 Yellow to grey sand and gravel overlaying bedrock Core MZZK04 (1132905500E, 223702400N, ground altitude: 1.5 m) 0.0e2.6 1.5 to 1.1 Brownish grey clay e disturbed pond sediment 2.6e27.3 1.1 to 25.8 Dark grey clay 27.3e33.5 25.8 to 32.0 Grey clay 33.5e33.9 32.0 to 32.4 Yellowish grey coarse sand overlaying bedrock Core BVC (11 40104600E, 222000900N, ground altitude: 7.6 m) 0.0e0.9 7.6 to 8.5 Disturbed sediment 0.9e2.4 8.5 to 9.8 Light brownish grey silt and clay 2.4e8.1 9.8 to 15.7 Greenish grey to grey silt and clay 8.1e12.3 15.7 to 19.9 Grey to brownish grey silt and clay with clasts of weathered clay 12.3e19.9 19.9 to 27.5 Grey to brownish grey silt and clay with plentiful plant fragments 19.9e21.7 27.5 to 29.3 Brown organic clayey sand 21.7e22.4 29.3 to 30.0 Brown to grey clayey sand with small gravel overlaying bedrock

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 6 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12

Table 2 Radiocarbon dates from key sediment cores.

Core Depth (m) Material Method Conventional age (yr BP) Calibrated date (yr BP) (2s) Central cal. age (yr BP) Laboratory code SS0901 2.96 Bulk organic AMS 4800 40 5600e5470 5540 Beta201319 SS0901 6.00 Plant fragments AMS 6460 50 7440e7270 7360 Beta291320 SS0901 6.32 Plant fragments AMS 6580 40 7520e7434 7480 Beta290247 SS0901 6.32 Bulk organic AMS 7310 40 8185e8020 8100 Beta289663 SS0901 7.30 Bulk organic AMS 7690 40 8552e8404 8480 Beta289664 SS0901 9.83 Bulk organic AMS 7480 40 8378e8198 8290 Beta289665 SS0901 10.24 Bulk organic AMS 8090 40 9132e8971 9050 Beta289666 SS0901 12.26 Plant fragments AMS 7677 36 8542e8407 8480 GZ3942 SS0901 13.41 Plant fragments AMS 7961 35 8991e8695 8840 GZ3943 SS0901 14.67 Plant fragments AMS 8037 36 9023e8846 8940 GZ3944 SS0901 14.73 Peat AMS 7990 36 9005e8717 8860 GZ3945 SS0904 1.82 Peat AMS 2090 20 1998e2120 2060 GZ4141 SS0904 3.54 Plant fragment AMS 2210 25 2151e2318 2230 GZ4142 SS0904 3.99 Plant fragments AMS 2250 40 2340e2150 2250 Beta291321 SS0904 4.61 Plant fragments AMS 2160 40 2310e2040 2180 Beta291322 SS0904 6.92 Plant fragments AMS 2560 30 2750e2700 2730 Beta291323 SDZK01 5.0 Bulk organic AMS 2455 28 2702e2361 2530 GZ3927 SDZK01 7.8 Bulk organic AMS 2528 29 2743e2492 2620 GZ3928 SDZK01 12.0 Bulk organic AMS 6429 45 7425e7274 7350 GZ3929 SDZK01 28.5 Bulk organic AMS 8095 33 9125e8987 9060 GZ3930 SDZK01 36.0 Bulk organic AMS 7989 36 9003e8717 8860 GZ3931 SDZK14 15.0 Bulk organic AMS 8040 45 9031e8725 8880 GZ3933 SDZK14 30.6 Bulk organic AMS 9135 44 10 258e10 154 10 210 GZ3934 SDZK14 37.8 Bulk organic AMS 10 848 49 12 900e12 803 12 850 GZ3935 MZZK04 5.2 Bulk organic AMS 1911 29 1929e1811 1870 GZ3936 MZZK04 10.3 Bulk organic AMS 3177 30 3453e3355 3400 GZ3937 MZZK04 16.6 Bulk organic AMS 3755 32 4183e4067 4130 GZ3938 MZZK04 27.9 Bulk organic AMS 8069 39 9092e8932 9010 GZ3939 BVC 3.0 Foraminifera AMS 4191 30 4597e4290 4450 GZ2210 BVC 7.5 Foraminifera AMS 6973 34 7673e7473 7580 GZ2209 BVC 8.8 Foraminifera AMS 8071 34 8919e8554 8690 GZ2208 BVC 20.1 Peat AMS 9320 40 10 560e10 410 10 500 Beta195332

