Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

YUAN Ping, WANG Houjie, 2), *, WU Xiao, 2), and BI Naishuang, 2)

Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

YUAN Ping1), WANG Houjie1), 2), *, WU Xiao1), 2), and BI Naishuang1), 2)

1),,,266100,2),,266237,

The grain-size distribution of surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments (mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion; whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Bohai Sea and North Yellow Sea; surface sediment; grain size; sedimentation rate; sediment supply; sedimentary dynamic environment

1 Introduction

Coastal ocean plays an important role in global material cycling (Holligan and Boois, 1993). The processes of sediment transport in the coastal ocean dominated by both riverine and ocean dynamics (., river discharge, the tidal and wave dynamics) have been widely recognized, since the distribution of sediment size has profound impacts on physical, chemical and biological processes (De-Master., 1986; Kuehl., 1996; Saito., 2001; Hu., 2009; Lin., 2011). Understanding the process of se- diment transport in the coastal ocean is crucial in interpreting the marine environmental change over geological time, elucidating the impacts on coastal morphology and ecosystem, and revealing significant land-ocean exchange of materials/energy with anthropogenic impacts.

The Bohai Sea (BS) and its adjacent northern Yellow Sea (NYS; nearby the Bohai Strait) are distinctly charac- terized by high turbidity since the Yellow River and some median to small-sized rivers (., Haihe River, Luanhe River, Liaohe River, Liugu River) deliver a large amount of terrestrial sediment to the coastal ocean (Milliman and Meade, 1983; Saito., 2001; Qiao., 2010; Wang., 2014).

Moreover, the hydrodynamic environment in the BS and NYS has significant seasonal variability as modulated by the monsoonal climate, which causes active re-suspension and transport of the fine-grained sediment in winter season and thus will have a major impact on the regional sediment size distribution, resulting in significant seasonal variation of the sediment transport (Li., 2010; Bi., 2011; Yang., 2011; Wang., 2014; Zeng., 2015). Therefore, the distribution and dispersal mechanism of the terrestrial sediment in the study area are complicated and the Bohai Sea and its adjacent Bohai Strait are believed to be the key areas for the study of sediment transport process.

The grain-size characteristics of surface sediment are indicative to the processes of sediment transport, deposition and redistribution, and thus contain considerable information about sediment provenance, sea level change, dynamic environment and transport pathway. The modern sedimentation rate is believed to be indicative to the se- dimentary environment (Hu, 1984; Alexander., 1991; Liu., 2004; Qi., 2004; Hu, 2010; Li., 2012). In recent years, investigations have been conducted in the BS and NYS to examine the size distribution of surface sediments and its relationship with sediment supply and hydrodynamic environment (Cheng., 2004; Qiao.,2010; Pang., 2016), magnetic properties (Wang., 2017b), geochemical compositions and clay minerals cha- racteristics (Liu., 2007; Wang., 2010; Huang., 2014; Li., 2014). However, most of previous studies based on sediment grain-size analysis only focused on some limited parts of the Bohai Sea (., the Liaodong Bay, the Yellow River Delta) or only at the Bohai Strait. The correlation between grain-size distribution of surface sediment and modern sedimentation rate still remains poorly understood at regional scale, and there lacks an overall picture of sediment distribution patterns over the whole BS and NYS.

In this paper, we analyzed the grain-size distribution characteristics of surface sediment in the whole BS and NYS, and examined the relationships with sediment supply and hydrodynamic environment. In addition, the cor-relation between grain-size distribution of surface sediment and modern sedimentation rate was discussed, and the impact of sediment transport pattern on the distribution of surface sediment was also analyzed. The work is thus of significance to understand the source-to-sink pro- cess and the sedimentary dynamic mechanism.

2 Study Area

The Bohai Sea is a shallow semi-enclosed epicontinental sea. It can be geographically divided into four parts such as the Bohai Bay, the Laizhou Bay, the Liaodong Bay and the central Bohai Basin (Fig.1). The area of the Bohai Sea is 77000km2and the average water depth is 18m (Qin and Li, 1983). The Bohai Strait between the Liao- dong and Shandong Peninsulas connects the Bohai Sea and the Yellow Sea (Fig.1). It is about 143km in width with water depth ranging from 20 to 86m, and it is separated into several channels by Miaodao Islands. The nor- thern channel (Laotieshan Channel) is the main channel which has a maximum depth of 86m. The Bohai Strait is an important transport pathway for the water and sediment exchange between the Bohai Sea and Yellow Sea.

Fig.1 Study area and general pattern of seasonal circulation systems in the BS and NYS (modified after Guan, 1994; Fang et al., 2000).

Many rivers (., the Yellow River, the Luanhe River, the Liaohe River, the Haihe River, the Liugu River and the Fuzhou River) (see Fig.1), delivered a large amount of terrestrial sediments to the coastal ocean, among which the Yellow River has the highest water discharge and sediment load to the sea and is thus a dominant source for the sediments in the BS (Qiao., 2010; Wang., 2011 and 2014). In addition, the majority of Yellow River- derived sediments appears to be temporarily trapped in the modern nearshore deltaic system, whereas the other fraction of fine-grained sediment was transported offshore over long distance and eventually settled in the central Yellow Sea to contribute to the distal shelf muddy deposit since the middle Holocene (., Qin and Li, 1983; Ale- xander., 1991; Martin., 1993; Bi., 2011).

Wind waves play an important role in the sediment transport in the coastal area. Wind waves are highly energetic in the shallow marine environment with a typical significant wave height varying from 0.3 to 0.7m in the nearshore of the BS to about 1.0m in the Bohai Strait and central basin (Jiang., 2000).

Mean tidal currents in the study area vary from 0.2 (at the mouth of Laizhou Bay) to 0.8ms−1(in the northern Bohai Strait and eastern part of Liaodong Bay) and are dominated by mixed semi-diurnal tide with two counter- clockwise semi-diurnal amphidromic systems (M2and S2) (Huang., 1999).

