Byong-Jae Ryu

Compositional and structural identification of natural gas hydrates collected at Site 1249 on Ocean Drilling Program Leg 204

In contrast to the structural studies of laboratory-grown gas hydrate, this study has been performed on naturally grown clathrate hydrates from the sea floor. The PXRD pattern of natural gas hydrate shows that the sample had a structure I hydrate. The13C NMR spectrum was obtained for the natural gas hydrate sample in order to identify the cage occupancy of guest molecules and determine the hydration number. The NMR spectrum reveal that the natural gas hydrates used in this study contain only methane with no noticeable amount of other hydrocarbons. The existence of two peaks at different chemical shifts indicates that methane molecules are encapsulated in both large and small cages. In addition, Raman spectroscopic analysis is also carried out to identify natural hydrates and compared with the NMR results. Investigating the composition and structure of natural gas hydrates is essential for applying natural gas hydrates as a novel energy source.

Framboidal pyrites in late Quaternary core sediments of the East Sea and their paleoenvironmental implications

The three piston cores, collected from the East Sea, were analyzed to detect changes of sedimentary environment during the late Quaternary. The cores consist mainly of muddy sediments that are interbedded with silty sands, lapilli and ash layers. The so-called dark laminated mud (DLM) layers are found during the last-glacial and interglacial transition. The geochemical results document that the DLM layers are enriched in Fe relative to Mn. Fe is present as a form of framboidal pyrite, which shows a proportion of about 2∶1 for S and Fe. Based on the morphology, the pyrites are regarded to have grown in the transitional state between high and low supersaturations of Fe and S. Furthermore, the framboidal size distribution implies that the DLM layers have deposited in euxinic environments.

Distribution of late Quaternary tephra layers in the western part of the Ulleung Basin, East Sea

Lithologies and stratigraphic relationships of late Quaternary core sediments were examined in three piston cores, recovered from the western Ulleung Basin. The approximately 8-m long cores consist of muddy sediments that are interbedded with lapilli and ash layers. The morphological and major element compositional studies suggest that volcanic glass shards of the tephra layers are equivalent to those of fallout deposits of the Ulleung—Oki (ca. 9.3 ka), Aira—Tanzawa (ca. 22–24.3 ka), and Ulleung—Yamato (ca. 25–33 ka). The occurrence of the lapilli layers in the western Ulleung Basin implies that the known fallout distribution of pumicetype glass shards can be extended about 50 to 100 km to the west.

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.

Introduction to special section: Exploration and characterization of gas hydrates

The characterization of marine gas hydrates from geophysical data has evolved significantly over the past decade. For many years, marine gas hydrate interpretation was largely limited to a determination as either “likely present” or “likely not present” based on the presence or absence of