## 4 Discussion
### Identification of the Sediment Provenance  
Previous studies have revealed that REEs, La, Sc, Ti, Co, Hf, Th, and Nb are especially useful for monitoring source-area composition [34, 35, 36, 37, 38]. In order to further study the sediment provenance, element ratios of Si/Al, Rb/K, Zr/Ti, Th/Sc, La/Co, \(\delta\)Ce, and so on were selected for identification of sediment sources of SO-31 (Figure 4), given that the ratios of these elements can better offset the grain size and carbonate dilution effect. Additionally, these elements show distinct differences during magmatic differentiation and can be more effective in discriminating their provenance [39,40]. It is evident from Figure 4 that the vertical distribution of element ratios aligns well with the distribution of individual element contents (Figures 2 and 3). Notably, at a depth of 250 cm, significant variations are observed in indicators such as Ni/Co, Th/Sc, Th/Cr, and \(\delta\)Ce. These changes further subdivide Unit 1 into two sub-units, namely, Unit 1A and Unit 1B, reflecting a substantial alteration in sediment sources within this region. Th/Sc and Th/Cr are commonly utilized as indicators for differentiating sediment sources [41,42]. [PERSON] (2007) demonstrated that Th/Cr and Th/Sc effectively distinguish sediments originating from the Pearl River, Red River, and Mekong River, which are the primary input rivers around the Beibu Gulf [25]. In the study area, Th/Sc and Th/Cr also exhibit marked differences among the three main source end-members: the Red River, the eastern part of the Qiongzhou Strait, and the western part of Hainan Island. Furthermore, the vertical variations in Th/Sc and Th/Cr correspond well with the unit division of SO-31, confirming their utility as effective indicators for analyzing sediment sources of SO-31. The scatter plots of ratios of Th/Sc and Th/Cr show that the three sedimentary units had different distribution characteristics. The scatter plots in Unit 1 (including Unit 1A and Unit 1B) had a similar distance with that of the Red River, the west nearshore of the Hainan Island, and the east side of the Qiongzhou Strait, indicating that the three possible source areas may have similar contributions to the sediments in the sea area. Compared with Unit1, the scatter plots in Unit 2 moved gradually away from the east side of the Qiongzhou Strait and coastal sediments of the Hainan Island, but closer to the Red River, indicating the increasing contributions of the Red River runoff. Unit 3 is closer to the Red River than Unit 2 and Unit 1, which indicates that the Red River was the main source area for the core sediments in this unit. The provenance analysis results of the SO-31 sediments differ from the STAT22 sediments reported by [PERSON] (2016) [18]. Geographically, the core STAT22 is situated on the eastern slope of the Beibu Gulf (Figure 1), and its sediments are predominantly influenced by sources from the western part of Hainan Island. Due to the \"central depression\" topography of the Beibu Gulf, sediments from the western part of the gulf, including those from the Red River, are less likely to be transported to the STAT22 location under the influence of the bottom current. In contrast, SO-31 is located in the central area of the Beibu Gulf, where materials from the surrounding area of the Beibu Gulf, including the Red River and the western part of the Beibu Gulf, can more easily reach this location. Therefore, the sediments at SO-31 provide a more comprehensive record of the provenance changes in the Gulf. In addition, it should also be noted that the scatter plot of Th/Sc vs. Th/Cr (Figure 5) shows that the sediments from the three units in SO-31 are not completely located among the Red River, the western nearshore of Hainan Island, and the eastern Qiongzhou Strait end-member, suggesting the presence of other significant sediment sources influencing this region, especially the western coastal area of the Beibu Gulf.