16 research outputs found
Occurrences of fossil planktonic foraminifera in cores PC-11 and ND-2
No full textWe investigated fossil planktonic foraminifera from cores ND-02 (Nakdong River delta) and PC-11 (inner shelf off southeast Korea), to assess millennial-scale variations in the surface oceanography. The coiling ratio of Neogloboquadrina pachyderma shows five millennial-scale near-cyclic variations (~8.7-8.3, ~7.5-6.2, ~4.7, ~2.8 and ~0.5 ka) since ~9 ka. These variations are similar to warm-water diatom variations at the Oki Ridge where a cyclicity of ~1.6 kyr is present, possibly associated with a Bond event. By contrast, the near-cyclic variations in our planktonic foraminiferal record were not fully consistent with the reported intensity variations of the Tsushima Warm Current in the East Asian margin and the warm Kuroshio Current in the Northwest Pacific. Our records from southeast Korea seem to reflect the more regional oceanographic variations in the marginal seas of East Asia. We suggest that the coastal upwelling of the subsurface cold water along the southeast coast of Korea plays an important role in generating near-cyclic variations of warm-water biota. These variations may have a connection to the Bond event periodicity in the North Atlantic Ocean
Occurrences of fossil benthic foraminifera in sediment core PC-11
No full textWe investigated fossil planktonic foraminifera from cores ND-02 (Nakdong River delta) and PC-11 (inner shelf off southeast Korea), to assess millennial-scale variations in the surface oceanography. In addition, we also analyzed benthic foraminiferal fauna in core PC-11. The coiling ratio of Neogloboquadrina pachyderma shows five millennial-scale near-cyclic variations (~8.7-8.3, ~7.5-6.2, ~4.7, ~2.8 and ~0.5 ka) since ~9 ka. These variations are similar to warm-water diatom variations at the Oki Ridge where a cyclicity of ~1.6 kyr is present, possibly associated with a Bond event. By contrast, the near-cyclic variations in our planktonic foraminiferal record were not fully consistent with the reported intensity variations of the Tsushima Warm Current in the East Asian margin and the warm Kuroshio Current in the Northwest Pacific. Our records from southeast Korea seem to reflect the more regional oceanographic variations in the marginal seas of East Asia. We suggest that the coastal upwelling of the subsurface cold water along the southeast coast of Korea plays an important role in generating near-cyclic variations of warm-water biota. The occurrence of the bathyal benthic foraminifera, such as Eilohedra nipponica and Psuedoparrella naraensis in core PC-11 also supports the scenario of the upwelling of subsurface water into the shallower depths at the timing of the more common sinistral specimens of N. pachyderma than the dextral specimens that implies the more intense upwelling. These variations may have a connection to the Bond event periodicity in the North Atlantic Ocean
Application of a deltaic ichnological model to Holocene deltaic deposits: examples from the Nakdong River delta in the Southeastern Korean Peninsula
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Origin of Compositional Diversity of Marine Tephra during the Late Middle Pleistocene B-KY1 Baekdusan Volcanic Eruption
No full textThe focus of this study was the Baekdusan-Kita 1 (B-KY1) eruption during the late Middle Pleistocene. We identified B-KY1 tephra between the Toya and Aso-1 tephras in the ODP 794A core from the Japan Basin of the East Sea/Japan Sea. The stratigraphic position of the B-KY1 tephra correlated exactly with the first B-KY1 to be identified, in the 20EEZ-1 core from the Kita–Yamato Trough. However, B-KY1 tephra in the ODP 794A core showed a wide range of geochemical compositions. The textural characteristics of B-KY1 tephra in the ODP 794A core was characterized by higher content of fine-grained bubble-wall shards than that of the B-KY1 tephra in the 20EEZ-1 core. The difference in B-KY1 tephra between the two coring sites may reflect shifts in wind direction during volcanic eruptions. We refined the eruption age of the B-KY1 tephra by examining distinct sedimentary facies related to the start of the penultimate deglaciation of this region at ca. 135 ka. The findings of this study suggest that the compositional diversity of B-KY1 tephra may have been influenced by subsequent mixing of comendite and comenditic trachyte magma with injected pantelleritic magma during the late Middle Pleistocene Baekdusan volcanic eruption
