洛川黄土碎屑锆石的粒度特征及其古气候意义

The grain size of the detrital zircon of Luochuan and its paleoclimate implication

目的通过对洛川剖面12个层位的锆石粒度分析测量,探讨洛川黄土-古土壤及新近纪红黏土中碎屑锆石的粒度特征及其反应的古气候变化特征。方法利用Quanta400 FEG场发射环境扫描电子显微镜获得锆石的二次电子图像,并用Scandium软件对锆石的二次电子图像进行粒径测量。结果第四纪黄土-古土壤及新近纪红黏土中存在大量碎屑锆石,除L24层黄土外,其余黄土层中的锆石粒度均显示出相似的分布特征,主要粒级分布于20～60μm之间,与全岩粒度相比,其中值粒径、平均粒径均有差异,但L9与L15层中锆石中值粒径与全岩差异最小,红黏土中差异最大。结论成壤作用对粉尘沉积物全岩粒度分析结果影响较大,而对稳定重矿物锆石粒径影响较小。第四纪黄土L9与L15层全岩粒度分析中粗颗粒含量明显增加,应归因于此时粉尘沉积物土壤化过程中成壤作用减弱。新近纪红黏土粗颗粒锆石百分含量的增加,并不预示着此时冬季风的明显加强,而是与周缘裸露的基底岩石提供了近距离的物源有关。L24层黄土在各项粒度指标均显示出其特殊性,可能与青藏高原的阶段性隆升有关。

Aim Through measuring the detrital zircon grain-size of 12 samples in the Luochuan Quaternary loess-paleosol sequence and Neogene red clay,it is found that the zircon grain-size has an excellent long-term proxy record of East Asian monsoon climate changes.Methods Using the Quanta400 FEG Environmental Scanning Electron Microscope（ESEM） to get the SEM image from samples,and measuring the grain-size base on the SEM image by using Scandium software.Results The gain-size data show that Quaternary loess-paleosol and Neogene red clay contain a lot of the contents of detrital zircon,besides loess layer L24,all loess layers in Luochuan area exhibit similar distribution characteristics,main content distribution in the area between 20~60 μm,the zircon median size and average size have differences in part of loess and red caly formation.But Quaternary loess layer L9 and L15 zircon median size have the minimial differences from loess formation,otherwise the red caly layer has the biggest differences.Conclusion Pedogenesis has more influence on the grain-size of loess-paleosol dust sediments than zircon（a stable heavy minerals） grain-size.In Quaternary loess layer L9 and L15,grain-size significantly increases in the contents of coarse particle,probably due to the Pedogenesis decrease;detrital zircon samples in Neogene red clay have similar change trend,but they have different controlling factors.The increase of coarse zircon grain content in Luochuan Neogene red clay did not indicate the monsoon became stronger,perhaps it was related to the near provenance provided by the exposed basement rock in the surrounding area.The L24 zircon gain-size shows obvious diflerence from other loess-paleosol layers in median size and average size,maybe it was dependent on the uplift of Qinghai-Tibet plateau.