## 5 Discussion
### Correlation between ND-3, KND-3, and ND-2 Cores in the Lower Delta Plain  
The ND-2 drilling site is located 4 km to the east of ND-3, and the KND-3 drilling site is located between ND-2 and ND-3. All three cores have the same elevation and the core samples from the three sites are obtained with the same drilling method. The analysis results of sedimentary facies in the three core sediments indicate that the paleosols of the ND-3 core appear at a depth of approximately 43.00 m, and the paleosols of the KND-3 and ND-2 core appear at a depth of 50.54 m and 60.00 m, respectively (Figure 5a). This suggests that when the sea level was low, the paleo-topography between the three cores was not flat and the elevation decreased from west to east. In addition, since the Yangsan Fault penetrates along the present Nakdong River channel, an incised valley had probably developed around the fault [18]. This is why core sediments of the ND-2 drilling site in the incised valley are thicker than other core sediments.  
The sediments of the three cores in the present lower delta plain show similar patterns at the middle and upper parts (Figure 5a). There are, however, big differences between the three cores at the lowest parts to a depth of approximately 32.00 m. In the sediments, which are interpreted as representing an estuarine environment, the ND-3 core sediments are mainly composed of fine to medium sand, whereas the KND-3 and ND-2 core sediments contain a large amount of mud content. This suggests that the relatively coarse-grained sediments in the ND-3 core are deposited in the bayhead delta setting, which is closer to the channel than the KND-3 and ND-2 core sediments. The lowest sedimentary Unit of the three cores were interpreted as being deposited in different environments (Figure 5a). Since the age at the lowest Unit of the KND-3 and ND-2 core was not measured and each core had different elevations to the former sea level, the three cores cannot be interpreted as being deposited at the same time. However, when the sea level was low, it can be assumed that the three cores were not deposited under the same environment setting.  
Since the dating results of the KND-3 core are very few in number, it is difficult to compare them with other cores. The dating results for the ND-2 and ND-3 core sediments show slight differences (Figure 5b). For both cores, the ages of the uppermost parts corresponding to the delta plain are around 0.6-0.5 cal ka BP. However, below a depth of approximately 12.00 m (corresponding to the delta front and prodelta), the ND-3 core sediment is found to be 800 to 1000 years older than the ND-2 core sediments. This indicates  
Figure 5: Comparison of present lower delta plain cores. (**a**) Sedimentary columns and (**b**) comparison of age between the ND-3 KND-3, and ND-2 cores (KND-3 data are modified from [PERSON] et al. [22] and ND-2 data are modified from [PERSON] [18]). The area filled with yellow color represents the different sedimentary environments between three cores.  
that the sediments supplied from the land to the ND-2 and ND-3 drilling sites were not accumulated simultaneously, or even within the same setting, due to topographic difference. The sediment was likely first supplied to the ND-3 area; subsequently, as the elevation of the ND-3 drilling site increased over time, the sediment route changed and sediment was supplied into the ND-2 area by lateral switching.