Age of the SKP-II and SKP-III tephras from the southern East Sea/ Japan Sea
No full textThe ages of two newly identified tephras in marine oxygen isotope stages(MIS) 3 and 4 from the southern East Sea/ Japan Sea(ES/ JS) have been determined1
Paleoclimate Signals of Lake Hovsgol, Mongolia, over the Last 19,000 Years Using Authigenic Beryllium Isotopes
No full textThe paleoclimatic signal of Lake Hovsgol in Mongolia was investigated using authigenic beryllium isotopes from 20 sediment samples. The beryllium isotopic records from this study indicate that there were three dry and cold periods at the lake around 9400, 12,700, and 17,100 cal yr BP. The significant 10Be signal during the Younger Dryas (YD) period is found to be lower. This indicates that the Lake Hovsgol area was subject to a period of cold and dry conditions with minimal dust input, which manifests itself as a low 9Be profile in this study. In warmer and wetter climatic conditions, the 10Be concentration was found to be higher, and this agrees with the sedimentation record of this study. The climatic signals from Lake Hovsgol appear to be similar to a decreased beryllium record during the deglacial period. The results of this study confirm that the Lake Hovsgol area also experienced the global paleoclimatic change associated with the YD event around 12,700 cal yr BP. Further investigation using worldwide lake records by the analytical technique of authigenic beryllium will be useful for understanding the global paleoclimatic change pattern in conjunction with lake-level change.DOI: 10.2458/56.1651
Evolution of Depositional Environments in Response to the Holocene Sea-Level Change in the Lower Delta Plain of Nakdong River Delta, Korea
No full textThe Nakdong River delta, located in southeastern Korea, preserves thick and wide sediments, which are suitable for the high-resolution study of the evolution of depositional environments in the lower delta plain area. This study traces the Holocene evolution of the Nakdong River delta using deep drill core (ND-3; 46.60 m thick) sediments from the present delta plain. Sedimentary units of the sediments were classified based on grain size compositions and sedimentary structures: (A) alluvial zone, (B) estuarine zone, (C) shallow marine, (D) prodelta, (E) delta front, and (F) delta plain. The weathered sediment, paleosol, was observed at 43.16 m below the surface. There is an unconformity (43.10 m) to separate a Pleistocene sediment layer in the lowermost part differentiating from a Holocene sediment layer in the upper part of the core. The shallow marine sedimentary unit (32.20~23.50 m), in which grain size decreases upward is overlain by the prodelta unit (23.50~15.10 m), which consists of fine-grained sediments and relatively homogeneous sedimentary facies. The boundary between the delta front unit (15.10~8.00 m) and the delta plain unit (8.00~0.00 m) appears to lie at 8.0 m, and the variation in grain size is different; coarsening upward in the delta front unit and fining upward in the delta front unit, respectively. These sediments are characterized by a lot of sand–mud couplets and mica flakes aligned along with cross-stratification, which may be deposited in relatively high-energy environments. Until 13 cal ka BP, the sea level was 70 m below the present level and the drilling site might be located onshore. At 10 cal ka BP, the sea level was located 50 m below the present level and the drilling site might be moved to an estuarine environment. From 8 to 6 cal ka BP, a transgression phase occurred as a result of coastline invasion by the rapid rise of the sea level. Thus, the drilling site was drowned in a shallow marine environment. After 6 cal ka BP, the sea level reached the present level, and, since then, progradation might begin to form, primarily by more sediment input. After this period, the progradation phase continues as the sediments have advanced and the delta grows
Late Quaternary depositional environmental changes from the middle part of Nakdong River delta core sediments
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