晚第四纪中国北方沙地风沙沉积的Zr/Rb比值特征及意义

Zr/Rb比值因不受后期成壤作用影响,在黄土高原黄土堆积中可作为较好的冬季风气候替代性指标。与黄土沉积相比,中国北方沙地的气候变化研究相对较弱,能够敏感反映沙地气候变化的替代性指标仍处于探索阶段。本文选取东亚季风控制区的毛乌素沙地、浑善达克沙地和科尔沁沙地11个典型的风成沉积序列,对Zr/Rb比值及其意义进行了研究。通过全岩样品Zr、Rb元素含量、Zr/Rb比值与平均粒径和>63 μm的粗颗粒百分含量对比分析,发现在毛乌素沙地,Zr/Rb比值与粗颗粒百分含量和平均粒径成正相关关系,与黄土高原的变化趋势相似,可作为冬季风强度的替代性指标。而在浑善达克沙地和科尔沁沙地,虽然Rb含量在砂质古土壤层中随着细颗粒物质的增加而增大,与毛乌素沙地的变化相同;但是,Zr含量也表现出增加趋势,导致Zr/Rb比值变化趋势与毛乌素沙地相反。其主要原因可能是沙地所处的构造地貌单元不同,物源区存在差异。因此,Zr/Rb比值作为冬季风的替代性指标在不同沙地应用需要慎重分析。

Variation of Zr/Rb Ratio from Aeolian Deposits and Its Implication in Dune Field in Northern China during the Late Quaternary

Zr/Rb ratio is the better climatic proxy for the East Asian winter monsoon than coarse grain size fraction which is widely used, just because the post-depositional weathering and pedogenesis does not modify the Zr/Rb ratio due to the immobility of Zr and Rb elements. Compared with in the Loess Plateau, the climatic change was studied poorly in dund fields in northern China. There is a serious need for concerted studies to a new and reliable proxy which can reflect climate change sensitively. This study proposes the different variation of Zr/Rb ratio and the implication in dund fields. 11 typical aeolian deposits profiles were selected from Mu Us dune field, Otindag dune field and Horqin dune field in northern China where climate is controlled by the East Asia monsoon. The Zr/Rb ratio, element content of Zr and Rb as well as grain size of bulk sample were measured. Variations of the Zr/Rb ratio in Mu Us dune field encompassing the last 60 ka displayed a generally similar pattern to that of >63 μm and mean grain size of bulk samples which was consistent with the trend of the Loess Plateau, so the Zr/Rb ratio recorded the change of the East Asia winter monsoon. However, in Otindag dune field and Horqin dune field, Rb content in sandy soil increased with fine particles while Zr content also raised resulting in the opposite shift trend of Zr/Rb ratio, which was different from Mu Us dune field. The cause may be due to the three dune fields located in different tectonic geomorphic units, the provenances of aeolian material are different from each other. Therefore, Zr/Rb ratio used as winter monsoon strength proxy applied in different dune fields requires careful analysis.