Grain size, geochemical characteristics, and transport patterns of surface sediments in the Dongsha sea area, South China sea

This study conducted a comprehensive analysis of grain size, major elements, and rare earth elements (REEs) in 34 surface sediment samples from the Dongsha area of the South China Sea to reveal their characteristics and material sources. Grain size analysis indicates that sedimentary processes are jointly controlled by inputs from multiple sources and complex hydrodynamic conditions. End-member modeling identified three main components corresponding to distantly transported riverine clay, riverine fine silt, and proximal shelf coarse clastics. Sediment transport predominantly occurs along northeast-southwest and northwest-southeast trends. The major element composition is primarily Al₂O₃, followed by CaO, indicating a primarily terrigenous clastic origin with minimal contributions from marine biogenic processes and authigenic sedimentation. The REE distribution patterns, characterized by light REE (LREE) enrichment, moderate negative Eu anomalies, and no Ce anomalies, support a predominantly terrigenous origin. Comprehensive provenance analysis demonstrates a high geochemical affinity between the sediments and materials derived from Taiwan, with a minor contribution from the Pearl River. Therefore, Taiwan is the dominant source of surface sediments in the study area, and the Pearl River is a secondary source. These findings provide a critical basis for understanding sedimentary processes and material transport mechanisms in the Dongsha area of the South China Sea.

1 Introduction

The South China Sea (SCS), the largest marginal sea in the western Pacific Ocean, is located in the East Asian monsoon belt. The sediment flux transported to the South China Sea by the surrounding islands and rivers reaches 700 Mt/yr, making it among the world’s marginal seas that receive the most terrestrial substances (Liu et al., 2016). At the same time, the South China Sea is under the action of the East Asian monsoon and ocean currents, forming a multilevel, complex, and changeable ocean current system, which profoundly affects the “source-sink” processes and transport mechanisms of sediments (Hu et al., 2012; Clift et al., 2014; Liu et al., 2016; Xu et al., 2024). The northern continental slope of the South China Sea is a key channel for the transport of material from the South China mainland and Taiwan Island to the abyssal basin, and it is also among the main deposition centers of terrigenous clastics. Within the South China Sea, the sedimentary environments differ significantly between the continental shelf and abyssal basin areas, which is reflected in the multisource characteristics of sediment sources and the complexity of hydrodynamic processes (Zhao et al., 2017). The Dongsha area is located on the northern continental slope of the South China Sea. It is characterized by complex geological structures, having undergone multiple phases of tectonic movement and complex fold deformation, including various structures such as compressional anticlines (Sun et al., 2018; Liu et al., 2022), rollover anticlines, and buckle folds. Furthermore, the region features extensive development of faults, fractures, and mud diapirs, which provide crucial pathways for the formation and migration of natural gas hydrates (Su and Liu, 2020). Intense marine dynamics, such as the strong bottom currents from a branch of the Kuroshio Current, further modify the seabed topography, resulting in complex geomorphic features like submarine erosion channels (Luan et al., 2011; Huang et al., 2018). This multi-source nature and process complexity make it an ideal area for studying sediment transport and deposition (Wang et al., 2022). However, the relative contribution rate of the Dongsha Sea Area sediment sources remains controversial and must urgently be further clarified through comprehensive multi-indicator analysis methods.

The grain-size parameters of sediments are primarily governed by sediment provenance, local hydrodynamic conditions, and regional seabed topography (Visher, 1969; Talas et al., 2023). Beyond traditional grade scales and grain-size parameters (Folk et al., 1970), various granulometric methods have been progressively developed and utilized in recent years. For instance, grain-size end-member modeling has proven highly valuable for analyzing sediment provenance and transport dynamics (Weltje, 1997; 2012; Peng et al., 2023). McLaren and Bowles (McLaren and Bowles, 1985) introduced a one-dimensional grain-size trend analysis model to determine the net transport pathways of seabed sediments. Subsequently, the two-dimensional model further refined by Gao and Collins (Gao and Collins, 1992; Gao, 1996) has been widely adopted. Concurrently, the geochemical characteristics of sediments play a crucial role in provenance studies (Sawant et al., 2017; Ramirez-Montoya et al., 2021; Sonfack et al., 2021; Wanas and Assal, 2021; Singh et al., 2022; Sangeeta et al., 2023; Galindo-Ruiz et al., 2025). Analysis of the composition and distribution of major and rare earth elements can effectively distinguish contributions from different source areas, determine tectonic settings, and thereby reveal the effects of mixing and sorting during sedimentary processes (Sun et al., 2024). Integrating grain-size analysis with geochemical indicators allows for a more comprehensive and accurate reconstruction of sediment source-to-sink processes.

