Central Japan Sea Research Articles

Оценка сдвигов частоты инерционных колебаний в центральной части Японского моря по данным поверхностных буёв

Оценка сдвигов частоты инерционных колебаний в центральной части Японского моря по данным поверхностных буёв

Revealing the distribution of active submarine faults off the coast of Oga Peninsula using high-resolution stereoscopic topographic images

This study aims to elucidate the importance of a combination of tectonic geomorphological interpretations using high-resolution stereoscopic topographic images and seismic profile interpretations as methodological standards for organizing maps of the distribution of active submarine faults. Furthermore, we provide topographic evidence of active submarine reverse faults off the coast of the Oga Peninsula, which lies on the eastern margin of the Japan Sea. First, the study reprocessed the point cloud data of multi-beam bathymetry data and produced a new digital bathymetry model and anaglyph images. These images and contour maps enabled the identification of broad deformation and other small-scale tectonic landforms, such as fault scarp, fault scarp on the flat surface of a basin, and flexure scarp. We first described detailed tectonic landforms along the NE–SW trending northwest-dipping faults across the Mogami Trough, which was identified by previous geological studies, to demonstrate the effectiveness of the proposed method. Moreover, we applied the method as an applicable example for tectonic landforms in the east-dipping fault at the offshore of the Oga Peninsula with seismic profile interpretation. The result demonstrated that an active fault across the central part of the Mogami Trough can be recognized on the bases of geology and topography. However, identifying an active fault that extends to the area with poor sediment deposition at the northern end of the trough is difficult. The study found that detecting active fault distribution by tectonic geomorphological interpretation is practical and effective using a stereoscopic image from high-resolution topographical data. Moreover, a new east-dipping reverse fault, that is, the Kyurokujima-nansei-oki fault, was discovered along the eastern margin of the northwestern part of the Mogami Trough by identifying flexure scarp and comparison with seismic profiles. The strike of the Kyurokujima-nansei-oki fault is parallel to that of the aftershock distribution of the southwest part of the 1983 Central Japan Sea Earthquake and resembles an east-dipping structure. Thus, this fault should be considered one of the source faults during assessments of seismic and tsunami hazards.

Variability of wave height in the Sea of Japan

The Sea of Japan is a semi-enclosed sea located in the northwest Pacific area. There has been no study conducted to examine the relationship between the interannual variability of the wind field and the spatial variability of wave height in the Sea of Japan. We examined the relationship between the spatiotemporal variability of the wave height and the wind field in the Japan Sea using monthly mean ERA5 reanalysis data. Firstly, it is demonstrated that the ERA5 wave height closely matches that of moored buoys. Following that, we investigated the relationship between the wave height and the wind anomaly distribution using empirical orthogonal function (EOF) and self-organizing map (SOM) analysis. This is the first study to examine the spatial variation of wave height using the SOM. The EOF’s first mode is a mode that oscillates in the central region of the Sea of Japan, while the second mode is a north-south dipole mode. In each season, the wave height anomaly EOF mode had a significant correlation with a variety of climatic indices. Although the correlation between EOF of wave height anomaly and EOF of wind speed anomaly was not high throughout the year, the correlation between them was high for each season. The SOM revealed a pattern consistent with the EOF analysis’s first and second modes. Throughout the year, the relationship between wave height and wind speed anomaly patterns is not strong. The link, however, varies with the seasons. The frequency of patterns with increased wave height anomalies in the central Japan Sea has increased in recent years. This is consistent with the EOF first mode’s long-term trend.

Drift of Surface Lagrangian Buoys in the Central Sea of Japan in October–November 2011

Drift of Surface Lagrangian Buoys in the Central Sea of Japan in October–November 2011

Paleoceanographic insights on meridional ventilation variations in the Japan Sea since the Last Glacial Maximum: A radiolarian assemblage perspective

