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The Ulleung Basin of the East Sea is a biological hotspot, but studies on the macrobenthos therein are lacking. To evaluate the macrobenthos species diversity and community structure in the Ulleung Basin, we conducted a survey from 2017 to 2021. A total of 262 species of macrobenthos was detected by the survey, and the average habitat density was 243 individuals/m2. The average biomass of macrobenthos was 43.7 g/m2, and the average Shannon and Wiener's diversity index was 2.3 (0.69–3.61). The dominant species were the polychaetes Terebellides horikoshii, Chaetozone setosa, Scalibregma inflatum, and Aglaophamus sp. and the bivalvia Axinopsida subquadrata. The community structure of macrobenthos differed according to water depth, and a correlation analysis using environmental variables showed that the community was affected by water temperature, salinity, and dissolved oxygen. The macrobenthic fauna in the Ulleung Basin was greatly influenced by water depth, the sedimentary environment was different, and the dominant species were also different. In addition, there was little seasonal change compared to the East Sea coast. Our findings will facilitate further investigation of benthic ecosystems throughout the East Sea. [ABSTRACT FROM AUTHOR]

The spatial distributions of dissolved lead (Pb) concentrations and stable Pb isotope ratios in the Ulleung Basin, East/Japan Sea, were investigated to identify the Pb source, the behavior of Pb in seawater, and the relationship between Pb isotopes and water masses. The Pb concentrations showed a surface maximum and rapidly decreased with increasing water depth. In the slope area, Pb concentrations increased near the seafloor due to diffusive flux from the sediments. The Pb concentrations in the surface layer were higher than in the Northwest Pacific Ocean and western Philippine Sea due to the proximity and downwind location of the sampling site from a Pb source area. The bottom Pb concentrations were lower in the study area than in the open ocean, indicating a higher scavenging rate due to the presence of well-developed bottom boundary layers from the continental slope to the basin. The Pb isotope ratios obtained from the continental slope and basin area were relatively constant among water masses. The Pb isotope ratios in the surface layer were similar to those of coal from northern China, which may indicate Pb supply from coal combustion in China. Meanwhile, Pb in the bottom water had a similar isotopic signature to Russian ore deposits and coal, indicating that bottom Pb is associated with thermohaline circulation in the East/Japan Sea. The Pb isotope ratios of the intermediate layer were constant across the depths of East Sea Central Water (ESCW) and were similar to aerosols measured previously (in 2003–2004) around Ulleung Island, which may indicate the timing of ESCW formation. [ABSTRACT FROM AUTHOR]

We characterized the biogeochemical organic carbon (Corg) cycles in the surface sediment layer of the Ulleung Basin (UB) of the East Sea. The total oxygen uptake (TOU) rate and the diffusive oxygen uptake (DOU) rate of the sediment were measured using an autonomous in situ benthic lander equipped with a benthic chamber (KIOST BelcII) and a microprofiler (KIOST BelpII). The TOU rate was in the range of 1.51 to 1.93 mmol O2 m−2 d−1, about double the DOU rate. The high TOU/DOU ratio implies that the benthic biological activity in the upper sediment layer is one of the important factors controlling benthic remineralization. The in situ oxygen exposure time was about 20 days, which is comparable to the values of other continental margin sediments. The sedimentary Corg oxidation rates ranged from 6.4 to 6.5 g C m−2 yr−1, which accounted for ~2% of the primary production in UB. The Corg burial fluxes ranged from 3.14 ± 0.12 to 3.48 ± 0.60 g C m−2 yr−1, corresponding to more than 30% of the deposited Corg buried into the inactive sediment deep layer. [ABSTRACT FROM AUTHOR]

Integrating seismic interpretation facilitates the discernment of tectonostratigraphic evolution in the eastern Korean continental margin, East Sea. The sedimentary succession of this margin is divided into three major seismic units based on distinct unconformities. These unconformities and their associated seismic characteristics indicate that the eastern Korean continental margin has experienced four evolutionary stages through extensional and subsequent two‐phase tectonic inversions. Early Miocene back‐arc opening of the East Sea triggered the extension of NNW–SSE and N–S trending rifts, resulting in non‐marine to deep‐marine deposition of typical rift‐related linked sedimentological systems in the eastern Korean continental margin. In the early Late Miocene, changes in plate motion and the subduction mode of the Japanese island (NW–SE compression) caused the positive inversion of extensional fault‐bounded half‐grabens in the eastern Korean continental margin, and the Hupo Basin was likely created by the regional flexural response. During this depositional period, hemipelagic sedimentation accompanied by episodic gravity‐controlled slope failures was predominant in the deep‐water environment. The subsidence of the Hupo Basin was enhanced by crustal shortening (E–W compression) that was induced by subduction initiation at the western margin of the Ulleung Basin during the late Early Pliocene (ca. 3.8 Ma). At that time, sedimentary environment gradually became shallower with time and coarse‐grained terrigenous input into the Hupo Basin began so that shallow‐ to deep‐marine sedimentation occurred. In the Quaternary period, the uplift of the Hupo Bank and coeval subsidence of the Hupo Basin were maintained by continued compressive neotectonics. The shallow‐ to deep‐marine deposition continued, but greater quantities of coarse‐grained terrestrial sediment were transported into the Hupo Basin due to combined effects on tectonics and eustasy. Based on the tectonostratigraphic reconstruction developed in this study, we propose that the Hupo Basin is likely a Late Miocene–Quaternary compression‐related basin induced by crustal or thrust loading. [ABSTRACT FROM AUTHOR]

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOMH) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m-3 d-1) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOMH, which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m-3 d-1) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m-3 d-1). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOMH in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

The Ulleung Basin of the East Sea is a biological hotspot, but studies on the macrobenthos therein are lacking. To evaluate the macrobenthos species diversity and community structure in the Ulleung Basin, we conducted a survey from 2017 to 2021. A total of 262 species of macrobenthos was detected by the survey, and the average habitat density was 243 individuals/m2. The average biomass of macrobenthos was 43.7 g/m2, and the average Shannon and Wiener’s diversity index was 2.3 (0.69–3.61). The dominant species were the polychaetes Terebellides horikoshii, Chaetozone setosa, Scalibregma inflatum, and Aglaophamus sp. and the bivalvia Axinopsida subquadrata. The community structure of macrobenthos differed according to water depth, and a correlation analysis using environmental variables showed that the community was affected by water temperature, salinity, and dissolved oxygen. The macrobenthic fauna in the Ulleung Basin was greatly influenced by water depth, the sedimentary environment was different, and the dominant species were also different. In addition, there was little seasonal change compared to the East Sea coast. Our findings will facilitate further investigation of benthic ecosystems throughout the East Sea.

