川西高原甘孜黄土微体炭屑的粒度特征与古环境意义

Grain size characteristics of microfossil charcoal and the environmental implications in loess deposits from Ganzi,Western Sichuan Plateau

选取川西高原末次间冰期以来甘孜黄土沉积序列进行微体炭屑分析,逐粒统计炭屑的长轴长度,得到微体炭屑的粒度分布曲线和累计分布曲线,计算炭屑的数密度、粒径和粒度特征参数.结果表明,黄土和古土壤样品中炭屑粒径在10~130μm,平均粒径为24μm.粒径10~30μm的炭屑占60%~80%;粒径10~50μm的炭屑占比>90%;粒径50~100μm的炭屑占2%~9%;粒径>100μm的炭屑占比极低,平均仅为0.14%.炭屑粒径的频率分布曲线基本呈正态分布,偏度在-0.54~-0.13,为负偏,峰度2.74~3.19,分选系数0.61~0.73,粒度参数表明,微体炭屑的分选性良好.末次间冰期以来甘孜黄土炭屑以细颗粒组分为主,且古土壤层中大颗粒炭屑的占比高于黄土地层,可能主要由区域性自然火灾事件引起.

A detailed analysis was conducted of the particle size of charcoals under the microscope on the Ganzi loess sequence in Western Sichuan Plateau since the last interglaciation.Our results showed that the mean grain size of charcoals was 24μm,and the main size fraction of charcoals in all samples was between 10 and 130μm.The size fraction of 10 to 30μm was 60%-80%,while the 10-50μm fraction more than 90%.The coarse fraction of 50 to 100μm was 2%to 9%,and the>100μm fraction with an average percentage of 0.14%.The frequency distribution of the particle size of charcoals was basically of a normal distribution possessing a negative deviation between from-0.54 to-0.13.The kurtosis was between 2.74-3.19,and the sorting coefficient was 0.61-0.73,indicating that the microcharcoal was of poor sorting.The character of particle size distribution mainly reflected that,since the last interglacial period,the micro charcoals in the Ganzi loess was dominated by a silt size fraction,which was mainly caused by regional natural fire events.