Holocene Delta Evolution and Sediment Flux of the Pearl River
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JOURNAL OF QUATERNARY SCIENCE (2016) 31(5) 484–494 ISSN 0267-8179. DOI: 10.1002/jqs.2873 Holocene delta evolution and sediment flux of the Pearl River, southern China XING WEI,1* CHAOYU WU,2 PEITONG NI3 and WENYUAN MO2 1State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Science, Guangzhou 510301, China 2Center for Coastal Ocean Science and Technology Research, Sun Yat-sen University, Guangzhou 510275, China 3Guangdong Research Institute of Water Resources and Hydropower, Guangzhou 510610, China Received 19 January 2016; Revised 19 April 2016; Accepted 14 May 2016 ABSTRACT: Delta progradation and sediment flux of the Pearl River Delta (PRD), southern China, during the Holocene are presented based on analyses of borehole data on the delta plain. Results indicate that the delta prograded into the drowned valley because of early Holocene inundation from 9 to 6 cal ka BP, as sea-level rise decelerated. The sea level reached its present level at about 6 cal ka BP and, as a consequence, a large portion of the drowned valley was covered by the estuary, with more than 160 rock islands and platforms. The scattered landmasses promoted active deposition and acted as deposition nuclei during deltaic evolution. Consequently, apart from exhibiting a general tendency towards progression, PRD development occurred less regularly over time and space because of deposition around island boundaries. During the last 2 ka, mainly because of significantly increased human activities, which have trapped sediments in the encircled tidal flats along the front of delta plains, the shoreline has advanced rapidly. Estimated sediment fluxes for the three periods (9–6, 6–2 and 2–0 cal ka BP), À based on the sediment volume analysis, were 17–25, 22–30 and 44–58 million t a 1, respectively. Copyright # 2016 John Wiley & Sons, Ltd. KEYWORDS: delta evolution; Holocene; Pearl River; sea-level change; sediment flux. Introduction estuary of about 1740 km2 (Zong et al., 2009; Wu et al., 2010). Because the receiving basin is semi-closed and no Deltas are recognized as discrete bulges in the shoreline offshore sediment dispersal to the deep ocean has occurred where the associated rivers deliver more sediment than (Huang et al., 1982; Wei and Wu, 2014), by quantifying the marine processes can redistribute (Elliot, 1978). Recent deltaic sediments of the Pearl River, we can calculate its studies have shown that on a millennial time scale, coastal Holocene sediment flux. hydrodynamics and past sediment discharges are the key In this paper, we present new data from three newly factors controlling delta morphology and progradation rates collected drill cores taken from the PRD. We then compile during the middle to late Holocene. For example, the data on sedimentary facies and radiocarbon dates from 19 sedimentary facies of the delta front became coarser grained, previously described sediment cores taken from the delta and the progradation rate of the Mekong Delta decreased at (Fig. 1). Based on the sedimentary facies data and radiocar- 3.0–2.5 cal ka BP, in large part owing to changes in the bon dates from all the sediment cores, the palaeogeography coastal oceanographic setting from a tide-dominated bay and evolution of the PRD and sediment flux during the last head to a wave-influenced open coast (Ta et al., 2002; 9 ka are discussed. Tanabe et al., 2003). The morphology of the Yellow River Delta also changed from straight to lobate because of an increase in river sediment discharge caused by human Study area activities in its drainage basin during the last 1000 years The Pearl River system, 2044 km long, is one of the most (Saito et al., 2001). An increase in sediment discharge during important river systems in China. The Pearl River catchment the late Holocene also created deltaic protrusions of the is located between 102˚150–114˚530E and 21˚500–26˚490N. shorelines at the mouths of the Po, Tevere, Krishna and Song The total area of the catchment within China is 44.21 Â 104 Hong rivers (Rao et al., 1990; Bellotti et al., 1994; Cencini, km2. Its delta, the PRD, is located in Guangdong province, 1998; Amorosi and Milli, 2001; Tanabe et al., 2006). Many South China. After entering the delta area, the West River, factors control the morphodynamics and sedimentary facies North River, East River, Liuxi River and Tang River, which are of deltas. Therefore, comprehensive, quantitative analyses of the main tributaries of the Pearl River, bifurcate continuously variable effects such as coastal hydrodynamics, sediment flux and form the extremely complicated network system of the and sea-level fluctuation are required to better understand Pearl River. The Pearl River discharges into