Geophysical evidence indicating the presence of gas hydrate has been found in the Ulleung Basin, which lies off the east coast of the Korean Peninsula; however, hydrate distribution in the basin is not well understood. Logging-while-drilling data for 13 sites in the Ulleung Basin, East Sea, were obtained to investigate the distribution pattern of gas hydrate. Most of the sites yielded log data indicating the presence of gas hydrate. Prominent fractures (both resistive and conductive fractures) were clearly identified on the resistivity borehole images, particularly at seismic chimney sites. Resistive fractures, which contain large amounts of gas hydrate, are prominent in the seismic chimney sites. The strike and dip of each fracture was calculated and displayed on a stereographic plot and rosette diagram. From the fracture orientations on the stereographic plots, the maximum horizontal stress is NW–SE, reflecting the regional stress regime around the Ulleung Basin, although the fracture orientations are broadly distributed, indicating that the fracture pattern is not well-ordered on the rosette diagram. The fracture dips are between 36.46° and 63.66°; the range of dip azimuths is 0.94°–359°, and exhibit little change with depth. The dip azimuths are generally westerly to southwesterly. [ABSTRACT FROM AUTHOR]

The area near the subpolar front of the East Sea has high primary productivity during the spring season. We conducted two surveys, one in early spring and another in late spring, to assess environmental factors that influence phytoplankton community structure during these times. During early spring, vertical mixing supplied abundant nutrients to the surface. Due to the well-mixed water column, there were high nutrient levels, but total phytoplankton abundances and diversity were relatively low and were dominated by the diatom Chaetoceros spp. During late spring, the water column gradually stratified, with relatively high levels of nutrients in the surface layers near the coastal areas. Phytoplankton abundance and diversity at that time were higher, and there were diatoms (Pseudo-nitzschia spp. and Chaetoceros spp.), cryptophytes, and small flagellates. Pseudo-nitzschia spp. were especially abundant in re-sampled areas. The presence of a stratified and stable water mass and sufficient nitrate led to high phytoplankton growth, even in the open sea during late spring. These findings provide a better understanding of how phytoplankton population dynamics in the East Sea depend on water column stability during both early and late spring seasons.

This study evaluates the application of biomarkerbased temperature proxy data (alkenone with its resultant U 37 K ′ index and glycerol dialkyl glycerol tetraether (GDGT) with its resultant TEX86and TEX index) in the Ulleung Basin of the East Sea. Based U 37 K ′ on the and TEX86 indices from particles trapped for 10 days in December, 2015 and core-top sediment as well as the comparison of these indices to in situ seawater temperature data (from 2012 to 2016) from the Korea Oceanographic Data Center, this study confirms that the U 37 K ′ index of core-top sediment indicates the annual mean SST of the surface water. This study also suggests that TEX86 data of core-top sediment may represent the annual mean temperature of the subsurface water. However, a reliable calibration equation for GDGT will be required in this regional environment for the proper application of GDGT-based paleotemperature reconstruction. [ABSTRACT FROM AUTHOR]

Understanding bathymetric patterns of deep-sea meiofauna is important for elucidating the influence of environmental variables on the characteristics of meiofaunal community structures. Depth-dependent meiobenthic community structures were investigated on the continental shelf, slope and deep basin of the Ulleung Basin in the southeastern East Sea. Meiobenthic samples were collected at 29 stations (water depth: 74–2155 m) from the continental shelf near the Korea Strait to the southern slope and basin areas of the Ulleung Basin during the summer cruise of R/V Eardo (June 2006). In total, 15 meiobenthic taxa (between two and nine taxa at each station) were identified. Nematodes formed the most abundant taxon (53–80%), followed by benthic foraminifera (7–39%). Meiobenthic abundance varied from 295 to 3697 ind. 10 cm−2 (nematodes, 106–3319 ind. 10 cm−2) at each station. The distribution of meiobenthic animals in the Ulleung Basin showed considerable spatial trends, with higher densities on the southern slope (1083 ± 249 ind. 10 cm−2) of the Ulleung Basin than the shelf (650 ± 220 ind. 10 cm−2) or deep basin (459 ± 214 ind. 10 cm−2) regions. Among the environmental parameters, TOC (total organic carbon) concentration was significantly correlated with meiobenthic abundance (r 2=0.297), indicating that high food supply promotes meiobenthic abundance. Meiobenthic community structures showed depth-related trends from the continental shelf to the deep basin, with several meiobenthic taxa (e.g., amphipods, bivalves, harpacticoids, kinorhynchs, nauplius, nematodes, ostracods and polychaetes) contributing to these variations in the meiofauna. • Depth-dependent meiobenthic community structures were investigated. • Meiobenthic samples were collected from 29 stations of the Ulleung Basin. • Fifteen meiobenthic taxa were identified; nematodes were the most abundant taxon. • TOC concentration was significantly correlated with meiobenthic abundance. • Meiobenthic community structures showed depth-related trends. [ABSTRACT FROM AUTHOR]

