Approach To The Formation Mechanism Of The Hard-green Hard Clay From Chenghu Region In The Central Taihu Plain Of China

Hard clay of the late Pleistocene in Yangtze River Delta is a valuable ancient layer for coastal and shelf area. It contains the most abundant, diverse and ancient environmental information. And the relationship of that information is very complex. The ancient environmental information is very important for understanding the ancient environmental evolution of the Late Quaternary.The bottom of Chenghu Lake in Suzhou is mainly composed of the 'hard clay' (a kind of sediments consisting of silt and clay), which is similar to other lakes in Taihu Plain.The hard clay in Chenghu Lake has different colors, consist of dark-green, yellowish-brown, gray-brown mixed and gray. Based on the section description, AMS14C dating, grain size, element-geochemistry, active iron, clay mineral analysises, preliminary results are as follows:(1)The hard clay of core SC6 (0-6m) was formed during 11kaBP～29kaBP. According to the AMS14C dating datas of core SC6, as the deposition of the same time, the core SC4 (5.15-4.15m) was formed in 27000～22290±65aBP and the age of the core SC5 (3.55～5.10m) is 22540～27000aBP.(2) AMS14C datas show that the age of the upper of the green hard clay is not consist with the age of the bottom of the overlying lake sediments (7025±40aBP), lack of significant sediments. Clay mineral analysis shows that the discontinuous are due to erosion environment, that 1kaBP～7025±40aBP is erosion environment in Chenghu Lake area. So the upper deposition of core SC4 and core SC5 were erosioned, then formed a paleo depression.(3) Comprehensive analysis of the lithology description, grain size, source discriminant function DF, TiO2/Al2O3 and. K2O/Al2O3 comparison chart, CIA index and the Na/K ratio show that hard clay of different colors in the bottom of the Chenghu have the same structural characteristics. The contrast among northwest typical eolian deposits, Xiashu loess in Zhenjiang, hard clay in the Yangtze River Delta, Sheng island loess, the eolian dust red earth in Xuancheng of Anhui show that they have similar composition before the chemical weathering, they are all eolian deposits, so the original source is not the reason of difference of hard clay color.(4) Analysis of element geochemistry and clay minerals, contrast to Xiashu loess, North-west eolian deposits, Xuancheng contrast to clarify the lake bottom show experienced a significant chemical weathering, show that hard clay in the bottom of Chenghu Lake had undergone moderate chemical weathering.Hard clay of SC6 has undergone intensively moderate chemical weathering. And chemical weathering intensity of hard clay from 3.50m to 6.00m is obviously different from the upper part (from 3.50 to 0 m). The sediment from 3.50m to 6.00m have undergone the primary process of chemical weathering characterized by leaching of Ca and Na, and the upper part has reached the secondary process characterized by leaching of K, which underwent more intensive chemical weathering intensity and soil forming process. This chemical weathering characteristic reflects the global climate change and regional climatic conditions.The hard clay in the Chenghu Lake was deposit at the same time. Hard clay plain was formed until 11kaBP, then they were exposed to accept chemical weathering, but due to erosion environment, the upper of SC4 and SC5 was eroded,with the climate conditions becoming better, the lower area was easy to be submerged.So the time of chemical weathering of SC4 and SC5 core was less than SC6 core. As a result, clay mineral show that mixture of chlorite and vermiculite are dominant of 1.4nm minerals, little chlorite and little vermiculite.1.4nm mineral of core SC6(3.5～0m) was vermiculite-based,1.4nm mineral of core SC6 (3.5～5.2m) was chlorite-based. So the chemical weathering of dark-green hard clay was between the upper and lower section of core SC6, in a transition phase.(5) Redox environment, high content of smectite and humic acid-rich humus dip effect are the mainly reasons of hard clay that showing dark-green. *This dissertation is supported by the Natural Science Foundation of China (NSFC), within the framework of the project" Case study on the formation and evolution of Holocene lake environment and human response in the Taihu flood plain (No.40571170)".