the onset of postglacial sedimentation at the site was represented indicate that the altitude of this sub-surface rises to c. 10 m in the by the brownish grey clayey peat near the base of the core northern deltaic plain around Guangzhou (Zong et al., 2009b). (Table 1). A date from the basal peat at the depth of 14.73 m yields The older sequence here is incised to 33 m at core SDZK01. Transect an age of 8860 cal. years BP. Over the 7.5 m of sediment between D, where the basin is nearly 60 km wide, reveals a more complex 14.83 and 7.30 m, 7 radiocarbon dates show ages ranging from lithostratigraphy and palaeo-landscape. Cores PRD05 and SDZK14 9050 to 8290, with several apparent reversals. This highlights the show two separate palaeo-valleys incised to about 25 m and 41 m limitations of radiocarbon dating methods in such a dynamic respectively. The older marine sequence remains between these environment. However, it also implies a high rate of sedimentation two incised valleys and is recorded from core SDZK14. during the early Holocene. Other than this matter, the chronology Again, the altitude of the sub-surface varies from 20 m to 15 m. In of the sedimentary sequences revealed by the four cores is Transect E, two incised valleys are also identified, down-cutting reasonably consistent (Table 2), although the radiocarbon ages to 25 m and 42 m respectively. The surface altitude of the older should not be taken as exact dates due to the possibility of older marine sequence appears at 20 m to 15 m. carbon matter input from the river catchment (Raymond and Bauer, 2001). 4.3. Bio-lithostratigraphy 4.2. The palaeo-landscape and sediments Core SS0901 was chosen to represent the sedimentary history In significant parts of the Pearl River deltaic basin, an older of the apex area. The lithology (Table 1) shows that overlying the marine sequence is preserved (e.g. Huang et al., 1982; Zong et al., basal peat is a unit of dark grey clay up to 5.83 m in depth. This unit 2009b). Sediment core records show that, in some places, the contains shell fragments and some silt (Fig. 4). From 5.83 m to older marine sequence has been incised by fluvial action during the 1.41 m, organic matter increases, and some large pieces of vege- last glacial period. Transect A in Fig. 3 reveals the lithostratigraphy at tation including Glypstostrobus pensilis, common in subtropical the apex area of the delta plain, where the North River runs through swampy wetlands of southeast China, are recorded at 1.41e1.67 m. a narrow gap (c. 3 km in width) of bedrock geology. Within a small Particle size results show a dominance of clay with minor sand embayment on the north side of the river, Holocene sediment and silt that increases from the base and reaches about 15% at sequence at SS0901 has been protected from being eroded by river 11.5e7.5 m before decreasing upwards. Diatoms are well preserved channel migration. The basin widens to c.15 km at Transect B, where between 14.83 m and 4.05 m. The results show a gradual increase in a large volume of sand and gravel is recorded. At present, the river marine and brackish water species from 14.83 m to 7.50 m, before channel is restricted to the eastern side by flood defence. A layer their numbers reduce upwards and disappear at c. 4.00 m. This of silt and clay is preserved, which is dated to the late Holocene. marineebrackish diatom community is dominated by Coscinodiscus The deltaic basin is much wider along Transect C (c. 25 km), which divisus, a planktonic type living in moderate salinity environments shows a sub-surface around 20 m to 15 m between the older within the Pearl River estuary today (Zong et al., 2010b). The marine sequence and the postglacial sequence. Other core records freshwater diatom community comprises many species, mainly

