Holocene Climate Change and Human Impact around Lake Pomoho on the East Coast of Korea

Abstract

Climate change continues to be at the forefront of scientific research and global political debate. The ability to understand and predict future climate patterns is an important concern. Reconstruction of the paleoclimate is a vital step in modeling future climate change. The Holocene epoch(11700cal yr BP-present) contains periods that were warmer and cooler than present conditions. Careful examination of past climate events can provide researchers with greater predictive accuracy for modelling future climate change. Until now, there have not been many paleolimnological studies carried out in Korea due a lack of natural lakes and undisturbed sediments. However, lakes on the east coast of Korea have remained relatively undisturbed and are providing a wealth of climate data for Korean researchers. The purpose of this study is to reconstruct mid- and late Holocene climate and environmental changes around Lake Pomeho on the east coast of Korea. Pollen analysis, grain-size analysis, organic content analysis, magnetic susceptibility analysis and radiocarbon dating were performed on an approximately seven meter long section of a core taken from the northern edge of Lake Pomeho. The sediment core was radiocarbon dated to approximately 6700 cal yr BP. Based on statistical analysis(CONISS) of the pollen data, five zones were created. The climate in Zone PM 1(??-6700 cal yr BP) had a high percentage of Quercus and other mesic hardwoods, likely representing the Holocene Climate Optimum(HCO) in Korea. In zone PM 2(6700-4000 cal yr BP), the percentage of Quercus pollen decreased while Pinus increased at around 5800-5500 cal yr BP. This likely represents the end of HCO in Korea. Magnetic susceptibility data and grain-size analysis indicate that erosion levels remained heavy until around 5600 cal yr BP, when both decrease, suggesting the stabilization of sea level rise. At around 4300-4000 cal yr BP, there was a sudden decrease in arboreal species and an increase in cold tolerant plants, indicating climate conditions deteriorated, corresponding to the 4.2 kiloyear cold event found in other parts of the northern hemisphere. This study presents the first evidence for it in Korea. Zone PM 3 (4000-2500 cal yr BP) is characterized by either a deterioration of the climate after 4000 cal yr BP due to decreased EASM activity or a localized amelioration of the climate possibly due to the Tsushima Warm Current(TWC). In the first scenario, Pinus, thought to be of the relatively warm weather species Pinus thunbergii, decreases while Quercus increases, indicating cooler conditions. The percentages of Artemisia and Poaceae both decrease, possibly due to shifting soil conditions. In the second scenario, a warm phase could be indicated by the return of several hardwood species after the 4.2 kiloyear event. Quercus showed a significant increase from the previous zone, possibly indicating warmer and wetter conditions. Zone PM 4 (2500-850 cal yr BP) represents the beginning of agriculture in the area around Lake Pomeho. Disturbance indicator species Artemisia, Poaceae and Amaranthaceae increase in this zone, as does Oryza type pollen. Pollen concentrations in PM 4 are very low, while mean grain size is relatively high. This data suggests agriculture began in the region around 2500-2100 cal yr BP. Zone PM 5 (850-350 cal yr BP) is characterized by the intensification of agricultural activity around Lake Pomeho. Wild and domesticated Poaceae both continue to increase, as does Artemisia. Quercus continued to decline while Pinus and Betula, both sunlight loving trees, increased. High mean grain size indicates increased erosion, probably due to land clearance resulting from intensified agriculture. This study attempts to reconstruct the mid- to late-Holocene environment around Lake Pomeho on the east coast of Korea. Because there are few areas in Korea with undisturbed sediments, this study can be a meaningful contribution to paleoclimate research in Korea and East Asia. The end of the HCO, stabilization of sea level rise and the intensification of agricultural activities around Lake Pomeho correspond with data from other nearby study sites, indicating continuity in the area. Furthermore, this study is the first in Korea to find evidence of the 4.2 ky event, making it a meaningful contribution to the study of paleoclimate reconstruction. Keyword: multi-proxy analysis, east coast of Korea, coastal lagoon, paleoenvironmental reconstruction, paleoclimate, mid- and late-Holocene