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Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9 DOI: 10.1007/s13131-014-0407-8 http://www.hyxb.org.cn E-mail: [email protected]

Dispersal of the Zhujiang () derived in the Holocene GE Qian1*, LIU J. P.2, XUE Zuo2, Fengyou1 1 Key Laboratory of Submarine Geosciences, State Oceanic Administration, Second Institute of Oceanography, State Oceanic Administration, 310012, 2 Department of Marine, Earth and Atmospheric , North Carolina State University, Raleigh, NC 27695, USA

Received 6 November 2012; accepted 23 July 2013

©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014

Abstract High-resolution Chirp profiling and coring reveals an elongated (ca. 400 km) Holocene Zhujiang River (Pearl River)-derived mud area (maximum thickness > 20 m) extending from the Zhujiang , southwest- ward off the coast, to the Peninsula. Two depo-centers, one proximal and one distal, are identified. On the continental shelf off the west Guangdong Province, the mud is deposited in water depth shallower than 50 m; while to the southeast of the Zhujiang River , the mud area can extend to the −120 m isobath. A combined analysis with the stratigraphic sequences of other muddy deposits in the West- ern Pacific marginal seas (mainly Changjiang () and Huanghe (Yellow) derived) indicates that the initiation of the Zhujiang River muddy deposit can be further divided into two stages: Stage 1 is before the mid-Holocene sea-level highstand (ca. 7.0 cal. ka BP), the proximal mud was mostly deposited after 9.0 cal. ka BP, when the sea-level rose slowly after the Meltwater Pulse -1C; Stage 2, after the mid-Holocene sea- level highstand, clinoform developed on the continental shelf off the west Guangdong Province, extending ca. 400 km from the Zhujiang River Estuary. The proximal clinoform thins offshore, from ca. 10 m thickness around 5–10 m water depth to less than 1–2 m around 20–30 m water depth. In addition, we also find a de- veloped distal clinoform in the east of the . Key words: Sea, Zhujiang River, mud, clinoform Citation: Ge Qian, Liu J. P., Xue Zuo, Chu Fengyou. 2014. Dispersal of the Zhujiang River (Pearl River) derived sediment in the Ho- locene. Acta Oceanologica Sinica, 33(8): 1–9, doi: 10.1007/s13131-014-0407-8

1 Introduction second largest river, after the River, which drains into Rivers are the major carriers for delivering terrigenous ma- the (Dai et al., 2008). It originates from the Yun- terials to the ocean (Milliman and Meade, 1983; Milliman and nan- Plateau, drains the , Guizhou, Syvitski, 1992; Syvitski et al., 2005). The total suspended sedi- and Guangdong , flows toward the east and ment delivered by rivers to the ocean is about 15×109 t annually, empties into the South China Sea through the Zhujiang River of which Asian rivers discharge nearly more than 70% (Liu et al., Delta, which is one of the most important economic centers in 2009). Most of the sediment is either trapped in the or China, embracing Hongkong, Macao, , , deposited on the continental shelves, with less than 5%–10% of and other . The Zhujiang River drains an area of more the fluvial escaping to the deep sea (Liu et al., 2009; than 0.45×106 km2 (Tong, 2007), and its average water discharge Meade, 1996). Among the large rivers in Asian, those originat- is 302×109 m3 annually (Huang et al., 1982). The Zhujiang River ed in the are major contributors to the huge is a compound river system and mainly comprises the Xijiang, amount of sediment delivered to the West Pacific marginal seas. Beijiang, and Dongjiang Rivers. Among these , the Xi- In recent , numerous efforts have been made to investi- jiang River is the largest branch, and has a length of 2 214 km gate the delivering process and distribution of the large-river- and a drainage area of 0.35×106 km2. The upper reach of the Xi- derived sediment (e.g., Huanghe River (Alexander et al., 1991; jiang River (Hongshuihe River), which drains a karst area, is the Liu et al., 2004; Yang and Liu, 2007), Changjiang River ( et main source for river sediment. The estimated Xijiang River-de- al., 2000; Liu et al., 2006, 2007a; et al., 2009), and Mekong rived sediment flux is ca. 67×106 t/a (Dai et al., 2008). The - River (Ta et al., 2002; Xue et al., 2010)). A series of distal depo- ang River Delta locates in the subtropical zone, and has a warm centers are revealed by acoustic profiling and coring, which and wet climate, with an annual mean temperature of 22°C, and provide profound insight into Holocene sea-level variations an annual precipitation of 1 600–2 200 mm (Huang et al., 1982). and climatic conditions as well. However, the research of those Rain is seasonal, with most of the annual rainfall arriving be- about the Zhujiang River is still limited. tween June and August. The average sedimentation rate in delta The Zhujiang River is the third largest river in China and the is about 2–3 mm annually (Huang et al., 1982).