The Ulleung Basin in the southwestern East/Japan Sea (hereafter East Sea) is known as a biologically productive “hot spot” but climate-associated changes in the physicochemical oceanographic conditions and some biological changes have been reported. In this study, our main objective was to determine the contribution of small phytoplankton to the total primary production, which is valuable information for detecting marine ecosystem changes in the Ulleung Basin. The small phytoplankton productivity contributions determined by Moderate-Resolution Imaging Spectroradiometer (MODIS)-derived monthly productivities using a phytoplankton community-based productivity algorithm was significantly consistent with the field-measured productivity contributions of small phytoplankton in this study. The daily primary productivity of small phytoplankton ranged from 42.7 to 418.7 mg C m −2 d −1 with an average of 172.9 mg C m −2 d −1 (S.D. = ±61.4 mg C m −2 d −1 , n = 120), and the annual contribution of small phytoplankton ranged from 19.6% to 28.4% with an average of 23.6% (S.D. = ±8.1%) in the Ulleung Basin from 2003 to 2012. Overall, large phytoplankton were a major contributor to the total primary production in the Ulleung Basin (76.4 ± 8.2%) from 2003 to 2012, which indicates that the Ulleung Basin is a highly productive region. A significantly negative relationship (p < 0.05) was found between the small phytoplankton primary productivity contribution and the annual primary production in this study. This finding revealed that the recent decreasing annual primary production in the Ulleung Basin could be a consequence of the increasing contribution of small phytoplankton. The response of phytoplankton to ongoing climate change depending on different-size phytoplankton compositions should be a subject for further investigation in the Ulleung Basin as a biologically highly productive region in the East Sea. [ABSTRACT FROM AUTHOR]

We investigated bacterial production (BP) and respiration (BR), as well as the physico-chemical properties of the water column, to elucidate the effect of upwelling on heterotrophic bacterial metabolic activities and growth efficiency (BGE) in July 2012 and May 2013 in the Ulleung Basin (UB), East/Japan Sea. The upwelled conditions were characterized by higher chlorophyll- a (Chl- a ) concentrations resulting from the upward shift of the nitracline compared to that of the non-upwelled condition. Analyses of the size fractions of Chl- a and pigment composition revealed that large size phytoplankton (> 20 µm), mainly consisting of diatoms, appeared to be the major phytoplankton component. BP and BR were significantly correlated with Chl- a ( P < 0.001), but the correlations with temperature were not significant ( P > 0.05). These results suggest that bacterial metabolic activities are stimulated by the availability of organic resources enhanced by upwelling in the UB. Further statistical analysis showed that the difference in BP and BGE with variations in upwelling intensity were significant ( P = 0.018 for BP, P = 0.035 for BGE), but the difference in BR was not significant ( P = 0.321). These results suggest that metabolic energy is partitioned more for BP under a strong upwelling condition, i.e. high nutrient and Chl- a conditions. In contrast, the energy generated via respiration was partitioned more for maintaining metabolism rather than for biomass production under weakly or non-upwelled conditions, i.e. stratified and low Chl- a conditions. Overall, our results suggest that any changes in upwelling intensity would significantly affect the carbon cycle associated with the fate of primary production, and the role of the microbial loop in the UB where changes in the intensity and frequency of upwelling associated with climatic changes are in progress. [ABSTRACT FROM AUTHOR]

We examined the genetic diversity and abundance of picocyanobacteria using barcoded amplicon sequencing approaches and flow cytometry in the East Sea and the East China Sea to determine the distribution patterns of diversity during spring in seas adjacent to the Korean Peninsula. Synechococcus clades I and IV, which have been known as coldadapted ecotypes, dominated at most stations. However, the relative abundances of the two dominant clades differed in their spatial patterns. Clade I was exclusively dominant in the shelf area of the East China Sea and the north East Sea. However, the dominant genotypes belonging to clade I had different spatial distributions in the two areas and responded oppositely to seawater temperature. The dominance of distinct genotypes under the different ecological conditions suggests the presence of ecologically different ecotypes within the clade. Abundances of clade IV were greater than those of clade I at most stations in the southwest East Sea, showing an apparently different pattern from that of the other areas. A warm-water adapted clade II was observed at significant levels only at stations located in the eastern East China Sea affected by a branch of the warm Kuroshio Current. These results suggest that the physicochemical properties of influencing water masses play an important role in determining the distribution of Synechococcus genotypes. [ABSTRACT FROM AUTHOR]

Seismic reflection data from the Ulleung Basin, East Sea, provide extensive evidence for Plio-Quaternary mass transport deposits (MTDs). We identified 23 MTDs embedded in 7 MTD bearing seismic units (U1–U7, from oldest to youngest) and estimated their geometric characteristics and source areas. The thicknesses and volumes of these MTDs range between 25 m and 392 m, and 13 km 3 –550 km 3 , respectively. The depth of the acoustic basement below sea level varies between about 100 m in the western margins to over 5900 m in the central part of the basin and highlights eight structural highs in the central and southern parts of the basin. These structural highs had a major role in controlling the pathway and distribution of the MTDs in the basin. In general, the thickest MTD units are sourced from the southern parts of the basin and the volume of these MTDs has remained relatively constant throughout the Plio-Quaternary. The seismic reflection data also reveal about 50 m high slide/slump scars formed on the seafloor by the latest submarine landslides which mainly concentrate on the southern and western margins of the basin. High-sedimentation rates and pore pressure build-up are likely the predominant factor controlling initiation of slope failures in the southern margin. Tectonics and gas hydrates likely play a role, too. The initial water depth of submarine landslides and volume and runout of the MTDs originating from the southern margin imply significant geohazard to the region. [ABSTRACT FROM AUTHOR]

Cyclic variations of density and gamma-ray logs achieved from the two drill sites of UBGH2-1_1 and UBGH2-1_2 in the western slope of the Ulleung Basin were investigated to reveal their paleoceanographic implications and establish cyclostratigraphy of the well logs. Integration of the core, log, and seismic data from the two sites indicates that the sediments of the drilled intervals mostly consist of hemipelagic muds, with a few sporadic mass transport deposits intercalated only in the downslope site of UBGH2-1_1. Comparison with variations in sediment composition suggests that ratios of terrigeneous to biogenic materials which were modulated by orbital-scale climate changes should be responsible to the well-log variations. The established cyclostratigraphy of the well logs based on correlation with marine oxygen isotope records indicates such climatic modulation of the sedimentation in the western slope of the Ulleung Basin has persisted at least since 2.6 Ma, consistent with the variation reported from the Oki Ridge. [ABSTRACT FROM AUTHOR]

