Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China: Implication for Sediment Provenance

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1 Journal of Earth Science, Vol. 29, No. 1, p , February 2018 ISSN X Printed in China Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China: Implication for Sediment Provenance Jinbao Su 1, Wenbo Rao * 1, Yigang Wang 2, Changping Mao 1 1. School of Earth Sciences and Engineering, Hohai University, Nanjing , China 2. School of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing , China Jinbao Su: Wenbo Rao: ABSTRACT: The radial sand ridge system (RSRS) located at Jiangsu coast of China attracts much attention on its origin and mechanic of formation for its special structure and potential land resource. Due to complicated hydrodynamic condition, the Jiangsu RSRS is a hot debated on its potential sources, Yangtze River or Yellow River? We collected ten sand samples from surface sediments along the west coast of Bohai Sea and Yellow Sea from the modern Yellow River estuary to Yangtze River estuary in summer, The samples are analyzed by method of detrital zircon age for source identification of the RSRS sediments. The U-Pb age spectra of detrital zircon grains of the samples show a wide range from Cenozoic to Late Archean with several age peaks. Comparing the age spectra between the Yangtze River and the Yellow River, the detrital zircons have younger age (<100 Ma) group in the Yangtze River. These age distribution of the Jiangsu coastal RSRS sediments are similar to that of the Yangtze River, but different from the Yellow River. The samples located adjacent to the old Yellow River Delta show more wide-range age distribution, implying a compounded origination from the both rivers. Based on these findings it is proposed that, contrary to common opinion, the main sediment source of the Jiangsu RSRS is the Yangtze River, rather than the Yellow River. By implication, there should be evidence of hydrodynamic mechanics of oceanic currents and tidal motion. This aspect awaits confirmation in future research. KEY WORDS: Yellow Sea, Jiangsu coast, radial sand ridge, zircon geochronology, sediment provenance. 0 INTRODUCTION Sand ridge system consists of large, elongate sand bodies and channels formed in shallow seas. The ridges reach heights of several meters to several tens of meters, widths of several hundred meters to several kilometers, and lengths of several kilometers to tens of kilometers (Wang et al., 2012). Sand ridges occur widely in Chinese shelf seas such as finger-shaped sandy ridges of Liaodong bank in the Bohai Sea and Qiongzhou Strait of the South China Sea; the parallel sandy ridges of the Yalu River estuary and the West Korean Bay (Wang et al., 2012). The largest ridge along the Chinese coast is the radial sand ridge system (RSRS) at the North Jiangsu coast in the South Yellow Sea (Fig. 1). Sand ridges are thought as the result of tidal current action and influenced by pre-existing morphology, river discharge (e.g., Giosan et al., 5), and storm wave processes (Li and King, 7). The RSRS has been considered to be formed during the Holocene transgression (Wang et al., 1999; Zhu and An, 1993; Yang, 1989) or regression (Li et al., *Corresponding author: raowenbo@163.com China University of Geosciences and Springer-Verlag GmbH Germany, Part of Springer Nature 2018 Manuscript received April 11, Manuscript accepted January 15, ). Wang et al. (2012) interpreted it as the product of river-sea interaction. Numerical simulation of sediment dynamics has been used in some local distributions of sandy and muddy sediments (e.g., Zhu and Chang, 0; Huthnance, 1982a, b). However, it is hard to simulate sediment transport and deposition across largescale shelves, under single or multiple hydrodynamic conditions (Bian et al., 2014; Chen and Zhu, 2012). Meanwhile, the hydrodynamic mechanisms are often lack of detailed data to be supported with different opinions due to complexity of sediment transport patterns and processes. Hence, the study of sources and evolution of shelf deposits are crucial to fully understand the link between present-day terrestrial processes and coastal marine deposition, and hydrodynamic processes. The development of coastal ridges reflects the interplay of several factors, such as secular changes in sea level, sediment supply and wave characteristics (Anthony, 1995), while the most important factor is sediment supply. In fact, the amount of sediment load determines how much propagradtion of sand ridge takes place and whether the sand ridge front may eventually maintain a stable or eroded shoreline position (Anthony, 1995). A definitive understanding to the sources of sand ridges can yield important information about transport pathways and anthropogenic impacts, littoral transport directions, and local erosion, which is essential for assessing the current and future effects of sediment-impacting activities for the forming of sand Su, J. B., Rao, W. B., Wang, Y. G., et al., Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China: Implication for Sediment Provenance. Journal of Earth Science, 29(1):

