Zong Y.
1 *, Huang K.
2 , Yu F.
3 , Zheng Z.
2 , Switzer A.
3 , Huang G.
4 , Wang N.
1 , Tang M.
1
1 Department of Earth Sciences, The University of Hong Kong, Hong Kong SAR, China
2 Department of Earth Sciences, Sun Yet-san University, Guangzhou, China
3 Earth Observatory of Singapore, Nanyang Technological University, Singapore
4 Guangzhou Institute of Geography, Guangzhou, China
*Corresponding author: Tel.: +852-22194815; Fax. +852-25176912; E-mail address: yqzong@hkucc.hku.hk
(Y Zong)
Key words: coastal evolution, sea-level rise, monsoonal discharge, palaeo-incised valleys, microfossil diatoms, Pearl River delta
Abstract: The early-Holocene history of the Pearl River delta is reconstructed based on a series of sediment cores obtained from one of the main palaeo-valleys in the basin. Sedimentary and microfossil diatom analyses combined with radiocarbon dating provide new evidence for the interactions between sea-level rise, antecedent topography and sedimentary discharge changes within the deltaic basin since the last glacial. These new records show that river channels of last glacial age incised down to c. -40 m into an older (possibly MIS5 age) marine sequence which forms the floor of the deltaic basin and exists primarily at c. 10 m to 15 m below present mean sea level. Rapid postglacial sea-level rise flooded the incised valleys by the beginning of the Holocene, and prior to c. 9000 cal. years BP, marine inundation was largely confined within these incised valleys. The confined available accommodation space of the incised valleys combined with strong monsoon-driven freshwater, high sediment discharge and a period of rapid rising sea level meant that sedimentation rates were exceptionally high. Towards c. 8000 cal. years BP as sea level rose to about -5 m, marine inundation spilled out of the incised valleys and the sea flooded the whole deltaic 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.

1.
Introduction
Deltas and estuaries are fast evolving landforms. It has long been proposed that changes of these landforms during the postglacial period have been largely controlled by sea-level movements
(e.g. Stanley and Warne, 1994). During the late Pleistocene, sea level rose rapidly (e.g. Hanebuth et al., 2000; Siddall et al., 2003; Bassett et al., 2005), and vast areas of now continental shelves were inundated by the sea (e.g. Steinke et al., 2003). As a result of the continuous rise in sea level in the early Holocene, marine water transgressed many coastal basins and palaeo-river valleys. In river mouth regions, deltas or estuaries formed during the middle and late Holocene as sea level stabilised. The characteristics of these coastal landforms 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 respective rivers (e.g. Woodroffe, 2000), other coastal processes including tides and waves (e.g.
Tanabe et al., 2006), and human activity in the late Holocene (e.g. Zong et al., 2009a).
Over the past decades, the evolutionary history of many large deltas has been investigated, e.g. the Yellow River delta (Saito et al., 2000, 2001), the Yangtze delta (Hori et al., 2001; Li et al.,
2000, 2002), the Han River delta (Zong, 1992), the Song Hong River delta ( Tanabe et al., 2003a ,
2006; Li et al., 2006a, b; Funabiki et al., 2007), the Mekong delta (Nguyen et al., 2000; Ta et al.,
2001, 2002), the Ganges-Brahmaputra delta (Goodbred and Kuehl, 2000), the Mississippi delta
(Coleman, 1988), the Nile delta (Chen et al., 1992), and other deltas (Tanabe et al., 2003b;
Woodroffe, 2000), with some studies expended to the subaqueous deltas (e.g. Chen et al., 2000;
Liu et al., 2004; Wang et al., 2010). These investigations have revealed the influence of different driving mechanisms for deltaic evolution, primarily highlighting the importance of sediment supply and sea-level change. A few recent studies have focused on a possible sea-level jump in the early
Holocene around 8200 cal. years BP and its effects on sedimentary infill in incised valleys and a widespread marine transgression (Yim et al., 2006; Hori and Saito, 2007; Tamura 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 time. Furthermore, a reflection in sea level rise was reported from Singapore, which indicates further details of sea level history between 8000 and 7000 cal. years BP (Bird et al., 2007,
2010). Despite the amount of research carried out in many large deltas, there is a lack of sedimentary records that reveal the detailed response of the landscape to rapid sea-level rise in the late Pleistocene and early Holocene.
In theory, the Pearl River should have incised into the older marine sequences when sea level was low during the last glacial. If such palaeo-valleys have incised down as far as to the bedrock, i.e. to c. -40 m (Zong et al., 2009b), these valleys may have been inundated as early as the beginning of the Holocene. Recent drilling in the Pearl River delta has revealed one possible palaeo-valley (e.g.
