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Tracking paleoenvironmental change and past event deposits is very important to evaluate the natural hazard spatially. This paper presents how the environment changes and implies the event deposit depending on the diatom assemblage change. To investigate paleoenvironmental change and identify the difference between tsunami and typhoon deposits, we analyze diatoms from the sediments in two coastal lakes in southern Japan where flood deposits have been linked to historical typhoon and tsunami events (Lakes Kawahara and Ryuo). The sediment cores extend from B.C.E. 500 to approximately C.E. 1000 and the downcore variation in diatom assemblages indicates a series of transitions from saline to fresher conditions in both Lake Kawahara and Lake Ryuo between approximately C.E. 500 and 1700. We observe an obvious deviation in diatom assemblages in event deposits previously identified to be either of tsunami or typhoon in origin. For the most prominent event deposit preserved in Lake Ryuo by the Hoei tsunami of C.E. 1707, the deposition of marine diatoms serves as evidence of marine flooding, while the subsequent deposition of soil and freshwater diatoms indicates the mobilization of terrigenous sediment during returning seaward flows. In contrast, the most prominent event deposit in Lake Kawahara is associated with freshwater flooding by the Kamikaze typhoon of C.E. 1281 and contains very low diatom abundances and a peak of freshwater taxa, followed by a peak in diatom counts potentially due to greater biological activity induced by a resultant influx of nutrients and re‐oxygenation during the event. [ABSTRACT FROM AUTHOR]

Detailed diatom records within core sediments from Maeho Lagoon along the Eastern coast of South Korea revealed that the ecological dynamics of the lagoon during the Holocene were associated with relative sea level and regional precipitation. Accelerator mass spectrometry ( AMS) 14 C dating indicates that sediment accumulation began prior to 8300 cal. year BP, and that the lagoon formed around 8000 cal. year BP. The salinity level of the lagoon increased until 5000 cal. year BP, and then decreased. Long-term trends in salinity were dependent upon changes in sea level: periods of high salinity in the lagoon were generally coincident with periods of high sea levels along the east coasts of Korea and Japan. On the other hand, multicentennial-scale fluctuations in diatom assemblages and magnetic susceptibility ( MS) suggest that a 400 year fluctuation in salinity was regulated by changes in precipitation in the area of this lagoon. Changes in the westerly jet stream, controlled by variation in solar irradiance, had an important influence on precipitation volume in South Korea, suggesting that the patterns of the westerly jet stream fluctuate on a 400 year cycle. [ABSTRACT FROM AUTHOR]

Detailed diatom records within surface and core sediments from the Southern Bering Continental Shelf (SBCS) reveal that the Holocene evolution of sea-ice distribution is associated with low pressure patterns. Holocene sea-ice distribution over the SBCS was mainly controlled by the location of the Aleutian Low. The corresponding paleoceanographic and paleoclimate conditions can be divided into three stages: (1) the early Holocene (before 7000 cal. yr BP) was characterized by extensive sea-ice distribution under two low-pressure cells, which covered the western Bering Sea and the Gulf of Alaska, respectively. (2) Between 3000 and 7000 cal. yr BP, the low-pressure system over the Gulf of Alaska became weak, causing total sea-ice mass over the SBCS to retreat. (3) In the past 3000 years, prevailing southwesterly winds over the SBCS due to the developing Aleutian Low have reduced further sea-ice cover on the SBCS. These paleoclimatic changes were probably a response to ENSO variation. The timings of water mass exchanges on the SBCS coincided with sea-level change along the Alaskan Peninsula. As a result, subsequent morphologic alterations have also influenced the paleoceanographic condition of the SBCS. The effect of the surface coastal water and bottom marine water on the SBCS intensified about 6000 cal. yr BP when sea level increased. [ABSTRACT FROM AUTHOR]

To identify temporal changes in the marine reservoir effect in Northeast Japan, the radiocarbon ages of terrestrial plants and marine shells were measured from two cores obtained from Lake Harutori in Kushiro City. This lake located where one of the highest marine reservoir effect was detected in East Asia, facing the Oyashio cold current. This drilling site is situated in a tectonically active region of the southwestern Kuril arc. Based on lithological analyses, molluscan assemblages, and radiocarbon dating, we interpreted five units in order of decreasing age: basement coastal sediments, estuary sediment, laminated sediment, inner bay sediment, and artificial fills. These sediments consisted of laminated mud beds with terrestrial plant fragments and intercalating tsunami sand beds. The structure was imaged using core scanning computed tomography and X-ray fluorescence. The reservoir ages of seven pairs were obtained from estuary sediment to inner-bay sediment over the past 9000 years. [ABSTRACT FROM AUTHOR]

To detect long-term change in the reservoir effect of the eastern coast of Korea, we measured the radiocarbon ages of terrestrial plant and marine shell pairs from the same horizons from approximately 12 m of Holocene sediments under reclaimed land around the Mae-ho Lagoon. The lagoon sediment was examined for lithology, and sedimentary structures, as well as mollusk and diatom assemblages. From bottom to top the sediment comprised three units: transgressive lagoon, restricted lagoon, and delta. The offset in the radiocarbon age of nine pairs were also divided into three stages: 300 ± 90 to 410 ± 120 years (transgressive lagoon sediment in 7350–5800 cal BP), 640 ± 150 to 800 ± 150 years (restricted lagoon sediment in 4320–1600 cal BP), and 120 ± 140 to 330 ± 100 years (deltaic sediment in 1200–390 cal BP). The reservoir ages (R) in the sediments of transgressive lagoon and the restricted lagoon include the variation associated with dietary of shells, herbivorous and deposit-feeding, respectively. The R values in the deltaic lagoon sediment were the most representative of those in the Mae-ho Lagoon because they were obtained from the shells of a filter-feeding taxon, Potamocorbula amurensis , in the most recent sediment. These values from the deltaic lagoon sediment are also consistent with previously reported modern reservoir ages from the Korean Peninsula and Peter the Great Gulf around Vladivostok. [ABSTRACT FROM AUTHOR]