To address the above scientific questions, in this study, 34 surface sediment samples collected from the Dongsha Sea Area of the South China Sea were subjected to grain-size analysis, major- and rare-earth-elemental analysis, and transport trend simulation. Additionally, the compositional characteristics and sediment sources of the surface sediments in this area were systematically investigated.

2 Regional overview

Characterized by a NE-SW orientation, the northern slope of the South China Sea continental margin extends from the southwest of Taiwan Island to the eastern mouth of the Xisha Trough. It is defined by a northern boundary with the shelf at −200 m water depth and a southern boundary with the deep-sea basin at 3,400–3,700 m (Xu, 1997). A series of west-east trending submarine canyons, including the Pearl River Mouth, Dongsha, South Taiwan Bank, and the Penghu Canyon cluster on the southwestern Taiwan slope, developed across this region (Liu et al., 2019). These canyons profoundly alter the marginal morphology, generating NW- and WNW-trending negative topographies on the slope that act as critical pathways for terrestrial sediment transport to the deep sea (Wang, 1991; Zhang et al., 2019).

There is abundant rainfall in the South China Sea, with the average annual rainfall ranging from 1,500–2000 mm, and the seasonal distribution is uneven and is concentrated mainly in summer and autumn. Affected by the monsoon, the northern portion of the South China Sea experiences abundant precipitation in summer and occasional extreme weather, such as torrential rain and typhoons. A complex ocean current system has developed in the northern portion of the South China Sea, including deep-water currents, the Kuroshio Current, surface currents influenced by the East Asian monsoon, and the South China coastal current (Shaw and Chao, 1994; Huang et al., 2016). The ocean currents of the Dongsha Sea Area are affected by monsoons, water exchange through straits, and topography, resulting in the formation of complex and changeable ocean current systems, including slope current, the South China Sea warm current, coastal current, upwelling (summer), and downwelling current (winter) (Shu et al., 2018). The shallow circulation in the South China Sea is controlled by the monsoon system. In winter, it is strongly affected by the northeast monsoon, forming a slope current that flows southwestward; in the summer southwest monsoon, the direction of ocean currents changes, and the slope current flows northeastward (Fang et al., 2012).

3 Samples and test methods

3.1 Sample collection

In 2019, surface sediment samples were collected from the Dongsha Sea Area (116.62°E−118.54°E, 19.38°N-21.5°N) at water depths between 1,400 and 2000 m using a stainless-steel box corer. The locations of the sampling stations are shown in Figure 1. At each station, sediment samples from the upper layer (0–5 cm) were collected, placed in polyethylene sample bags, and sealed for low-temperature storage.

Figure 1

Figure 1. Regional overview and sampling station locations in the study area (a) Geography, current system, and monsoon in the study area; (b) Locations of the surface sediment sampling points. The current system and monsoon direction were obtained from (Fang et al., 1998; Li et al., 2010; Liu et al., 2010). The surface ocean currents in the South China Sea are numbered as follows: 1. Loop Current; 2. SCS Branch of Kuroshio; 3. Luzon Cyclonic Gyre; 4. Luzon Cyclonic Eddy; 5. Luzon Coastal Current; 6. SCS Warm Current; 7. Guangdong Coastal Current. KC: Kuroshio Current; TWC: Taiwan Warm Current.