Wu et al., 2007; Liu et al., 2008) that contains sediment of early Holocene age. Subsequently, we carried out a drilling programme to identify palaeo-incised valleys within the Pearl River deltaic basin and to obtain sediments for examination. This paper reports new high-resolution palaeo-environmental data collected from an incised valley. With these new data, we explain the sedimentary processes in the early Holocene and examine the complex interactions between

sea-level rise, monsoonal discharge and the palaeo-landscape which took place in an important time period of contemporaneous rapid sea-level rising (Zong, 2007) and strengthened summer monsoon (Wang et al., 2005; Yu et al., in press).
2.
The Pearl River delta
The Pearl River delta lies in the transitional zone between the upland landscape of the catchment basin and the deposition centre of the northern continental shelf of the South China
Sea (Figure 1a). Since the collision between the Indian plate and the Eurasian plate around 34 Ma
(Aitchison et al., 2007), the upland region gradually uplifted, whilst the continental shelf subsided, the latter receiving sediment from the Pearl River. During the Late Quaternary, major faults were active (Huang et al., 1982), and the current deltaic basin was gradually formed through subsidence.
Along some faults, several valleys were formed. The lower part of these valleys was filled with the
MIS 5 marine sediment unit, and the surface of this unit is about -10 m to -15 m (Zong et al.,
2009b). The Holocene deltaic formation lies on the older marine unit (Zong et al., 2009a).
The Pearl River catchment is under the climatic influence of the Asian Monsoon. The history of monsoon climate of the region is revealed by the Dongge Cave record (Yuan et al., 2004; Wang et al., 2005), which shows a period of rapid warming, albeit with fluctuations, after the last glacial maximum. Strongest monsoon occurred between 9,000 and 7000 cal. years BP (Figure 2a), since which the monsoon became weakened towards the last 500 years. The diatom data and organic carbon isotope ratios from the mouth region of the Pearl River (Figure 2b, c) are in agreement with the stalagmite records, suggesting that freshwater discharge from the Pearl River was strong before c. 7000 cal. years BP (Zong et al., 2006) and gradually declined since (Zong et al., 2010b, c;
Yu et al., in press ). In contrast, eustatic sea level rose rapidly since the last glacial maximum
(Hanebuth et al., 2000; Siddall et al., 2003; Bassett et al., 2005) in the South China Sea region.
Importantly, sea level reached its postglacial maximum height by c. 7000 years BP (Figure 2d) (Zong
2007; Bird et al., 2007, 2010), i.e. 3000 years later than the initiation of a period of increased monsoon strength.
A number of Late Quaternary stratigraphic models have been put forward by various authors.
Yim (1994) proposed a model with up to five marine sequences possibly corresponding to the last five periods of high sea level, whilst others confirmed two (e.g. Huang et al., 1982 ; Fyfe et al., 1997) or three marine sequences which were related to the interglacial high sea levels in marine isotopic stages 1, 5 and 7 (e.g. Owen et al., 1998; Zong et al., 2009b). Between the two marine sequences lies a layer of weathered crust which formed during the last glacial when the older marine sequence was exposed to oxidation. Based on their stratigraphic records, Huang et al. (1982), Li et al. (1990) and Yim (1994) believed the postglacial marine inundation only started by c. 8000 years
BP and the Holocene deltaic sediment overlies directly on the older marine sequence. Other investigations suggested the onset of postglacial deltaic or marine sedimentation took place in palaeo-incised channels as early as c. 9500 years BP (e.g. Owen et al., 1998). Despite some seismic profiles indicating small incised channels in the Hong Kong area, there is little sedimentary

evidence to reveal major palaeo-incised valleys in the Pearl River deltaic basin (e.g. Zong et al.,
2009a).
3.
Methods
In order to investigate the early-Holocene stratigraphy of the palaeo-valleys drill cores in three series (the SS, SDZK and MZZK series) were collected from the deltaic basin. The drill holes were put down using a push corer to capture undisturbed sediment. Core samples were stored with PVC tubes in a refrigerator. Lithostratigraphic transects were reconstructed (Figure 3) using the new cores together with previous core records. These transects run across and down the deltaic basin
(Figure 1b) and reveal a palaeo-landscape that includes two incised valleys. Several cores captured sediment sequences from incised valleys (Table 1), and they were selected for additional laboratory analyses.
In this study, four new cores (SS0901, SDZK01, SDZK14 and MZZK04) from the deltaic plain were analysed, and one core (BVC) from the estuarine mouth region was re-examined, because they all contain the early-Holocene sediment sequence which was not well understood in previous studies. A total of 32 radiocarbon dates are reported in Table 2. All these dates are calibrated using the CALIB 5.10 software (Stuiver et al., 1998) with the Intcal04 programme for terrestrial samples and the Marine04 programme for marine samples with a correction factor, ΔR as -128±40 years according to Southon et al. (2002). Whilst the details of the radiocarbon dates are given in Table 2, the central calibrated ages to nearest decade are plotted on diagrams for easy reading.