Group velocity maps were derived for the southern East Sea (Japan Sea) and the Korea Strait (Tsushima Strait) for the 5-36 s period range, which is sensitive to shear wave velocities of the crust and the uppermost mantle. Images produced in our study enhance our understanding of the tectonic evolution of a continental margin affected by subducting oceanic slabs and a colliding continental plate. The seismic structure of the study area has not been described well because seismic data for the region are scarce. In this study, we applied the ambient noise tomography technique that does not rely on earthquake data. We calculated ambient noise cross-correlations recorded at station pairs of dense seismic networks located in the regions surrounding the study area, such as the southern Korean Peninsula and southwestern part of the Japanese Islands. We then measured the group velocity dispersion curves of the fundamental mode Rayleigh waves from cross-correlograms and constructed 2-D group velocity maps reflecting group velocity structure from the upper crust to uppermost mantle. The results show that three distinct anomalies with different characteristics exist. Anomalies are located under the Ulleung Basin (UB), the boundary of the Basin, and the area between Tsushima Island and the UB. 1-D velocity models were obtained by inversion of dispersion curves that represent vertical variations of shear wave velocity at locations of three different anomalies. The 1-D velocity models and 2-D group velocity maps of lateral variations in shear wave group velocities show that the high velocity anomaly beneath the UB originates from crustal thinning and mantle uplift. Confirming the exact causes of two low velocity anomalies observed under the UB boundary and between Tsushima Island and the UB is difficult because additional information is unavailable. However, complex fault systems, small basins formed by faulting, and deep mantle flow can be possible causes of the existence of low velocity anomalies in the region. [ABSTRACT FROM AUTHOR]

We analyzed the data consist of core digital images and X-rays, core-logs, LWD (logging-while-drilling), and sediment grain-size from the second Ulleung Basin Gas Hydrate Expedition (UBGH2) in the East Sea. Core digital images and X-rays were spliced as a complete composite core in meters below seafloor (mbsf) for five sites; UBGH2-1_1 (Hole D), 2_1 (B), 2_2 (B), 2-6 (B) and 2-10 (C–D), and were correlated with the core-log and LWD measurements showing that possible gas hydrate bearing layers are between the depths of about 60–180 mbsf at these sites. Bulk densities generally increase with depth from 1.3 to 2.0 g/cm 3 in LWD data, and from 1.1 to 1.8 g/cm 3 onboard which measured lower than in-situ. Gas hydrate bearing sediments respond with an increase of LWD densities (1.4–1.6 g/cm 3 ) and a decrease in core-logs (1.1–1.4 g/cm 3 ). P-wave velocity values of LWD increase (1400 to 1700 m/s) with depth for non-reservoirs, and are high (1500 and 2000 m/s) within the gas hydrate bearing intervals depending on the hydrate saturations.Resistivity values logged onboard range from less than 1.0 to over 10.0 Ω-m, while LWD records are around 1.0 Ω-m and between 5.0 and 30.0 Ω-m in background sediments and possible gas hydrate reservoirs, respectively. High resistivity values were observed (5.0–30.0 Ω-m) within coarse-grained turbidites (mean grain-size between 2.9 and 5.1 ϕ; laminated sandy mud or muddy sands). Medium resistivities were observed (5.0 Ω-m) within the silt-dominant hemi-pelagic and turbiditic sediments (5.1–7.4 ϕ; crudely laminated, bioturbated, homogeneous sand, and disintegrated sand and sandy mud facies) bearing pore-filling gas hydrates, or disseminated gas hydrates either formed in pores or small fractures of fine-grained sediments. Core-log measurements are highly fluctuating and sensitive but mostly lower (e.g., density and resistivity) than LWD records. [ABSTRACT FROM AUTHOR]

A total of 85 samples, collected from the UBGH1-9 core taken from the Ulleung Basin, East Sea, Korea, were analyzed using diatom assemblages. 111 diatom species belonging to 46 genera were identified, and three diatom assemblage zones were established on the basis of occurrence and distribution pattern of diatoms. Diatom assemblage zone I (134.10-174 m) is characterized by a relatively high abundance of marine species, while the increased number of the brackish species is recorded in diatom assemblage zone II (75-125 m). The assemblage zones IIIa became drastic drop of valve abundances and brackish planktons, whereas it became increase during the IIIb. High Td values which indicate an influence of warm current are recorded both in diatom assemblage zone I and III, and low Td values in diatom assemblage zone II. Analysis of diatom assemblages indicating that the depositional condition moved from oceanic to littoral-neritic environments and that paleotemperature underwent a shift from warm to cold condition at the middle interval, and from cold to warm condition in the upper interval of the UBGH1-9 core. This suggests that the lower (130-162 m) and upper intervals (0-20 m) of the UBGH1-9 core were deposited in the warm current condition (Tsushima Warm Current). [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on the tectonic, sedimentary, and volcanic evolution of a unique back-arc basin (Ulleung Basin) in the southwestern part of the East Sea (Sea of Japan). The basin consists of thick extended continental (or transitional) crust and an overlying sedimentary succession (4–8km thick), with interlayered volcanic flows and sills, defining a number of seismic units of variable reflection characteristics. The northern margin is bounded by faulted continental blocks (South Korea Plateau) with isolated basement lows and sub-basins with intruded and extruded volcanics, whereas the southern margin is underlain by deep-seated basement with a thick (>8km thick) sedimentary succession. The western margin is bounded by a series of strike-slip and normal faults produced by NNW–SSE-directed dextral movements. These structural features suggest a south–southeastward drift of the southwestern Japanese Arc away from the South Korea Plateau during the Early to Middle Miocene, involving large-scale right-lateral strike-slip deformation along the western margin, akin to pull-apart basin formation. During the back-arc opening, the thinned continental crust was largely modified by intrusive volcanics under tensional stress regime. The volcanics also extruded across the axis of extension as well as along the boundary faults on the west. In the Middle to Late Miocene, major faults in the southern and western margins were inverted, forming partial closure of the basin especially in the southern part, although the plateau in the northern margin experienced continuous subsidence. The Ulleung Basin thus provides an example of an immature back-arc basin which experienced a rather brief episode of rifting and extension followed by closure due to tectonic inversion. [Copyright &y& Elsevier]