3.2 Test methods

3.2.1 Grain size analysis

The surface sediment grain size was evaluated in the physical properties laboratory of Qingdao Sparta Analytical and Test Co., Ltd. An appropriate amount of sediment sample was placed in a 50 mL centrifuge tube, mixed with 15% H₂O₂, and reacted in a 60 °C water bath until completion. After removing the supernatant, 10% HCl was added, and the mixture was again heated at 60 °C until no further gas evolution was observed. Then, distilled water was added, and the sample was centrifuged repeatedly until the pH of the solution was neutral. Afterward, 0.5 mol/dm³ (NaPO₃)₆ solution was added, followed by ultrasonication to prepare the solution for grain size analysis. The grain size was determined using a Malvern Mastersizer 2000 laser grain-size-measuring instrument. The measurement range of the instrument is 0.02–2000 μm, the testing error is less than 2%, and the grain size resolution is 0.01 Φ.

3.2.2 Major elemental analysis

Major elemental analysis was performed at the Geochemical Laboratory of Qingdao Sparta Analytical and Test Co., Ltd., using Varian 720 ES inductively coupled plasma‒optical emission spectrometry (ICP‒OES) manufactured by Varian, Inc. (United States). A 0.04 g powder sample was accurately weighed into a Teflon cup, to which 1.5 mL of HF and 0.5 mL of HNO₃ were added. Next, the sample was sealed and placed in an oven for digestion at 180 °C for 12 h, after which it was cooled and placed on a 150 °C electric hot plate to evaporate to near dryness. Then, 1 mL of HNO₃ and 1 mL of water were added. After cooling, the Teflon beaker was removed, weighed, and diluted to 40 g (the dilution factor was approximately 1,000) to obtain the test solution for instrumental analysis. To monitor the precision and accuracy of the test, parallel samples and standard substances were prepared for every 10 samples, and the test results were within the allowable range. The standards used were GBW07314, GBW07315, GBW07316, BHVO-2, and BCR-2, the measurement deviations for all major elements were less than 3% (Table 1).

Table 1

Table 1. Comparison of measured values versus certified values for major elements by ICP-OES (%).

3.2.3 Rare earth elemental analysis

Rare earth elements were analyzed at the Geochemical Laboratory of Qingdao Sparta Analytical and Test Co., Ltd., using an iCAP RQ ICP‒MS. The bulk surface sediment sample was dried and ground to 63 μm. The test procedure was the same as that for the major elemental analysis. To monitor the precision and accuracy of the analysis, replicate samples and standard samples were analyzed, and the test results were within the allowable range. The standards used were GBW07314, GBW07315, GBW07316, BHVO-2, and BCR-2, the test results fall within the acceptable margin of error (Table 2).

Table 2

Table 2. Comparison of measured versus certified values for rare earth elements (μg/g).

4 Results

4.1 Grain-size component characteristics and end-member analysis

4.1.1 Characteristics of the grain size component

The surface sediment grain size analysis of the surface sediments in the Dongsha Sea Area revealed that the sediments in the study area were composed mainly of silt, clay, and sandy components (Figure 2). The silt content was the highest, ranging from 54.27% to 76.63%, with an average value of 62.42%. In terms of spatial distribution, the silt content gradually decreased from northwest to southeast. The clay component content was the next highest, ranging from 20.16%–42.31%, with an average value of 34.83%. Its spatial distribution characteristics were significantly negatively correlated with the silt component, showing a distribution pattern that increases from northwest to southeast. The overall content of the sand component in the study area was the lowest, ranging from 0.1%–10.14% with an average value of 2.75%. The spatial distribution of the sand component was relatively scattered, with relatively high-value localized areas on the west, northeast, and southeast sides, without obvious spatial distribution patterns.

Figure 2

Figure 2. Spatial distribution of surface sediment particle-size components.

