# Middle Holocene Coastal Environmental and Climate Change on the Southern Coast of Korea
## 2 Study Area and Methods  
The study area is located on the southern coast of Korea, which has been used for agriculture for the last 100 years. In particular, sediments are sourced from the surrounding mountains and accumulate in the basin, except for the southern part, where sediments precipitate in the marine environment. Deukryang Bay is an inner bay where seawater flows from south to north, and which shallows from the southeast to the northwest. A seawall has been built in the southern part of the bay, and freshwater flows into the bay from small streams, including the Deuknyang and Joseong streams. According to Korean Meteorological Administration, the annual precipitation is 1500-2000 mm, and summer precipitation accounts for 70% of the total annual precipitation.  
Using a general map of the area produced in the Japanese colonial era during the 1910s, we verified the locations of intertidal zones and coastal sands on the coastline before reclamation activity. Therefore, the sediment cores obtained include records of the depositional environment below the present sea level. Core BSL11 was obtained near the channel in the intertidal zone; the current tidal range is -3.5 m (Figure 1).  
Figure 1: (**a**) Location of the BSL11 core site and surface currents in the Yellow Sea and East China Sea. Blue (red) arrows represent cold (warm) currents. Dashed arrows indicate seasonally varying currents. TWC, Tsushima Warm Current; CWC, Cheju Warm Current; YSWC, Yellow Sea Warm Current; KCC, Korean Coastal Current; CCC, Chinese Coastal Current; CDW, Changjiang Diluted Water. The Heukasan Mud Belt (HMB) is marked by gray shading (modified from [PERSON] et al. [29]). (**b**) Aerial photo around Deukryang, Beology, and Goheung watersheds. (**c**) Aerial photo of Deukryang watershed which has more outlets than Beology, and Goheung watersheds. For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.  
Core BSL11 comprises 15 m of sediments; samples for grain-size and geochemical analyses were collected every 10 cm, while radiocarbon dating samples were collected at specific depth where plant or shell fragments appear (Table 1). Radiocarbon dating of samples was conducted using an accelerator mass spectrometer (AMS) at the Korean Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Korea. For grain-size analysis, the samples were treated with 35% H\({}_{2}\)O\({}_{2}\) to decompose organic matter and then boiled in 1 N HCl for 2 h to remove carbonates and iron oxides. Samples were then rinsed three times using distilled water, then treated with an ultrasonic vibrator for 30 s to keep them in suspension and facilitate dispersion. We then performed grain-size analysis using a Mastersizer 2000 laser analyzer (Malvern Instruments, Worcestershire, UK).  
For TOC and TS analyses, samples were treated with 1 N HCl for 1 h to remove carbonates and iron oxides then rinsed three times using distilled water. Approximately 3-5 mg of the treated sample was then loaded into a tin combustion cup. The TOC and TS were determined using a CNS elemental analyzer (vario Micro cube; Elementar, Langenselbold, Germany). The C/S ratio was calculated using TOC and TS. Stable carbon isotope (\(\delta^{13}\)C\({}_{\rm TOC}\)) analyses of HCl-treated samples were analyzed using an isotope ratio mass spectrometer (Isoprime100; G.V. Instruments, Manchester, UK) connected to a CNS elemental analyzer. Results are expressed in delta (\(\delta\)) notation of the Vienna Pee Dee belemnite standard (VPDB). The standard CH6 (sucrose, \(\delta^{13}\)C = \(-\)10.45 \(\pm\) 0.033%) was obtained from the International Atomic Energy Agency (IAEA) and has a precision of 0.2% or more for replicate measurements.