阿拉善地区古水下沉积物与风沙沉积物磁学特征及意义

Magnetic Characteristics of Aeolian Sand and Ancient Underwater Sediments and Their Implications in Alxa Region of China

风力搬运过程中存在的物质分选极可能导致沉积区与物源区沉积物性质出现差异,对于研究物源示踪具有重要的意义。选取阿拉善地区古水下沉积物和风沙沉积物,进行磁学参数测定,调查分选作用对沉积物性质的影响,并探讨对中国黄土高原黄土物源研究的启示。结果表明:古水下沉积物与风沙沉积物的细砂组分磁学性质基本一致,但粉砂组分则存在明显差别,古水下沉积物的磁性矿物浓度显著低于风沙沉积物,风力分选作用可能是风沙沉积物粉砂组分磁性强的主要原因;古水下沉积物中高矫顽力矿物浓度较低,受沉积后改造作用微弱,指示古水下沉积物可用于黄土物源示踪研究。阿拉善地区沉积物和黄土高原黄土的磁学性质对比表明:风沙沉积物与黄土高原黄土磁学性质存在明显差别,但古水下沉积物与黄土高原黄土磁学性质却十分相似。这一地质证据支持阿拉善地区是黄土高原黄土沉积物的重要物源区,同时暗示古水下沉积物在黄土物源示踪方面的巨大潜力。

Geological evidence and satellite observation usually obtained difference conclusion in tracing source of Chinese loess.It is widely observed that the southern Mongolia and Alxa Region are most frequent sandstorms outbreak regions where a huge number of loess-silt particles could be transported and deposited in Chinese Loess Plateau.However,environmental magnetism and geochemical evidences of district surface soil didn’t support this result.It is clear that the wind sorting exists in sandstorms which must provide most material of Chinese Loess Plateau and this process may lead to different characteristics between sedimentary deposits and provenances.In this paper,aeolian sand and ancient underwater sediment samples had been collected from Alxa Region,and the environmental magnetism parameters are determined.The results suggest that：（1）Aeolian sand and ancient underwater sediments show similar magnetic characteristics in coarse particles（63-1 000μm）,but obviously different in fine particles（≤63μm）.It may be caused by wind sorting.（2）Ancient underwater sediments show low concentration of high coercivity magnetic minerals which indicates weak mineral transformation during post-deposit period.It indicates that the ancient underwater sediments can be used to detect source in Alxa Region.（3）By comparing magnetic characteristics of fine particle between Alxa Region sediment and loess,we observed that loess show obviously different with aeolian sand but similarity with ancient underwater sediments,which reveals great potential to trace loess source.