For long-term sediment transport processes in coastal sea such as the BS, and the NYS, the tidal residual currents are of importance to transport the fine-grained sediments (Zhao., 1995; Delhez, 1996; Wei., 2004). As shown in Fig.1, the general circulation in the study area consists of an inflow (consisting of the Yellow Sea Warm Current and the Liaonan Coastal Current) through the northern Bohai Strait and an outflow (the North Shandong Coastal Current) through the southern part in winter and summer (Guan, 1994). The North Shandong Coastal Current originates from the west coast of Bohai Bay and flows along the coast of the Huanghe Delta and the Lai- zhou Bay in winter and summer, while the Liaodong Gyre, located in the northern part of Liaodong Bay, rotates anticyclonically in winter and cyclonically in summer (Guan, 1994; Fang., 2000).

3 Materials and Methods

3.1 Sampling of Surface Sediment

In this study, surface sediment samples at totally 360 stations and 102 short cores with measurement of210Pb activities were systematically collected from the BS and NYS, with datasets covering the entire study area (Fig.2). The data of modern sedimentation rate (cmyr−1) acquired at 102 stations (Fig.2B), based on the radioisotope (210Pb) measurements, were collected from the previous documents which generally depicted the spatial variability of sedimentation rates in the BS and NYS. Here we regarded the sedimentation rate of the topmost layer in the collected cores as the modern sedimentation rate.

During the cruises the surface sediment samples (0–3cm) were wrapped in labelled plastic sealing pockets and properly kept, until they were processed for grain-size analysis in laboratory. Both the grain-size data in this study and those in references were measured according to the analytical methods mentioned below.

3.2 Analysis of Sediment Grain Size

Since sand, silt and clay are dominant classes in the surface sediments in the study area, the grain-size compositions of sediment samples were determined by a Mastersizer 2000 laser particle size analyzer (Malvern Ins- truments Ltd., UK; the measurement range is 0.02–2000μm) in laboratory. Sediment samples were pretreated to decompose the organic matters and remove carbonates with 30% H2O2and 1molL−1HCl, respectively. Then the samples were dispersed and homogenized by an ultrasonic vibrator for 30s before passing through the particle size analyzer.

Here we described the size of sedimentary particles on the basis of Wentworth’s (1922) grain-size scale where sand is 2000μm (−1Φ) to 62.5μm (4Φ); silt is 62.5μm (4Φ) to 3.91μm (8Φ), and clay is <3.91μm (> 8Φ). And the classification and nomenclature of sediment types followed the Shepard scheme (Shepard, 1954).

Grain-size parameters,., mean grain size () in Φ scale, sorting coefficient (), skewness () and kurtosis (), were calculated with the statistic moment method (McManus, 1988) as following equations:

whererepresents theth size class,Pis the percentage of sizeS(in Φ units), andis the total number of size classes. The grading standard proposed by Jia. (2002) was then applied to qualitatively describe the grain size parameters derived from moment method.

3.3 Grain Size Trend Analysis

The two-dimensional model proposed by Gao and Col- lins (1991, 1992) is used to describe net sediment transport patterns. The model could be performed as the following steps:

The first step is to define trend vectors for each sampling station by comparing the grain size parameters (,and) of each sample with its neighbors in any direction within a characteristic distance (Der, representing the maximum distance between sampling stations). The dimen- sionless trend vector, with unit length, point to the direction of the net sediment transport which is defined toward the station with better sorted surface sediment, whether finer and more negatively skewed or coarser and more positively skewed.

Secondly, the trend vectors are summed to produce a single vector at each sampling station, maybe more than one unit length. Finally, a filtering procedure is performed to remove the background noise by averaging the resultant vectors of each sampling station and its surrounding neighbors.

It should be noted that the direction of the vector indicates the possible sediment transport direction, while the magnitude of the vector indicates the significance of grain size trend rather than the sediment transport rate (Gao and Collins, 1998). Moreover, the direction of the maximum gradient in grain-size characteristics does not necessarily coincide with the main sediment transport direction (Assel- man, 1999).

4 Results

4.1 Grain-Size Distribution of Surface Sediment

4.1.1 Classification of surface sediment

The percentage contents of sand, silt and clay in the surface sediment from the BS and NYS are shown in Fig.3. The surface sediment in the study area was clearly domi- nated by silt fractions with mean percentage content of 45.22%. The mean contents of sand and clay were 36.19% and 18.33%, respectively. It seems that the distribution of sand content is exactly opposite to the distribution of silt. In the areas with high sand content (., the river mouth of Daqing River, the Liugu River Estuary to Fuzhou River Estuary, the Liaodong-Bozhong shoals, the Laotieshan Channel and the southeast of Liaodong Peninsula), the content of silt is relatively low, while the silt content is high in the areas with low sand content. Besides, clay content has no apparent relationship with sand and clay contents. High values of clay content mainly distribute in upper Liaodong Bay, the Bohai Bay, the west part of the Laizhou Bay, and extend northward to the west coast of Liaodong Bay.

Fig.2 (A) Location of surface sediments sampled from the BS and NYS. Red circle () represents the sampling station for cruise in 2014, black star () represents the sampling station for cruise in 2008 (Hu, 2010), black cross (+) represents the sampling station for cruise in 2010, and diamond () represents the sampling station for data collection from 1998 to 2012 (Wang, 2001; Cheng et al., 2004; Kong et al., 2006; Dong, 2011; Jin, 2014); (B) Distribution of mud areas over the study area (modified from Saito and Yang, 1995) and core locations for 210Pb dating (Du et al., 1990; Alexander et al., 1991; Li et al., 1991; Yang et al., 1991; Zhao et al., 1991; Li, 1993; Yang et al., 1993; Dong et al., 1995; Li and Shi, 1995; Song et al., 1997; Liu et al., 2004; Qi et al., 2004; Li et al., 2007; Zhang et al., 2009; Zhao et al., 2009; Hu, 2010; Xu et al., 2012).

The classification and nomenclature of surface sediment types in the study area are shown in Fig.3D. Six primary sediment types were identified in the study area such as sand, silty sand, sandy silt, silt, and clayey silt. Silty sand and clayey silt are the dominant sediment types (with contents of 37.6% and 27.27%, respectively). Silty sand is mainly distributed in the areas with high sand contents such as the Liugu River Estuary, the Fuzhou River Estuary, the Liaodong and Bozhong shoals and the middle of the North Yellow Sea. Clayey silt mainly covers the Bohai Bay, the north of the Laizhou Bay and extends northward to the Liaodong Bay. Then sandy silt (17.77%) is found around the Shandong Peninsula.

