DARKNESS OF THE JAPAN SEA SEDIMENTS AS A PROXY FOR HIGH-RESOLUTION RECONSTRUCTION OF EAST ASIAN SUMMER MONSOON VARIABILITY

Asian monsoon is a regional phenomenon of global significance. More than 1/3 of world population lives in the area under the influence of Asian monsoon. Their lives are strongly influenced by the summer monsoon precipitation not only because it is the major source of their water resources but because it occasionally causes floods. The East Asian summer monsoon brings a large amount of precipitation in southeastern margin of Asia that influences surface water salinity and nutrient in East Asian marginal seas. Recent oxygen isotope study of stalagmite from Hulu Cave in China revealed millennial-scale variability of the summer monsoon intensity in association with the Dansgaard-Oeschger Cycles [DOCs] (Wang et al., 2001). However, the record is only 70 ky long, and there is no good proxy for longer term high-resolution reconstruction of East Asian summer monsoon intensity beyond 70 ka. The Pleistocene to Holocene fine-grained sediments in the deeper part of the Japan Sea are characterized with centimeter- to decimeter-scale alternations of darker, organic-rich, laminated layer and lighter, organic-lean, bioturbated layer. Deposition of the individual dark layer is synchronous and correlatable basin-wide. Tada et al. (1999) suggested that these dark and light layers were formed in association with DOCs with dark layers corresponding to interstadials. They speculated that deposition of the dark layers was resulted from increasing contribution of the lower salinity and nutrient-enriched East China Sea Coastal Water [ECSCW] during interstadials of DOCs. Present oceanographic data suggests that the discharge from the Yangtze River significantly contributes the formation of ECSCW, and that more than 70 % of the water from the Yangtze River flows into the Japan Sea. Nutrient budget calculation of the sea also suggests that approximately the same amount of phosphorous discharged from the Yangtze River is buried as organic matter in the Japan Sea. Thus it is likely that the Japan Sea act as a trap of nutrients discharged from the Yangtze River. Because the Yangtze River discharge is strongly controlled by the summer monsoon precipitation, the organic matter burial in the Japan Sea, and the gray scale of its sediments, could be an excellent high-resolution proxy for the East Asian summer monsoon and DOCs.