# Middle Holocene Coastal Environmental and Climate Change on the Southern Coast of Korea
## 4 Discussion
### Depositional Environments
#### 4.1.2 Stage 2 (ca 10,000 cal yr BP to ca 7080 cal yr BP)  
Stage 2 corresponds to Unit 2b (elevation \(-6.42\) to \(-12.52\) m) where shells are rare (Figure 3). The lower part of stage 2 has a larger grain size than the upper part. Stage 2 formed at 10,000-7080 cal yr BP and experienced a rapid rise in global sea level following the LGM. [PERSON] et al. proposed an abrupt increase in sea level, from \(-17\) m in 9500 cal yr BP to \(-3\) m in 8000 cal yr BP, in the Sunda shelf in Southeast Asia [1]. [PERSON] et al.reported that the relative sea level of Toyooka Basin rose from \(-31.05\) m in 10,000 cal yr BP to \(-4\) m in 7900 cal yr BP and to \(-0.47\) m in 6700 cal yr BP [6]. Recently, [PERSON] et al. suggested that sea level rose from \(-8.58\) m at 8260 cal yr BP to \(+0.17\) m at 7430 cal yr BP at a site in the Yellow Sea in China [7] (Figure 4). Therefore, Stage 2 in our study area was likely affected by marine transgression.  
In our record, the most pronounced effect from the marine transgression is the change in grain size. In the clay layer from \(-12.52\) to \(-10.82\) m in elevation, which is part of Unit 2b, TOC values range from 0.3% to 0.8%, and TS values exhibit two peaks from 0.14% to 0.53%. These changes may be related to sea level rise, but they are not consistent with known temporal changes in global sea level. In addition, the clay layer, which was likely deposited in the intertidal zone, has a relatively high age uncertainty due to the low number of radiocarbon dating analyses. Therefore, it is very difficult to estimate the age of the marine transgression from a specific elevation in the core. In addition, the sedimentation rate during sea level rise in this area was relatively low at 0.6 mm/year, compared with the high value of 0.8 mm/year observed at the same time in other studies [4]. However, the highest TS value occurs at an elevation of \(-9.42\) m, which is near the sample that was age dated from this interval. This TS peak (0.8%) occurs at ca. 8000 cal yr BP, which may be  
Figure 4: Comparison of estimated sea level curves from (**a**) the Deukryang area (core BSL11, this study; white circles on red line), (**b**) the Goheung area (core STP18-20 [11]; green squares on blackline), (**c**) the Beoglyo area (core BSL18 [4] purple diamonds on black line), and (**d**) simplified relative sea level curve of the Toyooka basin [6] turquoise inverted triangles on blue line). Other points refer to sea level fluctuations in the Yellow sea between China and around the coastal area of Korea [3,7] and global relative sea level [1].  
due to the marine transgression, since most studies indicate a sea level rise of 6 m. TOC is also the highest during this interval (0.8-0.9%), and the C/S ratio is very low (1.0). It is difficult to provide the exact timing of the transgression in this area, but it is clear that by ca. 8000 cal yr BP the area had been affected by the Holocene transgression. Therefore, the sedimentary environment had changed from a fluvial to an intertidal zone in response to sea level rise at ca. 8000 cal yr BP.  
A distinct peak where the grain size increases and TOC decreases markedly is observed at an elevation of \(-\)7.22 m at ca. 7280 cal yr BP, which records a dramatic decrease in \(\delta^{13}\)C\({}_{\text{TOC}}\) values to \(-\)25.3% (Figure 5). This value indicates that the organic matter formed in a terrestrial environment, whereas the TOC, TS, and C/S ratios indicate a marine depositional environment. Therefore, we interpret this interval as an event in which sediments from terrestrial environments were temporarily supplied to the marine environment.  
Figure 5: Comparison of sedimentary environmental proxies (\(\delta^{13}\)C\({}_{\text{TOC}}\), TS, C/S ratios, and red color intensity) from the Deukryang area with reconstructed climate change on the southwestern coast of Korea (Goheung area from core STP18-20 [11], and Beolgyo area from core BSL18 [4]).