## 3 Results
### Characteristics of Major and Trace Elements  
The major and trace element compositions, together with the mean grain size (MZ), for the core sediments are given in Tables 2 and 3. The mean grain size ranges from 6.18\(\Phi\) to 7.76\(\Phi\), with the average of 7.14\(\Phi\), showing that silt-sized or finer grains dominate the core sediments, and the study area had a relatively calm sedimentary environment. Among the major elements measured for the entire profile, SiO\({}_{2}\) is a dominant component, averaging 57.72% in the sediment, followed by Al\({}_{2}\)O\({}_{3}\), averaging 14.43%. The two elements amount to an average of more than 70%, indicating that the sediment is dominated by silicate and aluminosilicate. The CaO content is also relatively high, with an average of 4.0%, which is slightly lower than the average of the coastal area of the Hainan Island [23; 24], but far higher than the sediment content of the Red River [25; 26; 27], indicating that bicarbonate deposition is relatively active in this area. Content The content of Al\({}_{2}\)O\({}_{3}\) was slightly higher than the average of shallow sea sediments [18], but lower than that of the Red River [25; 26; 27], which may be related to the dilution of biocalcinous detritus in the study area, which may be due to the quiet mode of sedimentation. The contents of Fe\({}_{2}\)O\({}_{3}\), K\({}_{2}\)O, MgO, Na\({}_{2}\)O, TiO\({}_{2}\), P\({}_{2}\)O\({}_{5}\), and MnO in the sediments of SO-31 are close to the average values of the sediments from the shallow sea of the South China Sea, and the contents of most trace elements such as Co, Cu, Ni, Zn, V, Ba, Sc, Ga, Pb, Cr, and La in the core sediments are slightly higher than the average of the shallow area of the South China Sea [18], which may be caused by the difference in the sedimentary environment and material sources between the Beibu Gulf and other areas in the South China Sea. In general, geochemical composition similar to that of the upper crust (UCC) [28] indicates that the surrounding continent is the main source of sediments in this area. The SO-31 core sediments can be divided into 3 sedimentary units: Unit 1, Unit 2, and Unit 3, in ascending order, according to the vertical variation in grain size and geochemistry (Figures 2 and 3). In Unit 1 (0-501 cm, 0-7700 a BP), the content of SiO\({}_{2}\) in sediments showed a gradual increase with depth; conversely, the contents of Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), and Na\({}_{2}\)O showed a trend of decreasing with depth. At 501 cm depth, an obvious change in the contents of most of major elements (such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), P\({}_{2}\)O\({}_{5}\), MgO, CaO, and Na\({}_{2}\)O) and trace elements (such as Zn, Cu, Gd, and Sr) can be observed simultaneously. In Unit 2 (501-640 cm, 7700-11,600 a BP), SiO\({}_{2}\) content is slightly higher than the average, and the contents of K\({}_{2}\)O and TiO\({}_{2}\) are close to that of the average. In Unit 3 (640-738 cm, 11,600-12,800 a BP), compared with Unit 2, the contents of SiO2 in Unit 3 decreased rapidly, and conversely, the contents of Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), K\({}_{2}\)O, TiO\({}_{2}\), Co, Cu, Ni, Zn, V, Sc, Ga, Pb, Cr, and Zr increased significantly. Similarly to the major and trace elements, the rare earth elements (REEs and LREEs/HREEs) and the grain size of the core sediments also show similar variation in the vertical direction. The abrupt changes at depths of 501 cm and 640 cm indicate that the sedimentary environment has undergone significant changes.