## 4 Discussion
### Depositional Sedimentation History and Environmental Changes
Consequently, the contribution rate of the Red River to the provenance of the area was diminished, leading to a relative reduction in the Red River's influence on the regional provenance (Figure 5). Collectively, these observations suggest that the Qiongzhou Strait likely opened around 7700 a BP; this is consistent with [PERSON] et al. and [PERSON] et al. 's assumptions [43; 44]. From 7700-0 a BP, sediments originating from the eastern Qiongzhou Strait became one of the primary contributors to the core area sediments, as indicated by the proximity of Unit 1 sediments to Qiongzhou East sediments in Th/Sc vs. Th/Cr plots (Figure 5). Unit 1 sediments are positioned centrally among the three provenance end-members (the Red River, the western Hainan Island, and the eastern Qiongzhou Strait), indicating a mixed sedimentary product derived from all three sources (Figure 5); this aligns with previous interpretations by [PERSON] (2012) and [PERSON] (2015) [55; 56]. Compared to Unit 1B, the scatter plot of Unit 1A (2800-0 a BP) in Th/Sc vs. Th/Cr shows closer affinity to the western Hainan Island and Qiongzhou Strait. This stage also corresponds to stages 4-8 (7200-0 a BP), as defined by [PERSON] et al. However, their focus was primarily on the increase in the sedimentation rate during Stage 6, which they attributed to sea-level drop and abnormal hydrodynamic conditions, while the reasons for the overall increase in the sedimentation rate across stages 4-8 remain unaddressed [13]. According to [PERSON] (2007) [57], sea levels in the Beibu Gulf exhibited a downward trend during this period (2800-0 a BP). The continuous decline in sea level facilitated greater sediment transport to the study area, enhancing local sedimentation rates. Meanwhile, the Qiongzhou Strait channel underwent progressive incision and widening due to tidal currents, increasing sediment input from the east into the Beibu Gulf and, correspondingly, elevating sediment contribution rates in the region. Several prevailing perspectives exist regarding the formation and evolution of the Qiongzhou Strait, particularly concerning its opening time during the Holocene. [PERSON] et al. conducted a comprehensive analysis of regional geological, geophysical, and pale-ontological data, concluding that the east-west waterway of the Qiongzhou Strait formed between 10.5 and 7.1 ka BP [43]. [PERSON] et al., based on relative sea level change curves and sediment thickness data from the northwest South China Sea, determined that the east-west waterway of Qiongzhou Strait primarily developed between 11.0 and 8.5 ka BP, with full penetration occurring at approximately 8.5 ka BP [44]. [PERSON] et al., using grain size analysis and AMS \({}^{14}\)C dating of core sediments from both sides of the strait,estimated the penetration time of the Strait to be around 8.0 ka BP [11]. In this study, the significant increase in provenance contribution from Qiongzhou Strait, observed at 7.7 ka BP in core SO-31, aligns well with the aforementioned conclusion despite minor temporal discrepancies. These results further corroborate that the Qiongzhou Strait opened in the early Holocene, approximately 8.0-7.7 ka BP. Additionally, [PERSON] et al. also observed a significant increase in deposition rates at the C4 and B106 core sediment stations in the eastern Beibu Gulf around 7.0 ka BP [20], whereas no such notable increase was observed in the SO-31 sediments; this discrepancy suggests that the increased deposition rates in the two stations should not be attributed to the opening events of the Qiongzhou Strait.