Holocene Evolution of the Outer Lake of Hwajinpo Lagoon On the Eastern Coast of Korea; Environmental Changes with Holocene Sea-Level Fluctuation of the East Sea (Sea of Japan)

Abstract

The evolution of the outer lake of Hwajinpo Lagoon in Korea has been reconstructed using environmental proxies (lithologic, geochemical, and fossil data) with a chronology established using 7 accelerator mass spectrometry (AMS) radiocarbon dates. Grain size, water content, and X-ray analyses from the core of outer coastal lakes (HJ99) were used to reconstruct sedimentary environments by using total organic carbon, C/N, S, and C/S chemical proxies. Assemblages of mollusc remains also provided paleoenvironmental information. The environmental changes of the outer lake of Hwajinpo Lagoon can be divided into 6 depositional phases. The basin of the Hwajinpo was exposed and underwent a weathering process before the Holocene period. The muddy sand layer on the weathered bedrock indicated an estuarine system about 6000 BP. The laminated layer implies that the lagoonal system was anoxic between about 5500–2800 BP. The marl layer implies a relatively oxic lagoonal condition with mollusc presence about 2500 B P. The layer of very low sulfur content indicates a freshwater lake system isolated by a sand barrier about 1700 BP. Beginning about 1000 B P, the river system deposits progress progradation on the marl layer. Two erosional landforms could be related with a high standing sea level span during Holocene. These high-stands are dated at 5700 BP and 2200 BP and are supposed to have formed erosional landforms of about 1.6 amsl and 0.8 amsl, respectively. Environmental changes of the outer lake of Hwajinpo Lagoon are considered due mainly to the lake- and sea-level fluctuation during Holocene.

Earth Surface Processes and Environmental Changes in East Asia

“…Nine species of molluscs are present in the core HJ99, including five unidentified species. Unit H1 containing mainly P. amurensis presents in recent condition of Hwajinpo Lagoon (Yum et al , 2004(Yum et al , 2006 Especially, Potamocorbula amurensis is a typical brackish type mollusc in the coastal area of Korea (Kwon et al 1993).…”

“…9.9). According a radiocarbon datum from the corresponding unit of Yum et al ( , 2004Yum et al ( , 2006, sedimentation rate is assumed 1.35 mm/year in this unit (dot line in Fig. A straight line was fit between sections of constant sedimentation to construct this age-depth model.…”

“…The time when unit H6 was exposed could be before last glacial maximum period, because the bottom of unit H5 started deposition after 6,800 14 C yr BP (Yum et al , 2004(Yum et al , 2006. Unit H6 Weathered Deposit on the Exposed Basement (Before 6,800 14 C yr BP) The basement rock of the Hwajinpo Lagoon area is Gyeonggi Gneiss Complex intruded by Daebo granites that have been the most clastic sources of depositional material to Hwajinpo lagoon.…”

“…Unit H4 Stagnant Brackish Lagoonal Deposit (5,800 14 C yr BP) Yum et al ( , 2004 have suggested that the difference in bottom condition is a conse- (1) Normal exchange between the oxic waters of the depositional basin and the adjacent ocean, followed by (2) subsequent restriction due to fluctuations in sea level and/or tectonic activity. Finally, (3) the development of low oxygen conditions and laminite formation.…”

“…This study area of Hwajinpo and Songjiho lagoons reported tectonically stables and it means that the study areas provide the record of the eustatic sea level fluctuation. Jung and Park (1976) reported for bottom sediment distribution of the Hwajinpo, Park and Kim (1981) for sand barrier developments, and Yum et al ( , 2004 for recent depositional environment of the Hwajinpo Lagoon. And it could be useful for comparing the sea level fluctuation of Japanese Islands side that is tectonically active area in the East Sea (Japan Sea).…”