Sediment samples from cores SS0901, SDZK01 and BVC were chosen for diatom analysis in order to infer the palaeo-environmental conditions. For each sediment sample, a minimum of 300 diatom valves were counted and identified to species level according to Van der Werff and Huls
(1958-1966). They were then grouped into two categories (marine and brackish water – MB and freshwater – F) according to Denys (1991-1992). They were further divided into two sub-groups, planktonic and benthic. Such classification helps understand the relative strength of marine water and freshwater influence as well as water depth at the time of deposition, albeit qualitatively ( Vos and de Wolf, 1993 ). The sediment samples were selected at various intervals for analysis using a laser particle size analyser, and classified into clay, silt and sand fractions.
4.
Results
4.1
Sedimentary chronology
Deltas are a dynamic landform which involves considerable contrast in sediment deposition and considerable reworking. Adding to this complexity is the influx of terrestrial organic carbon matter which may affect the precision of age determination by radiocarbon dating methods
(Raymond and Bauer, 2001). This problem appears more severe for the early Holocene when sea level rose fast and deltaic sedimentation was rapid (Stanley and Chen, 2000). This has resulted commonly in age reversals from the early Holocene sediment sequences of the Mekong (Tamura et

al., 2009), The Song Hong (Tanabe et al., 2003a) and the Yangtze (Wang et al., 2010). In our core
SS0901, this situation occurs too (Table 2). It is clear that the onset of postglacial sedimentation at the site was represented by the brownish grey clayey peat near the base of the core (Table 1). A date from the basal peat at the depth of 14.73 m yields an age of 8860 cal. years BP. Over the 7.5 m of sediment between 14.83 and 7.30 m, 7 radiocarbon dates show ages ranging from 9050 to 8290, with several apparent reversals. This highlights the limitations of radiocarbon dating methods in such a dynamic environment. However, it also implies a high rate of sedimentation during the early
Holocene. Other than this matter, the chronology of the sedimentary sequences revealed by the four cores is reasonably consistent (Table 2), although the radiocarbon ages should not be taken as exact dates due to the possibility of older carbon matter input from the river catchment (Raymond and Bauer, 2001).
4.2
The palaeo-landscape and sediments
In significant parts of the Pearl River deltaic basin, an older marine sequence is preserved (e.g.
Huang et al., 1982; Zong et al., 2009b). Sediment core records show that, in some places, the older marine sequence has been incised by fluvial action during the last glacial period. Transect A in
Figure 3 reveals the lithostratigraphy at the apex area of the delta plain, where the North River runs through a narrow gap (c. 3 km in width) of bedrock geology. Within a small embayment on the north side of the river, Holocene sediment sequence at SS0901 has been protected from being eroded by river channel migration. The basin widens to c. 15 km at Transect B, where a large volume of sand and gravel is recorded. At present, the river channel is restricted to the eastern side by flood defence. A layer of silt and clay is preserved, which is dated to the late Holocene. The deltaic basin is much wider along Transect C (c. 25 km), which shows a sub-surface around -20 m to
-15 m between the older marine sequence and the postglacial sequence. Other core records indicate that the altitude of this sub-surface rises to c. -10 m in the northern deltaic plain around
Guangzhou (Zong et al., 2009b). The older sequence here is incised to -33 m at core SDZK01.
Transect D, where the basin is nearly 60 km wide, reveals a more complex lithostratigraphy and palaeo-landscape. Cores PRD05 and SDZK14 show two separate palaeo-valleys incised to about -25 m and -41 m respectively. The older marine sequence remains between these two incised valleys and is recorded northeast from core SDZK14. Again, the altitude of the sub-surface varies from -20 m to -15 m. In Transect E, two incised valleys are also identified, down-cutting to -25 m and -42 m respectively. The surface altitude of the older marine sequence appears at -20 m to -15 m.
4.3
Bio-litho-stratigraphy
Core SS0901 was chosen to represent the sedimentary history of the apex area. The lithology
(Table 1) shows that overlying the basal peat is a unit of dark grey clay up to 5.83 m in depth. This unit contains shell fragments and some silt (Figure 4). From 5.83 m to 1.41 m, organic matter increases, and some large pieces of vegetation including Glypstostrobus pensilis, common in subtropical swampy wetlands of southeast China, are recorded at 1.41-1.67 m. Particle size results show a dominance of clay with minor sand and silt that increases from the base and reaches about

15% at 11.5-7.5 m before decreasing upwards. Diatoms are well preserved between 14.83 m and
4.05 m. The results show a gradual increase in marine and brackish water species from 14.83m to
7.50 m, before their numbers reduce upwards and disappear at c. 4.00 m. This marine-brackish diatom community is dominated by Coscinodiscus divisus, a planktonic type living in moderate salinity environments within the Pearl River estuary today (Zong et al., 2010b). The freshwater diatom community comprises many species, mainly benthic types. Planktonic types generally make up about 10% throughout.
