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Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, China
Estuarine, Coastal and Shelf Science 71 (2007) 37e46 www.elsevier.com/locate/ecss
Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, China Taoyuan Wei a, Zhongyuan Chen b,*, Lingyun Duan a, Jiawei Gu a, Yoshiki Saito c, Weiguo Zhang b, Yonghong Wang d, Yutaka Kanai e a
b
Department of Geography, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, China 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 sedimentary 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 yr 1 were recorded in the fine-grained (silty) sediments, which were interbedded with coarsegrained (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 yr 1) and delta front slope (5.22 cm yr 1) facies. The DEM data also show that erosion (1.61e7.32 cm yr 1) dominates in the northern estuarine distributaries, and accumulation (3.01e4.97 cm yr 1) 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 yr 1 to 6.3e6.6 cm yr 1 in the delta front slope facies, which progressively decreases to <0.8 cm yr 1 in the prodelta facies and is unmeasurable 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 The Yangtze River delivers more than 470 Mt of sediment annually into its estuary to build a huge delta system (>40,000 km2, including subaqueous parts) that presently sustains intensifying human activities (Chen et al., 2001, in press). Fluvial sediment discharge into the estuary consists mainly of fine-grained particles that accumulate in the river
* Corresponding author. E-mail address: [email protected] (Z. Chen). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.08.014
mouth area and are dispersed further offshore in the form of freshwater plumes (Milliman et al., 1985; Chen et al., 1988, 1999; Shen et al., 2003; Yang et al., 2003). Previous studies have shown that sediments in the river mouth area (<10 m water depth) are mostly fine sand, silty sand, and silt, and sediments off the river mouth (10e60 m water depth) are silty clay and clayey silt (Chen et al., 2003; Yang et al., 2003; 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) estimated that about 70% of the annual sediment load is deposited near the coast, with about 30% being carried further offshore,
38
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46
of which a large proportion of the suspended sediment even approaches the nearshore areas off Zhejiang and Fujian provinces of southeastern China, driven by the Chinese Coastal Current (Liu et al., in press). Processes of sediment transport from estuary to continental shelf have largely shaped the coastal topography, and are a significant mechanism of coastal environmental change at human and global dimensions (Syvitski et al., 2005). To examine accumulation and erosion of subaqueous sediments in relation to the sediment transport pattern, measuring sedimentation rates in the river mouth area and along the coast is an effective means of gaining a better understanding of these processes. For example, Liu et al. (in press) have quantitatively defined the sediment budget with respect to sediment transport from the Yangtze River mouth southward to the coasts of Zhejiang and Fujian provinces. Kuehl et al. (1989) corroborate sediment bypassing from river-delta to the Bengal fan in the light of sedimentation rate by radioisotopic measurement. To clarify the sedimentation rate in the Yangtze Estuary is vital, considering the need to determine the fate and flux of the river-derived sediment discharged to the coast and the close linkage between fine-grained riverine sediment and geological and biological processes. Recent studies of nutrient (including pollutants due to human activity) delivery highlight the relationship between the fine-grained sediment flux and associated the food chain in the coastal zone and shelf area (Liu et al., 2003; Tsunogai et al., 2003). However, sedimentation rates, in particular in the delta front facies, and their changes in relation to sediment dynamics and sources, must still be confirmed, despite numerous studies in the Yangtze River mouth area during past decades (e.g., Mckee et al., 1983; Liu et al., 1984; DeMaster et al., 1985; Xia et al., 1999; Chen et al., 2004; Xia et al., 2004). From these studies, we know that most sedimentation rates were given in the prodelta muddy zone, a few were in the delta front sandy zone (Fig. 1A). Sedimentation rate of the topset sediments has been a long puzzle for coastal scientists, primarily due to strong erosional processes occurring seasonally in the nearshore face above NWB (Coleman, 1981; Yan and Xu, 1987; Stanley and Warne, 1993), and coarser (sandy) sediment unsuitable for radioisotope measurement (Appleby and Oldfield, 1978; Xiang, 1997). The objectives of the present study were through the integration of Pb-210 and Cs-137 measurements and a Digital Elevation Model (DEM) to: (1) conduct a thorough examination of sedimentation rates in the proximal to distal subaqueous delta; (2) determine the controls on sedimentation rates in the subaqueous delta facies; and (3) establish a sedimentological database that will highlight later studies of the impacts of the Three-Gorges Dam, scheduling to be completed in 2009. 2. Methods From 1995 to 2003, eighteen (18) vibrocores (C1eC12, Y4eY9) were recovered from various geomorphological units in the Yangtze Estuary, including tidal flat, estuarine distributary, sandy shoals, delta front slope, and prodelta (Fig. 1A).
Vibrocores were collected in PVC tubes with 6-cm in diameter. Five vibrocores (C1eC3, C8, and C9), ranging from 20 to 590 cm long, were collected from the upper tidal flat of the southern delta coast. C4eC6, C7, and C10, ranging from 48 to 135 cm long, were collected from the eastern upper tidal flat on Chongming Island. C11 and C12, 56 and 104 cm long, respectively, were collected from the estuarine distributary. Y4, 220 cm long, was collected from the delta front marginal slope at about 10 m water depth, which is contiguous seaward with the prodelta facies. Vibrocores Y5eY8, from 200 to 400 cm long, were collected at the water depth of 15e40 m from the prodelta facies, and Y9, 360 cm long, was from the delta-shelf transition zone, at about 50 m water depth. Sediment logging applied to all vibrocores, while splitting in the laboratory, to record sediment texture and structure, organic matter distribution, the presence of plant roots, and the occurrence of biogenesis, etc. Grain size was determined for 282 samples taken from the 18 vibrocores at intervals based on changes in sediment lithology. The grain-size distribution in these samples was examined using a laser particle analyzer (Beckman Coulter LS13,320). In the present study, sand particles are those >63 mm; silt, 63e2 mm; and clay, <2 mm; and mean grain size (Mz, 4) is also reported. An independent set of 237 samples was taken at 0.5 to 7.0 cm intervals for radioisotope analyses (Pb-210 and Cs137). From vibrocores C1eC12, 129 samples were prepared for Pb-210 measurement in the State Key Laboratory of Estuarine and Coastal Research (SKLEC), East China Normal University. From Y4eY9, 108 samples were analyzed for both Pb-210 and Cs-137 at the Geological Survey of Japan (GSJ), Tsukuba. SKLEC sample preparation procedures were as follows: ca. 10 g wet sediment of each sample was dried in an oven at 105 C for 2 h; 2e5 g dried sample was ground, sieved through 0.150 mm mesh to remove plant roots, and wax-sealed in a tube for 3 weeks. Then, the Pb-210 radioactivity was determined with a High-Purity Germanium Gamma Detector (ORTEC, GWL-120210-S). The ratio of dry to wet sample (%) was determined for some samples (vibrocores C2eC4; C6eC8), and then bulk density (g cm 3) was calculated (Black, 1965). The peak height at 46.5 KeV was considered to represent the total Pb-210 activity in the sediments, that at 351.92 KeV the supported Pb-210 activity (background radioactivity of Pb-210), and the difference between them the excess Pb-210 activity (Bq g 1). Both the Constant Initial Concentration (CIC) model and the Constant Rate of Supply (CRS) model were used to calculate the sedimentation rates (cm yr 1) in the upper tidal flat, estuarine distributary, and delta front slope subfacies (cf. Robbins and Edgington, 1975; DeMaster et al., 1985; Ye, 1991; Wan, 1997; Xiang, 1997; Xia et al., 2004). The sedimentation rate was calculated only for those sediment sections of vibrocores C1eC12 in which declining trend of Pb-210 radioactivity with vibrocore depth was detected. Similar procedures were followed to prepare the samples from vibrocores Y4eY9 for radioisotope analysis
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 121º
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1 Changxing island 2 Hengsha island 3 Jiuduansha shoal Sedimentaion rates in previous studies (cmyr-1)
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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 et al., 2004). Two bathymetric maps, made 42 years apart (in 1958 and 2000), of the Yangtze River mouth area (1:50,000 and 1:75,000; Maritime Bureau of China, 1958, 2000) were used for the DEM (Fig. 1A,B) as follows: (1) The maps were digitized and elevation information was retrieved from the 5-m isoclines
and from water depth measurements at numerous individual points; (2) a 300 300 grid was determined in the target area, on the basis of principle of grid number > the number of data points retrieved; and (3) the two resulting maps were overlaid to determine the sediment budget of selected subfacies. The results of the DEM, together with radioisotope measurement, compose the database for the present study.
