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Estuarine, Coastal and Shelf Science 71 (2007) 37e46

Sedimentation rates in relation to sedimentary processes of the Estuary,

Taoyuan Wei a, Chen b,*, Lingyun Duan a, Jiawei Gu a, Yoshiki Saito c, Weiguo Zhang b, Yonghong Wang d, Yutaka Kanai e

a Department of Geography, Normal University, North Zhongshan Rd. 3663, 200062, China b State Key Laboratory of Estuarine and Coastal Research, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, China c IGG, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan d College of Marine Geosciences, Ocean University of China, Qingdao, Shandong 266003, China e RCDME, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan Received 9 August 2006; accepted 10 August 2006

Abstract

Radioisotope analysis and Digital Elevation Model (DEM) method were combined to examine sedimentation rates and associated sedimen- tary processes in the Yangtze River Estuary. The major depocenter is validated at the delta front sedimentary facies above the normal wave base (NWB), where accumulation exceeds erosion. This alternated sedimentation does not accommodate Pb-210 and Cs-137 measurement, although sedimentation rates of less than 0.2e5.0 cm yrÀ1 were recorded in the fine-grained (silty) sediments, which were interbedded with coarse- grained (sandy) sediments. However, historical DEM data provide more detailed information on sedimentation in the delta front facies, where accumulation is dominant in the sandy shoals (1.73e8.30 cm yrÀ1) and delta front slope (5.22 cm yrÀ1) facies. The DEM data also show that erosion (1.61e7.32 cm yrÀ1) dominates in the northern estuarine distributaries, and accumulation (3.01e4.97 cm yrÀ1) prevails in the southern ones, primarily owing to the superimposed runoff and ebb tidal currents. Pb-210 and Cs-137 measurements reveal sedimentation rate from 2.0 cm yrÀ1 to 6.3e6.6 cm yrÀ1 in the delta front slope facies, which progressively decreases to <0.8 cm yrÀ1 in the prodelta facies and is un- measurable in the delta-shelf transition zone. DEM analysis detects minor erosion in the delta front slope and prodelta facies, although accumulation predominates there. The present sedimentological database will be useful for estuarine environmental assessment after the Three-Gorges Dam is completed in 2009. Ó 2006 Elsevier Ltd. All rights reserved.

Keywords: DEM; radioisotope measurement; deltaic depocenter; sedimentation rate; Yangtze Estuary

1. Introduction mouth area and are dispersed further offshore in the form of freshwater plumes (Milliman et al., 1985; Chen et al., 1988, The Yangtze River delivers more than 470 Mt of sediment 1999; Shen et al., 2003; Yang et al., 2003). Previous studies annually into its estuary to build a huge delta system have shown that sediments in the river mouth area (<10 m wa- (>40,000 km2, including subaqueous parts) that presently sus- ter depth) are mostly fine sand, silty sand, and silt, and sedi- tains intensifying human activities (Chen et al., 2001, in ments off the river mouth (10e60 m water depth) are silty press). Fluvial sediment discharge into the estuary consists clay and clayey silt (Chen et al., 2003; Yang et al., 2003; mainly of fine-grained particles that accumulate in the river Wang et al., 2005). Further offshore, relict sand of the late Pleistocene dominates (Niino and Emery, 1961; Chen et al., 2000). Milliman et al. (1985) and Shen and Pan (2001) esti- * Corresponding author. mated that about 70% of the annual sediment load is deposited E-mail address: [email protected] (Z. Chen). near the coast, with about 30% being carried further offshore,

0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.08.014 转载 中国科技论文在线 http://www.paper.edu.cn