Core SDZK01 was obtained from the central north part of the deltaic basin with the aim of revealing the full postglacial history. The sediment at the base of this core (Table 1) shows a thin sandy layer that drapes the sandstone bedrock. On top of the sandy layer is a unit of yellowish grey clay, and then dark grey clay, the latter containing oysters and marine gastropods and bivalves.
From 7.8 m upwards, the sediment changes to a yellowish clay. There may be a depositional hiatus at 7.8 m where the sediment changes colour sharply, however particle size results show no significant trend across this contact nor throughout the core (Figure 5). Diatom results indicate strong freshwater influence in the lower part of the core, corresponding to the yellowish grey clay and sandy clay between 15.4 m and 39.9 m. The assemblages are dominated by freshwater taxa commonly found in distributaries and at the head of the modern Pearl River estuary, including planktonic Aulacoseira granulata and benthic Fallacia subhamulata (Zong et al., 2006). The assemblages also contain significant numbers of marine brackish water species, such as Cos. divisus.
Within the dark grey clay, marine and brackish diatoms reach over 40%, comprising Cos. divisus,
Cos. blandus and Cyclotella striata. A similar assemblages is recorded in the mouth region of the
Pearl River estuary today (Zong et al., 2010b). In contrast the upper yellowish grey clay unit shows a significant increase in freshwater diatoms.
Cores SDZK14 and MZZK04 have not been analysed for diatoms, because their lithostratigraphies are largely comparable with that of SDZK01 (Table 1). Based on the three radiocarbon dates, core SDZK14 suggests a period of rapid sedimentation in the early Holocene as recorded by the light grey clay (19.20-43.70 m) and the grey to dark grey clay (15.10-19.20 m), both units exhibit considerable laminations, i.e. shallow water deposits. These two units are dated to the early Holocene (Table 2). The dark grey clay unit (12.5-15.10 m) may represent a phase of strong marine influence in the early middle Holocene. The grey clay with oyster shells and marine macrofauna increasing from 12.50 m upwards suggest a change from delta front to delta plain environment. Compared with core SDZK14, the stratigraphy of core MZZK04 is much simpler. The grey clay unit (27.30-33.50 m) was likely deposited during the early Holocene according to the date
(9010 cal. years BP) at 27.90 m. The dark grey clay unit (2.90-27.30 m) was deposited during the mid to late Holocene.
Core BVC was first reported in Zong et al. (2009b). Because this core is relevant to the discussion of the paper, both of the lithostratigraphic and diatom data are re-examined here. This core was taken from a shallow water area at the mouth of small valley from Lantau Island of Hong
Kong (Figure 1b). A sandy layer, rich in organic matter, at the base of the core is dated to 10500 cal. years BP (Table 2). On top of this sandy layer are three units of silt and clay (Table 1). Diatoms from

the sandy layer are all freshwater benthic species (Figure 6). The lower clay unit (12.3-19.9 m) contains mixed assemblages, dominantly planktonic marine and brackish water species including
Cos. divisus, Cos, blandus and Cyc. striata (Zong et al., 2009b), with about 20% of benthic freshwater taxa. In the middle clay unit (8.1-12.3 m), the diatoms are of almost fully marine and brackish water taxa, dominated by the planktonic marine Paralia sulcata. This species represents the marine component of the Pearl River estuarine diatom community (Zong et al., 2010b), and thus its dominance in the middle clay unit suggests a close-to-marine environment. Marine and brackish water diatoms remain dominant in the upper clay unit (0.9-8.1 m), but the percentages of
Paralia sulcata reduce to below 20% (Figure 6).
5 Discussion
5.1. Infilling of the deltaic basin
The new lithostratigraphic data has revealed the existence of palaeo-incised valleys in the
Pearl River deltaic basin (Figure 3). These valleys were likely formed during the last glacial when sea level was much lower than present. One such valley has incised through the older marine sequence and down to the bedrock as revealed by cores SDZK01 and SDZK14, reaching c. -40 m.
This valley widens from 2-3 km at Transect C to 7-8 km at Transects D and E. Sediments infilling this valley are about 25 m thick and dated to between c. 10,000 and 8500 cal. years BP, suggesting an average sedimentation rate of over 15 mm/a. This is much higher than the reported rate in the late
Holocene of 2-4 mm/a (Zong et al., 2009a). This unit of sediment is dominated by freshwater benthic diatoms (Phase I in cores SS0901 and SDZK01, Figures 4 and 5), indicating a shallow depositional environment influenced strongly by freshwater discharge. Minor marine influence is evident throughout this unit, and it became slightly stronger towards the end of this phase.