2 Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China º 124º 128ºE km 40ºN 34º 28º Bohai Sea HHK HH01SD A3 Yellow River Old Yellow River China Nj24 20 A6 A5 A7 Jiangsu coast A8 TZN A4 Yangtze River BCC Liaodong Peninsula YSCC LDCC YSWC Yellow Sea A9 G20 A10 G40 CDFW G62 Cx25 50 TWC East China Sea Dakdong Korea 50 KCC 100 G3 G7 Cheju G15 50 G30 TC KC Han 100 Keum Yeongsan Samples in Choi et al., 2013 Samples in He et al., 2013 Samples in Yang et al., 9 Samples in this paper 128 Sand ridge Figure 1. Regional map of the study area and sample locations (modified after Yuan D L et al., 8; Yang et al., 3). Contour unit is meter. Shaded areas showing distribution of mud patches at the ocean bottom, with darker colors indicating finer grain sediments. The arrows indicate ocean currents in summer. YSWC. Yellow Sea Warm Current; YSCC. Yellow Sea Coastal Current; KCC. Korea Coastal Current; TSWC. Tsushima Warm Current; TWC. Taiwan Warm Current; BCC. Bohai Sea Coastal Current; LDCC. Liaodong Peninsula Coastal Current; TC. Tushima Current; CDFW. Yangtze River fresh water. ridges (Barnard et al., 2013). It is considered that the Yangtze River (called Changjiang in Chinese) and the Yellow River (called Huanghe in Chinese) located along the Chinese coast, possess a large amount of freshwater. Besides, there are relatively small rivers of Korea and China providing a small quantity of freshwater (Chang and Isobe, 3). However, which river provides the main sediment provenance of coastal China sea, is much debated (Lim et al., 6; Yang et al., 3). Various methodologies have been applied to investigate the provenance of the Yellow Sea sediments such as clay mineralogy, heavy mineralogy, carbonate mineralogy, geochemistry, magnetic property, seismic profiles, and satellite observations (Hu et al., 2012; Dong et al., 2011; Shi and Wang, 2010; Xu et al., 9; Yang and Youn, 7; Chough et al., 4, 2; Wu et al., 1). Lim et al. (6) proposed that much of the Yellow Sea sediments are a mixture from Korea and China, with the East China Sea sediments predominately of Chinese river origin. The RSRS sediments of the Jiangsu coast was considered to be derived directly from the Yangtze River (Yang, 1989), the old Yellow River (Zhang and Chen, 1992), or both rivers (Rao et al., 2015; Wang et al., 2012; Yang et al., 2). Li et al. (1) proposed that the sediments of the RSRS initially came from the Yangtze River and later mainly from the Yellow River and partly from sea-floor erosion; Wang et al. (2012, 1999) suggested that the RSRS was supplied by the old Yangtze River sediment during the Late Pleistocene and later were affected by the input of the Yellow River during the Late Holocene. Recently, Rao et al. (2015) pointed out that the old Yellow River and the Yangtze River were the dominant sources of the RSRS but the effect of the Korean rivers could not be neglected. Obviously, oceanic circulation and sediment transport patterns in the Yellow Sea complicate our comprehension on sediment dispersal, and hence the further research on provenance discrimination for the coastal China sea is necessary. Zircons have since played a prominent role in the interpretation of the composition and history of modern and ancient sediments due to the fact that zircon is highly refractory at the Earth s surface and that it exists in virtually all sedimentary deposits (Su et al., 2017, 2014a, b; Cawood et al., 3). Hence, zircons provide critical links in understanding the source history of a deposit, sedimentary dispersal systems and tectonic reconstructions. This paper selects sand samples at the RSRS of the Jiangsu coast to present detrital U-Pb zircon age data. Based on the comparison of the age spectra of sands and provenance analysis, we propose a new depositional and formational processes of the RSRS in the Jiangsu coast.