Sediment grain-size parameters can effectively reflect grain-size characteristics and serve as a good indicator for analyzing depositional environments and provenance. The average grain size of the surface sediments in the Dongsha Sea Area ranged from 6.8–29.7 µm, with an average value of 13.4 µm, mostly ranging from 9–12 µm (Figure 3). The sediment grains were generally relatively fine and decreased in size from northwest to southeast, the mean grain size can reflect the average kinetic energy of the depositional medium, with coarser grains generally indicating stronger regional hydrodynamic conditions. Therefore, it is inferred that the overall hydrodynamic energy in the study area gradually weakens from the northwest to the southeast.; the sorting coefficient was between 1.3 and 2.1, with an average value of 1.6, the sorting coefficient shows an increasing trend from southeast to northwest, indicating a gradual deterioration in sorting, which is similar to the trend of the average grain size; Skewness reflects the symmetry of the grain-size frequency distribution curve, which provides insights into the origin of the sediment, the variation range of skewness was relatively small, and both positive and negative skewness were present, with positive skewness predominating and ranging between −0.099 and 0.216, the average skewness was 0.04; Most of the study area exhibits a platykurtic distribution, the kurtosis was in the range of 0.9–1.2 with an average of 1.1 and a gradual increase from the northwest to the southeast in the study area, the presence of anomalous kurtosis values indicates that the sediments are derived from mixed sources.

Figure 3

Figure 3. Surface sediment grain size parameters.

4.1.2 Grain-sized end-member analysis

The end-member analysis results for the surface sediments in the Dongsha Sea Area are shown in Figure 4. A relatively high multiple correlation coefficient (R²) and a low angle deviation value (θ) indicate better fitting results. When the number of end members was 2, R² was 0.97; when the number of end members was 3, R² was 0.99; and when the number of end members was 4, R² did not change significantly (Figure 4). To avoid excessive fitting and keep the angle deviation as small as possible (Zhang et al., 2020), three end members were selected in this study to perform end member inversion on the grain size data. The three end-member distribution curves obtained from the inversion all had an obvious main peak, and the overall performance was a lognormal distribution. The grain sizes of the main peaks from End Member 1 to End Member 3 increased progressively and were 2.76, 6.57, and 15.63 μm, respectively (Figure 4c).

Figure 4

Figure 4. Simulation of the surface sediment grain size end-member (a) Linear correlation; (b) Angle deviation; (c) Frequency distribution curve of each end-member.

4.2 Geochemical elemental content and distribution characteristics

4.2.1 Content and distribution characteristics of major elements

The results of the major elemental content and related parameters (Table 3) revealed that the major elements of the surface sediments in the study area are present as Al₂O₃ (7.31%), CaO (6.24%), Fe₂O₃ (3.85%), K₂O (2.14%), Na₂O (1.68%), MgO (1.29%), TiO₂ (0.36%), P₂O₅ (0.01%), and MnO (0.001%). Among them, the Al₂O₃ content was the highest but lower than that in Taiwan, the Pearl River, and Luzon Island. CaO, MnO, and P₂O₅ had large coefficients of variation, indicating strong variation, whereas the coefficients of variation for the rest of the elements were small.

Table 3

Table 3. Major elemental content and related parameters (unit: %).

The spatial distribution of the principal quantities in the study area (Figure 5) reveals that the spatial distribution of Al₂O₃, Fe₂O₃, Na₂O, MgO, TiO₂, and K₂O gradually increases from northwest to southeast, which is similar to the clay content distribution. The overall performance is that the content gradually increases as the depth increases and the grain size becomes finer. The areas with high CaO values are distributed mainly in the western portion of the study area, and the contents of MnO and P₂O₅ are very low. The distribution showed no obvious regularity.

Figure 5

Figure 5. Spatial distribution of major elements.

4.2.2 Content and distribution characteristics of rare earth elements

According to the rare earth elemental content and each characteristic parameter of the surface sediments (Table 4), the total concentrations of rare earth elements (ΣREE) ranged between 104.09 and 211.13 ppm, with an average value of 144.64 ppm, which was higher than that of Luzon Island (123.93 ppm) and lower than those of Taiwan Island (193.12 ppm) and the Pearl River (255.4 ppm). The coefficient of variation was approximately 12%. The ΣLREE/HREE ratio ranged from 10.53 to 13.49, with an average value of 11.87, indicating significant enrichment of light rare earth elements. The (La/Sm)_UCC values varied between 0.87 and 1.01, with an average value of 0.93, which was lower than those of Taiwan (0.98) and the Pearl River (1.65) and higher than that of Luzon Island (0.91). (Gd/Yb)_UCC values varied between 1.15 and 1.71, with an average value of 1.43, which is higher than that of Taiwan (1.26), the Pearl River (1.25), and Luzon Island (1.14). The (La/Yb)_UCC varied between 1.21 and 1.84, with an average value of 1.53, which was higher than those of Taiwan (1.04), the Pearl River (1.13), and Luzon Island (1.23). The (La/Yb)_UCC, (La/Sm)_UCC, and (Gd/Yb)_UCC changed relatively little, with the coefficients of variation all less than 10%.

