SEA LEVEL FLUCTUATIONS OF THE LAST FIVE MILLENNIA RECORDED IN MACROTIDAL FLATS ALONG THE WEST COAST OF KOREA

Alluvial or coastal plains are distributed widely along the western coast of the Korean Peninsula, especially in the low-lying coastal and river mouth areas. Examples include the Honam Plain around the Mankyeong and Dongjin rivers (3500 km2), the Naju Plain around the Yeongsan River (5210 km2) and so on. The recent Quaternary geology mapping project in Korea drilled numerous cores in and around the Honam and Naju Plains and revealed that the largest parts of the plains were originally tide-dominated macrotidal flats (KIGAM, 2014). Written records of tidal flat reclamation history remain from the Goryeo Dynasty (A.D. 1235); significant areas of reclaimed tidal flat along the western coast are now used, mainly, as arable paddy land.

Several studies of sea level have indicated that it reached its highest level between 7000 and 6000 years ago and then decreased with small fluctuations to the present level. Nevertheless, a more precise estimate of the amplitude and timing of the highstand apex during the mid-Holocene is very important, and detailed knowledge of the changes accompanying various fluctuations during the last five millennia is also crucial for understanding the evolutionary history of the diverse coastal geomorphology, including marine terraces, lagoons, sand dunes, and coastal plains, as well as the lifestyles of the people who lived there in the Bronze and Iron ages. However, the sea level changes during the last five millennia following the Holocene sea level highstand are difficult to delineate, partly because their amplitudes were small and indicators of exact sea level are sparse. Although some attempts have been made along the west coast of Korea, they have received little attention due to a dispute about the range of sea level fluctuation (Hwang, 1998; Hwang and Yoon, 2002; 2011).

The present study provides a description and chronology of three coastal plain sediment cores (YSDL-17, GML-01, YSML-07) and two cores (SB1, SB4) retrieved from the Naju Plain, and two cores (JUL-31, JUL-21) from the Honam Plain, southwestern Korea. Based on the lateral distribution of the sediments, we propose a sea level model over the last five millennia. The data are then compared with previously reported sea level studies to elucidate the changes of hydro-isostatic balance in the Korean peninsula.