中国东部的第四纪风尘堆积与季风变迁

THE QUATERNARY DUST-FALL ACCUMULATION AND THE MONSOON VARIABILITY IN EASTERN CHINA

历史时期的雨土及近代尘暴的研究证实,风尘堆积与冬半年强冷气流活动有关。中国东部第四纪风尘堆积(黄土和下蜀土)分布的时、空变化及其粒度的不断增粗表明,第四纪以来冬季风强度是不断增加的。季风强度的变化主要与青藏高原越来越快的整体隆升有关。青藏高原上升产生了高原季风,并对西风气流和蒙古冷高压发生影响,即从三方面对季风起到增强作用。另一方面是中新生代以来地球气候变冷,使中、高纬地带的海陆分布和地形对大气环流有更大的影响。

It has been confirmed that the Quaternary dust-fall accumulation,called loess in NorthChina and Xiashu soil in the middle and lower reaches of Changjiang River,was caused mainlyby shifting and deposition of material through the force of strong north wind during ice ages ofthe Pleistocene in eastern China.One of the principal features for the Pleistocene dust-fall accumulation is its clear changein spatial distribution with the lapse of time.For example,the loess accumulation formed dur-ing the early Pleistocene migrated from the border belt of the present Loess Plateau (from about(?)°E) eastwards to the foot of the Xinganling,Yanshan,Taihang and east Qinling Mts (toabout 121°E).It was expanded to the western side of Liaodong and Shandong Peninsulas,sou-thern Jiangsu and Anhui Provinces (from about 34°N to 31°N) in the Middle Pleistocene,then to the eastern side of Liaodong Peninsula,shelf,area of Huanghai Sea which was emer-ged as land in the full-glacial stage,and the region around the Poyang Lake south of theChangjiang River (about 28°N) in the Late Pleistocene.Similarly,the grain size of the Pleistocene dust-fall accumulation increases regularly fromthe early to late Pleistocene at the same site and from the northwest to southeast for the sametime.It is shown that the intensity of winter monsoon increased continually from the glacialperiod of the earlp to late Pleistocene in eastern China.It was stated up to now that the East Asia Monsoon,called the sea-land monsoon,resultedfrom the pattern of sea-land distribution and the disparity of the thermodynamic effect betweensea and land.But,the changes of monsoon intensity during the Quaternary,especially fromthe end of early Pleistocene to the beginning of the middle Pleistocene,show that the pat-tern of monsoon circulation is under the influence of other factors.The most important factor isconsidered as the increasingly rapid uplift of the Qinghai-Xizang Plateau as a whole since thebeginning of Quaternary.The uplift of the Plateau has been strengthening the winter monsoonin East Asia in the following three aspects.1) the emerging of plateau monsoon,2) the strength-ening of the Mongolian-Siberian cold high pressure,and 3) the increased orographic di-version of westerly and even more deflection of its northern branch southward after it roundedthe Plateau.Another factor is the cooling of global climate since the middle and late Ceno-zoic,which has increased the influence of sea-land distribution and morphologic elevation onatmospheric circulation,especially in the middle latitude zone.