Fig.3 Percentage contents of sand (A), silt (B) and clay (C) in the surface sediments, and classification and nomenclature of sediment types (D) in the BS and the NYS followed Shepard (1954) scheme.

4.1.2 Surface sediment grain size distribution

The spatial distribution of the mean grain size () of surface sediment has been shown in Fig.4A, ranging from 1.84Φ to 7.65Φ (mean: 5.27Φ). The results show that the surface sediment in the study area is mostly silt (4–8Φ), and contains very few coarse-grained sand (<4Φ). Several mud deposits with finer sediment (6–7Φ) were mainly found in the upper Liaodong Bay, the Bohai Bay, the west part of the Laizhou Bay that extends northward to the west coast of Liaodong Bay, and the northeast of Shandong Peninsula. A silt-dominated belt was evidently identified that extends eastward from the Laizhou Bay along the coast of north Shandong Peninsula (Fig.4A). Coarse- grained sand/silty sand (<5Φ) was found in the river mouth of Daqing River, the Liugu River estuary to Fuzhou River estuary, the tidally-dominated sand ridge system (., the Liaodong and Bozhong shoals, the Laotieshan Channel), and the southeast of Liaodong Peninsula. The distribution of fine sediment (clay and silt) approximately coincided with those descriptions of the depositional setting in these areas (see Fig.2B). The surface sediment is primarily com- posed by sand in the northern Bohai Strait and its adjacent area where the morphology is featured by the tidal sand ridge (Liu., 2007).

The surface sediments over the study area are poorly or very poorly sorted with a mean sorting coefficient of 2.29 (Fig.4B). The areas with coarse-grained surface sediment were usually very poorly sorted, revealing complicated provenance and highly energetic sedimentary environment. Except for these areas, the values of sorting coefficients were relatively lower in the study area, indicating relatively simple sediment source (., the Yellow River sediment or the Liaohe River sediment) or stable sedimentary dynamic conditions. The surface sediment in the study area was mostly characterized by extremely positive skewness (1.29 in average) and wide or relatively wide kurtosis (2.92 in average; Fig.4). The fine-grained sediment is generally better sorted with lower skewness and kurtosis compared with the coarse-grained sediment in the study area. However, the surface sediments from the southeast of Liaodong Peninsula were relatively coarser but well sorted, which might be ascribed to the local reworking by strong tidal dynamics (Wang., 2010).

4.2 Net Surface Sediment Transport Pattern

The net surface sediment transport pattern, represented by the grain-size trends, reveals several distinct characteristics in the study area (Fig.5). By comparing the sediments from the stations withdifferent characteristic distances (Der, representing the maximum distance between sampling stations), it was found that the net surface sediment transport pattern can be better produced when the Der=0.4. The result shows that there are three converg- ing centers of sediment transport in the study area: the upper Liaodong Bay, the mud area in the central Bohai Sea, and the mud area of northeast Shandong Peninsula (Fig.5), corresponding the deposition of silt-dominated fine-grained sediments (Figs.3 and 4).

Fig.4 Distribution of grain-size parameters in the surface sediments: (A) Mz, mean grain size; (B) σ, sorting coefficient; (C) Sk, skewness and (D) Kg, kurtosis.

However, the grain-size trend model still has some limitations (., edge effect, sampling depth and temporal- spatial-scale) that might impact the interpretations of the model results. For instance, sediments nearby the Yellow River mouth were primarily transported northward and northeastward as indicated by the grain-size trend analysis (Fig.5), which, however, is inconsistent with the previous conclusion that the longshore transport is dominated for the Yellow River-delivered sediment (Zang, 1996; Wang., 2007; Yang., 2011).

Fig.5 Net sediment transport pattern of surface sediments in the BS and NYS based on grain size trend analysis.

4.3 Distribution of Modern Sedimentation Rate

Large spatial variability of modern sedimentation rate was found in the study area (Fig.6). The rapid sedimentation around the subaqueous Yellow River Delta was evidently dominated by the accumulation of river-delivered sediments with sedimentation rates spatially varying from 2 to 9.6cmyr−1and a depo-center located at the present river mouth. High sedimentation rates (>0.8cmyr−1) were also found in the Bohai Bay and the upper Liaodong Bay, suggesting the river-dominated proximal accumulations. In contrast, the mud deposits in the central Bohai Sea and the north Shandong Peninsula had relatively low sedimentation rates although the grain-size trend analysis indicated they were convergent centers of fine-grained sediments (Figs.3, 4, 5 and 6). The fine-grained fractions of Yellow River-delivered sediments could be transported over long distance and eventually accumulated in a less energetic environment, which formed significant muddy deposits at centennial-to-millennial scales.

4.4 Correlation Between the Grain Size Distribution of Surface Sediment and Modern Sedimentation Rate

Based on the information obtained (as described above), it can be discovered that the direction of the maximum gradient of sedimentation rate and the net sediment transport trend basically coincide with each other (Figs.5 and 6). And there was an obvious spatial relationship among the modern sedimentation rate, the grain-size distribution of surface sediment and sediment transport trend (Figs.3, 4, 5 and 6), except for the southeast of Liaodong Peninsula where the sedimentation rate data was lacking. The compiled datasets indicated that the coarse surface sediments were generally corresponded relative low sedimentation rates except for the muddy deposit to the northeast Shandong Peninsula where the sedimentation rate was relatively low but with fine-grained surface sediment. Furthermore, the grain-size trend analysis indicated that the fine-grained sediment areas, such as the mud area of the central Bohai Sea and the upper Liaodong Bay where sedimentation rates are relatively higher than the coarse- grained sediment areas, were the convergent centers of surface sediment, except for the Bohai Bay and the sub- aqueous Yellow River Delta where offshore sediment transport was evident (Fig.5).

Fig.6 Distribution of modern sedimentation rate in the BS and NYS.

