苏州澄湖湖底硬粘土地球化学特征及其成因意义

位于太湖平原的苏州澄湖,湖底十分平坦,主要由硬粘土组成。在湖底获得18.00m长的柱样,0～6.00m为长江三角洲东部平原区的第一硬粘土层。通过AMS14C测年、元素含量测定及物源判别函数DF、n(Na)/n(K)与CIA指数计算和A-CN-K三角模型图等分析,探讨了该硬粘土的成因及堆积后的后期改造作用。研究结果表明,该硬粘土形成于大约29～10kaB.P,为晚更新世晚期的风成堆积物。湖底硬粘土自堆积后至澄湖形成,长期暴露于地表,经历了强烈的风化成土作用,为中等风化强度。且3.50m上下段化学风化过程明显不同,3.50m以下硬粘土段为早期去Na、Ca阶段;3.50m以上硬粘土段已经进入中期去K阶段,经历了比前期更加强烈的化学风化和成土作用。反映了对全球气候变化及区域气候条件的响应。与典型风成堆积物相比,化学风化强度从洛川黄土→洛川古土壤、西峰红粘土→镇江下蜀土→澄湖SC6硬粘土→宣城风成红土依次增强。

Geochemical and Genetic Significance of Hard Clay in the Central Area of Chenghu Lake (Suzhou), Jiangsu Province,China

The Chenghu Lake is situated in central Taihu Plain in the lower Yangtze region. The lake bottom is extremely hard and flat and is mainly composed of loessial materials called ‘hard clay’. A 18 meters-long sediment core (SC6) was taken in the central part of the lake, and its first 6.00m part is the Late Pleistocene uppermost hard clay of the Yangtze Delta. AMS14C, discriminant function (DF), CIA index and Na/K and A-CN-K triangle model diagram were analyzed in this paper, then the origin of the hard clay and post transformation were discussed. The results indicated that the hard clay was formed in about 29 kaB.P~10 kaB.P, belonging to Late Pleistocene aeolian deposits. The hard clay has been exposed to the surface for a long time from the accumulation of hard clay to matured Cheng Lake. CIA index and A-CN-K triangular diagram indicate that hard clay of SC6 has undergone intensive moderate chemical weathering. And the chemical weathering intensity of hard clay from 3.50 m to 6.00 m depths is obviously different from that in the upper part (from 3.50 m to 0 m).The sediments from 3.50 m to 6.00 m depths 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 and soil-forming process. This chemical weathering characteristics reflect the global climate change and regional climatic conditions. Compared to classic eolian deposits, the order of chemical weathering intensity is as follows: loess in Luochuan< paleosol in Luochuan and Late-Tertiary red-clay in Xifeng< Xiashu loess in Zhenjiang< SC6 hard clay in the Chenghu Lake< eolian dust red earth in Xuancheng of Anhui Province.