# Middle Holocene Coastal Environmental and Climate Change on the Southern Coast of Korea
## 1 Introduction  
After the last glacial maximum (LGM), sea level rose rapidly; coastal plains were affected by the transgression and are distributed worldwide. On the western and southern coasts of the Korean Peninsula, the high tide-affected areas of intertidal zones were particularly affected by this sea level rise. Most of this intertidal zone is now used as arable land through reclamation. The timing and altitude of sea level change around coastal Korea has been reported previously, with a focus on sea level high stands and their fluctuation after the maximum Holocene high stand [1; 2; 3; 4; 5; 6; 7; 8]. These studies have documented sea level rise and climate change from geochemical analyses of coastal sediments. Recently, several studies have focused on the southern coast of Korea with respect to changes in sea level and climate [4; 9; 10; 11]. These authors traced the mechanisms responsible for summer monsoon intensity and sea level changes using stable carbon isotopes (\(\delta^{13}\)C) and C/S ratios formed by different processes in the coastal area. They also studied environmental and climatic changes based on paleo-vegetation data from pollen records. These studies were conducted on coastal sediments in the Beolgyo [4] and Goheung areas [9; 11], which are near the study area (Figure 1). The sediments deposited during the Holocene transgression after the LGM are distributed on the coastal plains of Korea. However, it is very difficult to observe undisturbed sedimentary layers that were deposited after the sea level high stand, owing to human activities such as reclamation for farming and infrastructure development. Therefore, this study attempts to resolve this issue by comparing the results of the Beolgyo and Goheung areas to our new results from Deukryang. Several previous studies [4; 9; 10; 11], have used geochemical analysis to analyze the sedimentary environment. In particular, the total sulfur content (TS; wt.%), total organic carbon content (TOC; wt.%), and the ratio between total sulfur and carbon (C/S ratio) are very useful for distinguishing between terrestrial and marine environments, because the C/S ratio is related to the amount of pyrite formed in the two different sedimentary environments and can therefore be used as an indicator of salinity. Diagenetic pyrite forms more readily in marine sediments than in freshwater sediments, owing to the high concentrations of dissolved sulfate in seawater [1]. In general, where marine and freshwater environments are separated, marine sediments have low C/S ratios (0.5-5), whereas freshwater sediments have high ratios (\(\geq\)10) [12]. Brackish water that is more affected by saline water has C/S ratios of 4-11, whereas brackish water more affected by freshwater has ratios between 11 and 17 [4; 13]. The TS can also be used to classify terrestrial and marine sediments, as marine sediments typically fall between 0.3% and 3% [9; 11; 14; 15]. The \(\delta^{13}\)C\({}_{\text{TOC}}\) value is also useful for distinguishing between marine and terrestrial organic matter origin. Terrestrial sediments are generally between \(-\)32.5% and \(-\)24.9%, whereas values between \(-\)21.7% and \(-\)17.3% are typical in marine sediments [16]. Since the beginning of the Holocene, the sea level of the Korean Peninsula has risen more than 20-30 m above sea level [1; 6; 7; 17; 18], resulting in a prominent shift from terrestrial to marine environments on the current coastal area. The transgression in the coastal area supplied sulfate and formed anoxic sedimentary environments necessary for pyrite formation. Therefore, changes in the TS have been interpreted to reflect the incursion of seawater on the peninsula, owing to sea level changes (e.g., [6; 19]). The variation of \(\delta^{13}\)C\({}_{\text{TOC}}\) in sediments has been widely used for tracking paleo-ronmental changes associated with sea level changes in the Korean Peninsula [9; 10; 20; 21; 22; 23; 24; 25].