The Japan Sea is ideal for investigating deep water formation due to its unique topography and hydrography. However, because of the scarcity of reliable indicator and high-resolution ventilation records, the driving mechanisms behind ventilation changes in the entire Japan Sea since the Last Glacial Maximum (LGM) remain elusive. In this study, we analyze the radiolarian assemblage in three sediment cores from the southwestern, central, and northwestern Japan Sea to reveal ventilation variations over the last 25 ka. Our results suggest that weak ventilation was a widespread phenomenon in the Japan Sea during the LGM. This is inferred by the low relative abundances of species belonging into the Japan Sea Proper Water (JSPW) Assemblage, which are indicative of deep water ventilation in the Japan Sea. The JSPW Assemblage have gradually become the predominant species in studied cores from the last deglaciation to the early Holocene, suggesting an abrupt transition from oxygen poorer to oxygen richer conditions. All investigated regions show re-ventilation during the deglaciation, albeit at different times. The JSPW ventilation in the central Japan Sea started during the 19 ka Melt-water Pulse (MWP) and remained in an interglacial mode after MWP-1A (~14.7 ka), presumably due to the opening of the Tsushima Strait and intrusion of the Tsushima Warm Current (TWC). An abrupt increase in productive shallow water export during the Bølling-Allerød (B/A) was likely the key factor causing poorly oxygenated deep-water conditions in the southwestern Japan Sea. In contrast, the weakening ventilation in the northwestern Japan Sea during the B/A and Younger Dryas was probably caused by the blocking effect of the sea ice.

Evolution of the Upper Ocean Stratification in the Japan Sea Since the Last Glacial

AbstractPaleoceanographic evidence commonly indicates that Last Glacial Maximum surface temperatures in the Japan Sea were comparable to modern conditions, in striking difference to colder neighboring regions. Here, based on a core from the central Japan Sea, our results show similar UK′37‐ and TEXL86‐derived temperatures between 24.7 and 16.3 ka BP, followed by an abrupt divergence at ~16.3 ka BP and a weakening of divergence after ~8.7 ka BP. We attribute this process to a highly stratified glacial upper ocean controlled by the East Asian Summer Monsoon, increasing thermal gradient between surface and subsurface layers during the deglaciation and the intrusion of Tsushima Warm Current since the mid‐Holocene, respectively. Therefore, we suggest that threshold‐like changes in upper‐ocean temperatures linked to sea level rise and monsoon dynamics, rather than just sea surface temperatures, play a critical role in shaping the thermal and ventilation history of this NW Pacific marginal sea.

Behavior of Water Mass Beneath the Tsushima Warm Current in the Japan Sea

To better understand the behavior of water mass beneath the Tsushima Warm Current (TWC), we use the vertical cross-sections of potential temperature, salinity and dissolved oxygen in the Japan Sea obtained by the T/V Oshoro Maru of the Hokkaido University during 8–29 June in 2011 to analyze its origins and variations. The results show that the potential temperature and salinity beneath the TWC varies little, but the dissolved oxygen varies largely with the geographical location. There are two deep water masses with different dissolved oxygen content below the TWC. One is on the coastal side with the low dissolved oxygen, and the other is on the offshore side with the high dissolved oxygen. It is inferred that the former one is relatively old water and the latter is the new water. By using the phosphate (PO4) and the apparent oxygen utilization (AOU) relationship, we calculate the PO40 (preformed PO4) as a water mass tracer. These results suggest that the water masses beneath the TWC with high and low dissolved oxygen originate from the same surface water mass in the central Japan Sea.

Drastic hydrographic changes inferred from radiolarian assemblages in the central Japan Sea since the Last Glacial Maximum

Assessing modern radiolarian distribution in the marginal seas is essential for understanding its relationship to hydrographic and environmental variables, upon which subsequent interpretation of past radiolarian records can be made reliably. Here, we combine published radiolarian assemblages in 69 surface sediments from the Japan Sea with those obtained from 15 new surface sediments in this study to determine the modern distribution of radiolarians. The Tsushima Warm Current (TWC) taxa are only found in the south of the Subpolar Front. The results of redundancy analysis confirm that the temperature at 10 m depth (temperature10m) is the main factor controlling the spatial variability of TWC taxa. The water depth plays a key role in affecting the abundances of Japan Sea Proper Water (JSPW) Assemblage including Larcopyle buetschlii Dreyer (adult), Cycladophora davisiana Ehrenberg, and Actinomma group. The JSPW Assemblage can be used as a reliable indicator of ventilation changes in the Japan Sea. Based on the newly developed TWC taxa and JSPW Assemblage, we further reconstructed the water column structure in the central Japan Sea over the past 25 ka. There was weak ventilation in the central Japan Sea even during the Last Glacial Maximum (LGM), although a distinct decrease in the TWC inflow resulted in surface-water freshening and water column stratification. After the LGM, the TWC taxa together with JSPW Assemblage resumed in the central Japan Sea at the 19 ka meltwater pulse (MWP). The MWP-1A was clearly marked by an enhanced intrusion of the TWC. Synchronously, JSPW was ventilated deeply down after ~14.6 ka attaining to the similar level of the Holocene as a result of changing sea surface conditions. Along with the continuing sea-level rise, the matured TWC entered the Japan Sea at 7 ka and since then the modern hydrographic condition has been established. We suggest that the changes in the sea level and TWC strength predominantly determined the surface hydrography and deep ventilation in the central Japan Sea since 25 ka, and the TWC might have notably affected the Japan Sea since the Bølling-Allerød.