Foundation item: The National Natural Foundation of China under contract Nos 41106045 and 41206045; the Scientific Research Fund of the Second Institute of Oceanography, SOA under contract No. JT1102; the Basic Research Fund of State Oceanic Administration (named as Pale- oceanographic Research in the Western Pacific). *Corresponding author, E-mail: [email protected] 2 GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9

The Zhujiang River-derived muddy sediment is transported boreholes data (Huang and Ge, 1995; Owen et al., 1998; Fyfe et southwestward by the coastal current and deposited on the al., 1999; Yim, 2001; Xiao et al., 2006; Zhao et al., 2007a; Yim et continental shelf after delivering to the South China Sea (Fig. 1), al., 2006, 2008; Lan et al., 2008; Zong et al., 2009a, b) (Fig. 7), due to the influence of the geostrophic flow. However, little is we find an elongated (ca. 400 km), more than 20 m thick, pri- known about the morphological, stratigraphic, and other fea- marily Zhujiang River-derived muddy deposit extending from tures of the Zhujiang River-derived muddy deposit. In this the southwestward off the Guangdong coast to the study, we present the Chirp sonar sub-bottom profiles, which Leizhou Peninsula. We further divide this extensive deposit into obtained from the northern South China Sea geophysical in- two sub-systems: System 1 represents the Zhujiang River Delta, vestigation cruise in 2007. In conjunction with sediment char- which includes the subaerial delta plain, estuary, and the sub- acteristics analysis of four boreholes and previously published aqueous part (delta front and prodelta). System 2 is the Leizhou coring studies in the adjacent , we aim to understand Penisula Mud Wedge, which represents the distal part of the the distribution and delivering process of the Zhujiang River- Zhujiang River-derived mud deposits delivered by China Coast- derived sediment in the Holocene. al Current (Fig. 9). Muddy deposit in System 1 thins offshore in two directions: along-shelf to the west, the deposit pinches out 2 Methods and data at water depths 40 to 50 m off the west Guangdong continental Approximate 620 km high-resolution acoustic data was shelf; in the across-shelf direction, the mud deposit can extend retrieved using an EdgeTech 0512i Chirp Sonar Sub-bottom to water depth 120 m off the estuary (Fig. 9). Besides the seismic Profiler (frequency range: 0.5–12 kHz) in 2007 (Figs 1–6). Back sub-bottom profiles, boundary of the mud area is also based on in laboratory, all acoustic data were post-processed using the the data of relict sediment cores E602 (Xiao et al., 2006), KP43, Discover software (Version 3.0), and an acoustic velocity of KP83, KP234 (Yim et al., 2006), ZD1, and ZD2 (Zhao et al., 2007a) 1 500 m/s was assumed to calculate water depth and sediment (Fig. 7), and the preliminary distribution of the northern South thickness. Five of nine acoustic profiles are discussed in this China Sea sediment described by Liu et al. (2002). The high-res- paper (Fig. 1). The distribution or isopach map of the Zhujiang olution seismic profiles allow more closely examination of the River-derived mud on the Zhujiang River delta plain and estu- internal architecture of the northern South China Sea continen- ary is mainly based on 53 boreholes data from previous studies tal shelf mud deposit. (Owen et al., 1998; Fyfe et al., 1999; Yim, 2001; Yim et al., 2008; Overall, seismic sub-bottom profiles show a prominent sub- Lan et al., 2008; Zong et al., 2009a, b), while that on the north- surface acoustic reflector (Figs 2–5). Based on previous stud- ern South China Sea shelf is based on the acoustic profiles and ies of other systems in the Western Pacific (e.g., the other 24 boreholes data (Huang and Ge, 1995; Xiao et al., 2006; Changjiang subaqueous delta (Chen et al., 2000), distal deposits Yim et al., 2006; Zhao et al., 2007a) (Fig. 7). in the (Liu et al., 2004; Yang and Liu, 2007) and Sea (Liu et al., 2007a)), this prominent acoustic reflector 3 Results is referred as the base of the post-glacial transgressive surface Combined with the selected seismic sub-bottom profiles (TS), which is apparently caused by a rapid landward transgres- (Figs 2–6), lithological feature of AMS 14C-dated cores VB1, sion during a rapid sea-level rise. Above the TS, there is a ho- DEW42, KP72, and KP91 (Yim et al., 2006) (Fig. 8), and other mogeneous mud layer thinning offshore, which is referred to

