关键词: 黄土高原, 古季风, 晚更新世

Abstract: Studies on loess soil sequence, susceptibility, size and stable isotope in secondary carbonate at different profiles on the Loess Plateau reveal palaeo monsoon evolution law in time and space since the Late Pleistocene. During the last interglacial period, sediment sequence on northern margin of the Loess Plateau are composed by three thin palaeo soil layers and two loess layers. From northern edge to southern margin of the Loess Plateau, the three palaeo soil layers gradually merge into a compounded palaeo soil layer (S 1), its thickness and the degree of soilization distinctly increase. The susceptibility curve shape varies from three peaks to two peak to a wide peaks and the susceptibility value varies from 80~100SI to more than 200SI. During the last glacial period, from north to south or from northwest to southeast of the Loess Plateau, sediment sequence varies from three palaeo sand layers interbedded with two loess layers to single loess layer to two loess layers interbedded with a weak palaeo soil layer to three loess layers interbedded with two palaeo soil layers. Susceptibility value varies from 30~60SI to 100SI to 200~300SI from north to south, but susceptibility curves all show two peaks at different profiles. During Holocene, black mature soil was extensively developed, in which, susceptibility curves distinctly show a peak at different profiles and its value varies from 70SI to 80SI to150SI to 200~300SI from north to south. The δ 13 C value of carbonate in Xifeng area is heavier and δ 18 O value is higher in comparison with Luochuan and Xi'an area, which reflect that East Asia Monsoon effect gradually increases and rainfall decreases from east to west. The above mentioned facts indicate that there were three periods with distinct East Asia Monsoon effect during the last interglacial period on the Loess Plateau: 130~116 ka, 106~94 ka and 84~75 ka, it arrived at the strongest at 123 ka, 103 ka and 79 ka. The East Asia Monsoon effect was strongest at 103 ka eastward from Liupan Mountain and at 123 ka westward from Liupan Mountain. During the last glacial period, there were three periods with remarkable Northwestern Monsoon effect: 75~60 ka, 45~30 ka and 20~10 ka. The southern edge of Mu Us Desert located between Yulin and Dingbian Mizhi from 75 ka to 50 ka. From 20 ka to 10 ka, the southern edge of desert arrived at near Great Wall and the front of the Northwestern Monsoon pushed into the lower reaches of the Yangtz River, and Malan loess was extensively formed. The East Asia Monsoon effect was very pronounced and gradually increases since Holocene, its intensity was the strongest in the middle Holocene, the front of which pushed into the interior of the Mu Us Desert. Afterward, the front of the East Asia Monsoon moved back to now location.

Key words: Loess Plateau, Palaeo monsoon, The Late Pleistocene