青藏高原典型地区沉积物地球化学特征与矿物组成的粒度效应

Effect of sediments grain-size on geochemical characteristics and mineral composition in typical areas of the Qinghai-Tibet Plateau

地表松散沉积物中不同粒级的颗粒蕴含着不同的信息,这种差异与其物质来源、搬运和沉积过程中的分选作用以及沉积后的风化作用等有关。因此,可根据沉积物中不同粒级颗粒地球化学特征与矿物组成的差异,进行沉积物物源追踪或沉积环境重建。选择青藏高原在气候和地貌方面具有典型意义的柴达木盆地、错那湖、雅鲁藏布江流域作为研究区,分别采集了这几个地区的沙丘沙、黄土、湖积物、冲积物、洪积物、残坡积物等地表松散沉积物样品,用干筛法将沉积物样品分为两个部分,粗颗粒部分粒径范围为≥75~500μm,细颗粒部分粒径<75μm。对上述粗、细颗粒分别进行稀土与微量元素、Sr-Nd同位素、轻矿物组成的实验室分析测试,并对沉积物中粗、细颗粒的测试结果采用古典多维尺度方法进行相似性分析。结果表明:青藏高原松散沉积物中粗、细颗粒在轻矿物组成、微量和稀土元素含量、稀土特征、元素参数等方面存在差异;细颗粒部分蕴含了更多的环境信息,富含容易受化学风化影响的黏土矿物以及硬度较小、容易被磨蚀的方解石等矿物。在物源示踪中不同气候区的沉积物细颗粒不宜进行直接比较,应根据样品的粒度分布情况优先选择合适的粗颗粒组分进行物源示踪。

Different information is contained in fine or coarse among earth surface loose sediments.These differences were coursed by the sources of the sediments,processes of transport and deposition of the sediments,and the weathering of sediments after they deposited.We can trace the sources of the sediments,or reconstruct the past environment of study area by the different information.Tracing sources of aeolian sediment is key to reconstructing earth surface processes in arid areas and interpreting the paleoenvironmental significance of aeolian sequences.The Qaidam Basin,Co Nag Lake,and Yarlung Zangbo River in the Qinghai-Tibet Plateau(QTP)were chosen as our study area because of their important significances both in climate and landform.Loose sediment samples,including aeolian dune sand,loess,lacustrine deposits,fluvial sediments,proluvium,and eluvium-deluvium,were collected from the three areas.The sediment samples were separated into two parts by dry sieving:a coarse part with grain size between 75 and 500μm,and a fine part less than 75μm in grain size.Laboratory measurements of rare earth elements,trace elements,and Sr-Nd isotopic composition were made.Multidimensional scaling(MDS)represents the proximity of data(e.g.measures of similarity,closeness,relatedness)as distances among points in a multidimensional(typically two-dimensional)space.We calculated the Euclidean distance of every two samples in our samples and then applied classical MDS model to fit the data.The results show that the coarse and fine particles of sediment samples,the loose sediments in our study areas,QTP,differ in light mineral composition,trace and rare earth element composition.Compared to the coarse particles,the fine particles are rich in clay minerals that are easily affected by chemical weathering,and minerals with low hardness such as calcite,which is easily to be abraded.The fine part of the sediment contains more environmental information,suggesting that the fine part cannot be directly compared between different climate zones for provenance identification.To trace sediment sources,using the coarse part based on the grain-size distribution of the targeted sediment would be more appropriate.