N Zhujiang River China Kong

Yamen Guangdong Province Fig2.

Fig3.

Jianjiang River

Fig4.

Nandu River Fig5. Leizhou South China Sea Penisula Fig6. Island

Fig.1. Satellite image of the northern South China Sea taken on November 2001 (from NASA visible Earth at http://visibleearth. nasa.gov/view_rec.php?id=2301). Suspended-sediment plume off the Zhujiang River extended to the southwest. Red lines represent the positions of seismic tracklines which obtained from the northern South China Sea geophysical investigation cruise in 2007. GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9 3

clinoform N (−5 m) HST MFS S (−28 m) TST TS (−30 m) Gas

multiple 10 m 2 km

Fig.2. Seismic sub-bottom profile off the Zhujiang River Estuary, location of this profile is shown in Fig. 1. This profile shows HST, MFS, TST, TS, and signals of multiple reflection and biogenic gas.

NE (−10 m) clinoform HST MFS SW (−33 m) TST TS (−35 m) paleo-channel 10 m 2 km

Fig.3. Seismic sub-bottom profile off the Yamen, location of this profile is shown in Fig. 1. This profile shows HST, MFS, TST, TS and Zhujiang River paleo-channel.

NW (−12 m)

SE (−42 m) HST

multiple TS (−45 m) 10 m 2 km

Fig.4. Seismic sub-bottom profile off the west Guangdong Province coast, location of this profile is shown in Fig. 1. This profile shows HST, TS, and signal of multiple reflection.

S (−15 m) N (−15 m)

recent prograding sigmoid

sand waves HST

TS (−43 m)

multiple 10 m

2 km

Fig.5. Seismic sub-bottom profile and an enlargement off the west Guangdong Province coast, location of this profile is shown in Fig. 1. This profile shows HST, TS, and signals of multiple reflection and sand waves. 4 GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9

sand waves multiple

10 m 2 km

Fig.6. Seismic sub-bottom profile from the east of the Qiongzhou Strait, location of this profile is shown in Fig. 1. This profile shows signals of multiple reflection and sand waves.