38 T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46

of which a large proportion of the suspended sediment even Vibrocores were collected in PVC tubes with 6-cm in diame- approaches the nearshore areas off Zhejiang and Fujian prov- ter. Five vibrocores (C1eC3, C8, and C9), ranging from 20 to inces of southeastern China, driven by the Chinese Coastal 590 cm long, were collected from the upper tidal flat of the Current (Liu et al., in press). southern delta coast. C4eC6, C7, and C10, ranging from 48 Processes of sediment transport from estuary to continental to 135 cm long, were collected from the eastern upper tidal shelf have largely shaped the coastal topography, and are a sig- flat on . C11 and C12, 56 and 104 cm nificant mechanism of coastal environmental change at human long, respectively, were collected from the estuarine distribu- and global dimensions (Syvitski et al., 2005). To examine ac- tary. Y4, 220 cm long, was collected from the delta front mar- cumulation and erosion of subaqueous sediments in relation to ginal slope at about 10 m water depth, which is contiguous the sediment transport pattern, measuring sedimentation rates seaward with the prodelta facies. Vibrocores Y5eY8, from in the river mouth area and along the coast is an effective 200 to 400 cm long, were collected at the water depth of means of gaining a better understanding of these processes. 15e40 m from the prodelta facies, and Y9, 360 cm long, For example, Liu et al. (in press) have quantitatively defined was from the delta-shelf transition zone, at about 50 m water the sediment budget with respect to sediment transport from depth. Sediment logging applied to all vibrocores, while split- the Yangtze River mouth southward to the coasts of Zhejiang ting in the laboratory, to record sediment texture and structure, and Fujian provinces. Kuehl et al. (1989) corroborate sediment organic matter distribution, the presence of plant roots, and the bypassing from river-delta to the Bengal fan in the light of sed- occurrence of biogenesis, etc. imentation rate by radioisotopic measurement. Grain size was determined for 282 samples taken from the To clarify the sedimentation rate in the Yangtze Estuary is 18 vibrocores at intervals based on changes in sediment lithol- vital, considering the need to determine the fate and flux of the ogy. The grain-size distribution in these samples was exam- river-derived sediment discharged to the coast and the close ined using a laser particle analyzer (Beckman Coulter linkage between fine-grained riverine sediment and geological LS13,320). In the present study, sand particles are those and biological processes. Recent studies of nutrient (including >63 mm; silt, 63e2 mm; and clay, <2 mm; and mean grain pollutants due to human activity) delivery highlight the rela- size (Mz, 4) is also reported. tionship between the fine-grained sediment flux and associated An independent set of 237 samples was taken at 0.5 to the food chain in the coastal zone and shelf area (Liu et al., 7.0 cm intervals for radioisotope analyses (Pb-210 and Cs- 2003; Tsunogai et al., 2003). However, sedimentation rates, 137). From vibrocores C1eC12, 129 samples were prepared in particular in the delta front facies, and their changes in re- for Pb-210 measurement in the State Key Laboratory of Estu- lation to sediment dynamics and sources, must still be con- arine and Coastal Research (SKLEC), East China Normal Uni- firmed, despite numerous studies in the Yangtze River mouth versity. From Y4eY9, 108 samples were analyzed for both area during past decades (e.g., Mckee et al., 1983; Liu et al., Pb-210 and Cs-137 at the Geological Survey of Japan (GSJ), 1984; DeMaster et al., 1985; Xia et al., 1999; Chen et al., Tsukuba. 2004; Xia et al., 2004). From these studies, we know that SKLEC sample preparation procedures were as follows: most sedimentation rates were given in the prodelta muddy ca. 10 g wet sediment of each sample was dried in an zone, a few were in the delta front sandy zone (Fig. 1A). Sed- oven at 105 C for 2 h; 2e5 g dried sample was ground, imentation rate of the topset sediments has been a long puzzle sieved through 0.150 mm mesh to remove plant roots, and for coastal scientists, primarily due to strong erosional pro- wax-sealed in a tube for 3 weeks. Then, the Pb-210 radioac- cesses occurring seasonally in the nearshore face above tivity was determined with a High-Purity Germanium NWB (Coleman, 1981; Yan and Xu, 1987; Stanley and Warne, Gamma Detector (ORTEC, GWL-120210-S). The ratio of 1993), and coarser (sandy) sediment unsuitable for radioiso- dry to wet sample (%) was determined for some samples tope measurement (Appleby and Oldfield, 1978; Xiang, 1997). (vibrocores C2eC4; C6eC8), and then bulk density The objectives of the present study were through the in- (g cmÀ3) was calculated (Black, 1965). The peak height tegration of Pb-210 and Cs-137 measurements and a Digital at 46.5 KeV was considered to represent the total Pb-210 Elevation Model (DEM) to: (1) conduct a thorough exami- activity in the sediments, that at 351.92 KeV the supported nation of sedimentation rates in the proximal to distal sub- Pb-210 activity (background radioactivity of Pb-210), and aqueous delta; (2) determine the controls on sedimentation the difference between them the excess Pb-210 activity rates in the subaqueous delta facies; and (3) establish a sed- (Bq gÀ1). Both the Constant Initial Concentration (CIC) imentological database that will highlight later studies of model and the Constant Rate of Supply (CRS) model the impacts of the Three-Gorges Dam, scheduling to be were used to calculate the sedimentation rates (cm yrÀ1) completed in 2009. in the upper tidal flat, estuarine distributary, and delta front slope subfacies (cf. Robbins and Edgington, 1975; DeMaster 2. Methods et al., 1985; Ye, 1991; Wan, 1997; Xiang, 1997; Xia et al., 2004). The sedimentation rate was calculated only for those From 1995 to 2003, eighteen (18) vibrocores (C1eC12, sediment sections of vibrocores C1eC12 in which declining Y4eY9) were recovered from various geomorphological units trend of Pb-210 radioactivity with vibrocore depth was de- in the Yangtze Estuary, including tidal flat, estuarine distribu- tected. Similar procedures were followed to prepare the tary, sandy shoals, delta front slope, and prodelta (Fig. 1A). samples from vibrocores Y4eY9 for radioisotope analysis 中国科技论文在线 http://www.paper.edu.cn