Table 4

Table 4. REE content (ppm) and characteristic parameters of the surface samples.

After normalization of the rare earth elements to chondrite (Table 4; Figure 6), the variation in δEu in the Dongsha Sea Area among the various samples in the Dongsha Sea Area was relatively small, between 0.59 and 0.74, with an average value of 0.69. The coefficient of variation was 5%, indicating a moderate negative anomaly, and the content generally decreased gradually from the east to the west in the study area. The variation range of δCe was 0.97–1.05, the average value was 1, the coefficient of variation was only 2%, and no obvious abnormalities were found. The content generally gradually decreased from the north to the south in the study area.

Figure 6

Figure 6. δEu and δCe spatial distribution in the surface sediments.

Chondrite-normalized patterns (Pourmand et al., 2012) can reflect the degree of differentiation of samples relative to Earth’s primitive materials, thereby revealing characteristics of the sediment source area. Analysis shows that the surface sediments in the Dongsha area of the South China Sea exhibit generally consistent distribution patterns after chondrite normalization (Figure 7). These patterns align with those of the Upper Continental Crust (UCC) (Taylor and McLennan, 1985), world shale composites (Byrne and Sholkovitz, 1996), and the Post-Archean Australian Shale (PAAS) (Pourmand et al., 2012). All display light rare earth element (LREE) enrichment, low heavy rare earth element (HREE) concentrations, and significant fractionation between LREEs and HREEs (Figure 7). However, the rare earth element concentrations in the studied sediments are lower than the average values of world shales and PAAS, indicating strong terrigenous characteristics.

Figure 7

Figure 7. Chondrite-normalized distribution pattern of rare earth elements in surface sediments.

A comparison of chondrite-normalized rare earth elemental distribution patterns between the 34 surface sediment samples from the Dongsha Sea Area and potential sources from Taiwan, the Pearl River, and Luzon Island (Figure 8) revealed that the rare earth elemental patterns of the Dongsha Sea Area sediments are largely consistent with those of Taiwan and the Pearl River. All exhibited a gently right-sloping distribution, characterized by enrichment in light rare earth elements, flat heavy rare earth elemental curves, and a distinct “V”-shaped concave-downward anomaly for Eu, indicating a moderate negative Eu anomaly. In contrast, samples from Luzon Island show a different trend with no noticeable Eu anomaly. Therefore, the rare earth elements in the Dongsha Sea Area are likely primarily derived from Taiwan and the Pearl River, with no significant contribution from Luzon Island to the study area.

Figure 8

Figure 8. Chondrite-normalized rare earth elemental distribution patterns of the surface sediments and potential source area deposits. The data for the Pearl River were obtained from (Xu and Han, 2009b), the data for Taiwan were obtained from (Li et al., 2013), and the data for Luzon Island were obtained from (Soberano et al., 2024).

5 Discussion

5.1 Surface sediment transport trend analysis

The composition and distribution of surface sediments in the study area are primarily controlled by hydrodynamic sorting. According to the planar distribution pattern of the three grain sizes of the end members of the surface sediments in the study area (Figure 9), combined with the results suggested by skewness and kurtosis which indicate mixed sedimentary sources, a comprehensive analysis can be obtained. The planar distribution of EM1 increased from northwest to southeast. This component was the finest component among the end members in terms of grain size (less than 4 μm), which may represent the suspended load in the marine environment (Zhong et al., 2017), indicating that clay minerals, possibly mixed river mud (clay), have been transported over a long distance. The two-dimensional distribution of EM2 is similar to that of EM1, increasing from northwest to southeast. Notably, dense submarine canyons have developed in this area, which are important channels for sediment transport to the deep sea. The surface circulation is controlled by the seasonal current turn under the influence of the East Asian monsoon, with rainfall intensity increasing under the influence of the East Asian summer monsoon. Thus, the carrying capacity of the river is enhanced (Wehausen and Brumsack, 2002), and previous studies have shown that surface ocean currents and deep-water currents carry large amounts of fine-grained sediments from the southwestern portion near Taiwan Island and transport them to the northern portion of the South China Sea (Lüdmann et al., 2005; Zhong et al., 2017).