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46
the lower delta front slope to prodelta facies, distinguished from the upper delta front facies by the change in lithology at 10e15 m water depth, the depth of the NWB off the Yangtze River mouth (Chen, 1987). Abundant organic matter occurs as irregular patches in the sediments, which are massive and structureless. Shells (mostly bivalves) are well preserved and borrowings are common. Silty sand and clayey silt (Mz 4.2e4.9 4) are thinly interbedded (3e5 cm thick) in the delta-shelf transition zone, where highly fragmented shells are found and scouring surfaces prevail (Fig. 2A). The proportions of sand, silt, and clay in 282 samples from the above-mentioned sedimentary facies were plotted on a ternary diagram to characterize their distribution in relation to sediment dynamics (Fig. 2B). Clearly, silt dominates in the upper tidal flat facies, silty sediment (mostly, find sandy silt and silty fine sand) constituents the most delta front facies, and clayey silt takes over the lower delta front slope to prodelta facies (cf. Chen et al., 2000).
3. Data and observations 3.1. Estuarine sediments The Yangtze estuarine sediments can be characterized by their sedimentary facies, which were identified in the 18 vibrocores (Figs. 1A and 2A). From the coastline seaward are the delta front facies (i.e. the upper tidal flat, estuarine distributary, sandy shoal, and delta front slope), and prodelta facies, and the delta-shelf transition zone (cf. Coleman, 1981; Chen et al., 2000). The upper tidal flat sediments, about 30e40 cm thick, consist of yellowish gray massive clayey silt (Mz 5.0e5.7 4) with many silty lenses, abundant organic matter and root traces (Fig. 2A). Sandy sediment sections occur in the lower portions of the vibrocores (e.g., C8 in Fig. 2A). Gray to yellowish fine silt and clayey silt (Mz 4.8e5.5 4) dominate the estuarine distributary and sandy shoal subfacies (Fig. 2A). Relatively pure silty sediment sections are often intercalated by thin (5e10 cm thick) sandy silt layers, in which ripple cross-bedding occurs. The upper delta front slope facies is composed of fine sand (Mz 4.1e4.6 4), in which small-scale trough cross-bedding occurs, and shells (both whole and fragmentary) are present. Scouring surfaces are common at lithologic change boundaries. In contrast, grayish silty clay (Mz 6.3e6.9 4) dominates
3.2. Sedimentation rates The CIC model reveals that sedimentation rates in the delta front facies (topset or delta front platform) range from 0.17 to 1.94 cm yr 1 in the upper tidal flat (C2eC8, Fig. 3) and from
Y8
Y9
Clay Silt Fine sand
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Homogeneous silty clay, massive and structureless Clayey silt, massive and structureless Sandy silt
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Shell fragments
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Plant roots
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0 0
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75
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Clay Basal delta topography
5 m
Prodelta
Y5 Y6 Y7 Y8 Y9
590 cm
C
Delta-shelf transit zone
Relict sand
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).
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46
Age (yr)
Age (yr) 60
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Depth (cm)
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Depth (cm)
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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 yr 1 in the estuarine distributary (C11 and C12, Fig. 4). The CRS model indicates a rate of 0.16e 5.0 cm yr 1 in the upper tidal flat (determined only from vibrocores C2eC4 and C6eC8, where bulk density data were available; Fig. 3). The sedimentation rate could not be determined for the upper tidal flat in several vibrocores, including C1, C9, and C10. Further seaward, the sedimentation rate in Y4 (upper delta front slope) could not be determined from either the Pb-210 or the Cs-137 test. The results of the DEM practice show both sedimentation and erosion of the delta front sediments (topset; Fig. 5; Table 1). Accumulation rates in the many sandy shoals (Fig. 5A, C, D, F, and H) range from 1.73 to 8.30 cm yr 1, which contrast with erosion rates of 1.35e3.29 cm yr 1. The accumulation rates in the estuarine distributary (Figs. 5B, E, and G) appear 3.01e4.70 cm yr 1, and the erosion rates are 1.61e 7.32 cm yr 1. The accumulation rates on the delta front slope (Fig. 5I) are 2.72e4.97 cm yr 1. As determined from the DEM, accumulation (volume, 1294.70e133.76 106 m3) dominates in the sandy shoals, although erosion (28.11e 67.19 106 m3) also occurs there (Table 1). Both accumulation (164.05e133.77 106 m3) and erosion (47.45e 490.91 106 m3) are intensive in the estuarine distributaries. Accumulation prevails in the South Passage, and erosion in the North Channel and North Passage, on either side of Hengsha Island (Figs. 1B and 5). Accumulation (1233.67 106 m3) obviously exceeds erosion (35.81 106 m3) on the delta front slope (Fig. 5; Table 1). The Pb-210 results (CIC model) for cores Y5eY8 from the lower delta front slope to prodelta facies indicate accumulation
rates varying from 0.8 to 6.3 cm yr 1, and Cs-137 records rates ranging from 2.1 to 6.6 cm yr 1 (Fig. 6). Meanwhile, the accumulation rate of the prodelta facies determined from the DEM is 5.22 cm yr 1, with an accumulation budget of 1292.2 106 m3, whereas the erosion rate is 2.31 cm yr 1, relative to an erosion budget of 77.07 106 m3 (Table 1). The sedimentation rate could not be determined in vibrocore Y9, in the delta-shelf transition zone (Figs. 1A and 6). 4. Discussion and conclusions Radioisotope measurement and DEM analysis results systematically revealed accumulation and erosion rates in the Yangtze Estuary, thus characterizing the sedimentary processes and associated sediment transport near the river mouth area. This study highlights the effects of estuarine sediment dynamics, including fluvial discharge, littoral currents, and tidal and wave currents (Chen et al., 1988; Goodbred and Kuehl, 1998; Chen et al., 2000; Shen and Pan, 2001; Hori et al., 2002; Uehara et al., 2002; Chen et al., 2003, 2004). The results of the study give us a better understanding of the controls on deltaic sedimentation in the study area, which will be useful for evaluating recent deltaic morphological changes e accumulation and erosion budgets in relation to environmental modification at human and catchment dimension (Goodbred and Kuehl, 1999; Syvitski et al., 2005). The sediments sampled by vibrocores from the Yangtze Estuary indicate uneven sedimentation rates in the delta front (topset) facies, comprising the upper tidal flat, estuarine distributary, sandy shoal, and upper delta front slope subfacies,
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 river mouth (Fig. 2C). Fine sand and silt are the major sediment components of these subfacies, consistent with the dominant tidal current of 1.0e1.5 m s 1 recorded in the Yangtze River mouth area (Fig. 2A,B) (Chen et al., 1988; Wang et al., 2005). Interbedded sandy layers and scouring surfaces where lithologic changes are observed may reflect seasonal erosion by typhoons e a strong sediment dynamics that can reactivate sediment on the nearshore seabed above the NWB (Chen et al., 1988; Hori et al., 2002). In contrast, the clayey silt composing a large part of the lower delta front slope and prodelta facies below the NWB, where sedimentation in general is continuous, is massive and structureless with abundant organic matter and burrowings (Fig. 2A) (cf. Walker and James, 1992; Chen et al., 2004). The delta-shelf transition facies consists of thin (centi- to decimeter-thick) beds composed of a sand-silt-clay mixture with numerous fragmented shells, bounded by intensive scourings, indicating reactivation by submarine currents or even typhoon-triggered current flow (Chen, 1987; Xu, 1997; Chen et al., 2000, 2003). Recently, Wang et al. (2005) discussed similar highly laminated sediments in the Yangtze delta-shelf transition zone.
