## 2 Materials and Methods  
A gravity core, SO-31, was obtained in the central part of the Beibu Gulf, South China Sea, at coordinates 107\({}^{\circ}\)22\({}^{\prime}\)19\({}^{\prime\prime}\) E, 18\({}^{\circ}\)37\({}^{\prime}\)40\({}^{\prime\prime}\) N, by the Chinese-German joint cruise of R/V Sonne 219 in 2011. This sedimentary profile is 738 cm in length and was recovered at a water depth of 75 m. The core site is ~210 km from the Red River mouth in Vietnam and about 150 km from the Changhua River mouth on Hainan Island (Figure 1). The SO-31 core, situated in the central region of the Beibu Gulf, is susceptible to environmental events in its surroundings, which may influence its sedimentary environment and could be recorded in the core sediments. Moreover, no significant depositional discontinuities were observed within the vertical profile of SO-31, suggesting continuous recording of the depositional history since the Holocene epoch. Consequently, this sedimentary column serves as an excellent representative for studying regional sedimentary environmental evolution. The core was cut into two halves, photographed, described, and then subsampled for grain-size and geochemistry analysis. The core was sampled at 3 cm intervals, and the outer rims of the samples were removed to avoid contamination, yielding a total of 246 samples. Additionally, 16 surface sediment samples were collected from the Qiongzhou east (8 samples) and Hainan west nearshore (8 samples) by box sampler using the China R/V \"Fendou 5\" in 2012 (Figure 1), and the upper 5 cm of each sample was used for analysis of grain size and elements. All samples were stored in polyethylene bags and kept frozen at \(-\)20 \({}^{\circ}\)C until chemical analysis in the laboratory at Guangzhou Marine Geological Survey (GMGS, China). Grain size of the bulk samples was measured by laser particle size analyzer (Mastersizer 2000), after removing organic matter and biogenic carbonate from the samples with 10% H\({}_{2}\)O\({}_{2}\) and 1 N HCl, respectively. Prior to the geochemical analysis, the bulk samples were desalted with distilled water and dried at 60 \({}^{\circ}\)C in a clean oven, and then ground to 200 mesh (aperture 0.075 mm) or less in an agate mortar. The 10 major elements (Si, Al, Fe, Mg, Mn, Ca, Na, K, Ti, and P) and 12 trace elements (Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Sr, Ba, Pb, and Zr) were measured by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, 4300 DV). To analyze the contents of rare earth elements (REEs), about 100 mg of each sample was first digested by 1 mL 50% HNO\({}_{3}\) and 3 mL HF in a tightly closed Teflon vessel on a hotplate at about 160 \({}^{\circ}\)C for 48 h. After evaporation to dryness, 1 mL of 55% HClO\({}_{4}\) was added to the vessel, which was heated again to about 160 \({}^{\circ}\)C until the complete evaporation of the acid. After the sample cooled down to room temperature, 1.5 mL of 50% HNO\({}_{3}\) was added, and the sample was again heated at 160 \({}^{\circ}\)C for 12 h and then cooled to room temperature. Afterwards, the solution was moved to a measuring cylinder (including the solutions produced by rinsing the vessel with 50% HNO\({}_{3}\) to concentrate the remaining sample in the vessel), diluted with 10% HNO\({}_{3}\), and measured with an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, X Series 2). We conducted repeated analysis of several samples and standard sample (GSD-9) analysis to ensure testing accuracy and precision (relative error \(<\) 2%). The analysis of grain size and all the elements were done in the Laboratory of Guangzhou Marine Geology Survey (GMGS), Guangzhou, China. The trace elements, including REEs, from the sediments of  
Figure 1: Geographical setting of the Beibu Gulf and location of the gravity core SO-31. The black triangles and black dots show the surficial sediments from Qiongzhou east and Hainan west nearshore, respectively. The yellow line indicates the boundary between Vietnam and The People’s Republic of China.