Gas hydrates in the Ulleung Basin, East Sea of Korea

To develop gas hydrates as a potential energy source, geophysical and geological studies were carried out in the Ulleung Basin, East Sea. Bottom simulating reflectors (BSRs) were initially used indicator for the potential presence of gas hydrates across the basin. Based on these early results, 12367 line-km of multichannel seismic data, 38 piston cores, and multibeam data were collected. The cores showed high amounts of total organic carbon and high residual hydrocarbon gas levels. Gas composition and isotope ratios define it as of primarily biogenic origin. In addition to the BSRs, numerous chimney-structures were found in seismic data. These features indicate a high potential of the Ulleung Basin to host significant amounts of gas hydrate. Dedicated geophysical surveys, geological and experimental studies were carried out culminating in two deep drilling expeditions, completed in 2007 and 2010. Sediment coring (including pressure coring), and a comprehensive well log program complements the regional studies and were used for a resource assessment. Two targets for a future test-production are currently proposed: pore-filling gas hydrate in sand-dominated sediments and massive occurrences of gas hydrate within chimney-like structures. An environmental impact study has been launched evaluating any potential risks to production

Long-term annual primary production in the Ulleung Basin as a biological hot spot in the East/Japan Sea

Although the Ulleung Basin is an important biological hot spot in East/Japan Sea (hereafter the East Sea), very limited knowledge for seasonal and annual variations in the primary productivity exists. In this study, a recent decadal trend of primary production in the Ulleung Basin was analyzed based on MODIS-derived monthly primary production for a better annual production budget. Based on the MODIS-derived primary production, the mean daily primary productivity was 766.8 mg C m-2 d-1 (SD=+/- 196.7 mg C m-2 d-1) and the annual primary productivity was 280.2 g C m-2 yr-1 (SD=+/- 14.9 g C m-2 yr-1) in the Ulleung Basin during the study period. The monthly contributions of primary production were not largely variable among different months, and a relatively small interannual production variability was also observed in the Ulleung Basin, which indicates that the Ulleung Basin is a sustaining biologically productive region called as hot spot in the East Sea. However, a significant recent decline in the annual primary production was observed in the Ulleung Basin after 2006. Although no strong possibilities were found in this study, the current warming sea surface temperature and a negative phase PDO index were suggested for the recent declining primary production. For a better understanding of subsequent effects on marine ecosystems, more intensive interdisciplinary field studies will be required in the Ulleung Basin

Methane hydrate formation in Ulleung basin under conditions of variable salinity: reduced model and experiments

In this paper, we present a reduced model of methane hydrate formation in variable salinity conditions, with details on the equilibrium phase behavior adapted to a case study from Ulleung Basin. The model simplifies the comprehensive model considered by Liu and Flemings using common assumptions on hydrostatic pressure, geothermal gradient, and phase incompressibility, as well as a simplified phase equilibria model. The two-phase threecomponent model is very robust and efficient as well as amenable to various numerical analyses, yet is capable of simulating realistic cases. We compare various thermodynamic models for equilibria as well as attempt a quantitative explanation for anomalous spikes of salinity observed in Ulleung Basin

Sedimentary and structural evolution of the Eastern South Korea Plateau (ESKP), East Sea (Japan Sea)

