Jin Cheul Kim

Delta Formation in the Nakdong River, Korea, during the Holocene as Inferred from the Diatom Assemblage

ABSTRACT Cho, A.; Cheong, D.; Kim, J.C.; Shin, S.; Park, Y.-H., and Katsuki, K., 2017. Delta formation in the Nakdong River, Korea, during the Holocene as inferred from the diatom assemblage. The causes and succession of the Nakdong River delta formation related to sea-level change and precipitation in the early–middle Holocene were discussed based on diatom analysis. On the basis of ecology, stratigraphic changes of diatom flora were divided into five divisions, labeled zones I–V. According to the diatom assemblage, the first marine transgression occurred at about 10.5 ka at this research site. At that time, marine species gradually increased, and freshwater species had their highest values. This site was an estuary influenced by inputs of fresh and marine water until 9.8 ka (zone I). Zone II is dominated by bay and offshore species, meaning that sea level rose continuously after about 9.8 ka (zone II) when the Nakdong River mouth region was an inner bay. Then, the environment of this site changed to an outer bay after 8 ka (zone III) because of sea-level rise, which is supported by increasing numbers of oceanic species. At the end of this zone, at 6–7 ka, sea level highstand occurred. A prodelta began forming just after the highstand under slow sea-level rise (zone IV), and the delta front arrived at its present location at 5 ka by voluminous sediment input from the Nakdong River related to high precipitation during the Holocene Climate Optimum (zone V). After 5 ka, the delta front remained around its present site because of stable or decreasing sea level. Compared with other Asia deltas (Yangtze and Mekong), the initiation of the Nakdong River delta was delayed by about 1000–2000 years because of differences in relative sea-level changes.

The 8.2 ka cooling event in coastal East Asia: High-resolution pollen evidence from southwestern Korea

In this study, we present a high-resolution multi-proxy record (pollen, magnetic susceptibility, and mean grain size) from Bigeum Island, South Korea, which mainly showed early Holocene paleoenvironmental change. Bigeum pollen records indicated that early Holocene climate variations in coastal East Asia were principally controlled by the Atlantic meridional overturning circulation. Most importantly, the 8.2 ka cooling event was clearly recognized for the first time in coastal East Asia, where few high-resolution proxy data, such as ice core and stalagmite δ18O records, are available. The insular vegetation in the study site was extremely susceptible to even short-term climate changes, such as the 8.2 ka cooling event, which allowed a detailed climate reconstruction from pollen data. Early Holocene climate teleconnections between coastal East Asia and other regions were identified through regional comparisons of Greenland, China, Brazil, Spain, Madagascar, and Korea. Coastal East Asia is one of world’s most populated regions and will be particularly vulnerable to future climate change. Accurate and detailed paleoclimate proxy data, such as the Bigeum pollen record, will therefore be increasingly important in this region.

Characteristics of Marine Terrace Sediments Formed during the Marine Isotope Stage 5e in the West South Coast of the Korean Peninsula

It was firstly revealed in this research that the marine terrace of the Ijin-ri (Bukpyung-myeon, Haenam-gun) was formed during the last interglacial (Marine Isotope Stage 5e; MIS 5e). The marine terrace totally ranging from 4.8 m (asl) to 8.8 m (asl) is subdivided into 4 units; Unit I ranges 4.8-5.3 m, unit II ranges 5.3-6.9 m, unit III ranges 6.98.3 m, and unit IV ranges 8.3-8.8 m. Strong evidences that units II and III were formed during MIS5e were obtained based on OSL dating, the physical characterizations such as particle size distribution, magnetic susceptibility and water content, principal element and trace element analyses, and quantitative clay mineral analysis for samples at the 30 cm intervals. The rounded gravels on the marine terrace are regarded to be originated from the clastic materials transported directly from the surrounding mountains toward the marine and abraded in the coastal area, without any fluvial processes. During the warmest period (125k, unit II), the increase in rainfall, along with the rapid rise in sea level, was likely caused the high amount of clastic materials transported to the upper part of the beach. As a result of comparing clay mineral ratios of study site with those derived from sediments of either tidal flats, or the Yellow Sea, it is interpreted that the sediments of study site were influenced from the marine. The results will be used to investigate the hydrological activity and sedimentary environment during the high sea level in the past.