The CRS model can be more applicable than the CIC model for delineating the nature of sedimentation in the topset sedimentary facies, that is, the upper tidal flat, estuarine distributary, sandy shoal, and upper delta front slope subfacies (cf. Appleby and Oldfield, 1978; Xiang, 1997); in general, the CRS model yields higher sedimentation rates (0.16e 5.0 cm yr 1) than the CIC model (0.17e1.94 cm yr 1) (Figs. 3 and 4). However, given the situation of uneven sedimentation, rates derived from interbedded fine-grained sediments using the CRS model are rather inaccurate (Li et al., 1999), neither fully reflecting the nature of sedimentation in these facies, nor matching the development of the bathymetric geometry of the facies. In addition to the coastal sediment dynamics, which serve as the driving force for shifting estuarine topography, the bathymetric changes in the Yangtze Estuary have been sensitive to variations in sediment discharge from the drainage basin (cf. Yang et al., 2003). Undeterminable rates at the sites of C1, C9, C10, and Y4 can be interpreted to reflect discontinuous sedimentation or the presence of coarser (sandy) sediments in the delta front facies, which do not absorb enough radioactivity for Pb-210 and Cs-137 measurement (cf. Robbins and Edgington, 1975; Appleby and Oldfield, 1978; Wan, 1997) (Figs. 2 and 4).
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 121° 50′
122° 00′
122° 10′
43
122° 20′ E
31° 20′ N
A 0
B
Hengsha Island
N 0
C
0 0
D
0
31° 10′
E 0
10
0
8
0 4
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I
0 -4
Nanhui
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10km
-8
31° 00′
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0
-12
Thickness of deposition/erosion (m)
J
-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 sediments alternately accumulated on and were eroded from the nearshore seafloor above the NWB. In sandy shoals, accumulation predominates, 1.73e8.30 cm yr 1 with an accumulation budget of 133.76e1294.7 106 m3 in the past about 50 years, although minor erosion also occurs there (Fig. 5; Table 1). On the other hand, the DEM shows significant erosion in the estuarine distributaries: 1.61e7.32 cm yr 1 with an erosion budget of 47.45e490.91 106 m3 (Table 1). The major accumulation and erosion in the southern and northern
estuarine distributaries can be attributed to the dominant sedimentation by the superimposed Yangtze discharge and ebb tidal currents, and by flood tidal currents (Fig. 5) (Chen et al., 1988). Previous work on sediment dynamics in the river mouth area, showing that the major flood tide transmission is from the ocean landward at a bearing of 305 and that the ebb flow direction is southward, corroborates this observation (cf. Shen and Pan, 2001; Wang et al., in press). Doubtlessly, accumulation (4.97 cm yr 1 for 1233.7 106 m3) dominates over erosion (2.72 cm yr 1 for 35.81 106 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 (106 m2)
Accumulation area (106 m2)
169.48
140.75
28.73
403.65
C D F H
Chongming shoal (partial) Tiaozisha shoal Hengsha shoal Jiuduansha shoal Nanhui shoal
136.18 310.73 403.90 335.06
89.042 237.17 371.55 282.18
47.14 73.56 32.35 52.88
Distributary
B E G
North channel North branch South branch
227.51 256.65 192.40
67.75 101.69 122.38
Delta-front slope
I
5e10 m water depth
622.02
Prodelta
J
10e20 m water depth
668.44
Sandy shoal
A
Erosion area (106 m2)
Accumulation budge (106 m3)
Erosion budget (106 m3)
Sedimentation rate (cm yr 1)
Erosion rate (cm yr 1)
21.39
6.83
1.77
133.76 425.00 1294.69 205.23
67.19 62.38 28.11 29.97
3.58 4.27 8.30 1.73
3.39 2.02 2.07 1.35
159.76 154.96 70.02
133.77 164.05 154.65
490.91 398.21 47.45
4.70 3.84 3.01
7.32 6.12 1.61
590.67
31.35
1233.67
35.81
4.97
2.72
589.13
79.31
1292.22
77.07
5.22
2.31
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 0
100
200
100 Background
Y7
200
Depth (cm)
0
Depth (cm)
Y4
Depth (cm)
0
0
Depth (cm)
44
6.3 cmyr-1
200
Background 200
4.3-6.6 cmyr-1 1
0
100
150 0.001
2.8-2.9 cmyr-1
50
Y8 100
Background
0.1
0
1
0.004
0.008
200
300 0.001
100
Background
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2.4-4.5 cmyr -1
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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 prodelta facies, which lies at 10e50 m water depth (Figs. 1 and 2C). Radioisotope measurement indicates a lower sedimentation rate (2.0e2.9 cm yr 1) at Y5 (Fig. 6), probably due to stronger wave erosion at this site near the NWB (Fig. 2C) (Mckee et al., 1983; DeMaster et al., 1985). Also, Shen et al. (2003) and Wang et al. (in press) reported a strong plume front at this site off the river mouth, which greatly triggers resuspension of the seafloor sediments. Thus, the sedimentation rate increases from 2.0 to 6.3e6.6 cm yr 1 in vibrocores Y6 to Y7 as away seaward from the NWB (Figs. 2C and 6). The decreasing rate from Y8 (0.8e2.1 cm yr 1) to Y9 (unmeasurable) reflects the nature of sedimentation in the prodelta facies and the delta-shelf transition zone (Fig. 6), where the modern Yangtze subaqueous delta terminates (Chen et al., 2003; Wang et al., 2005). Nevertheless, our DEM data also indicate minor erosion (2.32 cm yr 1 for 77.07 106 m3) in the prodelta facies, although it is negligible in comparison with the accumulation (5.22 cm yr 1 for 1292.2 106 m3) (Table 1). We note that the sedimentation rate (5.22 cm yr 1) in the prodelta facies determined by DEM is close to that (6.3e6.6 cm yr 1) determined from Pb-210 and Cs-137 data (Fig. 6; Table 1).
