Provenance and Transport Patterns of Clay-Size and Silt-Size Sediments in the Jianggang Sand Ridges from the Southwestern Yellow Sea

Abstract

The Jianggang sand ridges (JSR) in the southwestern Yellow Sea are a radiating tidal sand ridge system that plays crucial roles in ecological preservation, coastal protection, and terrestrial resource supply. Clay and silt fractions constitute important sediment components of the Jianggang sand ridges. In this study, the Sr-Nd isotopes of clay fractions and the Pb isotopes of K-feldspar in the silt fractions, along with their elemental geochemistry, are investigated to reveal the provenance and transport patterns of clay-size and silt-size sediments in the study areas. The results show that in both the clay-size sediments and the K-feldspar of the silt-size sediments, Ba exhibits the highest content, with the ranges of 432.24 μg/g to 531.05 μg/g and 398.02 μg/g to 2822.36 μg/g, respectively. In contrast, Lu shows the lowest abundance (<0.5 μg/g and <0.1 μg/g, respectively). The ⁸⁷Sr/⁸⁶Sr and ε_Nd(0) values of the clay fraction vary from 0.7158 to 0.7265 and from −14.65 to −10.92, respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb of K-feldspar in silt fraction are 17.959~18.429, 15.450~15.689, and 38.066~38.551, respectively. Through the MixSIAR model, it is suggested that the Yangtze River Mouth is the dominant contributor to clay-size sediments in both the onshore and offshore sand ridges (53.9 ± 8.8% and 51.9 ± 8.4%, respectively), followed by the Modern Yellow River Mouth and the Old Yellow River Delta (sum of contributions: <36%). For the silt fraction, the primary sediment sources of the onshore and offshore sand ridges are the Yangtze River Mouth (46.8 ± 5.5%) and the Old Yellow River Delta (42.4 ± 5.3%), while the Modern Yellow River contributes less than 16%. The Northern Chinese Deserts and the Korean rivers make only minor contributions to both fractions. Elemental and isotopic tracers indicate that the silt-size and clay-size sediments derived from the Modern Yellow River are transported southward along the Jiangsu coast by the Subei Coastal Current. Meanwhile, the silt fraction from the Yangtze River Mouth is carried northward along the coast under the influence of the Subei Coastal Current, whereas the clay fraction of it has another longer path, which moves through the central Yellow Sea and migrates southward along the Jiangsu coast to the Jianggang sand ridges under the influence of the Yellow Sea Warm Current. This study enriches the geochemical dataset of the southern Yellow Sea.

1. Introduction

Tidal sand ridges are marine depositional landforms that develop on continental shelves under the influence of tidal currents. They are characterized by alternating ridge–trough structures and are widely distributed in shallow seas worldwide [1,2,3]. Among them, the Jianggang sand ridges (JSR), located along the Jiangsu coast in the southwestern Yellow Sea, have attracted extensive attention due to their unique morphology and vast extent (Figure 1) [4,5]. Over the past few decades, rapid economic growth and population expansion in Jiangsu Province have intensified land-use demands and highlighted the contradiction between the supply and demand of land resources. The Jianggang sand ridges have played a crucial role in alleviating this contradiction. It is also of valuable significance to coastal disaster resilience and resource development [5,6]. Furthermore, as a crucial component of the Chinese coastal system, these sand ridges contribute significantly to the maintenance of regional biodiversity [7]. Therefore, protecting and preserving the Jianggang sand ridges have become a key objective.

Sufficient sediment supply and hydrodynamic support are crucial to the development of the sand ridges. Due to variations in sediment discharge, supply patterns, and transportation directions from different sources, the study of multi-provenance and multi-pathway is crucial to the future development trends and morphological changes of sand ridges. The Yellow Sea is a broad marginal sea that receives abundant terrigenous sediments from surrounding rivers, making it challenging to identify the primary sources of sediments forming the Jianggang sand ridges. Early studies suggested that their sediments are predominantly supplied by the Yangtze River. With increasing attention to the evolution of the ancient Yellow River Delta, later research proposed a dual-provenance model involving both the Yangtze River and the Old Yellow River Delta [6,8]. At present, a multi-provenance view based on the Modern Yellow River Mouth, the Old Yellow River Delta, and the Yangtze River Mouth represents a valuable interpretation shared by the scientific community [5,9,10]. For example, based on zircon U-Pb chronology, Shang et al. [5] found that the sediments of the sand ridges are mainly from the Yangtze and Yellow River, while Mao et al. [10] supported the view of three sources mentioned above using Pb isotopes. However, these studies usually focus on bulk sediments and although the results are recapitulatory, they cannot accurately reflect the provenance information of each specific grain size fraction [11]. Silt-size sediments constitute the major component of the sand ridges and play a vital role in their accretion and development [12,13]. Clay-size sediments, which also represent an important fraction, serve as key carriers of organic matter and motivate the biogeochemical cycles of the southwestern Yellow Sea [14,15]. The study of single size fraction can eliminate the influence of particle size sorting on the provenance and obtain relatively accurate provenance information. Therefore, a systematic and detailed analysis for the provenance and transport patterns of both clay and silt fractions is essential to understand the dynamics in the Jianggang sand ridges [16].