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 7

Fig. 4. Sedimentary and microfossil diatom data from core SS0901 obtained from the apex of the Pearl River delta plain. Central calibrated ages of radiocarbon dates are plotted along the lithology column. benthic types. Planktonic types generally make up about 10% sharply, however particle size results show no significant trend throughout. across this contact nor throughout the core (Fig. 5). Diatom results Core SDZK01 was obtained from the central north part of the indicate strong freshwater influence in the lower part of the core, deltaic basin with the aim of revealing the full postglacial history. corresponding to the yellowish grey clay and sandy clay between The sediment at the base of this core (Table 1)showsathinsandy 15.4 m and 39.9 m. The assemblages are dominated by freshwater layer that drapes the sandstone bedrock. On top of the sandy layer taxa commonly found in distributaries and at the head of the modern is a unit of yellowish grey clay, and then dark grey clay, the latter Pearl River estuary, including planktonic Aulacoseira granulata and containing oysters and marine gastropods and bivalves. From 7.8 m benthic Fallacia subhamulata (Zong et al., 2006). The assemblages upwards, the sediment changes to a yellowish clay. There may be also contain significant numbers of marineebrackish water species, a depositional hiatus at 7.8 m where the sediment changes colour such as Cos. divisus. Within the dark grey clay, marine and brackish

Fig. 5. Sedimentary and microfossil diatom data from core SDZK01 obtained from the central part of the Pearl River delta plain. Central calibrated ages of radiocarbon dates are plotted along the lithology column.

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 8 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 diatoms reach over 40%, comprising Cos. divisus, Coscinodiscus diatoms remain dominant in the upper clay unit (0.9e8.1 m), but blandus and Cyclotella striata. A similar assemblages is recorded in the percentages of P. sulcata reduce to below 20% (Fig. 6). the mouth region of the Pearl River estuary today (Zong et al., 2010b). In contrast the upper yellowish grey clay unit shows asignificant increase in freshwater diatoms. 5. Discussion Cores SDZK14 and MZZK04 have not been analysed for diatoms, because their lithostratigraphies are largely comparable with 5.1. Infilling of the deltaic basin that of SDZK01 (Table 1). Based on the three radiocarbon dates, core SDZK14 suggests a period of rapid sedimentation in the early The new lithostratigraphic data has revealed the existence of Holocene as recorded by the light grey clay (19.20e43.70 m) and the palaeo-incised valleys in the Pearl River deltaic basin (Fig. 3). These grey to dark grey clay (15.10e19.20 m), both units exhibit consid- valleys were likely formed during the last glacial when sea level erable laminations, i.e. shallow water deposits. These two units was much lower than present. One such valley has incised through are dated to the early Holocene (Table 2). The dark grey clay the older marine sequence and down to the bedrock as revealed unit (12.5e15.10 m) may represent a phase of strong marine influ- by cores SDZK01 and SDZK14, reaching c. 40 m. This valley widens ence in the early middle Holocene. The