Jang Jun Bahk

Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan)

Abstract A detailed facies analysis of long piston-core sediments from the Ulleung Basin reveals four mud facies: laminated mud, crudely laminated mud, bioturbated mud, and homogenous mud. The laminated and homogeneous muds were most likely deposited from turbidity currents, whereas the bioturbated and crudely laminated muds were formed by hemipelagic/pelagic sedimentation under well-oxygenated and poorly oxygenated bottom-water conditions, respectively. A detailed analysis of vertical distribution of the mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages reveals paleoenvironmental changes during the late Quaternary. During the period between 49 and 23 ka, eustatic lowering of sea level facilitated frequent slope failure, resulting in repeated deposition of fine-grained turbidites on the basin floor. Normal hemipelagic sediments were intensively bioturbated under well-oxygenated bottom-water conditions. During the period between 23 and ∼13 to 11 ka, deposition from turbidity currents continued, but hemipelagic sediments were non-bioturbated under poorly oxygenated bottom water conditions. The bottom-water deoxygenation is attributed to further lowering of sea level during the last glacial period and consequent intensified stratification in the water column by freshwater supply to the nearly isolated sea. Primary laminae preserved in the crudely laminated mud generally exhibit low compositional bimodality in backscattered electron images, which suggests low surface primary productivity and presence of superficial micro-bioturbation. After rapid rise of sea level at ∼13 to 11 ka, the slopes were stabilized and pelagic sedimentation has prevailed under well-oxygenated bottom-water conditions.

Late Quaternary Sedimentation in the Eastern Continental Margin of the Korean Peninsula

The eastern continental margin of the Korean Peninsula shows complex topography, including Ulleung Basin (UB), South Korea Plateau (SKP), Oki Bank (OB) and Ulleung Interplain Gap (UIG). A detailed analysis of Chirp (2-7 kHz) subbottom profiles and piston cores reveals various sedimentary processes during the last- and post-glacial periods. The extensive mass movements occur along the slopes of the UB, OB and SKP. The dominant mass-flow deposits in Unit II (>∼15 ka) suggest frequent slope failures during the last-glacial period. The slope failures in the UB and OB were most likely triggered by the reducing of hydro-static pressure and gas-hydrate dissolution associated with sea-level lowering. In contrast, mass movements in the SKP, occurring at deeper than 1000 m water depth, were probably generated by earthquakes. The mass flows evolved from mass movements formed debrites and turbidites in the downslope areas. In the upper Unit II, the common association of non-bioturbated muds reflects a poorly oxygenated bottom-water condition by the last-glacial sea-level lowering. Erosion by bottom-currents is indicated by the truncated reflectors of channel walls and muddy or manganiferous contourites in the Ulleung Interplain Channel (UIC) along the UIG. The dominant muddy and manganiferous contourites and rare turbidites in Unit I (<∼15 ka) on the UIG and basin plain of the UB reflect intensified bottom currents and infrequent slope failures during the post-glacial period. These conditions have facilitated the formation of a thin, elongate mound of bottom-current drifts on the southeastern UIC flank and the more erosion in the UIC.

Relative role of productivity vs. stagnation in dark laminated mud formation during the last 25,000 years in the Ulleung Basin, East/Japan Sea

Records of redox-sensitive elements (Mo, Cd, Cu, V and U), δ18O of planktonic foraminifera (δ18Opf), and organic carbon and biogenic opal contents in dark laminated mud (DLM) layers of the East/Japan Sea reveal relative role of primary productivity versus bottom-water stagnation for the formation of these layers during the last 25,000 years. Enrichments of redox-sensitive elements in these layers indicate that bottom waters were reduced during the formation of the lower DLM (22,500-15,00014C yr B.P.) and the upper DLM (10,400-10,20014C yr B.P.), probably to a greater extent in the lower layer. Anomalous low δ18Opf values and minimal contents of both organic carbon and biogenic opal suggest that bottom-water stagnation due to density-stratified water column could be more important than primary productivity for the lower DLM formation. In contrast, the relatively high contents of the organic carbon in the upper DLM invoke a primary role of enhanced productivity in the formation. This inference is, however, questioned by the absence of comparable increase in biogenic opal contents which is expected from generally coupled production of organic carbon and opal in surface water. The discrepancy between the organic carbon and opal records suggests that a bottom-water stagnation, which might have caused the preferential preservation of organic carbon in the reducing bottom-water condition, could be also more important than primary productivity for the upper DLM formation.

Sedimentation and seafloor mound formation in the southern slope of the Ulleung Basin, East Sea, Korea, since the Last Glacial Maximum

Detailed analysis of core sediments and acoustic characteristics of Chirp sub-bottom profiles from a submarine ridge in the southern slope of the Ulleung Basin, East Sea, Korea, reveals changes in sedimentary processes and deformation of sedimentary sequence induced by seafloor mound formation since the Last Glacial Maximum (LGM). The core sediments consist generally of four sedimentary facies which represent hemipelagic sedimentation under a poorly-oxygenated bottom-water condition during the LGM (crudely laminated mud), shoreface erosion and seaward transport off the shelf break during the post-glacial transgression (bioturbated sandy mud or muddy sand), hemipelagic sedimentation under generally well-oxygenated bottom-water condition after the sea-level rise (bioturbated mud), and episodic precipitation of authigenic carbonates by methane venting since the LGM (authigenic carbonate-bearing mud), respectively. The formation of authigenic carbonate-bearing mud facies and associated accumulation of near-seafloor gas hydrates might have led to up-warping of the sediment cover since the LGM that resulted in the seafloor mounds protruding along the crest line of the ridge. The effects of up-warping by expansion of the subsurface gas hydrates are firstly manifested in this paper by the abrupt lateral changes in the sedimentary facies distribution and uplifted acoustic reflectors between the mounds and the background seafloor.