Jong-Hwa Chun

The origin of cold-water authigenic carbonates from deep-water, muddy sediments in the Ulleung Basin, East Sea of Korea

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 δ13C values (δ13C=−43 to − 27‰ PDB) suggest that methane was the main carbon source for their formation. Oxygen isotope values (δ18O = 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.

Late Holocene distal mud deposits off the Nakdong delta, SE Korea: evidence for shore-parallel sediment transport in a current-dominated setting

The distal mud deposits (DMDs) off the Nakdong delta represent a subaqueous delta on the inner continental shelf aligned parallel to the southeast coast of Korea and displaying a clinoform geometry. Hydrographically, the coast is characterized by a micro-tidal regime, the strong Korean Coastal Current (KCC) and the East Korean Warm Current (EKWC). Age models and sedimentary facies related to the clinoform geometries are based on high-resolution chirp subbottom profile data and have provided information on shore-parallel sediment transport and accumulation during the late Holocene sea-level highstand. The highest sedimentation rates (6.19–9.17 cm/year) produced steep foresets in the central DMDs at water depths of 35–50 m. Here, vertical burrows are repeatedly truncated by laminated mud packages displaying erosional surfaces. This region represents the main depocenter of the Nakdong subaqueous delta. The topset sediments of the southern DMD at ~40 m water depth closer to the river mouth show relatively low sedimentation rates (0.01–0.12 cm/year). Here, the muds have a predominantly mottled character. Similarly, the foreset sediments of the northern DMD at ~71–80 m water depth with sedimentation rates of 0.10–2.03 cm/year are also predominantly characterized by mottled muds. The spatial dispersal pattern of the DMDs is consistent with the coast-parallel front between the KCC and EKWC along the southeast Korean coast. In addition, the depocenter of the Nakdong subaqueous delta clinoform is affected by the near-bed turbulence generated by episodic storm events.

Relationships between Gas Hydrate Occurrence Types and Sediment Characteristics in the Ulleung Basin, East Sea

During the 2nd Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) in 2010, gas-hydrate-bearing sediment cores were recovered at 10 drill sites. Base, on Infrared (IR) thermal image and grain-size analysis of the cores, three distinct types of gas hydrate are classified: Type I (fracture-filling in mud layers), Type II (disseminated in mud layers), and Type III (pore-filling in sand layers). Types I and II gas hydrates occur in mud as discrete veins, nodules or disseminated particles. Type III fills the pore spaces of the sand layers encased in mud layers. In this case, the sand content of hosting sediments shows a general linear relationship with gas hydrate saturation. The degrees of temperature anomalies () from IR images generally increase with gas hydrate saturation regardless of gas hydrate occurrence types. Type I is dominantly found in the sites where seismic profiles delineate chimney structures, whereas Type II where the drill cores are composed almost of mud layers. Type III was mainly recovered from the sites where hemipelagic muds are frequently intercalated with turbidite sand layers. Our results indicate that gas hydrate occurrence is closely related to sedimentological characteristic of gas hydrate-bearing sediments, that is, grain size distribution.

A multi-factor approach for process-based seabed characterization: example from the northeastern continental margin of the Korean peninsula (East Sea)

This study investigates sediment transport and depositional processes from a newly collected dataset comprising sub-bottom chirp profiles, multibeam bathymetry, and sediment cores from the northeastern continental margin of Korea in the East Sea (Japan Sea). Twelve echo-types and eleven sedimentary facies have been defined and interpreted as deposits formed by shallow-marine, hemipelagic sedimentation, bottom current, and mass-movement processes. Hemipelagic sedimentation, which is acoustically characterized by undisturbed layered sediments, appears to have been the primary sedimentary process throughout the study area. The inner and outer continental shelf (<150 m water depth) have been influenced by shallow-marine sedimentary processes. Two slope-parallel canyons, 0.2–2 km wide and up to 30 km long, appear to have acted as possible conduits for turbidity currents from the shallower shelf into the deep basins. Bottom current deposits, expressed as erosional moats immediately below topographic highs, are prevalent on the southern lower slope at water depths of 400–450 m. Mass-movements (i.e., slides/slumps, debris flow deposits) consisting of chaotic facies characterize the lower slope and represent one of the most important sedimentary processes in the study area. Piston cores confirm the presence of mass-transport deposits (MTDs) that are characterized by mud clasts of variable size, shape, and color. Multibeam bathymetry shows that large-scale MTDs are chiefly initiated on the lower slope (400–600 m) with gradients up to 3° and where they produce scarps on the order of 100 m in height. Sandy MTDs also occur on the upper continental slope adjacent to the seaward edge of the shelf terrace. Earthquakes associated with tectonic activity and the development of fluid overpressure is considered as the main conditioning factor for destabilizing the slope sediments. Overall, the sedimentary processes show typical characteristics of a fine-grained clastic slope apron and change down-slope and differ within each physiographic province. Furthermore, the influence of geological inheritance (i.e., structural folds and faults) on geomorphology and sediment facies development is an important additional factor on the lower slopes. Together, these factors provide a rational basis for continental margin seabed characterization.

Near-seafloor Environment and Potential Fault around the Gas Hydrate Drill Site of the Ulleung Basin, East Sea

To perform environmental monitoring of deep-sea drill sites (e.g., gas leakage and seafloor deformation) and for optimal placement of monitoring equipment on the seafloor, the exact fault trace and any possible fault offsets need to known. We mapped the structures around a growth fault near the gas hydrate drill site. Then, based on the contour lines, we derived individual two-dimensional (2D) chirp data and merged 3D volumes. The chirp data revealed a clear growth fault slightly south (about 210 m) of drill site. The fault was mapped across all inlines of the 3D volumes. Fault offsets were determined for five individual layers and a clear change in offset with depth was observed. A reduced offset was noted below layer L2 (~1 m) but it was to 2.5 m for both underlying layers L3 and L4. Based on five piston cores across the survey area between North Zone and South Zone, the linear sedimentation rate for the Holocene showed almost no difference (16.5–16.7 cm/kyr). The rate below the Holocene showed a difference between the North Zone (lower; 12.6–12.8 cm/kyr) and the South Zone (higher; 14.8–16.1 cm/kyr). This change is explained by a sudden reduction of fault activity during the Holocene.