太湖平原碟形洼地沉积物记录的距今8000年以来植被、气候与地貌演变

利用DGY孔沉积物的岩性、粒度及孢粉分析资料,探讨了太湖平原碟形洼地沉积物记录的8000年以来植被演化、气候波动,流域地貌环境的演变及其对海平面波动的响应。孢粉记录表明:本区距今8000~5000年木本植被由以落叶阔叶乔木和针叶乔木为主变为落叶阔叶乔木和常绿阔叶乔木为主,气候由温和略干转为暖热湿润;距今5000~4000年森林退缩,气候温和偏干;距今4000年左右出现低温事件;距今4000~3,000年木本植被以常绿阔叶乔木为主,气候温暖湿润;距今3000年以来木本植被主要为常绿阔叶乔木、落叶阔叶乔木和针叶乔木,气候总体温暖湿润,有波动变凉干趋势。岩性、粒度及孢粉记录综合显示本区距今8000~5000年发育潟湖相泥质粉砂,反映流域环境开放、与海连通;距今5000~4000年发育淡水沼泽,未受海水作用,反映水域缩小、流域环境封闭;距今4000~3000年水域扩大,并出现海水倒灌,反映内低外高的碟形洼地已经形成;距今3000年以来,发育湖沼平原,发生数次海水倒灌,反映碟形洼地地貌的进一步发育。

Climate, vegetation and geomorphology evolution since 8 ka BP recorded by sediments from dish-like depression of Taihu Lake Plain

Based on the analyses of lithology, grain size and sporopollen of sediments from the DGY borehole in the Taihu dish-like depression, this paper discusses the evolutions of vegetation, climate and geomorphology in the Taihu dish-like depression since 8ka BP. The records of sporopollen show that the arbor vegetation changed from deciduous broad-leaf trees and conifer trees to deciduous and evergreen broad-leaf trees during 8 -5 ka BP. This indicates that the palaeoclimate changed from warm-dry to hot-wet during this period of time. The shrinkage of forests reflects a cool and dry climate during 5 -4 ka BP. A cooling event occurred at -4ka BP. During 4 -3ka BP, the forest was mainly composed of evergreen broad-leaf trees, reflecting a warm and wet climate. Since 3 ka BP, the forest was mainly composed of conifer trees and evergreen and deciduous broad-leaf trees, reflecting a warm-moist climate. Interpretations of sporopollen records also suggest a cooling trend with some fluctuations during this period. The records of sporopollen, lithology and grain size show jointly that muddy silt of lagoon facies prevailed in the area during 8 -5 ka BP, reflecting an open depositional environment effected by seawater. Freshwater marsh prevailed and the seawater effect vanished during 5 - 4 ka BP, reflecting the shrinkage and isolation of the lake area. The lake area growths and saltwater intrusion occurred during 4 - 3 ka BP, suggesting the formation of the dish-like depression of the Taihu drainage area. Freshwater marsh which was influenced occasionally by tidewater has prevailed since 3 ka BP, reflecting the further development of the dish-like depression in the study area.