By the time the incised valleys were filled c. 8500 cal. years BP, the deltaic basin appeared to be a broad, shallow (c. 15 m depth) embayment. The embayment width is about 60 km wide at
Transect D. On top of the palaeo-valley deposit (Phase I) and the older marine sequence, two more sedimentary units (Phases II and III, Figure 4 and 5) are found and they record the infilling of the accommodation space in this broad deltaic basin. These two units are dated to early-mid Holocene and mid-late Holocene respectively. Firstly, the dark grey clay in 7.8 – 15.4 m of core SDZK01 shows evidence of strong tidal influence as the diatom assemblages are dominated by marine-brackish planktonic species. Such strong tidal influence is also recorded in 6.0 – 8.6 m of core SS0901. This is a phase when marine influence in the Pearl River delta was strongest, and this phase occurred around 8500-7000 cal. years BP in the apex area of the delta plain (Figure 4). Above this unit is a layer of yellowish grey clay, which contains diatom assemblages dominated by freshwater species whose numbers increase towards the surface. This suggests a gradual increase in fluvial influence as the deltaic shoreline advanced.
The above three phases of sedimentation form the complete postglacial history of the Pearl
River delta. However, the middle phase (Phase II) is somewhat difficult to define without detailed microfossil and chronological analyses. Alterations in sediment colour in core SDZK01 seem to

coincide closely with changes of sedimentary conditions, but such association is not easily applied to other cores. Nevertheless, this study adds details of an important sedimentary phase to the evolutionary history of the Pearl River delta proposed previously (e.g. Zong et al., 2009a). This new model (Figure 7) suggests two stages of sedimentation. The first stage (Phase I) took place when sea level rose rapidly and monsoonal discharge was strongest during the early Holocene. The environmental conditions resulted in the rapid infill of the palaeo-incised valleys and the inundation of the broad deltaic basin. The second stage saw deltaic progradation (Phase III), details of which were discussed below.
5.2. The role of sea-level rise, monsoonal discharge and the palaeo-landscape
By c. 10,000 BP or the end of melt water pulse 1B (MWP 1B), sea-level rose to c. -30 m in the
South China Sea region (Figure 2). This resulted in the initial marine inundation in the deepest part of the incised valleys of the Pearl River deltaic basin. Evidence for this early marine inundation is recorded from cores BVC, MZZK04, SDZK14 and SDZK01. Sea level continued to rise, and by c. 8500
BP, it reached -10 to -15 m. During the 1500 years, the fluvial and marine interaction was largely confined within the incised valleys. The effect of strong monsoonal freshwater on sedimentation was magnified as freshwater outflow was restricted in the relatively narrow valleys. Such largely fluvial sedimentation is indicated by the dominant freshwater benthic diatoms from the sediment.
Similarly, the very high sedimentation rate is a result as terrestrial sediment being deposited in valleys with limited accommodation space. While the fluvial effects were magnified by the palaeo-landscape of the incised valleys, the sea-level signature was largely suppressed.
Sea level continued to rise after c. 8500 cal. years BP, resulting in marine inundation spilling from palaeo-incised valleys onto the broad deltaic basin. Around this time, monsoonal discharge was still at its highest level in the Holocene (Figure 2). However, as freshwater outflow was no longer confined within the incised valleys, the relative effect of freshwater discharge and sediment supply against the rise in sea level was greatly reduced. This resulted in rapid shoreline retreat towards the apex of the deltaic basin and an increase in water salinity across the deltaic basin as recorded in all cores from this study. The basin wide change is recorded as an increase in marine-brackish water diatoms in cores SDZK01 and SS0901 from Phase I to Phase II (Figures 4 and
5). Such an increase in salinity is also manifest by the high percentages of Paralia sulcata in core
BVC from a far field location (Figure 6). Several researchers have attributed this sudden increase in marine influence or deltaic shoreline retreat to a phase of rapid sea-level rise around 8200 cal. years BP (Yim et al., 2006; Hori and Saito, 2007; Tamura et al., 2009). The apparent increase in marine influence and salinity recorded in Pearl River delta cores, however, may not be entirely the result of a phase of accelerated rise in sea level. It is very likely that sea water spilling out of the palaeo-valleys and inundating the wider deltaic basin also played a primary role in the shoreline retreat and a relative increase in salinity.
From c. 8000 to 7000 cal. years BP, it is apparent that sea level rose 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 SS0901 recorded an increase in freshwater influence (end of Phase II, Figure 4). At the

same time, the majority of the deltaic basin was still under strong marine influence (e.g. Phase II,
Figures 5 and 6). The high percentages of marine brackish planktonic diatoms from cores SDZK01 and BVC suggest a tidal regime similar to the present mouth region of the Pearl River (Zong et al.,
2010b). In the southern part of the deltaic basin, a change from high sedimentation in the early
Holocene to a much reduced sedimentation in the middle Holocene is recorded at core PRD05
(Figure 3d) where salinity increased to its maximum at this location around 7000 cal. years BP according to the ostracod records of Liu et al. (2008). Nevertheless, this time marks the turning point, a change from transgression to regression.
