Palaeohydrological and palaeoenvironmental fluctuations of the historic Eurimji Lake

Abstract

Lake Eurimji is located in the northern part of Jaecheon, Chungbug Province, in South Korea. It is well known for its scenic beauty as a nationally registered landscape site (Yang GS, Jungwon Munhwa 14:1–38, 2010). Since the late Bronze Age, its water has been used for agricultural irrigation. The lake was initially formed at the mouth of a fanglomerate where Jurassic granites blocked the main course of a stream channel at about 305 m above sea level (m.a.s.l.). Natural levees grew laterally near the main outlets of the fan-valley mouth up to about 307 m.a.s.l. The lake accumulated organic sandy mud on the sand-andgravel streambeds extending toward the southern valley mouth near Yongdu Mt., the ages of which were dated as early as ca. 3,000 calibration years Before Present (cal-yr BP). The early evidence of lake deposits is supported by radiocarbon ages for organic muds below the lowermost artificial layers in Lake Eurimji, which vertically constitute the bottom of Eurimji bank or dyke at about 306.5 m.a.s.l. The height is about 15 m. The present study reconstructs the paleo-precipitation of Lake Eurimji by applying an age mode to core sediments in the ER3-1 borehole using the mean grain size of sediments trapped at the bottom of the lake. The results show that lake sediments accumulated at an extremely high rate, ca. 3.49 mm/year during the Early Three Kingdoms Period, around 2,120–1,340 cal-yr BP. The average grain size of the trapped sediments is proportional to the total amount of precipitation during the elapsed time of each monitoring session. Both the paleoclimatic cyclicity and historical documents indicate that droughts in the lake catchment areas prevailed at 80 AD, 120 AD, 160 AD, 310 AD, 350 AD, 460–500 AD, and 620 AD, while relatively wet conditions occurred at approximately 90 AD, 150 AD, 200 AD, 240 AD, 320 AD, 360 AD, 435 AD, and 530 AD. These drought records suggest a multi-decadal cyclicity, particularly during the Early Three Kingdoms Period. This may imply that multi-decadal to century scale fluctuations of winter and summer monsoons controlled the sedimentary regime of Lake Eurimji catchment areas during this period in South Korea.