Correlation of Dark and Light Layers, and the Origin of Their Cyclicity in the Quaternary Sediments from the Japan Sea

Abstract

Nearly continuous cores of Quaternary fine-grained sediments with distinct dark-light colored cycles were recovered from Sites 794, 795, and 797 in the basinal parts of the Japan Sea during Leg 127. A comparison of gray value (darkness) profiles supplemented by visual inspection of core photographs between sites indicated that most of the dark and light layers were correlatable between sites, and that two of the dark layers lie close to adjacent marker ash layers. These observations indicate that deposition of dark and light layers resulted from basin-wide synchronous events.In order to understand the origin of these dark-light cycles, petrographical, mineralogical, compositional, and paleontological studies were carried out on closely spaced samples from the upper Quaternary sediments recovered from Site 797. Age model was constructed based on comparison between variation in diatom abundance and the standard oxygen isotope curve of Imbrie et al. (1984), the latter was interpolated between the five age controlled levels established at Site 797. The two curves show similar patterns which enabled us to "tune" the sediment ages to the oxygen isotope stages. We have to use variation in diatom abundance as a substitute for oxygen isotope curve since oxygen isotopic data are not available at the studied sites.Bottom water oxygenation conditions were estimated based on two criteria: (1) the degree of lamina preservation and (2) the ratio of C org to S tot . The surface water productivity was deduced from the C org and biogenic silica content.Results suggest that the bottom water oxygenation level and the surface water productivity varied significantly in response to the glacial-interglacial cycles. Glacio-eustatic sea-level changes and subsequent changes in water circulation in the Japan Sea appear to have been responsible for these variations and consequent changes in sediment composition throughout the Quaternary.

Quantification of aeolian dust (Kosa) contribution to the Japan Sea sediments and its variation during the last 200 ky.

“…In the sequence we studied, three fining upward layers and 13 sharp contacts were also described as primary sedimentary structures within both dark and light layers (Shipboard Scientific Party, 1990). However, these fining upward layers and layers with sharp basal contact are not turbidite because they are correlatable basinwide, and the interruption of the sequence by turbidite is minimized because the site is located slightly above the basin floor (Tada et al, 1992(Tada et al, , 1999. These characteristics are shown in Figs.…”

“…The dark layers are mostly laminated whereas the light layers are homogeneous to bioturbated and the boundary between the dark and light layers are generally sharp (Tada et al, 1999). These dark-light cycle is considered to be formed by frequent repetition of anoxic and oxic bottom water environment basinwide (Tada et al, 1992(Tada et al, , 1999. In the sequence we studied, three fining upward layers and 13 sharp contacts were also described as primary sedimentary structures within both dark and light layers (Shipboard Scientific Party, 1990).…”

“…The deposits are composed of clay and silty clay which is occasionally biosiliceous and/or biocalcareous (Shipboard Scientific Party, 1990;Tada et al, 1992). They show centimeter to decimeter-scale alternations of the dark and light layers which are correlatable within the Japan Sea (Tada et al, 1992). The dark layers are mostly laminated whereas the light layers are homogeneous to bioturbated and the boundary between the dark and light layers are generally sharp (Tada et al, 1999).…”

“…In spite of the core disturbance observed in the core top, the core was considered to have caught the mudline because brownish oxidized layer of approximately 60 cm thick is preserved at the core top, which is comparable to the reported thickness of the oxidized layer of the Japan Sea sediments (Masuzawa, 1987). The deposits are composed of clay and silty clay which is occasionally biosiliceous and/or biocalcareous (Shipboard Scientific Party, 1990;Tada et al, 1992). They show centimeter to decimeter-scale alternations of the dark and light layers which are correlatable within the Japan Sea (Tada et al, 1992).…”

“…These estimations are about ten times larger than the flux to the central North Pacific between 30°a nd 40°N (0.1 to 0.2 g/cm 2 /ky) calculated as the mass accumulation rates of total detrital mineral component for core top samples (Hovan et al, 1991;Rea, 1994;Rea et al, 1998), and are comparable to the average total mass accumulation rate of 4.1 g/cm 2 /ky during Quaternary at site 797 (Shipboard Scientific Party, 1990). Thus, a significant contribution of Kosa to the sediments is expected (Tada et al, 1992).…”