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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: 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]

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]

Abstract: Diatom assemblages of the surface and in core sediment samples from Lake Saroma (Japan) were examined for the purpose of evaluating anthropogenic effect on the coastal environmental changes. Before the first inlet excavation, the lake''s water quality and ecology were controlled by water exchange with the Okhotsk Sea as well as lake-level variation. However, large-scale ecological modification occurred, mainly due to artificial excavation and shellfish industrial farms. A distinct record of the succession of the dominant diatom taxa was preserved in core sediments. Low-oxygen water was prevalent in the lake in 1929, before the first inlet excavation. Immediately after the first inlet excavation, the low-oxygen water in the western basin of the lake began to disappear, in a trend that became increasing transparent, which has been attributed to an increasing rate of water exchange. However, the lacustrine environment of bottom sediments resumes deterioration 20years after since the first artificial excavation: the resultant deposition of river-mouth materials into the deep basin caused eutrophication and environmental disturbance of the lake bottom. At the same time, the eutrophication of surface water became intensified with the onset of intense scallop culturing beginning in 1966. Increasing organic loads deposited onto the bottom layer in the form of excreta from the scallop nursery led to more oxygen deficiency and the elution of nitrogen and phosphorus from the sediment, which again brought about eutrophication of the surface layer. Such environmental change was reflected in a decrease of benthic diatom taxa and an increase of planktonic taxa, trends which have continued until today. Particularly, the numbers of diatom assemblage have been decreasing all over the lake during the last 10years, which suggests that Lake Saroma''s present-day deterioration and eutrophication will continue or become even worse. [Copyright &y& Elsevier]

Abstract: Diatom assemblages in suspended particulate matter (SPM), surface sediments, and piston core samples in the southern Bering Sea and the western subarctic Pacific were examined. The distribution of diatom assemblages in the southern Bering Sea shows not only differences in species composition between the SPM and surface sediment samples, but also preferential dissolution of different species in the upper water column or surface sediments. Furthermore, we present the downcore profiles of selected diatom taxa for four piston cores to indicate changes in paleoceanographic conditions, e.g., seasonal productivity, sea-ice and surface-water circulation, of the glacial and deglaciation periods during the late Quaternary. In particular, we propose five models for the annual cycling of the water masses in the western subarctic Pacific and the southern Bering Sea during the late Quaternary. Significant regional differences in diatom accumulation rates and taxonomic composition between the southern Bering Sea and the western subarctic Pacific reflect spatial shifts in water masses caused by the variable extent of sea-ice cover and cold low-salinity surface waters in these regions. [Copyright &y& Elsevier]

Abstract: High-resolution analyses of geochemical parameters (biogenic opal, calcium carbonate, organic carbon, and nitrogen) and microfossil assemblages (diatoms and radiolarians) on Core MD01-2412 clarified detailed paleoceanographic changes such as sea-ice cover and biological production in the southwestern Okhotsk Sea during the last 115kyr. An age model of Core MD01-2412 was established based on δ18O stratigraphy, accelerator mass spectrometer (AMS) 14C, and tephrochronology. Sea-ice history reconstructed by siliceous microplankton records indicated that the present sea-ice condition was formed during the last 8kyr. Only during Marine Isotope Stage (MIS) 2 was the duration of sea-ice cover in this region much longer than that of today (4–5 months a year). Two diatom species, Thalassionema nitzschioides and Fragilariopsis doliolus, revealed that the Soya Warm Current Water (SWCW) flowed into the Okhotsk Sea near the site of Core MD01-2412 during the last 12–14kyr and during MIS 5a, and was associated with sea-level rise. Biological productivity rapidly increased during MIS 1, associated with sea-ice retreat. Two major increases of organic carbon (OC) contents (wt%) and Corg/N ratios were observed, and the timings of these events were 15.8–16.7ka (Event 1) and 13.1–13.6ka (Event 2). Corresponding to these events, the abundance of Cycladophora davisiana, an intermediate water dwelling radiolarian species, increased. This high C. davisiana abundance can be correlated to the input of terrestrial organic matter from the submerged shelf to the intermediate water. Apart from the radiolarians, the production of diatoms in the surface waters was suppressed by the development of well-stratified surface water along with sea-ice melting during the early Holocene. Diatom production increased gradually during the last 10kyr with enhanced vertical mixing. [Copyright &y& Elsevier]