## 4 Discussion
### Depositional Sedimentation History and Environmental Changes  
During the Holocene, with the regional sea level rise associated with climate change, the Beibu Gulf experienced a series of drastic environmental changes. Especially, the Qiongzhou Strait opening event at the end of the Early Holocene [43; 44] had a great impact on the hydrodynamic environment and material input of the Beibu Gulf [45; 46]. At the beginning of the Holocene (12,800-11,600 a BP), the Beibu Gulf was in the period of relatively low sea level, and the regional sea level was \(\sim\)60 m lower than in the present [47; 48]. During a comparable period (12,900-11,700 a BP), [PERSON] et al. discovered intermittent occurrences of diatoms mainly composed of typical coastal species and nearshore planktonic species in the adjacent columnar sediment SO219/31-4, suggesting that it was in a fluvial or shallow-water sedimentary environment [13], this is consistent with the low Sr/Ba ratio (\(<\)0.25) found in the Unit 3 sediments in the present study. In the period, the Red River, being the largest river discharging into the Beibu Gulf, significantly influenced sedimentation in the region. It extensively incised the continental shelf, extending its channel southward even to the study area [49; 50; 51; 52], potentially allowing direct accumulation of Red River sediments on the mid-outer shelf. Additionally, smaller rivers such as the Changhua, Ma, and Naniu Rivers also contributed to regional sediment sources, though the Red River remained the predominant contributor. In contrast, the eastern side of the Qiongzhou Strait did not serve as a major sediment source during this period due to the strait's closure [43; 44]. Between 11,600-7700 a BP, the location of SO-31 progressively moved farther from the Red River outlet and surrounding terrestrial areas as regional sea levels rose, which is supported by the reduced abundance of shell remains and CaO, alongside an increase in Sr/Ba ratios in Unit 2 sediments compared to those in Unit 3. This stage corresponds to stage 2 and stage 3 (11,700-9500 a BP and 9500-7200 a BP) as defined by [PERSON] et al. based on diatom assemblages. Unfortunately, this only illustrated the lowest sedimentation rate observed in the 9500-7200 stage and was attributed to the slowing of sea level rise, while the  
Figure 5: Discrimination plot of Th/Cr vs. Th/Sc for SO-31 core sediments. overall low sedimentation rate in the two stages failed to provide a detailed explanation [13]. Throughout this interval, the Red River continued to dominate sediment supply to the study area (Figure 5). Previous studies indicate that sea levels in the Beibu Gulf were rising rapidly in this period [53; 54], and as sea levels increased, sediment deposition gradually shifted away from river mouths, including the Red River and other coastal rivers, leading to a decrease in sediment input and overall sedimentation rates in the region. Notably, at the end of Unit 2 (depth of 501 cm), there was a marked change in sedimentation rates and provenance, suggesting the potential west-to-east connection of the Qiongzhou Strait. Previous studies have shown that during the 11,600-7700 a BP period, the sea level in the Beibu Gulf was still rising rapidly. With the continuous rise in sea level, the sediment area gradually moved away from the estuary area of the Red River and other rivers entering the sea, and the provenance input of the Red River and other rivers entering the sea decreased, resulting in the overall decrease in the sedimentation rate in the area. It is worth noting that at the end of this unit (501 cm, 7700 a BP), there was a significant change in deposition rate and provenance in the area, which may indicate that the Qiongzhou Strait had been connected from east to west. With a significant influx of sediments from the eastern part of the Qiongzhou Strait into the Beibu Gulf, these materials were subsequently transported to the study area by ocean currents.