grey clay with oyster shells from 2e3 km at Transect C to 7e8 km at Transects D and E. Sedi- and marine macrofauna increasing from 12.50 m upwards suggest ments infilling this valley are about 25 m thick and dated to a change from delta front to delta plain environment. Compared between c. 10,000 and 8500 cal. years BP, suggesting an average with core SDZK14, the stratigraphy of core MZZK04 is much simpler. sedimentation rate of over 15 mm/a. This is much higher than the The grey clay unit (27.30e33.50 m) was likely deposited during the reported rate in the late Holocene of 2e4 mm/a (Zong et al., 2009a). early Holocene according to the date (9010 cal. years BP) at 27.90 m. This unit of sediment is dominated by freshwater benthic diatoms The dark grey clay unit (2.90e27.30 m) was deposited during the (Phase I in cores SS0901 and SDZK01, Figs. 4 and 5), indicating mid to late Holocene. a shallow depositional environment influenced strongly by fresh- Core BVC was first reported in Zong et al. (2009b). Because this water discharge. Minor marine influence is evident throughout this core is relevant to the discussion of the paper, both of the lithos- unit, and it became slightly stronger towards the end of this phase. tratigraphic and diatom data are re-examined here. This core was By the time the incised valleys were filled c. 8500 cal. years BP, taken from a shallow water area at the mouth of small valley from the deltaic basin appeared to be a broad, shallow (c. 15 m depth) Lantau Island of Hong Kong (Fig. 1b). A sandy layer, rich in organic embayment. The embayment width is about 60 km wide at Transect matter, at the base of the core is dated to 10 500 cal. years BP D. On top of the palaeo-valley deposit (Phase I) and the older marine (Table 2). On top of this sandy layer are three units of silt and clay sequence, two more sedimentary units (Phases II and III, Figs. 4 and 5) (Table 1). Diatoms from the sandy layer are all freshwater benthic are found and they record the infilling of the accommodation space species (Fig. 6). The lower clay unit (12.3e19.9 m) contains in this broad deltaic basin. These two units are dated to early mid mixed assemblages, dominantly planktonic marine and brackish Holocene and midelate Holocene respectively. Firstly, the dark water species including Cos. divisus, Cos. blandus and Cyc. striata grey clay in 7.8e15.4 m of core SDZK01 shows evidence of strong (Zong et al., 2009b), with about 20% of benthic freshwater taxa. tidal influence as the diatom assemblages are dominated by In the middle clay unit (8.1e12.3 m), the diatoms are of almost fully marineebrackish planktonic species. Such strong tidal influence is marine and brackish water taxa, dominated by the planktonic also recorded in 6.0e8.6 m of core SS0901. This is a phase when marine Paralia sulcata. This species represents the marine compo- marine influence in the Pearl River delta was strongest, and this nent of the Pearl River estuarine diatom community (Zong et al., phaseoccurredaround8500e7000 cal. years BP in the apex area of 2010b), and thus its dominance in the middle clay unit suggests the delta plain (Fig. 4). Above this unit is a layer of yellowish grey clay, a close-to-marine environment. Marine and brackish water which contains diatom assemblages dominated by freshwater species