Figure 9

Figure 9. Plane distribution of the end-member content of surface sediments with different grain sizes.

According to previous studies at adjacent stations in the Dongsha Sea Area, the grain-size components of the EM2 end members in the study area are mainly quartz and feldspar (Wan et al., 2007); therefore, EM2 can be interpreted as the fine silt component of the river. EM3 is the coarsest component among the particle-size end members, which is contrary to the planar distribution pattern of EM1 and EM2, and generally decreases from northwest to southeast. Based on the results of previous studies, this component may be related mainly to the reprocessing of high-energy natural events such as tropical storms caused by typhoons and landslides caused by earthquakes (Zhong et al., 2017). The strikingly distinct spatial distribution patterns of the three aforementioned end-members are a direct manifestation of hydrodynamic sorting in the study area. Under the influence of stable monsoon-driven circulation and bottom currents, the fine-grained clays (EM1) and silts (EM2), acting as suspended load, are transported over long distances steadily towards the southeastern deep-sea basin, resulting in a southeastward increase in their abundance. In contrast, the coarse-grained EM3 fraction, moving as saltation or bed load, is only entrained and transported over short distances during extreme high-energy events such as typhoons and earthquakes. It subsequently settles rapidly where energy attenuates, leading to its concentrated distribution in the northwestern region, closer to sediment sources or areas of stronger hydrodynamic forces. This phenomenon of “spatially segregated coarse and fine components” is a classic manifestation of hydrodynamic sorting.

Numerous studies have shown that the Dongsha Sea Area mainly receives sediments from the Pearl River, the rivers southwest of Taiwan Island, the Red River, the Mekong River, the rivers in central Vietnam, and the sediments from Luzon Island transported by ocean currents (Shao et al., 2009; Wang and Li, 2009).

On average, the Pearl River transports 69 × 10⁶ t of suspended solids to the South China Sea every year (Milliman and Syvitski, 1992). Taiwan’s unique geological setting-characterized by rapid tectonic uplift, steep topography, and widely distributed easily erodible rock formations resulting from active plate collision-is the fundamental cause of its enormous sediment discharge. Driven by heavy rainfall from monsoons and typhoons, Taiwanese rivers, particularly those in the southwest, transport as much as 187 × 10⁶ t of suspended sediments annually into the South China Sea, constituting a major terrigenous source for the region (Dadson et al., 2003; Liu et al., 2008). Owing to tectonic movement and monsoon rainfall, Luzon Island also strongly affects the transport of terrigenous debris; the sediment transport volume of the rivers on Luzon Island is 42 × 10⁶ t (Liu and Stattegger, 2014). The Red River transports 100–130 × 10⁶ t of sediments to the Beibu Gulf every year on average. However, owing to the obstruction of Hainan Island, its sediments rarely reach the Dongsha Sea Area (Tanabe et al., 2006). Many studies have shown that it is difficult for substances from the Mekong River and the central rivers of Vietnam to reach the northern shelf and slope of the South China Sea (Szczuciński et al., 2013). Did substances from the Changjiang River enter the South China Sea through the Taiwan Strait? To date, a unified conclusion has not been reached on this issue (Wan et al., 2007). Some studies have shown that it is difficult for modern substances from the Changjiang River to cross the Taiwan Strait and enter the South China Sea (Xu et al., 2009a). Therefore, the substance impacts of the Red River and Mekong River (Vietnam) and the Changjiang River are not considered in this study. With respect to the contribution of eolian matter to the South China Sea, some scholars have suggested that eolian sedimentation contributes to the northern portion of the South China Sea (Wang et al., 1999; Tamburini et al., 2003), but there is no clear structural evidence to support this argument. Most scholars agree that sediments from surrounding rivers are the main source of substances for the South China Sea, whereas the contribution of eolian sediments is negligible (Boulay et al., 2007).

