Paleoceanographic changes in the Ulleung Basin, East (Japan) Sea, during the last 20,000years: Evidence from variations in element composition of core sediments

A millennial-scale paleoceanographic record of the Ulleung Basin in the southern part of the East (Japan) Sea (ES) for the last 20,000years was reconstructed based on variations in the element compositions of core sediment. Most of the elements showed relatively wide content ranges throughout the core, largely because of different sediment types, their grain size spectra, biological components, and geochemical conditions (i.e., redox conditions). In particular, biogenic components (Ca, Sr, Ba, and CaCO₃) and some minor elements (Cu, Mn, Mo, U, Cd, Zn, and V) revealed three distinct intervals of major paleoceanographic changes. Interval 1 (∼18–9.3kyr BP) is characterized by a rhythmic alternation of dark- and light-colored sediment bands. Distinctive differences in the abundance of organic carbon, carbonate, and redox-elements between the dark- and light-colored-sediment layers are probably attributable to especially those associated with bottom-water redox variations combined with the enhanced primary productivity and water stratification in the Ulleung Basin. These alternations perhaps reflect fluctuations in paleo-discharge from Chinese and/or Korean rivers during the last deglaciation. From a global perspective, the repetition of dark sapropelic sediment layers suggests a possible teleconnection between oceanographic change and development of the East Asian summer monsoon and consequent fluctuations in fluvial discharge from Korean and Chinese rivers to the East Sea basin. In Interval 2 (9.3–6.5kyr BP), a remarkable increase in the concentrations of organic carbon and redox elements is probably linked to the intrusion of the warm/saline Tsushima Current to the basin, which would have enhanced ventilation and upwelling of nutrient-rich sub-surface waters. In parallel, a pronounced decrease in carbonate content indicates extensive carbonate dissolution due to the consumption of large amounts of free pore-water oxygen in surface sediments that would have accompanied by the decomposition of organic matter. In Interval 3 (6.5kyr BP–Present), most geochemical parameters are relatively constant, suggesting little changes in paleoenvironments; these observations indicate that modern oceanographic conditions were established in the East Sea at about 6.5kyr BP. In particular, a prominent decline in carbonate contents centered at around 3–4kyr BP appears to have been caused by a decrease in primary productivity in association with a suppression of the warm Kuroshio Current, rather than by geochemical processes.