The East Sea (Japan Sea) is a semi-enclosed back-arc basin that is thought to preserve a significant record of tectonic evolution and paleo-climatic changes of Eastern Asia during the Neogene. We use here 2-D regional multi-channel seismic reflection profiles and borehole data from Expedition 346 of the Integrated Ocean Drilling Program (IODP) to provide new constraints on the geological history of the Eastern South Korea Plateau (ESKP). The ESKP represents a structurally-complex basement high in the southwestern East Sea which formed during rifting of the back-arc basin. Our new observations show that the ESKP is composed of numerous horsts and grabens controlled by NE-trending normal faults. The acoustic basement is blanketed by Oligocene to recent sediments that have preferentially accumulated in topographic lows (up to 1.5 km thick) and have been cored during Expedition 346 at Site U1430 close to the southern margin of the ESKP. Seismic profiles in the ESKP reveal three units separated by regional unconformities. These seismic units closely correspond to IODP lithostratigraphic units defined at Site U1430, where biostratigraphic data can be used to constrain the timing of three main evolutionary stages of the ESKP. Stage 1 was related to rifting in the late Oligocene and middle Miocene, terminated by a regional uplift leading to an erosional phase in the middle Miocene. Stage 2 was associated with subsidence in the middle and late Miocene and uplift and accompanying erosion or non-deposition in the latest late Miocene. Stage 3 (Pliocene to present) recorded overall uniform hemipelagic-pelagic subsidence of the ESKP with short-lived tectonically-induced uplifts in the late middle Miocene and latest Miocene-early Pliocene. The three stages of evolution of the ESKP closely correlate to sedimentary changes since the Oligocene and suggest a direct control of regional/local tectonics on sedimentation patterns in the southwestern East Sea, with secondary influence of regional climatic and paleo-oceanographic processes

Rapid Variability in the Japan/East Sea: Basin Oscillations, Internal Tides, and Near-Inertial Oscillations

Many processes contribute to the variations of currents, sea surface height (SSH), and thermocline depth in marginal seas. Energetic examples range broadly over time scales from slow mesoscale and interannual variations to rapid basin oscillations, internal tides, and near-inertial oscillations. Our measurement array in the Japan/East Sea (JES) offered a special opportunity to study these processes simultaneously, revealing important interconnections among them