Near-inertial internal waves and multiple-inertial oscillations trapped by negative vorticity anomaly in the central Sea of Japan

This study aims to reveal the interaction between wind-induced internal waves and mesoscale features in a frontal region of the central Sea of Japan. A mooring system with an upward-looking acoustic Doppler current profiler was deployed over a meandering pathway of the Tsushima Warm Current. The annual record of horizontal current showed an extraordinary event of near-inertial internal waves (NIWs) that were trapped in a mesoscale warm-core eddy, characterized by negative vorticity anomaly. Near the critical depth inside the eddy, where background vorticity vanishes, the amplitude of downward-traveling NIW reached 50–70 cm s−1 in root-mean-square current magnitude. The amplified NIWs were concurrent with the emergence of superinertial oscillations, peaking at multiple inertial (MI) frequencies, i.e., double, triple, and quadruple inertial. The MI oscillations are regarded as arising through nonlinear resonance of NIW. Examining local surface winds suggest that a fast-moving cyclone passing over the site initially generated the NIW event. The trapping and amplification processes of wind-generated NIW are examined by a series of ray-tracing simulations, where background vorticity is reconstructed from in situ hydrographic observations. Based on the results above, a specific picture model of kinetic energy circulation around the Sea of Japan can be presented: the atmospheric event provides momentum energy into the ocean surface, resulting in the generation of NIWs and trapping within anticyclonic vortices. A part of the near-inertial kinetic energy is then transferred into the MI oscillations if NIW is vigorously amplified near the critical depth; then it likely dissipates into turbulence.

Observations of oceanic fronts and water-mass properties in the central Japan Sea: Repeated surveys from an underwater glider

Abstract Repeated glider surveys were conducted in the Japan Sea during April–June 2016. Observations revealed mesoscale hydrographic structures of the subpolar front, the Tsushima Warm Current, and eddies formed around flows in the central Japan Sea off Sado Island, in addition to fine-scale water properties associated with mesoscale variation. Data collected from satellite altimeters and thermometers, profiling floats, and ship conductivity–temperature–depth profiles were also analyzed to elucidate spatiotemporal characteristics in multiscale phenomena: mesoscale to fine scale. We present new quantitative estimates of the geostrophic volume transports of the Tsushima Warm Current and the subpolar front current in the central Japan Sea, based on hydrographic data from the glider's transects: they are, respectively, 2.5–2.7 Sv and 1.1–1.4 Sv. The mesoscale frontal/eddy structures varied greatly on a timescale of less than one month. We classified water masses, which revealed that the upper-layer, the low-salinity water within an anticyclonic eddy, was characterized in terms of salinity 100 m in this region, for which there are few hydrographic measurement data.

High-resolution Quaternary record of marine organic carbon content in the hemipelagic sediments of the Japan Sea from bromine counts measured by XRF core scanner

The marine organic carbon content in sediments is a useful tool for reconstructing past productivity in the ocean. Bromine has been proposed as a useful proxy for marine organic carbon, since bromine is more concentrated in marine organic matter compared to terrestrial organic matter.Here, we present a high-resolution Quaternary record of marine organic carbon in the hemipelagic sediments of the Japan Sea obtained during Integrated Ocean Drilling Program (IODP) Expedition 346. We measured the bromine content in the sediments using an XRF core scanner (ITRAX). The total organic carbon content, total nitrogen, and stable carbon isotope values were also measured for discrete samples from the same sediments. The total organic carbon/total nitrogen ratio and the stable carbon isotope ratio of the total organic carbon are used to estimate the marine organic carbon content within the total organic carbon.The Br counts and marine organic carbon content show a high correlation, which we used to construct a calibration equation of the marine organic carbon content from the Br counts. Using the calibration equation, we reconstructed the changes in the marine organic carbon content during the Quaternary with a time resolution of ~ 50 years using sediment cores from IODP Site U1424 in the east central Japan Sea. The method to estimate the marine organic carbon content from the Br counts measured by the XRF core scanner proposed in this paper will be a useful tool to reconstruct the organic carbon content with high resolution and high speed.

