Estuarine, Coastal and Shelf Science

Transcription

1 Estuarine, Coastal and Shelf Science (1) Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: Dynamics and spatial variability of near-bottom sediment exchange in the Yangtze Estuary, China Hong Liu a, Qing He a, *, Zhengbing Wang b,c, Gert Jan Weltje c, Jing Zhang a a East China Normal University, State Key Laboratory of Estuarine and Coastal Research, Shanghai, People s Republic of China b Deltares, WL j Delft Hydraulics, P.O. Box 177, NL-MH Delft, The Netherlands c Delft University of Technology, Faculty of Civil Engineering and Geosciences, P.O. Box 5, NL-GA Delft, The Netherlands article info abstract Article history: Received February 9 Accepted 1 April 9 Available online April 9 Keywords: Yangtze (Changjiang) Estuary grain-size distributions non-linear sediment mixing selective deposition sediment exchange rate This study was conducted to examine the spatial variations in the exchange between near-bottom suspended and sea-bottom sediments in the Yangtze Estuary and adjacent region, as well as to explore the fate of suspended sediments in the study area. The relationship between the sand, silt, and clay contents of the sediments was analyzed by log-ratio analysis, which revealed a non-linear function for selective deposition and a wide range of grain-size distributions in the estuary. This finding does not conform to the nearly constant clay/silt ratios reported for other tidal basins around the world, due to non-linear sediment mixing under complex hydrodynamic conditions. The sediment exchange processes in the Yangtze Estuary were quantified based on the principle of mass balance. The average grain-size distribution of near-bottom suspended sediments from the estuary showed that approximately 9% of the riverine sediments accumulated in the mouth bar area, while the rest, which is primarily composed of fine-grained sediments, is transferred to the outer estuary and deposited in the form of flocs. The spatial distribution of the sediment exchange ratios demonstrated that small amounts of suspended sediment were deposited onto the seabed of the upper estuary (exchange ratio <.1), because the finegrained suspended sediments in this region were transported to the mouth bar area by the ebb-dominated tidal flow. The sediment exchange ratios in the outer estuary also showed very low values (<.1) due to the oceanic currents offshore that prevented the diffusion of riverine sediments further seaward. Intensive sediment exchange occurred in the inner estuary due to the sand-mud mixing which was controlled by bidirectional tidal flows. In addition, a high sediment exchange ratio occurred in the muddy area (>.) seaward of the river mouth, which implies that this is the present-day depocenter of Yangtze mud. The sediment exchange rates obtained by combining the dimensionless exchange ratios and bulk sediment accumulation rates, were found to be cm/yr in the muddy depocenter, which extends to the south of the river mouth (from 1 Eto1 E longitude, at 1 N latitude). Crown Copyright Ó 9 Published by Elsevier Ltd. All rights reserved. 1. Introduction Sediment grain-size distributions (GSDs) have been extensively studied because they provide valuable information regarding the origins of sediments. In addition, analysis of GSDs is an effective method of evaluating sediment transport, deposition, and sizeselective erosion. Early studies of grain size primarily focused on establishing links between sedimentary environments and summary statistics of GSDs, such as the mean, standard deviation, and skewness (Shepard, 195; Folk and Ward, 1957; Friedman, 1979). In later studies, the search for environmental fingerprints * Corresponding author. address: qinghe@sklec.ecnu.edu.cn (Q. He). was abandoned, and a more dynamic concept in which spatial patterns of summary statistics were assumed to reflect the net sediment transport pathways was adopted (McLaren, 191; Gao et al., 199; Le Roux and Rojas, 7). Parametric curve-fitting (Visher, 199; Leys et al., 5) has also been a popular method of characterizing GSDs. This technique is based on the assumption that observed (polymodal) GSDs can be regarded as mixtures of specific analytical functions, such as the normal, lognormal, or log-hyperbolic distributions (see Weltje and Prins, 7, for a critical review). Alternative approaches to the analysis of GSDs have been developed based on the principles of compositional data analysis (Aitchison, 19). In this approach, GSDs are regarded as spectra rather than distributions falling into a particular parametric class. These spectra are represented by a series of mass fractions that correspond to discrete size ranges. Traditionally, classification of 7-771/$ see front matter Crown Copyright Ó 9 Published by Elsevier Ltd. All rights reserved. doi:1.11/j.ecss.9..