5 Discussion

5.1 Impacts of Sediment Supply

Several rivers around the Bohai Sea carried a considerable amount of terrestrial sediments into the Bohai Sea that account for about 90% of the total sediments in the Bohai Sea (Qiao., 2010). Because the Yellow River delivers the highest amount of sediments to the sea, it is thus believed to be a dominated source for the sediments in the Bohai Sea. Over the recent decades, the Yellow River and its delta have experienced some significant changes, such as the reduction of water and sediment discharge (Fig.7), the migration of river mouth in 1996 (Fig.8A) andthe operation of Water-Sediment Regulation Scheme since 2002. The water and sediment discharge from the Yellow River into the Bohai Sea have decreased significantly (Fig.7), while the annual variation tendency of the median grain size is unapparent but a coarsening trend since the operation of Water-Sediment Regulation Scheme in 2002. Decadal-scale changes of the subaqueous profiles at the present river mouth (CS23) and the abandoned river mouth (CS27) from 1976 to 2012 presented evident responses to the changing sediment input and channel migration (Fig.8; Wang., 2017a). Bathymetric data along section CS23 (Fig.8B) showed that the Yellow River subaqueous delta prograded seaward after the channel migration in 1976 with steepened subaqueous slope; whereas bathymetric data along section CS27 revealed that the Yellow River subaqueous delta prograded seaward before 1996, and it then retreated landward after the migration of river mouth in 1996 (Fig.8B). These changes seemed to be closely related to the water-sediment discharge from the Yellow River to the sea and the grain size compositions of river- delivered sediments (Fig.7). Despite the water and sediment discharge to the sea have reduced recently, there was still a coarsening trend on the river-delivered sediment composition. In addition, at long-term scale, the dynamic environment near the two sections did not have significant differences except for the significantly changing of sediment supply from the Yellow River. Therefore, sub- aqueous delta at section CS23 kept prograding seaward with the rapid deposition of coarser sediments near the ri- ver mouth, shallower than 10m (Fig.8B), whereas thefine- grained sediments could be transported offshore. However, subaqueous delta at section CS27 suffered significant erosion since 1996 (Fig.8C) partially due to the lack of sediment supply after the river mouth migration, which indicated the important impact of sediment supply on the grain-size distribution of sediments.

Evidences from mineralogy and geochemistry analysis confirmed that sediments delivered from the Yellow River dominated the areas including the northern Laizhou Bay to the central Bohai Sea, the south part of the Bohai Bay and the southern Bohai Strait (Qin.,1985; Chen, 1989). Wang. (2014) investigated the seasonal distribution of suspended sediments in the Bohai Sea and suggested that the subaqueous Yellow River Delta acted as a major sink in the summer seasons that trapped most of the Yellow River-delivered sediments, but transited to be a primary source in winter-spring seasons for sediment redistribution in the Bohai Sea and sediment export to the Yellow Sea. Moreover, the regions with high concentration of suspended sediments in winter-spring season were found in the northern Laizhou Bay, the south part of the mud area of central Bohai Sea and the southern Bohai Strait (Qin., 1985; Chen, 1989). In addition, the sedimentation rate in the southern part of the mud area in the central Bohai Sea was higher than that in the northern part (Fig.6), which might be ascribed to the northeastward transport of the Yellow River sediment (Fig.5). Liu. (2007) proposed that the southwest and middle part of the mud area in central Bohai Sea were affected by the Yellow River sediments, whereas sediments in its northern part differed from the Yellow River sediments in the geochemical composition. The sedimentation rate around the Yellow River mouth was relatively high (>3cmyr−1), but decreased northeastward and extended to the south part of the Bohai Strait (Fig.6), which was consistent with the se- diment transport trend based on grain-size analysis (Fig.5) and the transport pathway of the Yellow River sediments as identified from the remote sensing datasets (Bi., 2011).

Fig.7 Time series of annual water discharge, sediment load and median grain size at station Lijin since 1950 (data from Yellow River Conservancy Commission).

Fig.8 Map of the modern Huanghe Delta, river courses, bathymetry, and two offshore sections for bathymetric surveys (A), changes of the subaqueous profiles at the two offshore sections CS23 (B) and CS27 (C) from 1976 to 2012 (after Wang et al., 2017a). Yellow arrow indicates progradation while red arrow represents erosion.

The sediments delivered from the Luanhe River, the Liu- gu River and Fuzhou River to the Bohai Sea were mainly sand and mixed with gravels, with few fine-grained fractions (Qin., 1985). The coarse-grained sediments most- ly deposited locally near the river mouths (Fig.4A). In contrast, sediments derived from the Yellow River, the Liaohe River and the Haihe River were mainly composed by fine-grained fractions as the sediments with grain size <0.01mm (or >6.6Φ) accounted for more than 60% in the Bohai Bay, Yellow River mouth and the upper Liaodong Bay (Qin., 1985). The mineral assemblages and their distribution patterns in the surface sediments from the Bo- hai Sea indicated that the Liaohe River-delivered sediments (relatively fine) mainly deposited at the upper Liaodong Bay (about 40˚10΄N), and most of the sediments delivered from the Luanhe River and Liugu River (relatively coarse) deposited in the nearshore areas (Chen., 1980), which was confirmed by the distribution pattern of surface sediments (Fig.4A). In summary, fine-grained sediments delivered from the Yellow River, the Liaohe River and the Haihe River can be transported over long distance, while sediments from the Luanhe River, the Liugu River and the Fuzhou River were mainly composed by sand and mostly deposited near the river mouth, indicating the dominant role of sediment supply on the grain-size distribution of surface sediments in the Bohai Sea.