Isotopic and geochemical tracers, such as Sr-Nd isotopes, Pb isotopes of K-feldspar, and elemental compositions, are widely recognized as reliable indicators of sediment provenance due to their stability, high discrimination, and mild particle size sorting properties [17,18]. In this study, we analyze the Sr-Nd isotopes and elemental geochemistry of clay-size sediments, as well as the Pb isotopes of K-feldspar and the elemental geochemistry of silt-sized sediments from the Jianggang sand ridges. Combining these data with the MixSIAR model, we quantify the relative contributions of different sediment provenance to the onshore and offshore sand ridges and construct the source-to-sink transport patterns. This study aims to provide theoretical support for the management of coastal resources and environmental protection in the southwestern Yellow Sea.

Figure 1. Locations of sampling sites in the potential source rivers (a) and in the research area (b). JSR in (b) denotes Jianggang sand ridges. The dotted square in (a) means the research area, its zoomed-in view is shown in (b). The coastal currents are modified from [3,4,19,20,21].

Figure 1. Locations of sampling sites in the potential source rivers (a) and in the research area (b). JSR in (b) denotes Jianggang sand ridges. The dotted square in (a) means the research area, its zoomed-in view is shown in (b). The coastal currents are modified from [3,4,19,20,21].

2. Regional Setting

The Jianggang sand ridges are located in the southwestern Yellow Sea. These sand ridges radiate eastward, northward, and southward from Jianggang, forming a fan-shaped area with an angle of approximately 160° [8]. The sand ridges are located in the Northern Jiangsu-Subsidence Zone [8], they span approximately 260 km from east to west and roughly 150 km from north to south, covering a total area of more than 20,000 km² (Figure 1) [3]. The sand ridges consist of more than 70 individual sand ridges (brown areas in Figure 1b) and over 20 tidal channels (cyan areas in Figure 1b). The ridges are generally less than 100 km in length and 10–15 km in width, with water depths ranging from 0 to 25 m (Figure 1) [3,13]. Ridges and tidal channels are arranged roughly parallel to each other. Because of the influence of tidal currents and other hydrodynamic processes, the overall configuration of the Jianggang sand ridges exhibits notable differences from that of typical linear tidal sand ridges, which is reflected in the fact that the tail of the sand ridge is mostly parallel to the direction of the tide [13].

Tidal waves represent a key hydrodynamic factor in the southern Yellow Sea [8]. Under the influence of the M₂ tidal constituent, a standing tidal wave is formed in the central South Yellow Sea, which drives the tidal currents to converge southwestward during the flood stage [19,20]. In addition to tides, ocean currents also play an important role of the hydrodynamic system in the southern Yellow Sea [21]. The Korean Coastal Current in the western part of the Korean Peninsula flows southward. The Yellow Sea Warm Current enters from the southeastern part of the southern Yellow Sea and moves northward. After reaching the southeastern part of the Shandong Peninsula, part of it turns westward (Figure 1) [20,21]. The Yellow Sea Coastal Current develops along the eastern coast of the Shandong Peninsula and migrates southward along the coastline to Haizhou Bay [21]. The Subei Coastal Current originates in Haizhou Bay and subsequently flows southward along the Jiangsu coast toward the Yangtze River Mouth. Previous studies have also suggested that the Subei Coastal Current flows northward along the Jiangsu coast during the summer [9,21]. The Yangtze River diluted water exhibits three main branches flowing northeastward, southward, and northward, with its northern branch extending up to approximately 33.5° N during summer and autumn [22].

The main rivers flowing into the Bohai Sea and the Yellow Sea are the Yangtze River and the Modern Yellow River (Figure 1). Their drainage basin areas are approximately 1.7 × 10⁶ km² and 6.8 × 10⁵ km², respectively, with average annual discharges of about 9.17 × 10¹¹ m³ and 3.16 × 10¹⁰ m³, respectively. Over the past decade, the average annual sediment loads have been approximately 1.13 × 10⁸ t for the Yangtze River and 1.82 × 10⁸ t for the Yellow River [23]. During 1128 and 1855 AD, the ancient Yellow River flowed into the southern Yellow Sea, depositing vast amounts of sediment and forming the subaqueous delta. Currently, hydrodynamic erosion transports approximately 7.9 × 10⁸ t of sediment from the old delta each year [24]. The Yangtze River Basin is rich in strata, including Proterozoic to Quaternary. Among them, the upper reaches include the Yangtze Craton, which is dominated by metamorphic mudstone, acidic volcanic rocks, and carbonates, and the middle and lower reaches are mainly covered with medium acid granites [25]. The Yellow River Basin spans multiple tectonic belts, and the source rock attributes of the basin are rich and diverse, including Paleozoic limestone, Mesozoic carbonate, Quaternary loose sediments, etc., involving ancient metamorphic rock systems. The Yellow River mainly flows through the Loess Plateau, and the huge Quaternary aeolian loess is its most important and characteristic source [25]. The characteristics of the source rock of the Old Yellow River Basin are similar to those of the Modern Yellow River, and the difference between the two rivers is that the downstream of the Old Yellow River flows through the Sulu orogenic belt and the Yangtze craton [24]. In addition to riverine inputs, aeolian sediments from the northern China (i.e., the Kubuchi, Tengger, Mu Us, Badain Jaran, and Ulan Buh deserts) are also considered potential sediment sources to the Yellow Sea affected by the prevailing East Asian winter monsoon [26].