Surface sediment transport is a multifaceted and long-term process that is influenced by various factors, including topographic conditions, hydrodynamic processes, and sediment sources (Bhattacharya and Giosan, 2003; Yu et al., 2019; Liang et al., 2020). In the grain trend analysis, arrows indicate the net sediment transport direction (Zhang et al., 2019), and the length of the composite vector represents the significance of the grain size trend. Based on the surface sediment transport trends in the Dongsha Sea Area (Figure 10), source trends are observed along the northeast-southwest and northwest‒southeast directions, whereas the grain size trend in the southeast-northwest direction was not significant, indicating that the surface sediments in the Dongsha Sea Area were mainly from the northwest direction and that the sediment sources in the northeast and southeast directions were fewer and could be ignored. Therefore, the Dongsha Sea Area received mainly sediments from the Pearl River and Taiwan Island, and Luzon Island was not considered a potential source area for subsequent provenance discrimination analyses.

Figure 10

Figure 10. Transport trend of the surface sediments in the study area.

5.2 Geochemical provenance identification

5.2.1 Identification of major elemental sources

The chemical elemental content in sediments usually varies regularly with grain size, i.e., the “elemental grain size effect” (Förstner and Wittmann, 1983). Al₂O₃ is present mostly in terrigenous sediments, which originate from weathering and erosion of continental rocks and are transported to the sedimentary environment by rivers or other transportation processes (Taylor and Mclennan, 1985). Since the study area is located in the Dongsha Sea Area, where biological activity is high, the sediments are enriched in CaCO₃. This phenomenon is related to abundant biological carbonate generation, especially in upwelling areas, where nutrients are abundant and promote the growth of plankton, which in turn increases the precipitation of calcium carbonate. The biological sources of calcium carbonate are mainly from the shells and skeletons of plankton, which settle to the seafloor to form sediments with high CaCO₃ content (Wang and Li, 2009), indicating that the CaO in sediments is mainly of authigenic or biogenic origin and is present in calcareous biogenic detrital deposits. TiO₂ is relatively stable in supergenesis and is an inert element that does not readily form soluble compounds after weathering; therefore, it is an important indicator of the composition of terrestrial clastics (Cullers, 1994). Therefore, the study area received mainly terrestrial detritus input, followed by input from biological sources.

Al₂O₃ and TiO₂ are robust proxies for terrigenous input, as they are primarily concentrated in weathering-resistant minerals. Their elevated concentrations typically indicate significant contributions from continental crustal materials (Norman and Deckker, 1990). To trace sediment provenance in the study area, we compared major element ratios in surface sediments from the Dongsha area of the South China Sea with those from Taiwan and the Pearl River (Figure 11). The results show that elemental ratios from the Dongsha area closely cluster with Taiwanese samples and are distinct from those of the Pearl River. This distribution pattern suggests a stronger geochemical affinity to Taiwanese sources, with a relatively limited contribution from the Pearl River. Based on this major element evidence, we infer that Taiwanese materials are the primary source for the Dongsha area. This transport process is likely governed by the following dynamic systems: the prevailing winter northeasterly monsoon provides the initial impetus for southwestward sediment transport, while the current system consisting of the Kuroshio Warm Current and its branches serves as the principal carrier, delivering materials from southwestern Taiwan to the study area (Liu et al., 2010). The sediment transport trend map (Figure 9) further corroborates this pathway, as its vector direction closely matches the “Taiwan → Dongsha” trajectory driven by the aforementioned mechanisms. In summary, by integrating geochemical proxies and physical transport evidence, we conclude that the sediments in the Dongsha area are primarily derived from Taiwan, with a subordinate influence from the Pearl River.

Figure 11

Figure 11. Dongsha Sea Area and potential source areas in the South China Sea (a) Comparison of MgO/TiO₂ - Al₂O₃/MgO (b) Comparison of Al₂O₃/TiO₂ - MgO/TiO₂.

