黄土高原风尘序列的碳酸盐成因及其风化过程

碳酸盐是黄土高原风尘序列的主要组成部分,其含量与原生碳酸盐溶解、再沉淀,以及含钙硅酸盐化学风化强度等密切相关。碳酸盐在表生环境下极易移动,且其迁移变化受大气降水、温度等古气候所制约,因此黄土次生碳酸盐记录了古气候的重要信息。甘肃西峰赵家川剖面碳酸盐含量、磁化率的系统测定和风尘序列地质特征研究表明,在整个剖面中,碳酸盐含量和磁化率曲线呈显著的负相关,并从剖面上部到红粘土底部,碳酸盐含量有增加趋势。黄土、古土壤和红粘土沉积分别处在不同风化阶段:黄土化学风化最弱,处于钙质残积阶段,古土壤处于强烈钙质淋溶阶段和硅铝残积阶段,红粘土化学风化最强烈,处于硅铝残积阶段或红土化阶段;不同的化学风化阶段古气候性质有别。

Genesis of the Carbonate in the Loess-Paleosol-Red Clay Sequence of the Chinese Loess Plateau and Its Weathering Phases

Carbonate is an important component in the loess-paleosol-red clay sequence of the Chinese Loess Plateau. Its content is closely related to original carbonate dissolution and re-deposition and the chemical weathering intensity of calc silicates. Carbonate is easily remobilized in surficial environment, with migration affected by the quantity of paleo-meteoric water and paleo-temperature. Thus the secondary carbonate recorded important information of paleo-climate. This paper has systematical measured the carbonate contents and magnetic susceptibility values of the Xifeng loess section in Gansu Province, and studied geological characteristics of the loess-paleosol-red clay sequence. The results indicate that there is negative correlation between the carbonate contents in the section and the magnetic susceptibility values, with the carbonate contents increasing from the top (loess) to the bottom (the red clay) of the section. The loess, paleosol, and red clay of the section are weathered in different levels respectively, with the loess formed in weak weathering stage of calc sedentary, the paleosol formed in strong calcium leaching and Si-Al sedentary stage, and the red clay layer formed in the most strong chemical weathering stage of the Si-Al sedentary stage or lateritization stage. Different phases of chemistry weathering are in accordance with different paleoclimatic characteristics.