元素地球化学揭示的长江中下游下蜀黄土物源及其环境意义

GEOCHEMICAL EVIDENCE FOR PROVENANCE OF XIASHU LOESS IN THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER AND ITS ENVIRONMENTAL IMPLICATION

中晚更新世以来长江中下游地区粉尘堆积广泛发育,这些广义上可称之为"下蜀土"的区域粉尘堆积,典型代表有"九江下蜀土"、"安徽下蜀土"、"南京下蜀土"。其形成的年代及其粉尘堆积属性已基本明了,但是其物质来源却存在较大争议。详细分析了这些粉尘的元素地球化学特征,并与黄土高原黄土和红土进行了对比,结果表明:1)长江中下游的下蜀黄土均非来自北方的黄土高原,且九江、安徽、南京三地物源相互之间也存在显著差异,均为近源堆积,北方粉尘的作用过去被严重高估了;2)冰期背景下,长江中下游地区气候普遍恶化,区内大面积沙化和厚层粉尘堆积是现代亚热带湿润季风区对冰期气候环境响应的结果,是特殊地形条件和气候背景下冬季风近地面气流侵蚀—搬运裸露的河(湖)漫滩而形成的区域风沙—粉尘堆积体系,长江流域这种区域堆积体系及其地环境与地生态效应在以往的研究中没有得到应有的重视和认识。

Dust deposits have been widely developed in the middle and lower reaches of Yangtze River since the middle Pleistocene age. These regional dust deposits, known as Xiashu loess in a broad sense, are typically represented by ＂Jiujiang Xiashu loess＂, ＂Anhui Xiashu loess＂ and ＂Nanjing Xiashu loess＂. Their depositional age and sedimentary properties have been clarified, but their provenance remains controversial. Geochemical characteristics of the Xiashu loess was analyzed in this paper and compared with loess of red earth from CLE Results are stated as： 1） The Xiashu loess in the middle and lower reaches of the Yangtze River was not derived from far north of China, and there are significant differences in the origin of loess from Jiujiang, Anhui and Nanjing areas. The role of dust from North China has been greatly overestimated in the past. 2） During glacial periods, the climate of the middle and lower reaches Yangtze River region was generally deteriorated. The large area of desertification and thick dust deposition in the above-mentioned region represents a respone of the modem subtropical humid monsoon climate zone to global climate change. These were the result of detrital materials transported from proximate upwind fluvial plains by winter monsoon under special topographic and climate conditions. But the effects of regional geomorphology have not received due attention in the past.