T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 39

121º 122º 123º 124º E 32 º N3 N Province

Yellow Sea Yangtze River Chongming lsland B Study area

C11 A CHINA East C4-C6 China C1 C12 Yangtze River Sea 1 2 Shanghai C7 Y4 C2-C3 C10 Y5 C8 3 Y6 Legend Y7 C9 Y8 0 50km 1 º Y9 Vibrocore site 20 A Delta front 10 B Prodelta D 30 C Delt-shelf transition B 40 D Relict sand zone 50 10 Bathymetry 1 Changxing island C 2 Hengsha island 3 Jiuduansha shoal Sedimentaion rates in previous studies (cmyr-1) A

121º 50' 122º 00' 122º 10' 122º 20' 122º 30' E

2 2 10 10 0 N 5 North 31 º 20' N Hengsha Channel Island 2 5 0 0 Hengsha Shoal 20 0 2 0 10 2 0 North Branch 5 0 10 Jiu 31 Duan 2 º 10' Sha 5 South 0 Island

Branch 2 0 Nanhui Shoal 5

2 2 5 31 B º 00'

Fig. 1. A) The Yangtze Estuary and the locations of the vibrocores; and B) river mouth area selected for application of the digital elevation model.

in the Laboratory of the Geological Survey of Japan (Chen and from water depth measurements at numerous individual et al., 2004). points; (2) a 300 Â 300 grid was determined in the target area, Two bathymetric maps, made 42 years apart (in 1958 and on the basis of principle of grid number > the number of data 2000), of the Yangtze River mouth area (1:50,000 and points retrieved; and (3) the two resulting maps were overlaid 1:75,000; Maritime Bureau of China, 1958, 2000) were used to determine the sediment budget of selected subfacies. The re- for the DEM (Fig. 1A,B) as follows: (1) The maps were digitized sults of the DEM, together with radioisotope measurement, and elevation information was retrieved from the 5-m isoclines compose the database for the present study. 中国科技论文在线 http://www.paper.edu.cn