3 146 Jinbao Su, Wenbo Rao, Yigang Wang and Changping Mao 1 REGIONAL SETTINGS The Yellow Sea is an epicontinental sea located between China and the Korean Peninsula with an average depth of 45 m. It is bounded by the Bohai Sea to the north and by the East China Sea to the south. These sea basins belong to a Cenozoic rift basin with continental basement (Cai et al., 2014; Su et al., 2014c, d). It is considered that the coastline has prograded ~40 km over the last thousand years, most rapidly between 1128 and 1855 year before the Yellow River mouth migrated from the North Jiangsu coast northwards to the Bohai Sea (Wang et al., 2012). Thus, the coastal plain in the northern Jiangsu Province, located between the old Yellow River in the north and the modern Yangtze River in the south, covers an area about km 2 (Fig. 1). The RSRS is located at the Jiangsu coast with a radiative fan pattern (Fig. 1). The length of RSRS is km and the width is 140 km, consisting of more than 70 sand ridges and tidal channels (Wang et al., 1999). The sediments of the RSRS mainly consist of well-sorted fine sands with less percentage of silts, whereas the sediment composition in the tidal channels is fining seaward with more silts. The rivers around the Yellow Sea bear remarkably different sizes, water, and sediment discharges (Yang et al., 3). At present, the Yangtze River and the Yellow River do not directly empty into the Yellow Sea, however, they were considered as the main sources of the sedimentation of major parts of the Yellow Sea during Holocene (Saito et al., 1; Martin et al., 1993; Zhu and An, 1993). The Yangtze River and the Yellow River supply tremendous sediment loads, about ~ and ton/year, respectively (Yang et al., 3). Total sediment discharge from small Chinese rivers into the Yellow Sea is less than ton/year. Whereas the total sediment discharge from Korean rivers into the Yellow Sea is relatively meager, generally less than ton/year (Ren and Shi, 1986; Schubel et al., 1984). There are two general circulation patterns in the Yellow Sea, which are considered to play an important role in the formation of the RSRS. One is basin-size counterclockwise gyre with a northward inflow of the Yellow Sea Warm Current (YSWC) along the eastern margin, and the other is a southward outflow of the Jiangsu Coastal Current (JSCC) or the Yellow Sea Coastal Current (YSCC) along the west coast (Fig. 1, Hu and Li, 1993; Beardsley et al., 1985). The YSWC intrudes into the northern Yellow Sea during winter, whereas the YSCC flow southward during both summer and winter. Meanwhile, a southward outflow of the Korea Coastal Current (KCC) together with YSWC constitute a clockwise gyre in the eastern part of the Yellow Sea (Yang et al., 3). In addition, the Yellow Sea is also under the influence of two types of tidal waves, a progressive tidal wave from the Pacific propagating from the southeast towards the North Yellow Sea, and a local reflected tidal wave formed by the obstruction of the Shandong Peninsula in the northwest (Wang et al., 2012). The both tidal waves converged at the Jiangsu coast in the western Yellow Sea, form a standing wave (Zhang, 1998), where the semidiurnal rotary tidal currents are very strong with maximum velocities exceeding 2.5 m s -1 (Wang et al., 2012). 2 ANALYTICAL METHODS The field expedition was conducted between April and July, We collected samples from the RSRS of the Jiangsu coast, the Yangtze River and the Yellow River estuaries by grab sampler and small shovel (Fig. 1). Samples A3 and HHK are silty sands collected from the Yellow River estuary. Sample A5 is sand from the old Yellow River Delta. Sample A6 is sand collected from the coast of the north of the old Yellow River. Samples A7, A8, A9 and TZN collected from offshore of the RSRS. Samples A4 and A10 are sands collected from the downstream Yangtze River. The sampling locations are chosen not only to characterize individual regions but also to avoid contamination from industries, harbors and local residents, and are thus representative. Hydrodynamic sorting can influence the distribution of various-sized minerals in the fluvial sediment and cause significant differentiation in zircon grain size and U-Pb age (Garzanti et al., 2010, 9; Lawrence et al., 2010). Many researchers suggested that choosing a proper-sized sand fraction could avoid hydrodynamic fractionation for provenance studies, and the zircons of the μm fraction considerably avoided the bias of the size-effect on detrital zircon age (e.g., Morton and Chenery, 9; Andersen, 5; Morton et al., 5; Morton and Hallsworth, 1994). However, this type of narrow-fraction can produce