Horizontal distribution of copepod nauplius community structure around the Noto Peninsula, central Sea of Japan during May 2005

Horizontal distribution of copepod nauplius community structure around the Noto Peninsula, central Sea of Japan during May 2005

Phylogeography of the sandy beach amphipod Haustorioides japonicus along the Sea of Japan: Paleogeographical signatures of cryptic regional divergences

Recent findings of genetic breaks within apparently continuous marine populations challenge the traditional vicariance paradigm in population genetics. Such “invisible” boundaries are sometimes associated with potential geographic barriers that have forced divergence of an ancestral population, habitat discontinuities, biogeographic disjunctions due to environmental gradients, or a combination of these factors. To explore the factors that influence the genetic population structure of apparently continuous populations along the Sea of Japan, the sandy beach amphipod Haustorioides japonicus was examined. We sampled a total of 300 individuals of H. japonicus from the coast of Japan, and obtained partial sequences of the mitochondrial COI gene. The sequences from 19 local populations were clustered into five groups (Northwestern Pacific, Northern, Central, Southern Sea of Japan, and East China Sea) based on a spatial genetic mixture analysis and a minimum-spanning network. AMOVA and pairwise Fst tests further supported the significant divergence of the five groups. Phylogenetic analysis revealed the relationship among the haplotypes of H. japonicus and outgroups, which inferred the northward range expansion of the species. A relaxed molecular-clock Bayesian analysis inferred the early-to middle-Pleistocene divergence of the populations. Among the five clusters, the Central Sea of Japan showed the highest values for genetic diversity indices indicating the existence of a relatively stable and large population there. The hypothesis is also supported by Bayesian Skyline Plots that showed sudden population expansion for all the clusters except for Central Sea of Japan. The present study shows genetic boundaries between the Sea of Japan and the neighboring seas, probably due to geographic isolation during the Pleistocene glacial periods. We further found divergence between the populations along the apparently continuous coast of the Sea of Japan. Historical changes in the geographic range of H. japonicus in relation to sandy beach habitat availability, account for the genetic breaks among the three populations in the Sea of Japan. The present results infer that the past geographic events influenced the population formation of H. japonicus.

Seasonality and occurrence of the dominant mysid Neomysis awatschensis (Brandt, 1851) in the Yura River estuary, central Sea of Japan

Mysids play an essential role as the dominant prey species for coastal fishes. In Japan, the mysid Neomysis awatschensis (Brandt, 1851) is commonly distributed in estuarine and shallow coastal waters with high productivity due to supply of nutrients from the watershed. The present study investigates the distribution and life history of the mysid N. awatschensis in relation to biotic and abiotic factors by monthly sampling from March 2014 to July 2015 at stations along the Yura River estuary, the central Sea of Japan.The Yura River physical conditions change seasonally and are affected by its discharge. Salt-wedge-intrusions were frequently observed and found up to 18 km from the river mouth from April to December 2014 when river discharge was low, whereas in 2015 intrusions were less pronounced, possibly due to river mouth clogging. Clear seasonality was observed in mysid density, showing highest densities (up to 202 inds/m2) in spring and early summer in contrast with lowest densities (<16 inds/m2) from late summer to winter. Neomysis awatschensis appeared to change its reproduction characteristics in response to changes in the environment, large-sized females occurred in spring to produce large numbers of eggs and larvae during the high primary production period. In late summer, the population declined due to increasing mortality of juveniles at high temperatures. In autumn and winter, high river discharge and low temperature suppressed reproduction and survival in the mysid population. It is notable that N. awatschensis occurred within a salinity range of 0–32 and no clear influence of salinity on the mysid density was found.Those results suggest that reproduction is partially suppressed by variable and oligotrophic conditions in the Yura River estuary. Combined with the flexibility to seasonal changes in temperature and food availability, the great tolerance to acute salinity fluctuations allows N. awatschensis to maintain its population in the estuary.