3. Materials and Methods

3.1. Sampling

In 2019, six surface sediment samples were collected from the onshore sand ridges and seven surface sediment samples are obtained from the estuary of each potential source (the Yangtze River Mouth, the Modern Yellow River Mouth and the Old Yellow River Delta) using a clean wooden shovel. The samples of these areas were collected at the water depth of about 50 cm. Meanwhile, a boat trip was undertaken to the offshore sand ridges where 12 surface sediment samples were collected using a Peterson sampler (DT-101B, Qingdao Mingcheng Environmental Protection Technology Co., Ltd., Qingdao, China) at the water depth of 6~8 m. All samples were taken from 0~20 cm on the surface. The sites of the selected samples were roughly evenly distributed in space, and covered each area (Figure 1b). In order to avoid the impact of contamination on the provenance research, the sampling sites were selected to avoid areas with significant human pollution, such as ports, waterways, and tourist attractions (Figure 1), and nitrile gloves were worn when sampling. After collection, branches, shells, and other debris were removed, and the samples were stored in polyethylene resealable bags.

3.2. Pre-Treatments and Analytical Methods

Clay-size sediments (<2 μm) were extracted using Stokes’ sedimentation law for Sr-Nd isotope composition and trace element analysis. The pre-treatment procedure is presented in Supplementary Materials. For trace element (containing the rare earth element, REE) analysis of the 37 samples (including all the collected samples), the dissolution method is presented in Supplementary Materials, the instrument used was an iCAP RQ inductively coupled plasma mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). GSD-9 and GSD-10 were used as reference standards for quality control of sample testing. For Nd isotope composition testing, 0.2 g of each sample was weighed and placed in a Teflon dissolution vessel (CEM Corporation, Matthews, UK). Next, the sample was dissolved at a constant temperature of 180 °C using a 1:6:3 mixture of HNO₃ + HF + HClO₄. Subsequently, Nd purification of the sample was carried out on a cation exchange resin column. Finally, the Nd isotope composition was tested using a VG354 multi-collector thermal ionization mass spectrometer produced by VG Instruments Ltd. (Manchester, UK). To ensure experimental accuracy, calibration was performed using La Jolla standards (NIST SRM 3137, Neodymium (Nd) Isotopic Standard Reference Material, National Institute of Standards and Technology (NIST); Gaithersburg, MD, USA, 2020) after testing five unknown samples [27]. The ¹⁴³Nd/¹⁴⁴Nd ratio was standardized using the CHUR based on the following formula:

ε_Nd(0) = [(¹⁴³Nd/¹⁴⁴Nd)_sample/(¹⁴³Nd/¹⁴⁴Nd)_CHUR − 1] × 10⁴

(1)

of which, (¹⁴³Nd/¹⁴⁴Nd)_CHUR = 0.512638 [27]. The determination instrument and procedure of Sr isotope are the same as those of Nd isotope, and the NBS 987 material was adopted to control quality.

According to the analysis by particle analyzer (Mastersizer-2000, Malvern Instruments Limited, Malvern, UK), the median grain size of the surface sediments from the study area ranged from 4.5 to 7.2 Φ (mainly coarse silt grade). The sorting coefficient ranged from 1.2~2.5, with a moderate deviation, indicating complex sediment sources or inadequate hydrodynamic sorting. Therefore, to limit the grain size sorting on silt-size sediment geochemistry, for silt-size sediment analysis, the samples with the grain size between 32 and 63 μm (4 to 5 Φ) were extracted through wet sieving, which is more accurate than the Stokes’ law. After that, at least 1 g K-feldspar of each silt-size sediment sample was separated through the magnetic separation–flotation method. Subsequently, the K-feldspar were soaked in 10% hydrogen peroxide for twelve hours to remove adsorbed organic contaminants and the Pb isotopic compositions and trace element (for the K-feldspar, the elements except for Si and O are all trace elements) contents were measured, with the determination method for trace element contents conducted following the same procedures as those used for the clay-size sediments. Before Pb isotopic composition determination of K-feldspar, 0.1 g of each sample was weighed and dissolved. The solution was then poured into AG1-X8 anion exchange resin (Bio-Rad Laboratories, Hercules, CA, USA) for Pb separation. After elution with 6 mol/L HCl, the sample analysis was performed using a VG354 mass spectrometer (VG Instruments Ltd., Manchester, UK). To ensure the quality of detection, NBS981 was used for standardization correction.