strongly-biased and misleading results by enhancing rather than minimizing grain-size effects (Garzanti et al., 2010, 9), and it might not reflect the entire age information of all detrital zircons (Lawrence et al., 2010). In fact, there is still no consensus on how to eliminate or minimize the hydraulic sorting effect on zircon size and age to provide more reliable and comprehensive geochronological information, although the zircons of μm have very similar age populations with the bulk zircons (Yang et al., 2012). Therefore, this paper, zircons were extracted randomly from the whole samples crushed using conventional heavy liquid and magnetic techniques and then handpicked under a binocular microscope. Among the collected zircons, 60 grains were randomly selected from each sample and mounted onto slide glasses, which were subsequently heated to temperature of ca. 310 ºC to embed the zircon grains into PFA Teflon sheet. Embedded zircon grains were polished to expose mineral centers. In situ isotopic measurements of these samples were carried out at the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) Laboratory of the Tianjin Institute of Geology and Mineral Resources, China. The analysis procedures are the same to that described by Wu et al. (6b) and Yuan H L et al. (8). The U-Pb ages and concordia plots were dealed by the software ISOPLOT 4.0 (Ludwig, 3). Due to the large amount of radiogenic Pb, the 207 Pb- 206 Pb age is more reliable for the zircons with ages older than Ma. On the contrary, due to the low content of radiogenic Pb and the uncertainty of common Pb correction, the 206 Pb- 238 U age is more reliable for zircons with ages younger than Ma (Anderson, 7; Jordan, 1988). The LA-ICP-MS U-Pb isotopic age determinations from the 10 samples analyzed in this study are listed in supplementary Table S1. It is not considered further for the result that are >10% discordant (by comparison of 206 Pb- 238 U and 206 Pb- 207 Pb ages). Acceptance of analyses with up to 10% discordance can yield a more complete and accurate description of provenance components. To ensure that grains with a complex history (e.g., inheritance, Pb loss or overgrowths) do not compromise data quality, the time-resolved pattern of 206 Pb/ 238 U was monitored closely during acquisition, and any analysis that showed unusual patterns were rejected (Su et al., 2014a). In addi-

4 Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China 147 tion, provenance interpretations are based primarily on age clusters that include at least three analyses, as inheritance, Pb loss and/or multi-domain analyses will almost always increase scatter (Gehrels, 2011). 3 RESULTS The analysis data are presented in concordia plots (Fig. 2). The Th/U ratios offer a broad estimate of the zircon origin, although it is difficult to distinguish the origins of zircons based exclusively on the Th/U ratios (Su et al., 2014a). Most zircons have Th/U values greater than 0.1 (Fig. 3), implying an igneous origin (Corfu et al., 3; Belousova et al., 2). Most zircon grains from the samples A3 and HHK are rounded and fragmented, μm long and μm wide, which may indicate a long transport distance (Figs. 4a and 4b). A total of 123 analyses were conducted on these zircon grains. Some zircon grains are discordant due to Pb loss (Figs. 2a and 2i). The analysis plot gives a wide range of U-Pb apparent ages from to Ma with several groups. The detrital zircon U-Pb ages are mostly clustered at and Ma, and subordinate age group is Ga (Figs. 5b and 5c). Zircon grains from sample A5 are μm in length and μm in width (Fig. 4c). Most zircon grains are rounded and partly fragmented, and a few of them are euhedral, which may be suggestive of a variable distance of transport. A total of 60 analyses were conducted on these zircon grains. Some zircon grains are discordant due to Pb loss (Fig. 2c). The results indicates a dominant age group of ca. 100 Ma and other weak peaks of Ma and 1 Ma (Fig. 5e). Zircon grains from sample A6 are 100 μm long and μm wide (Fig. 4d). Most zircons are euthedral with less rounded and fragmented implying a short distance of transport. A total of 60 analyses were conducted on these zircon grains. The zircon ages are mainly ca. 150,, and Ma (Fig. 5f). Zircon grains from samples A7, A9 and ZTN are μm long and μm wide (Figs. 4e, 4g and 4j), whereas sample A8 shows smaller size with μm long and μm wide (Fig. 4f). Most zircons are rounded and partly fragmented, and a few of them are euthedral, which may be suggestive of variable sources. Samples A7, A9 and TZN have similar age groups with peaks of ,, Ma (Figs. 5g, 5h, 5i). The age groups of A8 are different with the youngest age peak of Ma and older than other samples (Fig. 5d). Zircon grains from samples A4 and A10 are μm long and μm wide (Figs. 4h and 4i). Most zircons are rounded and fragmented with a few of euthedral structures, which may represent a long distance of transport. These ages of zircon grains yield an obvious extent between Ma and age groups of ,, and Ma (Figs. 5j and 5k). 