西安蓝田杨家湾黄土中第一层古土壤(S_1)元素含量与环境

THE ELEMENT CONTENTS OF S_1 PALEOSOL IN YANGJIAWAN, LANTIAN,XI'AN AND THE ENVIRONMENTAL IMPLICATION

研究地区处在关中平原的中南部,蓝田杨家湾剖面距西安市东约50km。在第一层古土壤(S_1)剖面中以10cm间隔采样,共采集样品41个。利用X-荧光元素分析仪进行了元素分析,根据野外观测确定了Fe_2O_3和CaCO_3的迁移深度、风化淋滤黄土的分层及厚度。野外观察识别出了S_1古土壤粘化层之下发育了深部风化黄土层,当时风化淋滤深度比现今大约2m;该层土壤剖面可分为粘化层(Bt)、含红色铁质粘粒胶膜的风化黄土层(Cs)、不含红色铁质粘粒胶膜的风化黄土层(Cl)和CaCO_3结核淀积层(Ck),剖面构成为Bt-Cs-Cl-Ck型。实验结果表明,S_1粘化层中SiO_2,Al_2O_3和Fe_2O_3平均含量分别为63.97%,15.33%和6.03%,比下部的风化淋滤黄土层高,显示有一定的富集。S_1古土壤的CIA平均值为69.25,表明S_1古土壤的风化程度中等,红色铁质粘粒胶膜的明显迁移指示当时达到了酸性风化阶段。蓝田杨家湾S_1土壤剖面中Fe_2O_3和CaCO_3的分布特征及风化黄土层的发育指示该层土壤为亚热带黄棕壤,属于酸性淋溶土壤,当时该区为亚热带气候,年均降水量和年均温度比现今明显高。S_1古土壤发育时东亚夏季风活动显著增强,并能够较频繁越过秦岭山脉,给该地区带来了丰富的降水。

Yangjiawan section（34°03′41″N, 109°32′51″E ）, 50km to the east of Xi＇an, lies in the south central area of Guanzhong Plain. Fourty-one samples were collected from the section of S1 paleosol at a interval of 5cm. The major oxides and trace elements are analyzed by X-ray fluorescence analysis. The migration depth of Fe2O3 and CaCO3 , the layer and thickness of weathering and leaching loess has been observed in the field. It is observed that the deep weathering and leaching loess has developed under the S1 paleosol, more than 2m than nowadays. The St paleosol section contains Bt, Cs, Cl and Ck, and therefore belonging to Bt-Cs-Cl-Ck type soil. The results show that the average contents of SiO2,Al2O3 ,and Fe2O3 of S1 paleosol are 63.97% ,15.33% and 6.03% ,higher than that of the deep weathering and leaching loess. It does not only indicate that these oxides have a certain enrichment in the Sl paleosol,but also suggest that these trace elements have a certain loss in the S1 paleosol. The average CIA of S1 paleosol is 69.25, and it reveals that the weathering grade of S1 paleosol is medium middling. The apparent migration of red ferruginous clay film indicates that the soil has undergone acidic weathering stage. The distributive characteristics of Fe2O3, CaCO3 and the development of deep weathering and leaching loess indicate that the S1 paleosol was subtropical yellow brown forest soil, belonging to acidic alfisol. When the S1 paleosol was developing, the climate of the region was subtropical, with humid monsoon, and the mean annual precipitation and temperature were obviously higher than nowadays.