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

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 similar to 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 similar to 13 to 11 ka, the slopes were stabilized and pelagic sedimentation has prevailed under well-oxygenated bottom-water conditions. (C) 2000 Elsevier Science B.V. All rights reserved.