동해 울릉해저간극 심층 해수 특성과 수송량의 십년 규모 변화

학위논문(박사) -- 서울대학교대학원 : 자연과학대학 지구환경과학부, 2023. 8. 남성현.Long-term changes in the physical characteristics of the deep waters and the volume transports of the deep waters through the Ulleung Interplain Gap (UIG), which is a unique passageway for exchanges in deep waters (>1500 m) between the Japan Basin to Ulleung Basin, are crucial for understanding long-term changes in the East Sea meridional overturning circulation. In particular, the western part of the UIG is very suitable for examining the change in the deep water transports from the Japan Basin to the Ulleung Basin considering the path of the deep and abyssal circulation due to the bottom topographic effect. The boundary depth and physical characteristics of the deep water masses in the Ulleung Interplain Gap (UIG), the Ulleung Basin, and the Japan Basin, as well as the meridional volume transports of the deep waters from 1500 m to 2300 m, were all examined in this study. The data used in this study are surface moored current meter data collected at the UIG from 1997 to 2019 and hydrographic data collected from the Japan Basin, Ulleung Basin, and the UIG from 1993 to 2019. The volume transports of the deep waters from 1500 m to 2300 m through the western UIG from 1997 to 2019 were obtained by correlating the along-channel current component at 1800 m at EC1 from 1997 to 2019 with the volume transports of the deep waters from 1500 m to 2300 m through the western UIG from November 2002 to April 2004 obtained from five subsurface moored current meter data. The equatorward volume transports of the deep waters from 1500 m to 2300 m through the western UIG continued to increase from the late 1990s to the 2010s. In addition, it was revealed that the deep water transported from 1500 m to 2300 m through the western UIG was composed of the Deep Water until 2000s, but the portion of the Central Water was increasing in the 2010s based on the changes in deep salinity minimum (boundary between CW and DW) and upper boundary homogeneous layer (boundary between DW and BW). Considering that the portion of the CW increases not only in UIG but also in the Japan Basin, it is believed to related to the increase in CW formation and the increase in equatorward volume transports of the deep waters from 1500 m to 2300 m through the western UIG. However, it is necessary to compare the relationship with the volume transports of the deep waters from 1500 m to 2300 m through the western UIG through additional studies such as quantitative CW formation estimation. Deep water warming has recently intensified, as evidenced by the potential temperature of the deep waters in the Japan Basin, Ulleung Basin, and the UIG being twice as high from the 2000s to the 2010s as it was from the 1990s to the 2000s. On the other hand, it was established that the potential density of the deep waters increased in the 2000s due to the salinity effect and reduced once more in the 2010s, as well as that the salinity of the deep water increased from the 1990s to the 2000s and decreased again. Not only from the Japan Basin to the Ulleung Basin, but also from the Ulleung Basin to the Yamato Basin and from the Yamato Basin to the Japan Basin, the deep and abyssal circulation can be strengthened from 1500 m to 2300 m form the 1990s to the 2010s, which suggests that the amount of the Deep Water in the depth range may have decreased and the amount of the Central Water may have increased. The fact that CW formation is strengthening independently of BW development further indicates that the future East Sea meridional overturning circulation may evolve into a double circulation or a more complicated circulation than a single circulation.동해 자오면 순환 경로의 주요 길목 중 하나인 독도와 울릉도 사이의 심층 수송 통로인 울릉해저간극에서의 남북 방향 심층 수송량과 심층 해수의 물리적 특성 변화는 동해 전체 자오면 순환의 변화를 이해하는데 있어 매우 중요하다. 특히, 울릉해저간극의 서쪽 해역은 해저지형효과에 의한 심층 순환 경로를 고려했을 때 일분 분지로부터 울릉 분지로 유입하는 심층 수송량의 변화를 살펴보기에 매우 적합한 해역이다. 이 연구에서는 울릉해저간극 서쪽 해역의 1500m부터 2300m 수심 범위에서 심층 해수의 남북 방향 수송량과 일본 분지, 울릉 분지, 울릉해저간극에서 동해 심층 해수의 경계 수심과 물리적 특성의 10년 규모의 변화를 연구하였다. 이 연구에서는 1997년부터 2019년까지 울릉해저간극에서 수집된 아표층 계류 유속 자료와 1993년부터 2019년까지 일본 분지, 울릉 분지, 울릉해저간극에서 수집된 물리적 특성을 관측한 자료를 사용하였다. 1997년부터 2019년까지의 울릉해저간극 서쪽 해역의 수송량은 2002년 11월부터 2004년 4월까지 울릉해저간극에서 5개의 아표층 계류 유속 자료로 구한 울릉해저간극 서쪽 해역의 심층 수송량과 1997년부터 2019년까지 울릉해저간극의 한 가운데 위치한 EC1 정점에서의 수심 1800m 유속과의 상관관계를 통해 계산하였다. 울릉해저간극 서쪽 해역을 통해 일본 분지에서 울릉 분지로 수송되는 심층 해수 수송량은 1990년대 후반부터 2010년대까지 지속적으로 증가하였다. 이와 함께 수송되는 심층 해수는 심층 해수의 경계 수심인 심해염분최소층과 해저균질층상부수심 변화를 통해 2000년대까지는 심층수로만 구성되어 있던 반면, 2010년대는 중앙수의 비율이 증가하고 있음을 확인하였다. 일본 분지, 울릉 분지, 울릉해저간극의 심층 온위는 1990년대에서 2000년대로의 증가보다 2000년대에서 2010년대로의 증가가 2배 높아 심층 해수의 온난화가 최근 더욱 가속화 되었음을 확인하였다. 반면 심층 해수의 염분은 1990년대에서 2000년대에 증가한 후 2010년대에 다시 감소하였고, 염분 효과에 의해 2000년대 심층 해수의 밀도는 증가 후 2010년대 다시 감소하였다. 1990년대부터 2010년대까지 1500m에서 2300m 수심 범위에서의 심층 순환 강화는 일본 분지에서 울릉 분지로 뿐만 아니라 울릉 분지에서 야마토 분지로, 그리고 야마토 분지에서 일본 분지로까지 강화될 수 있고, 해당 수심 범위에서 심층수의 비율은 감소하고 중앙수의 비율은 증가할 수 있음을 시사한다. 또한 저층수 생성과는 별개로 중앙수 생성이 강화되고 있음을 확인하였고, 이는 향후 동해 자오면 순환이 저층수 생성에 의한 단일 순환보다는 이중 순환 또는 그 이상의 복잡한 순환 형태가 될 수 있음을 시사한다.Abstract i Table of Contents iv List of Figures vi List of Tables xii 1. Introduction 1 1.1. Background 1 1.2. Purpose of study 5 2. Data and processing 8 2.1. Data 8 2.1.1. Subsurface moored current meter data 8 2.1.2. Hydrographic data 14 2.2. Data processing 18 2.3. Estimation of annual-mean volume transports of the deep waters below 1500 m through the western UIG from 1997 to 2019 23 2.4. Estimation of boundaries between the deep waters (CW/DW/BW) 31 3. Results 33 3.1. Interannual to decadal variation of the volume transports of the deep waters below 1500 m through the western UIG from 1997 to 2019 33 3.2. Decadal variation in the depth and the potential temperature of DSM, UBHL and the upper boundary depth of CW from the 1990s to 2010s 35 3.3. Decadal variation of physical characteristics of the deep waters from 1993 to 2019 42 4. Discussion 44 4.1. Comparison between the equatorward volume transports of the deep waters below 1500 m in western UIG (VT15) and zonal geostrophic flows below 1500 m in the latitude of 38–39 °N along the baseline 44 4.2. Decadal changes of the volume of the deep waters in relation to decadal changes in the equatorward volume transport of the deep waters below 1500 m in western UIG (VT15) from the 1990s to the 2010s 50 4.3. Decadal changes in the physical characteristics of the deep waters from the 1990s to the 2010s 53 5. Conclusion 55 References 59 Abstract in Korean 62 Appendix 64박