2 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) gravel-free marine sediments has been based on the proportions of sand, silt, and clay, which lend themselves to visualization in ternary diagrams (Pejrup, 19; Flemming, ). Such compositional representations assume that GSDs are mixtures of a limited number of fixed sediment types, and that as such, they are very similar to parametric curve-fitting exercises. Generalization of the mixture representation of GSDs has been achieved through the use of so-called end-member models that have been employed to statistically unmix compositional data (Weltje, 1997). Endmember modeling has been applied to GSDs of deep-sea cores and aeolian deposits in order to quantify the interplay of provenance and selective deposition (Weltje and Prins, ; Prins et al., 7). Spatial patterns of sea-bottom GSDs are important to sedimentologists and civil engineers because studies of estuarine geomorphology and validation of numerical models of sediment transport are based on such patterns. Moreover, the grain-size characteristics of sea-bottom sediments are of increasing concern to biogeochemical researchers because the nutrients and pollutants carried by fine-grained sediments play a significant role in biogeochemical processes (McKee et al., ). Many studies of the relation between sedimentation processes and GSDs of sea-bottom sediments in the Yangtze Estuary have been conducted (Chen et al., 7). These studies include an evaluation of sea-bottom sediment types in the estuary (Dong and Ding, 19), estuarine sedimentary characteristics (Liu et al., 1995), a particle compositions of sea-bottom sediment (Li et al., 1995), sedimentation on the subaqueous Yangtze Delta (Chen et al., ), and dynamic sedimentation environment and sediment transport patterns in the North Branch of the Yangtze Estuary (Jia et al., 1; Yang and Liu, ). Sea-bottom sediments of the outer estuary are relict, and represent reworked, nearshore deposits of the late- Pleistocene sea-level lowstand (Yan and Xu, 197; Qin et al., 197; Chen et al., ). Previous studies in the Yangtze Estuary have focused on the sediment grain-size distributions and sedimentation processes rather than sediment dynamics. In the present study, the characteristics of sea-bottom sediment grain size were evaluated by log-ratio analysis to gain a better understanding of the sediment dynamics of input, exchange and output areas in the Yangtze Estuary. Ongoing discussions regarding the relation between sediment dynamics and GSDs (Hartmann and Flemming, 7) demonstrate that many problems associated with grain-size analysis and interpretation have not been resolved, despite more than 7 years of sedimentological research. One of the outstanding issues is determination of the exact nature of the exchange between suspended and sea-bottom sediments, which is particularly important for understanding sediment transport and the fate of suspended sediments in estuarine and coastal areas (Hossaina and Eyreb, ; Jiang and Wang, 5; Ren and Packman, 7). In this paper, we present an integrated data set of the GSDs of suspended and sea-bottom sediments to analyze sediment dynamics in the Yangtze Estuary, and apply the principle of mass balance to analyze sediment exchange processes. The results allow us to trace the Yangtze River sediments along the estuary and into the adjacent shelf. In addition, the combination of our results with long-term accumulation rates based on 1 Pb dating allows us to identify the current depocenter of the Yangtze Estuary mud belt.. Study area The Yangtze River is the fourth largest river in the world in terms of water discharge and sediment load (Chen et al., 1; Yang et al., 7). The mean annual total runoff at Datong Hydrological Gauging Station is m /yr (195 5) and the mean annual suspended sediment load from the Yangtze River approaches kg/yr (1951 5). The mean concentration of suspended sediments in the estuary is.1 kg/m (1951 5), and the mean grain size of the suspended sediments is 9 mm (197 5) (CMWR, ). The tidal limit of the Yangtze River is located at Datong, and the tidal flow limit is located at Jiangyin. The modern Yangtze Delta dates back to the mid-holocene epoch. At around 7 yr BP, the rate of post-glacial sea-level rise had decreased sufficiently to initiate progradation of deltaic deposits (Chen and Stanley, 199; Chen et al., ; Wang et al., 5). Hydrographic conditions in the Yangtze Estuary and the adjacent region are governed by a southward current of relatively cold and brackish water known as the Jiangsu Coastal Current in the north and the Zhejiang-Fujian Coastal Current in the south. These currents, which are most active during winter, carry water and sediments from the Yangtze River southward along the inner shelf. In addition, there is a northward flow of warm and saline water on the outer shelf that is known as the Taiwan Warm Current. This current intensifies during summer in response to the prevailing southeast monsoon (Qin et al., 197; Liu et al., ). Large amounts of fine-grained sediments from the Yangtze River Basin are deposited in the estuary and on the subaqueous delta. Indeed, about half of the sediments entering the estuary are deposited in the river-mouth area (Chen et al., 195), while the rest are believed to be carried southward along the coast by littoral currents. The Yangtze Estuary is a mesotidal estuary with a mean tidal range of. m. The estuary consists of complex shoal-channel systems (He et al., 1). The morphology of the Yangtze Estuary is characterized by three bifurcations and four outlets (Fig. 1a). Analysis of sediment accumulation rates based on 1 Pb dating of sediment cores indicates low sedimentation rates in the outer estuary, and high sedimentation rates in the muddy areas of the Yangtze Estuary (Wei et al., 7) (Fig. 1b). To evaluate the vertical sediment exchange in the Yangtze Estuary and the adjacent region, the average GSDs of near-bottom suspended sediments in the areas of input, exchange, and output were calculated. For this study, the input area was defined as the upper reaches of the estuary from Jiangyin to Changxing Island (Fig. 1a). The exchange area was defined as the estuarine mouth from Changxing Island to the m isobath. The output area was defined as the area seaward of the Yangtze Estuary.. Materials and methods Sea-bottom sediment samples (n ¼ 57) were collected from the Yangtze Estuary and the adjacent region between and (Fig. 1a) using a grab (clamshell) sampler. The cm thick surface layer of the bottom samples was used for grain size analysis. Suspended sediment samples (n ¼ ) were collected from the inner estuary in February (over a period including the spring and neap tides; sampling sites CJ1 CJ15; Fig. 1b), and from the outer estuary in July (sampling sites LJ1 LJ; Fig. 1b). The estuarine samples were collected using a horizontal iron water sampler at relative heights of.h,.h,.h,.h,.h and 1.H, where H is the water depth above the bed. The outer estuary samples were collected at relative heights of.h,.5h, and 1.H. For the present study only the suspended sediment samples taken at relative depths of 1.H were used. Preparation was identical for the suspended and sea-bottom sediment samples. Prior to grain-size analysis, organic matter was removed by adding H O (1%). Aggregates were then dispersed by the addition of (NaPO ) and subsequent ultrasonic treatment. Grain-size analyses were carried out on a Coulter LS 1Q grain-size analyzer, which subdivides each sample into 5 size fractions between. mm and 1 mm. Because GSDs represent compositional data which are governed by constant-sum and non-negativity constraints, the raw data