estuarine bays delta evolution. through eight major outlets. The network and the estuarine The Pearl River delta (PRD), located in southern China, is bays are affected by tidal rise and freshwater dilution; thus the third-largest estuarine delta in China after the Yangtze they are both ‘estuaries’ by definition (Samoylov, 1958; and Yellow River Deltas, in terms of delta plain area. Pritchard, 1967). Following the terminology of Galloway (1975), the PRD can The hydraulic features of the Pearl River are summarized be classified as a tide-dominated system. For the last 6 ka, in Table 1. Water discharge levels from the Pearl River during the Holocene sea-level highstand, the delta has vary seasonally because most of the drainage area exists prograded more than 160 km seaward because of the volumi- within a subtropical monsoon climate regime. Runoff and nous supply of sediment from the Pearl River. At present the sediment discharge during the flood season (April–September) receiving basin is not completely filled and drains into a large account for 74–84 and 91–95% of the total annual amount, ÃCorrespondence to: X. Wei, as above. respectively. Suspended sediments consist primarily of silt E-mail: [email protected] and clay (Huang et al., 1982; Zhao, 1990). Although bedload Copyright # 2016 John Wiley & Sons, Ltd. DELTA EVOLUTION AND SEDIMENT FLUX OF THE PEARL RIVER, SOUTHERN CHINA 485 Figure 1. (A) The drainage basin of the Pearl River. (B) Map showing the Pearl River delta plain, estuary and bathymetry. Water-depth contour interval is 5 m. Open circles indicate the borehole sites. A–A’, B–B’, C–C’ and D–D’ are the locations of the profiles shown in Fig. 5. À sediments consist of medium and coarse sand, these sedi- average flood tide volumes reach as high as 73 500 m3 s 1, ments are thought to account for <10 % of the total sediment which is nearly 13 times the average freshwater discharge load (Li et al., 1990). (Zong et al., 2009). Tides in the Pearl River estuary are irregular and semi- Wind-driven waves and currents have a minimal impact on diurnal with amplitudes that range from approximately sediment transport within the estuary relative to tidal currents 0.8–0.9 m near the Wanshan Islands to approximately 1.7 m (Owen, 2005). Protected from storm waves by offshore near Humen (Zhao, 1990). Despite this small tidal range, rocky islands, wave energy levels within the estuary are low, Table 1. Flow and sediment discharge of the Pearl River (averaged from 1954 to 2000). Flow Suspended load À À Tributary Annual amount (108 m3) Mean discharge (m3 s 1) % Concentration (kg m 3) Annual amount (104 t) % West River 2220 7020 73.29 0.32 7100 80.03 North River 413 1310 13.63 0.13 647 7.29 East River 233 737 7.69 0.13 296 3.34 Sui River 68.4 217 2.26 0.16 109 1.23 Zeng River 38.2 121 1.26 0.14 56.1 0.63 Liuxi River 18.7 59.4 0.62 0.06 10.2 0.11 Tan River 20.7 65.5 0.68 0.11 23.0 0.26 The rest 17.1 0.56 630.7 7.11 Total 3029.1 9584.3 100 8872 100 Copyright # 2016 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 31(5) 484–494 (2016) 486 JOURNAL OF QUATERNARY SCIENCE although not during the passage of typhoons. Under typhoon deltaic sediments). In ascending order, these consist of two conditions, wave heights in excess of 1.5 m are common (channel-fill and floodplain sediments), two (tide-influenced (Zong et al., 2009). channel-fill and sub- to intertidal flat sediments) and five (tide-influenced channel-fill, delta front platform, tidal flat, Materials and methods abandoned channel-fill sediments and floodplain sediments) 0 00 0 00 sedimentary facies, respectively (Fig. 2). Each sedimentary Three borehole cores, PRD04 (22˚29 23 N, 113˚11 38 E), unit is characterized by a combination of its lithology, colour, PRD10 (22˚4302200N, 113˚1404200E) and PRD15 (22˚5404900N, 0 00 sedimentary structures, textures, contact character, lithologi- 113˚31 02 E), were obtained from the present delta plain cal succession, fossil components, grain size, and radiocar- during 2004–2005. The cores were split, described and bon dates. Details of the sedimentary units are described photographed. Radiographs of slab samples were taken below. throughout all split cores. Sand and mud (silt and clay) contents were measured every 20 cm throughout the core Unit 0 (late Pleistocene undifferentiated sediments) using 5-cm-thick sand and 2-cm-thick silt and clay samples. Fossils and microfossils such as diatoms, foraminifers and This unit is the lowest part of the section, approximately À mollusc species were identified. Twenty-seven samples were below 19.52 m (below present sea level) at PRD10 and À collected for 14C dating; analyses were undertaken on plant approximately 17.79 m at PRD15; the unit was not recov- fragments and molluscan shells by conventional 14C dating at ered at the PRD04 site.