3.3. Quantitative Analysis of the Sediment Provenance

The Bayesian-based MixSIAR model (version 3.1.7) was employed to quantitatively estimate the source contributions to different fractions in the Jianggang sand ridges. MixSIAR explicitly incorporates uncertainties in tracer measurements, source variability, and process errors. The principal formulas are as follows:

$$X_{ij}={\sum }_{k=1}^n{P}_k({\mathrm{S}}_{\mathrm{j}\mathrm{k}}+{\mathrm{C}}_{\mathrm{j}\mathrm{k}})+{\mathsf{\epsilon }}_{\mathrm{i}\mathrm{j}}$$

(2)

$${\mathrm{S}}_{\mathrm{j}\mathrm{k}}~\mathrm{N}({\mathsf{\mu }}_{\mathrm{j}\mathrm{k}},{\omega }_{jk}^2)$$

(3)

$${\mathrm{C}}_{\mathrm{j}\mathrm{k}}~\mathrm{N}({\mathsf{\lambda }}_{\mathrm{j}\mathrm{k}},{\mathsf{\zeta }}_{jk}^2)$$

(4)

$${{\mathsf{\epsilon }}_{\mathrm{i}\mathrm{j}}~\mathrm{N}(0,\sigma }_j^2)$$

(5)

where X_ij is the value of tracer index j in sink sediment i, P_k is the contribution of potential source region k, S_jk is the value of index j in potential source region k, with an average value of μ and a standard deviation of ω, C_jk is the fractionation value of index j in potential source region k, with an average value of λ and a standard deviation of ζ, and ε_ij is the error of index j in sink sediment i, with an average value of 0 and a standard deviation of σ. The model algorithm was implemented using R Studio 4.1.3. Considering the computational efficiency and running time of the model, the error structure and the Markov chain Monte Carlo running step size were set to “Process + Residual” and “Long”, respectively. The convergence of the model results was determined by Gelman–Rubin and Geweke diagnostic tests, and the fractionation value in the model was set to 0.

3.4. Data Processing and Plot Editing

Box plots, line plots, dotted line plots, and principal component analysis were all generated using Origin 2022, developed by OriginLab, Inc. (Northampton, MA, USA) Maps and index distribution plots were generated using ArcGIS 10.3, developed by ESRI, Inc. (Redland, CA, USA) Image editing was performed using CorelDRAW 2019, developed by Corel Corporation, (Ottawa, ON, Canada). Multidimensional scaling analysis was performed using IsoplotR 6.1, a program based on the R language.

4. Results

4.1. Elemental Geochemical Characteristics of Sediments

The clay-size sediments of the Yangtze River Mouth have the highest Ba content, ranging from 439.06 μg/g to 495.76 μg/g (Figure 2a and Supplementary Table S1). Compared to the Yangtze River Mouth, the sediments of the Old Yellow River Delta show significantly higher contents of Ba, Cu, Y, Ta, and Pb. The sediments of the Modern Yellow River Mouth exhibit higher contents of Sr, Sc, Cu, and Pb (Figure 2a and Supplementary Table S1).

The onshore sand ridges show the highest Ba content, ranging from 453.88 μg/g to 531.05 μg/g, followed by Cr and Rb. Compared to the Yangtze River Mouth, the Modern Yellow River Mouth and the Old Yellow River Delta, sediments in these areas show significantly higher contents of Sr, Cr, and Ni (Figure 2a–c and Supplementary Table S1). The sediments of the offshore sand ridges exhibit the highest content of Ba, which range from 432.24 μg/g to 516.01 μg/g. This is followed by Cr and Zr. Compared to the Yangtze River Mouth, the Modern Yellow River Mouth and the Old Yellow River Delta, sediments in these regions exhibit higher contents of Sr, Cr, and Co (Figure 2a–c and Supplementary Table S1).

For the REE of the clay-size sediments, in the Yangtze River Mouth, the content of Ce has the highest value, while the Lu has the lowest content with the value of lower than 0.4 (Figure 3a and Supplementary Table S1). Compared with this river, the Modern Yellow River Mouth shows higher contents of all the REE, while the Old Yellow River Delta shows the lower contents of La, Tm, Yb, and Lu. The REE contents of the onshore and offshore sand ridges are the same as those of the Yangtze River Mouth as a whole (Figure 3b,c and Supplementary Table S1).

The Yangtze River Mouth shows the highest Ti content, ranging from 236.08 to 2360.24 μg/g, followed by Ba (Figure 2d and Supplementary Table S2). Compared to the Yangtze River Mouth, the Old Yellow River Delta has lower contents of Ti, Hf, and Ta; however, the Modern Yellow River Mouth has higher contents of Sr (Figure 2d,f and Supplementary Table S2).

The onshore sand ridges sediments show the highest Ba content, ranging from 398.02 to 2716.62 μg/g, followed by Fe. Compared to the river sediments, the sediments of the onshore sand ridges show higher Rb and Ba contents (Figure 2e,f and Supplementary Table S2). The K-feldspar in silt-size sediments from the offshore sand ridges show the highest Ba content, which ranged from 638.46 to 2822.36 μg/g. Compared to the river sediments, the K-feldspar of silt-size sediments has significantly higher contents of Ge, Rb, Cs, and Ba (Figure 2e,f and Supplementary Table S2). These differences show the diversity of onshore and offshore sediment provenance and demonstrate the feasibility of using mineral geochemistry for tracing clay and silt components.

For the REE of the K-feldspar of silt-size sediments, the Yangtze River Mouth has the highest contents of Ce and the lowest content of Lu. Compared with this source, the Old Yellow River Delta has the higher contents of La, Ce, and Eu (Figure 3d–f and Supplementary Table S2), the Modern Yellow River Mouth has the higher contents of Ce, Eu, Yb, and Lu. As a whole, the REE contents of the onshore and offshore sand ridges are similar to those of the Old Yellow River Mouth (Figure 3d–f, and Supplementary Table S2).