By c. 7000 cal. years BP, sea-level reached its present height in southern China (Zong, 2004) , and the palaeo-incised valleys of the Pearl River delta were fully filled as indicated by records of cores SDZK01, SDZK14, MZZK04 and PRD05 (Figure 3). The deltaic basin appeared as a broad, shallow estuary with many isolated bedrock islands (Zong et al., 2009a). Water depth varied between a few meters in the apex area and 10-15 m in the central and outer parts of the basin.
High sea level and a wide deltaic basin at this time saw strong tidal processes, which helps disperse the incoming terrestrial sediment across the entire deltaic basin and beyond. As a result, sedimentation at locations such as NL and UV1 in the mouth region commenced around 6500 cal. years BP (Zong et al., 2009a). By c. 5500 cal. years BP, diatom assemblages indicate that the apex area had become a freshwater wetland environment (Phase III, Figure 4). However, a change from brackish water to freshwater conditions indicated by diatoms at c. 5 m depth in core SDZK01
(Phase III, Figure 5) was as late as c. 2500 cal. years BP. This suggests the deltaic shoreline reached
Transect C (Figure 1b) by c. 2000 cal. years BP. The slow advancement of deltaic shoreline from
7000 to 2000 cal. years BP is likely a consequence of the strong tidal regime. Under this regime, vertical sedimentation took place in distal locations around rocky islands (e.g. Wu et al., 2007). In other words, significant amounts of sediment were transported to the subaqueous delta front area rather than being deposited at the contemporaneous river mouth. The deltaic shoreline further advanced to Transect D by c. 1000 years ago as indicated by the oyster shell deposits near the top of core SDZK14 (Table 1) and sedimentary/microfossil records of core JT81 (Zong et al., 2009a,
2010b) and core PRD05 (Liu et al., 2008). During the late Holocene, the sedimentation rate accelerated in the area of Transects D and E (Figures 1b and 3). Such acceleration in deltaic sedimentation was considerably influenced by human activity including active land reclamations for the creation of numerous fish ponds and paddies (Zong et al., 2009a ; 2010a ).
5.3. Comparison with other Asian deltas
It is common that many Asian deltas record a phase of rapid sedimentation in the early
Holocene followed by a much slower sedimentary rate in the mid- and late Holocene. For example, rapid sedimentation is recorded in the Mekong delta where c.19 m of sediment was deposited between c. 9100 and 8000 cal. years BP in core KS, (Tamura et al., 2009) and in the Yangtze where c.
28 m of sediment was accumulated between 13200 and 8400 cal. years BP in core ZK9 (Wang et al.,
2010). A similar sequence at the age of 12310 cal. years BP is also recorded in the Han River delta
(Zong, 1989). During the last 7000 cal. years, sedimentation in the apex area of the Mekong delta

decreased as result of the development of saltmarshes and mangroves (Tamura et al., 2009). In the
Yangtze, a slowdown in sedimentation rate (Wang et al., 2010) is associated with the development of the Chenier ridges along the southern edge of the deltaic plain. These deltas have been under the same monsoonal climate and experienced a similar sea-level history to the Pearl River delta.
However, the characteristics of their evolutionary histories are slightly different from each other.
The palaeo-incised valley of the Yangtze is a wide, elongate channel (Li et al., 2002). The accommodation space above the palaeo-valley is less than 5 m in depth, and this space was quickly transformed into freshwater marshy environment after c. 7000 cal. years BP (Zong et al., 2011).
Therefore, the Pearl River delta is unique because of its palaeo-landscape: a broad accommodation space of c. 15 m deep and c. 60 km wide on top of much narrower incised valleys of c. 25 m in depth (Figure 8). This unique palaeo-landscape has magnified the effects of sea-level rise and monsoonal discharge on deltaic sedimentation, resulting in a strong contrast between the dominantly fluvial processes in the early Holocene and the strongly tidal processes in the middle
Holocene.
6 Conclusions
This study has investigated the evolutionary history of the Pearl River delta with a particular focus on the sedimentary processes between the early Holocene and the mid-late Holocene. The study focussed on the palaeo-landscape effects in the history of early Holocene environmental change. The core records show that an older (possibly MIS5 in age) marine sequence formed the floor of the deltaic basin. This marine sequence was incised during the last glacial period. By the end of the Pleistocene, the deltaic basin appeared to have a wide basin which was dissected by a series of narrow incised valleys. Results of sedimentary/diatom records and radiocarbon dating from key cores obtained from one of the palaeo-incised valleys suggest three phases of environmental change. Phase I saw a period of largely freshwater sedimentation primarily confined within the incised valleys that is associated with rapid sea-level rise and strong monsoonal discharge. Fluvial processes dominated this phase because monsoonal discharge was confined within the incised valleys. Phase II recorded much slower sedimentation but strong tidal influence over the wide deltaic basin. This change is a result of sea level spilling out of the incised valleys and sea water inundating the wider deltaic basin. This transgressive flooding changed the sedimentary regime from dominantly fluvial to dominantly tidal. During this transgressive phase, the mouth of the Pearl River retreated to the apex area. Sediment and freshwater discharge were dispersed across the basin and beyond by strong tidal processes. As the basin shifted to a regime dominated by strong tidal processes, water salinity increased markedly from previous phase. Phase II culminated by c. 7000 cal. years BP when sea level reached its present height. Phase III records a period where deltaic shoreline progradation was slow at first and accelerated during the last 2000 years due to human activity that filled up much of the accommodation space. As a result, the sedimentary processes changed gradually from tide dominated to fluvial dominated at present.