3 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) Fig. 1. Study area and sampling sites of sea-bottom and suspended sediments. (a) Bathymetry and sampling sites of sea-bottom sediments (subdivided into five types). (b) Sampling sites of suspended sediments and sedimentation rate [cm yr 1 ] based on 1 Pb dating of sediment cores. Samples of sites of CJ1 CJ15 were collected in February, while those from sites of LJ1 LJ were collected in July. Sedimentation rate data are from Duan et al. (5) and Liu et al. (). collected in this study are difficult to interpret. However, compositional data may be transformed to log-ratios (Aitchison, 19) to give a representation that is unconstrained. Such transformed data may be subjected to standard methods of statistical analysis because log-ratios can take on any value between C N;þND. In this study, we utilized the log-ratio method to analyze the relationships between the sediment components of C Y, C T, and C S, which represent the clay (Y), silt (T), and sand (S) contents of the sea-bottom sediments. Two log-ratio values were defined: lnfc Y =C T g, which is the log-ratio of clay to silt, and lnfc S =ðc Y þ C T Þg, which is the logratio representation of the sand content. The observed sea-bottom sediment grain size distribution is the sum of the flux of the suspended sediment mass to the seabed in the form of single grains and flocs. The grain diameter at which the flux of mass to the seabed is equal to the single grain and floc depositions is the floc limit (d f ). Thus, sediment grains larger than d f are primarily deposited as single grains, while those smaller than d f are primarily deposited as flocs (Curran et al., ). An integrated analysis of current velocities, salinity, and in-situ floc-size distributions shows that the critical diameter for the flocculation of fine cohesive sediments in the Yangtze Estuary is.5 mm (Tang, 7). Therefore, in this study, we adopted.5 mm as the floc limit (the critical diameter for the flocculation). This allowed us to estimate the mass fractions of flocculated and cohesionless sediments for all measured (disaggregated) GSDs, which were denoted as m f and m c, respectively. In this study, sediment exchange refers to the vertical exchange between the near-bottom suspended and the seabottom sediments and reflects the results of the suspended sediment deposition characteristics and sediment dynamics. A quantitative analysis of the vertical sediment exchange process was conducted by evaluating the constraints imposed by the sediment budget, according to which the sum of the masses of the exchange and output sediments should equal the mass of the input sediments: G I ¼ pg E þ ð1 pþg O (1) where G I, G E, and G O are the GSDs of the input, exchange, and output sediments, respectively. Each GSD is a vector of k elements (mass fractions of grain-size classes) which sum to unity, or 1%. The unknown, p, is a proportion ( p 1) termed the exchange ratio. The least-squares solution to this constrained linear mixing problem is given by:

4 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) 5 p ¼ P k j ¼ 1G Ej G Oj G Ij G Oj P k j ¼ 1 G Ej G Oj () To convert these proportions to rates of sediment exchange between the water column and the bed (V E ), we need independent information regarding the bulk sediment accumulation rates (V B ): V E ¼ pv B () where V E is the rate of sediment exchange, p is the exchange ratio, and V B is the sediment accumulation rate. The sediment accumulation rate data were obtained from Duan et al. (5) and Liu et al. (), and are shown in Fig. 1b.. Results.1. Sea-bottom sediment types and mean grain size Sea-bottom sediments were classified using the method described by Shepard (195) and plotted in a ternary diagram of clay (.5 mm), silt (.5 mm), and sand (.5 mm) contents. Five primary sediment types occur in the Yangtze Estuary and the adjacent region: clayey silt, sand, silty sand, sandy silt, and silt (Fig. ). Clayey silt is the dominant sediment type (% of all 57 sea-bottom sediment samples) and is primarily deposited in the lower reaches of the Yangtze Estuary (Turbidity Maximum Zone), in the muddy area, and on the outer estuary. Sand (%) is present in the upper reaches of the estuary, on the shoals, and in the primary channels of the Turbidity Maximum Zone. The relict deposits are also composed of sand. Silty sand (15%), sandy silt (11%), and silt (%) are present in the estuarine mouth bar area and in the upper North Branch. The spatial distribution of the mean diameter of the sea-bottom sediments (Fig. a) shows a clear coarse fine-coarse trend from the upper estuary to the outer estuary. On average, the diameter of sediments on the sea-bottom in the South Branch vary from 15 mm to mm and displays a general fining towards the mouth of the estuary. The mean grain size in the North Branch is finer than that of the South Branch, with values in the range of 5 1 mm in the upper reaches, and 15 mm in the lower reaches. Fig.. Ternary diagram of sand/silt/clay proportions of 57 samples taken from the study area (sampling sites referring to Fig.1a). (Y, clay; T, silt; S, sand; TY, silty clay; SY, sandy clay; YT, clayey silt; ST, sandy silt; YS, clayey sand; TS, silty sand; S T Y, sand silt clay). The mean diameter of sea-bottom sediment in the mouth bar area of the estuary is more variable and reflects the feedback between morphology and sediment exchange. The coarsest seabottom sediments in the three main outlets are present in the North Channel, while the finest are present in the South Passage (Fig. a). In the southeastern portion of the estuary, sea-bottom sediments are fine, with a mean diameter of less than mm. The mean diameter of the outer continental shelf deposits ranges from 15 mm tomm (Fig. a). The clay component is transported out of the estuary and deposited in the outer estuary (Fig. b). The silt component is transported eastward and southward through the estuary and deposited in the South Passage and further southward along the coast (Fig. c). The sand component is primarily deposited in the upper reaches of the estuary (Fig. d)... Log-ratio analysis of sea-bottom sediment grain size To examine the relations between the three sediment