# Middle Holocene Coastal Environmental and Climate Change on the Southern Coast of Korea
## 4 Discussion
### Climate Change and Driving Forces in the Beolgyo Area during the Middle Holocene  
As discussed in Section 1, changes in the \(\delta^{13}\)C\({}_{\text{TOC}}\) values are important for freshwater input interpretation in this area. In order to interpret the factors causing changes in the sedimentary environment, the origin of sediment and changes in vegetation must be considered simultaneously. [PERSON] et al. reported a rapid change in vegetation and environment in the Goheung area on the southern Korean coast at ca. 7500 cal yr BP and surmised that the influence of the EASM prevailed until ca. 6000 cal yr BP, before the influence of the EASM weakened, affecting climate change associated with SST in the southwestern Pacific and typhoons related to El Nino conditions [11]. This coincides with the period of decreasing SST in the southwestern coast of the Korean Peninsula from 6600 to 8400 cal yr BP [29]. Therefore, changes in the vegetation environment (as evidenced by carbon isotopes) reported at the same time as sea level change on the west coast are  
Figure 6: Comparison of \(\delta^{13}\)C\({}_{\text{TOC}}\) curves from the Deukryang area cores STP16-20 [11], BSL18 [4], and BSL11 (this study) with the Asian monsoon index [30], sea surface temperature data [31], and alkenone temperature [29].  
interpreted to be related to the effect of atmospheric circulation due to climate change. This period can be interpreted as indicative of climate change and the southward migration of the westerly jet, which was suggested as the cause of the decrease in SST [29].  
Between 5100 and 4900 cal yr BP, the intensity of EASM tended to decrease, and carbon isotopes increased in the Deukryang, Boolgo, and Goheung areas (Figure 6). This period corresponds to a rapid decrease in summer SST in the Yellow Sea. In addition, the red color intensity trend shows that the TS peak of Deukryang area (core BSL11) corresponds to the time period from 5000-3000 cal yr BP (Figure 5). The climate change induced by the weakening EASM reported by [PERSON] et al. coincides with decreases in the summer SST, and sea level decreased gradually with temperature after the Holocene optimum period [11]. This is also consistent with the interpretation that the influence of El Nino and La Nina conditions was caused by changes in SSTs in the South Pacific, which became stronger as the EASM weakened [4; 9; 10; 11]. Since the interpretation of the westerly jet migration during this period was suggested by [PERSON] et al. [29], further study is needed to relate the influences of El Nino and La Nina conditions with the westerly jet migration. Nevertheless, it is possible to infer that changes in land hydrology caused by the influences of El Nino and La Nina conditions resulted in increased freshwater input, because changes in land hydrological events, such as typhoons and freshwater input, have been reported in many areas between 5100 and 3000 cal yr BP. However, it is not clear how the westerly jet migration relates to both hydrological change in the southern coast and the weakening of the EASM. If the cause of the weakening of the EASM on the west coast is due to the migration of the westerly jet, it is possible to better understand the changing hydrology on the west and south coast of the Korean Peninsula in the middle Holocene. In addition, further research is needed to understand the difference in the intensity of the hydrological changes recorded by the sediments in the area, which is dependent on the pathway of the typhoon.