110° 111° 112° 113° 114° 115° 116° E

23° Zhujiang River N

Guangdong Province

22°

21°

South China Sea

20°

Hainan Island

Fig.7. Distribution map of the used boreholes on the Zhujiang River Delta and northern South China Sea continental shelf (Huang and Ge, 1995; Owen et al., 1998; Fyfe et al., 1999; Yim, 2001; Xiao et al., 2006; Zhao et al., 2007a; Yim et al., 2006, 2008; Lan et al., 2008; Zong et al., 2009a, b). the transgressive system tract (TST) (Figs 2 and 3). The TST is indicates the presence of biogenic gas. capped by the maximum flooding surface (MFS), indicating it Above the TST, there is a MFS (Figs 2 and 3), which was was formed after the post-glacial transgression, but before the formed during the mid-Holocene sea-level highstand. The AMS sea-level reached its last highstand. However, there is no TST in 14C dates and lithological feature (Yim et al., 2006) (Fig. 8) in Figs 4 and 5. cores VB1, DEW42, KP72, and KP91 indicate that the mid-Holo- Close inspection of the seismic sub-bottom profiles suggests cene sea-level highstand happened around 7.0 cal. ka BP. Above that, the TST in Figs 2 and 3 is well preserved above the TS, thins the TS or MFS, a prominent finer-grained clinoform, which is offshore and pinches out at water depth of 30 m. Below the TS, termed as highstand system tract (HST), thins offshore to the a similar acoustic reflector appears, which formed by multiple east (Figs 2–5). Close inspection of the clinoform profile in Fig. reflection. We find a paleo-channel in Fig. 3, and an acoustically 5 shows a rather complex inner structure—which suggests there “turbid” layer beneath the foreset in Fig. 2. Compared with seis- is a distinct sigmoidal units in the HST. The seismic sub-bottom mic sub-bottom profiles from other areas (Diaz et al., 1996; Nit- profile in Fig. 6 is dominated by well-developed coarser-grained trouer et al., 1996; Liu et al., 2004), this turbid layer presumably sediment sand waves, with mud deposit deficit, indicating a dif- GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9 5

VB 1 DEW 42 KP 72 KP 91

0.9 m

2.3 m 0.2 m 8 338–8 022 6 404–6 097 0.3 m 11 124–10 209

7.0 m

11.5 m

calender age (cal. a BP) 0.8 m 8 096–7 852

post-glacial mud facies

6 058–5 605 post-glacial transgressive 0.5 m sand facies

Fig.8. Lithological feature of the boreholes near the Zhujiang River Estuary (Yim et al., 2006). ferent depositional environment with strong dynamics. ang River-derived sediment in the (Yang et al., 1992). -driven coastal currents flow southward with 4 Discussion numerous sediment, then the flow is oriented obliquely off- The fate of sediment dispersed from the river into the coastal shore in the bottom Ekman layer (Cookman and Flemings, ocean involves at least four processes: supply via plumes, initial 2001), and causes downwelling of nearshore water. At the same deposition, re-suspension and transport by marine processes, time, northward-flowing South China Sea warm current causes and long-term net accumulation (Wright and Nittrouer, 1995). upwelling (Su, 2004). With these two vertical circulation cells, Previous boreholes data show that the Zhujiang River-derived the Zhujiang River-derived mud is mostly trapped within −50 mud is mostly deposited on the west part of the delta plain and m isobath (Fig. 9). estuary. The high-resolution seismic sub-bottom profiles reveal The post-glacial sea-level rise provides a large accommoda- that the mud deposits extend ca. 400 km off the estuary to the tion space for terrigenous sediment to fill (Driscoll and Karn- southwest, around the Leizhou Peninsula (Fig. 9). Scholars ana- er, 1999). However, the sea-level rise is not a smooth process. lyzed the clay mineral assemblage and neodymium isotope to Several episodic rapid rising events have been identified in trace the source of the mud in the northern South China Sea. the Western Pacific. For instance, sea-level curve based on the Liu et al. (2007b) pointed out that kaolinite content from the studies in the South China Sea (Hanebuth et al., 2011) and the continental shelf off the west Guangdong Province (ca. 40%) southern Yellow Sea (Liu et al., 2004) shows two rapid rises dur- was higher than that from the continental shelf off the east ing the Holocene: the first one occurred around 11.5–11.2 cal. Guangdong Province (ca. 30%), and similar with that from the ka BP, namely Meltwater Pulse-1B (MWP-1B), is a rapid rising Zhujiang River drainage area, suggesting that the source of clay from −58 to −43 m; the second one, namely Meltwater Pulse-1C mineral on continental shelf off the west Guangdong Province (MWP-1C), is referred as another rapid rise from −36 to −16 m was Zhujiang River. Shao et al. (2009) presented neodymium occurred between 9.8 and 9.0 cal. ka BP. In the 1.4 ka interval isotopic records in the nearby area, which indicated that mud between the MWP-1B and 1C, sea-level rose slowly from −43 to on the continental shelf off west Guangdong Province was con- −36 m. trolled by Zhujiang River-derived materials. As an unconformity (Liu et al., 2007a) or acoustic reflector, A calculation by Geyer et al. (2004) indicates that a surface the TS has been referred as an indicator of sediment starvation river plume can only transport flocculated sediments for 5 km during the rapid sea-level rising events like MWP-1B and 1C. while a bottom re-suspension can maintain sediment in the The rapid sea-level rise flooded previous deposits, left an un- water at large distances from the river mouth. The Zhujiang Riv- conformity and furthermore created the accommodation space er-derived sediment is firstly deposited on the continental shelf for the later sediment accumulation. The majority of seismic near the estuary, and then, the southwestward Chinese Coastal sub-bottom profiles in this study show a prominent TS ranging Current strengthened by the strong northeast winter monsoon from 45 to 30 m below sea level (Figs 2–5). For instance, the TS plays an important role locally in southwestward longshore depth in two distal profiles shown in Figs 4 and 5 are around transport of re-suspended sediment (Fig. 1). The mud can be −45 m, whereas on others the TS depth are around −30 to −35 transported and deposited ca. 400 km away from the Zhujiang m. From the sea-level history and seismic sub-bottom profiles, River Estuary. This sediment transport pattern of “summer de- the TS in Figs 4 and 5 may be associated with the rapid MWP- posit and winter transport” is similar with that of the - 1B event and subsequent slow rise, and the others may be re- 6 GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9