4 DISCUSSION The Jiangsu coastal zone and its surrounding rivers, Korean Rivers, south-central and southeastern Yellow Sea sediments have great difference in grain size distributions (Rao et al., 2015; Lim et al., 2013, 6; Yang and Youn, 7). Compared with this, the zircon sizes of coastal sediments from the Yangtze River to modern Yellow River estuary change greatly too based on analysis from CL image (Fig. 4). The Yangtze River and old Yellow River have a similar zircon size of μm in length, whereas zircon grains in the modern Yellow River sediments are μm long. Except for sample A8 with μm, all the grain-size diameters of samples located in the RSRS fall in a range of μm, which are similar to that of the Yangtze River and the old Yellow River, whereas the sample A8 has similar zircon sizes with that of the modern Yellow River. It is noted that, most zircon sizes of the Yangtze River sediments are coarser than that of the modern Yellow River, which is contrary to mean grain size of the sediments reported by Rao et al. (2015). That may be caused by different mineral composition for different sources with influence of hydrodynamic fractionation. Yang et al. (2012) suggested that younger zircons have coarser and more variable sizes than the older zircons in the Yangtze River. Although the hydrodynamic sorting can influence the distribution of varioussized minerals in the fluvial sediment (Garzanti et al., 2010, 9; Lawrence et al., 2010), the zircon sizes seem to illustrate a consistent result with that of the geochronological data of detrital zircons in this paper as following. The data of detrital zircons of the Yangtze River and the Yellow River have been reported by several researchers (He et al., 2013; Yang et al., 9). Their age distributions are attributed to widespread drainage systems and cover regions. The Yangtze River drainage covers several tectonic units, including the Qamdo Block, the Songpan-Garze terrane, the Qinling- Dabie orogenic belt and the South China Block (Su et al., 2014a; He et al., 2013). The South China Block comprises the main part of the Yangtze River drainage, and it records all the tectonic assembly and orogenic events occurred in or around the Yangtze Block, e.g., Columbia-age, Grenvillian, Jinningian, Caledonian, Hercynian, Indo-Sinian, Yanshanian and Himalayan (Su et al., 2014a; Li et al., 2013). The Yellow River drainage flows through the Qinling Orogen and the North China Block. The peak ages of detrital zircons in the Yellow River sediments show main age groups of , , , and Ga (Fig. 5a), showing multiphase crustal growth of the North China Block (Yang et al., 9; Wu et al., 6a). The detrital zircons are stochastic selection, and thus all the age distributions of samples have representation. The age distributions of the fluvial sediments from the Yangtze River and the modern Yellow River display similar age spectra to that of the samples located in the two rivers estuaries, respectively (Fig. 5). The age population of detrital zircons from the modern Yellow River are subdivided into age groups of 300, 300, 1 000, and Ma (Fig. 5a). All groups are older than Ma and the prominent age group is 500 Ma. The zircon ages in the Yangtze River fall largely in 100, 300, 300, 1 000, and Ma. The obvious difference of age distributions between the Yangtze River and the Yellow River is that there is the youngest age group of 100 Ma among the detrital grains of the Yangtze River (Figs. 5a, 5l and 5m). Furthermore, the subordinary age groups falling and 300 Ma in the Yangtze River is different from that of the Yellow River. The Yangtze River has an obvious age peak of 700 Ma,

5 148 Jinbao Su, Wenbo Rao, Yigang Wang and Changping Mao A3 n=63 n= A (e) A7 3 0 n= A9 (g) n= (a) (c) (i) HHK 0.16 n=26 n= A A6 n= (f) A n= A10 3 n= (b) (d) (j) (h) TZN n= n= Figure 2. U-Pb concordia plots for zircons from 10 sand samples. (a) (j) represent A3-A10, HHK and TZN.

6 Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China Th/ 238U TZN HHK A3 A4 A5 A6 A7 A8 A9 A Age (Ma) Figure 3. Age (Ma) versus Th/U ratio for 10 samples of study area. Figure 4. Cathodoluminescence (CL) images of representative zircons. (a) (j) represent correspond samples A3, HHK, A5, A6, A7, A8, A9, A10, A4 and ZTN.