The markedly increase of water salinity during 9000-8000 cal. years BP as recorded in sediment

cores in the Pearl River deltaic basin is likely to suggest a change in the hydrological conditions as sea water spilled over the incised valleys and inundated a much wider deltaic basin. Thus, the palaeotopography of the basin is the key driver of the early Holocene evolution and the relative increase in marine influence recorded in this basin cannot be solely attributed to a period of accelerated rise in eustatic sea level.
Acknowledgements
This research is funded by the Research Grant Funds (HKU707109P) and Hong Kong University seed grants (200902159004 and 200911159014) awarded to Zong, and Guangdong Natural Science
Foundation (Grant No. 10451027501005648), Foundation for Distinguished Young Talents in Higher
Education of Guangdong (FDYT: LYM10009) and the Fundamental Research Funds for the Central
Universities (Grant No. 11lgpy53) awarded to Huang and Zheng. The contribution of Switzer is based on research supported in part by the Singapore National Research Foundation (NRF RF
Award NRFRF2010-04). For Yu and Switzer this paper is Earth Observatory of Singapore contribution number 7. The authors thank the Geological Survey of Guangdong Province for providing core samples and records of the SDZK and MZZK series. This paper is a contribution to
IGCP Project 588 ‘Preparing for coastal change: A detailed process-response framework for coastal change at different timescales’ and the INQUA working group on ‘‘Coastal and Marine Processes’’ through project INQUA 1001: ‘Quaternary Coastal Change and Records of Extreme Marine
Inundation on Coastal Environments’.
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Captions:
Figure 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.
Figure 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 revision), 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.
Figure 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
Figure 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).
Figure 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.
Figure 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.
Figure 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.
Figure 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 middle-late Holocene deltaic progradation (deltaic regressive unit). This is an improved model to the one proposed by Zong et al. (2009a).

Figure 8. Key stages of the evolutionary history of the Pearl River delta during the late
Pleistocene – early 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.
Table 1. Lithological details of key new sediment cores
Depth (m) Altitude (m) Description
Core SS0901 (112 ° 50’30”E, 23 ° 10’05”N, ground altitude: 4.8 m)
0.00 – 0.41
0.41 – 1.41
1.41 – 1.67
1.67 – 5.83
5.83 – 14.83
14.83 – 15.60
15.60 – 15.98
4.80 to 4.39
4.39 to 3.39
3.39 to 3.13
3.13 to -1.03
-1.03 to -10.03
-10.03 to -10.80
-10.80 to -11.18
Filled material
Greenish light grey clay, with organic matter increasing downwards
Blackish grey clay with rich organic matter and large pieces of Glypstostrobus sp.
Dark grey clay, with organic matter increasing from 4.10 m upwards
Dark grey clay, with shell fragments in various depths and plant roots at the base
Brownish grey clayey peat
Yellowish grey sand and gravel
15.98 – 16.25 -11.38 to -11.65 Reddish weathered crust of sandstone
Core SS0904 (112 ° 56’03”E, 23 ° 05’15”N, ground altitude: 4.4 m)
0.00 – 0.41
0.41 – 1.00
1.00 – 2.14
2.14 – 3.21
4.40 to 3.99
3.99 to 3.40
3.40 to 2.26
2.26 to 1.19
Filled material
Yellowish grey clay
Grey silt with a thin band of peat
Grey find sand
3.21 – 4.28
4.28 – 9.40
1.19 to 0.12
0.12 to -5.00
Grey clay with organic rich bands
Grey clay with thin bands of silt
9.40 – 9.63 -5.00 to -5.23 Yellow coarse sand
