基于碎屑锆石的黄土高原—毛乌素交界区石峁黄土粉尘物源估算*

对于黄土高原粉尘物质的来源以及沙漠和河流对粉尘的贡献,仍未达成一致的认识。本研究以毛乌素东神木市石峁剖面为研究对象,利用碎屑锆石年龄谱,对末次冰消期以来的风成粉尘堆积和河流沉积物展开源区分析,并使用逆蒙特卡罗模型进行物源的定量计算。结果显示,粒径分布和阴极发光(CL)图像显示石峁黄土为典型风成沉积。其粗颗粒碎屑锆石(>40 μm)年龄峰值集中在190~300 Ma、300~600 Ma、1700~1900 Ma和2300~2600 Ma,年龄谱显示近源(局地基岩碎屑)、中源(黄河河沙)和远源(阿拉善地区沙漠)均有贡献; 定量计算表明,三者分别贡献71%、21%和8%的风成粗颗粒。中、近源中,黄河及其支流侵蚀基岩后搬运沉积的漫滩物质成为粉尘输移的中转站,为黄土的形成提供了主要的物源。光释光测年结果表明,石峁剖面河流沉积物及其上覆的黄土物质分别形成于倒数第2次冰消期和末次冰消期,两者之间的沉积间断可能指示了末次冰期至冰消期的侵蚀事件,导致了河滩物质暴露,从而形成了潜在的局地尘源。这些结果表明,包括黄河干支流在内的河流系统对黄土粉尘有重要的物源贡献。

Estimation of dust source of Shimao loess in Loess Plateau-Mu Us junction area based on detrital zircon

There is still no consensus on the source of dust in the Chinese Loess Plateau,especially the contribution of deserts and rivers. Focusing on the Shimao section of Shenmu city in the eastern Mu Us Desert,this study analyzes the source of aeolian dust deposits and river sediments since the last deglaciation by using detrital zircon age,as well as Inverse Monte Carlo model to quantify the provenance distribution. The results of particle size distribution and Cathodoluminescence images show that Shimao loess is composed of typical aeolian deposits. The peak ages of coarse detrital zircons(>40 μm)in Shimao loess occur at 190~300 Ma,300~600 Ma,1700~1900 Ma and 2300~2600 Ma,demonstrating that the proximal(local bedrock detritus),intermediate(Yellow River sediments)and the distal(Alashan Desert)sources all contribute to the detrital zircon ages;quantitative calculations show that the three source areas contribute 71%,21% and 8% of the aeolian coarse particles,respectively. For the proximal and intermediate sources,the Yellow River and its tributaries eroded the bedrock and transported the deposited floodplain materials,which became the transfer station for dust transportation and provided the main material source for the formation of loess. The results of optically stimulated luminescence dating show that the river sediments and the overlying loess in the Shimao sectoin were formed in the penultimate deglaciation and the last deglaciation respectively. This in turn suggests that the sedimentary discontinuity between the river sediments and the overlying loess layer may indicate the erosion events from the last glacial to deglaciation period,which led to the exposure of river floodplain materials and the formation of potential local dust sources. These results indicate that river systems including the Yellow River and its tributaries have an important provenance contributions to dust loess.