components, the data were plotted using two different methods (Fig. ). Fig. a and b provides a straightforward visualization of the relations between the three sediment components, which appear to vary with sand content. Three domains can be distinguished: (1) Segment 1: C S < 1%; C Y and C T are constant (approximately % and 7%, respectively). () Segment : 1%< C S < %; C T is constant (approximately 7%); C Y is negatively correlated with C S and decreases to about %. () Segment : C S > %; C Y and C T are negatively correlated with C S and decrease to %. The log-ratio plot, which is shown in Fig. c, provides an alternative to Fig. a and b, as well as to the conventional ternary representation shown in Fig.. Of particular interest is the relationship between sand content and the clay/silt ratio in segments and, which changes from a negative correlation to a positive correlation. The sediments in segment 1 are present in the exchange and output areas, which are characterized by relatively fine sediments. The sediments in segment are dominant in the exchange and output areas, which are characterized by segregation of fine and coarse sediments. The sediments in segment are present in the input and exchange areas, which are characterized by relatively coarse sediments. The variation among the different segments suggests that non-linear sediment mixing is reflected in the spatial changes of sea-bottom sediments from the upper estuary (input area) via the mouth area (exchange area), to the outer estuary (output area). To evaluate the mode of deposition of the sea-bottom sediments, the relationship between the log-ratio of the flocculated fraction to the cohesionless fraction, lnfm f =m c g, and the mean diameter, Md, were analyzed. A clear linear trend was observed between lnfm f =m c g and Md (Fig. 5). Relatively fine-grained sediments, which are described by lnfm f =m c g >, display a nearly perfect correlation between Md and lnfm f =m c g. These findings indicate simple linear mixing of two sediment fractions with fixed characteristics. Relatively coarse-grained sediments, which are defined by lnfm f =m c g <, display an increased scatter, which implies size sorting within the cohesionless sediment population. Points that fall near the line in Fig. 5 represent mixtures of the flocculated sediment population and relatively coarse cohesionless sediments. The sea-bottom sediments in the input area are primarily deposited in the form of single grains, which are mainly deposited as flocs in the output area (Fig. 5). The sediments in the

5 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) Fig.. Spatial distribution of the mean diameter (a), contents of clay (b), silt (c) and sand (d) in sea-bottom sediments. exchange area are deposited in the form of single grains as well as flocs. The spatial distribution of lnfm f =m c g in the study area (Fig. ) demonstrates that the sediments in the upper reaches of the estuary and the mouth bar of the North Channel are primarily deposited in the form of single grains, as implied by the negative value of lnfm f =m c g. The sea-bottom sediments in the South and North Passages, as well as the southeast portion of the estuary are primarily deposited as flocs, as demonstrated by the positive values of lnfm f =m c g... Sediment exchange characteristics Sediments supplied to the estuary play an important role in the evolution of estuarine morphology and nearshore biogeochemical processes. The vertical exchange between the near-bottom suspended and sea-bottom sediments in the estuary involves repeated cycles of flocculation, deposition, and resuspension and governs the spatial segregation of sediments according to grain size. The sediments that remain in the estuary contribute to the evolution of estuarine morphological structures, such as channel-bank systems, shoals, mouth bars, and nearshore subaqueous deltas. The sediments that escape from the estuary are transported to the outer estuary and along the coast. The GSDs of near-bottom suspended and sea-bottom sediments were analyzed to examine the vertical sediment exchange processes in one estuarine outlet from the South Branch to the North Channel. Great differences were observed in the GSDs of the suspended and sea-bottom sediments in the input and output areas, which indicate that few suspended sediments were deposited onto the seabed (Fig. 7). However, the GSDs of the sea-bottom sediments in the exchange area are similar to those of the suspended sediments due to the intensive vertical sediment exchange. As a result, quasi-bimodal patterns are present in some GSDs of seabottom sediments in this area, and the finer peak is the result of sediment exchange (Fig. 7). The average GSDs of near-bottom suspended sediments clearly illustrate the exchange process in the exchange area (Fig. ). The distribution of the suspended sediments reveals a fine coarse-fine trend in the Yangtze Estuary that is converse to the trend of the seabottom sediments. On average, the suspended sediments in the exchange area are coarser than those of the input or output areas. Figs. 7 and describe the variation in the suspended sediment GSDs from the sediment transport processes in the estuary. The results of this study indicate that the vertical sediment exchange in the mouth bar area (the exchange area) occurs more frequently than in the upper estuary (the input area) or the outer estuary (the output area). Substitution of the GSDs of sea-bottom sediments, as G E, into Eq. (1) gives the local exchange ratio of sea-bottom and suspended sediments. The average suspended sediment GSDs of the input and output areas, represented by G I and G O, are shown in Fig.. This calculation was conducted for all 57 GSDs (Fig. 9a). High values of p indicate that the suspended sediments are deposited on the local seabed and contribute to the estuarine morphology, whereas low values indicate that the suspended sediments are not deposited on the local seabed, but are transported further seaward. The highest sediment exchange ratio, which was observed in the muddy area of the Yangtze Estuary, was approximately.. This finding indicates that the exchange between suspended and sea-bottom sediments occurs frequently in this area. The exchange ratio of the upper estuary and the outer estuary was found to be lower than.1, which indicates that only small amounts of suspended sediments are deposited onto the seabed (Fig. 9a). The data describing the sedimentation rate are shown Fig. 1b. These data were derived from 1 Pb dating of sediment cores and then converted to exchange rates using Eq. (). The resulting plot is shown in Fig. 9b. The low V E values indicate that the depositional rate of the suspended sediments is relatively low and that most suspended sediments are transported out of the estuary. The high V E values indicate that most suspended sediments are deposited onto the seabed of the muddy area (the distal part of the South Passage).