Core SDZK01 (113 ° 12’22”E, 22 ° 55’08”N, ground altitude: 3.5 m)
0.0 – 2.8
2.8 – 7.8
7.8 – 15.4
15.4 – 36.4
36.4 – 39.9
3.5 to 0.7
0.7 to -4.3
-4.3 to -11.9
-11.9 to -32.9
-32.9 to -36.4
Filled material
Yellowish grey clay
Dark grey clay, with fragments of oyster shell and sea shell
Yellowish grey clay
Yellowish grey sandy clay
39.9 – 41.0 -36.4 to -37.5 Reddish weathered crust of sandstone bedrock
Core SDZK14 (113 ° 21’35”E, 22 ° 44’25”N, ground altitude: 2.5 m)
0.0 – 1.8
1.8 – 12.5
12.5 – 15.1
15.1 – 19.2
2.5 to 0.7
0.7 to -10.0
-10.0 to -12.6
-12.6 to -16.7
Filled material
Grey clay with oyster shells and shell fragments increasing towards the top
Dark grey clay with plant fragments
Grey to dark grey clay with laminations and occasional plant fragments

19.2 – 43.7 -16.7 to -41.2 Light grey clay with laminations and thin silt and sandy layers
43.7 – 49.0 -41.2 to -46.5 Yellow to grey sand and gravel overlaying bedrock
Core MZZK04 (113 ° 29’55”E, 22 ° 37’24”N, ground altitude: 1.5 m)
0.0 – 2.6
2.6 – 27.3
27.3 – 33.5
33.5 – 33.9
1.5 to -1.1
-1.1 to -25.8
-25.8 to -32.0
-32.0 to -32.4
Brownish grey clay – disturbed pond sediment
Dark grey clay
Grey clay
Yellowish grey coarse sand overlaying bedrock
Core BVC (11401’46”E, 2220’09”N, ground altitude: -7.6 m)
0.0 – 0.9 -7.6 to -8.5 Disturbed sediment
0.9 – 2.4
2.4 – 8.1
8.1 – 12.3
12.3 – 19.9
-8.5 to -9.8
-9.8 to -15.7
-15.7 to -19.9
-19.9 to -27.5
Light brownish grey silt and clay
Greenish grey to grey silt and clay
Grey to brownish grey silt and clay with clasts of weathered clay
Grey to brownish grey silt and clay with plentiful plant fragments
19.9 – 21.7 -27.5 to -29.3 Brown organic clayey sand
21.7 – 22.4
Table 2. Radiocarbon dates from key sediment cores
Core
-29.3 to -30.0
Depth Material
Brown to grey clayey sand with small gravel overlaying bedrock
Method Conventional Calibrated date Central cal. age
(m) age (yr BP) (yr BP) (2σ) (yr BP)
SS0901
SS0904
SS0904
SS0904
SS0904
SS0904
SDZK01
SDZK01
SDZK01
SS0901
SS0901
SS0901
SS0901
SS0901
SS0901
SS0901
SS0901
SS0901
SS0901
2.96
3.54
3.99
4.61
6.92
5.0
7.8
12.0
Bulk organic AMS
6.00 Plant fragments AMS
6.32 Plant fragments AMS
6.32
7.30
Bulk organic AMS
Bulk organic AMS
9.83 Bulk organic AMS
10.24 Bulk organic AMS
12.26 Plant fragments AMS
13.41 Plant fragments AMS
14.67 Plant fragments AMS
14.73 Peat
1.82 Peat
AMS
AMS
Plant fragment AMS
Plant fragments AMS
Plant fragments AMS
Plant fragments AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
7990±36
2090±20
2210±25
2250±40
2160±40
2560±30
2455±28
2528±29
6429±45
4800±40
6460±50
6580±40
7310±40
7690±40
7480±40
8090±40
7677±36
7961±35
8037±36
8860
2060
2230
2250
2180
2730
2530
2620
7350
5540
7360
7480
8100
8480
8290
9050
8480
8840
8940
9005-8717
1998-2120
2151-2318
2340-2150
2310-2040
2750-2700
2702-2361
2743-2492
7425-7274
5600-5470
7440-7270
7520-7434
8185-8020
8552-8404
8378-8198
9132-8971
8542-8407
8991-8695
9023-8846
GZ3945
GZ4141
GZ4142
Beta291321
Beta291322
Beta291323
GZ3927
GZ3928
GZ3929
Laboratory code
Beta201319
Beta291320
Beta290247
Beta289663
Beta289664
Beta289665
Beta289666
GZ3942
GZ3943
GZ3944

SDZK01
SDZK01
SDZK14
28.5
36.0
15.0
SDZK14
SDZK14
30.6
37.8
MZZK04 5.2
MZZK04 10.3
MZZK04 16.6
MZZK04 27.9
BVC
BVC
3.0
7.5
BVC
BVC
8.8
20.1
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Bulk organic AMS
Foraminifera AMS
Foraminifera AMS
Foraminifera AMS
Peat AMS
8095±33
7989±36
8040±45
9135±44
10848±49
1911±29
3177±30
3755±32
8069±39
4191±30
6973±34
8071±34
9320±40
9125-8987
9003-8717
9031-8725
10258-10154
12900-12803
1929-1811
3453-3355
4183-4067
9092-8932
4597-4290
7673-7473
8919-8554
10560-10410
4130
9010
4450
7580
8690
10500
9060
8860
8880
10210
12850
1870
3400
GZ3930
GZ3931
GZ3933
GZ3934
GZ3935
GZ3936
GZ3937
GZ3938
GZ3939
GZ2210
GZ2209
GZ2208
Beta195332