110° 111° 112° 113° 114° 115° 116° E

Zhujiang River 23° N

Guangdong Province 22°

System 1 System 2 21°

Leizhou Penisula South China Sea Qiongzhou Strait 20° Hainan Island

Fig.9. Isopach of the Holocene-age Zhujiang River-derived sediment distribution. The dash line and question mark mean that if the Zhujiang River-derived sediment can be transported to the is still unknown. lated to MWP-1C. Thus, the mud area above the TS in this study the cores located off the Zhujiang River paleo-estuary, lots of should be formed during the Holocene (started from 11.5 cal. sediment can easily be transported and deposited in this area. ka BP). Previous studies indicated that the TST deposition of the In the early Holocene, the Zhujiang River delivers abundant Changjiang (Liu et al., 2007a) and Huanghe (Liu et al., 2004) Riv- fine-grained sediment to the northern South China Sea as a ers-derived mud is mainly consisted of silt and clayey silt, which result of the plentiful precipitation associated with the strong has a similar acoustic feature with that in this study (Figs 2 and summer monsoon (Wang et al., 1999), although the coastal 3). However, Yim et al. (2006) presented that the mud formed currents do not resume completely. Above the TS, a TST was after the mid-Holocene sea-level highstand directly covers on documented in Figs 2 and 3, suggesting that there is no obvi- the post-glacial transgressive sand facies (Fig. 8). We consider ous sediment deficit during the sea-level slowly rising stages that the location of the cores in their paper is near the estuary, after the MWP-1C. But there is no TST apparently developed in the grain size of sediment in proximal area is coarser than that Figs 4 and 5 after the MWP-1B transgression. Generally speak- in distal area. Therefore, the sand facies may also contain the ing, as the rate of relative sea-level rise increase, distal parts of TST, in addition to the post-glacial transgressive sand. The Zhu- the shelf become sediment-starved because the locus of sedi- jiang River discharged into the South China Sea through Yamen mentation moves quickly landward. Due to the presence of during the sea-level lowstand and eroded the bed rock to form the prominent Zhujiang River Shoal extending eastward from a paleo-channel (Bao, 1995), and furthermore filled by the Zhu- the modern Zhujiang River Mouth, the Zhujiang River-derived jiang River-derived fine-grained sediment during the transgres- sediment might not have been able to be transported to the sion (Fig. 3). south between MWP-1B and the middle Holocene sea-level Most of the modern deltas formed after the mid-Holocene highstand. Combined with the rapid increase in accommoda- sea-level highstand (Stanley and Warne, 1994). Compared with tion space the Zhujiang River's early- to middle-Holocene del- the seismic sub-bottom profiles from the northern South China taic deposits apparently were concentrated inland, and east of Sea continental shelf and lithological feature of AMS 14C-dated the present Zhujiang River Mouth nearly, not along the South cores VB1, DEW42, KP72, and KP91 (Yim et al., 2006) (Figs 7 and China Sea shelf. 8), we find the sea level reached an altitude close to its maxi- However, Yim et al. (2006) pointed out that the ages of mud mum around 7.0 cal. ka BP in the northern South China Sea, facies in Cores KP103, KP142, and KP183 (Fig. 7) exceeded 9.0 with the appearance of the MFS in the seismic profiles (Figs 2 cal. ka BP, older than the TST in Figs 2 and 3. There are two and 3). Above the MFS, the clinoform thins offshore, from ca. reasons for the age difference. Firstly, the water depth of these 10 m thickness around 5–10 m water depth to less than 1–2 cores is more than 65 m (Yim et al., 2006). Therefore, after the m around 20–30 m water depth (Figs 2 and 3). The evolution- MWP-1B, the former water depth was more than 20 m, it pro- ary history of the Zhujiang River Delta also validates this con- vided favorable conditions for the mud deposition; secondly, clusion: the sedimentation rate was less than 1 mm/a before GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9 7