Abstract: We investigated character and distribution of the gas hydrate zone in the western part of the Ulleung Basin in the East Sea (Japan Sea) from seismic data using stochastic sparse spike deconvolution, analysis of frequencies, and acoustic-elastic coupled full-waveform inversion. The presence of gas hydrates is primarily indicated by a bottom-simulating reflector (BSR) with negative polarity at approximately 240–250 ms below the seafloor. The instantaneous frequency section defines a high-frequency anomaly above the BSR underlain by a less clear low-frequency anomaly. Sparse spike deconvolution indicates that the BSR occurs horizontally along the boundary between the high- and low-frequency anomalies. Compared to well logs obtained from the seafloor through the BSR, the high-frequency anomaly, maximally 50 ms thick (ca. 45 m), appears to outline the hydrate-bearing zone. The results from full-waveform inversion appear to delineate the hydrate-bearing zone and the underlying gas-bearing zone, although their depth ranges do not coincide accurately with those inferred from well logs. The hydrate-bearing zone is characterized by an increase in both p- and s-wave velocities, while the gas-bearing zone is marked by a decrease in p-wave velocity. The very narrow free gas zone is less than 30 m thick and appears to be concentrated immediately below the gas hydrate stability zone. The hydrate-bearing zone is also defined by decreased Poisson's ratio which suggests appreciable variations of the hydrate-bearing zone in thickness. [Copyright &y& Elsevier]

Abstract: During drilling in 2010 in the Ulleung Basin, East Sea, Logging-While-Drilling (LWD) was conducted for thirteen sites. LWD data shows various characteristics indicating presence of gas hydrate-bearing sediments. In particular, a series seismic chimney sites are characterized by anomalous log data (i.e. high resistivity and velocity values), compared to surrounding marine sediments. At chimney sites, the resistivity and velocity log values are over 200 Ω-m and 3000 m/s, respectively. Moreover, log values of low density (less than 1.1 g/cm3) indicating the presence of massive hydrates also correlate with intervals with the highest resistivity and velocity. Gas hydrates at the seismic inferred chimney sites occurred within inclined fractures in the mud dominated sediments. These gas hydrate-filled fractures were identified on LWD resistivity images and X-ray images of pressure cores. The gas hydrate-filled fracture intervals coincide with high measured resistivity intervals at three sites (Sites UBGH2-3, UBGH2-7, UBGH2-11). In most cases, high measured resistivity translated into high hydrate saturations via Archie's relationship; but the high saturations derived from the Archie's relationship appear to overestimate gas hydrate saturations when compared to pressure core samples and acoustic log measurements. Also, in intervals with inclined gas hydrate-filled fractures (e.g., UBGH2-3), there is notable separation between phase-shift and attenuation resistivity logs, with 2 MHz resistivity measurements being significantly higher than 400 kHz resistivity measurements. In this study, the analysis of fractures with dip angles greater than 30° on the resistivity log-images show dip angles between 43 and 63° in average for the three sites examined. The dip azimuth of the fractures at Sites UBGH2-3 and UBGH2-7 dominate westerly to southwestern direction, while the fractures at Site UBGH2-11 are characterized by no preferred orientation. This fracture pattern indicates that maximum horizontal stress direction at the time of fracture formation was not constant. In addition, the differential compaction of the sediments after deposited may be contributed to the development of fracture. [Copyright &y& Elsevier]

Examinations of core and well-log data from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) drill sites suggest that Sites UBGH2-2_2 and UBGH2-6 have relatively good gas hydrate reservoir quality in terms of individual and total cumulative thicknesses of gas-hydrate-bearing sand (HYBS) beds. In both of the sites, core sediments are generally dominated by hemipelagic muds which are intercalated with turbidite sands. The turbidite sands are usually thin-to-medium bedded and mainly consist of well sorted coarse silt to fine sand. Anomalies in infrared core temperatures and porewater chlorinity data and pressure core measurements indicate that “gas hydrate occurrence zones” (GHOZ) are present about 68–155 mbsf at Site UBGH2-2_2 and 110–155 mbsf at Site UBGH2-6. In both the GHOZ, gas hydrates are preferentially associated with many of the turbidite sands as “pore-filling” type hydrates. The HYBS identified in the cores from Site UBGH2-6 are medium-to-thick bedded particularly in the lower part of the GHOZ and well coincident with significant high excursions in all of the resistivity, density, and velocity logs. Gas-hydrate saturations in the HYBS range from 12% to 79% with an average of 52% based on pore-water chlorinity. In contrast, the HYBS from Site UBGH2-2_2 are usually thin-bedded and show poor correlations with both of the resistivity and velocity logs owing to volume averaging effects of the logging tools on the thin HYBS beds. Gas-hydrate saturations in the HYBS range from 15% to 65% with an average of 37% based on pore-water chlorinity. In both of the sites, large fluctuations in biogenic opal contents have significant effects on the sediment physical properties, resulting in limited usage of gamma ray and density logs in discriminating sand reservoirs. [ABSTRACT FROM AUTHOR]

Abstract: The Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) recovered various forms of gas-hydrate bearing sediments from 10 drill sites in the lower slope and basin floor of the Ulleung Basin. To characterize the gas-hydrate occurrences and the properties of the host sediments, whole-round core samples were taken from portions of recovered cores determined to be hydrate-bearing based on infrared (IR) scanning. These samples were further characterized by a variety of shipboard experiments such as imaging of the sediments with hand-held IR and visual cameras, measurements of pore water chlorinity within and around IR inferred cold regions in the core and grain-size analysis of pore-water squeeze cakes. Sediment compositions of selected samples were further characterized by X-ray diffraction and scanning electron microscopes during post-cruise analysis. The shipboard and post-cruise analysis results collectively indicate that the recovered gas hydrates mainly occur as 1) “pore-filling” type bounded by discrete silty sand to sandy silt layers, 2) “fracture-filling” veins and nodules, or 3) “disseminated” type in silt. In addition, minor but significant variation in gas hydrate concentrations were observed in diatomaceous silt where gas hydrates occur as “pore-filling” material in layers dominated by intact diatom frustules. Gas hydrate accumulations of “fracture-filling” type occur predominantly in regions where acoustic blanking features in the seismic record suggest gas migration from below the gas hydrate stability zone. Results from the UBGH2 core studies along with the analysis of similar samples from other expeditions, including those executed by the Ocean Drilling Program, the Integrated Ocean Drilling Program, and the First Ulleung Basin Gas Hydrate Drilling Expedition, greatly improved our understanding of lithologic controls on marine gas hydrate occurrences. [Copyright &y& Elsevier]