Compared to the upper continental crust (UCC), the clay-size sediments of the modern Yellow River Mouth, the old Yellow River Delta, the Yangtze River Mouth, and the Jianggang sand ridges are significantly enriched in Li, Rb, V, Cr, Cu, Zn, Y, Pb, Th, U, and REE, while depleted in Sr, Ba, and Ta (Figure 4a,b). The UCC-normalized distribution patterns display large variations in Li, Rb, Pb, Th, and U, while in the rare earth elements they show relatively flat and slightly left-leaning characteristics, which is manifested by the higher UCC normalization value of Tm-Lu and the lower normalization value of LREE (Figure 4a,b).

After normalization by the UCC, the K-feldspar in the silt-size sediments of these regions show significant depletion of all elements except for Ge, Rb, Cs, Sr, Ba, and Pb (Figure 5c,d). Meanwhile, the sediments of the Jianggang sand ridges show positive Eu anomalies, which are similar to those of the Yangtze River Mouth, the Old Yellow River Delta, and the Modern Yellow River Mouth (Figure 5c,d), reflecting the silt-size sediments of the study areas are mainly from the above-mentioned terrigenous rivers. Usually, in the process of magma evolution, part of Eu²⁺ can replace Ca²⁺ and preferentially enter the plagioclase lattice. With the separation of a large number of plagioclase, the Eu in the melt is consumed in large quantities, resulting in significant negative Eu anomalies in the residual melt and its minerals (such as K-feldspar). The positive Eu anomaly can reflect the capture of Eu by K-feldspar during diagenesis [17].

4.2. Isotopic Characteristics of Sediments

The ⁸⁷Sr/⁸⁶Sr of clay-size sediments in the Yangtze River Mouth ranges from 0.7195 to 0.7257, with an average of approximately 0.7222, while the ε_Nd(0) ranges from −15.41 to −11.57, with an average of approximately −13.68 (Figure 6a,b and

Table 1. Statistics of Sr-Nd isotopic compositions in clay-size sediments from the JSR and potential sources.

Sampling Area	87Sr/86Sr		143Nd/144Nd		εNd(0)
	Average	Range	Average	Range	Average	Range
Onshore JSR (6)	0.7226	0.7190~0.7265	0.511968	0.511892~0.512013	−13.07	−14.55~−12.19
Offshore JSR (12)	0.7202	0.7158~0.7255	0.511978	0.511887~0.512078	−12.87	−14.65~−10.92
Yangtze River Mouth (7)	0.7222	0.7195~0.7257	0.511937	0.511848~0.512045	−13.68	−15.41~−11.57
Old Yellow River Delta (7)	0.7189	0.7159~0.7241	0.512035	0.511892~0.512128	−11.75	−14.55~−9.95
Mouth Yellow River Mouth (7)	0.7215	0.7176~0.7254	0.511993	0.511838~0.512141	−12.56	−15.60~−9.68

Note: The numbers in the parentheses are the sample quantities.

). The average ⁸⁷Sr/⁸⁶Sr and ε_Nd(0) values of sediments from the old Yellow River Delta are lower and higher than those of sediments from the Yangtze River Mouth, respectively. Those isotope values of sediments from the Modern Yellow River Mouth are between those of the Yangtze River Mouth and the Old Yellow River Delta (Figure 6a,b and Table 1). In the onshore sand ridges, the range and average values of ⁸⁷Sr/⁸⁶Sr of sediments are similar to those in the Yangtze River Mouth, while those of the ε_Nd(0) are located between the Yangtze River Mouth and the Modern Yellow River Mouth (Figure 6a,b and Table 1). The ⁸⁷Sr/⁸⁶Sr range of the offshore sand ridges sediments is similar to that of the Old Yellow River Delta, with an average value slightly higher than that of the Old Yellow River Delta. For ε_Nd(0), the range is slightly wider than that of the Yangtze River Mouth and onshore sand ridges, with the average value slightly lower than that of the Modern Yellow River Mouth (Figure 6a,b and Table 1).

The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb of K-feldspar in the silt-size sediments from the Yangtze River Mouth are 18.01~18.29, 15.53~15.74, and 37.77~38.59, respectively (Figure 6c–e and

Table 2. Statistics of K-feldspar Pb isotopic compositions in silt-size sediments from the JSR and potential sources.

Sampling Area	206Pb/204Pb		207Pb/204Pb		208Pb/204Pb
	Average	Range	Average	Range	Average	Range
Onshore JSR (6)	18.26	18.07~18.43	15.56	15.45~15.69	38.33	38.26~38.46
Offshore JSR (12)	18.19	17.96~18.33	15.57	15.46~15.69	38.29	38.07~38.55
Yangtze River Mouth (7)	18.17	18.01~18.29	15.64	15.53~15.74	38.23	37.77~38.59
Old Yellow River Delta (7)	18.39	18.23~18.51	15.58	15.49~15.65	38.47	38.28~38.72
Modern Yellow River Mouth (7)	17.85	17.56~18.04	15.47	15.36~15.61	37.65	37.46~38.09

Note: The numbers in the parentheses are the quantities of samples.