the mid-Holocene sea-level highstand, and after that the rate and Ke (2000) pointed out that both the suspended load sedi- speeded up to more than 2 mm/a (Huang et al., 1982). Before ment and bedload sediment are transported from east to west the 7.0 cal. ka BP, the sediment from the Zhujiang River was in the Qiongzhou Strait. Therefore, the Gulf of Tonkin-derived mostly delivered to the ocean, rarely involved in the formation sediment hypothesis is unacceptable. The satellite image (Fig. 1, of delta. During this time, rapid sea level rise was the dominant obtained in the November 2001 by NASA), shows that the Zhu- driving mechanism, with variation of monsoon as the second- jiang River-derived fine-grained sediment was transported to ary driving mechanism for the Zhujiang River Delta evolution. the Gulf of Leizhou by the coastal current, and then continued After the mid-Holocene sea-level highstand, monsoon became southward delivery along the Leizhou Peninsula. In the east of the dominant controlling variable for sedimentary processes, the Qiongzhou Strait, the coastal current converged with an ir- the Zhujiang River Delta advanced quickly (Zong et al., 2009a). regular diurnal tide (Li and Ke, 2000), these two reverse current The paleo-shoreline rate is 10.5 m/a between 6.8 directions form a local cyclonic circulation (Li et al., 2011), the and 4.5 cal. ka BP, then slows down to 6.4 m/a between 4.5 and sediment load of seawater reduced rapidly, which lead to the 2.0 cal. ka BP, and finally increases to 29 m/a after 2.0 cal. ka BP fast deposition to form this distal depo-center. Therefore, the (Zong et al., 2009a). The slowing in progradation rate is possibly main material source of the clinoform in Fig. 5 is the Zhujiang a result of a gradual reduction in sediment supply because of River, and Jianjiang River and may also contribute a weakening summer monsoon and consequently decreased to this mud area. However, if the Zhujiang River-derived sedi- freshwater/sediment discharge (Wang et al., 2005). The acceler- ment can be transported to the Gulf of Tonkin is still unknown ated progradation rate after 2.0 cal. ka BP has been correlated to (question mark in Fig. 9), we need more works in the Qiongzhou the increased human activities (Zong et al., 2009a) or middle- to Strait and Gulf of Tonkin. late-Holocene sea level fall and then regressive processes. Al- though the monsoon-driven water discharge reduced, as a re- 5 Conclusions sult of the weaken summer monsoon (Zong et al., 2006), the re- The acoustic profiling and reexamination of previously pub- claimant in the Zhujiang River catchment increased erosion lished boreholes reveal an elongated (ca. 400 km) Holocene and sediment supply (Owen, 2005). This shift can be seen in Fig. Zhujiang River-derived muddy deposit (maximum thickness 5. Most of the sediment may be trapped in the delta plain and > 20 m) extending from the Zhujiang River delta plain south- involved in formation of the Zhujiang River Delta, which leads westward off the Guangdong coast to the Leizhou Peninsula. to the reduction of sediment discharge into the ocean (Zong et On the continental shelf off the west Guangdong Province, the al., 2009a). mud is deposited in water depth shallower than 50 m; while to Compared with the mud delivery capacity of the Changjiang the southeast off the Zhujiang River Estuary, the mud area can (ca. 800 km) (Liu et al., 2006, 2007a) and Amazon (ca. 1 500 km) extend to the −120 m isobath. A prominent subsurface acoustic (Nittrouer et al., 