Holocene environmental change at the southern coast of Korea based on organic carbon isotope (δ13C) and C/S ratios

To trace the environmental change that occurred at a coastal area during the Holocene, we performed radiocarbon dating and geochemical analyses using sedimentary cores recovered from the Beolgyo tidal flats on the south coast of Korea. The δ13C values of total organic carbon (TOC), which represent relative inputs of terrestrial (or freshwater) and marine organic sources, revealed a long-term increasing trend from −27.5 to −21.7‰ during the period between 10,000 and 3000 cal BP, suggesting an evolutionary series of fluvial-dominant to marine-dominant environments, coupled with weakened summer monsoon intensity. From 3000 cal BP to the present, the δ13CTOC decreased gradually from −21.7 to −24‰, indicating enhancement of freshwater input during this time. The observed long-term environmental change was divided into six stages based on the averages of the total sulfur content (TS%) and the organic carbon to sulfur ratio (C/S), proxies for paleosalinity. On multi-centennial to millennial timescales, higher C/S ratios and lower TS% were linked to higher δ13CTOC values, indicating strong freshwater input at Stage 2 (∼8300–7400 cal BP), Stage 4 (∼4500–3300 cal BP), and Stage 6 (∼1100 cal BP to present). Strong freshwater discharge events synchronously occurred at 4300 cal BP and 3600 cal BP along the south coast of Korea. Further testing should be carried out in other East Asian coastal areas. This study suggests that sedimentary δ13CTOC values and C/S ratios can be used in Asian coastal regions to trace past freshwater input events and summer monsoon intensity during relatively stable sea level periods (e.g., the middle to late Holocene).