Interestingly, the sedimentation rates recorded in the thinner sediment section at Y5 and Y6 by Pb-210 are generally lower than those recorded by Cs-137, determined from thicker sediment sections of the same vibrocores (Fig. 6). This difference may reflect a reduction in sediment discharge in the river mouth area as a result of the construction of numerous dams during the 1960s and 1970s (Chen et al., 2001; Yang et al., 2003). The Yangtze Estuary is a huge ecosystem, which has been nourished by the large amount (470 Mt, on a multi-yearly basis) of sediment input from the upper drainage basin. The extensive delta plain (about 30,000 km2), on which millions of inhabitants engage in industry and agriculture, has been prograding for the last ca. 7000 years (Stanley and Chen, 1996; Chen and Stanley, 1998). However, it is likely that the coastline and coastal morphology will be greatly modified by the reduction in the sediment supply resulting from the ongoing Three-Gorges Dam project, which will be completed in 2009. This reduction will inevitably lead to coastal erosion and associated environmental degradation and disasters. Therefore, this study provides a useful sedimentological reference for monitoring estuarine morphological preservation in the near future.
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Acknowledgements The authors cordially thank Mr. B. Xue and Mr. C. Zhu, the Second Institute of Oceanography, SOA, Hangzhou, for assisting in the sampling in the Yangtze Estuary. The manuscript was kindly reviewed by Dr. J.P. Liu, who offered many constructive suggestions. This project was funded by the China National and Natural Science Foundation (Grant No.40341009) and the National Key Project (#973 e 200CB 412505). References Appleby, P.G., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1e8. Black, C.A. (Ed.), 1965. Methods of Soil Analysis: Part I Physical and Mineralogical Properties, Part II. American Society of Agronomy, Madison, Wisconsin, USA, pp. 374e390. Chen, Z., 1987. The storm deposits of the Yangtze subaqueous delta. In: Yan, Q.S., Xu, S.Y. (Eds.), Recent Yangtze Delta Deposits. East China Normal University Press, Shanghai, pp. 246e257 (in Chinese, with English summary). Chen, Z., Stanley, D.J., 1998. Rising sea level on eastern China’s Yangtze delta. Journal of Coastal Research 14, 360e366. Chen, J.Y., Shen, H.T., Yun, C.X., 1988. Process of Dynamics and Geomorphology of the Changjiang Estuary. Shanghai Scientific and Technical Publishers, Shanghai, pp. 205e283 (in Chinese, with English summary). Chen, J.Y., Li, D.J., Chen, B.L., Hu, F.X., Zhu, H.F., Liu, C.Z., 1999. The processes of dynamic sedimentation in the Changjiang Estuary. Journal of Sea Research 41, 129e140. Chen, Z., Song, B.P., Wang, Z.H., Cai, Y.L., 2000. Late Quaternary evolution of the sub-aqueous Yangtze Delta, China: sedimentation, stratigraphy, palynology, and deformation. Marine Geology 162, 423e441. Chen, Z., Yu, L.Z., Gupta, A., 2001. Yangtze River, China. Geomorphology 41 (2e3), 248. Chen, Z., Saito, Y., Hori, K., Zhao, Y., Kitamura, A., 2003. Early Holocene mud-ridge formation in the Yangtze offshore, China: a tidal-controlled estuarine pattern and sea-level implications. Marine Geology 198, 245e257. Chen, Z., Saito, Y., Kanai, Y., Wei, T.Y., Li, L.Q., Yao, H.S., Wang, Z.H., 2004. Low concentration of heavy metals in the Yangtze estuarine sediments, China: a diluting setting. Estuarine, Coastal and Shelf Science 60, 91e100. Chen, Z., Gupta, A., Yin, H.F. (Eds.), Large monsoon rivers of Asia. Geomorphology, in press. Coleman, J.M., 1981. Deltas: Processes of Deposition and Models for Exploration, Second ed. International Human Resources Development, Boston, 124 pp. DeMaster, D.J., McKee, B.A., Nittrouer, C.A., Qian, J.C., Chen, G.D., 1985. Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea. Continental Shelf Research 4, 143e158. Goodbred Jr., S.L., Kuehl, S.A., 1998. Floodplain processes in the Bengal Basin and the storage of Ganges-Brahmaputra river sediment: an accretion study using 137Cs and 210Pb geochronology. Sedimentary Geology 121 (3e4), 239e258. Goodbred Jr., S.L., Kuehl, S.A., 1999. Holocene and modern sediment budgets for the Ganges-Brahmaputra river: evidence for highstand dispersal to floodplain, shelf, and deep-sea depocenters. Geology 27 (6), 559e562. Hori, K., Saito, Y., Zhao, Q.H., Wang, P.X., 2002. Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China. Sedimentary Geology 146, 249e264. Kuehl, S.A., Hariu, T.M., Moore, W.S., 1989. Shelf sedimentation off the Ganges-Brahmaputra river system: evidence for sediment bypassing to the Bengal fan. Geology 17, 1132e1135.