). In comparison, the average values of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb in the K-feldspar of the silt fraction from the Old Yellow River Delta are higher, while that of ²⁰⁷Pb/²⁰⁴Pb is lower (Figure 6c–e and Table 2). The mean values of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb in K-feldspar from the Modern Yellow River Mouth sediments are about 17.85, 15.47, and 37.65, respectively, which are lower than those of the previous two river sediments, reflecting the differences in fluvial source rocks between the Modern Yellow River and others rivers. The ranges of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb in K-feldspar of the silt-size sediments from the onshore sand ridges are 18.07~18.43 and 15.45~15.69, respectively, similar to those from the Old Yellow River Delta. The range (38.26~38.46) and average value (38.33) of ²⁰⁸Pb/²⁰⁴Pb in the sediments from these regions (38.26~38.46) are higher than those from the Yangtze River Mouth (Figure 6c–e and Table 2). For the offshore sand ridges, the range and average value of ²⁰⁶Pb/²⁰⁴Pb in K-feldspar of silt-size sediments are similar to those from the Yangtze River Mouth, while the ranges of ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb are close to those of the Yangtze River Mouth and the Old Yellow River Delta (Figure 6c–e and Table 2). Overall, the Sr-Nd-Pb isotopic composition of the sediments from the Jianggang sand ridges differed from those from the Yangtze River Mouth, the Modern Yellow River Mouth and the Old Yellow River Delta, which reflects the constraints imposed by diverse sources on the clay and silt components of the sand ridges.

5. Discussions

5.1. The Influence of Particle Size Sorting on Elemental Geochemistry

Compared to the bulk sediments [15], the contents of transition metal elements and large ion lithophile elements in the clay-size sediments of the onshore and offshore sand ridges are higher (Figure 7a), reflecting their enrichment in the clay mineral lattice [30]. The contents of rare earth elements, Y, Zr, Nb, Hf, and Ta are comparable in clay and bulk sediments (Figure 7a) because these elements are mainly associated with heavy minerals, which are widely present in sediments with different grain sizes [31]. In addition, it can be seen that Sr and Ba are depleted in clay-size sediments (Figure 7a) because these elements are mostly present in coarse-grained feldspar of sediments [30] and, therefore, have relatively lower contents in the clay fraction.

Compared with the bulk sediments, the K-feldspar of silt-size sediments in the Jianggang sand ridges have higher contents of Rb, Sr, Ba, and Pb, and an obviously positive Eu anomaly (Figure 7b). This reflects the capture of the above elements during the crystallization process of K-feldspar. Rb⁺ and Ba²⁺ have the same charge as K⁺ and can easily enter the crystal lattice of K-feldspar and other minerals in proportion [32]. Sr²⁺ and Pb²⁺ have the same charge and similar ionic radiuses, so K-feldspar can easily capture them in the melt, causing Sr enrichment [32]. The above findings indicate that elements with different geochemical properties are sorted by minerals, as well as clay minerals, which is beneficial for the source identification based on elements.

5.2. Sediment Provenance of Clay and Silt Fractions in the Jianggang Sand Ridges

5.2.1. Qualitative Estimation

It is widely accepted that the elemental compositions of sediments, including V, Rb, Sc, Y, Hf, La, and Sm, and the Sr-Nd isotope compositions, are highly discriminative and stable in the surface environment [30,33,34]. Therefore, this study combines these elements for provenance tracing (Table S3).

The UCC-normalized distribution patterns of the clay-size sediments in the Jianggang sand ridges are significantly different from those in the Korean rivers (Figure 4a,b). Furthermore, discrimination diagrams of elemental ratios and Sr-Nd isotope show that the clay fractions of the Jianggang sand ridges, the Korean rivers, and the northern Chinese deserts fell in different regions (Figure 8), indicating that these two potential sources are not the main provenance of clay-size sediments in the Jianggang sand ridges. This may be because these two potential sources are far from the sand ridges, and some sediments migrate to other regions during long-distance transport [35].

It can be further observed from Figure 8 that the sediments of the onshore sand ridges are almost entirely located close to the Yangtze River Mouth, while some of the offshore sediments are close to the Yangtze River Mouth, and others are distributed within the regions of the Modern Yellow River Mouth and the Old Yellow River Delta. Multidimensional scaling analysis and principal component analysis also show the close relationship between the sediments from the onshore or offshore sand ridges and the Yangtze or Yellow River, as well as a distant relationship between the Jianggang sand ridges and the Korean rivers or Northern Chinese Deserts (Figure 8e,f). Therefore, the Yangtze River Mouth, the Modern Yellow River Mouth, and the Old Yellow River Delta have a significant influence on onshore and offshore clay-size sediments, while the Korean rivers and the Northern Chinese Deserts are not the main contributors to the clay-size sediments of the sand ridges.

Previous studies have found that the Pb isotopes, (Ce/Lu)_n ratio, and Rb/Cs ratio of K-feldspar, as well as oxygen isotopes, Mg/Fe ratio, and Ge/Ti ratio of quartz are good provenance tracers, and these indicators can effectively represent the compositions of source area [17,18]. Therefore, these indicators are used for source tracing of silt-size sediments (Table S3). Element ratios and oxygen isotope of the quartz are from the unpublished data. The provenance discrimination results show that the silt-size sediments of the Korean rivers and the Northern Chinese Deserts are far from the scopes of Jianggang sand ridges in the diagram (Figure 9). Therefore, they have limited sediment supplies to the Jianggang sand ridges. Further analysis of Pb-O composition shows that the onshore sand ridges sediments mainly overlap with the scope of the Yangtze River Mouth. In contrast, the offshore sand ridges sediments are located near the Yangtze River Mouth and the Old Yellow River Delta (Figure 9c,d). However, both the onshore and offshore sand ridges are at a certain distance from the Modern Yellow River Mouth (Figure 9). Based on the UCC normalized patterns, principal component analysis, and multidimensional scaling analysis, the results demonstrate that the silt-size sediments of the onshore sand ridges are mainly derived from the Yangtze River Mouth. In contrast, those of the offshore sand ridges have two dominating sources: the Old Yellow River Delta and the Yangtze River Mouth.