Fig. 6. Sedimentary and microfossil diatom data from core BVC obtained from the mouth region of the Pearl River estuary. Central calibrated ages of radiocarbon dates are plotted along the lithology column.

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 9 whose numbers increase towards the surface. This suggests a gradual shoreline retreat towards the apex of the deltaic basin and an increase in fluvial influence as the deltaic shoreline advanced. increase in water salinity across the deltaic basin as recorded in The above three phases of sedimentation form the complete all cores from this study. The basin wide change is recorded as postglacial history of the Pearl River delta. However, the middle an increase in marineebrackish water diatoms in cores SDZK01 and phase (Phase II) is somewhat difficult to define without detailed SS0901 from Phase I to Phase II (Figs. 4 and 5). Such an increase in microfossil and chronological analyses. Alterations in sediment salinity is also manifest by the high percentages of P. sulcata in colour in core SDZK01 seem to coincide closely with changes of core BVC from a far field location (Fig. 6). Several researchers have sedimentary conditions, but such association is not easily applied to attributed this sudden increase in marine influence or deltaic other cores. Nevertheless, this study adds details of an important shoreline retreat to a phase of rapid sea-level rise around 8200 cal. sedimentary phase to the evolutionary history of the Pearl River years BP (Yim et al., 2006; Hori and Saito, 2007; Tamura et al., 2009). delta proposed previously (e.g. Zong et al., 2009a). This new model The apparent increase in marine influence and salinity recorded (Fig. 7) suggests two stages of sedimentation. The first stage (Phase in Pearl River delta cores, however, may not be entirely the result of I) took place when sea level rose rapidly and monsoonal discharge a phase of accelerated rise in sea level. It is very likely that sea water was strongest during the early Holocene. The environmental spilling out of the palaeo-valleys and inundating the wider deltaic conditions resulted in the rapid infill of the palaeo-incised valleys basin also played a primary role in the shoreline retreat and a relative and the inundation of the broad deltaic basin. The second stage saw increase in salinity. deltaic progradation (Phase III), details of which were discussed From c. 8000 to 7000 cal. years BP, it is apparent that sea level rose below. at a much reduced rate and was close to the present height. This saw a change in sedimentary processes in the apex area where core 5.2. The role of sea-level rise, monsoonal discharge SS0901 recorded an increase in freshwater influence (end of Phase II, and the palaeo-landscape Fig. 4). At the same time, the majority of the deltaic basin was still under strong marine influence (e.g. Phase II, Figs. 5 and 6). The By c. 10,000 BP or the end of melt water pulse 1B (MWP 1B), high percentages of marineebrackish planktonic diatoms from cores sea-level rose to c. 30 m in the South China Sea region (Fig. 2). SDZK01 and BVC suggest a tidal regime similar to the present mouth This resulted in the initial marine inundation in the deepest part of region of the Pearl River (Zong et al., 2010b). In the southern part of the incised valleys of the Pearl River deltaic basin. Evidence for the deltaic basin, a change from high sedimentation in the early this early marine inundation is recorded from cores BVC, MZZK04, Holocene to a much reduced sedimentation in the middle Holocene SDZK14 and SDZK01. Sea level continued to rise, and by c. 8500 BP, it is recorded at core PRD05 (Fig. 3d) where salinity increased to its reached 10 to 15 m. During the 1500 years, the fluvial and marine maximum at this location around 7000 cal. years BP according to the interaction was largely confined within the incised valleys. The effect ostracod records of Liu et al. (2008). Nevertheless, this time marks of strong monsoonal freshwater on sedimentation was magnified as the turning point, a change from transgression to regression. freshwater outflow was restricted in the relatively narrow valleys. By c. 7000 cal. years BP, sea-level reached its present height Such largely fluvial sedimentation is indicated by the dominant in southern China (Zong, 2004), and the palaeo-incised valleys freshwater benthic diatoms from the sediment. Similarly, the of the Pearl River delta were fully filled as indicated by records of very high sedimentation rate is a result as terrestrial sediment being cores SDZK01, SDZK14, MZZK04 and PRD05 (Fig. 3). The deltaic basin deposited in valleys with limited accommodation space. While the appeared as a broad, shallow estuary with many isolated bedrock fluvial effects were magnified by the palaeo-landscape of the incised islands (Zong et al., 2009a). Water depth varied between a few valleys, the sea-level signature was largely suppressed. meters in the apex area and 10e15 m in the central and outer parts of Sea level continued to rise after c. 8500 cal. years BP, resulting the basin. High sea level and a wide deltaic basin at this time saw in marine inundation spilling from palaeo-incised valleys onto the strong tidal processes, which helps disperse the incoming terrestrial broad deltaic basin. Around this time, monsoonal discharge was still sediment across the entire deltaic basin and beyond. As a result, at its highest level in the Holocene (Fig. 2). However, as freshwater sedimentation at locations such as NL and UV1 in the mouth region outflow was no longer confined within the incised valleys, the commenced around 6500 cal. years BP (Zong et al., 2009a). By c. relative effect of freshwater discharge and sediment supply against 5500 cal. years BP, diatom assemblages indicate that the apex area the rise in sea level was greatly reduced. This resulted in rapid had become a freshwater wetland environment (Phase III, Fig. 4).

Fig. 7. A stratigraphic transect along an incised valley from the apex to the mouth region of the Pearl River delta showing the early Holocene infill (marine transgressive unit) and the middleelate Holocene deltaic progradation (deltaic regressive unit). This is an improved model to the one proposed by Zong et al. (2009a).

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 10 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12

Fig. 8. Key stages of the evolutionary history of the Pearl River delta during the late Pleistoceneeearly Holocene: (a) river down-cutting the older marine sequence during the last glacial, (b) initial marine inundation by the end of MWP 1B, (c) sea-level rise and rapid sedimentation within palaeo-valleys during the early Holocene, (d) marine inundation of the broad deltaic basin during the early middle Holocene.

