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Annual-scale environmental and ecosystem changes since the mid-19th century were reconstructed in a seasonally frozen lagoon, Lake Mokoto, located along the Okhotsk Sea coast of Hokkaido, northern Japan. X-ray fluorescence (XRF) and diatom analyses of laminated sediment were conducted to determine the impact of short-term climate oscillations and anthropogenic eutrophication on the lagoon ecosystem. In this lagoon, eutrophication has progressed since the early 20th century due to deforestation and human activity in the catchment area, such as development of the cattle industry and field agriculture. In particular, the sedimentation rate of Lake Mokoto and potassium and phosphorus contents of lake sediment have increased rapidly since 1970, in association with an increase in the number of cattle. In addition, the Lake Mokoto ecosystem shows interannual and decadal cyclic fluctuations that are separate from the anthropogenic effects. Prior to anthropogenic eutrophication, the nutrient condition of Lake Mokoto showed a close relationship with the Pacific decadal oscillation (PDO). The growth of seagrass, inferred from epiphytic diatom abundance, showed a clear decadal cycle following variation in solar irradiance. [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]

This study reconstructs a record of typhoon frequency over the Korean Peninsula during the mid-Holocene using mineral components and diatom assemblages in deposits of Lagoon Hyangho, located on the east coast of the peninsula. The lagoon deposits confirm the occurrence of cyclical, multi-centennial scale episodes of low salinization induced by typhoon-derived heavy rain. Although shifts in typhoon frequency broadly follow El Niño/Southern Oscillation (ENSO) conditions during the Holocene, evidence from the typhoon-induced deposits in Hyangho Lagoon suggests that the path of the polar westerly jet controls the effect of ENSO on multi-centennial-scale typhoon patterns across the mid-latitude region of East Asia. The influence of ENSO is limited when the westerly jet passes through low latitudes. Fluctuations in solar activity play a key role in regulating movement of the westerly jet. Multi-centennial scale changes in typhoon frequency in mid-latitude East Asia are, therefore, influenced by changes in solar activity and ENSO conditions. [ABSTRACT FROM AUTHOR]

The western coast of the Korean Peninsula has some of the highest tidal ranges in the world. Gomso Bay, located along the western coast of Korea, is a semi-enclosed embayment. Tidal deposits occur under most lowland rural areas along Gomso Bay. In this study, the diatom assemblage and grain size distributions of surface sediments along a tributary of Gomso Bay called Jujin Stream were examined to identify their relationships with tidal levels. Consequently, riverside sediments of Jujin Stream could be separated into four tidal level zones based on their diatom assemblage and grain size distribution. Clear diatom and grain size distribution gaps were found between the tidal (zones 1 and 2), fluvio-tidal transition (zone 3), and freshwater (zone 4) zones. The tidal zone was characterized by allochthonous marine diatom taxa. These marine taxa, and the sand content, gradually decreased in the upstream direction. Sediment in the fluvio-tidal transition zone has particularly distinctive features. Extremely high numbers of diatoms are present, which are dominated by freshwater taxa, with rare marine and marine-brackish taxa. Grain sizes in this zone increase sharply in the upstream direction. Combining diatom and grain size data can provide accurate paleo sea-level information for strongly tidal areas. • Diatom and grain size distributions in an area of large tidal range along Jujin Stream, which flows into Gomso Bay. • Limit of sea-water incursion is appear by the fluvio-tidal transition zone, which has extremely high diatom abundance with a majority of freshwater diatoms and rare marine diatom. • The elevation of the fluvio-tidal transition zone is between the extreme high-water level of spring tide and the maximum tidal level of neap tide, although the distance from the river mouth is stronger factor. • In an area of large tidal range, appearance of marine diatom in sediment suggests the elevation was under extreme high-water level of spring tide. [ABSTRACT FROM AUTHOR]

Abstract: Diatom fossils from core sediments and living diatoms from water samples of Notoro Lagoon in northern Japan were examined to evaluate natural climate effects on lagoon environmental changes. In 1974, the artificial inlet was excavated. Immediately after, the anoxic bottom water in Notoro Lagoon began to disappear due to an increasing water exchange rate. However, chemical oxygen demand (COD) in the bottom water of Notoro Lagoon gradually increased, with fluctuations, during the last 30 years. In addition, the dominant diatom assemblages in Notoro Lagoon shifted to ice-related and spring bloom taxa after the excavation. The dominant taxa of each year in the sediment core were also strongly related to the timing of lagoon ice melting. This is because the COD in Notoro Lagoon was affected by the deposited volume of blooming diatoms, which was controlled by the duration of ice cover and the timing of ice discharge to the Okhotsk Sea likely due to an air pressure pattern change over the northern North Pacific like the Pacific Decadal Oscillation (PDO). [Copyright &y& Elsevier]

Abstract: Sea-ice diatom taxa (Fragilariopsis cylindrus and Fragilariopsis oceanica) and their relative abundance in the Okhotsk Sea were used to reconstruct the history of sea-ice distribution and atmospheric pressure patterns since the Last Glacial Maximum (LGM). The temporal state of sea-ice distribution and atmospheric pressure patterns since the LGM can be divided into three modes: northern Aleutian Low mode, southern Aleutian Low mode, and strong Siberian High mode. The Southern Aleutian Low mode was dominant before 15ka and after 6.5ka, respectively, showing expanded sea-ice distribution into the central and southern Okhotsk Sea. During the deglaciation period (15ka to 10ka), sea-ice retreated from the southern Okhotsk Sea because of the pronounced westerly winds under the strong Siberian High mode. However, sea-ice distribution expanded in the northern Okhotsk Sea, which favors the development of extensive polynyas on the northern continental shelf. Occurrences of northern Aleutian Low mode were frequent between 10 and 6.5ka, while sea-ice distribution expanded into the eastern Okhotsk Sea. Formation of the Okhotsk Sea Intermediate Water, inferred from radiolarian species Cycladophora davisiana, intensified under both northern Aleutian Low mode and strong Siberian High mode. [Copyright &y& Elsevier]

Paleoenvironment and paleoclimate changes during the Little Ice Age (LIA) were investigated using sediments from Lake Hamana. This lagoon in central Japan is under a complicated climate system influenced by the East Asian summer monsoon (EASM), East Asian winter monsoon (EAWM), and strong typhoon events. Diatom assemblages and chemical components of a 1.3 m sediment core suggest historical events of rapid salination and cyclical ecosystem shifts accompanied by monsoon condition changes. Most research has indicated that Lake Hamana changed from a freshwater lake to a brackish lagoon because of the Meio earthquake in 1498 CE. However, based on results from the diatom assemblage and chemical components, this salination occurred because of changes in the Hamana paleoriver flow, and this change happened before the earthquake, around the mid-15th century. Furthermore, it is clear that Lake Hamana was an oligohaline lagoon before salination. The lithology of the lake sediments and the diatom assemblage changed during the LIA; sand, water, and total diatom contents decreased. However, the relative abundance of marine diatom taxa increased during the LIA. These changes were introduced by a weakening EASM coupled with low precipitation under low solar activity during the LIA. Several flood events represented by freshwater diatom taxa and magnetic susceptibility peaks co-occurred with strong El Niño events during the LIA. It is inferred that this strong El Niño during the LIA induced frequent typhoons causing floods in central Japan. [ABSTRACT FROM AUTHOR]