40 T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46

3. Data and observations the lower delta front slope to prodelta facies, distinguished from the upper delta front facies by the change in lithology 3.1. Estuarine sediments at 10e15 m water depth, the depth of the NWB off the Yang- tze River mouth (Chen, 1987). Abundant organic matter oc- The Yangtze estuarine sediments can be characterized by curs as irregular patches in the sediments, which are massive their sedimentary facies, which were identified in the 18 vibro- and structureless. Shells (mostly bivalves) are well preserved cores (Figs. 1A and 2A). From the coastline seaward are the and borrowings are common. delta front facies (i.e. the upper tidal flat, estuarine distribu- Silty sand and clayey silt (Mz 4.2e4.9 4) are thinly inter- tary, sandy shoal, and delta front slope), and prodelta facies, bedded (3e5 cm thick) in the delta-shelf transition zone, and the delta-shelf transition zone (cf. Coleman, 1981; Chen where highly fragmented shells are found and scouring sur- et al., 2000). faces prevail (Fig. 2A). The upper tidal flat sediments, about 30e40 cm thick, con- The proportions of sand, silt, and clay in 282 samples from sist of yellowish gray massive clayey silt (Mz 5.0e5.7 4) with the above-mentioned sedimentary facies were plotted on a ter- many silty lenses, abundant organic matter and root traces nary diagram to characterize their distribution in relation to (Fig. 2A). Sandy sediment sections occur in the lower portions sediment dynamics (Fig. 2B). Clearly, silt dominates in the up- of the vibrocores (e.g., C8 in Fig. 2A). Gray to yellowish fine per tidal flat facies, silty sediment (mostly, find sandy silt and silt and clayey silt (Mz 4.8e5.5 4) dominate the estuarine dis- silty fine sand) constituents the most delta front facies, and tributary and sandy shoal subfacies (Fig. 2A). Relatively pure clayey silt takes over the lower delta front slope to prodelta silty sediment sections are often intercalated by thin (5e10 cm facies (cf. Chen et al., 2000). thick) sandy silt layers, in which ripple cross-bedding occurs. The upper delta front slope facies is composed of fine sand 3.2. Sedimentation rates (Mz 4.1e4.6 4), in which small-scale trough cross-bedding occurs, and shells (both whole and fragmentary) are present. The CIC model reveals that sedimentation rates in the delta Scouring surfaces are common at lithologic change bound- front facies (topset or delta front platform) range from 0.17 to aries. In contrast, grayish silty clay (Mz 6.3e6.9 4) dominates 1.94 cm yrÀ1 in the upper tidal flat (C2eC8, Fig. 3) and from

Tidal flat <10 m 10 - 50m

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 Y4 Y5 Y6 Y7 Y8 Y9

Clay Clay Clay Silt Silt Clay Fine Silt Silt sand Fine Clay Clay sand Fine Fine sand Clay Silt Clay Clay Silt Clay Clay sand Silt Fine Silt Silt Fine Silt Silt sand Fine sand Fine Fine sand Fine sand Fine sand sand sand Clay Clay Clay Clay Clay Silt Clay Silt Silt Silt Silt Fine Silt Fine Clay Fine Fine sand Silt Fine sand sand Fine sand sand sand Fine sand 0

20 cm 20 cm

48 cm 56 cm 62 cm 56 cm

85 cm 1 A 105 cm 104 cm

135 cm 150 cm

B Silt Tidal flat Legend 2 0 <10m water depth 100 Homogeneous silty clay, 200 cm 10-50m water depth massive and structureless Clayey silt, massive 220 cm and structureless 25 Sandy silt 75 Silty to fine sand 285 cm 3 Shell fragments Plant roots 310 cm 50 50 Organic matters Erosional surface 360 cm

75 25 C1 C2 C4C5C6 400 cm 4 C3 C9 C7 C8C10 C11 Mean sea level Delta plain C12 Y4 Normal Wave Base 100 Delta front (NWB) 0 Y5 0255075100 Sand Clay Y6 Basal delta 5 topography Y7 m Prodelta Y8 Delta-shelf Y9 transit zone Relict sand 590 cm C

Fig. 2. A) Sediment sections of the 18 vibrocores shown sequentially from the river mouth seaward; B) ternary diagram showing grain-size distributions in the different deltaic sedimentary facies, primarily the delta front and prodelta facies; and C) diagram showing the topset, foreset, and bottomset sedimentary facies in relation to the normal wave base (NWB) at 10e15 m water depth (cf. Chen, 1987). 中国科技论文在线 http://www.paper.edu.cn