6 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) 7 5 Input Exchange Output 1 C Y ( ) C T ( ) ln (C Y /C T ) C S ( ) a b C S ( ) c ln{c S /(C Y +C T )} Fig.. Relationship between the contents of sand (C S ), clay (C Y ), and silt (C T ) in seabottom sediments. (a) Conventional display of C Y to C S ; (b) conventional display of C T to C S ; (c) log-ratio display (Input, exchange and output legend see Fig.1; 1,, in the figure are bottom grain-size segments, discussed in text). Md (Φ) Input Exchange Output ln {m f /m c } Fig. 5. Relationship between the mean diameter [F] and lnfm f =m cg of sea-bottom sediments (Input, exchange and output legend see Fig. 1). 5. Discussion Fig.. Spatial distribution of lnfm f =m cg of sea-bottom sediments Interpretation of sea-bottom grain size characteristics The South Branch is currently the most active channel of the Yangtze Estuary (Fig. 1). The majority of the sea-bottom sediments in the South Branch are relatively coarse, and represent bed load derived from the Yangtze River Basin. Due to selective deposition, the fine particle sediments are transported into the mouth bar area and the outer estuary, while the coarse sediments are deposited in the upper reaches of the estuary (Fig. ). Consequently, the mean diameter of the sea-bottom sediments is larger than that of other channels (Fig. ). Prior to the 1th Century, the North Branch was the major channel of the Yangtze Estuary. However, flow has gradually decreased over the past years. By the early th Century, the water discharge from the North Branch had been reduced to approximately 5% of the total discharge of the estuary, and by the end of the 195s less than % of the total discharge passed through the North Branch (Chen et al., 19). There is also a net influx of salt water into the North Branch, and the sea-bottom sediments primarily originate from the deposition of fine-grained suspended sediments. Moreover, the distribution of sea-bottom GSDs in the upper reaches of the North Branch is affected by the tidal bore (Chen et al., ). Therefore, the mean diameter of the sea-bottom sediments in the upper reaches is larger than that of the lower reaches (Fig. ). The GSD pattern in the mouth bar area is more complex. This is because the vertical sediment exchange between the suspended sediments and the sea-bottom sediments occurs more frequently than in other regions (Fig. ). Due to the Coriolis effects, the majority of fine-grained sediments are transported to the outer estuary through the South Passage (Chen et al., 19), where the grain size of sea-bottom sediments is relatively low. The discharge ratio of the North Channel is much larger than that of the South or North Passages (Chen et al., 19). Furthermore, the current velocities of the North Channel are much larger than those of other channels (Liu et al., 7). Consequently, the grain size of the seabottom sediments is largest in the North Channel. The southern portion of the estuary is one of the primary depocenters of fine-grained suspended sediments escaping from the Yangtze Estuary, and the sea-bottom sediments in that area are relatively fine (Fig. ). 5.. From grain size to sediment dynamics A nearly constant ratio between clay and silt has been reported for various tidal systems worldwide. Ternary diagrams presented by Flemming () show a constant clay/silt ratio in the following

7 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) Frequency/ d Frequency/ d Frequency/ d 1 1 CJ1 Nearly bottom 1 1 CJ1 1 suspended sediment Sea-bottom sediment CJ LJ Grain diameter (µm) Frequency/ d Frequency/ d Frequency/ d CJ1 LJ Grain diameter (µm) Fig. 7. Comparison of the grain size distribution of near-bottom suspended and sea-bottom sediments in the Yangtze Estuary. Sampling names are shown in the top left corner. See Fig. 1b for the sampling sites. Samples from CJ1 and CJ1 were collected from the in the input area, which those from CJ11, CJ1 and LJ1 were collected from the exchange area, and LJ was collected from the output area. five intertidal environments: the macrotidal flats in Jiangsu Province, China; the Wadden Sea in Denmark; the Dyfi Estuary in Wales; the Minas Basin in the Bay of Fundy, Canada; and the Mugu Lagoon in the USA. The phenomenon also occurs in two open-shelf environments: the Bering Shelf and the Central Gulf of the Alaska Shelf. An analysis conducted by Van Ledden () also shows that clay/silt ratios in various tidal basins of the North Sea (the Wadden Sea, the Ems-Dollard Estuary, and the Western Scheldt) vary within a narrow range, between.1 and.5. To determine if this principle also applies to the Yangtze Estuary system, clay/silt ratios were analyzed in the estuary and the adjacent region. The results revealed that the ratio between silt and clay in the Yangtze Estuary is not constant, but varies systematically across the estuary (Fig. c). The clay/silt ratio in the Yangtze Estuary Frequency/ d 1 Input Exchange Output Grain Diameter (µm) Fig.. Average grain-size distributions of near-bottom suspended sediments in Yangtze Estuary (Input, exchange and output legend see Fig. 1). Fig. 9. Spatial distribution of the vertical sediment exchange ratio p (a) and sediment exchange rate [cm yr 1 ] (b).