Evolution of westerly jet during the last 60 ka: Evidence from core deposits in the central Japan (East) Sea

Aeolian dust has a significant environmental impact on the ocean biogeochemical cycles in different timescales. However, spatial and temporal variations of dust transported on long distances in the upper troposphere by the westerly jet, in particular over orbital-millennial timescales, have not been resolved yet. In this study, we present the evidence for changes in the main axis and path of westerly jet over East Asia at millennial-orbital scale during the last glacial period (~60 ka). We examine the grain size characteristics of sediment core LV53-23 collected from the central Japan (East) Sea, which lies the downwind of westerly jet to discuss the evolution of westerly jet since the last glacial period. Our results show that core sediments are composed mainly of silt characterized of eolian dust, which is consistent with those sediment cores discussed in previous studies from the Japan Sea. The median grain-size of silt fraction has similar pattern with environmentally sensitive grain size extracted by standard deviation/grain-size method. The median grain size in silt fraction increased in stadial periods and decreased in interstadial periods, which might be related to changes in the path of westerly jet. During the stadial, the westerly jet mostly located in the the south of the Tibetan Plateau. It can influence the northern part of the Tibetan Plateau extending to 50°N with high speed, which is prerequisite of dust raised to westerlies in northern China and thus carried the coarser dust from nearby Mongolian Gobi and northeastern China to the Japan Sea. During the interstadial, the provenance of the finer aeolian dust was from Taklimakan Desert when westerly jet migrated northward and concentrated within a narrow band at northern Tibetan Plateau with a longer duration. We found the multi-millennial time scales variations during DOI 8, 12, 14, 16 in cores LV53-23 and MD01-2407 located in the southwestern Japan Sea based on the results of NGRIP and GISP 2 Greenland ice cores. Contrasting with grain size variations of the loess-paleosol records (e.g., Gulang Loess sequence), the lowest grain size during the Last Glacial Maximum confirm that the silt fraction of Japan Sea sediment was mostly transported by westerlies instead of East Asian Winter Monsoon. The mechanism driving the orbital scale variability in grain size of aeolian dust in the Japan Sea is mainly forced by variations of ice volume due to summer insolation in the Northern Hemisphere. A southward migration in the main axis of westerly jet and its influential area was affected by a decrease of summer insolation and increase of ice volume in Northern Hemisphere. During the Last Glacial Maximum, although the cold surges and dust storms were intensive, the southward path of westerly jet may decreased the dust entrainment ability transported by westerlies so that downwind received finer grain size eolian dust. This argument is consistent with findings showing that the location of subarctic front in North Pacific during the Last Glacial Maximum was caused by a southward shift of the path of Kuroshio Current which is controlled by westerly jet. During the glacial period, the decreasing grain size of dust in northern site is explained by weaker westerlies in higher latitudes (40°–50°N) caused by the southward shift of westerly jet main axis, while the increase grain size in the southern site is controlled by the increasing of dust transport path because of westerlies speed in lower latitudes.

Records of Mytilus trossulus (Bivalvia: Mytilidae) and Its Possible Distribution along the Northern and Central Japan Sea Coast of Honshu Island

Records of <i>Mytilus trossulus</i> (Bivalvia: Mytilidae) and Its Possible Distribution along the Northern and Central Japan Sea Coast of Honshu Island

Migration area of the Tsushima Warm Current Branches within the Sea of Japan: Implications from transport of 228Ra

We investigated lateral profiles of 228Ra (half-life; 5.75 years) activity and 228Ra/226Ra (1600 years) activity ratio using 241 surface water samples collected in/around the Sea of Japan and the East China Sea (ECS) during June–October of 2009–2014. In the ECS, the 228Ra/226Ra ratio in the surface waters exhibited markedly wide variation (<0.05–3.5) in June, predominantly reflecting the mixing between the 228Ra-rich continental shelf water and the 228Ra-depleted Kuroshio Current water. In July, the surface waters of the central Sea of Japan (135–138°E) became separated into three currents: the Offshore Branch of the Tsushima Warm Current (OBTWC) (228Ra/226Ra =0.7–1.2) at 39–41°N, the Coastal Branch of the TWC (CBTWC) (~0.7) on the southern side, and sub-Arctic Current (~0.7) on the northern side. From the central to northeastern Sea of Japan, the 228Ra/226Ra ratio at the surface (0.8–1.0) was within a range between that of the CBTWC and OBTWC. The fraction of continental shelf water in the CBTWC, OBTWC, and in their combined current was estimated to be 11–16%, ~8%, and 10–11%, respectively.