T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 41

Age (yr) Age (yr) Age (yr) 0030 60 90 30 60 90 120 0306090120 0 0 0 0.26 C2 0.25 C3 C4 2 2 5 0.44 4 4 0.39 10 0.40 0.36 6 6 15 0.75

8 0.44 8 0.42 20 10 10 25 0.40

Depth (cm)

Depth (cm)

Depth (cm) 12 12 30 0.16 0.75 14 0.23 14 0.17 35 0.17 0.16 16 16 40

Age (yr) Age (yr) Age (yr) 03060900306090120 0306090120 0 0 0 C6 1.16 C7 C8 5 10 1.89 20 1.72 40 10 20 2.66 1.70 1.08 60 30 1.74 1.66 15 2.70 2.55 80 40 1.60 2.50 20 0.56 100 50 0.64 0.85

Depth (cm)

Depth (cm) Depth (cm) 120 1.35 25 1.68 60 0.51 140 30 5.00 70 0.47 160 0.71 1.03 1.94 35 80 180 Constant Initial Concentration Model (CIC) Constant Rate of Supply Model (CRS) 0.23 Sedimentation Rate (cmyr-1)

Fig. 3. Sedimentation rates in vibrocores C2eC8 (topset sediments), recorded by Pb-210, determined by the CRS and CIC models.

0.39 to 0.86 cm yrÀ1 in the estuarine distributary (C11 and rates varying from 0.8 to 6.3 cm yrÀ1, and Cs-137 records rates C12, Fig. 4). The CRS model indicates a rate of 0.16e ranging from 2.1 to 6.6 cm yrÀ1 (Fig. 6). Meanwhile, the accumu- 5.0 cm yrÀ1 in the upper tidal flat (determined only from lation rate of the prodelta facies determined from the DEM is vibrocores C2eC4 and C6eC8, where bulk density data 5.22 cm yrÀ1, with an accumulation budget of 1292.2 Â 106 m3, were available; Fig. 3). The sedimentation rate could not be whereas the erosion rate is 2.31 cm yrÀ1, relative to an erosion determined for the upper tidal flat in several vibrocores, in- budget of 77.07 Â 106 m3 (Table 1). cluding C1, C9, and C10. Further seaward, the sedimentation The sedimentation rate could not be determined in vibro- rate in Y4 (upper delta front slope) could not be determined core Y9, in the delta-shelf transition zone (Figs. 1A and 6). from either the Pb-210 or the Cs-137 test. The results of the DEM practice show both sedimentation 4. Discussion and conclusions and erosion of the delta front sediments (topset; Fig. 5; Table 1). Accumulation rates in the many sandy shoals (Fig. 5A, C, Radioisotope measurement and DEM analysis results sys- D, F, and H) range from 1.73 to 8.30 cm yrÀ1, which contrast tematically revealed accumulation and erosion rates in the with erosion rates of 1.35e3.29 cm yrÀ1. The accumulation Yangtze Estuary, thus characterizing the sedimentary pro- rates in the estuarine distributary (Figs. 5B, E, and G) appear cesses and associated sediment transport near the river mouth 3.01e4.70 cm yrÀ1, and the erosion rates are 1.61e area. This study highlights the effects of estuarine sediment 7.32 cm yrÀ1. The accumulation rates on the delta front slope dynamics, including fluvial discharge, littoral currents, and (Fig. 5I) are 2.72e4.97 cm yrÀ1. As determined from the tidal and wave currents (Chen et al., 1988; Goodbred and DEM, accumulation (volume, 1294.70e133.76 Â 106 m3) Kuehl, 1998; Chen et al., 2000; Shen and Pan, 2001; Hori dominates in the sandy shoals, although erosion (28.11e et al., 2002; Uehara et al., 2002; Chen et al., 2003, 2004). 67.19 Â 106 m3) also occurs there (Table 1). Both accumula- The results of the study give us a better understanding of the tion (164.05e133.77 Â 106 m3) and erosion (47.45e controls on deltaic sedimentation in the study area, which 490.91 Â 106 m3) are intensive in the estuarine distributaries. will be useful for evaluating recent deltaic morphological Accumulation prevails in the South Passage, and erosion in changes e accumulation and erosion budgets in relation to en- the North Channel and North Passage, on either side of Heng- vironmental modification at human and catchment dimension sha Island (Figs. 1B and 5). Accumulation (1233.67 Â 106 m3) (Goodbred and Kuehl, 1999; Syvitski et al., 2005). obviously exceeds erosion (35.81 Â 106 m3) on the delta front The sediments sampled by vibrocores from the Yangtze slope (Fig. 5; Table 1). Estuary indicate uneven sedimentation rates in the delta front The Pb-210 results (CIC model) for cores Y5eY8 from the (topset) facies, comprising the upper tidal flat, estuarine dis- lower delta front slope to prodelta facies indicate accumulation tributary, sandy shoal, and upper delta front slope subfacies, 中国科技论文在线 http://www.paper.edu.cn