5.2.2. Quantitative Calculation

The MixSIAR model was used to accurately evaluate the contributions of potential sources to the Jianggang sand ridges. Since the isotopic and elemental ratios of clay-size sediments from the Old Yellow River Delta and the Modern Yellow River Mouth overlapped, they were combined into a single endmember. Calculations based on ε_Nd(0)-Rb/Sc show that the Yangtze River Mouth had the most primary contribution to the clay-size sediments of the onshore and offshore sand ridges, reaching 53.9 ± 8.8% and 51.9 ± 8.4%, respectively. Followed by the Modern Yellow River and the Old Yellow River (34.6 ± 8.7% for onshore sand ridges, 35.8 ± 8.1% for offshore sand ridges), while the efforts from the Northern Chinese Deserts and Korean rivers were the lowest (Figure 10a).

For silt-sized sediments, calculations show that the Yangtze River Mouth contributed less to offshore sand ridges (36.5 ± 5.7%) than the onshore (46.8 ± 5.5%) ones. The Old Yellow River Delta shows the opposite trend, with the contributions of 29.1 ± 6.1% and 42.4 ± 5.3% to the onshore and offshore sand ridges, respectively. Moreover, the Modern Yellow River Mouth contributes more to offshore than to onshore sand ridges. The Northern Chinese Deserts contribute similarly to onshore (3.6 ± 1.4%) and offshore (5.0 ± 1.9%) sand ridges, while the Korean rivers contribute 6.9 ± 2.5% to offshore sediments, higher than that to the onshore sediments (Figure 10b).

Compared to the clay fraction, the Yangtze River Mouth and the Modern Yellow River Mouth contribute less to the silt component of the Jianggang sand ridges. After excluding the dominant effects of calculation errors, this can be attributed to the sorting effect of coarse grains, which settle quickly through migration.

5.3. Source-Sink Transport Patterns of Sediments in the Jianggang Sand Ridges

The Sr-Nd isotopic compositions of the onshore clay-size sediments from the Jianggang sand ridges are similar to those from the Yangtze River Mouth (Figure 8 and Table 1), suggesting northward supplement of these sediments from this potential source (Figure 11a). This view is consistent with the studies based on hydrodynamic simulations and mineralogical analysis [9,21]. Further comparison shows that the Sr-Nd isotopic ratios of the clay-size sediments from the Yangtze River Mouth are similar to those from the central Yellow Sea and southeastern Shandong Peninsula (Figure 11a). Therefore, the results demonstrate that the clay-size sediments from the Yangtze River Mouth carry a long-distance migration path, which pass through the central Yellow Sea sedimentary area, reach the eastern Shandong Peninsula, and then move southward along the coast to the Jianggang sand ridges through the Yellow Sea Warm Current and Subei Coastal Current (Figure 11a).

The Pb isotopic ratios of silt-sized sediments in the Yangtze River Mouth are similar to those of the onshore sand ridges (Figure 9 and Table 1), reflecting the northward supply of silt-size sediments from this potential source (Figure 11b), which is consistent with previous studies on sediment transport [45]. Through the analysis and comparison of previous research data, it is found that the contents of Zn, Rb, Sr, Pb, and Ba in K-feldspar from silt-size sediments in the central South Yellow Sea differed from those in the Yangtze River Mouth [44]. In addition, the ternary phase diagram shows that the projection distance of the Yangtze River Mouth and the central South Yellow Sea is relatively far (Figure 11b). Therefore, although silt-size sediments in the Yangtze River Mouth can reach the central Yellow Sea under hydrodynamic force [46], long-distance circulation may not be the main pattern of supply to the Jianggang sand ridges. Compared to the clay fraction, the long-distance transport of silt-size sediments in the Yangtze River Mouth is limited because hydrodynamic sorting during circulation makes it difficult to maintain migration.

Under the influence of the Bohai Coastal Current, part of the sediments in the Modern Yellow River Mouth settle in the Shandong Peninsula sedimentary area and the central Yellow Sea sedimentary area [47]. In contrast, another part continues to migrate southward along the Shandong Peninsula and reaches the Old Yellow River Delta under the influence of the Bohai Coastal Current [48]. Subsequently, sediments of the Modern and the Old Yellow River are carried by the Subei Coastal Current to the Jianggang sand ridges [3,5,45], which is the only route for them to reach the study area and has been supported by previous studies [9,20].

The sediments of the Korean rivers reach the southeastern Yellow Sea sedimentary area under the influence of the Korean Coastal Current [30], after which part of them is driven northward by the Yellow Sea Warm Current and eventually reaches the Jianggang sand ridges under the influence of the Subei Coastal Current. In northern China, dust storms occur most frequently during the spring [49]. During this period, dust from the Northern Chinese Deserts is transported to the eastern coast of China driven by the East Asian Winter Monsoon [49]. This transport pattern has been supported by previous model-based and observational studies [49].