Yellow Sea water flows into the Bohai Sea through the northern Bohai Strait, and Bohai Sea water flows out to the Yellow Sea through the southern part (Qin., 1985; Guan, 1994). Thus, sediments from the Bohai Sea, particularly those from the Yellow River impacted the distribution of surface sediments in the north Yellow Sea. The sediments derived from the Yellow River were trans- ported from the Bohai Sea to the northern Yellow Sea through the Bohai Strait (Qin and Li, 1986; Martin., 1993), and most of them were conveyed southward and then transported into the South Yellow Sea along the east coast of Shandong Peninsula under the influence of Shan- dong Coastal Current (Liu., 2004; Yang and Liu, 2007). The muddy depocenter off the east coast of Shandong Peninsula could represent a direct escape route for Yellow River-derived sediments delivered into the Yellow Sea (Alexander., 1991; Cai., 2003). Evidences from elemental geochemistry confirmed that the calcite- enriched Yellow River sediment, largely different from the surface sediments from the north part of the North Yellow Sea, mainly distributed around the Shandong Peninsula, which indicated the dominant role of sediment supply on the sedimentary pattern (Chen, 1989).

Previous studies have suggested that the central mud deposit zone in the North Yellow Sea is mainly supplied by the Yellow River (Park and Khim, 1992; Liu., 2001; Yang and Liu, 2007). Meanwhile, the distribution of silt content (Fig.3B) also confirmed the important contribution of the Yellow River-derived sediments to the shelf muddy deposit. Chemical elemental composition of the surface sediments from the northern Yellow Sea indicated that the Yellow River sediments mainly influenced the coastal area of the Shandong Peninsula, whereas three dominant sediment sources to the central mud in the nor- thern Yellow Sea are the Yalujiang River (10%–17%), the Yangtze River (10%–17%) and the Yellow River (66%– 80%) (Qi., 2004). And the annual depositional fluxes of heavy metals (Huang.,2014) and the clay mineralogical characteristics (Li., 2014) in the surface sedi- ments from the northern Yellow Sea also showed that the fine-grained sediment in the western North Yellow Sea was mostly derived from the Yellow River, whereas the sediment in the eastern part of the North Yellow Sea was supplied by the Yalujiang River (Wang., 2011; Li., 2014). Therefore, the sediment in the northern Yellow Sea was featured by mixed sources from the Yellow River, the Yangtze River and the Yalujiang River (Wang., 2001; Qi., 2004; Wang., 2017b).

It is noteworthy that the impacts of sediment supply on the grain-size distribution of surface sediments and modern sedimentation rates are comparatively restricted and would be confined to local regions near the source without dynamic reworking. For example, the subaqueous delta (section CS27) was eroded continuously since 1996 not only due to the lack of sediment supply after the river channel shift, but also the impacts of the dynamic reworking (Fig.8C).

5.2 Impacts of Sedimentary Dynamic Environment

Besides the river input, ocean dynamic factors play a critical role in distributing the sediments in the marginal seas including water masses, tidal currents, circulations, and storm waves (Sündermanm and Feng, 2004). Both tidal currents and wind waves are critical to the sediment suspension or resuspension, transport and deposition in the Bohai Sea and Yellow Sea, presenting strong seasonal variability as modulated by the monsoonal climate (Zhu and Chang, 2000; Jiang., 2000, 2004; Lu., 2011; Bian., 2013a, 2013b; Wang, 2014; Zeng., 2015).

In the shallow marine environment, as impacted by mon- soonal climate, the suspended sediment concentration (SSC) was higher in winter than summer although the water and sediment discharged to the sea in summer were much greater. Moreover, there was a close relationship between the SSC and wave actions in the southern Bohai Sea (Fig.9). In winter, the strong northeasterly or northwesterly wind produced very strong waves, resulting in active coastal resuspension; while in summer, the prevailing sou- therly wind could not produce significant waves due to the limited fetch, which could have little impacts on the coastal resuspension along the Yellow River Delta (Wang., 2014). The coherent seasonal variability of significant wave height and suspended sediment concentration indicated the evident dominance of active coastal resuspension and enhanced coastal currents on the sediment distribution and transport in the Bohai Sea (Li., 2010; Yang., 2011; Zeng., 2015). And in winter, associated erosion-resuspension due to high bottom shear stress along the northern Shandong Peninsula make it possible for Yellow River-derived sediment to deliver from the Bohai Sea to the Yellow Sea (Zeng., 2015). Therefore, during winter seasons, highly turbid water mass is formed off the Yellow River Delta and extends along the coast of Shandong Peninsula to the Yellow Sea via the southern Bohai Strait (Bi., 2011). Generally, the sub- aqueous Yellow River Delta acts as a major sink for the terrestrial sediments in summer seasons, but as a primary source for sediment redistribution in the Bohai Sea and export to the Yellow Sea in winter seasons (Bi., 2011; Yang., 2011; Wang., 2014).

The distribution pattern of surface sediment grain size was indicative to the local hydrodynamic conditions. For example, the surface sediments in the river mouth of Da- qing River and the Liugu River estuary to Fuzhou River estuary were relatively coarse as largely impacted by the river input (Figs.3 and 4A). However, coarse-grained sur- face sediments in the Liaodong-Bozhong shoals, the Lao- tieshan Channel and the southeast of Liaodong Peninsula were mainly associated with the effective physical sorting from strong tidal dynamics (Zhu and Chang, 2000).

Two trends of sediment transport were identified in the Liaodong Bay with high sedimentation rate (Fig.5), coinciding with the general patterns of winter circulation (Fig.1). Sediments from the Bohai Bay and the Luanhe River Estuary were transported eastward and southeastward, respectively, converging toward the mud area in the central Bohai Sea (Fig.5). Sediments from the subaqueous Yellow River Delta tended to transport northward to the mud area or northeastward along the northern coast of the Shandong Peninsula to the Yellow Sea.

There is a distinct convergent trend of sediment transport to the mud area in the North Yellow Sea (Fig.5), as confirmed by the geophysical observations along the Shandong Peninsula coast (Alexander., 1991; Liu., 2001; Cheng., 2004). Additionally, previous works also confirmed that most of Yellow River-delivered sediments were conveyed southward and then transported into the South Yellow Sea along the Shandong Peninsula under the influence of Shandong Coastal Current (Liu., 2004; Yang and Liu, 2007).

Fig.9 Spatially averaged daily significant wave height versus monthly surface suspended sediment concentration (SSC) in the southern Bohai Sea that covers Laizhou Bay and the Yellow River Delta in 2010 (after from Wang et al., 2014). The data of daily wave heights are derived from the WAVEWATCH III Global model with a spatial resolution of 0.5˚.