7 150 Jinbao Su, Wenbo Rao, Yigang Wang and Changping Mao Figure 5. Age histograms for detrital zircons of study area. (a) HH01SD (Yang et al., 9); (b) (k) HHK, A3, A8, A5, A6, A7, TZN, A9, A10, and A4; (l) Nj24; (m) Cx25 (He et al., 2013). whereas it is weak or absent in the Yellow River. The Yellow River s has a main age peak of 500 Ma, however, it is not prominent in the Yangtze River s. The width and discrepancy of age distributions with numerous peaks observed in the samples of the Yellow River and the Yangtze River, suggest that the provenances of zircon grains are spatially large in extent. Based on the age-distribution characters, the samples HHK, A3, and A8 have two main age peaks concentrating in Ma, especially that of 500 Ma. In contrast, the samples A7, TZN, A9 and A10 show more successive age scatters in Ma, particularly in existence of age groups of 100 and 700 Ma (Fig. 5). Samples A7, TZN, A9 and

8 Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China 151 A10 seem to come from the Yangtze River according to their youngest ages, while samples HHK, A3 and A8 should be derived from the modern Yellow River. Samples A5 and A6 have young age peaks similar to that of the Yangtze River, however, their Paleozoic age groups are different (Figs. 5e and 5f). Thus, we infer that samples A5 and A6 may originate from either mixture of the two rivers or coastal rocks. Away from coast to the east, the Yellow Sea sediments varied in provenance between the Yangtze River and the South Korean rivers (Fig. 1). Based on Choi et al. (2013), the sediments of the middle Yellow Sea (G40, G62, G30, and G20; see Fig. 1 for locations) may originate from the Yangtze River, whereas sediments adjacent to the Korean Peninsula (G3, G7, and G15) may derive from the South Korean rivers (Fig. 1). That is to say, the sediments derived from the Korean Peninsula have less transportation to the west. The sediments in the old Yellow River and the modern Yellow River should have similar grain sizes and age distribution characters due to their similar drain areas. However, the old Yellow River has more analogous to the Yangtze River. This means the Yangtze River may provide more sediments, which influences the depositional region around the old Yellow River mouth. Hence, we speculate that the Yangtze River may be the dominate source of the RSRS sediments, whereas the old and the modern Yellow River may provide a small quantity of sediments for the RSRS. In addition to the grain sizes and ages of detrital zircons, the geochemical analysis of the Jiangsu RSRS sediments also illustrates a similar result that the RSRS sediments may originate mainly from the Yangtze River and partly from the Yellow River (Rao et al., 2015). However, different from our present zircon data, our previous geochemical result supports that the old Yellow River is an important provenance and the Korean rivers provide minimal sediment input for most RSRS sediments (Rao et al., 2015). We reappraise the contribution of the old Yellow River for that it was influenced by the Yangtze River s input as above discussion. Meanwhile, the samples are coarse-grained sediments near the onshore, therefore it is hard to reflect the influence from far-distance Korean rivers. Nevertheless, Korean rivers have provided sandy sediments to the northern Yellow Sea between the Shandong Peninsula and the Korean Peninsula (Kim et al., 1999). Hence, it is possible that the Korean rivers supply minimal suspended material to the Jiangsu coastal RSRS as suggestion in Rao et al. (2015). Sediment supply determines the plan-view patterns, ridge and swale morphology and spacing, internal structures, facies arrangements, and the texture and mineralogy of constituent ridge sediments (Taylor and Stone, 1996; Tanner, 1993, 1987). Meanwhile, the relatively shallow sea sediments are strongly affected by monsoons, massive freshwater outflows, sediment input from the surrounding landmass, and the hydrodynamic regime in response to sea-level rise (Chen, 9; Lee et al., 9; Chough et al., 4). Tidal currents, sea currents, and waves all contribute to hydrodynamic condition in the Yellow Sea, while the tidal currents have dominated shelf dynamics since the Last Glacial Maximum transgression and eventually control the deposition of sediments (Dong et al., 1989; Sternberg et al., 1985). It is suggested that most of the sandy shelf sediments in the Bohai and Yellow seas are relicts deposited during Late Pleistocene sea level low stands (Emery, 1968), or tidal deposits since Last Glacial Maximum (e.g., Chen and Zhu, 2012; Zhu and Chen, 5; Liu et al., 1998). Chough et al. (4) pointed out that the Jiangsu coastal RSRS represented high stand