5.2.2 Source identification of rare earth elements

To further clarify the contributions of the two potential provenance areas, i.e., Taiwan and the Pearl River, to the Dongsha Sea Area, the rare earth elements of 34 surface sediment samples were analyzed and compared with those of the two potential source areas, i.e., Taiwan and the Pearl River (Figure 12). δEu-δCe, (La/Sm)_UCC -(La/Nd)_UCC, and (Gd/Lu)_UCC -(Gd/Yb)_UCC intersection plots were used as tools for provenance identification to help determine the sediment contribution of each provenance area on the study area. The REEs between Taiwan and the study area were more similar, indicating that Taiwan Island may be the supply area of the main substance for the study area. The Pearl River also provides a certain amount of substance for the Dongsha Sea Area. The standardized distribution model of chondrites of rare earth elements reveals that the Pearl River and the study area have similar element enrichment and distribution trends, and the surface sediment transport trend in the study area differ. However, after the Pearl River discharged in the sea, the substance from the Pearl River was transported mainly westward under the influence of the Guangdong coastal current and the South China coastal current and deposited on the coastal shelf between the Pearl River estuary and the northeastern side of Hainan Island (water depth less than 50 m) (Ge et al., 2014); therefore, the supply was relatively low. In summary, the main substance of the study area is Taiwan, followed by the Pearl River.

Figure 12

Figure 12. Dongsha Sea Area and potential source areas in the South China Sea (a) Comparison of δEu-δCe; (b) Comparison of (La/Sm)_UCC-(La/Nd)_UCC; (c) Comparison of (Gd/Lu)_UCC-(Gd/Yb)_UCC.

6 Conclusion

An analysis of grain size, major elements, and rare earth elements performed on 34 surface samples from the Dongsha area of the South China Sea has led to the following conclusions.

1. The spatial distribution and compositional parameters of sediment grain size reflect a depositional process in the study area controlled by the combined effects of multi-source inputs and complex hydrodynamic conditions. End-member modeling of the grain-size data identified three end members: EM1 likely represents distantly transported mixed fluvial clay, EM2 may indicate fluvial fine silt, and EM3 is interpreted as coarse-grained, shelf-derived detritus. The overall sediment transport pathways are predominantly oriented NE-SW and NW-SE, while the SE-NW direction shows no significant trend.

2. The major element composition of the sediments is dominated by Al₂O₃, while the CaO content exhibits a wide variation range (1.94%–14.9%). The high Al₂O₃ content typically indicates strong terrigenous clastic input, whereas the broadly varying CaO content likely reflects the admixture of biogenic carbonate. When interpreting provenance information, we primarily rely on elemental indicators sensitive to terrigenous clastic materials. These indicators reveal a close geochemical affinity between the study area and the source region of Taiwan, suggesting that Taiwan is the primary source of terrigenous clastic material in the Dongsha area of the South China Sea.

3. The concentrations and distribution patterns of rare earth elements (REEs) in the study area indicate that the sediments are characterized by light REE (LREE) enrichment, a moderate negative Eu anomaly, and the absence of a Ce anomaly. These features collectively suggest a dominant terrigenous origin. A systematic comparison of chondrite-normalized REE patterns with potential source regions demonstrates that Taiwan is the predominant source of the sediments. While a contribution from the Pearl River is also identified, its input flux is relatively limited. Consequently, we conclude that the surface sediments in the Dongsha area of the South China Sea are primarily derived from Taiwan, with the Pearl River acting as a secondary source.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

AW: Data curation, Formal Analysis, Writing – original draft. HY: Conceptualization, Formal Analysis, Writing – review and editing. XT: Formal Analysis, Investigation, Methodology, Resources, Supervision, Writing – review and editing. ZiZ: Formal Analysis, Funding acquisition, Resources, Writing – review and editing. GX: Conceptualization, Formal Analysis, Funding acquisition, Writing – review and editing. ZhZ: Formal Analysis, Investigation, Supervision, Writing – review and editing. FM: Formal Analysis, Methodology, Supervision, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was financially supported by the Scientific Research Fund of Qingdao Vocational and Technical College of Hotel Management, the Shandong Provincial Natural Science Foundation (Grant No. ZR2022QD042), the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering (Grant No. MEGE2024011), and the Hainan Provincial Natural Science Foundation (425RC851).

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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