Because of the uneven distribution of the sampling sites and the presence of ‘edge effect’ (., the sites situated on the boundaries have fewer neighboring sites than others in interior area), the grain size trend analysis cannot reveal detailed characteristics and could be distorted at the boundary (Gao and Collins, 1998; Cheng., 2004).

6 Conclusions

Owing to the influence of sediment supply and hydrodynamic environment with significant seasonal variability, the surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS) were primarily composed of silt-sand and clay-silt with a dominant class of silt. The surface sediments were mostly poorly to very poorly sorted, with positive skewness, and wide to relatively wide kurtosis. In the Bohai Sea the Yellow River supply not only dominated the distribution of coarse-grained sediments near the river mouth, but also impacted the distribution of fine-grained surface sediments (mainly silt) in the Bohai Bay, the center of the Bohai Sea, and the north coast of Shandong Peninsula. The fine-grained sediment could be transported to the Yellow Sea over long distance by the coastal currents.

The distribution of surface sediment grain size was close- ly associated with the modern sedimentation rates estima- ted by210Pb-dating. The area with coarser-grained surface sediments generally had the low sedimentation rate except for the river mouths where river-delivered sediments were dynamically sorted. In the areas where surface se- diments were fine-grained, the sedimentation rates were relatively high. Furthermore, the grain-size trend analysis showed that the fine-grained sediment areas, such as the mud area in the central Bohai Sea and the upper Liaodong Bay where sedimentation rates are high, were the convergent center of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 41525021), and the Ministry of Science and Technology of People’s Republic of China (Nos. 2016YFA0600903 and 2017YFC0405502). We are grateful to Dr. Jun Li from the Qingdao Institute of Marine Geology, China Geological Survey for providing the datasets on surface sediments.

Alexander, C. R., DeMaster, D. J., and Nittrouer, C. A., 1991. Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea., 98 (1): 51-72.

Asselman, N. E. M., 1999. Grain-size trends used to assess the effective discharge for floodplain sedimentation, River Waal, the Netherlands., 69 (1A): 51-61.

Bi, N. S., Yang, Z. S., Wang, H. J., Fan, D., Sun, X., and Lei, K., 2011. Seasonal variation of suspended-sediment transport through the southern Bohai Strait., 93 (3): 239-247.

Bian, C., Jiang, W., and Greatbatch, R. J., 2013a. An exploratory model study of sediment transport sources and deposits in the Bohai Sea, Yellow Sea, and East China Sea., 118 (11): 5908-5923.

Bian, C., Jiang, W., Quan, Q., Wang, T., Greatbatch, R. J., and Li, W., 2013b. Distributions of suspended sediment concentration in the Yellow Sea and the East China Sea based on field surveys during the four seasons of 2011., 121-122: 24-35.

Cai, D. L., Shi, X. F., Zhou, W. J., Liu, W. G., Zhang, S. F., Cao, Y. N., and Han, Y. B., 2003. Sources and transportation of suspended matter and sediment in the southern Yellow Sea: Evidence from stable carbon isotopes., 48 (S1): 21-29.

Chen, L. R., 1989. A study on mineral assemblages in sediments of the Bohai Sea, the Huanghai Sea and the East China Sea., 13 (2): 1-8 (in Chinese with English abstract).

Chen, L. R., Luan, Z. F., Zheng, T. M., Xu, W. Q., and Dong, T. L., 1980. Mineral assemblages and their distribution pattern in the sediments of the Gulf of Bohai Sea., 11 (1): 46-64 (in Chinese with English abstract).

Cheng, P., Gao, S., and Bokuniewicz, H., 2004. Net sediment transport patterns over the Bohai Strait based on grain size trend analysis.,60 (2): 203-212.

Delhez, E. J. M., 1996. On the residual advection of passive con- stituents., 8 (3-4): 147-169.

DeMaster, D. J., Kuehl, S. A., and Nittrouer, C. A., 1986. Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: Examination of biological silica uptake and the fate of particle-reactive elements., 6 (1): 107-125.

Dong, A. G., 2011. Source, sink and its environmental record of sediments in Yellow Sea and East China Sea. PhD thesis. Ocean University of China, 1-154.

Dong, T. L., Yang, G. F., and Xu, S. M., 1995. Modern sedimentary characteristics in the south of the Bohai Sea., 15 (4): 131-134 (in Chinese with English abstract).

Du, R. Z., Liu, G. X., and Yang, S. L., 1990. Modern sedimentation rate and sedimentation process in Bohai Bay., 10 (3): 15-22 (in Chinese with English abstract).

Fang, Y., Fang, G. H., and Zhang, Q. H., 2000. Numerical simulation and dynamic study of the wintertime circulation of the Bohai Sea., 18 (1): 1-9.

Gao, S., and Collins, M., 1991. A critique of the ‘McLaren Me- thod’ for defining sediment transport paths–Discussion., 61 (1): 143-146.

Gao, S., and Collins, M., 1992. Net sediment transport patterns inferred from grain-size trends, based upon definition of ‘trans- port vectors’., 81 (1-2): 47-60.

Gao, S., and Collins, M., 1998. The use of grain size trends in marine sediment dynamics., 12 (4): 241-246 (in Chinese with English abstract).

Guan, B. X., 1994. Patterns and structures of the currents in Bohai, Huanghai and East China Seas. In:. Zhou, D.,., eds., Springer, Dordrecht, 17-26.

Holligan, P. M., and Boois, H. D., 1993. Land ocean interactions in the coastal zone (LOICZ). Science plan. Global Change Report, Sweden, 1-50.

Hu, D., 1984. Upwelling and sedimentation dynamics., 2 (1): 12-19.

Hu, L. M., 2010. Sources and sink of sedimentary organic matter in the river-dominated continental shelves: A case study in the Bohai and Yellow Seas. PhD thesis. Ocean University of China, 1-190.

Hu, L. M., Zhang, G., Zheng, B. H., Qin, Y. W., Lin, T., and Guo, Z. G., 2009. Occurrence and distribution of organochlorine pesticides (OCPs) in surface sediments of the Bohai Sea, Chi- na., 77 (5): 663-672.