deposits and were largely shaped by tidal currents when sea level reached the present position at about 6 ka, whereas Li et al. (1) put forward that the RSRS sediments were deposited during the regressive phase of sea level and formed by tidal currents and long shore transportation of sands from the old Yellow River and the modern Yangtze River. As the rate of sea-level rise decreased, depositional processes were affected by sediment supply, topography and prevailing currents (Liu et al., 1; Park et al., 0; Jin and Chough, 1998). Anyway, it is considered that most of the Yangtze River-derived sediments are transported southward along the coast to the East China Sea (Li et al., 1). Further north, along the Jiangsu coast, sandy mud and muddy sand were considered as deriving from the old Yellow River Delta prior to 1855 (Chough et al., 4). Different from above, our previous geochemical analysis and detrital zircon data support that the Yangtze River sediment is transported to the Jiangsu coastal RSRS under some tidal and current effects. In fact, the Yangtze River mouth has been continually shifted eastward and southward to its present position since 2 ka ago, and thus the Yangtze River might have been a significant earlier source of sediments for the southwestern Yellow Sea (Yang et al., 2). Wu et al. (2014) demonstrated that the northward branch of the Yangtze plume arrived at ~33.5ºN in modern time. This means the Yangtzederived sediments could arrive and influence the old Yellow River mouth before the Yangtze River migrating southward to the present position. That is accordant with our zircon data. Although the old Yellow River emptied directly into the Yellow Sea from 1128 to 1855, it should be far less than that of input from the Yangtze River since Pleistocene. By the Bohai Coastal Current and Yellow Sea Coastal Current, the modern Yellow River sediments in part may pass around the Shandong Peninsula and thereby may also reach the RSRS area off the Jiangsu coast as well as the old Yellow River Delta sediments (Fig. 1). However, it is difficult to discern the sediments derived from the old Yellow River Delta and the modern Yellow River for their similar origin. Though the sediment contribution of the Korean field to the RSRS off the Jiangsu coast is insignificant (Wang et al., 2012; Yang et al., 2; Li et al., 1), it is should not be neglected as potential source for fine-particle sediments and its possible effect on the hydrodynamic currents round the Yellow Sea. Overall, the input of the Yangtze River feeds the main sediments of the RSRS, whereas the Yellow River including the old and the modern provides a small part of sediments for the RSRS according to zircon data. It is conceivable that the oceanic currents and tidal motions control the formation of the RSRS with the supply rate of sediments, and the intensity of bottom stress (Uehara and Saito, 3). The spatial distribution of the bottom stress together with sediment supply may result in the present structure and plan-view patterns of the RSRS. 5 CONCLUSION Ten sand samples were analyzed to interpret provenance

9 152 Jinbao Su, Wenbo Rao, Yigang Wang and Changping Mao of the radial sand ridge system of the Jiangsu coast. The zircon U-Pb ages show a wide range of age distribution, with successive age scatters in Ma. Most samples collected from the RSRS sediments have similar age distribution of detrital zircons with that of the Yangtze River. One sample from the RSRS sediments seems to come from the Yellow River for their youngest age group of older than Ma. The samples located at the old Yellow River mouth have younger age peaks similar to the Yangtze River sediments and Paleozoic age group similar to the modern Yellow River sediments. They are inferred to originate from mixture sands of the Yangtze and Yellow Rivers. It is suggested that the RSRS sediments of the Jiangsu coast are supplied mainly by the Yangtze River, forming under the hydrodynamic effect from oceanic currents and tidal motion etc. ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos , ), the National Key Technology Research and Development Program (No. 2012BAB03B01) and the Fundamental Research Funds for the Central Universities (No. 2015B16914). We would like to thank Drs. Xiao Wang and Yaodong Pan for field and laboratory helps and also acknowledge the crew of the sampling cruise in 2013 for RSRS sediment samples. Thanks are also extended to the anonymous reviewers and the editors for their constructive comments. The final publication is available at Springer via Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at REFERENCES CITED Andersen, T., 5. 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