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Lim, Jaesoo,Lee, Jin-Young,Hong, Sei-Sun,Kim, Ju-Yong,Yi, Sangheon,Nahm, Wook-Hyun Cambridge University Press 2017 Quaternary research Vol.87 No.1
<B>Abstract</B><P>To trace past changes in flooding frequency, we investigated fluvial sediments in the middle reach of the Nakdong River, South Korea. Sediments with larger grain size, lower total organic carbon percentage, and depleted δ<SUP>13</SUP>C values in the recovered sediment cores were interpreted as periods of more frequent flooding. Patterns of decreased long-term flooding frequency and vegetation changes during the early to late Holocene were similar to the decreasing regional summer monsoon intensity. Multicentennial frequent flooding periods in the study area (2900-3400 cal yr BP, 3600-3900 cal yr BP, 4600-5300 cal yr BP, and 5800-6400 cal yr BP) corresponded to stronger El Niño-Southern Oscillation (ENSO) activity periods. Based on previous studies showing that high-frequency tropical typhoon-driven coastal inundation along the western coast of Japan during the middle to late Holocene was coupled with stronger ENSO activity, it is likely that the observed centennial-to-millennial-scale flood frequency change in South Korea was influenced mainly by changes in the genesis and tracks of tropical typhoons at centennial-to-millennial time scales. This suggests that the centennial-to-millennial-scale hydrologic changes in East Asia were linked to the remote atmospheric-oceanic circulation changes represented by an ENSO-like pattern.</P>
Megascopic evidence for impact cratering in the Jeokjung–Chogye Basin, Hapcheon, South Korea
Jaesoo Lim,Sei-Sun Hong,Min Han,Sangheon Yi,Sung Won Kim 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
The 4-km-diameter Jeokjung–Chogye Basin in Hapcheon, South Korea is famous for its bowl-shaped geomorphology. In the basin, as a macroscopic evidence for past impact cratering, the shatter cone was found in the 6-cm long shale clast in the crater-fill impact melt rocks, at 130 m depth of the drilled core, and its appearance is characterized by striated fractures that typically make partial to complete cones. The striations of the surface are directional and form a distinctive pattern in which the acute angle of the intersection points toward the apex of the cone. It is interesting that the shatter cones point upward, and their axes lie at high angle to the bedding of the shale clast. Shatter cones have been considered as the only distinctive and unique shock-metamorphic feature that can be found in hand specimen to outcrop. Shatter cones can be formed in all kinds of target rocks including sandstone, shale and carbonates, and more delicate and well-formed cones form in fine-grained rocks. Recently several studies suggested that the shatter cones can be found in several stratigraphic settings within and around impact structures, for example, in the central uplifts of complex impact craters, in the rim of complex impact craters, and in a breccia dike and crater-fill impact melt rock. Based on the past studies, shatter cones form under relatively low shock pressures with 2~10 GPa in large volume of target rock below the crater floor. The shatter cones found in this study suggest that at least shock pressure around 130 m depth was 2~10 GPa during the impact cratering.
Jaesoo Lim,In-Kwon Um,Sangheon Yi 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
To trace past extreme hydroclimatic events (e.g., flooding, drought and typhoon) in East Asia, where population density is high and new climate norms are emerging due to global warming, we investigated fluvial sediments in the middle reach of the Nakdong River, South Korea, as an analogue of past regional climate changes. Based on strong correlations among lithogenic elemental ratios (e.g., Zr/Ti and Sr/Si) measured with an X-ray fluorescence core scanner, grain sizes and carbon isotope values of organic matter in the sediments, we reconstructed changes in decadal-scale flooding and drought events over the period 7,700-1,700 calibrated years before present (cal BP). Flooding events were associated with higher sea surface temperatures in the western Pacific and strong El Niño-Southern Oscillation (ENSO)-like climatic conditions; Nakdong extreme flooding (NEF) events occurred at 7,700, 7,200, 6,000, 5,000, 3,700, 3,200, 3,000, 2,600, 2,000, and 1,700 cal BP. Nakdong extreme drought (NED) events occurred at 5,300, 4,500, 4,000, 3,500, 2,100, and 1,900 cal BP, mainly under weakened ENSO-like conditions. We found a significant climatic shift around 3,700 cal BP (or 3.7 ka). During the period of 7,700-3,700 cal BP, the hydroclimate fluctuated, with dominant periodicities of 950, 540, 400, 300, 65, 45, 30, and 24 years, while hydroclimate during the subsequent period of 3,700-1,700 cal BP showed shorter centennial to decadal cycles (320, 110-120, and 60-75 years). This 3.7-ka climatic shift is consistent with a marked southward shift of the intertropical convergence zone, intensified ENSO activity, increased frequency of recurving typhoons, and deep-ocean circulation changes in both the northern and southern hemispheres, demonstrating the urgent need for investigation of the critical role of deep-water circulation in past and future climate changes.