Huang, D. J., Su, J. L., and Backhaus, J. O., 1999. Modelling the seasonal thermal stratification and baroclinic circulation in the Bohai Sea., 19 (11): 1485-1505.

Huang, P., Li, T., Li, A., Yu, X., and Hu, N., 2014. Distribution, enrichment and sources of heavy metals in surface sediments of the North Yellow Sea., 73: 1-13.

Jia, J. J., Gao, S., and Xue, Y. C., 2002. Grain-size parameters derived from graphic and moment methods: A comparative study., 33 (6): 577-582 (in Chinese with English abstract).

Jiang, W. S., Pohlmann, T., Sun, J., and Starke, A., 2004. SPM transport in the Bohai Sea: Field experiments and numerical modelling., 44 (3-4): 175-188.

Jiang, W. S., Pohlmann, T., Sündermann, J., and Feng, S. Z., 2000. A modelling study of SPM transport in the Bohai Sea., 24 (3): 175-200.

Jin, Y. X., 2014. The tidal geomorphology in the eastern Bohai Sea and its controlling factors. PhD thesis. Ocean University of China, 1-131.

Kong, X. H., Liu, J., Li, W. R., Wang, S. L., Zhou, L. Y., and Yuan, L., 2006. Grain size characters and mineral components of surface sediments in the offshore area of northeastern Shan- dong Peninsula., 26 (3): 21-29 (in Chinese with English abstract).

Kuehl, S. A., Nittrouer, C. A., Allison, M. A., Faria, L. E. C., Dukat, D. A., Jaeger, J. M., Pacioni, T. D., Figueiredo, A. G., and Underkoffler, E. C., 1996. Sediment deposition, accumulation, and seabed dynamics in an energetic fine-grained coastal environment., 16 (5): 787-815.

Li, F. Y., 1993. Modern sedimentation rates and sedimentation feature in the Huanghe River Estuary based on210Pb technique., 11 (4): 333-342.

Li, F. Y., and Shi, Y. L., 1995. Accumulation rates of sediment and sedimentary environment in the South Bohai Sea., 13 (2): 33-37.

Li, F. Y., Qi, J., Song, J. M., and Li, X. G., 2007. Chemical elements accumulation characteristics of the sediment drill core in the Jiaozhou Bay., 53 (5): 681-690.

Li, F. Y., Yuan, W., DeMaster, D. J., and Nittrouer, C. A., 1991. Profile model of210Pb in the South China Sea, South Huanghai Sea and Bohai Sea., 11 (3): 35-43 (in Chinese with English abstract).

Li, G. S., Xue, X. H., Liu, Y., Wang, H. L., and Liao, H. P., 2010. Diagnostic experiments for transport mechanisms of suspend- ed sediment discharged from the Yellow River in the Bohai Sea., 20 (1): 49-63.

Li, J., Hu, B. Q., Dou, Y. G., Zhao, J. T., and Li, G. G., 2012. Modern sedimentation rate, budget and supply of the muddy deposits in the East China Seas., 58 (4): 745-756 (in Chinese with English abstract).

Li, Y., Li, A., Huang, P., Xu, F., and Zheng, X., 2014. Clay minerals in surface sediment of the North Yellow Sea and their implication to provenance and transportation., 90: 33-40.

Lin, T., Hu, L. M., Guo, Z. G., Qin, Y. W., Yang, Z. S., Zhang, G., and Zheng, M., 2011. Sources of polycyclic aromatic hydrocarbons to sediments of the Bohai and Yellow Seas in East Asia., 116 (D23): 1-11.

Liu, J. P., Milliman, J. D., and Gao, S., 2001. The Shandong mud wedge and post-glacial sediment accumulation in the Yellow Sea., 21 (4): 212-218.

Liu, J. P., Milliman, J. D., Gao, S., and Cheng, P., 2004. Holocene development of the Yellow River’s subaqueous delta, North Yellow Sea., 209 (1-4): 45-67.

Liu, Z., Berné, S., Saito, Y., Yu, H., Trentesaux, A., Uehara, K., Yin, P., Liu, J. P., Li, C., Hu, G., and Wang, X., 2007. Internal architecture and mobility of tidal sand ridges in the East China Sea., 27 (13): 1820-1834.

Lu, J., Qiao, F. L., Wang, X. H., Wang, Y. G., Teng, Y., and Xia, C. S., 2011. A numerical study of transport dynamics and seasonal variability of the Yellow River sediment in the Bohai and Yellow Seas., 95 (1): 39-51.

Martin, J. M., Zhang, J., Shi, M. C., and Zhou, Q., 1993. Actual flux of the Huanghe (Yellow River) sediment to the Western Pacific Ocean., 31 (3): 243-254.

McManus, J., 1988. Grain size determination and interpretation. In:. Tucker, M., ed., Wiley Blackwell, Oxford, 63-85.

Milliman, J. D., and Meade, R. H., 1983. World-wide delivery of river sediment to the oceans., 91 (1): 1-21.

Pang, C. G., Li, K., and Hu, D. X., 2016. Net accumulation of suspended sediment and its seasonal variability dominated by shelf circulation in the Yellow and East China Seas., 371: 33-43.

Park, Y. A., and Khim, B. K., 1992. Origin and dispersal of recent clay minerals in the Yellow Sea., 104 (1-4): 205-213.

Qi, J., Li, F. Y., Song, J. M., Gao, S., Wang, G. Z., and Cheng, P., 2004. Sedimentation rate and flux of the North Yellow Sea., 24 (2): 9-14 (in Chinese with English abstract).

Qiao, S. Q., Shi, X. F., Wang, G. Q., Yang, G., Hu, N. J., Liu, S. F., Liu, Y. G., Zhu, A. M., and Li, C. X., 2010. Discussion on grain-size characteristics of seafloor sediment and transport pattern in the Bohai Sea., 32 (4): 139-147 (in Chinese with English Abstract).

Qin, Y. S., and Li, F., 1983. Study of influence of sediment loads discharged from the Huanghe River on sedimentation in the Bohai Sea and the Huanghai Sea. In:. China Ocean Press, Beijing, 83-92 (in Chinese).