1986; Allison et al., 2000), that of the Zhujiang reflector, referred as transgressive surface (TS), is identified in River (ca. 400 km) is limited. The main reasons of this phe- the majority of the profiles. Above the TS, there is a homoge- nomenon are the low sediment discharge and the block of the neous mud deposit, which is referred as the transgressive sys- Leizhou Peninsula. However, the mud delivery capacity of the tem tract (TST). The TST in profiles, is probably formed after 9.0 Zhujiang River is similar with Mekong River's (ca. 200 km) (Xue cal. ka BP, during the sea-level rising was slow after the MWP- et al., 2010). Although the annual sediment discharge of Mekong 1C. After the mid-Holocene sea-level highstand (ca. 7.0 cal. ka River is large (ca. 160×106 t/a) (Milliman and Syvitski, 1992), the BP), clinoform developed on the continental shelf off the west gradient of the delta plain is gentle (ca. 0.000 03 m/km) (Syvitski Guangdong Province, extending ca. 400 km southwestward and Saito, 2007), which is not conducive to the mud delivery. from the Zhujiang River Estuary. The proximal clinoform thins Furthermore, the locates at the junction of two offshore, from ca. 10 m thickness around 5–10 m water depth different tidal systems (irregular South China Sea semi-diurnal to less than 1–2 m around 20–30 m water depth. In addition, tide with 3 m tidal range from the east direction, and Gulf of we also find a developed distal depo-center in the east of the diurnal tide with 1 m tidal range from the west direc- Leizhou Peninsula. The convergence of coastal current and ir- tion), the sediment load of seawater is limited, and numerous regular diurnal tide in the east of the Qiongzhou Strait leads to Mekong-derived materials are trapped and deposited on Me- the reduction of the sediment load, rapid sediment deposition, kong delta front (Cape Camau) to form the high-angle clino- and formation of the clinoform. form (Xue et al., 2010). The seismic sub-bottom profiles in Figs 5 and 6 are dominat- Acknowledgements ed by sand waves, which indicate a strong dynamic deposition The authors would like to thank the two anonymous review- environment (Peng, 2000). A similar stratigraphy has also been ers for their constructive comments. documented in the dynamic Strait (Liu et al., 2008). The sand was formed by eroding the old stratum in sea floor (Wang, 2000). Geographically, the Qiongzhou Strait forms during the References early- to mid-Holocene (Zhao et al., 2007b), the clinoform in Alexander C R, DeMaster D J, Nittrouer C A. 1991. Sediment accumu- Fig. 5 may be the result of the sediment exchange between the lation in a modern epicontinental-shelf setting: the Yellow Sea. northern South China Sea continental shelf and Gulf of Tonkin Marine Geology, 98(1): 51–72 after the mid-Holocene sea-level highstand. The fine-grained Allison M A, Lee M T, Ogston A S, et al. 2000. Origin of Amazon mud- banks along the northeastern coast of . Marine sediment from Gulf of Tonkin was delivered into the study area, Geology, 163(1–4): 241–256 and directly deposited above the Zhujiang River-derived mud Bao Caiwang. 1995. Buried ancient channels and deltas in the Zhujiang after the sorting in dynamic environment. However, based upon River mouth shelf area. Marine Geology & Quaternary Geology the research of the tidal sediment load in Qiongzhou Strait, Li (in Chinese), 15(2): 25–34 8 GE Qian et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 8, P. 1–9

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