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Li, C., Wang, P., Fan, D., Li, T., 1999. Sedimentation rate and sedimentary break in tidal deposits. Marine Geology and Quaternary Geology 19 (2), 11e18 (in Chinese, with English summary). Liu, G.X., Yang, S.L., Zhou, Y.H., 1984. Sedimentation rates in the Changjiang Estuary determined with Pb-210 method. Marine Geology and Quaternary Geology 4 (1), 113e116 (in Chinese, with English summary). Liu, K., Peng, T., Shaw, P., 2003. Circulation and biogeochemical processes in the East China Sea and the vicinity of Taiwan. Deep Sea Research Part II: Tropical Studies in Oceanography 50 (6e7), 1055e1333. Liu, J.P., Xu, K.H., Li, A.C., Milliman, J.D., Velozzi, D.M., Xiao, S.B., Yang, Z.S. Flux and Fate of Yangtze River sediment delivered to the East China Sea. Geomorphology, in press. Maritime Bureau of China, 1958. Bathymetry of the Yangtze River mouth and adjacent area (1:50,000) (unpublished, in Chinese). Maritime Bureau of China, 2000. Bathymetry Map: Jigujiao e Hengsha (1:75,000), Beijing (unpublished, in Chinese). Mckee, B.A., Nittrouer, C.A., DeMaster, D.J., 1983. Concepts of sediment deposition and accumulation applied to the continental shelf near the mouth of the Yangtze River. Geology 11, 631e633. Milliman, J.D., Shen, H.T., Yang, Z.S., Meade, R.H., 1985. Transport and deposition of river sediment in the Changjiang Estuary and adjacent continental shelf. Continental Shelf Research 4, 37e45. Niino, H., Emery, K.O., 1961. Sediments of shallow portions of East China Sea and South China Sea. Geological Society of America Bulletin 72, 731e762. Robbins, J.A., Edgington, D.N., 1975. Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137. Geochimica et Cosmochimica Acta 39, 285e304. Shen, H.T., Pan, D.A. (Eds.), 2001. Turbidity Maximum in the Changjiang Estuary. China Ocean Press, Beijing, p. 194 (in Chinese, with English summary). Shen, H.T., Mao, Z.C., Zhu, J.R., 2003. Saltwater Intrusion in the Changjiang Estuary. China Ocean Press, Beijing, 175 pp (in Chinese, with English summary). Stanley, D.J., Chen, Z., 1996. Neolithic settlement distributions as a function of sea level-controlled topography in the Yangtze delta, China. Geology 24, 1083e1086. Stanley, D.J., Warne, G.A., 1993. Nile delta: recent geological evolution and human impact. Science 260, 628e634. Syvitski, J.P.M., Vo¨ro¨smarty, C.J., Bettner, A.J., Green, P., 2005. Impact of humans on the flux of terrestrial sediment to the global coastal ocean. Science 208, 376e380. Tsunogai, S., Iseki, K., Kusakabe, M., Saito, Y., 2003. Biogeochemical cycles in the East China Sea: MASFLEX Program. Deep Sea Research Part II: Topical Studies in Oceanography 50 (2), 321e529. Uehara, K., Saito, Y., Hori, K., 2002. Paleotidal regime in the Changjiang (Yangtze) Estuary, the East China Sea, and the Yellow Sea at 6 ka and 10 ka estimated from a numerical model. Marine Geology 183, 179e192. Walker, R.G., James, N.P., 1992. Facies Models: Response to Sea Level Change. Geological Association of Canada, Ontario, 409 pp. Wan, G.J., 1997. 210Pb dating for recent sedimentation. Quaternary Sciences 3, 230e239 (in Chinese, with English summary). Wang, Z.H., Saito, Y., Hori, K., Kitamura, A., Chen, Z., 2005. Yangtze offshore, China: highly laminated sediments from the transition zone between subaqueous delta and the continental shelf. Estuarine, Coastal and Shelf Science 62, 161e168. Wang, Z.H., Li, L.Q., Chen, D.C., Xu, K.Q., Wei, T.Y., Gao, J.H., Zhao, Y.W., Chen, Z., Masabate, W. Fine-sediment dynamics off the Changjiang River mouth, Chinaea case study. Geomorphology, in press. Xia, X.M., Xie, Q.C., Li, Y., Li, B.G., Feng, Y.J., 1999. 137Cs and 210Pb profiles of the seabed cores along the East China Sea coast and their implications to sedimentary environment. Donghai Marine Science 17 (1), 20e27 (in Chinese, with English summary). Xia, X.M., Yang, H., Li, Y., Li, B.G., Pan, S.M., 2004. Modern sedimentation rates in the contiguous sea area of Changjiang Estuary and Hangzhou Bay. Acta Sedimentologica Sinica 22 (1), 130e135 (in Chinese, with English summary).
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Xiang, L., 1997. Dating of recent Dianchi lake sediments by gamma ray analysis. Nuclear Techniques 20 (2), 100e104 (in Chinese, with English summary). Xu, S.Y., 1997. Storm Deposits in the Yangtze Delta. Science Press, Beijing, 150 pp. Yan, Q.S., Xu, S.Y. (Eds.), 1987. Recent Yangtze Sediments. East China Normal University Press, Shanghai, 438 pp (in Chinese, with English summary).
Yang, S.L., Belkin, I.M., Belkina, A.I., Zhao, Q.Y., Zhu, J., Ding, P.X., 2003. Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next halfcentury. Hydrology 56, 1e11. Ye, C.K., 1991. Contrasting investigation by 137Cs method and 210Pb method for the present sedimentation rate of Poyang Lake, Jiangxi. Acta Sedimentologica Sinica 9 (1), 106e114 (in Chinese, with English summary).
Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, China Taoyuan Wei a, Zhongyuan Chen b,*, Lingyun Duan a, Jiawei Gu a, Yoshiki Saito c, Weiguo Zhang b, Yonghong Wang d, Yutaka Kanai e a
b
Department of Geography, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, China 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 sedimentary 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 yr 1 were recorded in the fine-grained (silty) sediments, which were interbedded with coarsegrained (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 yr 1) and delta front slope (5.22 cm yr 1) facies. The DEM data also show that erosion (1.61e7.32 cm yr 1) dominates in the northern estuarine distributaries, and accumulation (3.01e4.97 cm yr 1) 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 yr 1 to 6.3e6.6 cm yr 1 in the delta front slope facies, which progressively decreases to <0.8 cm yr 1 in the prodelta facies and is unmeasurable 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 The Yangtze River delivers more than 470 Mt of sediment annually into its estuary to build a huge delta system (>40,000 km2, including subaqueous parts) that presently sustains intensifying human activities (Chen et al., 2001, in press). Fluvial sediment discharge into the estuary consists mainly of fine-grained particles that accumulate in the river
* Corresponding author. E-mail address: [email protected] (Z. Chen). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.08.014
mouth area and are dispersed further offshore in the form of freshwater plumes (Milliman et al., 1985; Chen et al., 1988, 1999; Shen et al., 2003; Yang et al., 2003). Previous studies have shown that sediments in the river mouth area (<10 m water depth) are mostly fine sand, silty sand, and silt, and sediments off the river mouth (10e60 m water depth) are silty clay and clayey silt (Chen et al., 2003; Yang et al., 2003; 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) estimated that about 70% of the annual sediment load is deposited near the coast, with about 30% being carried further offshore,
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of which a large proportion of the suspended sediment even approaches the nearshore areas off Zhejiang and Fujian provinces of southeastern China, driven by the Chinese Coastal Current (Liu et al., in press). Processes of sediment transport from estuary to continental shelf have largely shaped the coastal topography, and are a significant mechanism of coastal environmental change at human and global dimensions (Syvitski et al., 2005). To examine accumulation and erosion of subaqueous sediments in relation to the sediment transport pattern, measuring sedimentation rates in the river mouth area and along the coast is an effective means of gaining a better understanding of these processes. For example, Liu et al. (in press) have quantitatively defined the sediment budget with respect to sediment transport from the Yangtze River mouth southward to the coasts of Zhejiang and Fujian provinces. Kuehl et al. (1989) corroborate sediment bypassing from river-delta to the Bengal fan in the light of sedimentation rate by radioisotopic measurement. To clarify the sedimentation rate in the Yangtze Estuary is vital, considering the need to determine the fate and flux of the river-derived sediment discharged to the coast and the close linkage between fine-grained riverine sediment and geological and biological processes. Recent studies of nutrient (including pollutants due to human activity) delivery highlight the relationship between the fine-grained sediment flux and associated the food chain in the coastal zone and shelf area (Liu et al., 2003; Tsunogai et al., 2003). However, sedimentation rates, in particular in the delta front facies, and their changes in relation to sediment dynamics and sources, must still be confirmed, despite numerous studies in the Yangtze River mouth area during past decades (e.g., Mckee et al., 1983; Liu et al., 1984; DeMaster et al., 1985; Xia et al., 1999; Chen et al., 2004; Xia et al., 2004). From these studies, we know that most sedimentation rates were given in the prodelta muddy zone, a few were in the delta front sandy zone (Fig. 1A). Sedimentation rate of the topset sediments has been a long puzzle for coastal scientists, primarily due to strong erosional processes occurring seasonally in the nearshore face above NWB (Coleman, 1981; Yan and Xu, 1987; Stanley and Warne, 1993), and coarser (sandy) sediment unsuitable for radioisotope measurement (Appleby and Oldfield, 1978; Xiang, 1997). The objectives of the present study were through the integration of Pb-210 and Cs-137 measurements and a Digital Elevation Model (DEM) to: (1) conduct a thorough examination of sedimentation rates in the proximal to distal subaqueous delta; (2) determine the controls on sedimentation rates in the subaqueous delta facies; and (3) establish a sedimentological database that will highlight later studies of the impacts of the Three-Gorges Dam, scheduling to be completed in 2009. 2. Methods From 1995 to 2003, eighteen (18) vibrocores (C1eC12, Y4eY9) were recovered from various geomorphological units in the Yangtze Estuary, including tidal flat, estuarine distributary, sandy shoals, delta front slope, and prodelta (Fig. 1A).
Vibrocores were collected in PVC tubes with 6-cm in diameter. Five vibrocores (C1eC3, C8, and C9), ranging from 20 to 590 cm long, were collected from the upper tidal flat of the southern delta coast. C4eC6, C7, and C10, ranging from 48 to 135 cm long, were collected from the eastern upper tidal flat on Chongming Island. C11 and C12, 56 and 104 cm long, respectively, were collected from the estuarine distributary. Y4, 220 cm long, was collected from the delta front marginal slope at about 10 m water depth, which is contiguous seaward with the prodelta facies. Vibrocores Y5eY8, from 200 to 400 cm long, were collected at the water depth of 15e40 m from the prodelta facies, and Y9, 360 cm long, was from the delta-shelf transition zone, at about 50 m water depth. Sediment logging applied to all vibrocores, while splitting in the laboratory, to record sediment texture and structure, organic matter distribution, the presence of plant roots, and the occurrence of biogenesis, etc. Grain size was determined for 282 samples taken from the 18 vibrocores at intervals based on changes in sediment lithology. The grain-size distribution in these samples was examined using a laser particle analyzer (Beckman Coulter LS13,320). In the present study, sand particles are those >63 mm; silt, 63e2 mm; and clay, <2 mm; and mean grain size (Mz, 4) is also reported. An independent set of 237 samples was taken at 0.5 to 7.0 cm intervals for radioisotope analyses (Pb-210 and Cs137). From vibrocores C1eC12, 129 samples were prepared for Pb-210 measurement in the State Key Laboratory of Estuarine and Coastal Research (SKLEC), East China Normal University. From Y4eY9, 108 samples were analyzed for both Pb-210 and Cs-137 at the Geological Survey of Japan (GSJ), Tsukuba. SKLEC sample preparation procedures were as follows: ca. 10 g wet sediment of each sample was dried in an oven at 105 C for 2 h; 2e5 g dried sample was ground, sieved through 0.150 mm mesh to remove plant roots, and wax-sealed in a tube for 3 weeks. Then, the Pb-210 radioactivity was determined with a High-Purity Germanium Gamma Detector (ORTEC, GWL-120210-S). The ratio of dry to wet sample (%) was determined for some samples (vibrocores C2eC4; C6eC8), and then bulk density (g cm 3) was calculated (Black, 1965). The peak height at 46.5 KeV was considered to represent the total Pb-210 activity in the sediments, that at 351.92 KeV the supported Pb-210 activity (background radioactivity of Pb-210), and the difference between them the excess Pb-210 activity (Bq g 1). Both the Constant Initial Concentration (CIC) model and the Constant Rate of Supply (CRS) model were used to calculate the sedimentation rates (cm yr 1) in the upper tidal flat, estuarine distributary, and delta front slope subfacies (cf. Robbins and Edgington, 1975; DeMaster et al., 1985; Ye, 1991; Wan, 1997; Xiang, 1997; Xia et al., 2004). The sedimentation rate was calculated only for those sediment sections of vibrocores C1eC12 in which declining trend of Pb-210 radioactivity with vibrocore depth was detected. Similar procedures were followed to prepare the samples from vibrocores Y4eY9 for radioisotope analysis
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 121º
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in the Laboratory of the Geological Survey of Japan (Chen et al., 2004). Two bathymetric maps, made 42 years apart (in 1958 and 2000), of the Yangtze River mouth area (1:50,000 and 1:75,000; Maritime Bureau of China, 1958, 2000) were used for the DEM (Fig. 1A,B) as follows: (1) The maps were digitized and elevation information was retrieved from the 5-m isoclines
and from water depth measurements at numerous individual points; (2) a 300 300 grid was determined in the target area, on the basis of principle of grid number > the number of data points retrieved; and (3) the two resulting maps were overlaid to determine the sediment budget of selected subfacies. The results of the DEM, together with radioisotope measurement, compose the database for the present study.
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the lower delta front slope to prodelta facies, distinguished from the upper delta front facies by the change in lithology at 10e15 m water depth, the depth of the NWB off the Yangtze River mouth (Chen, 1987). Abundant organic matter occurs as irregular patches in the sediments, which are massive and structureless. Shells (mostly bivalves) are well preserved and borrowings are common. Silty sand and clayey silt (Mz 4.2e4.9 4) are thinly interbedded (3e5 cm thick) in the delta-shelf transition zone, where highly fragmented shells are found and scouring surfaces prevail (Fig. 2A). The proportions of sand, silt, and clay in 282 samples from the above-mentioned sedimentary facies were plotted on a ternary diagram to characterize their distribution in relation to sediment dynamics (Fig. 2B). Clearly, silt dominates in the upper tidal flat facies, silty sediment (mostly, find sandy silt and silty fine sand) constituents the most delta front facies, and clayey silt takes over the lower delta front slope to prodelta facies (cf. Chen et al., 2000).
3. Data and observations 3.1. Estuarine sediments The Yangtze estuarine sediments can be characterized by their sedimentary facies, which were identified in the 18 vibrocores (Figs. 1A and 2A). From the coastline seaward are the delta front facies (i.e. the upper tidal flat, estuarine distributary, sandy shoal, and delta front slope), and prodelta facies, and the delta-shelf transition zone (cf. Coleman, 1981; Chen et al., 2000). The upper tidal flat sediments, about 30e40 cm thick, consist of yellowish gray massive clayey silt (Mz 5.0e5.7 4) with many silty lenses, abundant organic matter and root traces (Fig. 2A). Sandy sediment sections occur in the lower portions of the vibrocores (e.g., C8 in Fig. 2A). Gray to yellowish fine silt and clayey silt (Mz 4.8e5.5 4) dominate the estuarine distributary and sandy shoal subfacies (Fig. 2A). Relatively pure silty sediment sections are often intercalated by thin (5e10 cm thick) sandy silt layers, in which ripple cross-bedding occurs. The upper delta front slope facies is composed of fine sand (Mz 4.1e4.6 4), in which small-scale trough cross-bedding occurs, and shells (both whole and fragmentary) are present. Scouring surfaces are common at lithologic change boundaries. In contrast, grayish silty clay (Mz 6.3e6.9 4) dominates
3.2. Sedimentation rates The CIC model reveals that sedimentation rates in the delta front facies (topset or delta front platform) range from 0.17 to 1.94 cm yr 1 in the upper tidal flat (C2eC8, Fig. 3) and from
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4
C2 C3C4C5C6 C9
C7 C8 C10
Delta plain
400 cm
Mean sea level
C11 C12
Normal Wave Base (NWB)
Y4 100
Sand
Delta front
0 0
25
50
75
100
Clay Basal delta topography
5 m
Prodelta
Y5 Y6 Y7 Y8 Y9
590 cm
C
Delta-shelf transit zone
Relict sand
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).
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46
Age (yr)
Age (yr) 60
90
0.26
4
0.40
Depth (cm)
Depth (cm)
60
6 0.44
10 12
0.39 0.36
6 8
0.42
10
25
35
5.00
20 0.40
25
0.51
0.16
0.75
40
0
10
Depth (cm)
Depth (cm)
2.70
120
0.75
15
35
0.17
0.16
30
60
1.16 1.89
Age (yr) 90
120
0
C7
1.08 1.74 2.55 1.60 40 0.56
30
50
0.64
0.85
60
30
60
90
120
C8
40 2.66 60 80 100
1.66 2.50
120
1.35
1.68
140
70
1.03
0.47
160
1.94
0.71
180
80 Constant Initial Concentration Model (CIC)
0
20
20 1.72 1.70
20
30
0
C6
5
15
90
90
C4
Age (yr) 60
60
30
Age (yr) 30
30
5
16
0
0
10
14
0.23
0.17
16
10
0
C3
12
14
0
Age (yr) 120
90
2
0.44
8
30
0.25
C2
2 4
0
0
Depth (cm)
30
Depth (cm)
0
0
41
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 yr 1 in the estuarine distributary (C11 and C12, Fig. 4). The CRS model indicates a rate of 0.16e 5.0 cm yr 1 in the upper tidal flat (determined only from vibrocores C2eC4 and C6eC8, where bulk density data were available; Fig. 3). The sedimentation rate could not be determined for the upper tidal flat in several vibrocores, including C1, C9, and C10. Further seaward, the sedimentation rate in Y4 (upper delta front slope) could not be determined from either the Pb-210 or the Cs-137 test. The results of the DEM practice show both sedimentation and erosion of the delta front sediments (topset; Fig. 5; Table 1). Accumulation rates in the many sandy shoals (Fig. 5A, C, D, F, and H) range from 1.73 to 8.30 cm yr 1, which contrast with erosion rates of 1.35e3.29 cm yr 1. The accumulation rates in the estuarine distributary (Figs. 5B, E, and G) appear 3.01e4.70 cm yr 1, and the erosion rates are 1.61e 7.32 cm yr 1. The accumulation rates on the delta front slope (Fig. 5I) are 2.72e4.97 cm yr 1. As determined from the DEM, accumulation (volume, 1294.70e133.76 106 m3) dominates in the sandy shoals, although erosion (28.11e 67.19 106 m3) also occurs there (Table 1). Both accumulation (164.05e133.77 106 m3) and erosion (47.45e 490.91 106 m3) are intensive in the estuarine distributaries. Accumulation prevails in the South Passage, and erosion in the North Channel and North Passage, on either side of Hengsha Island (Figs. 1B and 5). Accumulation (1233.67 106 m3) obviously exceeds erosion (35.81 106 m3) on the delta front slope (Fig. 5; Table 1). The Pb-210 results (CIC model) for cores Y5eY8 from the lower delta front slope to prodelta facies indicate accumulation
rates varying from 0.8 to 6.3 cm yr 1, and Cs-137 records rates ranging from 2.1 to 6.6 cm yr 1 (Fig. 6). Meanwhile, the accumulation rate of the prodelta facies determined from the DEM is 5.22 cm yr 1, with an accumulation budget of 1292.2 106 m3, whereas the erosion rate is 2.31 cm yr 1, relative to an erosion budget of 77.07 106 m3 (Table 1). The sedimentation rate could not be determined in vibrocore Y9, in the delta-shelf transition zone (Figs. 1A and 6). 4. Discussion and conclusions Radioisotope measurement and DEM analysis results systematically revealed accumulation and erosion rates in the Yangtze Estuary, thus characterizing the sedimentary processes and associated sediment transport near the river mouth area. This study highlights the effects of estuarine sediment dynamics, including fluvial discharge, littoral currents, and tidal and wave currents (Chen et al., 1988; Goodbred and Kuehl, 1998; Chen et al., 2000; Shen and Pan, 2001; Hori et al., 2002; Uehara et al., 2002; Chen et al., 2003, 2004). The results of the study give us a better understanding of the controls on deltaic sedimentation in the study area, which will be useful for evaluating recent deltaic morphological changes e accumulation and erosion budgets in relation to environmental modification at human and catchment dimension (Goodbred and Kuehl, 1999; Syvitski et al., 2005). The sediments sampled by vibrocores from the Yangtze Estuary indicate uneven sedimentation rates in the delta front (topset) facies, comprising the upper tidal flat, estuarine distributary, sandy shoal, and upper delta front slope subfacies,
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 river mouth (Fig. 2C). Fine sand and silt are the major sediment components of these subfacies, consistent with the dominant tidal current of 1.0e1.5 m s 1 recorded in the Yangtze River mouth area (Fig. 2A,B) (Chen et al., 1988; Wang et al., 2005). Interbedded sandy layers and scouring surfaces where lithologic changes are observed may reflect seasonal erosion by typhoons e a strong sediment dynamics that can reactivate sediment on the nearshore seabed above the NWB (Chen et al., 1988; Hori et al., 2002). In contrast, the clayey silt composing a large part of the lower delta front slope and prodelta facies below the NWB, where sedimentation in general is continuous, is massive and structureless with abundant organic matter and burrowings (Fig. 2A) (cf. Walker and James, 1992; Chen et al., 2004). The delta-shelf transition facies consists of thin (centi- to decimeter-thick) beds composed of a sand-silt-clay mixture with numerous fragmented shells, bounded by intensive scourings, indicating reactivation by submarine currents or even typhoon-triggered current flow (Chen, 1987; Xu, 1997; Chen et al., 2000, 2003). Recently, Wang et al. (2005) discussed similar highly laminated sediments in the Yangtze delta-shelf transition zone.
The CRS model can be more applicable than the CIC model for delineating the nature of sedimentation in the topset sedimentary facies, that is, the upper tidal flat, estuarine distributary, sandy shoal, and upper delta front slope subfacies (cf. Appleby and Oldfield, 1978; Xiang, 1997); in general, the CRS model yields higher sedimentation rates (0.16e 5.0 cm yr 1) than the CIC model (0.17e1.94 cm yr 1) (Figs. 3 and 4). However, given the situation of uneven sedimentation, rates derived from interbedded fine-grained sediments using the CRS model are rather inaccurate (Li et al., 1999), neither fully reflecting the nature of sedimentation in these facies, nor matching the development of the bathymetric geometry of the facies. In addition to the coastal sediment dynamics, which serve as the driving force for shifting estuarine topography, the bathymetric changes in the Yangtze Estuary have been sensitive to variations in sediment discharge from the drainage basin (cf. Yang et al., 2003). Undeterminable rates at the sites of C1, C9, C10, and Y4 can be interpreted to reflect discontinuous sedimentation or the presence of coarser (sandy) sediments in the delta front facies, which do not absorb enough radioactivity for Pb-210 and Cs-137 measurement (cf. Robbins and Edgington, 1975; Appleby and Oldfield, 1978; Wan, 1997) (Figs. 2 and 4).
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 121° 50′
122° 00′
122° 10′
43
122° 20′ E
31° 20′ N
A 0
B
Hengsha Island
N 0
C
0 0
D
0
31° 10′
E 0
10
0
8
0 4
F
I
0 -4
Nanhui
G
0
10km
-8
31° 00′
H
0
-12
Thickness of deposition/erosion (m)
J
-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 sediments alternately accumulated on and were eroded from the nearshore seafloor above the NWB. In sandy shoals, accumulation predominates, 1.73e8.30 cm yr 1 with an accumulation budget of 133.76e1294.7 106 m3 in the past about 50 years, although minor erosion also occurs there (Fig. 5; Table 1). On the other hand, the DEM shows significant erosion in the estuarine distributaries: 1.61e7.32 cm yr 1 with an erosion budget of 47.45e490.91 106 m3 (Table 1). The major accumulation and erosion in the southern and northern
estuarine distributaries can be attributed to the dominant sedimentation by the superimposed Yangtze discharge and ebb tidal currents, and by flood tidal currents (Fig. 5) (Chen et al., 1988). Previous work on sediment dynamics in the river mouth area, showing that the major flood tide transmission is from the ocean landward at a bearing of 305 and that the ebb flow direction is southward, corroborates this observation (cf. Shen and Pan, 2001; Wang et al., in press). Doubtlessly, accumulation (4.97 cm yr 1 for 1233.7 106 m3) dominates over erosion (2.72 cm yr 1 for 35.81 106 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 (106 m2)
Accumulation area (106 m2)
169.48
140.75
28.73
403.65
C D F H
Chongming shoal (partial) Tiaozisha shoal Hengsha shoal Jiuduansha shoal Nanhui shoal
136.18 310.73 403.90 335.06
89.042 237.17 371.55 282.18
47.14 73.56 32.35 52.88
Distributary
B E G
North channel North branch South branch
227.51 256.65 192.40
67.75 101.69 122.38
Delta-front slope
I
5e10 m water depth
622.02
Prodelta
J
10e20 m water depth
668.44
Sandy shoal
A
Erosion area (106 m2)
Accumulation budge (106 m3)
Erosion budget (106 m3)
Sedimentation rate (cm yr 1)
Erosion rate (cm yr 1)
21.39
6.83
1.77
133.76 425.00 1294.69 205.23
67.19 62.38 28.11 29.97
3.58 4.27 8.30 1.73
3.39 2.02 2.07 1.35
159.76 154.96 70.02
133.77 164.05 154.65
490.91 398.21 47.45
4.70 3.84 3.01
7.32 6.12 1.61
590.67
31.35
1233.67
35.81
4.97
2.72
589.13
79.31
1292.22
77.07
5.22
2.31
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46 0
100
200
100 Background
Y7
200
Depth (cm)
0
Depth (cm)
Y4
Depth (cm)
0
0
Depth (cm)
44
6.3 cmyr-1
200
Background 200
4.3-6.6 cmyr-1 1
0
100
150 0.001
2.8-2.9 cmyr-1
50
Y8 100
Background
0.1
0
1
0.004
0.008
200
300 0.001
100
Background
Y9 200
2.4-4.5 cmyr -1
300 0.01
0.1
Pb-210ex(Bqg-1)
1
0
0.004
0.008
Cs-137(Bqg-1)
0.008
0
400
Background
200
0-75 cm 0-2.1 cmyr-1
400
600 0.01
0.1
0
1
0.005 0.01 0.015 0.02
Cs-137(Bqg-1) 0
0
Depth (cm)
Depth (cm)
25-125 cm 2.2 cmyr-1
0.004
Pb-210ex(Bqg-1)
0
100
0
Cs-137(Bqg-1)
200
Cs-137(Bqg-1)
0
1
0-55 cm 0.8cmyr-1
600 0.001
150 0.01
Pb-210ex(Bqg-1)
Depth (cm)
Depth (cm)
Depth (cm)
Depth (cm)
2.0 cmyr-1
0.1
0
0
50
400 0.01
Pb-210ex(Bqg-1)
Cs-137(Bqg-1)
0
Y6
0.001 0.002 0.003
Depth (cm)
0.1
Pb-210ex(Bqg-1)
Y5
400 0.001
300 0.01
Depth (cm)
300 0.001
200
400 0.001
200
Background
400 0.01
0.1
1
0
Pb-210ex(Bqg-1)
0.001
0.002
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 prodelta facies, which lies at 10e50 m water depth (Figs. 1 and 2C). Radioisotope measurement indicates a lower sedimentation rate (2.0e2.9 cm yr 1) at Y5 (Fig. 6), probably due to stronger wave erosion at this site near the NWB (Fig. 2C) (Mckee et al., 1983; DeMaster et al., 1985). Also, Shen et al. (2003) and Wang et al. (in press) reported a strong plume front at this site off the river mouth, which greatly triggers resuspension of the seafloor sediments. Thus, the sedimentation rate increases from 2.0 to 6.3e6.6 cm yr 1 in vibrocores Y6 to Y7 as away seaward from the NWB (Figs. 2C and 6). The decreasing rate from Y8 (0.8e2.1 cm yr 1) to Y9 (unmeasurable) reflects the nature of sedimentation in the prodelta facies and the delta-shelf transition zone (Fig. 6), where the modern Yangtze subaqueous delta terminates (Chen et al., 2003; Wang et al., 2005). Nevertheless, our DEM data also indicate minor erosion (2.32 cm yr 1 for 77.07 106 m3) in the prodelta facies, although it is negligible in comparison with the accumulation (5.22 cm yr 1 for 1292.2 106 m3) (Table 1). We note that the sedimentation rate (5.22 cm yr 1) in the prodelta facies determined by DEM is close to that (6.3e6.6 cm yr 1) determined from Pb-210 and Cs-137 data (Fig. 6; Table 1).
Interestingly, the sedimentation rates recorded in the thinner sediment section at Y5 and Y6 by Pb-210 are generally lower than those recorded by Cs-137, determined from thicker sediment sections of the same vibrocores (Fig. 6). This difference may reflect a reduction in sediment discharge in the river mouth area as a result of the construction of numerous dams during the 1960s and 1970s (Chen et al., 2001; Yang et al., 2003). The Yangtze Estuary is a huge ecosystem, which has been nourished by the large amount (470 Mt, on a multi-yearly basis) of sediment input from the upper drainage basin. The extensive delta plain (about 30,000 km2), on which millions of inhabitants engage in industry and agriculture, has been prograding for the last ca. 7000 years (Stanley and Chen, 1996; Chen and Stanley, 1998). However, it is likely that the coastline and coastal morphology will be greatly modified by the reduction in the sediment supply resulting from the ongoing Three-Gorges Dam project, which will be completed in 2009. This reduction will inevitably lead to coastal erosion and associated environmental degradation and disasters. Therefore, this study provides a useful sedimentological reference for monitoring estuarine morphological preservation in the near future.
T. Wei et al. / Estuarine, Coastal and Shelf Science 71 (2007) 37e46
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