5.4. Recommendation to Maintain the Area of Jiangsu Tidal Flats

Jiangsu tidal flat encompasses a vast region of land and sea. As a key area for land resource development and management, it has a constant impact on local construction and sustainable development. Previous studies detected that from 2009 to 2017, the total area of Jiangsu tidal flat had decreased [50]. In the future, due to the combined effects of climate warming, ocean expansion, and glacial melting, the sea level of China is expected to rise. Moreover, the disturbance of storm surges will cause the coastline of Jiangsu to retreat. Therefore, the total area of Jiangsu tidal flats is likely to continue to decrease hereafter. According to our study, sediment transport from the Yangtze River and the Yellow River has a significant impact on the changes of this coastal region. Therefore, in the future, it is necessary to maintain the stability of river sediment discharge by river management, dam dredging and long-term monitoring.

6. Conclusions and Prospects

In this study, the Sr-Nd isotopes of clay-size sediments, K-feldspar Pb isotopes of silt-size sediments and their elemental geochemistry are explored to investigate the source contributions and transport patterns of clay and silt fractions in sediments from the Jianggang sand ridges based on the multi-dimensional scaling analysis and the MixSIAR model. The main conclusions are as follows:

(1)

The clay-size sediments and K-feldspar of silt-size sediments of the Jianggang sand ridges have the highest content of Ba. Compared with the Yangtze River Mouth, the Modern Yellow River Mouth, and the Old Yellow River Delta, the clay fraction of Jianggang sand ridges has significantly higher contents of Sr, Cr, Ni, Co, and Lu, while the K-feldspar of silt fraction have higher contents of Rb and Ba. Compared with the bulk sediments, the clay-size sediments are enriched in Sc, Co, Ni, Cu, Li, Rb, Pb, and Th, while the K-feldspar of silt-size sediments are enriched in Rb, Sr, Ba, and Pb, indicating the adsorption of characteristic elements by clay minerals and K-feldspar. The ranges of the Sr and Nd isotopic compositions in clay-size sediments from the Jianggang sand ridges are wider and narrower than those from the Old Yellow River Delta, respectively, while the ranges of Pb isotopic compositions in K-feldspar from the research areas are only narrower than those from the Modern Yellow River Mouth. These isotopic and geochemical differences indicate that the provenance of different size fractions in the Jianggang sand ridges are diverse.

(2)

Provenance analysis shows that the Yangtze River Mouth is the main source of clay-sized sediments in the onshore and offshore sand ridges, contributing approximately 53.9 ± 8.8% and 51.9 ± 8.4%, respectively. The Modern and the Old Yellow River are the secondary ones (approximately 34.6 ± 8.7% and 35.8 ± 8.1%, respectively). The contributions from the Northern Chinese Deserts and the Korean rivers are both less than 7%. For silt-size sediments, the Yangtze River Mouth contributed 46.8 ± 5.5% and 36.5 ± 5.7% to the onshore and offshore sand ridges, respectively, making it the main provenance. The Old Yellow River Delta, which is the subordinate one, contributed 29.1 ± 6.1% and 42.4 ± 5.3% to the onshore and offshore regions, respectively. The Modern Yellow River Mouth contributes less than 16%, while the contributions from the Northern Chinese Deserts and Korean rivers are less than 7% to the study areas. The reason for the conclusions may be that the Yangtze River and the Old Yellow River are close to the study area, and there are stable circulations around them. Moreover, their sediment transport is relatively sufficient.

(3)

Based on the distribution of tracer indicators, it was shown that the clay-size sediments in the Yangtze River Mouth have a specific migration path, which moved through the central Yellow Sea to the Jianggang sand ridges under the influence of the Yellow Sea Warm Current and the Subei Coastal Current. In contrast, this path is limited for silt-size sediments from the Yangtze River Mouth, which mainly rely on the northward Subei Coastal Current and the Yangtze River Diluted Water to reach the sand ridges. The clay and silt fractions of the Modern and the Old Yellow River sediments migrate to the study areas mainly through the southward coastal currents. At the same time, sediments of the two fractions in the Korean rivers reach the Jianggang sand ridges with the support of the Yellow Sea Warm Current and the southward coastal currents, while those from the Northern Chinese Deserts reach the study area mainly through the influence of the East Asian Winter Monsoon.

This study has played a practical role in the management of coastal tidal flat resources and the coordinated and sustainable development of the region. It has also demonstrated the feasibility of using single mineral geochemistry to trace the provenance of marginal sea sediments. In the future, it is suggested to utilize high-resolution sedimentary records and numerical modeling to further investigate the long-term evolution of source-to-sink processes in response to sea-level changes and human activities. Meanwhile, considering that the number of samples collected in this study is relatively limited, it is recommended to arrange sampling points on a larger scale and in higher density in future studies, which will improve the accuracy of the research conclusions. Moreover, it is also suggested to add a variety of trace indicators including organic matter, clay minerals, and biological sign, and construct a coupling model based on particle size–hydrodynamics-indexes to further enhance the reliability of provenance tracing results.