8 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) 9 and the adjacent region varies from.11 to., which is a much larger range than has been reported from other areas (Flemming, ; Van Ledden, ). This increased range likely occurs due to the non-linear sand mud mixing under complex hydrodynamic conditions. Apparently, the interplay of erosion, deposition, and mixing in the bed itself produces non-linear effects that are more pronounced in the Yangtze Estuary system than in other areas, in which constant silt/clay ratios seem to be a reasonable assumption. A possible explanation for our results is provided by a study conducted by Mitchener and Torfs (199), which revealed that adding sand to mud or vice versa increased the erosion resistance of the resulting sediment mixture and led to reduced erosion rates when the critical shear stress for erosion was exceeded. The results of that study also demonstrated that the most significant effect on erosion resistance occurred when small percentages of mud were added to sand. Although non-linear sediment mixing appears to be common in the Yangtze Estuary fine-grained sediments ðlnfm f =m c g> Þ display a simple linear mixing of the flocculated and cohesionless fractions, which are primarily deposited as flocs (Fig. 5). The coarsegrained sediments ðlnfm f =m c g> Þ display a selective deposition of the cohesionless sediment fraction, which is primarily deposited as single grains. High rates of floc deposition are recorded in the outer estuary between 1 E and 1 E along the coast (Fig. ). The finegrained sediment belt along the south coast indicates the deposition of southward-transported suspended sediments from the Yangtze Estuary in the form of flocs. 5.. Integration of exchange ratios and accumulation rates The GSDs of the suspended and sea-bottom sediments visually illustrate the vertical sediment exchange processes in the different sections of the Yangtze Estuary (Fig. 7). The bulk suspended sediment exchange ratio in the Yangtze Estuary can be calculated if the average GSD of suspended sediments at the exchange area (Fig. )is substituted into Eq. (1) as G E. The results of this calculation gives an exchange ratio of p ¼ 9.%. This means that 9.% of the suspended sediments supplied from the river basin are deposited in the Turbidity Maximum Zone and contribute to the morphological evolution of the estuary. These results are similar to those of studies in Yangtze Estuary conducted by Milliman et al. (195), Shen (1), and Liu et al. (), which provided estimates of %, %, and 7%, respectively. Hence, more than 5% of the suspended sediments escape from the estuary and are transported southward to be deposited in the outer estuary. More detailed information regarding the sediment exchange in the study area may be obtained by analysis of the spatial variation of sediment exchange ratios. Small amounts of suspended sediments were deposited onto the seabed of the upper estuary (sediment exchange ratio p <.1) because the fine-grained suspended sediments in this region were transported to the mouth bar area by the ebb-dominated tide flow (Fig. 9a). The sediment exchange ratios in the outer estuary also show very low values (p <.1) because the diffusion of suspended sediments in the outer estuary was blocked by the oceanic currents offshore (the southward Jiangsu Coastal Current and the northward Taiwan Warm Current). However, intensive sediment exchange occurred in the inner estuary due to sand mud mixing, which was controlled by the bidirectional tidal flow (Fig. 9a). The most frequent sediment exchange in the muddy area, which was evidenced by an exchange ratio of about., was observed at the depocenter of the suspended sediment. The results also show that the southward transport pathway of suspended sediments in the Yangtze Estuary is located between 1 E and 1 E longitude along the coast (Fig. 9a). This coast-parallel belt of high sediment exchange ratios is consistent with the pattern of longshore currents. The sediment exchange ratio, p, is a relative measure that describes the local balance between suspended and sea-bottom sediments, which can be transformed to the sediment exchange rate (V E ) using sedimentation rate. The low value of V E in the upper estuary indicates that a small amount of suspended sediments are deposited in this area (Fig. 9b). Conversely, the high sediment exchange rate in the muddy area with a value of greater than cm/ yr implies that this area is the depocenter of the Yangtze mud. Beyond the depocenter, the sediments are transported southward along the coast. These findings provide information regarding the transport pathway and the fate of suspended sediments in the Yangtze Estuary. On the decadal to centennial time scale associated with our results, a clearly defined depocenter of Yangtze mud is present in the south of the estuary, between 1 E and 1 E longitude, and around 1 N latitude (Fig. 9b).. Conclusion The near-bottom sediment exchange in the Yangtze Estuary was investigated through analysis of the GSDs of the sea-bottom and near-sea-bottom suspended sediments. In addition to the conventional approach to grain-size analysis, which involves classification and the calculation of summary statistics such as Md, we applied concepts from compositional data analysis to delineate specific patterns in the data. The relations between sand, silt, and clay content, as determined by log-ratio analysis, indicates that selective deposition is a non-linear function of the sediment mixture composition. In addition, the nearly constant clay/silt ratios that