Detailed analysis of core sediments and acoustic characteristics of Chirp sub-bottom profiles from a submarine ridge in the southern slope of the Ulleung Basin, East Sea, Korea, reveals changes in sedimentary processes and deformation of sedimentary sequence induced by seafloor mound formation since the Last Glacial Maximum (LGM). The core sediments consist generally of four sedimentary facies which represent hemipelagic sedimentation under a poorly-oxygenated bottom-water condition during the LGM (crudely laminated mud), shoreface erosion and seaward transport off the shelf break during the post-glacial transgression (bioturbated sandy mud or muddy sand), hemipelagic sedimentation under generally well-oxygenated bottom-water condition after the sea-level rise (bioturbated mud), and episodic precipitation of authigenic carbonates by methane venting since the LGM (authigenic carbonate-bearing mud), respectively. The formation of authigenic carbonate-bearing mud facies and associated accumulation of near-seafloor gas hydrates might have led to up-warping of the sediment cover since the LGM that resulted in the seafloor mounds protruding along the crest line of the ridge. The effects of up-warping by expansion of the subsurface gas hydrates are firstly manifested in this paper by the abrupt lateral changes in the sedimentary facies distribution and uplifted acoustic reflectors between the mounds and the background seafloor. [ABSTRACT FROM AUTHOR]

The high primary production enhanced by anticyclonic eddies and hourly variation pattern in the productivity during the spring season in the East Sea were first investigated using the first Korean Geostationary Ocean Color Imager (GOCI). Even though the stratification for a seasonal spring bloom is not well developed in the water column in early April in the East Sea, a physical upward water flux movement at the periphery of the anticyclonic eddies could remain the phytoplankton in euphotic zone to sustain high chlorophyll-a concentration conditions in the Ulleung Basin. At this time, nutrients were no major controlling factor for phytoplankton growth since concentrations of major nutrients (nitrate, silicate, and phosphate) were relatively high in the observed eddy sites based on the observation data from the Korean Oceanographic Data Center (KODC). The estimated mixed layer depth (MLD) significantly shallower at the periphery supports for this mechanism. The hourly primary productivity estimation based on a Carbon-based Productivity Model (CbPM) provides a bimodal pattern along the time especially in L1 with an approximately one order magnitude difference between the lowest and highest values of productivities on 5 April, 2011. Potential possibilities for this large discrepancy in the hourly productivity and some thoughts on a short time in situ incubation method were discussed. [ABSTRACT FROM AUTHOR]

Abstract: Palynofloral analysis was performed for the first time on sediment from the Ulleung Basin (East Sea, offshore Korea) to locate the Pliocene–Pleistocene boundary, which is very important in determining the depositional age of a stratigraphic unit that contains methane hydrate. Cores from the drill sites Ulleung Basin Gas Hydrate 1–9 (UBGH1–9) and Ulleung Basin Gas Hydrate 1–10 (UBGH1–10) in the Ulleung Basin produced abundant to common pollen grains and organic-walled dinoflagellate cysts. Age-diagnostic palynomorphs were present in certain intervals: 120–175mbsf at site UBGH1–9 and 170–205mbsf at site UBGH1–10. The biostratigraphically meaningful taxa were the pollen genera Carya, Liquidambar, and Fagus and the dinoflagellate cysts Capillicysta fusca, Filisphaera filifera subsp. pilosa, and Selenopemphix quanta. The latest stratigraphic occurrence of these pollen taxa in northeast Asia is the late Pliocene, and that of the dinoflagellate cysts is regarded as the late Pliocene in all aquatic areas, especially in the Pacific. The last appearance datum (LAD) of the age indicators in the two cores studied suggests that the Pliocene–Pleistocene boundary is at 120mbsf at site UBGH1–9 and 170mbsf at site UBGH1–10. [Copyright &y& Elsevier]

Carbonate concretions associated with recently discovered gas hydrates from the East Sea (Sea of Japan), Korea, were investigated to delineate their origin based on textural, mineralogical and stable isotopic data. Authigenic carbonates were sampled from the 8 m-long piston core recovered from a water depth of 2072 m during a gas hydrate exploration cruise in June 2007. The carbonate concretions occur at three intervals from a depth of 220 to 280 cm from the top of core within hemipelagic muds above massive and vein-filling gas hydrate layers from depth of 329 to 655 cm. Semi-lithified carbonates occur as concretions with knobby, rounded and irregular morphologies. Textural observation of the concretions reveals porous surfaces with a spongy texture and distinctive internal layering. These concretions are mostly composed of aragonite with relatively minor contribution of calcite, with transition from calcite- to aragonite-dominating phase towards the periphery. Highly negative δC values (δC=−43 to − 27‰ PDB) suggest that methane was the main carbon source for their formation. Oxygen isotope values (δO = 1.1 to 3.1‰ PDB) are slightly enriched due to the relatively stable cold temperature of the East Sea bottom water (0-4 °C). Distinctively separate and narrow ranges of stable isotope compositions from each concretion and the enrichment of carbon isotopes with depth indicate that the concretions at three intervals formed independently during different periods, but under similar anoxic diagenetic conditions by anaerobic oxidation of methane. In addition, distinctive geochemical clusters of each concretion with depleting oxygen isotope trend toward the periphery strongly suggest that the concretions in this study were formed separately by pore water interactions with adjacent surrounding pelagic muds. [ABSTRACT FROM AUTHOR]