Qin, Y. S., and Li, F., 1986. Study of the influence of sediment loads discharged from Huanghe River on sedimentation in the Bohai and Yellow Seas., 27: 125-135.

Qin, Y. S., Zhao, Y. Y., and Zhao, S. L., 1985.. Science Press, Beijing, 19-133 (in Chinese).

Saito, Y., and Yang, Z. S., 1995. Historical change of the Huang- he (Yellow River) and its impact on the sediment budget of the East China Sea. In:. Tsu- nogai, S.,., eds., Hokkaido University, Sapporo Hokkai- do, 7-12.

Saito, Y., Yang, Z. S., and Hori, K., 2001. The Huanghe (Yellow River) and Changjiang (Yangtze River) Deltas: A review on their characteristics, evolution and sediment discharge during the Holocene., 41 (2-3): 219-231.

Shepard, F. P., 1954. Nomenclature based on sand-silt-clay ratios., 24 (3): 151-158.

Song, Y. X., Zhan, X. W., and Wang, Y. G., 1997. Modern sedimentation features of estuary region of the Liaodong Bay., 19 (5): 145-149.

Sündermann, J., and Feng, S., 2004. Analysis and modelling of the Bohai Sea ecosystem–A joint German-Chinese study., 44 (3): 127-140.

Wang, G. Z., 2001. Sedimentary characteristics and mechanism of mud deposits in the northern Yellow Sea. Master thesis. Institute of Oceanology, Chinese Academy of Sciences, 1-104.

Wang, H. J., Saito, Y., Zhang, Y., Bi, N. S., Sun, X. X., and Yang, Z. S., 2011. Recent changes of sediment flux to the western Pacific Ocean from major rivers in East and Southeast Asia., 108 (1-2): 80-100.

Wang, H. J., Wang, A. M., Bi, N. S., Zeng, X. M., and Xiao, H. H., 2014. Seasonal distribution of suspended sediment in the Bohai Sea,China., 90: 17-32.

Wang, H. J., Wu, X., Bi, N. S., Li, S., Yuan, P., Wang, A. M., Syvitski, J. P. M., Saito, Y., Yang, Z. S., Liu, S. M., and Nittrouer, J., 2017a. Impacts of the dam-orientated water-sediment regulation scheme on the lower reaches and delta of the Yellow River (Huanghe): A review., 157: 93-113.

Wang, H. J., Yang, Z. S., Li, Y. H., Guo, Z., Sun, X., and Wang, Y., 2007. Dispersal pattern of suspended sediment in the shear frontal zone off the Huanghe (Yellow River) mouth., 27 (6): 854-871.

Wang, K. S., Shi, X. F., Cai, S. W., Qiao, S. Q., and Jiang, X. L., 2010. Distribution and provenance of the surface sediments of the Yellow River mouth and Laizhou Bay deduced from heavy minerals., 30 (6): 1-8 (in Chinese with English abstract).

Wang, Y. H., Wang, S., and Liu, M., 2017b. Magnetic properties indicate sediment provenance and distribution patterns in the Bohai and Yellow Seas, China., 140: 84-95.

Wei, H., Hainbucher, D., Pohlmann, T., Feng, S., and Suender- mann, J., 2004. Tidal-induced Lagrangian and Eulerian mean circulation in the Bohai Sea., 44 (3-4): 141-151.

Wentworth, C. K., 1922. A scale of grade and class terms for clastic sediments., 30 (5): 377-392.

Xu, D. H., Li, J., Zhao, J. T., Hu, B. Q., Bai, F. L., and Dou, Y. G., 2012. Grain-size distribution of surface sediments of the Liaodong Bay, Bohai and sedimentary environment restoration., 32 (5): 35-42 (in Chinese with English abstract).

Yang, S. L., Liu, G. X., Du, R. Z., and Zhang, B., 1991. Research on sedimentation rate in the Laizhou Bay and southern part of central basin of the Bohai Sea., 13 (6): 804-812.

Yang, S. L., Liu, G. X., Du, R. Z., and Zhang, B., 1993. Study on the modern sedimentation rate through210Pb age dating, Liaodong Bay., 11 (1): 128-135.

Yang, Z. S., and Liu, J. P., 2007. A unique Yellow River-derived distal subaqueous delta in the Yellow Sea., 240 (1-4): 169-176.

Yang, Z. S., Ji, Y. J., Bi, N. S., Lei, K., and Wang, H. J., 2011. Sediment transport off the Huanghe (Yellow River) Delta and in the adjacent Bohai Sea in winter and seasonal comparison., 93 (3): 173-181.

Zang, Q. Y., 1996.. China Ocean Press, Beijing, 1-166.

Zeng, X. M., He, R. Y., Xue, Z., Wang, H. J., Wang, Y., Yao, Z. G., Guan, W. B., and Warrillow, J., 2015. River-derived sediment suspension and transport in the Bohai, Yellow, and East China Seas: A preliminary modeling study., 111: 112-125.

Zhang, R. M., Zhang, G. L., Huang, B., and Zhang, X. J., 2009. Modern sedimentation rate analysis of the coastal area near the Shandong Penisula., 25 (9): 15-18.

Zhao, B. R., Zhuang, G. W., Cao, D. M., and Lei, F. H., 1995. Circulation, tidal residual currents and their effects on the sedimentations in the Bohai Sea., 26 (5): 466-473 (in Chinese with English abstract).

Zhao, G. M., Ye, S. Y., Li, G. X., and Gao, G. Y., 2009. Sedimentary geochemical records: Indications of environmental changes in Bohai Bay., 29 (5): 51-57 (in Chinese with English abstract).

Zhao, Y. Y., Li, F. Y., Chen, Y. W., DeMaster, D. J., Nittrouer, C. A., and Milliman, J. D., 1991. On sedimentation intensity in South Yellow Sea., 15 (4): 28-31.

Zhu, Y., and Chang, R., 2000. Preliminary study of the dynamic origin of the distribution pattern of bottom sediments on the continental shelves of the Bohai Sea, Yellow Sea and East China Sea., 51 (5): 663- 680.

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