However, a change from brackish water to freshwater conditions edge of the deltaic plain. These deltas have been under the same indicated by diatoms at c. 5 m depth in core SDZK01 (Phase III, Fig. 5) monsoonal climate and experienced a similar sea-level history to the was as late as c. 2500 cal. years BP. This suggests the deltaic shoreline Pearl River delta. However, the characteristics of their evolutionary reached Transect C (Fig. 1b) by c. 2000 cal. years BP. The slow histories are slightly different from each other. The palaeo-incised advancement of deltaic shoreline from 7000 to 2000 cal. years BP is valley of the Yangtze is a wide, elongate channel (Li et al., 2002). likely a consequence of the strong tidal regime. Under this regime, The accommodation space above the palaeo-valley is less than 5 m vertical sedimentation took place in distal locations around rocky in depth, and this space was quickly transformed into freshwater islands (e.g. Wu et al., 2007). In other words, significant amounts of marshy environment after c. 7000 cal. years BP (Zong et al., 2011). sediment were transported to the subaqueous delta front area rather Therefore, the Pearl River delta is unique because of its palaeo- than being deposited at the contemporaneous river mouth. The landscape: a broad accommodation space of c. 15 m deep and c. deltaic shoreline further advanced to Transect D by c. 1000 years ago 60 km wide on top of much narrower incised valleys of c. 25 m in as indicated by the oyster shell deposits near the top of core SDZK14 depth (Fig. 8). This unique palaeo-landscape has magnified the effects (Table 1) and sedimentary/microfossil records of core JT81 (Zong of sea-level rise and monsoonal discharge on deltaic sedimentation, et al., 2009a, 2010b) and core PRD05 (Liu et al., 2008). During resulting in a strong contrast between the dominantly fluvial the late Holocene, the sedimentation rate accelerated in the area processes in the early Holocene and the strongly tidal processes in of Transects D and E (Figs. 1b and 3). Such acceleration in deltaic the middle Holocene. sedimentation was considerably influenced by human activity including active land reclamations for the creation of numerous fish 6. Conclusions ponds and paddies (Zong et al., 2009a, 2010a). This study has investigated the evolutionary history of the Pearl 5.3. Comparison with other Asian deltas River delta with a particular focus on the sedimentary processes between the early Holocene and the midelate Holocene. The study It is common that many Asian deltas record a phase of rapid focussed on the palaeo-landscape effects in the history of early sedimentation in the early Holocene followed by a much slower Holocene environmental change. The core records show that sedimentary rate in the mid- and late Holocene. For example, rapid an older (possibly MIS5 in age) marine sequence formed the floor of sedimentation is recorded in the Mekong delta where c.19 m of the deltaic basin. This marine sequence was incised during the sediment was deposited between c. 9100 and 8000 cal. years BP in last glacial period. By the end of the Pleistocene, the deltaic basin core KS (Tamura et al., 2009), and in the Yangtze where c. 28 m of appeared to have a wide basin which was dissected by a series of sediment was accumulated between 13 200 and 8400 cal. years BP narrow incised valleys. Results of sedimentary/diatom records and in core ZK9 (Wang et al., 2010). A similar sequence at the age of radiocarbon dating from key cores obtained from one of the palaeo- 12 310 cal. years BP is also recorded in the Han River delta (Zong, incised valleys suggest three phases of environmental change. Phase 1989). During the last 7000 cal. years, sedimentation in the apex I saw a period of largely freshwater sedimentation primarily confined area of the Mekong delta decreased as result of the development of within the incised valleys that is associated with rapid sea-level rise saltmarshes and mangroves (Tamura et al., 2009). In the Yangtze, and strong monsoonal discharge. Fluvial processes dominated this a slowdown in sedimentation rate (Wang et al., 2010)isassociated phase because monsoonal discharge was confined within the incised with the development of the Chenier ridges along the southern valleys. Phase II recorded much slower sedimentation but strong

Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12 11 tidal influence over the wide deltaic basin. This change is a result of Funabiki, A., Haruyama, S., Nguyen, V.Q., Pham, V.H., Dinh, H.T., 2007. Holocene sea level spilling out of the incised valleys and sea water inundating delta plain development in the Song Hong () delta, . Journal of Asian Earth Sciences 30, 518e529. the wider deltaic basin. This transgressive flooding changed the Fyfe, J.A., Selby, I.C., Shaw, R., James, J.W.C., Evans, C.D.R., 1997. Quaternary sea-level sedimentary regime from dominantly fluvial to dominantly tidal. change on the continental shelf of Hong Kong. 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Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002 12 Y. Zong et al. / Quaternary Science Reviews xxx (2012) 1e12

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Please cite this article in press as: Zong, Y., et al., The role of sea-level rise, monsoonal discharge and the palaeo-landscape in the early Holocene evolution of the Pearl River delta, southern China, Quaternary Science Reviews (2012), doi:10.1016/j.quascirev.2012.01.002