Currents, Eddies, and a Fish Story in the Southwestern Japan/East Sea

As part of the Japan/East Sea (JES) initiative supported by the U.S. Office of Naval Research, we conducted an observational experiment to understand the physics of the mesoscale circulation in the Ulleung Basin, located in the southwestern corner of the JES. The current passing through the Korea Strait divides upon entering the JES, with portions of the current flowing along the Korean and Japanese coasts. The variability of these currents is especially energetic in our study region. Our objectives were to measure the time-varying currents in the upper and deep levels of the JES. We relate the population density of vertically migrating fish or squid to the time-varying locations of fronts

Ambient seismic noise tomography of the southern East Sea (Japan Sea) and the Korea Strait

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.This work was funded by the Korean Meteorological Administration and Development Program under Grant CATER 2012-5051

The Dok Cold Eddy

Current and temperature patterns in the Ulleung Basin of the Japan/East Sea are examined using acoustic travel-time measurements from an array of pressure-gauge-equipped inverted echo sounders moored between June 1999 and July 2001. The focus here is the formation and behavior of a persistent cold eddy observed south of Dok Island, referred to as the Dok Cold Eddy (DCE), and meandering of the Subpolar Front. The DCE is typically about 60 km in diameter and originates from the pinching off of a Subpolar Front meander between Ulleung and Dok Islands. After formation, the DCE dwells southwest of Dok Island for 1–6 months before propagating westward toward Korea, where it deflects the path of the East Korean Warm Current (EKWC). Four such DCE propagation events between January and June 2000 each deflected the EKWC, and after the fourth deflection the EKWC changed paths and flowed westward along the Japanese shelf as the “Offshore Branch” from June through November 2000. Beginning in March 2001, a deep, persistent meander of the Subpolar Front developed and oscillated with a period near 60 days, resulting in the deformation and northwestward displacement of the Ulleung Eddy. Satellite-altimeter data suggest that the Ulleung Eddy may have entered the northern Japan/East Sea. The evolution of this meander is compared with thin-jet nonlinear dynamics described by the modified Korteweg–deVries equation