Abstract: The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by . The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin. [Copyright &y& Elsevier]

Abstract: The presence of gas hydrate in the Ulleung Basin, East Sea (Japan Sea), inferred by various seismic indicators, including the widespread bottom-simulating reflector (BSR), has been confirmed by coring and drilling. We applied the standard AVO technique to the BSRs in turbidite/hemipelagic sediments crosscutting the dipping beds and those in debris-flow deposits to qualitatively assess the gas hydrate and gas concentrations. These BSRs are not likely to be affected by thin-bed tuning which can significantly alter the AVO response of the BSR. The BSRs crosscutting the dipping beds in turbidite/hemipelagic sediments are of low-seismic amplitude and characterized by a small positive gradient, indicating a decrease in Poisson’s ratio in the gas-hydrate stability zone (GHSZ), which, in turn, suggests the presence of gas hydrate. The BSRs in debris-flow deposits are characterized by a negative gradient, indicating decreased Poisson’s ratio below the GHSZ, which is likely due to a few percent or greater gas saturations. The increase in the steepness of the AVO gradient and the magnitude of the intercept of the BSRs in debris-flow deposits with increasing seismic amplitude of the BSRs is probably due to an increase in gas saturations, as predicted by AVO model studies based on rock physics. The reflection strength of the BSRs in debris-flow deposits, therefore, can be a qualitative measure of gas saturations below the GHSZ. [Copyright &y& Elsevier]

Abstract: A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO2, as indicated by C1/C2+, , and analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent. [Copyright &y& Elsevier]

Abstract: Structural mapping and restoration of depth-converted seismic profiles from the southwestern Ulleung Basin in the East Sea (Japan Sea) back-arc basin reveal the structural evolution of the area since the Middle Miocene. The back-arc closure began in the earliest Middle Miocene (ca. 15Ma) and caused compressional deformation that has propagated southwestward from the Dolgorae to the Gorae structures and further to the unnamed inversion structure immediately adjacent to SE Korea. The accelerated convergence of the young (<20Ma) and hot Shikoku Basin lithosphere into SW Japan at the culmination (15Ma) of the regional plate reorganization induced flat subduction along the Nankai Trough. Strong interplate coupling due to the flat subduction probably led to tectonic switching from back-arc opening to back-arc closure in the Middle Miocene. The NE–SW trending Dolgorae structures indicate NW–SE or N–S compression, orthogonal to the direction of convergence along SW Japan, whereas the younger, NW–SE trending Gorae V structure indicates NE–SW or E–W compression. We propose that this apparently abrupt change in stress regime is due to the eastward-movement of the Amur plate that began in the Pliocene. [Copyright &y& Elsevier]

Despite its importance in carbon mineralization pathways, relatively little is known about sulfate reduction along a continental slope and rise. We investigated the temporal and spatial variations and controls of sulfate reduction rates (SRRs) in the continental margin sediment of the Ulleung basin (UB), off the southeast Korean upwelling system in the East Sea. SRRs ranged from 1.22 to 8.07 mmol m-2 d-1 at the slope sites and from 0.69 to 3.18 mmol m-2 d-1 at the basin sites. These values were exceptionally higher than those observed within other marginal seas and were comparable to SRR values at the same depth range in the Peruvian and Chilean upwelling systems. The high sulfate reduction in the UB was attributable to enhanced primary production in the water column associated with coastal upwelling and a high export flux of large organic particles into the basin via the Ulleung warm eddy. The depth integrated SRR was approximately 4 times higher in the highly productive spring (4.91 ± 2.55 mmol m-2 d-1) than during the summer, which exhibited a stratified water column (1.28 ± 0.48 mmol m-2 d-1). In the meantime, despite the high organic carbon content (> 2.5% dry wt.) in the UB, the SRR was consistently suppressed in the Mn oxide-enriched (174 μ mol cm-3) surface sediments of the continental rise. Overall, these results indicate that the production of organic carbon in the water column and its subsequent deposition on the seafloor is the primary source controlling the temporal variability of sulfate reduction, whereas the Mn oxides that were enriched in the basin are responsible for the spatial variability of the SRR in the UB. Carbon oxidation in the UB accounted for approximately 30% of primary production and approximately 60% of the export flux, indicating that the continental slope and rise of the UB is a region of rapid organic carbon turnover and nutrient regeneration. [ABSTRACT FROM AUTHOR]

During the site survey for drill sites of the Ulleung Basin Gas Hydrate Expedition 1, near-seafloor gas hydrates were discovered in core sediments from basin plain (2010–2130 m water depth) and southern slope (820–1160 m). The gas hydrates were exclusively retrieved from the sites of high backscatter intensity anomalies of 13 kHz multi-beam echosounding, implying high seafloor reflectivity. In high-resolution (2–7 kHz) subbottom profiles, the coring sites are also characterized by narrow (250–850 m wide) acoustic blank zones reaching seafloor, where they have surface expressions of low-relief (

Abstract: The late Quaternary tephra layers discovered in piston cores from the Ulleung Basin of the East Sea (Sea of Japan) were studied using X-ray fluorescence spectrometry (XRF), inductively coupled plasma atomic emission (ICP-AES) and inductively coupled plasma mass spectrometer (ICP-MS) in order to identify and differentiate the tephra layers and to characterize their major and trace element compositions. The morphology and geochemistry of glass shards, combined with 14C dating and statistical methods, confirm the Ulleung and Aira-Tanzawa eruptions as the origin of the tephra layers, respectively. The so-called Ulleung-Oki (U-Oki; 10.1 cal. ka) and Ulleung-Yamato (U-Ym; 30.9 cal. ka) tephra layers consist predominantly of pumice-type glass shards. In contrast, the Aira-Tanzawa (AT; 23 cal. ka) tephra layer consists mainly of plane-type and/or bubble-wall glass shards. The U-Oki and U-Ym tephra layers contain K-rich glasses (R =K2O/SiO2 >0.09) ranging from phonolitic to trachytic alkali compositions, whereas the AT tephra layer which contains K-poor glasses (R =ca. 0.04) is rhyolitic. According to trace element content, the relative amounts of the elements Ti, Zr, Y, Nb and P from the tephra layers evidently indicate the different tectonic zones between the Ulleung (back-arc) and Kyushu (island-arc) islands. Especially, the relatively depleted concentrations of FeO, V, Co, and Ni in the U-Oki tephra layer most likely reflect more fractional crystallization of olivine, pyroxene, hornblende, and magnetite than those in the U-Ym tephra layer. [Copyright &y& Elsevier]