have been reported for many other tidal basins do not apply to the Yangtze Estuary due to the non-linear sand mud mixing which is governed by the complex hydrodynamic conditions in the system. The floc limit (d f, estimated as.5 mm) was used to distinguish the mass fractions of flocculated (m f ) and cohesionless (m c ) sediments. The spatial distributions of lnfm f =m c g showed that the sediments in the upper reaches of the estuary are primarily deposited in the form of single grains, while the sea-bottom sediments in the South and North Passages and the outer of the estuary are mainly deposited as flocs. Subsequent analyses were conducted in an attempt to quantify the sediment exchange processes in the estuary and the adjacent region based on the principle of mass balance. Based on the average GSDs of the suspended sediments, approximately 9% of the sediments from the Yangtze River accumulate in the inner estuary, while the rest are transferred to the outer estuary. The spatial distribution of the sediment exchange ratios demonstrated that small amounts of suspended sediment are deposited onto the seabed of the upper estuary (exchange ratio <.1), because the fine-grained suspended sediments in this region are transported to the mouth bar area by the ebb-dominated tidal flow. The sediment exchange ratios in the outer estuary also show very low values (p <.1) due to the oceanic currents offshore that prevent the diffusion of riverine sediments further seaward. However, intensive sediment exchange occurs in the inner estuary due to the sand mud mixing, which is controlled by the bidirectional tidal flows. In addition, the high sediment exchange ratio that occurs in the muddy area (>.) at the river mouth implies that this area is the depocenter of the Yangtze mud. Conversion of the dimensionless exchange ratio, p, to the exchange rates of the fine-grained sediments was achieved by considering the bulk sediment accumulation rates. On the decadal to centennial time scale associated with our results, a clearly defined depocenter of Yangtze mud with the exchange rates of

9 H. Liu et al. / Estuarine, Coastal and Shelf Science (1) greater than cm/yr is present in the south of the estuary. This depocenter, which extends to the south of the estuary, is located between 1 E and 1 E longitude, and around 1 N latitude. Our study of sediment exchange rates provides a clear picture of suspended sediment transport pathways in the Yangtze Estuary and the adjacent region, and allows us to trace the fate of suspended sediment supplied by the Yangtze River. Acknowledgements We thank the field work group of the 97 Program for their contribution. This study was funded by the funds for Creative Research Groups of China (Grant No. 71), the 97 Program (Grant No. DFB9), the Shanghai Science and Technology Foundation (Grant No. 7DJ1-1), and the PhD Program Scholarship Fund of ECNU (Grant No. 9). The authors also thank Prof. Zhongyuan Chen and Dr. Maotian Li for their constructive comments, which substantially improved the original manuscript. References Aitchison, J., 19. The Statistical Analysis of Compositional Data. Chapman and Hall, London, 1 pp. Chen, J.Y., Shen, H.T., Yun, C.X., 19. Introduction. In: Chen, J.Y., Shen, H.T., Yun, C.X. (Eds.), Processes of Dynamics and Geomorphology of the Changjiang Estuary. Shanghai Scientific and Technical Publishers, Shanghai, pp. 1 (in Chinese). Chen, J.Y., Zhu, H.F., Dong, Y.F., Sun, J.M., 195. Development of the Changjiang Estuary and its submerged delta. Continental Shelf Research, 7 5. Chen, Z., Stanley, D.J., 199. Yangtze delta, eastern China:. Late Quaternary subsidence and deformation. Marine Geology 11, 1 1. Chen, Z., Song, B., Wang, Z., Cai, Y.,. Late Quaternary evolution of the subaqueous Yangtze Delta, China: sedimentation, stratigraphy, palynology, and deformation. Marine Geology 1, 1. Chen, Z., Yu, L., Gupta, A., 1. The Yangtze River: an introduction. Geomorphology 1, Chen, Z., Watanabe, M., Wolanski, E., 7. Sedimentological and ecohydrological processes of Asian deltas: The Yangtze and the Mekong. Estuarine, Coastal and Shelf Science 71, 1. Chen, S.L., Gu, G.C., Liu, Y.S.,. Formation conditions and initial site of tidal bore in the North Branch of Yangtze River estuary. Journal of Hydraulic Engineering 11, (in Chinese with English abstract). CMWR (China Ministry of Water Resources),. Chinese River Sediment Bulletin. China Waterpower Press, Beijing, pp. (in Chinese). Curran, K.J., Hill, P.S., Schell, T.M., Milligan, T.G., Piper, D.J.W.,. Inferring the mass fraction of floc-deposited mud: application to fine-grained turbidites. Sedimentology 51, Dong, Y.F., Ding, W.J., 19. Relationship between the grain size characteristics and hydrodynamics of sedimentation in the Changjiang Estuary. In: Chen, J.Y., Shen, H.T., Yun, C.X. (Eds.), Processes of Dynamics and Geomorphology of the Changjiang Estuary. Shanghai Scientific and Technical Publishers, Shanghai, pp. 1 (in Chinese). Duan, L.Y., Wang, Z.H., Li, M.T., Pan, J.M., 5. 