The results of studies of the red snow crab Chionoecetes japonicus from the bathyal zone of the central Sea of Japan

This study of the deep-sea red snow crab Chionoecetes japonicus is based on materials that were collected from its poorly surveyed or earlier unknown habitats in the central Sea of Japan, including the KitaYamato Bank and the top of the Peter the Great Seamount, during a research cruise aboard the R/V Vladimir Safonov in April and May of 2012. The results are compared to the data of the previous studies aboard the fishing vessels Seisho-maru 68 (September and October 1992), Viera (May and June 2005), and the Arctic Orion (April and May 2007). The red snow crab stock on the Kita-Yamato Bank has declined by one-third within 2 decades. Both males and females formed the densest aggregations at depths of 800–1040 m. The total biomass of legal-sized males in 2012 is estimated at 7400 t. The first data on red snow crab that inhabit the top of Peter the Great Seamount show that its total biomass over the area of 1.5 km2 is 14 t. The mean carapace size of crabs from the Peter the Great Seamount is smaller (males, 83.0 mm and females, 58.6 mm) than that of individuals from the Kita-Yamato Bank (males, 99.9 mm and females, 65.0 mm).

Impact of autumn SST in the Japan Sea on winter rainfall and air temperature in Northeast China

We studied the impact of sea surface temperature anomaly (SSTA) in the Japan Sea and the sea area east of Japan on the winter rainfall and air temperature in Northeast (NE) China using the singular value decomposition (SVD) and empirical orthogonal function (EOF). The monthly-mean rainfall data observed at 160 stations in China, monthly-mean sea surface temperature (SST) of the Hadley Center for Climate Prediction and Research and monthly-mean air temperature from the NCEP reanalysis during 1960–2011 were used. Correlation analysis indicates that the SSTAs in the Japan Sea in September may last for three or four months and are an important index for forecasting the winter rainfall and air temperature in NE China. Positive SSTAs in the central Japan Sea and in the sea area east of Tokyo correspond to positive rainfall anomaly and negative air temperature anomaly in NE China. With the rise of SST in the Japan Sea, a weak cyclone appears over the Japan Sea. The northeasterly wind transports water vapor from the Okhotsk to NE China, resulting in more rainfall and lower air temperature. Negative SSTA years are accompanied by warmer air temperature and less snow in NE China. The 1000 hPa geopotential height anomaly and wind anomaly fields are simulated by IAP-9L model, which supports the analysis results.

Fine structure of dark layers in the central Japan Sea and their relationship with the abrupt climate and sea level changes over the last 75 ka inferred from lithophysical, geochemical and pollen results

High resolution of the color lightness, productivity proxies (TOC, chlorin, CaCO3, Ba-bio, Mo and U), and lithological description defined alternation of dark layers (DLs) and light layers in sediment core LV53-23 recovered from the central Japan Sea (west Yamato Rise) over the last 75ka. Age model was constructed by AMS 14C data, tephrochronology, δ18O of planktonic foraminifera record, and the correlation of DLs with lightness of Japan Sea core MD01-2407 and with millennial interstadials recorded in the Greenland ice cores and East Asia stalagmites. Regional interstadials were also independently established and dated by vegetation amelioration of the surrounded land, inferred from the pollen results. Correlation of the DLs with millennial scale climate and sea level changes indicate that both interstadial warming and sea level fluctuations influence on its formation providing their individual features. During MIS 3, with sea level above −90m, DL was formed mostly consistent with the Tada et al. (1999)’s hypotheses. During MIS 2 and 4, when sea level was nearly and below −90m, sea level descending led to water column stratification, oxygen depletion in bottom water and facilitates the DLs formation. High-resolution lightness and productivity proxies defined the subtle structure of pronounced interstadials 14 and 12.