42 T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46

Fig. 4. Pb-210 radioactivity distribution in 12 vibrocores (C1eC12) of topset sediments. Note, both measurable and unmeasurable sediment sections are shown. Alternating accumulation and erosion and coarser (sandy) sediments unsuitable for radioisotope measurements made it impossible to determine the sedimentation rate in some sediment sections.

which all lie above the NWB (10e15 m water depth) off the The CRS model can be more applicable than the CIC river mouth (Fig. 2C). Fine sand and silt are the major sedi- model for delineating the nature of sedimentation in the topset ment components of these subfacies, consistent with the dom- sedimentary facies, that is, the upper tidal flat, estuarine dis- inant tidal current of 1.0e1.5 m sÀ1 recorded in the Yangtze tributary, sandy shoal, and upper delta front slope subfacies River mouth area (Fig. 2A,B) (Chen et al., 1988; Wang et al., (cf. Appleby and Oldfield, 1978; Xiang, 1997); in general, 2005). Interbedded sandy layers and scouring surfaces where the CRS model yields higher sedimentation rates (0.16e lithologic changes are observed may reflect seasonal erosion 5.0 cm yrÀ1) than the CIC model (0.17e1.94 cm yrÀ1)(Figs. by typhoons e a strong sediment dynamics that can reactivate 3 and 4). However, given the situation of uneven sedimenta- sediment on the nearshore seabed above the NWB (Chen tion, rates derived from interbedded fine-grained sediments us- et al., 1988; Hori et al., 2002). In contrast, the clayey silt ing the CRS model are rather inaccurate (Li et al., 1999), composing a large part of the lower delta front slope and neither fully reflecting the nature of sedimentation in these prodelta facies below the NWB, where sedimentation in facies, nor matching the development of the bathymetric ge- general is continuous, is massive and structureless with ometry of the facies. In addition to the coastal sediment dy- abundant organic matter and burrowings (Fig. 2A) (cf. namics, which serve as the driving force for shifting Walker and James, 1992; Chen et al., 2004). The delta-shelf estuarine topography, the bathymetric changes in the Yangtze transition facies consists of thin (centi- to decimeter-thick) Estuary have been sensitive to variations in sediment discharge beds composed of a sand-silt-clay mixture with numerous from the drainage basin (cf. Yang et al., 2003). fragmented shells, bounded by intensive scourings, indicating Undeterminable rates at the sites of C1, C9, C10, and Y4 reactivation by submarine currents or even typhoon-triggered can be interpreted to reflect discontinuous sedimentation or current flow (Chen, 1987; Xu, 1997; Chen et al., 2000, the presence of coarser (sandy) sediments in the delta front fa- 2003). Recently, Wang et al. (2005) discussed similar highly cies, which do not absorb enough radioactivity for Pb-210 and laminated sediments in the Yangtze delta-shelf transition Cs-137 measurement (cf. Robbins and Edgington, 1975; Ap- zone. pleby and Oldfield, 1978; Wan, 1997)(Figs. 2 and 4). 中国科技论文在线 http://www.paper.edu.cn

T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 43

121° 50′ 122° 00′ 122° 10′ 122° 20′ E

A N

0

31° 20′ N Hengsha B Island 0

C 0 0 D 0

Thickness of deposition/erosion (m)

J 10 E 31° 10′ 0 8 0 0 4

F I 0

-4 Nanhui G 0 0 10km H -8

-12

31° 00′ -16 A-H morphologcal sites on the basis of bathymetric contour

Fig. 5. Accumulation and erosion rates in the Yangtze River mouth determined by DEM. A, C, D, F, and H, sandy shoals; B, E, and G, estuarine distributaries; I, delta front slope; J, prodelta.

In fact, the DEM data of the present study confirmed that estuarine distributaries can be attributed to the dominant sed- sediments alternately accumulated on and were eroded from imentation by the superimposed Yangtze discharge and ebb the nearshore seafloor above the NWB. In sandy shoals, accu- tidal currents, and by flood tidal currents (Fig. 5)(Chen mulation predominates, 1.73e8.30 cm yrÀ1 with an accumula- et al., 1988). Previous work on sediment dynamics in the river tion budget of 133.76e1294.7 Â 106 m3 in the past about mouth area, showing that the major flood tide transmission 50 years, although minor erosion also occurs there (Fig. 5; Ta- is from the ocean landward at a bearing of 305 and that the ble 1). On the other hand, the DEM shows significant erosion ebb flow direction is southward, corroborates this observation in the estuarine distributaries: 1.61e7.32 cm yrÀ1 with an ero- (cf. Shen and Pan, 2001; Wang et al., in press). Doubtlessly, sion budget of 47.45e490.91 Â 106 m3 (Table 1). The major accumulation (4.97 cm yrÀ1 for 1233.7 Â 106 m3) dominates accumulation and erosion in the southern and northern over erosion (2.72 cm yrÀ1 for 35.81 Â 106 m3) in the delta

Table 1 Result of DEM e sedimentation and erosion rate, and accumulation and erosion budget in the topset sedimentary facies (AeJ refers to Fig. 5) Subfacies Area Total area Accumulation Erosion Accumulation Erosion Sedimentation Erosion rate (106 m2) area area budge budget rate (cm yrÀ1) (106 m2) (106 m2) (106 m3) (106 m3) (cm yrÀ1) Sandy shoal A Chongming shoal 169.48 140.75 28.73 403.65 21.39 6.83 1.77 (partial) C Tiaozisha shoal 136.18 89.042 47.14 133.76 67.19 3.58 3.39 D Hengsha shoal 310.73 237.17 73.56 425.00 62.38 4.27 2.02 F Jiuduansha shoal 403.90 371.55 32.35 1294.69 28.11 8.30 2.07 H Nanhui shoal 335.06 282.18 52.88 205.23 29.97 1.73 1.35 Distributary B North channel 227.51 67.75 159.76 133.77 490.91 4.70 7.32 E North branch 256.65 101.69 154.96 164.05 398.21 3.84 6.12 G South branch 192.40 122.38 70.02 154.65 47.45 3.01 1.61 Delta-front I5e10 m 622.02 590.67 31.35 1233.67 35.81 4.97 2.72 slope water depth Prodelta J 10e20 m 668.44 589.13 79.31 1292.22 77.07 5.22 2.31 water depth 中国科技论文在线 http://www.paper.edu.cn

44 T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46

0 0 0 0

100 100 Background 6.3 cmyr-1 Y4 Background Y7 200 200 200 200 Depth (cm) Depth (cm) Depth (cm) Depth (cm) 4.3-6.6 cmyr-1

300 300 400 400 0.001 0.01 10.1 0.0010 0.002 0.003 0.001 0.01 0.1 1 0 0.004 0.008 Pb-210ex(Bqg-1) Cs-137(Bqg-1) Pb-210ex(Bqg-1) Cs-137(Bqg-1)