1 Pb distribution of the Changjiang Estuarine sediment and the implications to sedimentary environment. Acta Sedimentologica Sinica (), Flemming, B.W.,. A revised textural classification of gravel-free muddy sediments on the basis of ternary diagrams. Continental Shelf Research, Folk, R.L., Ward, W.C., Brazos River bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology 7,. Friedman, G.M., Differences in size distribution of populations of particles among sands of various origins. Sedimentology,. Gao, S., Collins, M.B., Lanckneus, J., De Moor, G., Van Lancker, V., 199. Grain size trends associated with net sediment transport patterns: an example from the Belgian continental shelf. Marine Geology 11, Hartmann, D., Flemming, B.W., 7. From particle size to sediment dynamics: an introduction. Sedimentary Geology,. He, Q., Li, J.F., Li, Y., 1. Field measurement of bottom boundary layer processes and sediment resuspension in Changjiang Estuary. Science in China (Series B),. Hossaina, S., Eyreb, B.,. Suspended sediment exchange through the subtropical Richmond River estuary, Australia: a balance approach. Estuarine, Coastal and Shelf Science 55, Jia, H.L., Liu, C.Z., Yang, O., 1. Dynamic sediment environment of the North Branch of Changjiang Estuary. Journal of East China Normal University (Natural Science) 1, 9 9 (in Chinese with English abstract). Jiang, W.S., Wang, H.J., 5. Distribution of suspended matter and its relationship with bed sediment particle size in Laizhou bay. Oceanologia et Limnologia Sinica, 97 1 (in Chinese with English abstract). Le Roux, J.P., Rojas, E.M., 7. Sediment transport patterns determined from grain size parameters: overview and state of the art. Sedimentary Geology, 7. Leys, J., McTainsh, G., Koen, T., Mooney, B., Strong, C., 5. Testing a statistical curvefitting procedure for quantifying sediment populations within multi-modal particle-size distributions. Earth Surface Processes and Landforms (5), Li, J.F., Shen, H.T., Xu, H.G., The bedload movement in the Changjiang River Estuary. Oceanologia Et Limnologia Sinica, 1 15 (in Chinese with English abstract). Liu, C.Z., Wu, L.C., Hua, D., The sediment structure, structural characteristics and sediment effect mechanism of Changjiang River submerged deltaselections of the Studies of the Turbidity Maximum and Estuarine Front in Changjiang Estuary. Journal of East China Normal University, (in Chinese with English abstract). Liu, J.P., Li, A.C., Xu, K.H., Velozzi, D.M., Yang, Z.S., Milliman, J.D., DeMaster, D.J.,. Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea. Continental Shelf Research, Liu, H., He, Q., Meng, Y., Wang, Y.Y., Tang, J.H., 7. Characteristics of surface sediment distribution and its hydrodynamic responses in Changjiang Estuary. Acta Geographica Sinica (1), 1 9 (in Chinese with English abstract). McKee, B.A., Aller, R.C., Allison, M.A., Bianchi, T.S., Kineke, G.C.,. Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes. Continental Shelf Research, McLaren, P., 191. An interpretation of trends in grain-size measurements. Journal of Sedimentary Petrology 51, 11. Milliman, J.D., Shen, H.T., Yang, Z.S., Meade, R.H., 195. Transport and deposition of river sediment in the Changjiang Estuary and adjacent continental shelf. Continental Shelf Research (1/), 7 5. Mitchener, H., Torfs, H., 199. Erosion of mud/sand mixtures. Coastal Engineering 9, 1 5. Pejrup, M., 19. The triangular diagram used for classification of estuarine sediments: a new approach. In: de Boer, P.L., van Gelder, A., Nio, S.D. (Eds.), Tide-Influenced Sedimentary Environments and Facies. Reidel, Dordrecht, pp. 9. Prins, M.A., Vriend, M., Nugteren, G., Vandenberghe, J., Lu, H., Zheng, H., Weltje, G.J., 7. Late Quaternary aeolian dust flux variability on the Chinese Loess Plateau: inferences from unmixing of loess grain-size records. Quaternary Science Reviews, 5. Qin, Y.S., Zhao, Y.Y., Chen, L.R., Zhao, S.L., 197. Geology of the East China Sea. China Science Press, Beijing, 9 pp. Ren, J., Packman, A.I., 7. Changes in fine sediment size distributions due to interactions with streambed sediments. Sedimentary Geology, Shen, H.T., 1. Material Flux of the Changjiang Estuary. China Ocean Press, Beijing, pp. 17(in Chinese). Shepard, F.P., 195. Nomenclature based on sand silt clay ratios. Journal of Sedimentary Petrology, Tang, J.H., 7. Characteristics of fine cohesive sediment flocculation in the Changjiang Estuary and adjacent sea area. Master s Thesis of East China Normal University, 19 pp. Van Ledden, M.,. Sand mud segregation in estuaries and tidal basins. Ph.D. Thesis, Delft University of Technology, Delft, Netherlands, 17 pp. Visher, G.S., 199. Grain size distributions and depositional processes. Journal of Sedimentary Petrology 9, Wang, Z., Saito, Y., Hori, K., Kitamura, A., Chen, Z., 5. Yangtze offshore, China: highly laminated sediments from the transition zone between subaqueous delta and the continental shelf. Estuarine, Coastal and Shelf Science, Wei, T.Y., Chen, Z.Y., Duan, L.Y., Gu, J.W., Saito, Y., Zhang, E.G., Wang, Y.H., Kanai, Y., 7. Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, China. Estuarine, Coastal and Shelf Science 71, 7. Weltje, G.J., End-member modeling of compositional data: numerical statistical algorithms for solving the explicit mixing problem. Journal of Mathematical Geology 9, Weltje, G.J., Prins, M.A.,. Muddled or mixed? Inferring palaeoclimate from size distributions of deep-sea clastics. Sedimentary Geology 1, 9. Weltje, G.J., Prins, M.A., 7. Genetically meaningful decomposition of grain-size distributions. Sedimentary Geology, 9. Yan, Q.S., Xu, S.Y., 197. Recent Yangtze Delta Deposits. East China Normal University Press, Shanghai, pp. (in Chinese with English abstract). Yang, O., Liu, C.Z.,. Analysis of sediment transport patterns and sediment sources of the north branch of the Changjiang estuary. Shuili Xuebao, 79 (in Chinese with English abstract). Yang, S.L., Zhang, J., Dai, S.B., Li, M., Xu, X.J., 7. Effect of deposition and erosion within the main river channel and large lakes on sediment delivery to the estuary of the Yangtze River. Journal of Geophysical Research 11, F5. doi:1.19/jf.