Evolution Of The East Asian Monsoon Since The Last Deglaciation: Sporopollen Records In The East China Sea And The North Yellow Sea

In this study a giant gravity core MD05-2908 from the south Okinawa Trough (SOT), two shorter cores B-U35 and B-U41 from the north Yellow Sea (NYS) and hundreds of surface sediment samples were used to trace the original of material and the climatic variation histories since the last deglaciation. Palynological analysis, together with organic geochemistry methods such as carbon and nitrogen isotopic composition and elemental ratios were hired as proxies for climate changes and source indicators. Combined with AMS14C dating technique, we reconstructed the evolution history of the East Asian Monsoon since the last deglaciation. The major conclusions are as follows:The palynological records in core MD05-2908 reveal that the climate was relatively warm and dry during 6800-3800a BP, two cold and humid intervals between 6800-6600a BP and 5800-4850a BP punctuated this warm period. After 3800a BP, the climate became colder and wetter.Maps of pollen percentage isopolls of the NYS reveal that the sporopollen is mainly come from Yalu in the east of NYS, and the distribution of pollen is controlled by the diluted water of Yalu; the east branch of the Yellow Sea Warm Current(YSWC) and a branch of the coastal current along the Liaodong peninsula control the distribute of pollen in the central of NYS; the west branch of YSWC impacts with the western coast of the Yellow Sea, a cyclonic eddy is generated over the west of NYS, and the eddy control the distribute of pollen at here.The palynological records in core B-U35,B-U41 reveal that the last deglaciation ended at 10100a BP, the climate cold and wet during the last deglaciation; the climate of early Holocene(10100-6600a BP) warm and dry, after 6600a BP, the temperature begin decline, the precipitation is abundance between 6600 to 4000a BP, after 4000a BP, the precipitation decreasing. The significant change in arboreal and herbaceous pollen after 800a BP reflects widespread intensive human influence.The spore content at MD05-2908 core were adopted as proxies for East Asian monsoon evolution. The spore record shows that the East Asian monsoon intensity between 6800-3800a BP, two weaken intervals between 6800-6600a BP and 5800-4850a BP punctuates this strength period. After 3800a BP, the East Asian monsoon obviously weakening, this may have been produced by the combined effects of the decrease of the insolation and the density at El Nino frequencies.The percentage of Compositae, Chenopodiaceae and Ephedraceae at cores B-U35 and B-U41 were adopted as proxies for East Asian monsoon evolution. The conclusion suggested that the East Asian monsoon obviously intensified during 10100-6600a BP, while since 6600a BP, the East Asian monsoon become weaker and after 4000a B. P. the East Asian monsoon sharply decreased.Compared the East Asian summer monsoon records derived from pollen and spore proxies in the East China Sea and the North Yellow Sea, we found that they show the contrary trend. The humidity in south and north part of China shows reverse characters. Which may be cased by the intensity variation of the East Asian summer monsoon and its related rain-front latitudinal migration.