Holocene shoreline displacement on the Gungnam Plain, Buyeo, near the central west coast of South Korea inferred from diatom assemblages

Diatom analyses of core GN-2 and trench Tr-1 taken from Gungnam Plain, on the lower reach of Geum River, are conducted to verify the Holocene shoreline displacement near the central west coast of South Korea. Age-controlled diatom profiles of GN-2 and Tr-1 record the history in responses of depositional environments to changes in sea level. Four diatom zones in GN-2 and two in Tr-1 were established. During Zone I (ca. 8400–6500 BP in Tr-1 and ca. 7100–6800 BP in GN-1), the depositional environment was an inner bay to a mud tidal flat. From 6800 to 4300 BP (Zone II of GN-1 and Tr-1), the former environments were replaced by brackish conditions with sediments deposited under a sea level high stand. Zone II also contains reworked sediments resulting from a temporary drop in sea level. Between 4300 BP and 400 BP (middle to upper part of Zone III of GN-1), there were stable freshwater environments with few diatoms after sea level fluctuations. Finally, during Zone IV (400 BP – Present), the depositional environments changed to freshwater marshlands. The multi-proxy data reflect the tendency of sea level variations in the Gungnam Plain from 8410 to 4330 BP. Around 8400 BP, the sea level reached an elevation of 1.14 m a.s.l. due to an early Holocene transgression. Overall, the sea level around the Gungnam Plain continued to rise from ca. 8400 to 6500 BP, although there was a temporary drop between 6800 BP and 6500 BP. At present, the spring tide has a range of 6.64 m (mean tide level 3.32 m) in Kunsan Harbor and local flood tides. The mean sea level in the Gungnam Plain about 6500 BP may have reached an elevation of 3.40 m a.s.l., corresponding to or slightly higher than the present sea level.