0 0 0 0 0-55 cm 2.8-2.9 0.8cmyr-1 -1 cmyr-1 Background 50 2.0 cmyr 50 200 200

Y5 Y8 0-75 cm 0-2.1 cmyr-1 100 100 400 400 Depth (cm) Depth (cm) Background Depth (cm) Depth (cm)

150 150 600 600 0.001 0.01 0.1 1 0 0.004 0.008 0.001 0.01 0.1 1 0 0.005 0.01 0.015 0.02 Pb-210ex(Bqg-1) Cs-137(Bqg-1) Pb-210ex(Bqg-1) Cs-137(Bqg-1)

0 0 0 0

100 25-125 cm 100 Background 2.2 cmyr-1 Y6 Y9 200 200 Background 200 200

Depth (cm) Depth (cm) 2.4-4.5 cmyr -1 Depth (cm) Depth (cm)

300 300 400 400 0.001 0.01 0.1 1 0 0.004 0.008 0.001 0.01 0.1 1 0 0.001 0.002 Pb-210ex(Bqg-1) Cs-137(Bqg-1) Pb-210ex(Bqg-1) Cs-137(Bqg-1)

Fig. 6. Radioactivity distribution and sedimentation rates of Pb-120 and Cs-137 in vibrocores Y4eY9.

front slope (Fig. 5), from where it changes gradually to the Interestingly, the sedimentation rates recorded in the thin- prodelta facies, which lies at 10e50 m water depth (Figs. 1 ner sediment section at Y5 and Y6 by Pb-210 are generally and 2C). lower than those recorded by Cs-137, determined from thicker Radioisotope measurement indicates a lower sedimentation sediment sections of the same vibrocores (Fig. 6). This differ- rate (2.0e2.9 cm yrÀ1)atY5(Fig. 6), probably due to stronger ence may reflect a reduction in sediment discharge in the wave erosion at this site near the NWB (Fig. 2C) (Mckee et al., river mouth area as a result of the construction of numerous 1983; DeMaster et al., 1985). Also, Shen et al. (2003) and dams during the 1960s and 1970s (Chen et al., 2001; Yang Wang et al. (in press) reported a strong plume front at this et al., 2003). site off the river mouth, which greatly triggers resuspension The Yangtze Estuary is a huge ecosystem, which has been of the seafloor sediments. Thus, the sedimentation rate in- nourished by the large amount (470 Mt, on a multi-yearly creases from 2.0 to 6.3e6.6 cm yrÀ1 in vibrocores Y6 to Y7 basis) of sediment input from the upper drainage basin. The as away seaward from the NWB (Figs. 2C and 6). The extensive delta plain (about 30,000 km2), on which millions decreasing rate from Y8 (0.8e2.1 cm yrÀ1) to Y9 (unmeasur- of inhabitants engage in industry and agriculture, has been able) reflects the nature of sedimentation in the prodelta facies prograding for the last ca. 7000 years (Stanley and Chen, and the delta-shelf transition zone (Fig. 6), where the modern 1996; Chen and Stanley, 1998). However, it is likely that the Yangtze subaqueous delta terminates (Chen et al., 2003; Wang coastline and coastal morphology will be greatly modified et al., 2005). Nevertheless, our DEM data also indicate minor by the reduction in the sediment supply resulting from the on- erosion (2.32 cm yrÀ1 for 77.07 Â 106 m3) in the prodelta going Three-Gorges Dam project, which will be completed in facies, although it is negligible in comparison with the accu- 2009. This reduction will inevitably lead to coastal erosion and mulation (5.22 cm yrÀ1 for 1292.2 Â 106 m3)(Table 1). We associated environmental degradation and disasters. Therefore, note that the sedimentation rate (5.22 cm yrÀ1) in the prodelta this study provides a useful sedimentological reference for facies determined by DEM is close to that (6.3e6.6 cm yrÀ1) monitoring estuarine morphological preservation in the near determined from Pb-210 and Cs-137 data (Fig. 6; Table 1). future. 中国科技论文在线 http://www.paper.edu.cn

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