Dae Choul Kim

Paleoceanographic change during the last deglaciation, east Sea of Korea

Foraminiferal assemblage and stable isotopic data are presented for three Quaternary piston cores from Ulleung Basin, East Sea of Korea ((ESK) Japan Sea) near the Korean Peninsula. Major changes in both temperature and salinity strongly affected surface and deep waters of the ESK during the transition from the Last Glacial Maximum (LGM) to the middle Holocene. Local environmental effects dominated during the LGM and the Bolling/Allerod (B/A) when the ESK became semi-isolated from the Pacific Ocean. Regional/global influences dominated following the B/A, after sufficient reconnection with the Pacific. This is reflected in the foraminiferal δ18O record which was largely salinity-controlled before the Younger Dryas (YD) and temperature-controlled after the YD. Paleoceanographic changes in the ESK during the last deglaciation reflect sequential reconnection with the Pacific Ocean, through gateways, first (B/A) in the north (Tsugaru Strait) and later (Holocene) in the south (Korea Strait).

Distribution and Acoustic Characteristics of Shallow Gas in the Korea Strait Shelf Mud off SE Korea

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area.

Chirp (2–7 kHz) echo characters and geotechnical properties of surface sediments in the Ulleung Basin, the East Sea

High-resolution (chirp, 2–7 kHz) reflection profiles from the southern continental slope and basin plain of the Ulleung Basin reveal three echo types. Sediment texture and geotechnical properties were analyzed using 22 piston cores. Echo type 1 shows a sharp bottom echo with discrete, continuous subbottom reflectors with a flat or undulatory upper surface interpreted as turbidite/hemipelagic sediment. It is dominant in the basin plain. Echo type 2 is characterized by laterally wedged, an acoustically transparent masses (debrites). This type occurs in the southern lower slope and base-of-slope area. Echo type 3 shows irregular blocky/lumpy or hyperbolic masses with various amount of internal deformation, (interpreted as slide/slump deposits). It occurs in the middle to upper slope area. These echo types show a zonal distribution.Sediment texture in the basin plain, especially mean grain size appears to be the most important parameter for the variations in geotechnical properties (wet bulk density, porosity, velocity, and shear strength). The variations in geotechnical properties with burial depth in the southern slope are dependent strongly on sediment compaction and/or consolidation. Overall, geotechnical properties and sediment texture change significantly from the southern slope area to the basin floor, suggesting that sediments were probably originated from the southern margin. The slope sediments especially are relatively compacted due to the mass flow processes. Therefore, the geotechnical properties of the plain and slope sediments show significantly different values.

Sequence Stratigraphy of Late Quaternary Deposits in the Southeastern Yellow Sea of Korea

High-resolution seismic profiles were analysed to investigate the sequence stratigraphy of late Quaternary deposits in the southeastern Yellow Sea of Korea. Approximately 1500 line-km data of chirp and sparker profiles were acquired. We also used deep drill cores to interpret the sedimentary sequence. High-resolution seismic profiles reveal the Holocene mud deposits and the complex sedimentary structure. The late Quaternary deposits in the study area can be divided into four sedimentary units (units A, B, C, and D from oldest to youngest) bounded by an erosional surface and internal seismic reflector: (1) incised channel fill/lowstand deltaic wedge (unit A); (2) transgressive sand ridges and sand sheets (unit B); (3) estuarine/deltaic or shallow-water mud (unit C); and (4) distal mud patch (unit D). Unit C is divided into two sedimentary subunits on the basis of high amplitude and unconformity: (i) estuarine/deltaic mud (unit C-a); and (ii) mud redeposited by sea-level rise and strong tidal currents (unit C-b). The evolution of units C-a and C-b is closely related to postglacial sea-level changes, sediment erosion, and reworking. These late Quaternary deposits have been controlled by sea-level change, tidal currents, and sediment erosion.

CUBE Filtering of Multibeam Echo Sounder Data

A MBES (multibeam echo sounder) survey around Yokji Island, Korea, was conducted to find an effective method for removing error data. Two post-processing software programs, PDS2000 (RESON) and HIPS (CARIS), were used to remove the error data using an interactive editing method and the CUBE algorithm filter. The post-processing with the PDS2000 and HIPS programs, using the interactive editing method, took 120 and 168 hours, respectively, and there was little difference in the seafloor images. The processing time of the PDS2000 and HIPS programs using the CUBE algorithm filter was 36 and 60 hours, respectively. Nevertheless, there was little difference in the seafloor images because of differences in the factor parameters in each of the post-processing programs. Therefore, post-processing using CUBE filtering can save time in data processing and provide consistent results, excluding the subjective decisions of the operator. This method is more effective than other methods for rejecting erroneous multibeam echo sounder data.

Characterization of physical properties for sediments and sedimentary rocks from Japan Trench, ODP Leg 186

Two sites (Sites 1150 and 1151) located in the landward deep-sea terrace of the Japan Trench were drilled during the Ocean Drilling Program (ODP) Leg 186. Site 1150 is seismically active, whereas Site 1151, aseismic. Measurements of sediment physical properties were made on both whole-round core and split-core sections. Bulk density is relatively uniform with similar variation with depth, although its absolute values are slightly different. It is most likely attributable to coring disturbance. Magnetic susceptibility is characterized by the highest value at the lithologic unit I. Depth variation of water content and porosity shows gradual decreasing pattern, except for lithologic unit I, similar to that of natural Gamma radiation. The abnormal characteristics of unit I are likely related to the under-consolidation caused by rapid deposition (100–200 m/myr) as well as impermeable nature of the sediment preventing dewatering. Grain density is higher in the unit I, but below which is relatively constant with depth. P-wave velocity gradually increases from 1500 to 2000 m/s with depth, caused by sediment compaction and/or consolidation. Sediment physical properties of both sites are very similar, although p-wave velocity is slightly higher (≈20 m/s on the average) at Site 1151. This indicates that sediment source, mineral composition and sediment diagenesis are not significantly different at both sites, implying that seismic activities such as earthquake do not likely cause marked changes of sediment physical properties.