In the deepest region of Korea Strait, the surface temperature is highest in August (lowest in March), while the near-bottom temperature is lowest in September (highest in May). Cross-spectral analysis of the monthly temperature data between the two layers shows high coherence at the annual frequency with phase of 154°. Why and how does such a nearly opposite phasing occur between the surface and the near-bottom temperatures there? This study aims at answering these questions using historical and recently observed data.Cold and relatively fresh subsurface water flowing southward along the east coast of Korea and, known as the North Korean Cold Water (NKCW), becomes noticeable in April near the Sokcho coast. The zonal temperature gradient there is largest around June. The width of the NKCW becomes larger from April to August. After October, the NKCW retreats back toward the coast. The southward movement of the NKCW is thus strong over a period of six to seven months and weak in winter, especially in February. The NKCW flows southward relatively quickly along the coast in April to October and arrives at the Ulleung Basin within one to two months. Because of the sill between the Ulleung Basin and Korea Strait, this water cannot continue to flow to south, but piles up for about two to three months before it moves over the sill. The convergence of the subsurface cold water in the Ulleung Basin displaces the isopycnals upward and this water then intrudes over the sill along the isopycnals. This explains why in April or May, when this water appears noticeably at the Sokcho coast, the near-bottom water in Korea Strait is warmest and in August or September when the NKCW, which is piled up enough at the southern end of the Ulleung Basin, intrudes to Korea Strait, the near-bottom temperatures there are at their lowest.The origin of the NKCW seems to be the water of salinity less han 34.1 psu and surface density of 27σθ or higher, which sinks in the northwestern East Sea in January–March. The sinking of the water results from surface cooling in winter and is intensified due to the strong negative windstress curl. The cold and relatively fresh water, formed in the northwestern East Sea, is hypothesized to flow to the Ulleung Basin along three major paths, along the east coast of Korea, through the channel north of Ulleung-do Island, and through the channel between Ulleung-do and Dok-do Islands. [Copyright &y& Elsevier]

The lithologic characteristics, stratigraphic relationships, and areal distribution of late Quaternary tephra layers have been determined based on four piston cores, recovered from the western Ulleung Basin of East/Japan Sea. The results show that, using morphological and major element compositional data, volcanic glass shards dispersed in tephra layers are identical to those of fallout deposits of the Ulleung–Oki (ca. 9.3 ka), Aira–Tanzawa (ca. 22 ka), and Ulleung–Yamato (ca. 33 ka). The lapilli tephra layers (Ulleung–Oki and Ulleung–Yamato) originating from Ulleung Island consist predominantly of pumice-type glass shards associated with minor amounts of alkali-feldspar, biotite, and plagioclase. On the other hand, the ash layers (including the Aira–Tanzawa ash) derived from the Japanese islands are mainly composed of bubble-wall and/or plane-type glasses that contain higher SiO2 and lower Al2O3 than the lapilli tephra layers. The occurrence of tephra layers in the western Ulleung Basin and their stratigraphic correlation extend the distribution of pumice-type glass shards about 50–100 km west of the previously known fallout zone. [Copyright &y& Elsevier]

The area near the subpolar front of the East Sea has high primary productivity during the spring season. We conducted two surveys, one in early spring and another in late spring, to assess environmental factors that influence phytoplankton community structure during these times. During early spring, vertical mixing supplied abundant nutrients to the surface. Due to the well-mixed water column, there were high nutrient levels, but total phytoplankton abundances and diversity were relatively low and were dominated by the diatom Chaetoceros spp. During late spring, the water column gradually stratified, with relatively high levels of nutrients in the surface layers near the coastal areas. Phytoplankton abundance and diversity at that time were higher, and there were diatoms (Pseudo-nitzschia spp. and Chaetoceros spp.), cryptophytes, and small flagellates. Pseudo-nitzschia spp. were especially abundant in re-sampled areas. The presence of a stratified and stable water mass and sufficient nitrate led to high phytoplankton growth, even in the open sea during late spring. These findings provide a better understanding of how phytoplankton population dynamics in the East Sea depend on water column stability during both early and late spring seasons. [ABSTRACT FROM AUTHOR]

Single and multichannel reflection profiles from the Ulleung Basin and the eastern Korean continental margin reveal that the acoustic basement is cut by two major N-S trending fault systems: (1) the Hupo and Yangsan faults in the continental shelf and the southeastern Korean Peninsula and (2) the Ulleung fault along the western margin of the Ulleung Basin. The right-stepping Hupo-Yangsan fault system experienced coeval dextral strike-slip deformation in the Early Miocene with a lateral displacement of about 35 km. This fault movement resulted in pull-apart opening of the Pohang-Youngduk Basin. The Ulleung Fault is expressed as a steep (> 30°) and relatively smooth basement escarpment along the present base of slope. In the southwestern margin of the Ulleung Basin, the fault merges with the Tsushima Fault. This fault is interpreted as part of the western boundary fault system resulting from an extensile shear deformation during the back-are opening of the East Sea (Sea of Japan) during Early Miocen time. The fault kinematics of these fault systems supports the hypothesis that the East Sea formed as a result of pull-apart opening accompanied by a southward drift of the SW Japanese Arc. During the major opening of the Ulleung Basin, the SW Japanese Arc apparently migrated 250–300 km southward, detached from the east Asian continent. This latter block movement induced major dextral shear deformation along the Ulleung Fault which, in turn, triggered secondary shear deformation in the juxtaposed Hupo-Yangsan fault system within the rigid continental crust. [ABSTRACT FROM AUTHOR]

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (