长江三角洲第四纪沉积记录与古环境响应

RECORDS FROM QUATERNARY SEDIMENT AND PALAEO-ENVIRONMENT IN THE YANGTZE RIVER DELTA

选择长江三角洲南东部贯穿第四系沉积的钻孔SG7,进行沉积物测年、沉积特征、粒度、磁化率、孢粉和主量元素的系统研究,以揭示长江三角洲在第四系地层的源区变化及其环境意义。研究表明,长江三角洲第四系的物源区在1.2MaB.P.前后存在显著差异,2.6～1.2MaB.P.主要为现代长江口南西部的小流域,母岩以上侏罗统凝灰岩为主;1.2MaB.P.开始古长江自北向南发生改道并于现代长江口贯通入海,使现代长江流域成为长江三角洲的主要源区;1.2MaB.P.以来长江三角洲沉积物的磁化率随地层层序呈周期性变化,其变化周期与地球轨道参数的变化周期吻合,高值代表孢粉数据指示的寒冷干燥的气候条件,而低值对应于温暖潮湿的气候,推测长江流域1.2MaB.P.以来的古气候演化主要受天文因素——米兰科维奇旋回所控制。大约1.2MaB.P.和3.8kaB.P.,分别记录了两次向冷干突变的气候转型事件,前者可能与青藏高原早更新世晚期的强烈隆升有关,其不仅影响了贯穿中国腹地的长江流域,更可能是影响东亚的气候事件,也成为1.2MaB.P.长江在研究区改道入海的主要驱动因素;后者为全球性气候事件,除受到天文因素的控制,可能还与人类活动的影响有关。

The SG7 borehole （30°57＇32＂N, 121°52＇03＂E） is located at Lin-Gang-Xin-Cheng （means near harbor new town） in Nanhui district in Shanghai city in southeastern part of the Yangtze River Delta（YRD）. This borehole has a thickness of 322m which is filled with the whole Quaternary and part of Pliocene strata. In order to distinguish the sediment change in Quaternary strata in the YRD and to figure out its palaeo-environment, the synthetical experiments were carried out, including grain size analyses, magnetic susceptibility measure, geochemical testing, spore and pollen analysis, OSL dating and paleomagnetic dating. The following results are obtained ： （ 1 ） there is a large-scale fining sedimentary sequence upwards in Quaternary strata in SG7 ,composed of six fining sub-sequences upwards,indicating six different evolution phases in this area during Quaternary. （2） The sediment of the Quaternary strata in the YRD was changed dramatically at 1.2MaB. P. During 2.6MaB. P. to 1.2MaB. P., the paleao-Yangtze River flowed through Jiangsu province and entered into the Yellow Sea,while the main sediment from a local small drainage basin, situated southwest to the modern YRD, discharged to the Yangtze River estuary, with the major components originated from upper Jurassic tuff. During 1.2MaB. P., because of palaeo-climate change and neotectonic movements, the palaeo-Yangtze River channel migrated southwards, and flowed through the present estuary area, and entered into the East China Sea. As a result, current Yangtze River Drainage Basin （YRDB） became the main sediment source and discharged the sediment to the Yangtze River estuary to form the modern YRD. （3）Since 1.2MaB. P. ,the magnetic susceptibility of the strata showed the periodic variation accompany with the stratigraphic and geochronological sequences, and its periods could compare with the orbital parameter periods of ,the Earth. Thus, it＇s high probably that the palaeo-climate change in the YRDB was prominently controlled by Milankovitch cycles after 1.2MaB. P. （4）Comparing with the studies on spore and pollen analysis,the magnetic susceptibility of the strata after 1.2MaB. P. could obviously indicate the palaeo-climate changes in the YRDB. Its high and low values corresponded to the cold-dry and warm-humid climate conditions respectively. However, it is hardly to find out the relationship between the variation of grain-size of the sediment and the palaeo-elimate changes. （5）According to the changes of magnetic susceptibility, two abrupt cooling and drying climate change events could be recognized in the YRDB at 1.2MaB. P. and 3.8kaB. P. The former event was probably related to the intense uplift of the Tibetan Plateau during Early Quaternary,which affected not only the YRDB but also the whole East Asia. Furthermore, it＇s the major driving mechanism to cause the channel migration southwards in the lower Yangtze River reaches considerably. Whereas the latter palaeo-climate event represented a global climate event, controlled by both astronomical factors and human activities.